Estimation of in-vivo muscle moment arms around the MCP joint using ultrasound

Since in the finger most muscle-tendons cross multiple joints, moment arms (MA’s) of these muscle-tendons have a major influence on the distribution of joint moments over these joints. This is crucial for finger balance and therefore finger functioning. Recent developments in the technique of ultrasonic measurements have made it possible to accurately measure tendon displacements over a large range of motion in-vivo. The change in tendon displacement divided by the change in corresponding joint angle (dl/dθ) was used to estimate joint angle dependent MA’s (MAest) of the flexor digitorum superficialis (FDS), flexor digitorum profundus (FDP) and extensor digitorum communis (EDC) tendon at the metacarpophalangeal (MCP) joint of the long finger. In addition, MA’s were obtained from the joint geometry for each individual subject (MAgeo). Two sessions of 3 repetitions of active flexion-extension motion of the long finger were conducted by each of the five subjects enrolled in this study. The intra subject repeatability between the two sessions at 00 of flexion for the FDS and EDC tendon was good (ICC> 0.95, p = 0.004 and 0.101 respectively) but rather weak for the FDP (ICC = 0.630, p = 0.157). The obtained MAest values were underestimated in comparison to the MAgeo values. Thus, this method proved to be promising, but should be further validated and developed to yield sufficiently reliable results. Once fully developed, the in-vivo estimation of MA’s can be used to determine the subject specific muscle MA balances around the finger joints. These MA balances can be an important parameter for a musculoskeletal finger model. In addition, it can be a generic measure for finger pathologies where finger balance is disturbed