Download Poster No: 0581 - Orthopaedic Research Society

ORIGINS OF THE THENAR AND HYPOTHENAR MUSCLES
*, **Kung, J; *, +** Budoff, J.; *Yeh, M.L.; *, **Gharbaoui, I; *Luo, Z.P.
+*Baylor College of Medicine, Houston, TX
[email protected]
INTRODUCTION:
Pain and weakness are well-recognized problems following carpal
tunnel release (CTR). [1,2] It is well known that the three thenar muscles
(flexor pollicis brevis, abductor pollicis brevia and opponens pollicis)
and the three hypothenar muscles (flexor digiti minimi, abductor digiti
minimi and opponens digiti minimi) take part of their origin from the
transverse carpal ligament (TCL).[2] It has been postulated that
alteration or instability of these muscle origins may be at least partially
responsible for postoperative weakness. [1]
The cause of pillar pain is unclear. While its etiologies may be
multifactorial, detachment or relaxation of the thenar and/or hypothenar
muscles, pulling of the released TCL on the thenar and hypothenar
origins, and periostitis of the bony origins of the thenar and hypothenar
muscles (the hamate and scaphoid tubercles) have all been suggested as
contributing factors.
While release of the TCL during CTR may result in a clinically
important disturbance of the origins of the thenar and hypothenar
muscles, a review of the literature failed to note the percentage of these
intrinsic muscles that originate from the TCL, as compared to the
percentage that originate from the adjacent carpal bones. The purpose of
this study is to quantify the contributions of the various origins to the
thenar and hypothenar muscles.
METHODS:
Ten fresh-frozen cadaveric arms were sectioned through the distal
forearm. All hands and wrists were examined and radiographed to
ensure that they were free from trauma or surgery. The superficial soft
tissues were removed, and the thenar and hypothenar muscles identified
(Figure 1). The thenar muscle insertions were transected to allow
unobstructed visualization of the origins. The boundaries of the muscle
origins were marked, the muscles excised, and the entire surface of all
origins digitized using a stylus connected to a Fastrak Digitizer
(Polhemus; Colchester, VT). As the muscles took origin from both bone
and soft tissue, their origins were irregular, with significant elevations in
the bony architecture. In order to accurately analyze the origin of the
muscles in all three dimensions, the Rapidform 2004 program was used
(INUS Tech; Seoul, Korea). This program can accurately calculate the
two dimensional area of the origins from three-dimensional input. This
process was repeated for the hypothenar muscle group.
RESULTS:
The thenar muscles took origin from the TCL, the scaphoid tubercle
and the trapezial ridge. The mean size of the thenar origins from the
TCL, scaphoid tubercle, and trapezial ridge are shown in Table 1. The
hypothenar muscles took origin from the TCL, hook of the hamate, the
volar carpal ligament (VCL) and the pisiform. The mean size of the
hypothenar origins from the TCL, hook of the hamate, VCL, and
pisiform was shown in Table 2.
DISCUSSION:
While there was a significant degree of individual variation between
specimens, as evidenced by the standard deviations noted in Tables 1
and 2, our results show that the majority (68%) of the thenar muscle
origin is from the TCL, while almost half (49%) of the hypothenar
muscle origin is from the TCL. As the volar carpal ligament is also
relaxed by CTR [3] a total of 72% of the hypothenar origin (from both
the TCL and the VCL) may be lengthened by CTR. Therefore, CTR
may lead to alteration of over 2/3 of the thenar and hypothenar muscle
origins.
The results of this study may help explain the origin of postoperative
pinch and grip weakness. Pinch strength, in which the thumb opposes
the index, and occasionally the middle finger, is related to strength of the
thenar muscles. So is grip strength. The flexor power of the four fingers
is most specifically measured by hook strength, which is considerably
greater than grip strength. The limiting power of grip strength, which
measures the opposing force of the thumb against the four fingers, is that
of the thenar muscles. Therefore, the most important muscles in grip are
those of the thenar eminence.[4] Any changes in grip strength should be
most directly related to its limiting factor, which is the opposition power
of the thumb generated by the thenar muscles.
Pillar pain is another common problem following CTR. The results
of this study lend support to the hypotheses that pillar pain, in whole or
in part, may be due to alterations of the thenar and/or hypothenar
origins.[5] It is possible that differential tensioning of the released
origins (from the TCL and VCL) and non-released origins (from the
bony carpus) may contribute to pillar pain. As 2/3 of the thenar and
hypothenar origins are lengthened, the forces of grip and pinch will be
preferentially transmitted to the relatively shorter 1/3 of the muscles
originating from the bony carpus. This could increase the stress on these
small myotendinous units, potentially leading to pain.
In conclusion, this study has found that over 2/3 of the thenar
muscles originate from the TCL, and that over 2/3 of the hypothenar
muscles originate from the TCL and VCL. Both of these ligaments, and
the intrinsic muscle fibers that originate from them, may be functionally
lengthened during CTR. These findings lend support to the theory that
altered intrinsic muscle origins may be a contributing factor in the
etiology of pinch and grip weakness, as well as pillar pain, following
CTR.
Area (cm2)
Average
TCL
3.60
67.52%
Scaphoid
0.52
10.36%
Trapezium
1.18
22.13%
Table 1: Area of the Thenar Origin.
S.D.
0.90
0.20
0.50
Area (cm2)
Average
TCL
1.73
48.55%
Hamate
0.35
11.86%
VCL
0.78
23.71%
Pisiform
0.50
15.88%
Table 2: Area of the Hypothenar Origins.
S.D.
1.24
16.53%
0.17
6.54%
0.61
12.63%
0.23
6.12%
8.10%
5.08%
7.28%
Fig.1 The thenar and hypothenar muscles
REFERENCES:
1.
Netscher D, Steadman AK, Thornby J, Cohen V. Temporal
changes in grip and pinch strength after open carpal tunnel release
and the effect of ligament reconstruction J Hand Surg (Am) 1998;
23:48-54.
2.
Ludlow KS, Merla JL, Cox JA, Hurst LN: Pillar pain as a
postoperative complication of carpal tunnel release: a review of
the literature. J Hand Ther 10:277-282, 1997.
3.
Richman JA, Gelberman RH, Rydevik BL, Hajek PC, Braun RM,
Gylys-Morin VM, Berthoty D. Carpal tunnel
syndrome:morphologic changes after release of the transverse
carpal ligament. J Hand Surg 1989;14A:852-857.
4.
Bechtol CO. Grip test: Use of a dynamometer with adjustable
hand spacing. J Bone Joint Surg 1954;36A: 820-824.
5.
Hunt TR, Osterman, AL. Complications of the treatment of carpal
tunnel syndrome. Hand Clinics 1994; 10: 63-71.
AFFILIATED INSTITUTIONS FOR CO-AUTHORS:
**Houston VAMC, Houston TX
51st Annual Meeting of the Orthopaedic Research Society
Poster No: 0581