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THE ANATOMICAL RECORD 250:475–479 (1998)
Biomechanical Stability of Abductor Pollicis Longus Muscles
With Variable Numbers of Tendinous Insertions
MAHMOUD MELLING,1* ROLAND REIHSNER,2 MARTIN STEINDL,3 DANIELA
KARIMIAN-TEHERANI,1 MARTINA SCHNALLINGER,1 AND MARK BEHNAM1
1Department of Anatomy I, University of Vienna, Vienna, Austria
2Ludwig Boltzmann Institute for Experimental Plastic Surgery, Vienna, Austria
3Martin Steindl, Department of Orthopedics, Orthopädisches Krankenhaus der Stadt
Wien-Gersthof, Vienna, Austria
ABSTRACT
Background: In the course of a study of different variants
of the tendon of the abductor pollicis longus (APL) muscle, the unusual
finding of a tendon with six subdivisions was observed in the first
compartment. Using this preparation and others with various numbers of
tendons (2, 3, 4, and 6), we intended to establish whether the multitendoned muscles increased the strength of the thumb.
Methods: Fifty-two upper extremities were examined with attention to
the tendons of the APL muscle. The strain on each tendon was biomechanically determined using a microcomputer and potentiometer. The stress
was measured continuously and plotted against strain.
Results: Following treatment with elastase, it is seen that the significance of changes in stability and Young’s modulus is inversely proportional to the number of tendons arising from the APL.
Conclusions: Division of the tendon into several parts leads to better
mechanical distribution of stress and extension on thumb abduction. In
addition, palmar-flexion in the wrist may be supported. Anat. Rec. 250:475–
479, 1998. r 1998 Wiley-Liss, Inc.
Key words: abductor pollicis longus muscle tendon; pancreas elastase;
stress extension relationship; Young’s modulus; transplant
material
Mainly clinical interests, and in particular surgical
ones, led to the present investigations. Repeated observations of supernumerary abductor pollicis longus (APL)
muscle tendons in the first compartment and of variants in the forearm opened questions about the frequency of such anomalies. Meckel (1832), e.g., described the division of the tendon of the APL muscle
into three tendon slips and Schmidt (1987), its division
into four. This study is a detailed report of a form of this
anomaly that has not yet been described in the literature.
The importance of these observations cannot be
assessed without some idea of the frequency of such
anomalies.
MATERIALS AND METHODS
In all, 52 upper extremities were prepared, taken
from cadavers of persons who had died at ages ranging
from 78 to 93 years. All were obtained from the dissection material held by the First School of Anatomy,
University of Vienna.
Formol-carbol fixation had already been performed
on 46 of the cadavers, whereas six were in a nonfixed,
i.e., fresh condition. Great care was taken to prepare
only material on which there were no surgical scars.
r 1998 WILEY-LISS, INC.
The machine as well as the measuring device and
control circuit is shown in Figure 3. All specimens were
loaded at a constant strain rate of 10% per minute,
achieved by a thyristor controlled gear-box (0.04–1
RPM). During the tests the specimens (original length
for all specimens: 10 mm) were kept in a bath of
phosphate-buffered saline at room temperature. Small
sheets of abrasive paper were fixed to the jaws of the
clamps to prevent the samples from slipping out. The
strain was measured using a potentiometer (range: 100
mm; sensitivity: 0.025 mm). Because of the very small
cross-sectional area, we developed a load cell for small
loads (0.005–100N). Length and load were recorded by
a digital-multimeter (DMM). Strain was calculated by
the ratio: increase in length/original length and stored
together with the measured load. The control unit was
adjusted by a microcompuer (Hewlett-Packard HP 86).
The stress was measured continuously and plotted
against the strain.
The curve for stress plotted against extension allows
determination of the stability (­max, maximal load capac-
*Correspondence to: Dr. M. Melling, Haslingergasse 29/1/12, A-1170
Vienna, Austria.
