Finger kinematics during grasp were measured on 5 cadaveric hands. To simulate finger flexion, the FDP was activated by a motor while intrinsic muscles were loaded at various levels (0, 125, 250, 375 or 500g). Finger movement was characterized by the order of metacarpophalangeal, proximal interphalangeal, and distal interphalangeal joint flexion and by the maximal fingertip-to-palm distance during finger closure.
Results
Without any intrinsic muscle contribution (0g load), FDP activation resulted in flexion of all 3 joints, whereby flexion began at the proximal interphalangeal joint, followed by distal interphalangeal joint and then metacarpophalangeal joint. With increasing intrinsic muscle load, finger flexion was initiated at the metacarpophalangeal joint, followed by the proximal interphalangeal and distal interphalangeal joints. This altered joint flexion order resulted in a larger maximal fingertip-to-palm distance during finger flexion. The difference between the 2 extreme conditions (0g vs. 500g of intrinsic muscle load) was 19mm.
Discussion
These findings demonstrate that simultaneous activation of the FDP and the intrinsic muscles results in an apparently more functional hand closing compared to FDP activation alone because of altered kinematics and larger fingertip-to-palm distances.
Clinical Relevance
These findings suggest that intrinsic muscle balancing during reconstruction of grasp in tetraplegic patients may improve function. Keywords: grasp, hand, intrinsic muscles, tendon transfer, tetraplegia
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Introduction
Regaining arm and hand function has been identified as the highest priority for persons with tetraplegia.1,2 Recovering even partial function can have an enormous impact on independence, which enhances quality of life.3 Restoration of finger flexion is an achievable goal in reconstructive surgery of many tetraplegic hands. It can be performed if both the extensor carpi radialis longus and brevis are fully innervated, in which case the extensor carpi radialis longus is transferred to the flexor digitorum profundus (FDP),4 which enables patients to grasp and hold objects. However, by restoring FDP function only, fingertips coming into full flexion approach the bases of the fingers rather than the center of the palm.5 This happens because finger flexion begins at the distal interphalangeal (DIP) joint, and fingers curl into flexion rather than following a large arc, which would provide a broad sweeping movement.6 The roll-up finger flexion tends to push large objects out of grasp5 and is therefore considered less functional in daily life.
In normal hand function, the intrinsic muscles, both the lumbricals and interosseus muscles balance finger movement7 and create this broad sweeping movement. Besides abducting and adducting the fingers, they are responsible for coupling metacarpophalangeal joint (MCP) flexion with interphalangeal joints (IP) extension.8,9 For 2-dimensional finger movement during grasp, the lumbricals and the interossei provide the same function, as shown by Leijnse et al.10 Since their function is redundant in the sagittal plane, we refer to them as 1 entity, the intrinsics. Intrinsic function is so important that even when patients have neither functional intrinsic muscle nor a sufficient number of transferable muscles to reconstruct them with an active tendon transfer, passive tenodeses are used to substitute for intrinsic muscle function.11,12
We characterize functional hand movement for tetraplegic persons as a large fingertip-to-palm distance during flexion because this enables them to grasp objects of varying sizes and shapes necessary for activities of daily life. The exact contribution of the intrinsic muscles to this function is incompletely understood. Therefore, the purpose of this study was to quantify the role of intrinsic muscle force in creating a functional grasp. We hypothesized that increasing intrinsic muscle contribution would result in a more functional grasp.
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