Biomechanics and Pinch Force of the Index Finger Under Simulated Proximal Interphalangeal Arthrodesis.

PURPOSE: To analyze the effect of simulated proximal interphalangeal (PIP) joint arthrodesis on distal interphalangeal (DIP) joint free flexion-extension (FE) and maximal voluntary pinch forces.

METHODS: Five healthy subjects were tested with the PIP joint unconstrained and constrained to selected angles to produce (1) free FE movements of the DIP joint at 2 selected angles of the metacarpophalangeal joint, and (2) maximal voluntary tip (thumb and index finger) and chuck (thumb, index, and middle fingers) pinch forces. Kinematic data from a motion analysis system, pinch force data from a mechanical pinch meter, and electromyography (EMG) data recorded from 2 flexor and extensor muscles of the index finger were collected during free FE movements of the DIP joint and pinch tests for distinct PIP joint constraint angles.

RESULTS: The EMG root mean square (RMS) values of the flexor digitorum profundus (FDP) and extensor digitorum (ED) did not change during free FE of the DIP joint. The extension angle of the range of motion of the DIP joint changed during free FE. It increased as the PIP constraint angle increased. The EMG RMS value of FDP and ED showed maximum values when the PIP joint was unconstrained and constrained at 0° to 20° of flexion during tip and chuck pinch. Neither the index finger metacarpophalangeal and DIP joint positions nor pinch force measurements differed with imposed PIP joint arthrodesis.

CONCLUSIONS: The PIP joint arthrodesis angle affects DIP joint extension. A minimal overall impact from simulated PIP arthrodesis in muscle activity and pinch force of the index finger was observed. The EMG RMS values of the FDP and ED revealed that a PIP arthrodesis at 0° to 20° of flexion leads to a more natural finger posture during tip and chuck pinch.

CLINICAL RELEVANCE: This study provided a quantitative comparison of free FE motion of the DIP joint, as well as FDP and ED forces during pinch, under simulated index finger PIP arthrodesis angles.