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Morphologic changes in the TMJ following splint wear.

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THE ANATOMICAL RECORD 266:167–176 (2002)
DOI 10.1002/ar.10050
Morphologic Changes in the TMJ
Following Splint Wear
BETTY J. SINDELAR,1* STAN EDWARDS,2 AND SUSAN W. HERRING2
1
School of Physical Therapy, Grover Center, Ohio University, Athens, Ohio
2
Department of Orthodontics, University of Washington, Seattle, Washington
ABSTRACT
Intraoral splints are a commonly used dental treatment for a variety of conditions.
Because such splints alter the condyle– disc–fossa relationship, they probably change the
loading status of the temporomandibular joint (TMJ), including the TMJ disc. Collagen, a
major constituent of the disc, acts to resist tensile loading, and it is presumed that the fiber
orientations of the individual disc bands reflect their functional loading. Therefore, the
purpose of this study was to examine effects of intraoral splint wear on TMJ morphology in
general, and collagen orientation of the intra-articular disc in particular. Young adult, female
miniature pigs were divided into three groups: open-bite splint, protrusive-bite splint, and
unsplinted control. Splints were worn for 2 months, after which the TMJ discs were harvested
for histological examination and stereological analysis, and the skulls were cleaned. Although
the splints had no effect on skull dimensions, changes were seen in the TMJs. The discs of the
protrusively-splinted group showed an increased thickness of the posterior band (P ⬍ 0.015)
and minor changes in collagen orientation of the anterior band. The most striking change was
the presence of a degenerative osseous defect on the medial side of the mandibular condyle in
half of the splinted animals. These results indicate that prolonged splint wear can induce
remodeling and even injury of TMJ tissues. Anat Rec 266:167–176, 2002.
©
2002 Wiley-Liss, Inc.
Key words: temporomandibular joint disc; porcine; morphology; intraoral
splints; collagen
Intraoral splints are used in dentistry to treat a variety
of conditions, such as occlusal malalignments, temporomandibular joint (TMJ) dysfunctions, and sleep apnea
(Bondemark, 1999; Carels and van der Linden, 1987; Gianelly et al., 1970; Grim, 1995; Johal and Battagel, 1999;
Kimmel, 1994; Major and Nebbe, 1997; Nitzan, 1994;
Wright et al., 1995). All splints change the occlusal relationship of the teeth and, hence, the spatial relationship of
the TMJ components, although the precise effect varies
with the splint (Ito et al., 1986). Additionally, splint wear
must affect disc structure, as indicated by studies showing
changes in shape (Ferrari and Herring, 1995) and glycosaminoglycan (GAG) content (Mao et al., 1998; Sindelar et
al., 2000). However, detailed morphology has not previously been assessed.
The TMJ disc is composed of approximately 80% water
(Sindelar et al., 2000); the remaining 20% is mostly collagen. Prior studies on the human, bovine, dog, rabbit, monkey, sheep, and rat TMJ disc have shown that the fibers of
the intermediate zone are oriented predominantly in the
anterior-posterior (AP) direction, while the fibers of the
anterior and posterior bands are more multidirectional or
“basket-woven” (Berkovitz, 2000; Desai et al., 1996;
Gillbe, 1973; Landesberg et al., 1996; Mills et al., 1988;
©
2002 WILEY-LISS, INC.
Mills et al., 1994b; Minarelli et al., 1997; Minarelli and
Liberti, 1997; Strauss et al., 1960; Taguchi et al., 1980;
Teng and Xu, 1991). The arrangement or structural organization of the collagen fibers is thought to correspond to
the mechanical function of the disc with fibers paralleling
the direction of applied tensile loading (Smith et al., 1981).