Received 8 July 1997; Accepted 11 November 1997
476
M. MELLING ET AL.
Fig. 1. Schematic illustrating preparation of the tendon of the APL muscle of a fresh (nonfixed) female
left hand. 1. APL tendons marked by 3 arrows; 2. main tendon of the APL muscle; 3. APB muscle was
detached at the origin to reveal the supernumerary tendon of the APL muscle.
ity), maximal extension (the extension corresponding to
the stability, emax) and the rigidity (Emax, tangents at
maximum slope of the stress-extension curve). In addition, the extension was determined where the onset
point of the linear portion of the stress-extension curve
begins (eon). To obtain characteristic values for the
material that are independent of the cross section, it is
necessary to standardize on the primary cross section.
To this end, the dry weight per unit of length was
determined. This represents the collagen content in the
cross-sectional area, which in turn represents the loadbearing portion (Weinans et al., 1992). If the stability
and rigidity are now divided by the dry weight per unit
of length, the respective quotients are ­max and Emax;
that is to say, Youngs’s modulus in the steepest section
of the stress-extension relationship. To establish the
influence of the elastic fibers, which admittedly have a
much smaller effect on the stability than do the collagen
fibers, the preparations were divided into two groups:
those in one group were left untreated, wheras the
preparations in the other group were digested with
pancreas elastase (Sigma 1250, St. Louis, MO). This
involved treating 10 mg of tissue in 1.0 ml phosphatebuffered saline with 10 U elastase (3 h at 37°C). The
elastase activity was 80 U/mg protein and 1.7 U/mg
trypsin (Fyhrie and Schaffler, 1995). On the basis of an
earlier study, no trypsin inhibitor was used, as from the
mechanical aspect no differences were seen between
preparations with and without use of the inhibitor
(Fyhrie and Schaffler, 1995).
RESULTS
The variant of the tendon of the APL muscle (Fig. 1)
described here was observed during preparation of a
fresh (nonfixed) left arm taken from the cadaver of a
woman who died at the age of 83 years. The tendon of
this well-developed APL muscle, which itself followed a
normal course, was divided ,2.7 cm proximal to the
first metacarpal bone into six parts; some of these were
round in section and others flattened, and their individual thickness varied from 1.7-4.2 mm. All the accessory tendons were to the ulnar side of the main tendon,
but with it in the common tendon sheath. Precise
separation of the accessory tendons was only present in
the distal segment of the tendon ,3.2 cm before the
insertion point; in contrast, there was no clear differentiation in the proximal segment. The accessory tendons
were quite thick in parts and the same length and
thickness as the main tendon in other parts.
477
ABDUCTOR POLLICIS LONGUS MUSCLES
TABLE 1. Number of supernumerary tendons of the APL
muscle in the upper extremities
Male
Female
Total
Right
Left
No. of tendons
3
5
4
7
—
—
19
5
6
7
11
3
1
33
2
3
2
3
4
6
52
TABLE 2. Mechanical properties of preparations with
the tendon of the APL muscle split into 2, 3, 4, and 6 slips
[emax elasticity in %, eon onset of linear portions of stressstrain relations in %, smax stability (Nm/g), Emax
(Nm/g) maximal Young’s modulus in the linear
portion of the curve]
No.
emax
eon
smax
Emax
1a
1b
2a
2b
2c
3a
3b
3c
3d
4a
4b
4c
4d
5a
5b
6a
6b
6c
6d
6e
6f
9.5
10.0
9.8
10.3
8.8
12.5
11.8
8.0
6.0
13.8
9.8
10.0
12.5
11.3
13.3
10.5
11.3
9.8
10.8
9.0
9.5
1.00
0.75
0.50
0.50
0.50
0.75
0.50
1.00
1.00
0.75
0.50
0.50
0.50
2.00
1.00
1.00
1.25
1.00
1.00
0.75
1.00
13.2
25.2
10.1
28.6
38.3
9.40
27.1
9.58
10.1
27.9
55.4
24.9
17.6
50.0
14.2
12.7
13.0
15.7
11.1
22.8
28.6
166.2
511.0
692.4
655.8
533.3
126.6
274.2
151.5
215.9
315.7
641.0
302.3
141.2
538.2
127.0
186.5
195.9
247.3
181.2
385.8
461.2
Of the six divisions of the split abductor tendon
(described in order from palmar to dorsal), one inserted
in the radial margin of the abductor pollis brevis (APB)
muscle immediately adjacent to its origin at the tubercle of the scaphoid bone. The next four tendon slips,
which had the same thickness in parts, took a course to
the palmoradial third of the base of the first metacarpal
bone.
