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Asymmetrical spondylolysis.

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Asymmetrical Spondylolysis
Charles F. Merbs*
Department of Anthropology, Arizona State University, Tempe, Arizona 85287-2402
unilateral spondylolysis; incomplete spondylolysis; fatigue fracture;
interarticularis; pedicle; spina bifida
The objective of this study was to examine examples of spondylolysis in which the pattern of
separation was clearly asymmetrical, in order to learn
more about the process of bone separation that produces
this condition. Although the primary focus was on unilateral complete separation, examples of asymmetry represented by incomplete separation and by complete bilateral
separation where the separation sites are in different locations on the two sides were included. Two collections
were used, one consisting of Canadian Inuit skeletons
curated at the Canadian Museum of Civilization, and the
other of 48 individual examples of asymmetrical spondylolysis from sites in a variety of localities curated by
several different institutions. The first collection was
studied primarily to observe early manifestations of spondylolysis, particularly incomplete separation, while various patterns of asymmetrical complete separation were
the focus of the second. The results indicate that asymme-
try is part of the earliest osteological picture of spondylolysis, with right-sidedness predominating, a condition perhaps related in some way to handedness. The right-side
predominance appears to decrease with age. The ratio of
unilateral to bilateral separation ranges from 3–33% in
different studies, and a significant number of the unilateral separations have spina bifida occurring in the same
vertebra. Overall, the specimens examined here, considered along with clinical cases, nicely illustrate a progression of spondylolysis. A unilateral separation may heal, it
may progress to bilateral separation, or it may remain as
a permanent condition, producing a pattern of degenerative changes that can include spondylolisthesis. A unilateral healing of bilateral complete separation is likely a
rare phenomenon, at least after the separations have
reached a certain level of maturity. Am J Phys Anthropol
119:156 –174, 2002. © 2002 Wiley-Liss, Inc.
Spondylolysis in its classic form is a fracture of the
lower lumbar region related to erect posture (Neugebauer, 1885; Poirier, 1911; Thieme, 1950; Merbs,
1996a). As such, it is a uniquely hominid condition.
It has also been observed that spondylolysis occurs
with higher frequency in individuals engaged in certain activities that appear to put unusual stress on
the lower back (Merbs, 1996a,b).
The classic presentation of spondylolysis is complete separation through the isthmus between the
superior and inferior articular processes (interarticularis) on both sides of the arch of a lower lumbar
vertebra (Merbs, 1996a). The result of this symmetrical fracturing is a “separate neural arch” and a
potential for anterior slippage (spondylolisthesis)
above the point of defect. This is consistent with the
stresses of erect posture being transmitted equally
to the two sides of the neural arch. Spondylolysis
has other presentations, however, particularly with
respect to vertebra affected, location of defect, degree of separation, and unilateral involvement.
Many of these include some expression of asymmetry, a situation more likely to reflect specific activity
patterns than simple erect posture.
Asymmetrical spondylolysis has been given little
attention in anthropological studies, usually being
relegated to passing mention or descriptions of individual specimens. With the exception of a study by
Waldron (1992), unilateral spondylolysis is simply
included among the totals obtained in specific studies. Miles (1975) noted, however, that unilateral
spondylolysis could be associated with spondylolisthesis, despite the fact that the arch is not completely separated. In his study of Alaskan Eskimos
and Aleuts, Stewart (1953) recorded examples of
spondylolysis in which the separation took place
through a pedicle rather than interarticularis, noting that such separations were nearly always paired
with separation through the isthmus on the opposite
side, thus making them asymmetrical. It was noted
in a study of Canadian Inuit skeletons (Merbs, 1995)
that complete separation on one side could be paired
with incomplete separation on the opposite side, and
that incomplete separation could begin from above
downward on one side of an arch, and from below
upward on the opposite side. It thus seems likely
*Correspondence to: Charles F. Merbs, Department of Anthropology, Arizona State University, Tempe, AZ 85287-2402.
Received 8 October 2001; accepted 5 February 2002.
DOI 10.1002/ajpa.10100
Published online in Wiley InterScience (www.interscience.wiley.
that asymmetry can be a property of spondylolysis
from its earliest expression, and that in some cases
it can remain as a permanent condition in adults,
even contributing to olisthesis.
The objective of this study was to examine asymmetrical expressions of spondylolysis to better understand the morphology and dynamics of this condition. Of particular interest was unilateral
spondylolysis, and its development and progression.
Questions considered included: 1) to what extent
does unilateral separation represent an early stage
of bilateral separation, 2) how does it become established as a permanent condition, and 3) is it ever the
result of bilateral separation where one side has
healed? Excluded from the objectives were any attempts to identify the kinds of activities or habitual
behaviors that could have produced the asymmetries observed, except for noting the predominance
of right-handedness in humans generally.
The stimulus for the study came from an initial
finding in a study of Canadian Inuit skeletons
(Merbs, 1983, 1995, 1996b) that classic spondylolysis (bilateral complete separation through interarticularis) accounted for barely more than half of all
affected intact vertebrae, and that asymmetry of
some type was obvious even in some of these. The
focus was thus directed toward discovering the significance of this asymmetry in the Inuit and what it
could contribute to a greater understanding of spondylolysis in general. Also included as a separate
series are additional examples of asymmetrical
spondylolysis that came to the author’s attention
during other studies, or were brought to his attention by other researchers.
Archaeological skeleton collections
In a study of 786 Aleut and Eskimo skeletons from
Alaska, Stewart (1953) found 259 (33%) vertebrae
with spondylolysis, 25 (9.7%) of these showing unilateral separation. Nine (36%) of the 25 unilateral
cases included spina bifida, compared with just 5 of
234 (2.1%) with bilateral separation (from Stewart,
1953, Table IV, p. 941). Stage of development of the
individual does not appear to be a factor in the
occurrence of spina bifida here, since only two individuals, both with bilateral separation, were less
than 18 years of age. The distribution of unilateral
cases without spina bifida is L2 ⫽ 1, L4 ⫽ 3, and
L5 ⫽ 12, while all nine with spina bifida were L5.
Among the bilateral cases were 15 which combined complete separation through a pedicle (anterior or posterior to the transverse process) with complete separation through interarticularis on the
opposite side, thus producing an extremely asymmetrical separate neural arch. Even the one example in Stewart (1953) of separation through a pedicle
on both sides (p. 944, his Fig. 6) is very unusual,
resulting in a markedly asymmetrical arch.
Gunness-Hey (1982) found unilateral separation
in 3 of 45 (6.7%) vertebrae with spondylolysis from
Kodiak Island, Alaska, and Simper (1986) found a
higher frequency of unilateral cases (4 of 34; 11.8%)
in Greenland Eskimo skeletons. Two of the Greenland cases (L1 and L3) included spina bifida.
The study by Snow (1948) of Archaic Indian skeletons from Indian Knoll, Kentucky, produced only
two examples of unilateral involvement among 62
(3.2%) vertebrae with spondylolysis. In both cases,
the defect was on the left side in L5. Stewart (1979)
found 47 vertebrae with spondylolysis in a study of
Arikara Indian skeletons from South Dakota, nine
(19.1%) of them unilateral. All vertebrae involved
were L5, with the defect on the left side in 3 and the
right side in 6.
Among European studies, Vyhnánek (1989) found
42 cases of spondylolysis in 227 Slavic-Avaric skeletons dating to the 7th– 8th centuries in Slovakia
and 49 in 803 ancient Slavic skeletons from Slovakia
and the Czech Republic. The percentage of cases
that are unilateral is given only for males: 7.1% for
Slavic-Avaric, and 15.3% for ancient Slavic. All of
the unilateral cases involved interarticularis, most
of the affected vertebrae were L5, and no significant
side difference was reported. Waldron (1992) found 5
examples of unilateral involvement among 54 (9.3%)
spondylolytic vertebrae from an Anglo-Saxon and
several Medieval sites in England. Three were male
and two female, all with ages in the 15–35-year
range. The left side was involved three times (all
L5), and the right twice (L5 and L2). Gaballah and
Badawy (1980) found 5 examples of unilateral separation among 17 (29.4%) spondylolytic vertebrae,
and one separated arch in 90 ancient Egyptian skeletons. The specific lumbar vertebrae were not always identifiable, but unilateral lysis was combined
with spina bifida in both cases of L5 involvement.
Most archaeological series contain too few examples of unilateral spondylolysis to provide useful
frequency values, but sometimes descriptions of individual cases can be informative. An example is
illustrated by Miles (1975, p. 33) from Mug House
(Mesa Verde, CO), involving a female estimated to
have been approximately 29 years of age when she
died. Pictured is an L5 with complete separation
through interarticularis on the right side and a
much expanded intact interarticularis on the left
side. A greatly enlarged inferior zygapophysial facet
on the left side indicates that the L5-S1 joint on this
side had undergone considerable degenerative
change, and first-degree olisthesis (less than 1/3 the
anteroposterior diameter of the body) relative to the
sacrum was reported (Miles, 1975, p. 31). Although
the author felt that spondylolysis had originally
been bilateral with the left side eventually healing,
this is not supported by the published radiograph
(Miles, 1975, p. 33). However, the vertebra does
have the appearance of a long-standing unilateral
separation, resulting eventually in severe osteoarthritis and olisthesis.
