An Analysis of Factors That Support Early Active Short Arc Motion of the Repaired Central Slip

Abstract

This study defines precise parameters for tendon excursion, force application, and exercise position for an early active short arc motion protocol for the repaired central slip. Recommended active excursion for the extensor digitorum communis (ED) in zone III is 3.75 mm during the early healing phase. Based on the radian concept, the proximal interphalangeal joint (PIP) is actively flexed and extended 28.65° (approximately 30°) or one-half radian to effect this tendon excursion. Resistance applied to the central slip with active extension from 30° to 0° is calculated mathematically at approximately 290 g of force. Tensile strengths for various repairs are reviewed to establish the safety margin between tensile strength of the repair site and force application. Force application for the active extension protocol is considered in terms of anatomic position. The position of wrist flexion at 30° reduces ED work requirement 1) by reducing viscoelastic flexor forces and 2) through a contribution from the interossei. The position of MP at 0° 1) transmits extensor forces to the central slip and 2) reduces ED work requirement through lumbrical and interossei action. The distal interphalangeal joint (DIP) is unrestrained during PIP flexion to allow volar slide of the lateral bands. Isolated DIP exercises with the PIP held at 0° creates a distal glide of the ED in zone IV while reducing tensions in zone III through the action of the lateral bands. The short arc motion protocol as defined in this paper and supported by a companion clinical study is safe and physiologically desirable as determined by this study.