A further tendon slip followed the classic course to
the base of the first metacarpal bone, which made it
seem logical to designate this one as the main tendon.
Only these six slips were found together with the
extensor pollicis brevis (EPB) muscle tendon in the first
compartment.
In Table 1 we also provide the distributions of the
number of supernumerary tendons of the APL muscle
in 52 upper extremities by sex and localization. The
individual values revealed by the mechanical analysis
are displayed in Tables 2 (untreated tendons) and 3
(elastase-treated). A typical measurement curve is
shown in Figure 2, and the mechanical parameters are
explained there as well.
With regard to elasticity (emax) and the extension at
the point where the linear portion of the stress-
Fig. 2. Typical graph of a stress-strain relation of an APL muscle
tendon, where ­max is the stability of the tendon, emax the extensibility,
and Emax the Young’s modulus in the steepest portion of the curve. The
transition point between the initial area and the linear part of the
curve, i.e., the upper limit of the physiological range of the biomechanical stability, is marked with eon.
extension relationship begins (eon), there were no great
differences among the subjects, that is to say among the
different numbers of tendon slips. After elastase treatment, there were significant differences (p , 0.05)
between the elasticity and the onset of the linear
portion of the stress-strain relationship (Table 4). With
regard to the stability (­max) and the maximal Young’s
modulus (Emax), there were significant differences between the proband with the tendon split into six slips
and the others. The influence of the elastase treatment
was minimal for the tendon split into six. This was not
true for the other cases, in which a reduction of the
stability was observed after the elastase treatment
(Table 5).
DISCUSSION
Duplication of the APL muscle was reported previously (Henle, 1871; Krause, 1880; Walsh, 1897; von
Bardeleben, 1906; Stein, 1951; Coleman et al., 1953;
Baba, 1954; Williams et al., 1989). Division of the
abductor tendon into three slips was also described
478
M. MELLING ET AL.
Fig. 3. Computer-assisted tensile testing machine and control circuit.
TABLE 3. Mechanical properties of the preparations
with the tendon of the APL muscle into 2, 3, 4, and 6 slips
after elastase treatment [emax elasticity in %, eon onset of
linear portions of stress-strain relations in %, smax
stability (Nm/g), Emax (Nm/g) maximal Young’s modulus
in the linear portion of the curve]
No.
emax
eon
smax
Emax
1a
1b
2a
2b
2c
3a
3b
3c
3d
4a
4b
4c
4d
5a
5b
6a
6b
6c
6d
6e
6f
9.0
9.3
8.3
9.8
9.0
11.3
11.5
8.8
7.5
11.5
7.8
8.8
10.5
9.5
11.0
9.5
10.0
8.3
9.3
8.8
8.0
0.50
0.50
0.25
0.50
0.25
0.50
0.50
0.75
0.50
0.50
0.50
0.25
0.50
1.25
0.75
0.50
0.75
0.50
0.25
0.50
0.50
13.1
36.9
9.36
24.9
22.3
10.1
27.1
10.9
9.62
28.9
45.4
23.6
21.3
38.6
14.7
14.8
16.3
18.3
8.96
30.8
28.2
145.5
562.3
651.9
507.6
365.3
137.9
316.3
148.0
189.4
287.6
497.5
297.9
176.6
383.9
130.0
222.9
224.6
282.0
158.1
423.1
304.0
(Meckel, 1832; Lacey et al., 1951; Loomis, 1951; Lapidus and Fenton, 1952; Walsh, 1955; Bergman et al.,
1988) as well as division into four slips (Wood, 1867 and
Ders 1868, according to Henle, 1871; Rauber and
Kopsch, 1914; Bell and Bell, 1811, according to Lacey et
al., 1951; Verdan, 1952, according to Wulle, 1974;
Backhouse, 1981; Schmidt, 1987) and into five slips
(Bunnell, 1948; Bunnel and Böhler, 1958; Schmidt and
TABLE 4. Differences between the mechanical
properties of tendons of the APL muscle with and without
elastase treatment, with special reference to elasticity
and onset of the linear portion of the
stress-strain relationship
Untreated
Elastase-treated
No.