Thus, if altered loading occurs, the disc may adapt by
reorientation of the collagen fibers to accommodate the
direction of the newly applied tensile stresses (Scapino
and Mills, 1997). In previous studies, surgically induced
anterior displacement of the rabbit TMJ disc did in fact
lead to changes in the collagen fiber orientation of the
anterior bands and intermediate zones (Mills et al., 1994a;
Mills and Scapino, 1993). In particular, the fibers of the
intermediate zone became more multidirectional, resem-
Grant sponsor: NIH/NIDR; Grant number: 5 RO1 DE11236;
Grant sponsor: NIH; Grant number: T35 DE07150.
*Correspondence to: Betty Sindelar, Ph.D., School of Physical
Therapy, Grover Center, Rm W295, Ohio University, Athens, OH
45701. Fax: (740) 593-0292. E-mail: sindelar@ohio.edu
Received 29 November 2000; Accepted 10 December 2001
Published online 14 February 2002
168
SINDELAR ET AL.
Fig. 1. Intraoral splints. Top: Chrome-cobalt ramp splints, occlusal view. Splints were
cemented onto occlusal surface of premolars
and molars of 8-month-old animals. Bottom:
Protrusive splint (PS), lateral view. Anterior is to
the left.
bling the posterior band. However, it is always difficult to
assess the effect of surgery on the involved tissues. Therefore, we proposed to alter the load on the TMJ disc in a
nonsurgical way, by changing the fossa– disc– condyle relationships through the use of intraoral splints.
The purpose of this study was to examine the effect of
intraoral splint wear on the collagen fiber orientation of
all disc bands. Two types of splints were used. One, which
protruded the mandible, was assumed to bring the condyle
to a more anterior functional position on the disc. The
SPLINT-INDUCED TMJ MORPHOLOGIC CHANGES
second was a simple opening splint that presumably left
the joint components in their normal positions. The control group wore no intraoral splint. It was expected that in
the protrusively splinted group the collagen fibers of the
intermediate zone would become more multidirectional,
while the fibers of the anterior band would become more
unidirectional. We hypothesized that, because of loading
in the altered position, the anterior band would functionally become the intermediate zone, and would therefore
orient more of its fibers along the AP axis of the disc.
The miniature pig was chosen as the animal model
because like humans, pigs are omnivorous, have bunodont
molars, and use a transverse chewing stroke (Bermejo et
al., 1993; Herring, 1976; Weaver et al., 1962). The disc of
the TMJ is quite similar to man in gross structure, histologic and biochemical composition, and response to mechanical loading (Berg, 1973; Christensen, 1975; Fontenot, 1985; Meister et al., 1973; Kopp, 1976; Sindelar et
al., 2000).
Because of the limited availability of fully mature miniature pigs, young adults were used, creating a possible
confounding problem due to growth. Previous studies
noted not only dental changes but facial growth alterations in monkeys and pigs as a result of intraoral bite
block wear (Altuna and Woodside, 1985; Carlson and
Schneiderman, 1983; Ferrari and Herring, 1995; Schneiderman, 1989). Therefore, an additional objective of this
project was to examine the skull and mandible for bony
growth or adaptive changes that could have occurred as
the result of intraoral splint wear. Because the animals
used in this study were post-pubertal and experiencing
slow growth, we did not expect to find size differences
among the groups. However, we did expect some evidence
of injury to the TMJ tissues because most previous studies
noted pathologic effects (whereas Ramfjord and Blankenship (1981) did not).
MATERIALS AND METHODS
Nineteen female Hanford miniature pigs were acquired
in sibling sets at 7 months of age. The subjects were
randomly divided into three groups: control (C), openingonly (control) splint (CS), and protrusive splint (PS). Impressions using custom-made forms and bite registrations
were taken on all pigs. Each animal received surgical
implantation of a small radio-opaque screw near the root
of the left maxillary and mandibular canines. These
screws were distant from the TMJ (approximately 14 cm)
and jaw muscles, and did not interfere with chewing function (Sindelar, unpublished EMG data). They allowed the
AP positioning of the mandible to be checked on videofluoroscopy during chewing. Prior to splint delivery, all pigs
were examined with a baseline videofluoroscopy chewing
session to determine the relative position of the maxilla
and the mandible in the AP direction. At 8 months of age,
the animals in the CS and PS groups were fitted with
chrome-cobalt ramp splints that covered the occlusal surfaces of the maxillary and mandibular molars (Fig. 1). The
CS and PS splints increased the height of the bite by 5 mm
at the first molariform tooth while the PS splint also
positioned the mandible 7 mm anteriorly. All splints were
cemented onto the teeth and were worn continuously for 2
months. Splints were checked daily and, when necessary,
replaced as quickly as possible. As stated previously, the C
group received no intraoral appliances. Additional videofluoroscopy sessions occurred within 2 days of splint de-
169
Fig. 2. Superior view of disc. Portions marked “C” were used for this
study. The center hatched area (11 mm wide) was used in another study.