Vyhnánek (1989, p. 76) illustrates an unusual
case of unilateral spondylolysis affecting L5 in an
adult male from the 11th century Slavic site of Abraham. The separation is through an extremely narrow interarticularis which curves inward to constrict the neural canal (stenosis), while the right
isthmus, although unusually broad, is more normal
in appearance. The transverse process on the left
side is short and tapering, while that on the right
side is longer and blunt. Also, the left pedicle contains what the author calls a retrosomatic foramen.
Part of the remarkable asymmetry of this vertebra
may be due to its possessing a transitional morphology, essentially lumbar on the left side, but with
some sacral features on the right. If the affected
vertebra is correctly identified as L5, it means that it
is the 24th vertebra below the skull and thus an
example of partial cranial shift at the lumbosacral
border. How much of the asymmetry can be attributed to the border-shifting and how much to the
unilateral nature of the spondylolysis is then difficult to say.
Snow (1948, p. 522) illustrates a bilateral separation from Indian Knoll which appears to have existed for a time as a unilateral separation, based on
the markedly different appearance of interarticularis on the two sides. The isthmus on the left side
appears relatively normal in width, with the separated edges well-corticated and only slightly irregular, while that on the right side is twice as wide,
poorly corticated, and extremely irregular. This suggests that separation occurred first on the left side,
with the separation edges undergoing considerable
cortication, but no significant remodeling, before
separation developed on the right side. The great
thickening of the right isthmus, due apparently to
callus development, and the incomplete cortication
of the separation edges on this side, suggest that
attempted repair was taking place when death intervened. The right transverse process appears to
contain a foramen similar to that reported by Vyhnánek (1989, p. 76), but it could be just an artifact.
In what appears to be the only study dealing specifically with unilateral spondylolysis in archaeological skeletons, albeit just five cases, Waldron (1992)
made the following observations with respect to the
side showing the spondylolysis: 1) the lamina may
be underdeveloped, 2) the body may show asymmetrical posterior wedging, 3) the spinous process may
be deviated toward the opposite side, and 4) radiological evidence of sclerosis may be observable on
the opposite side. He concluded, however, by saying
that “these findings do not invariably accompany the
condition” (p. 180).
Anatomical skeleton collections
Willis (1923) found 8 of 31 (25.8%) spondylolytic
vertebrae to have unilateral separation in a study of
748 skeletons in the Todd Collection at the Smithsonian Institution. All involved L5, and in each case
the lysis was on the right side, in 2 of the 8 (12.5%)
in combination with spina bifida. Increasing his
sample to 1,520 individuals, Willis (as reported by
Stewart, 1931) found 20 of 80 (25%) spondylolytic
vertebrae to have unilateral involvement, but it is
not clear how many of these included spina bifida.
The side distribution for the lysis was 17 (85%) right
and 3 (15%) left, and the vertebrae involved were
L1 ⫽ 1, L3 ⫽ 2, L5 ⫽ 15, and L6 ⫽ 2.
By adding data from the Terry Collection at the
Cleveland Museum of Natural History to those previously published by Willis (1923), Roche and Rowe
(1951) were able to report on the vertebrae of 4,200
skeletons representing European-Americans and
African-Americans. In this expanded series, unilateral separations accounted for 32 of 183 (17.5%)
spondylolytic vertebrae, with right-side separations
occurring more frequently than left (22 to 10). The
vertebrae affected were L2 ⫽ 3, L3 ⫽ 1, L4 ⫽ 1, and
L5 ⫽ 27. The L5 cases probably included two previously reported by Stewart (1931) as L6. Six of the 32
(18.8%) unilateral cases included spina bifida, compared with just 3 of 151 (2%) with bilateral separation. All nine spina bifida cases involved L5. The
authors noted that “in almost all unilateral separations encountered the arch was asymmetrical, apparently because of the difference in length between
the two isthmi. Hence those with wide separations
were more asymmetrical than those with narrow
separations” (Roche and Rowe, 1952, p. 491). Roche
and Rowe (1952, p. 492) further noted that “no significant difference could be demonstrated between
the ages of those with bilateral separations and
those with unilateral separations.”
In a report on South African Bantu skeletons in
what is now known as the Raymond Dart Collection
at the University of the Witwatersrand, South Africa, Shore (1929) described a 45-year-old Xosa male
with unilateral (right) spondylolysis of L4. He noted
the presence of a rough line on the bone surface in
the position of the defect on the opposite side,
marked asymmetry with the left lamina being much
thicker and longer than the right, and slight deflection of the spinous process to the left. The asymmetry, he concluded, demonstrates that “the intervertebral stresses were transmitted mainly by way of
the left arch and pedicle” (Shore, 1929, p. 437). He
also described a similar case involving the right side
of L5 in a 36-year-old Shangaan male. Again the left
lamina is wider and the left pedicle thicker than the
right, and the spinous process is deflected to the left,
but in this case the spondylolysis is bilateral. He
suggested that the defect on the right side predated
and probably existed for some time before the appearance of that on the left. Eisenstein (1978) found
6 unilateral examples among 18 (33.3%) spondylolytic vertebrae from 485 skeletons in the Dart Collection. The defects were equally divided between
right and left sides, and all of the identified vertebrae were L4 (2/4, 50%) or L5 (4/13, 30.8%). Eisenstein (1978, p. 489) noted that “there was no reason
to suspect that the intact opposite pars interarticularis represented a healed fracture.”
Clinical studies
Spondylolysis including unilateral involvement
has been of considerable interest to the field of
sports medicine because of its possible association
with specific athletic activities. Letts et al. (1986),
for example, described 10 cases of unilateral spondylolysis in “highly competitive adolescent athletes”
engaged in repetitive training and competition exercises involving flexion/extension of the lumbar spine.
All defects occurred at interarticularis in L4 (1 case)
or L5 (9 cases). Rossi (1978) found 30 examples of
unilateral separation among 225 (13.3%) vertebrae
with spondylolysis in athletes, most of them male.
Both studies found the lesion associated most commonly with gymnastics, but also with hockey (Letts
et al., 1986) and diving (Rossi, 1978).
In their study of living North Alaskan Eskimos,
Kettelkamp and Wright (1971) found 8 vertebrae
with unilateral separation among 45 (17.8%) with
spondylolysis in 43 individuals. The units affected
were L4 (2/4, 50%) and L5 (6/41, 14.6%), and all
separations occurred at interarticularis. They also
recorded a narrowing of interarticularis on the defective side in 5 of the 8 cases.
Lowe et al. (1987) reported that seven of their
patients with unilateral lysis showed changes in the
opposite hemiarch, including hypertrophy and a
healed defect at interarticularis, along with arch
deformity and sclerosis. They also found increased
scintigraphic uptake at interarticularis in some of
the affected vertebrae, indicating that bone activity
was taking place.
Maldague and Malghem (1976) approached the
problem from a different direction, looking at 50
individuals with unilateral arch hypertrophy of unknown origin. In 32 (64%) of these, the apparent
cause turned out to be a separation at interarticularis on the opposite side. They also observed that a
vertebra affected by spondylolysis on one side and
arch hypertrophy on the other side also showed a
distinctive asymmetry of the arch, which included a
tilting of the spinous process. They were concerned
that a defect like this could cause rotational instability. Porter and Park (1982) also observed asymmetry of the neural arch in the five cases of unilateral spondylolysis that they studied. The asymmetry
does present one advantage in the clinical context,
however: it can easily be detected on simple anteroposterior radiographs, thus allowing prediction of
the presence of a unilateral arch defect (Maldague
and Malghem, 1976; Porter and Park, 1982).
Although it is clear that separation can develop in
both hemiarches simultaneously (Merbs, 1995), clinical studies have also confirmed that unilateral separation can progress to bilateral separation. Hadley
(1963) described a 14-year-old male with relatively
mild back pain, but no sign of spondylolysis when he
was first examined. Two years later the symptoms
had become more severe, and radiographs showed a
well-established lysis on the left side of L5. The right
interarticularis was “crossed by a band of decreased
density” that could be related to stress fracturing.
Aland et al. (1986) described a 25-year-old individual in which complete separation at interarticularis on the left side of L4 ultimately led to pedicle
separation on the right. Radiographs identified
spondylolysis along with sclerosis of the right pedicle, and computed tomography (CT) showed a lucency in this pedicle. A follow-up 4 months later
showed massive overgrowth of bone on the pedicle,
similar to the callus development seen at a fracture
site, and an excisional biopsy of the pedicle did indeed reveal an established fracture and nonunion.
The authors concluded that spondylolysis through
the left interarticularis produced instability, leading
to hypertrophy and a stress fracture of the right
pedicle. Persistence of the instability resulted in
nonunion of this fracture. The pain was apparently
due primarily to a continued overgrowth of bone
impinging on the root of the fourth lumbar nerve.