emax (%)
eon (%)
emax (%)
eon (%)
21
10.42 6 0.4
0.85 6 0.08
9.41 6 0.26
0.52 6 0.05
TABLE 5. Differences between the mechanical
properties of tendons with and without elastase
treatment, with special reference to stability (smax in
Nm/g) and Young’s modulus (Emax in Nm/g) measured in
the linear portion of the stress-strain relationship
Untreated
Elastase-treated
No.
smax
Emax
smax
Emax
All
21
22.2 6 0.22
335.7 6 41.7
21.6 6 2.3
296.0 6 35.6
A. Without 6-fold split of the tendon of the APL muscle
15
39.2 6 13.3
359.5 6 46.5
22.5 6 0.21
320.5 6 37.3
B. With 6-fold split of the tendon of the APL muscle
6
17.3 6 2.8
276.3 6 48.6
19.6 6 3.39
269.3 6 37.4
Lahl, 1988); Melling et al., (1996) reported seven slips.
However, division of the APL muscle tendon into as
many as six slips in this form has been neither reported
nor investigated before this study. It is our opinion that
the division described here is clinically significant,
especially with reference to the diagnosis and treatment of de Quervain’s stenosing tenosynovitis affecting
the radius.
479
ABDUCTOR POLLICIS LONGUS MUSCLES
Unexplained persistence of postsurgical pain following a revision of the first tunnel could be avoided by a
thorough initial inspection resulting in the immediate
recognition of any multiple tendons enclosed in separate compartments and ensuring their free movement.
Clinical experience has shown that the lateral position
of supernumerary tendons also increases the risk of
injury (Schmidt et al., 1968).
One disadvantage of the aberrant tendons is that
they lead to the condition first described by de Quervain
(1895) as chronic stenosing tenosynovitis. From a mechanical point of view, however, division into several
parts leads to better distribution of stress and extension on abduction of the thumb. In the extreme case of
the sixfold split, an improvement of the stress distribution on abduction of the thumb might be obtained, and
in addition, the palmar flexion in the wrist may be
supported. Even within the physiological range, then,
there are smaller loads on the individual tendon slips
with resultant better distribution of the stress when
the tendons are considered together as a stresstransmitting complex. The relation between extension
and load capacity of the tendons is also contingent on
the insertions of the musculature.
Lacey et al. (1951) and Neviaser et al. (1980) have
also pointed out the clinical significance of accessory
end tendons of the APL muscle inserting into the
trapezoid bone or the abductor pollicis brevis muscle. It
is perfectly feasible to use these in plastic reconstruction of the interosseuse-dorsalis -I muscle or the extensor pollicis brevis muscle.
The thumb is useless without the tendon of the
abductor pollicis muscle, because the metacarpal phalangeal joint will overextend whenever the pincer grip
between the thumb and index finger is attempted. This
means that the arch of the thumb is destroyed, the
entire thenar eminence sinks into the concave palm of
the hand, and a right-angle bend arises along the
thenar fold (Bunnell and Böhler, 1958).
Although Bunnell (1948) regards the deviant mechanism of the aberrant tendon as causing the development of pain in de Quervain’s chronic tenosynovitis, the
accessory tendon also should be considered. He explains the occurrence of a supernumerary, aberrant
tendon in the human as an example of atavism, since in
most primates—e.g., chimpanzees, gorillas, and gibbons—this muscle normally exhibits two tendons, one
inserting at the first metacarpal bone and the other at
the trapezoid bone.
The observations of Bunnell (1948) in 22 patients
indicate the frequency of aberrant APL muscle tendons.
He reported accessory tendons in 12 of 22 patients who
underwent surgery. Lacey et al. (1951) found accessory
tendons in 82% of their cases and Stein (1951) in 68%,
whereas Baba (1954) found aberrant tendons in 132
(98,5%) of 134 wrists examined.
Considering the aforementioned research, continuing descriptions of aberrant tendons of the APL muscle
as a variant seems unwarranted. Our notes on the
number, thickness, course, and insertions of these
supernumerary tendons are meant to aid in determining the anatomical properties of the APL muscle as well
as to assist clinicians by highlighting the great significance of tendon deviations as a potential cause of
chronic tenosynovitis.
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