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 middle and distal phalanges. 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 extensor medial and interosseous lateral bands of the extensor mechanism, 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 fiber bundles 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 extensor medial and interosseous lateral bands. We find that including bundles of intercrossing fiber bundles significantly affects force transmission, which itself depends on finger posture. We conclude that the intercrossing fiber bundles-traditionally left out in prior models since Zancolli's simplification-play an important role in force transmission and variation of the latter with posture.

Keywords: extensor apparatus; extensor assembly; extensor mechanism; finger biomechanics; finger extensor tendons.

FIGURE 1

The view of the extensor mechanism modelled in a developed environment. (a): the full model, which contains the principal tendon and ligaments of the extensor mechanism: 1—the extensor hood, 2—interosseous medial fibers (red), 3—the extensor lateral fibers (blue), 4—interosseous lateral band, 5—extensor medial band, 6—medial tendon and middle phalanx attachment, 7—transverse retinacular ligament, 8—triangular ligament, 9—terminal tendon and distal phalanx attachment; (b): the trivial model. The trivial model does not contain the structures connecting the interosseous lateral bands (4) with the extensor medial band (5); (c): flexor tendons: 10—flexor digitorum superficialis tendon, 11—flexor digitorum profundus tendon (same for both models); (d) The schematic view of the intercrossing fiber bundles. Red: interosseous medial fibers; blue: extensor lateral fibers.

FIGURE 2

The influence of posture on the forces in ulnar intercrossing fiber bundles. Red: Ulnar interosseous medial fibers; blue: Extensor lateral fibers. The first row (panels a and b) corresponds to extension (MCP = 10°, PIP = 10°, DIP = 10°), the second row corresponds to mid‐flexion (MCP = 45°, PIP = 45°, DIP = 10°, panels c and d), and the third row corresponds to flexion (MCP = 90°, PIP = 90°, DIP = 80°, panels e and f). Two loading conditions are shown by bars of light (Φ = 2.9 N) and dark (Φ = 5.9 N in UI, EDC, RI, and LU muscles) colors, respectively. Within each loading condition, the same force magnitude was applied to all muscles. The median of 60 simulations, along with the 5th and 95th percentiles, is shown.

FIGURE 3

The effect of the posture on feasible tendon force set. Left column corresponds to a full extensor mechanism model, right column corresponds to a trivial model. The first row corresponds to extension (MCP = 10°; PIP = 10°; DIP = 10°), the second row corresponds to mid‐flexion (MCP = 45°; PIP = 45°; DIP = 10°), and the third row corresponds to flexion posture (MCP = 90°; PIP = 90°; DIP = 80°). The full‐loading state, which corresponds to loading of the extensor mechanism models by all four muscles, is shown by a circle in each feasible tendon force set. The middle and distal phalanx attachment force values in full‐loading state are comparable for both models, but the areas of the feasible tendon force set are smaller for the full model. Also for a full model, the shape and orientation of the feasible tendon force set change with posture. Dark area corresponds to 5.9 N loading and light area corresponds to 2.9 N loading.

FIGURE 4

Influence of the posture on sagital plane projection of the feasible force set. Left column corresponds to a full extensor mechanism model, right column corresponds to a trivial model. First row corresponds to extension posture (MCP = 10°; PIP;= 10°; DIP = 10°), second row corresponds to mid‐flexion posture (MCP = 45°; PIP = 45°; DIP = 10°), third row corresponds to flexion posture (MCP = 90°; PIP = 90°; DIP = 80°). Dark area corresponds to 5.9 N loading and light area corresponds to 2.9 N loading.