Garber and Wright (1986) described a case involving
a 26-year-old which appears identical to that of
Aland et al. (1986), even to the vertebra and side
affected. Four years later, the condition had progressed to complete bilateral separation through interarticularis.
In a restudy of young athletes, Letts et al. (1986)
discovered that 5 unilateral cases had healed, 3 were
basically unchanged, and 2 had progressed to bilateral separation. Albers and Yochum (1980) also described a case of unilateral separation progressing to
bilateral. A 21-year-old female with a 5-year history
of low back pain aggravated by athletic activities
was identified as having a radiolucent defect at left
interarticularis identified as spondylolysis, with
sclerosis and enlargement of the right pedicle. Radiographs taken 3 years later showed bilateral separation at interarticularis. The pedicle was still enlarged but not sclerotic. This case suggests that a
change in stress focus had occurred on the right side,
from the pedicle to interarticularis, or that interarticularis was simply the more vulnerable of the two
The progression from unilateral to bilateral separation is not limited to young individuals, as illustrated in the study by O’Beirne and Horgan (1988) of
a 40-year-old man with a 10-year history of lower
back pain who presented with complete spondylolysis of the right hemiarch. The left side was still
essentially intact, but the lamina showed a thickening on either side of the defect, suggesting the existence of a stress fracture that was progressing toward nonunion. “In the presence of unilateral
spondylolysis,” according to O’Beirne and Horgan
(1988, p. 221), “the remainder of the neural arch is
exposed to abnormal stresses, and it seems plausible
to suggest that the defect in the contralateral lamina was, in fact, a stress fracture sustained secondary to spondylolysis. This view is supported by the
hypertrophic appearance of the bone on either side
of the defect, consistent with repeated attempts at
Although the time necessary for unilateral separation to produce observable changes in the opposite
hemiarch is uncertain, Maldague and Malghem
(1976) felt that in the cases they studied the separation was “not of recent origin.” Nevertheless, Sherman et al. (1977) found such changes even in very
young individuals. Among patients with unilateral
spondylolysis and “reactive sclerosis and hypertrophy” of the opposite pedicle and lamina whom they
studied was one as young as 8 years of age, and 10
between 12–21 years. The vertebrae and side affected were L2 (1 left), L3 (1 left), L4 (2 left), and L5
(2 right, 5 left). All but the youngest had reported
back pain.
Lusins et al. (1994) used standard CT and singlephoton emission computed tomography (SPECT) to
evaluate three cases of unilateral spondylolysis. In
the youngest, a 16-year-old male with a 5-year history of back pain after exercise, CT showed L5 lysis
on the left side and degenerative changes in the
right L5-S1 zygapophysial joint, while SPECT
showed increased activity in this joint. In the second, a 33-year-old male with a 10-year history of
chronic back pain, CT showed L5 lysis on the left
side and some degree of hypertrophy of interarticularis on the right. Although the L5-S1 zygapophysial
facets appear normal, SPECT disclosed increased
diffuse bone activity in this area. In the third, a
46-year-old female, CT showed L5 lysis on the left
side and hypertrophy of interarticularis on the right.
Although the zygapophysial joints at this level appeared normal on standard radiographs, SPECT
showed increased activity anteriorly and to the right
of the midline. The authors concluded from the three
cases that following unilateral separation, stress
will be transmitted unevenly to the pedicle, interarticularis, and articular facets on the opposite side,
and that this can be confirmed by positive SPECT
If the process observed by Lusins et al. (1994)
were to continue beyond the stage they observed, the
result presumably would have been zygapophysial
degeneration on the side opposite the spondylolysis,
and perhaps olisthesis. Gunzburg and Wagner
(1988) reported on a case of L4 right unilateral spondylolysis in a 41-year-old Turkish woman with an
olisthesis of approximately 20%. The authors noted
the difference between the typical olisthesis associated with complete bilateral spondylolysis and degenerative olisthesis which occurs in the absence of
spondylolysis, and they decided to apply the “degenerative” label to their case. Present also were degenerative zygapophysial changes and osteophyte development on vertebral disk margins.
Based on the case of a 32-year-old woman with a
12-year history of lower lumbar pain and spondylolysis on the left side of L5, Kornberg (1988) felt that
unilateral spondylolysis was responsible for prema-
ture degenerative changes. Besides L5-S1 zygapophysial degeneration on the right side and grade I (less
than one-third the anteroposterior body diameter)
olisthesis, the affected vertebra also showed spina
bifida. Slippage allowed by the lysis on one side
appeared to produce greater stress on the opposite
intact side, leading to joint remodeling that could
include the equivalent of degenerative olisthesis. A
rotation of L5 to the right was also seen on CT.
The data analyzed here are derived from two different sources. The first is a study of vertebral columns from Inuit archaeological sites in the Canadian Arctic which included detailed observations on
spondylolysis, spondylolisthesis, and related phenomena. The second consists of individual cases of
asymmetrical spondylolysis from archaeological contexts encountered by the author during other studies or brought to his attention by other researchers.
The Canadian Inuit skeletons are primarily from
the Nunavut Territory (formerly the eastern part of
the Northwest Territories). Most are from sites in
the Kivallo district west of Hudson Bay, with others
from Labrador (part of the province of Newfoundland) and the Northwest Territories (District of
Mackenzie). In all, 417 Inuit skeletons (children
through old adults) were found to include vertebrae
that could be used in the study. Sites producing the
largest number of skeletons with observable vertebrae include Tunermiut (Native Point, Southampton Island), 118; Silumiut, 112; Kamarvik, 68; Saglek Bay (Labrador), 38; Kulaituijavik, 15;
Inuksivik (including Ukusialik), 12; and Naujan, 9.
Also included are 11 skeletons representing the Caribou Inuit who lived in the Barren Grounds west of
Hudson Bay. The material ranges in time from the
Thule culture period, beginning about AD 1000 in
Arctic Canada, through the historic period (beginning 17th century), to the first half of the 20th century. The Sadlermiut of Southampton Island represent an isolated group that became extinct during
the winter of 1902–1903.
Sex and age determinations are based on standard criteria (Buikstra and Ubelaker, 1994), with
the condition of vertebrae specifically eliminated as
an age determinant. Sex in nearly all cases was
based on shape of the pelvis. Where possible, skeletons were placed in four approximate age categories:
adolescent, 13–18 years; young adult, 18 –30 years;
middle adult, 30 – 45 years; and old adult, over 45
The second series is composed of individual examples of asymmetrical spondylolysis involving at least
one complete separation. The specimens come from
the states of Alaska, Arizona, California, and New
Mexico, the Canadian provinces of British Columbia
and Newfoundland, and the Sudan in Africa. Where
direct observation of the entire skeleton was not
possible, information on age and sex obtained from
the primary observer was used. To this series were
TABLE 1. Canadian Inuit lumbar vertebrae with complete or incomplete spondylolysis listed by pattern of symmetry or asymmetry
Vertebra affected
Spondylolysis type1
1A, incomplete separation, unilateral; 1B, incomplete separation, bilateral; 2A, complete separation on one side, incomplete
separation on other side; 2B, complete separation, unilateral; 3A, complete bilateral separation at interarticularis on both sides (classic
form); 3B, complete bilateral separation at interarticularis on one side, through a pedicle or lamina on the other side.
added Canadian Inuit cases that met the criteria.
This second series totaled 48 cases which were
grouped as follows: unilateral complete separation
(31 cases), complete separation combined with incomplete separation (5 cases), complete separation
at interarticularis combined with complete separation through a pedicle or lamina (9 cases), and three
complete separations in the same vertebra (3 cases).
Data collected for vertebrae with spondylolysis
included the following where possible: vertebra affected and evidence of border-shifting (based on
morphology and number of units below the skull),
side and location of defect, complete vs. incomplete
separation, cortication and width of interarticularis
at defect site, symmetry of the arch and shape of the
spinous process, presence of spina bifida, degenerative changes involving disk or zygapophysial joints,
and spondylolisthesis. Specimens were radiographed when necessary.
The Canadian study produced 119 lumbar vertebrae with some manifestation of spondylolysis, but
five had to be eliminated from the asymmetry study
because one hemiarch was missing or too damaged
to make the required observations. The remaining
114 were then divided into three basic categories as
follows: type 1, showing incomplete separation, but
no complete separation; type 2, complete separation
on just one side; and type 3, complete separation on
both sides. The categories were also subdivided (Table 1). Based on whether separation was incomplete
or complete, as well as the part and side affected, 69
(60.5%) of the affected vertebrae are considered
symmetrical (types 1B, 3A) and 45 (39.5%) asymmetrical (1A, 2A, 2B, 3B). If the direction of incomplete separation (downward from the superior margin or upward from the inferior margin) is also
considered, two of the type 1B cases (Table 2, cases
1 and 5) would also be classified as asymmetrical,
increasing this category to 47 (41.2%).
