Original Articles: Fractures
Predicted effects of metacarpal shortening on interosseous muscle function1

https://doi.org/10.1016/j.jhsa.2004.03.002Get rights and content

Abstract

Purpose

Metacarpal fractures are common in hand surgery. Metacarpal shortening ranging from 2 mm to as much as 10 mm has been deemed acceptable in the literature. We examined the effect of metacarpal shortening on interosseous muscle architecture and predicted force production capacity based on the standard muscle length-tension curve (commonly known as the Blix curve).

Methods

The dorsal interosseous muscles between the middle and ring finger metacarpals from 9 adult human cadaver hands were exposed and studied. The ring finger metacarpal was translated proximally in 2-mm increments in relation to a stationary middle finger metacarpal. Digital images were obtained and analyzed to define the length and orientation of individual muscle fibers with each incremental change in position.

Results

Interosseous muscle fiber length increased and pennation angle decreased uniformly with increasing proximal translation of the ring finger metacarpal. At 10 mm of shortening the fiber length had increased to 20.8 ± 1.8 mm, or to approximately 125% of optimum fiber length, and the pennation angle had decreased to 6.7° ± 2.2° or by approximately 50%.

Conclusions

The interosseous muscles have been shown to have a high fiber-to-muscle length ratio. This ratio indicates that these muscles function optimally over a short range of lengths, leaving them vulnerable to derangement in function owing to alteration in the surrounding bony architecture. Based on the standard muscle length—-tension relationship we had predicted a steady linear decrease in interosseous power with proximal translation of the metacarpal. The results indicate an initial linear progression with a plateau at approximately 8 mm of shortening. At 2 mm of shortening there is an approximately 8% loss of power generation, at 10 mm of metacarpal shortening we predict the interosseous muscle to be capable of only approximately 55% of its optimum power compared with the resting position.

Section snippets

Materials and methods

Power analysis was performed and indicated 8 hands would be required. Ten hands were initially selected, allowing for errors in technique. One hand showed ulnar neuropathy, leaving 9 fresh-frozen adult human cadaver specimens without notable musculoskeletal defect or deformity and intact from the midhumeral level. The hand was amputated at the level of the radiocarpal joint. The dorsal skin was incised from the wrist to the middle finger proximal interphalangeal joint and reflected to the

Results

Interosseous muscle fiber length increased and pennation angle decreased monotonically with increasing proximal translation of the ring finger metacarpal Figure 2, Figure 3. Both linear and second-order polynomial regression revealed that fiber length and pennation angle changed significantly with metacarpal translation (p < .01). There was no significant difference in the behavior of the proximal muscle fibers compared with the distal muscle fibers (Fig. 3) based on the lack of significant

Discussion

Skeletal muscle function is related quantitatively to skeletal muscle architecture.9 In general, muscles with long fibers appear to be adapted for excursion whereas muscles with short fibers function better for force generation. The palmar and dorsal interossei are architecturally similar and have short fiber lengths suggesting adaptation for force generation in the hand.6 Loss of intrinsic muscle function owing to low ulnar nerve palsy has been shown to lead to a loss of grip strength of

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