Abstract
This study investigated the effects of the finger extensor mechanism on the bone-to-bone contact forces at the interphalangeal and metacarpal joints and also on the forces in the intrinsic and extrinsic muscles during finger pressing. This was done with finger postures ranging from very flexed to fully extended. The role of the finger extensor mechanism was investigated by using two alternative finger models, one which omitted the extensor mechanism and another which included it. A six-camera three-dimensional motion analysis system was used to capture the finger posture during maximum voluntary isometric pressing. The fingertip loads were recorded simultaneously using a force plate system. Two three-dimensional biomechanical finger models, a minimal model without extensor mechanism and a full model with extensor mechanism (tendon network), were used to calculate the joint bone-to-bone contact forces and the extrinsic and intrinsic muscle forces. If the full model is assumed to be realistic, then the results suggest some useful biomechanical advantages provided by the tendon network of the extensor mechanism. It was found that the forces in the intrinsic muscles (interosseus group and lumbrical) are significantly reduced by 22% to 61% due to the action of the extensor mechanism, with the greatest reductions in more flexed postures. The bone-to-bone contact force at the MCP joint is reduced by 10% to 41%. This suggests that the extensor mechanism may help to reduce the risk of injury at the finger joints and also to moderate the forces in intrinsic muscles. These apparent biomechanical advantages may be a result of the extensor mechanism's distinctive interconnected fibrous structure, through which the contraction of the intrinsic muscles as flexors of the MCP joint can generate extensions at the DIP and PIP joints.
Highlights
The structural and functional complexities of the human finger have long been recognised [1,2,3,4,5]
This is in general agreement with the posture-dependent pattern of MCP joint contact force reported by Harding et al [21]
Comparing the joint contact forces generated by the minimal model and the full model in Figure 6, it appears that including the extensor mechanism does not have a significant effect on the calculated DIP and PIP joint contact forces
Summary
The structural and functional complexities of the human finger have long been recognised [1,2,3,4,5]. Effective function of the finger requires precise coordination of multiple muscles and the resulting finger motion is constrained by the forces exerted by the joint capsules, ligaments and joint articular surfaces. Finger mechanics is complicated by the finger extensor mechanism ( referred to as the extensor apparatus, extensor assembly or extensor expansion), which is a complex tendon network that brings together the forces of the lumbrical, interossei, and long extensor to produce precise functional movements of the phalanxes (see Figure 1). Despite this, little is known about how the extensor mechanism affects the mechanical loadings at finger joints and muscles
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