Comparison of the efficiency between the extrinsic and intrinsic extensor muscles for extension of the proximal interphalangeal

Abstract

Introduction Extension of the proximal interphalangeal (PIP) joint of the finger is consisted of both the extrinsic and intrinsic muscle function. We aimed to analyze combined role of both contribution in extension of the PIP joint of the finger with fresh cadaver.

Materials and methods Seven middle fingers of fresh cadaver were used in current study. The extensor digitorum communis (EDC) and the intrinsic tendons were dissected and the Krackow suture was done for each tendon. Each finger is mounted with Kirschner wires on the custom-made experiment system. The EDC tendon was connected to one motorized actuator, and the intrinsic tendons were connected to the other actuator. One pendulum of 120g was connected to the flexor digitorum profundus to give continuous flexion load on the PIP joint. Extension force was loaded on each tendon and the load was gradually increased from 0g. The load was gradually increased until full extension of the PIP was achieved with use of the control software. Angle of the PIP joint was measured with the sensor connected to the K-wire inserted at the distal portion of the middle phalanx. Three investigations were done for each finger unit; first, each motorized actuator was linked to the EDC tendon and the intrinsic tendons, respectively, second, two actuators were all linked to the EDC tendon, third, two actuators were all linked to the intrinsic tendons. Each investigation was designated as Group 1, 2, 3, respectively. These three investigations were done with two different modes, metacarpo-phalangeal (MP) joint with 20 degree flexion, and with 60 degree flexion. Linear regression analyses were done to determining regression formula between force loaded on each tendon and extension angle of the PIP joint. The slope of the formula considered as an efficiency of the each tendon for extension of the PIP joint. Physiologic cross sectional area (pCSA) of each muscle was used to incorporate maximal contraction power of each muscle. Actual contribution of each extensor system was determined with use of the extension efficiency of each extensor system and pCSAs.

Results With MP joint of 20˚ flexion, efficiency of the EDC for extension of the PIP joint was 26% greater than the intrinsic muscles. With MP joint of 60 ˚ flexion, efficiency of the EDC for extension of the PIP joint was 41% greater than the intrinsic muscles. Intrinsic muscles showed trends of decreasing efficiency of the PIP joint extension with MP joint flexion. With the MP joint flexion of 20˚, estimated actual contribution of the extrinsic and the intrinsic muscle were 51.4%, 48.5%, respectively, and with the MP joint flexion of 60˚, estimated actual contribution of the extrinsic and the intrinsic muscle were 54.1%, 45.9%, respectively.

Conclusions Extrinsic muscle has greater contribution for PIP joint extension compared to the intrinsic muscle. With MP joint flexion, role of the intrinsic tendons for extension of the PIP joint decreased.