The “slice” section was used to measure disc thicknesses.
livery, 1 month post-delivery, and 2 months post-delivery.
These sessions were used to assess whether the animals
were chewing into the splint position (i.e., the ramp portions were coming into contact during the closed portion of
the chewing cycle). Qualitative comparisons of the relative
positions of the implanted screws at the closed portion of
the chewing cycle were made to confirm that the mandible
was advanced in the PS group but not in the CS and C
groups. Food and water were provided to all animals per
breeder guidelines (Charles River, Wilmington, MA).
After 2 months of splint wear, the animals were euthanized with an intracardiac injection of pentobarbital (10
cc; Guidelines of the Animal Care Committee, University
of Washington, Seattle, WA). The left TMJ disc was dissected out and frozen. The heads were harvested and
cleaned.
Disc
Left discs were thawed at room temperature. Approximately 40% of each disc was removed from the middle by
two sagittal cuts (Fig. 2) and were used for a different
study. The remaining medial and lateral portions were
used for this study. A 1-mm sagittal slice was taken from
the lateral edge of the medial section and coded to blind
the investigators to treatment group. Images of these
slices were captured using a videocamera and analyzed
using NIH Image. The thinnest portion of the intermediate zone and the thickest portions of the anterior and
posterior bands were measured three times and averaged.
The slice was then fixed with ethanol and processed for
paraffin vacuum-embedding using a standard protocol
(Bancroft and Stevens, 1996). To assess for the effects of
processing on shrinkage, these slices were then remeasured.
Collagen Pattern
The medial and lateral disc portions were fixed in ethanol, embedded in paraffin, and sectioned in the horizontal plane from the superior surface at 7.0 ␮m (Bancroft
and Stevens, 1996). Sections were stained with Direct
Red, which identifies collagen almost exclusively (Junqueira et al., 1979; Sweat et al., 1964). Three separate
sections taken from the disc core were analyzed for each
region: lateral anterior, intermediate, and posterior
bands; and medial anterior, intermediate, and posterior
bands. The images were captured (Nikon T4, Kodak Ektachrome 160T color film) while being viewed under a
microscope at 50⫻ (Nikon Eclipse 5100) (Fig. 3). These
images were then converted to black and white bitmap
170
SINDELAR ET AL.
Fig. 4. Average angle frequency distributions. C ⫽ control group,
CS ⫽ control splint group, and PS ⫽ protrusive splint group. Angles of
0° and 180° indicate AP orientation, whereas 90° indicates mediolateral
orientation.
format for stereological analysis using a custom software
program, MacAzimuth (written by Prof. John Rensberger,
Geological Sciences, University of Washington). This software determines the orientation of all strings of six contiguous pixels relative to a fixed axis; further information
about the software can be found in Teng et al. (1997) and
Rensberger and Watabe (2000). The fixed axis was the AP
axis of the disc. Output is given as a frequency distribution of angles and constitutes a direct measure of the
orientation of the collagen fibers in the image (Fig. 4). For
each image, average orientation (mean angle) was calculated. The standard deviation of the mean angle was
taken as a measure of anisotropy, the nonrandom directionality of the collagen fibers.
Skull Measures
Fig. 3. Representative horizontal sections from the lateral portion of
a control group disc. Top: Anterior band. Middle: Intermediate zone.