Although spondylolysis may occur in older individuals, it seems more a condition of youth, and for
this reason incomplete spondylolysis (types 1A, 1B,
and 2A) in adolescents and young adults was chosen
as the starting point for studying early stages of
asymmetry. A previous study (Merbs, 1995) found at
least one vertebra with incomplete separation in 18
Canadian Inuit skeletons, 16 of them adolescent or
young adult. These 16 became a focus of the present
study. Expansion of the series to include Canadian
Inuit skeletons from other sites produced no additional examples of incomplete separation in the relevant age group. Among the 16 skeletons are 31
vertebrae with spondylolysis (Table 2). For study
purposes, however, 7 affected vertebrae were eliminated, 5 because the only separations present are
complete (case 4, L4; case 5, L5 and S1; case 7, L5;
case 11, L5), and 2 (case 8, L5; case 10, L5) because
one side is missing, thus making it impossible to
determine if the separation was asymmetrical. That
brought the final total of vertebrae in this series
with asymmetrical separation to 24.
Thirty-one incomplete separation sites are represented in these 24 vertebrae, 16 of them proceeding
downward from the superior margin, 12 proceeding
upward from the inferior margin, and 3 proceeding
from both margins simultaneously (Fig. 1). The ratio
of downward to upward separations (counting the
last three as both) varies by level, with upward
occurring more frequently only at L3 (3:2). The two
occur equally at L4 (8:8) and L5 (4:4), and all five
separations at S1 are downward.
Incomplete separations occur more frequently on
the right side than the left (19:11), and this is true at
every level: L3 ⫽ 3:1; L4 ⫽ 9:5; L5 ⫽ 4:3; and S1 ⫽
3:2. In case 4 (Table 2), an incomplete separation on
the left side of L5 is combined with complete separation on the right, thus adding L5 to the list of
vertebrae showing greater expression on the right
side and changing the ratio to 20:10. In both cases
where the fracture lines proceed in opposite directions on the two sides (cases 1 and 5), it is downward
on the left side and upward on the right side (Fig. 2)
(Table 2).
TABLE 2. Incomplete spondylolysis in adolescent and young adult Canadian Inuit1
Burial no.
18–20 years
18–20 years
18–20 years
21–30 years
21–30 years
21–30 years
21–30 years
21–30 years
21–30 years
21–30 years
21–30 years
21–30 years
Case numbers correspond to those in Merbs (1995, p. 2329). Sites: KA, Kamarvik; KU, Kulaituijavik; SIL, Silumiut; SN, Silumiut
North. Defect location: IA, pars interarticularis; PD, pedicle; LM, lamina. NS, no separation (neural arch intact); IS, incomplete
separation; , defect begins on superior margin and proceeds downward; Œ, defect begins on inferior margin and proceeds upward; CS,
complete separation; X, missing or damaged (no observation possible).
Fig. 1. Distribution by vertebra and side of incomplete and
complete spondylolysis in adolescent and young adult Canadian
Although the incomplete separations provide interesting information on fatigue fracture activity
that was taking place at time of death, 57.7% (41/71
separation sites) of the spondylolysis observed in
these adolescents and young adults was already
complete (Fig. 1). The greatest contributor of complete separations is L5 (28), followed by L4 and S1 (5
each), and L3 (2 cases). This suggests that most
spondylolysis begins at L5, then proceeds upward
and downward from that level.
Cases of complete bilateral separation among the
Canadian Inuit (Table 1, type 3A) in general provide
little information about the relative timing of the
separations on the two sides. In nearly all, the parts
involved had undergone considerable resorption and
remodeling, which obscured any evidence that one
side separated significantly ahead of the other.
Among the 13 young adults in this category, more
give the impression of having separated first on the
right side, but the evidence is inconclusive. A clearer
example is a case of L5 bilateral separation in a
young adult male. Unfortunately, the morphology of
the superior aspects of the separated edges had been
erased through contact with acidic soil that formed
in the grave (Merbs, 1997), but the inferior aspects
(arch portion) are nicely preserved and show complete cortication. Nevertheless, the right side displays noticeably more remodeling than the left, suggesting that separation occurred first on the right
Based on a matching of osteophytes and other
degenerative changes, eight clear examples of olisthesis following bilateral complete spondylolysis can
be identified in the Canadian Inuit series, with slippage ranging from 3–14 mm. The L4/5 level is affected three times, L5/S1 four times, and L5/L6 once.
The last, involving L5 and a lumbarized sacral vertebra, shows anterior slippage of 37% relative to the
diameter of the vertebral bodies (Merbs, 1983, p.
Fig. 3. Anterior view of fifth lumbar vertebra and sacrum
from Point Hope, Alaska, showing asymmetrical spondylolisthesis.
Fig. 2. Case 1, Table 2. Bilateral incomplete spondylolysis of
fifth lumbar vertebra. Top: Dorsal view shows separation extending laterally from superior margin of left interarticularis. Bottom: Inferior view shows separation on inferior margin of right
125, his Fig. 63B). Since olisthesis associated with
separate neural arch can be identified only by degenerative changes that take time to develop, and
then only under ideal conditions, it is likely that
other cases went unidentified, especially in younger
individuals. The Canadian series contains no obvious examples of asymmetrical olisthesis, but one
was observed involving L5/S1 in an old adult female
from Point Hope, Alaska (Fig. 3). Although the
amount of slippage appeared to be approximately
equal on both sides, L5 was tilted sharply downward
on the left side and large osteophytes had developed.
The opposing body surfaces were actually eburnated
on this side, indicating the absence of any disk material. This is indicative of unequal pressure on the
two sides, that on the left being much greater than
that on the right.
In the next phase of the analysis, Canadian Inuit
cases of spondylolysis types 2A, 2B, and 3B were
added to similar cases obtained from outside this
area. Excluded were the six 3B cases involving L1 or
L2. Although the mechanical forces and their effects
are similar in these two vertebrae to those seen in
the lower lumbar vertebrae, the etiology of the condition may be quite different. The defect in each case
presents as typical complete separation through interarticularis on one side or the other, but is very
atypical in that the margins of the defect are faceted,
giving the appearance of a joint in a situation where
no movement is possible. The etiology of this situation is not clear, but an error in fetal development
was suggested (Miki et al., 1991).
The addition of the Canadian Inuit cases involving L3–L6 brings the total number of specimens in
this series to 48 (Table 4). Included are 31 vertebrae
with complete separation on one side and the other
side intact (series I, numbers 1–31), 5 vertebrae with
complete separation on one side and incomplete separation on the other side (series II, numbers 32–36),
9 vertebrae with complete separation through interarticularis on one side and complete separation
through some other part of the arch on the other side
(series III, numbers 37– 45), and 3 vertebrae with
two complete separations on one side and one on the
other side (series IV, numbers 46 – 48). In only one
case are two vertebrae (numbers 4 and 33) from the
same individual.
The 48 affected vertebrae are distributed as follows: L3 ⫽ 2, L4 ⫽ 5, L5 ⫽ 37, and L6 ⫽ 4. Males
outnumber females 28 to 19 (sex not determined for
one). Involved are 68 separation sites located in the
following parts: interarticularis ⫽ 51, pedicle ⫽ 11,
lamina ⫽ 4, superior articular process ⫽ 1, and
inferior articular process ⫽ 1. Separation is complete at 63 sites and incomplete at 5.
In series I and II, complete separation occurs more
often on the right side (20:16). There is a marked age
difference, however, with right separations predominating (13:6) in younger individuals (numbers
1–16, 32–34), and left separations (7:10) in older
individuals (17–31, 35–36). In 11 cases of unilateral
separation (series I and II), the laminae of the affected vertebra had not fused (spina bifida), resulting in a separated hemiarch. The separated hemiarch was missing for eight of these, presumably not
recovered, not identified as human, or lost during
many decades of curation. In three cases of unilateral separation, the opposite side had broken postmortem, and only one of these arches was available
TABLE 3. Summary of vertebrae affected in adolescent and young adult Canadian Inuit skeletons with incomplete spondylolysis
Vertebrae affected
Incomplete ⫹ no separation
Incomplete ⫹ complete separation
Incomplete ⫺ direction difference
Incomplete ⫺ symmetrical
Complete separation - unilateral
Complete separation - bilateral
One side damaged or missing
for study. Although spina bifida cannot be ruled out
for the two missing arches, its occurrence seems
unlikely. In all cases of missing arches or hemiarches, the presence of zygapophysial facets on the
vertebra below indicated that they once did exist.
Based on information limited to recovered parts,
spina bifida occurred in 33.3% (11/33) of vertebrae
with unilateral spondylolysis (series I and II), but in
47.1% of the younger age group (8/17) compared
with 18.8% (3/16) in the older group.
Vertebrae with unilateral separation may also
show evidence of spondylolisthesis. This should be
relatively easy to identify in series I and II individuals, but observations are incomplete because of
poor preservation or missing adjacent vertebrae. It
was observed in just 1 of 18 (5.6%) younger individuals and 6 of 17 (35.3%) older individuals, with the
amount of slippage ranging from 3– 8 mm (Fig. 4).
