Download the fundamental thumb-tip force vectors produced by the

THE FUNDAMENTAL THUMB-TIP FORCE VECTORS
PRODUCED BY THE MUSCLES OF THE THUMB
Jonathan L. Pearlman1, Michal Weisman1, Francisco J. Valero-Cuevas1,2 and Stephanie Roach1
1
Neuromuscular Biomechanics Laboratory, Cornell University, Ithaca, NY, U.S.A.
2
The Hospital for Special Surgery, New York, NY, U.S.A.
E-mail: [email protected]
Web: http://www.mae.cornell.edu/nmbl
METHODS
Figure 1: Experimental Apparatus.
INTRODUCTION
A rigorous description of the magnitude and
direction of the 3D force vector each thumb
muscle produces at the thumb-tip is
necessary to understand the biomechanical
consequences to pinch of a variety of
paralyses and surgical procedures. Available
descriptions of the biomechanical function
of thumb musculature include muscle
architectural parameters and moment arms;
and graphical descriptions of the relative
lines of action of intact and transferred
tendons at each joint (Brand and Hollister,
1999). These descriptions, however, do not
translate directly or unequivocally into
descriptions of the 3D force vector each
muscle produces at the thumb-tip—where
pinch forces occur and where force vectors
need to be restored by tendon transfers, for
example. In this study, we characterized the
3D force vector each muscle produces at the
thumb-tip, and investigated if these thumbtip force vectors scaled linearly with tendon
tension.
We measured the output 3D thumb-tip force
vector produced by each tendon acting on
the thumb, plus two common tendon
transfers, as a function of input tendon
tension (n = 13). After fixing the hand to
frame (Fig 1), we mounted the thumb by
configuring it in standardized key or
opposition pinch posture and coupling the
thumb-tip to a rigidly-fixed 3D force/torque
sensor. Computer-controlled linear actuators
applied tension to the distal tendons of the
four extrinsic thumb muscles, and to six
Nylon cords reproducing the lines of action
of i) the four intrinsic thumb muscles and ii)
two tendon transfers commonly used to
restore thumb opposition following low
median nerve palsy. Transfer A (TRa), is
performed by routing the extensor indicis
proprius muscle to the insertion of the failed
abductor pollicis brevis (AbPB ) via the
pisiform bone. Transfer B (TRb) is done by
routing the flexor digitorum superficialis of
the ring finger to the insertion of the failed
AbPB via a slip in the flexor carpi ulnaris
(Hentz and Leclercq, 2002). We measured
the 3D force vector at the thumb-tip while
each actuator ramped tendon tension from 0
to 1/3 of predicted maximal muscle force
expected at each tendon, and back to zero.
RESULTS AND DISCUSSION
Many thumb-tip force vectors act in
unexpected directions (e.g., the opponens
force vector is parallel to the distal phalanx,
Fig 2). This underscores how standard
anatomical nomenclature can be of little
value in describing fingertip force
production for the purposes of the clinical
restoration of pinch function. The two
tendon transfers produced patently different
force vectors, demonstrating how alternative
(but presumably equivalent) tendon transfers
to restore thumb opposition can be
compared and contrasted by analyzing the
3D thumb-tip output force they produce.
TRb better reproduces the radial component
of force lost after low median nerve palsy
(i.e., previously provided by AbPB). For
most muscles, the increase of the thumb-tip
force vectors with tendon tension is best
described by a quadratic function
(p<0.05)—likely due to load-dependent
viscoelastic tendon paths and joint seating.
Thumb-tip force vectors were sensitive to
mounting procedure, as the moderate
sensitivity to joint seating was compounded
by inaccuracies in thumb posture. We are
now studying the effect of these
nonlinearities and sensitivities on the
production of thumb-tip forces by the
simultaneous action of multiple muscles.
REFERENCES
Brand, P. W. and Hollister, A. (1999)
Clinical mechanics of the hand (3rdEdn).
Mosby, St. Louis, Mo.
Hentz, V. R. and Leclercq, C. (2002)
Surgical rehabilitation of the upper limb
in tetraplegia. W B Saunders.
ACKNOWLEDGEMENTS
This material is based upon work supported
under a National Science Foundation
Graduate Research Fellowship (JLP) and a
Whitaker Foundation Grant (FVC).
Figure 2. Average
thumb-tip output vectors
(N) for each functional
posture in anatomical
projections. Flexor
Pollicis Longus (FPL),
Extensor Pollicis
Longus (EPL), Extensor
Pollicis Brevis (EPB),
Abductor Pollicis
Longus (AbPL),
Abductor Pollicis Brevis
(AbPB), Flexor Pollicis
Brevis (FPB), Opponens
Pollicis (OPP), Adductor
Pollicis oblique and
transverse heads (ADDo
and ADDt), and first
dorsal interosseous
(DIO). Transfers A and
B (TRa and TRb,
respectively) are also
shown.