Bottom: Posterior band. Direct Red staining. Anterior is to the top;
medial is to the right.
Prior studies (Altuna and Woodside, 1985; Carlson and
Schneiderman, 1983; Ferrari and Herring, 1995) have
indicated that intraoral splint wear alters the dimensions
of the maxilla and mandible. Therefore, the following
widths and lengths were measured on the maxilla and
mandible of the dried skulls (Fig. 5):
Right (RML) and left mandibular length (LML): infradentale to the most posterior point on the condyle.
171
SPLINT-INDUCED TMJ MORPHOLOGIC CHANGES
Fig. 5. Upper left: Lateral view of pig skull. RML ⫽ right mandibular length. Upper right: Posterior view of mandible. MW ⫽ maximum mandibular
width, CNW ⫽ condylar neck width, and CW ⫽ coronoid width. Lower left: Superior view of mandible. MA ⫽ mandibular arch width. Lower right:
Palatal view of skull. MXA ⫽ maxillary arch width and PL ⫽ palate length.
Maximum mandibular width (MW): taken near the
mandibular angles.
Width at condylar neck (CNW): taken immediately below the scar from the attachment of the posterior capsule.
Width at coronoid processes (CW): taken at the superior-most point.
Mandibular arch (MA) and maxillary arch (MXA) width:
taken between the lingual surfaces of the first molars at
the posterior cemento-enamel junction on the mandible
and maxilla, respectively.
Palate length (PL): hard palate length.
Two different standardization measures were employed: 1) cube root of weight in kilograms, and 2) basicranial axis length (Radinsky, 1984). The basicranial axis
length was considered preferable to total skull length,
which would have been affected by changes in upper jaw
dimensions. All measurements were taken three times
and averaged for final analysis.
Articular Surfaces
The articular surfaces of the temporal fossa and the
mandibular condyle were examined under the dissecting
microscope for signs of degenerative changes. Images of
the condylar articular surfaces were captured using a
videocamera and analyzed using NIH Image. All images
were blinded as to treatment group. The medial-lateral
width and AP length of the articular surface were measured, and surface areas were calculated using NIH Image. Measurements were taken three times, averaged for
final analysis, and standardized to appropriate power of
weight.
For all measurements, a nested analysis of variance
(ANOVA) was used. The individual pigs were randomly
assigned to treatment group, so “pig” was the random
effect whereas “treatment group” was the fixed effect. A
Fisher’s exact test was used to examine the degenerative
changes in the condylar articular surface.
RESULTS
The fluoroscopy sessions showed that the animals fully
occluded into their splints in almost all masticatory cycles
(85% or more). Examination of the screw position at occlusion verified that the PS group did have protruded
mandibular positions.
Overall, average disc dimensions were 26.8 ⫾ 2.2 mm
for the medial-lateral width and 14.5 ⫾ 1.5 mm for the AP
length, with no difference among groups. Regardless of
group, the posterior band was always the thickest and the
172
SINDELAR ET AL.
intermediate zone was the thinnest (P ⬍ 0.01) (Table 1).
Additionally, a significant treatment/band interaction was
present for the posterior band, which was thicker in the
PS than in the other groups (P ⬍ 0.015). Histologic preparation of the disc led to a linear shrinkage in the tissues
of 21.5%.
The mean angles of the collagen fiber orientation within
the intermediate zones of all groups were aligned around
the AP axis or 0° in our system (Table 2). Orientation of
the anterior and posterior bands ranged from 81° to 96° in
all groups. Within each treatment group, the orientation
of the intermediate zone was significantly different from
the other bands (P ⬍ 0.001), whereas the anterior and
posterior bands could not be distinguished. A band-location interaction was present in the C and CS groups (P ⬍
0.001) (Fig. 6). In the anterior bands, the medial locations
were inclined slightly posterolaterally, while the lateral
locations were inclined slightly posteromedially (Fig. 7).