The spondylolisthesis associated with unilateral
separation closely resembles that seen in degenerative spondylolisthesis (Merbs, 2001). In both cases,
the arch remains attached to the body, unlike in
classic spondylolisthesis following bilateral spondylolysis, where the arch becomes separated from
the body. As in degenerative olisthesis, the amount
of slippage associated with unilateral separation appears to be closely related to the severity of the
osteoarthritis present. Unlike in degenerative olisthesis, however, where both sides are affected, it is
only the side opposite the separation that exhibits
severe osteoarthritis in unilateral spondylolysis. As
indicated by the grooving and eburnation seen in
severe cases of degenerative olisthesis, considerable
linear vertical movement is taking place, but little if
any lateral movement. With time, this movement
and the pathological alteration of the joints that it
produces can lead to a forward migration of the
joints (olisthesis). Throughout the process, however,
the articular facets retain the curvilinear shape
(when viewed from above) normally associated with
joints at this level in the column. Because the condition is relatively stable with limited anterior
movement other than that associated with the olisthesis, the amount of slippage occurring is relatively
easy to measure (Merbs, 2001).
Based on the cases of long-standing unilateral
separation studied here, joint movement at the affected level appears to be much less stable. The
normally curvilinear zygapophysial facets become
flattened, thus allowing more lateral movement. The
shape assumed by the facets also indicates that vertical movement at the joints is not entirely linear, as
in degenerative olisthesis, but is combined with anterior movement. The inferior facets actually become convex when viewed from the side, reflecting
the superior anterior arc of movement taking place.
Because of the anterior movement taking place in
unilateral separation which is not directly associated with olisthesis, the amount of slippage occurring is more difficult to measure, depending to a
greater extent than in degenerative olisthesis on
how the vertebrae are positioned when the measurement is taken. It seems likely that the amount of
slippage observed in living patients with respect to
unilateral olisthesis would also vary somewhat depending on the positioning of the patient.
The basic asymmetry of long-standing unilateral spondylolysis may also be reflected in a wedging of the body of the affected vertebra, with the
intact side reduced in height. In case 24 (Table 4),
for example, the body measures 26 mm in height
on the side with the separation, compared with 21
mm on the intact side. Osteophyte formation indicative of disk degeneration is also more prevalent on the intact side.
All four cases of L6 spondylolysis (cases 6, 18, 24,
and 30) involved vertebrae that showed partial caudal shift, with essentially lumbar morphology on one
side and sacral morphology on the other (Fig. 5). The
transitional vertebra was separate from the sacrum
and appeared relatively mobile. In each case the
separation occurred on the “lumbar side,” even in
the two cases where the hemiarches had not united
dorsally (Merbs, 1983, p. 126, Fig. 64).
The morphology of separation involving interarticularis in series I cases (Table 4) ranges broadly in
length, regularity, and cortication of the edges. The
appearance at one end of the range is that of a fresh
fracture with rough edges, minute projections on one
edge usually matching quite well with indentations
on the other edge. The edges are in contact over their
entire length, and the fit is often too tight to observe
what might have been taking place along this line.
The defective interarticularis appears broader than
the intact one because of the addition of reactive
bone at the ends of the fracture line. Although these
early-stage cases are usually associated with
younger individuals, they may also appear in old
TABLE 4. Lumbar vertebrae with asymmetrical patterns of spondylolysis
Series I, unilateral complete separation
1. Pueblo Bonito, NM
2. Native Point, NT2
3. Amchitka Island, AK
Young adult
4. Silumiut, NT2
Young adult
5. Native Point, NT
Young adult
6. Kagamil Island, AK
Young adult
7. Unalaska Island, AK
Young adult
8. Grasshopper, AZ
Young adult
9. Paako, NM
Young adult
10. Pottery Mound, NM
Young adult
11. Puye, NM
Young adult
12. Sapawe, NM
Young adult
13. Silumiut, NT2
Young adult
14. Grasshopper, AZ
Young adult
15. Old Walpi, AZ
Young adult
16. Semna South, Sudan
Young adult
17. St. Lawrence Island, AK3
Middle adult
18. Abiquiu, NM
Middle adult
19. Kuskokwim, AK3
Middle adult
20. Mackenzie, NWT
Middle adult
21. Gran Quivira, NM
Middle adult
22. Kamarvik, NT2
Middle adult
23. Pueblo Bonito, NM
Middle adult
24. Semna South, Sudan
Middle adult
25. Native Point, NT2
Middle adult
26. Silumiut, NT
Middle adult
27. Puye, NM
Middle adult
28. Port au Choix, NFL
Old adult
29. Navajo Reservoir, NM
Old adult
30. Hawikuh, NM
Old adult
31. Pastolik, AK3
Old adult
Series II, complete separation ⫹ incomplete separation
32. Silumiut, NT2
Young adult
33. Pecos Mission, NM
Young adult
34. Ridley Island, BC
Young adult
35. Seward Peninsula, AK
Middle adult
36. Native Point, NT
Old adult
Series III, complete separation/interarticularis ⫹ complete separation pedicle or lamina
37. Amchitka Island, AK3
Male (?)
38. Umnak Island, AK
Young adult
39. Inuksivik, NT2
Young adult
40. Silumiut NT
Middle adult
41. Kamarvik, NT2
Middle adult
42. Saglek, LAB
Middle adult
43. Mocho, CA
Middle adult
44. Kamarvik, NT2
Old adult
45. Umnak Island, AK
Old Adult
Series IV, one complete separation ⫹ two complete separations
46. Pastolik, AK
Young adult
47. Native Point, NT2
Young adult
48. St. Lawrence Is., AK
Middle adult
3 mm
7 mm
3 mm
8 mm
3 mm
8 mm
6 mm
11 mm
7 mm
AK, Alaska; AZ, Arizona; BC, British Columbia, Canada; CA, California; LAB, Labrador, Newfoundland, Canada; NFLD, Newfoundland, Canada; NM, New Mexico; NT, Nunavut Territory, Canada; NWT, Northwest Territories, Canada; CS, complete separation; IS, incomplete separation; IA, interarticularis; PD, pedicle; LM, lamina; SP, superior articular process; IP, inferior articular
Included in Canadian Inuit portion of study.
Illustrated in Stewart (1953, p. 943–944).
adults (Fig. 6). Many of these early-stage cases probably reunite with subsequent remodeling, eventually making them difficult or impossible to detect.
If the separation persists, the osseous picture can
change dramatically. Through resorption and remodeling, the affected interarticularis may become
very narrow and the edges of the separation smooth
and well-corticated. A distinct gap may develop (Fig.
7). In sharp contrast, the intact side, particularly the
pedicle, may increase markedly in size through new
bone development, and the zygapophysial joint on
this side develops arthritis. As the arthritis becomes
more severe, the affected vertebra slips forward.
Perhaps the most variable feature of the changes
taking place is the appearance of the “intact” interarticularis which may undergo both fracture development or attempted repair, perhaps even in episodic fashion.
Fig. 4. Case 24, Table 4. Superior view of fifth lumbar vertebra and sacrum from Semna South, Sudan, showing complete
separation through right interarticularis and spondylolisthesis of
7 mm (exposed marker). Note deviation of spinous process toward
intact side.
Fig. 7. Case 26, Table 4. Inferior view of fifth lumbar vertebra
from Silumiut, Nunavut Territory, Canada, showing complete
separation through left interarticularis. Note severe osteoarthritis of right zygapophysial joint.
Fig. 5. Case 21, Table 4. Superior posterior view of sixth
lumbar vertebra from Gran Quivira, New Mexico, with complete
separation through left interarticularis. In terms of morphology,
vertebra is lumbar on left side and sacral on right. Note resorption of left lamina and protrusion of defect area into neural canal.
Fig. 6. Case 28, Table 4. Posterior view of fifth lumbar vertebra from Port au Choix, Newfoundland, showing complete separation through right interarticularis.
The arch in series I cases (Table 4) usually shows
some asymmetry attributable to the factors just described, although the range may vary considerably,
from barely detectable to considerable. A distinctive
feature of this asymmetry when the vertebra is
viewed dorsally is a deviation of the spinous process
away from the midline. Several vertebrae show no
obvious deviation and others are too damaged to
make the observation, but 13 of the 14 that do show
deviation are deflected toward the intact side. Sometimes deviation of the spinous process may also be
observed in vertebrae with unilateral incomplete
The vertebrae in this study provide little information regarding the spinous process tilt reported by
clinicians (Maldague and Malghem, 1976). In some
cases the process was too damaged to make the
observation, but most often it simply lacked a distinct vertical aspect that could express tilt. In those
cases where it was possible to make an observation
(cases 9, 16, 19, and 24), the top of the process tilted
toward the defect, as in the clinical cases.
