Analysis of intrinsic-extrinsic muscle function through interactive 3-dimensional kinematic simulation and cadaver...

To revisit the analysis of intrinsic muscle function and its relationship to the overall flexion moment at the index metacarpophalangeal (MCP) joint. Prior studies reported inconsistent levels of intrinsic contribution varying from 12.5% to 73% of the total flexion moment at the index MCP joint. This study hypothesized that 26% is a more realistic figure using computer simulation moment arms combined with known muscle-tension fractions. This study tested the hypothesis using a 2-part fresh cadaver study, the objectives of which were to measure actual muscle moment arms throughout index MCP range of motion and then combine these measurements with tension fractions to calculate normal intrinsic minus and low ulnar nerve palsy moments and then using the same specimens to apply loads to the muscles and measure resultant moments directly.

An interactive computer simulation was constructed to replicate the position and loading orientation of an earlier experiment reported in the literature and known muscle-tension fractions were combined with the resultant moment arm structure to calculate the moment at the index MCP joint for maximum isometric contraction of the normal, intrinsic minus, and low ulnar nerve palsy finger. Eleven fresh cadaver specimens were used. For experiment A the excursion and angle data were used to determine moment arms of each muscle and moments were calculated. For experiment B the muscles were connected to weights based on muscle-tension fractions and the resultant moment was measured at the fingertip.

Experiment A determined the muscle-tendon moment arms at the index MCP joint throughout the flexion-extension range of motion. Combining the flexor moment arms with known tension fractions determined that the intrinsics contributed 24% to the total flexion moment, the dorsal and palmar interossei contributed 22%, and the lumbrical contributed 2%. The results of experiment B confirmed the results of experiment A.

Intrinsic muscle contribution to flexion moment at the index MCP joint is not as high or as low as previously reported. The relative moment production is governed by both the moment arms and tension produced by the muscle; thus a more precise knowledge of moment arms is essential to understand muscle balance at a joint. Clinically the hand surgeon should expect a loss of 24% of index MCP flexion moment-generating capacity in the intrinsic-minus index finger and a loss of 22% in the index finger on a hand with low ulnar nerve palsy. Furthermore the flexion moment from the lumbrical of only 2% appears to be adequate to prevent clinical clawing of the digit.