In the posterior bands, the medial locations showed collagen to be almost perfectly mediolateral (90°), but the lateral locations sloped slightly anteromedially. In contrast,
the PS group showed no significant band-location interaction. In the posterior band, the medial-lateral difference
was reduced, and in the anterior band both locations were
oriented very closely to the mediolateral axis.
Visual examination of the intermediate zones showed
that the collagen fibers displayed a marked crimping (Fig.
3B), similar to the pattern found in ligament and tendon.
Much less crimping was seen in the anterior and posterior
bands. The magnitude of the waviness in the intermediate
zone made the analysis of parallelness impossible because
the program measured the direction of the crimps rather
than the direction of the fibers. Therefore, the analysis of
fiber direction was only performed on the anterior and
posterior bands, both medial and lateral components. Results of this analysis indicated that no significant differences were present relative to band, location, or treatment
(Table 3).
The splints had no effect on skeletal dimensions, including those of the condyle with or without either standard-
ization technique (Table 4). However, there was a trend
for the condylar measurements (width, perimeter, and
area) to be smaller in the PS group. Furthermore, visual
TABLE 1. Mean disc thickness measurements
ⴞ S.D. (mm)
Groups
Anterior
band
Intermediate
zone
Posterior
band
C
CS
PS
4.0 ⫾ 0.7
3.9 ⫾ 0.5
4.1 ⫾ 0.5
1.2 ⫾ 0.1
1.3 ⫾ 0.2
1.3 ⫾ 0.3
5.2 ⫾ 0.5
6.1 ⫾ 1.4
7.6 ⫾ 1.5*
Within each treatment group, bands were significantly different (P ⱕ 0.001). In addition, the posterior band of the PS
group was thicker than the other groups (P ⬍ 0.02).
C, control (n ⫽ 6); CS, control splint (n ⫽ 6); PS, protrusive
splint (n ⫽ 7).
Fig. 6. Mean collagen fiber orientation (⫾ standard error). The C and
CS groups showed significant mediolateral differences in the anterior
and posterior bands (P ⬍ 0.001) but the PS group did not.
TABLE 2. Mean angles (degrees) of collagen fibers relative to the AP axis ⴞ standard error
Anterior band
C
CS
PS
Intermediate zone
Posterior band
Medial
Lateral
Medial
Lateral
Medial
Lateral
85.2 ⫾ 2.0
87.5 ⫾ 2.0
88.9 ⫾ 1.9
94.4 ⫾ 2.0
96.2 ⫾ 2.0
88.4 ⫾ 1.8
⫺0.5 ⫾ 2.0a
1.5 ⫾ 2.0a
⫺1.5 ⫾ 2.0a
0.7 ⫾ 2.0a
1.6 ⫾ 2.0a
1.6 ⫾ 2.0a
89.3 ⫾ 2.1
88.2 ⫾ 2.1
90.4 ⫾ 1.9
82.2 ⫾ 2.0
80.6 ⫾ 2.0
85.1 ⫾ 1.9
Indicates significance within treatment group (P ⬍ 0.001).
C, control group (n ⫽ 6); CS, control splint group (n ⫽ 6); PS, protrusive splint group (n ⫽ 7).
a
173
50.4 ⫾ 6.0
50.6 ⫾ 3.4
50.9 ⫾ 5.6
48.7 ⫾ 5.0
49.8 ⫾ 6.9
49.0 ⫾ 5.3
No significant differences noted.
C, control group (n ⫽ 6); CS, control splint group (n ⫽ 6); PS,
protrusive splint group (n ⫽ 7).
Mean standard deviation of the mean angle ⫾ S.D.
inspection of the condyles showed a defect (Fig. 8) present
on the medial aspect of the articular surface in 53.8% of
the splinted animals (26 condyles inspected) (Table 5). No
defects of this type were found in any of the 10 mandibular
condyles of the control group. No significant difference
was noted between the splinted groups. When present, the
defect was found bilaterally, appeared to be pre-mortem,
and appeared to be indicative of degenerative joint disease. The cranial components of the joints appeared normal in all cases.
