The Bundles of Intercrossing Fibers of the Extensor Mechanism of the Fingers Greatly Influence the Transmission of Muscle Forces

Abstract

The extensor mechanism is a tendinous structure that plays an important role in finger function. It transmits forces from several intrinsic and extrinsic muscles to multiple bony attachments along the finger via sheets of collagen fibers. The most important attachments are located at the base of the second and third phalanges (proximal and distal attachments, respectively). How the forces from the muscles contribute to the forces at the attachment points, however, is not fully known. In addition to the well-accepted medial and lateral bands, there exist two layers of intercrossing fiber bundles (superficial interosseous medial fiber layer and deeper extensor lateral fiber layer), connecting them. In contrast to its common idealization as a minimal network of distinct strings, we built a numerical model consisting of fiber bundles to evaluate the role of multiple intercrossing fibers in the production of static finger forces. We compared this more detailed model of the extensor mechanism to the idealized minimal network that only includes the medial and lateral bands. We find that including bundles of intercrossing fibers significantly affects force transmission, which itself depends on finger posture. In a mid-flexion posture (metacarpal joint MCP = 45{degrees}; proximal interphalangeal joint PIP = 45{degrees}; distal interphalangeal joint DIP = 10{degrees}) the force transmitted by the lateral fibers is 40% lower than in a more pronounced flexed posture (MCP = 90{degrees}; PIP = 90{degrees}; DIP = 80{degrees}). We conclude that the intercrossing fiber bundles -- traditionally left out in prior models since Zancollis simplification -- play an important role in force transmission and variation of the latter with posture.