Improving the extensor lag and range of motion following free vascularized joint transfer to the proximal interphalangeal joint: Part 1. An observational and cadaveric study.

Background: Extension lag is a common occurrence following free vascularized joint transfer reconstruction of the proximal interphalangeal joint, thus limiting the range of motion that is achievable. In this Part 1 study, the anatomical and biomechanical arrangements of the toe proximal interphalangeal joint extensor mechanism were investigated.

Methods: Twelve second toes from 12 fresh cadavers were dissected for examination of the extensor mechanism. Similar observations were performed in nine clinical cases. A total of 21 toes were therefore examined with dynamic and static testing.

Results: In 17 toes, the central tendon attenuated before inserting onto the middle phalangeal base (type I). In four toes, a thicker tendinous insertion analogous to a central slip was identified (type II). In type I toes, traction of the extensor digitorum brevis and digitorum longus did not correct the extensor lag. The extension lag decreased when the metatarsophalangeal joint was flexed passively or when both extrinsic and intrinsic tendons were pulled simultaneously. In type II toes, a full extension of the proximal interphalangeal joint was achieved when the extensor tendons were pulled with or without pulling the intrinsic tendons. In both types of toes, there was an increase in the extension lag caused by dorsal bowstringing when separated from the extensor sling.

Conclusions: The lesser toe proximal interphalangeal joint adopts a naturally flexed posture for evolutionary reasons, with corresponding adaptations in extensor mechanism arrangements. The most significant limiting factor to full extension could be the presence of an attenuated central slip in the majority of toes.