154.4 ⫾ 75.7
149.5 ⫾ 32.5
125.7 ⫾ 20.9
71.2 ⫾ 21.8
71.2 ⫾ 15.7
59.0 ⫾ 5.2
23.0 ⫾ 1.6
23.2 ⫾ 1.9
23.0 ⫾ 0.4
†
P
37.6 ⫾ 1.8
38.1 ⫾ 1.9
38.7 ⫾ 2.1
36.6 ⫾ 1.4
34.7 ⫾ 2.2
36.1 ⫾ 2.1
155.7 ⫾ 3.8
155.3 ⫾ 4.7
157.4 ⫾ 5.7
†
76.4 ⫾ 4.2
76.5 ⫾ 3.2
77.5 ⫾ 4.1
PL
MXA
MA
CW
CNW
Numbers given are the raw values of the measurements in mm or mm2. RX ⫽ treatment group where C ⫽ control (n ⫽ 6 except for † where n ⫽ 5), CS ⫽ control splint
(n ⫽ 6), and PS ⫽ protrusive splint (n ⫽ 7). W ⫽ medial-lateral width of the articular surface of the condylar head. P ⫽ perimeter of the articular surface of the condylar
head. A ⫽ surface area of the condylar head. For all other abbreviations, see text. No medial-lateral differences were noted in the condylar articular surface
measurements, so side measures were averaged for this table. No significant differences were noted.
47.8 ⫾ 2.4
46.2 ⫾ 7.9
50.5 ⫾ 3.4
95.0 ⫾ 2.9
98.5 ⫾ 3.0
98.1 ⫾ 4.0
51.7 ⫾ 7.6
48.7 ⫾ 4.0
48.8 ⫾ 6.1
104.5 ⫾ 4.4
106.0 ⫾ 3.4
104.8 ⫾ 4.0
Lateral
204.8 ⫾ 5.8
203.0 ⫾ 5.7
208.3 ⫾ 4.0
Medial
205.4 ⫾ 5.1
202.3 ⫾ 5.3
207.3 ⫾ 4.6
Lateral
C
CS
PS
Medial
MW
C
CS
PS
Posterior band
LML
Anterior band
RML
TABLE 3. Measure of randomness
RX
Fig. 7. Schematic representation of the mean angle orientation of the
collagen fibers of the left disc. Solid squares represent areas examined
for fiber orientation. Thin solid lines across the disc surface represent 90°
orientation. Anterior is to the top.
TABLE 4. Measurements of the skull and mandible (mean ⴞ S.D.)
†
W
A
SPLINT-INDUCED TMJ MORPHOLOGIC CHANGES
174
SINDELAR ET AL.
Fig. 8. Example of the observed
defects. Superior surface of the
right condyle from a PS animal. Arrow points to the defect present on
the medial articular surface in 55%
of the TMJs of the splinted animals.
Bar length ⫽ 1 cm.
TABLE 5. Presence of defect on the medial aspect of
the articular surface of the mandibular condyle*
Defect
presence
Yes
No
Total
Control
a
0
10
10
Control
splint
Protrusive
splint
Total
6
6
12
8
6
14
14
22
36
*See Fig. 8 for a typical example.
Different from other groups (P ⱕ 0.005). Mandibular condyles from one of the control animals were unavailable for
observation.
a
DISCUSSION
The splints altered the occlusal relationship by changing the position of the mandible, not by affecting growth.
The lack of change in the measured skeletal parameters
among the treatment groups is undoubtedly related to the
age of the subjects. When the splints were delivered, the
animals were 8 months of age, which is past puberty.
Therefore, although these splints were similar to those
used in other studies (Altuna and Woodside, 1985; Carlson
and Schneiderman, 1983; Ferrari and Herring, 1995;
Schneiderman, 1989), they did not induce bony growth
adaptations. However, the splints apparently had an adverse impact on the TMJ structures, as indicated by the
defect on the medial condylar articular surface and the
trend of the PS to have smaller condylar articular surfaces. That the changes resulted from splint wear rather
than the other experimental procedures (e.g., marker
screw placement) is indicated by the absence of any
changes in the otherwise identically treated control animals. The defects observed suggest that the splints altered
the loading on the medial side of the joints in a way that
the animal was unable to accommodate, resulting in extensive bony destruction.