Another source of asymmetry in vertebrae with
unilateral separation is a deviation of the arch into
the neural canal at the defect site, thus reducing the
size of the neural canal (stenosis). This appears be
due, in part at least, to a buildup of bony callus at
the fracture site (Fig. 5). Series II (Table 4), consisting of five vertebrae which combine complete separation on one side with incomplete separation on the
other side, are particularly interesting because they
potentially represent vertebrae in transition from
unilateral to bilateral separation.
In four of these (cases 32–35), the incomplete separation appears to have been in the process of developing at time of death. In the three young adults
represented (cases 32–34), the complete separation
looks to be of recent origin, while in the fourth, a
middle adult, it likely existed for a much longer
period before death.
In case 32 (cross-listed in Table 2 as case 4), the
complete separation has the irregular shape of a
recent fracture and is poorly corticated along its
entire length. The separation line appears to have
three distinct parts when viewed from the side, suggesting that it developed as the coalescence of two
separate partial separations, one extending downward from the superior margin of articularis and the
other up from the inferior margin, eventually joining
across the center of the isthmus by a horizontal
fracture line (Merbs, 1995, p. 2,331, his Fig. 4). Case
33 is similar to 32, but with the addition of spina
bifida. Also, the fracture line is less complex.
Case 34 exhibits complete spondylolysis through
the pedicle between the superior articular and
transverse processes of L5 on the right side. The
arch also fractured postmortem between the pedicle
and the body on the left side, and the body part was
not recovered. The surfaces of the two separations
present an interesting contrast, the fracture edges of
the cancellous bone at the postmortem break still
being sharp, while those associated with the spondylolysis show the effects of resorption (Fig. 8). The
left hemiarch is intact, but the surface of the bone
shows evidence of an incomplete separation through
the pedicle extending from the margin of the neural
canal to near the tip of the transverse process.
Viewed radiographically, the separation appears as
a narrow band of radiolucency bordered by irregular
patches of sclerotic bone. If allowed to continue, the
result would likely have been complete bilateral separation through both pedicles. With the passage of
additional time, the separation surfaces would have
become corticated and the separation sites remodeled, perhaps, given the large surface areas involved, even developing into pseudarthroses.
Case 35 is similar to that just described, but it
involves L3 in an older individual, and the complete
separation is at interarticularis, not through a pedicle. Although a gap did not develop at the site of the
complete separation, the surfaces are well-corticated, and considerable resorption and remodeling
took place, to produce an extremely narrow isthmus.
A radiograph of the affected vertebra published by
Stewart (1953, p. 944, his Fig. 7) shows a radiolucent
line extending across at least part of the pedicle,
from the margin of the neural canal to near the tip of
the transverse process, similar to that in case 34.
There appears to be a big difference in the timing of
the separations in the two individuals, however,
with the complete separation in case 34 barely in
advance of the incomplete separation, while the
complete separation in case 35 had to exist long
enough for major changes to take place at the separation site.
Case 36, which involves L4 in an old adult male
with severe osteoporosis, is similar to case 35, but
with the incomplete separation far more advanced
(Fig. 9). Although the complete separation on the
right side retains the irregular appearance of a fracture line, some cortication and resorption have
Fig. 8. Case 34, Table 4. Fifth lumbar vertebra from Ridley
Island, British Columbia, showing complete separation through
left pedicle and partial separation through right pedicle. Top:
Posterior view, showing surface of recent separation (upper left)
and postmortem break (right center), and partial separation (arrow) through left pedicle. Middle: Inferior view, showing incomplete separation (arrows) extending from margin of neural canal
to near end of tip of transverse process. Bottom: Radiograph,
showing opacity (sclerosis) in region of incomplete separation
taken place, and a narrow gap (1–3 mm) has formed.
Interarticularis on the left side is much broader, due
to a lateral extension of new bone that gives the
Fig. 9. Case 36, Table 4. Posterior view of fourth lumbar
vertebra from Native Point, Nunavut Territory, Canada, showing
complete separation through right interarticularis and nearly
complete separation through left interarticularis. Note extensive
development of new bone on left side, giving appearance of a
second transverse process.
appearance of a second transverse process. A separation extends laterally from the margin of the neural canal, through interarticularis, and through all
but the lateral tip of the process formed by the new
bone. The separated surfaces are broad, irregular,
and poorly corticated. Considerable bone activity appears to have been taking place along the fracture
line when death occurred, but whether the defect
was healing or proceeding to complete separation is
difficult to say based on its morphology. An interesting feature of this vertebra is that the inferior articular processes form a more oblique angle with the
body, at 125° (measured between the inferior surface of the body and the facet surface of the process)
compared with the 100 –105° usually seen. A raised
facet had formed on the inferior surface of the L4
spinous process and on the superior surface of the
L5 process, suggesting that the unusual angulation
of the L4 arch may have been caused by forces generated by the spinous process contact. This also implies that movement was possible between the arch
and the body portion of L4, allowing the increased
angulation to take place, and that the small amount
of healing seen on the left side occurred after complete separation. The L4 described here is actually 1
of 3 vertebrae affected by spondylolysis in this individual. Bilateral complete separation through interarticularis is present in L5 and incomplete separation at interarticularis on the left side in L6, a
lumbarized sacral unit with essentially lumbar morphology on the left side and sacral morphology on
the right. The spondylolysis at L5 resulted in an
estimated anterior olisthesis of 14 mm (37%) relative to L6 (Merbs, 1983, p. 125, Fig. 63B), which in
turn led to extreme osteophyte development (disk
degeneration) on the superior anterior margin of the
body. The problem at L5 likely contributed to that at
The nine vertebrae that make up series III (Table
4) combine complete separation through interarticu-
Fig. 10. Case 39, Table 4. Posterior view of fifth lumbar
vertebra from Inuksivik, Nunavut Territory, Canada, showing
complete separation through right interarticularis and complete
separation through left lamina adjacent to inferior articular process, with extensive new bone formation.
laris on one side with separation through a pedicle
(cases 37, 38, 40, 41, 42, 44, and 45), a lamina (case
39), or an inferior articular process (case 43) on the
other side. In five of the pedicle cases the separation
is located where the pedicle attaches to the vertebral
body (cases 37, 38, 41, 42, and 45); in the other two
it lies between the superior articular and transverse
processes. In some cases the line of separation is
irregular, having the appearance of a recent fracture, and the separation site has been enlarged by
reactive bone. In case 39, for example, the separation through interarticularis on the right side appears to have been in existence for some time before
death, with considerable remodeling having occurred (Fig. 10), while that through the lamina immediately adjacent to the inferior articular process
on the left side presents the appearance of a healing
fracture. The separation line is extremely irregular,
with its edges in tight contact, and the entire area
has been considerably expanded by new bone. Case
44 presents a very different appearance, with both
the interarticularis and pedicle separations showing
considerable maturity (Fig. 11). An indication of this
is that a large faceted area has formed between the
portion of the pedicle still attached to the body and
that attached to the arch, indicating that considerable movement occurred at this unusual joint
Case 43 is the most unusual in series III, because
neither pedicle nor lamina is involved. The separation on the right side extends through the inferior
articular process, while that on the left side is
through interarticularis, but at the extreme lower
margin of the superior articular facet (Fig. 12). Although the left separation site still has the irregular
appearance of a fracture line, cortication and minimal remodeling have taken place. In contrast, the
right side looks like an active fracture site, with
considerable new bone contributing to a very thick
Fig. 11. Case 44, Table 4. Fifth lumbar vertebra from Kamarvik, Nunavut Territory, Canada, showing complete separation
through right interarticularis and left pedicle between superior articular and transverse processes. Right: Posterior view of arch. Left:
Anterior view of arch, showing facet that formed at pedicle separation site.
arch in this area. What is clearly an unstable situation has led to 11 mm of olisthesis.
All three vertebrae in series IV (Table 4) combine
a single complete separation on one side with two on
the other. Cases 46 and 47 are virtually identical,
each involving a young adult male with bilateral
complete separation of L5, with an additional separation through the right lamina midway between
the spinous process and the inferior articular process. The separations through interarticularis are
completely corticated, but neither displays any obvious signs of olisthesis. The two vary with respect
to laminar separation, however. In case 46, the
younger of the two, this separation appears to be of
recent origin, presenting as an irregular fracture
line with poor cortication and some reactive bone
still present, while in case 47 the fracture site has
smooth edges and the separation surface is completely corticated (Merbs, 1983, p. 123, Fig. 60). In
addition, L4 in this individual has bilateral complete
separation at interarticularis and is estimated to
have slipped forward 4 mm relative to L5.
Case 48 was described by Stewart (1953, p. 944,
his Fig. 6) as having defects “through the pedicle on
both sides,” but the total effect is still markedly
asymmetrical. The separation line on the right side
begins at the neural margin of the arch just anterior
to the superior articular process and proceeds laterally and dorsally in approximately a straight line,
leaving most of the transverse process still attached
to the body. The pedicular separation on the left side
also begins at the neural margin just anterior to the
superior articular process, but it is shorter and
curves sharply following the anterior margin of the
process. A second, horizontal separation crosses interarticularis, but well above the usual location seen
in classic spondylolysis. These two separations effectively isolate the superior articular process from the
rest of the vertebrae. Unfortunately, this small part
was not available for study, but most of its morphology could be reconstructed from its adjacent parts.
