Changes in the flexor digitorum profundus tendon geometry in the carpal tunnel due to force production and posture of metacarpophalangeal joint of the index finger: An MRI study
Introduction
During finger actions like in grasping, typing or pressing tasks, fingertip forces result mainly from changes in the activation of the extrinsic muscles of the hand (Gonzalez et al., 2005, Goodman and Choueka, 2005). The extrinsic muscles are multi-joint muscles that span the distal interphalangeal (DIP), proximal interphalangeal (PIP), metacarpophalangeal (MCP) and wrist joints. A better understanding of the mechanics of the tendons at the wrist joint is important for understanding the functioning of the hand and fingers.
Several studies on the effects of joint position on the tendons geometry at the wrist joint have been performed without applying a force to the tendons (Brand and Hollister, 1999, Chao et al., 1989) and with artificial load or springs simulating active force (An et al., 1983, Armstrong and Chaffin, 1978, Brand et al., 1975 Youm et al., 1978). Very few authors have investigated the in-vivo tendon behavior under varying joint angles and muscle contractions (Keir and Wells, 1999, Martin et al., 2012). It was previously documented that the paths of the extrinsic flexor tendons about a joint change due to change in position of that joint and activation level of that muscle (Keir and Wells, 1999, Martin et al., 2012).
The purpose of this study has been to measure changes in the paths of the flexor digitorum profundus (FDP) at the carpal tunnel level due to changes in: (1) position of the MCP joint of the index finger and (2) force produced by the index finger. Based on the study by Keir and Wells (1999) we expect that we will observe changes in the tendon paths qualitatively similar to those reported in that study. Whether or not the magnitudes of the changes will be comparable is unknown.
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Subjects
Five male subjects volunteered for the study. The subjects were young and reported no history of musculoskeletal injury or disease of the upper limbs. The average (standard deviation) age, mass, and height of the subjects were 21.8 (2.2) years, 71.7 (8.3) kg, and 179.9 (9.0) cm, respectively. None of the subjects had a history of long-term involvement in hand or finger activities such as professional typing or playing musical instruments. The subjects gave informed consent according to the
Ellipse area
The first result concerns the ellipse area. No significant difference (P > 0.1) of the ellipse area due to any of the factors was found. The average ellipse area was 24.5 (9.5) mm2.
Radius of curvature
The second result concerns the radius of curvature in the Y–Z plane (Fig. 4). There is a significant effect of the POSTURE (F2, 8 = 4.737, P < 0.05): in extension the radius is larger, with a value of 191.0 (148.9) mm. There is also a significant effect of the FINGER (F3, 12 = 12.601, P < 0.05). The general trend is that
Discussion
The major finding of this study was that changes in posture and force produced by a single finger can cause significant changes in the FDP tendon geometry within the carpal tunnel (Fig. 7). This finding is relevant to clinicians concerned with carpal tunnel syndrome: the change in tendon geometry can induce different levels of pressure on the median nerve (Lopes et al., 2011) or can affect the relative tendon movements after a carpal tunnel release surgery (Yoshii et al., 2008). Biomechanists
Conclusions
In conclusion, this study demonstrated that the tendon path in the carpal tunnel is very sensitive to the finger activity. The compliance of the carpal transverse ligament allows the tendons crossing this joint to be affected by a bowstringing phenomenon. A change in the moment arm (i.e. the tendon distance to joint center of rotation) occurs not only during wrist movements but also during finger movements and/or extrinsic finger muscle activity — even for small force values (~ 10–15% of MVC).
Acknowledgments
We would like to thank Dr. Susan Lemieux, Amanda Gearhart, Brian Johnson and Andrew Georgeson for their assistance with this project. This study was supported in part by NIH grants AG-018751, NS-035032, and AR-048563.
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