It is clear that the protrusive splint caused remodeling
of the disc. Within the disc, the most obvious morphological change was the increased thickness of the PS posterior band. This finding is similar to that reported previously (Ferrari and Herring, 1995; Scapino and Mills,
1997), and suggests an adaptation to a larger posterior
joint space created by the anterior displacement of the
mandible via the splint. However, the internal structure of
the posterior band was not altered by splinting and continued to feature multidirectional collagen fibers.
Even with splints, the intermediate zone retained its
strong alignment with the AP axis of the disc, which is
consistent with previous reports of the collagen alignment
of this zone in other species (Mills et al., 1988; Mills et al.,
1994b; Strauss et al., 1960; Taguchi et al., 1980; Teng and
Xu, 1991). It would be reasonable to assume that this zone
sees a large tensile force in the AP direction during physiologic function with or without splints. Of interest is the
fact that the intermediate zone also has a very large
concentration of highly sulfated GAGs, indicating that
this zone also sees a large compressive force during physiologic function (Sindelar et al., 2000). These extremes of
compressive and tensile loading during function make this
tissue unique.
Our principal hypothesis, that the anterior band would
develop an AP collagen orientation, was not borne out.
SPLINT-INDUCED TMJ MORPHOLOGIC CHANGES
Indeed, the CS and PS anterior bands showed fewer AP
oriented collagen fibers than the controls (Figs. 3 and 7).
That the PS anterior band had even more transverse
fibers than normal is further indicated by the loss of the
usual medial-lateral differences (Figs. 6 and 7). Although
these alterations are subtle, they suggest that functional
loading did change, albeit not in the expected way.
Other studies have shown that when a disc band or even
the posterior discal attachment becomes loaded like the
intermediate zone, that area begins to remodel itself into
a pseudo-intermediate zone (Mills et al., 1994a; Scapino
and Mills, 1997; Scapino, 1983). While expected, major
orientation changes were not observed in the PS anterior
band in this study. Given the ages of the experimental
animals and the relatively short duration of the splint
wear, there simply may not have been enough time for
remodeling to occur. Additionally, since the splints only
function when the teeth are in occlusion, the experimental
animals did not spend the entire 2 months in the altered
loading condition. It is important to note here that in
addition to chewing, pigs spend a considerable amount of
time bruxing. Yet there certainly was a large proportion of
time with no tooth-to-tooth contact. Thus, a longer wear
time or an experimental modification to control condyle
position at all times might have resulted in more obvious
histologic changes.
Alternatively, in the absence of direct observation of the
functioning joints, we cannot be certain that the PS splint
actually forced the condyle to function against the anterior
band rather than the intermediate zone. We emphasize at
this point that the relationship of the loading among the
condyle, disc, and fossa is not clear in the splinted positions, especially the PS state. Based on the results of this
study, our initial assumption that the condyle would simply be translated anteriorly and the mechanics of movement would not be different from the normal state appears
to be false. Perhaps additional splint wear time would
clear up this matter, but we would also suggest that monitoring the condyle– disc–fossa relationship during in vivo
movement is absolutely necessary to understand altered
loading mechanics.
In summary, the 2-month splint wear resulted in degenerative changes in the medial aspect of the mandibular
condyle, an increase in the superior-inferior thickness of
the posterior disc band, and a greater similarity in the
angulation of medial vs. lateral collagen fibers in the anterior band. While the first result was found in both
splinted groups, the latter two results were found only in
the protruded group.
ACKNOWLEDGMENTS
We thank John Rensberger, Ph.D., for help with the
MacAzimuth program, Ms. Patricia Emry for lab assistance, and Todd Alonzo, Ph.D., for help with the statistical
analysis.
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