Based on the presence of reactive bone, level of cortication, and degree of remodeling, the separation
dividing the arch from the body on the left side
appears to have occurred first with the development
of a pseudarthrosis similar to that seen in cases
40 – 42 and 44 (Table 4). The separation on the right
side and that isolating the left superior articular
process occurred later but in undetermined order.
Both of the separations on the left side were still
experiencing bone activity when death occurred.
Although the traditional view of spondylolysis is
that of bilateral complete separation through interarticularis, a process that effectively divides the
arch from the body in relatively symmetrical fashion, this study indicates that most spondylolysis
probably presents an asymmetrical picture as it develops, with the final symmetrical effect achieved
only after considerable remodeling has taken place.
Some cases, however, such as those involving unilateral complete separation and bilateral complete
separation where different parts of the arch are
affected on the two sides, may exhibit asymmetry as
a permanent condition.
Fig. 12. Case 43, Table 4. Fifth lumbar vertebra and sacrum
from Mocho, California, showing complete separation through left
superior articular process and right inferior articular process.
Top: Inferior view of L5, with parts separated to show edges.
Bottom: Superior view of L5 and sacrum, with parts articulated.
Most cases of spondylolysis are thought to result
from fatigue or stress fracturing of some part of the
vertebral arch, and the bare-bone appearance of the
condition in its incomplete form, and even frequently in its complete form, is certainly consistent
with this etiology. Stress fractures (spondylolysis
included) characteristically have a “subradiological”
period during which radiographs will appear normal
(Elliott et al., 1988). Presumably this is true of their
bare-bone appearance as well. However, the presence of a defect may be detected using forms of
imaging that visualize bone activity at a particular
site. For example, Bellah et al. (1991) found an abnormal focus of radiotracer uptake using single-photon emission computed tomography in the lumbar
spines of 71 young patients with symptoms of low
back pain possibly related to spondylolysis. In 66 of
these, the abnormalities appeared in the lower lumbar region, localized to interarticularis.
Perhaps even before the fractures become identifiable on radiographs, they may be visible on the
bare bone surface of archaeological specimens
(Merbs, 1995). They are seldom noted, however,
probably for the following reasons: 1) spondylolysis
usually occurs with low frequency, and only a fraction of these are likely to represent incomplete separation; 2) incomplete separation occurs primarily
in adolescents and young adults, age groups that
may be poorly represented in archaeological series;
or 3) they are simply overlooked. The high frequency
of this condition observed in the Hudson Bay Inuit
collections thus represents an unusual opportunity
to study this phenomenon in the context of asymmetrical expressions and spondylolysis formation. It
should be noted, however, that even in the young
individuals of these collections, incomplete separations occur primarily above and below L5, with complete separations occurring most frequently at L5.
This suggests that the primary activity resulting in
spondylolysis has already taken place, even in these
young individuals, and the near absence of incomplete separations in older individuals seems to confirm this (Merbs, 1995). Nevertheless, the incomplete separations that are present in the young
individuals, despite their transient nature, can provide interesting insight into the entire process.
The pattern that emerges suggests that the
stresses responsible for spondylolysis in its early
stages tend to be greater on the right side than the
left. They also appear to be greater on the inferior
margin at the L4 level and the superior margin at
S1, and greater on the inferior margin on the right
side above S1. The right-sidedness at this level of
involvement suggests that the asymmetry of the
stresses may in some way be related to handedness,
with activities emphasizing the dominant hand,
usually the right, being somehow transmitted to the
lower back. In the two vertebrae where the separation lines go in opposite directions, the pattern is
consistent with greater stress on the left superior
margin and the right inferior margin at interarticularis.
Limiting the asymmetry analysis to complete separation (i.e., the general approach of archaeological
and anatomical studies) has produced rates of unilateral separation to bilateral separation that range
from a high of 33.3% (6/18) in the anatomy skeletons
of the South African Dart Collection (Eisenstein,
1978), to a low of 3.2% (2/62) in Archaic Indian
skeletons from the Indian Knoll site in Kentucky.
The Canadian Inuit frequency of 21.2% (18/85) lies
near the midpoint of this range.
Although the literature contains little information
on sidedness in unilateral separation, the only relatively large series, the combined Todd and Terry
Collections, produced a right to left ratio of 22 to 10
(Roche and Rowe, 1951). In the unilateral series
analyzed here (Table 4), defects are also more numerous on the right side (20 to 16). Age may be a
factor, however, as right defects are much more numerous (13:6) among younger individuals but not
among older individuals (7:10).
Spina bifida and spondylolysis occur together with
higher-than-expected frequency according to the
clinical literature (Amuso and Mankin, 1967;
Fredrickson et al., 1984; Oakley and Carty, 1984),
but this association has been difficult to replicate in
studies of archaeological skeletons (Waldron, 1993).
When limited to unilateral separation, however,
particularly involving L5, an association with spina
bifida does seem likely. Stewart (1953), working
with archaeological skeletons from Alaska, and
Roche and Rowe (1951), working with the Todd and
Terry Collections, came up with spina bifida-unilateral spondylolysis distributions easily significant at
the 0.001 level, using the ␹2 test. This association is
supported by the present study, where the two conditions occur in the same vertebrae in one-third of
cases (Table 4, series I and II). Age may be a significant factor, however, as the frequency is much
higher in younger individuals than in the older
group (47% to 19%). All of the unilateral spondylolysis-spina bifida cases found by Stewart (1953), Simper (1986), and Roche and Rowe (1951) involved L5,
a common site for spina bifida, and all 11 in the
present study involve the last lumbar vertebra, L5
(9 cases) or L6 (2 cases). Given the different etiology
of the two conditions, the relationship between spina
bifida and unilateral spondylolysis seems purely mechanical, the discontinuity in the arch tending to
weaken it, but also acting as a block to the transmission of stresses from one side to the other.
Little information is available on asymmetrical
spondylolysis involving transitional lumbosacral
vertebrae, particularly when the morphology of the
affected vertebra is already asymmetrical, i.e., lumbar on one side, sacral on the other. To some extent,
the case described by Vyhnánek (1989, p. 76, his Fig.
8) meets this description, with the single separation
through interarticularis occurring on the lumbar
side. Four vertebrae in the present study also involve transitional lumbosacral vertebrae, in each
case a partially lumbarized sacral unit, and in all
four (two with spina bifida), the defect is on the
“lumbar” side (Table 4). Because of its basic morphology, the sacral side allows less movement than
the lumbar side, and thus there is less likelihood of
stresses that could lead to spondylolysis concentrating at interarticularis on the sacral side.
An intact arch may progress to unilateral spondylolysis and then to bilateral separation, but extracting information regarding the exact nature of
this process from skeletons is difficult. Here the
clinical data can be especially valuable. The identification of unilateral separation in a patient allows
for a close monitoring of the intact side of the arch,
to record any changes that take place. The earliest
changes that have been observed are scintigraphic
uptake at interarticularis, indicating that bone activity was taking place (Lowe et al., 1987), and the
appearance on radiographs of a band of decreased
density probably related to fatigue fracturing (Hadley, 1963). Archaeological equivalents of this would
appear to be cases 32 and 35 (Table 4), both of which
display narrow bands of lucency on radiographs as
well as external changes consistent with fatigue
fracturing. More general clinical observations included hypertrophy, sclerosis, and arch deformity
(Lowe et al., 1987). In some cases, what appears to
have been a developing defect healed with no apparent lasting effect (Letts et al., 1986); in others, the
condition progressed to bilateral separation (Albers
and Yochum, 1980; Aland et al., 1986; Garber and
Wright, 1986; Letts et al., 1986). In the presence of
unilateral separation, it would appear that the opposite side of the neural arch is exposed to abnormal
stresses that could lead to fatigue fracturing. The
hypertrophic appearance in the vicinity of the fracture site reported by several clinicians is consistent
with attempts at healing (Lowe et al., 1987;
O’Beirne and Horgan, 1988). An excellent example
of this is the 25-year-old individual described by
Aland et al. (1986), in whom sclerosis and lucency
were observed radiographically on the side opposite
a unilateral spondylolysis, followed by massive overgrowth of bone on the pedicle. Four months later,
the bone mass was excised and revealed an established fracture with nonunion. Although the phenomenon of unilateral separation leading to abnormal stresses and fracturing in the opposite
hemiarch appears to be associated primarily with
younger individuals, it was also observed in a 40year-old man with a 10-year history of lower back
pain (O’Beirne and Horgan, 1988).
Bilateral complete separation with subsequent
healing on just one side to produce unilateral spondylolysis has been reported in both the clinical and
archaeological literature, and it appears to be a real
phenomenon as long as the fracture site involved
remains active. After cortication and remodeling of
the separation edges, and given the potential for
movement between the separated arch and the body,
healing would seem an unlikely occurrence.
Asymmetry of the neural arch is commonly associated with unilateral spondylolysis, but the nature
and degree of asymmetry seem to vary considerably.
