Measurement of thumb-tip forces produced by individual muscles:
application to restoring key pinch following tetraplegia
Tendon transfer surgery, involving the muscles of the thumb, is the most
common way to treat grasp impairments resulting from high-level injury to the
cervical spinal cord. Despite the frequent success of transfer surgery,
surgical outcomes are inconsistent. Inconsistencies exist, in part, because we
do not have an appreciation for the thumb-tip forces that thumb muscles
produce. The aims of this dissertation were to measure such forces, to estimate
the extent to which anatomic factors contribute to force variability and to
assess common surgical procedures designed to restore the ability to key pinch.
We applied 10 N of tendon force to the flexed, cadaveric thumb and measured
thumb-tip force individually produced by nine thumb muscles: extensor pollicis
longus/brevis (EPL, EPB), abductor pollicis longus/brevis (APL, APB), flexor
pollicis longus (FPL), the radial and ulnar heads of flexor pollicis brevis
(FPBr, FPBu), adductor pollicis (ADP) and opponens pollicis (OPP). We also
developed a planar, three-link model-based on the forces collected-to determine
the sensitivity of thumb-tip force to muscle moment arms and bone lengths.
Lastly, we measured the effects of two common surgical procedures-fusion of the
interphalangeal (IP) joint and release of the A1 and oblique pulleys-that
sometimes complement tendon transfer surgery involving FPL.
In the plane of flexion/extension, the magnitudes of the thumb-tip forces
produced ranged from 7% to 30% of the applied force. Also, muscles produced
force in a variety of directions. The flexor pollicis longus and FPBu produced
force with sizeable palmar components. The adductor pollicis, APB, FPBr and OPP
were the major distal force producers. The abductor pollicis longus produced
force in the proximal direction, and EPB and EPL, mainly in the dorsal
direction. Variations in the proximal and palmar force components ranged
(interquartile) from 0.6 N and 0.3 N in APB to 1.2 N and 1.5 N in OPP. A
model-based sensitivity analysis showed that peak sensitivity of thumb-tip
force direction to moment arms exceeded 8.0 for a 30% variation in moment arms.
Also, peak sensitivity to bone lengths was approximately 5.0 for a 10%
variation. Lastly, pulley release and joint fusion both increased the thumb-tip
force produced by FPL. Pulley release made the force less palmarly-directed;
while fusion tended to do the opposite.
The low gain in force from tendon to thumb-tip is common and may be
associated with relatively small muscle moment arms. The directions of
thumb-tip forces produced were sometimes inconsistent with how muscles move the
thumb because contact and contact-free mechanics differ. Contrary to common
dogma, for example, many of the intrinsic flexors-OPP, APB, ADP, FPBr-and APL-a
thumb extensor-produced disproportionately large distal and proximal forces.
The extreme sensitivity of force direction to anatomic factors may partly
explain the relatively large variability in force direction for many muscles
(e.g., coefficient of variation of FPL's force direction: 27%).
Tendon transfer surgeries involving FPL, specific to persons with injury to
the fifth or sixth cervical spinal nerves, are designed to restore stable key
pinch. Stability is achieved when the thumb-tip does not slip on the object
being grasped. Our measurements of force directions suggest that FPL's
thumb-tip force may be too oblique to prevent slip on many objects that could
be grasped. This notion is consistent with post-surgical observations of
Froment's sign when some patients attempt to grasp objects. A more
palmarly-directed force would facilitate stable or slip-free contact. Our
measurements indicate that the direction of the resultant force produced by
FPL, FPBr and FPBu would produce a palmarly-directed force. Typically, the
functions of FPBr and FPBu are not restored. Therefore, this work suggests an
alternate procedure for restoring key pinch in the neurologically impaired
thumb. Lastly, our measurements of the effects of IP joint fusion and the
release of the A1 and oblique pulleys suggest that IP joint fusion may increase
the stability of key pinch; while pulley release may decrease the stability.
Completion of this work represents a critical step in the effort to improve
surgeries to restore stable key pinch following high-level injury to the
cervical spinal cord.