The kinds of asymmetry usually noted are as follows: 1) tilting of the vertical axis of the spinous
process with the anterior aspect closer to the side
with the defect; 2) deviation of the spinous process
from the midline toward the intact side; 3) a shorter,
smaller lamina, usually on the side of the defect; and
4) a wider (or narrower) interarticularis at the defect
site compared with the opposite side.
Arch asymmetry has the advantage of being easily
detected on simple anteroposterior radiographs of
living individuals, thus allowing the clinician to predict the presence of a unilateral arch defect. Arch
asymmetry was actually used as a sorting device by
Maldague and Malghem (1976), who discovered that
unilateral arch hypertrophy of unknown cause, as
identified in 32 of 50 individuals, turned out to be
associated with a separation at interarticularis on
the opposite side. Roche and Rowe (1952) attributed
asymmetry to the gap in the isthmus on the defective side, noting that those with a wide gap were
more asymmetrical than those with a narrow gap.
At best, however, this explanation can account for
only part of the asymmetry observed, as even cases
where the separated edges are in contact usually
show some arch asymmetry.
Spinous process tilt turned out to be the most
elusive of the factors of asymmetry examined in the
present study. Part of this was due to missing arches
and poor preservation, but some was due to the
morphology of the spinous processes involved, which
lacked a readily recognizable vertical linear dimension. In the four cases where it was possible to make
an observation, the top of the process did tilt toward
the defect.
Deviation of the spinous process away from the
midline provided more information, most of it consistent with clinical expectations. All but 1 of 14
showing deviation did so toward the intact side. This
appears consistent with the intact side bearing the
brunt of stresses generated in the arch following
unilateral separation.
The size and shape of the lamina on the defective
side relative to the intact side proved to be highly
variable and difficult to quantify. Narrower, shorter
laminae on the defective side appeared to correlate
roughly with the longevity of the defect: the longer
the separation existed, the smaller the lamina on
the side of the defect. The size discrepancy thus
appears due to resorption and remodeling with reduced stress on the defective side, contrasted perhaps with hypertrophy on the intact side.
The width of the isthmus at the site of separation
was found to vary greatly, ranging from much
broader than that on the intact side to much narrower. In those cases where a distinct gap is visible
at the separation site, the isthmus tends to be narrow. Most of the unilateral separations observed in
this study were consistent with lying along a continuum representing duration of the defect, and (perhaps even more important) phase of healing or nonhealing. Even in cases where the fatigue fracture
appears to still be developing, bone reaction at the
site tended to increase the diameter of the isthmus.
With progression of the fracture to some separation,
and for some time thereafter, the bone reaction may
be intense, resulting in an even greater widening of
the isthmus. The separation line tends to be very
irregular, giving the appearance of a fresh fracture.
The minute projections on one edge tend to match
quite well with indentations on the other edge, and
cortication, which is difficult to observe because of
the tight fit, is slight to nonexistent.
If union does not occur, reactive bone is no longer
produced, and the separation edges begin to lose
their complex topography and develop a cortex. This
is followed by resorption and remodeling, not only of
the new bone that formed, but of the original bone as
well. The result is a narrowing of the isthmus which
may or may not be accompanied by a gap forming at
the separation. Although the early stages of this
process tend to be found in younger individuals, they
may even be found in an old adult, as illustrated by
case 28.
With the stresses normally shared by both zygapophysial joints shifted to the side opposite the
defect, the joint on that side becomes severely
stressed. Using CT imaging, Lusins et al. (1994)
observed degenerative changes in the zygapophysial
joint on the intact side in an individual only 16 years
of age. With SPECT imaging they also demonstrated
the presence of bone activity on the intact side in
this and two other patients, even in the absence of
observable changes on CT images. The authors concluded from their cases that following unilateral
separation, stress to the back will be transmitted
unevenly to the pedicle, interarticularis, and articular facets on the opposite side, ultimately affecting
the zygapophysial joint on this side. A similar situation was described by Gunzburg and Wagner
(1988) and Kornberg (1988), with the similarity to
degenerative spondylolisthesis noted in both reports. The study of archaeological cases disclosed
some differences, however; the degenerative olisthesis associated with unilateral separation occurred
upward and forward along a curvilinear plane, while
that associated with bilateral separation was essentially a simple anterior migration of the joint. Other
indicators of increased stress on the intact side in
unilateral separation are a reduction in body height
and greater disk degeneration. Some of this asymmetry may account for what Kornberg (1988) referred to as a rotation of L5 seen on the CT scans of
one of his patients.
The clinical literature frequently associates pain
with unilateral separation; in fact, it is often that
pain which brings a case to the attention of a clinician (Lusins et al., 1994; Kornberg, 1988; Albers and
Yochum, 1980; O’Beirne and Horgan, 1988; Sherman et al., 1977; Hadley, 1963). Although archaeological specimens are hardly ideal for assessing the
pain factor, it can be noted that several elements of
osseous morphology are likely sources. The first of
these is the stress fracturing itself, a painful condition wherever it occurs. The second is the inward
curvature of the arch at the defect site that could
impinge on the root of the lumbar nerve at that
level, a possibility noted by Aland et al. (1986). Pain
at later stages could be associated with degenerative
Spondylolysis is more a process than a condition,
and its classic presentation in archaeological skeletons, i.e., bilateral separation through interarticularis, is essentially the end product of that process.
Trying to understand the process by studying only
its final effect has severe limitations. The study of
incomplete spondylolysis can provide greater insight
into the process by which a separation develops in
the vertebral arch, but few examples of this stage of
separation in archaeological specimens have been
described. Asymmetrical spondylolysis, most likely
produced by unequal stresses being applied to the
two sides, presents another opportunity to examine
those stresses, particularly as they affect the two
sides of a vertebra in different ways.
The earliest examples of spondylolysis recognizable in bare bone, based on the study of this phenomenon in Canadian Inuit skeletons, are frequently asymmetric, usually from side to side, but
sometimes in vertical direction as well (downward
from above or upward from below). Based on the
presence of complete separation in Inuit adolescent
and young adult skeletons, the process usually begins at the L5 level, with additional separations
then developing above (L3 and L4) and below (S1).
These additional separations in the adolescent and
young adult Inuit appear as incomplete spondylolysis, or stress fractures in progress. The presence of
bony callus at the separation sites reflects an effort
at healing, and clinical studies suggest that fracturing and healing may be episodic, resulting ultimately in complete healing or complete separation.
The incomplete separations occur more frequently
on the right side, suggesting that activities reflecting handedness may play a role. The study of examples of asymmetrical spondylolysis in archaeological
skeletons involving complete separation on at least
one side produced a similar picture with respect to
side involvement, the separation occurring more frequently on the right side, particularly in younger
It is clear from the clinical literature that cases of
unilateral spondylolysis may heal, progress to bilateral separation, or develop into a permanent unilateral separation. Although it is difficult to demonstrate these dynamic processes in dead bone, the
presence of external discontinuities and opacities on
radiographs suggestive of stress fracturing, along
with callus development in the vicinity of the defects, indicates that the defect sites were indeed
active when death occurred.
Although attempts to find correlations between
spondylolysis and spina bifida in the archaeological
context similar to those noted in clinical studies
have been largely unsuccessful, unilateral spondylolysis would appear to be an exception. In this
study, the two conditions were found to occur together with a much higher frequency than expected.
This may be due to greater stress being concentrated
on one side of the arch or the other because of the
discontinuity in the midline, thus increasing the
likelihood of separation on the stressed side. Another important factor is that if separation does
occur on one side, the discontinuity of the arch will
minimize its effect on the opposite side, making it
less likely that separation will occur on that side as
The examples of unilateral separation that remain as such, whether they are of archaeological or
clinical origin, eventually develop a distinctive pat-
tern of morphology that reflects the unequal stresses
on the two sides. The extent to which unequal
stresses contributed to the unilateral separation to
begin with is unclear, but the separation is clearly
related to morphological changes that subsequently
took place. The severe osteoarthritis that develops
in the zygapophyseal joint opposite the defect attests to the fact that this side bears a disproportionate amount of stress. Ultimately this can lead to an
anterior slippage, similar in many respects to that
seen in degenerative olisthesis. Other changes taking place that contribute to the overall asymmetry of
the affected vertebra include size differences in the
thickness of the laminae, width of the isthmi at
interarticularis and height of the body, deviation
and tilt of the spinous process, and impingement of
the arch on the neural canal. Taken along with
clinical examples, the archaeological specimens of
unilateral separation, whether incomplete or complete, present a dynamic picture of the stresses occurring in the lower lumbar region in humans and
the bony response that takes place.
Special thanks go to Ethne Barnes, Jerome Cybulski, Michael Fong, and Sonja Jerkic for bringing
fascinating specimens to my attention and allowing
me to include them in this study. Thanks also go to
the various institutions that allowed me to study the
vertebrae used here, and especially the Canadian
Museum of Civilization in Hull (Quebec, Canada).
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