Changes in geometry of the ®nger ¯exor tendons in the carpal tunnel with wrist posture and tendon load: an MRI study on normal wrists Peter J. Keir a, * , Richard P. Wells b a Department of Kinesiology and Health Science, York University, 364 Bethune College, 4700 Keele Street, North York, Toronto, Ontario, Canada M3J 1P3 b Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 Received 10 November 1998; accepted 10 February 1999 Abstract Objectives. (1) To develop a methodology to determine the trajectories of the digital ¯exor tendons using MRI. (2) To examine changes in tendon trajectories due to wrist posture, with and without pinch force. (3) To calculate the radius of curvature of the ¯exor tendons and note implications for contact forces on the median nerve. (4) To assess the use of LandsmeerÕs models at the wrist. Design. Finger ¯exor tendon centroids were digitized from magnetic resonance images of the carpal tunnel and the tendon paths were determined analytically. Radii of curvature were calculated from the tendon paths. Background. LandsmeerÕs models of joint-tendon interaction (Landsmeer, 1961) have been used to determine moment arms and radius of curvature of the tendon paths about articulations. An explanation for a biomechanical cause of work-related carpal tunnel syndrome originated from these models. Methods. Three healthy male participants had their right wrist scanned while splinted in four wrist postures (¯exed to 20°, 45°, neutral, extended to 20°) with and without maintaining a 10 N pinch grip. 20±24 cross-sectional images were used for each con- dition. Results. Volar movement of the tendons was seen with wrist ¯exion and the opposite was true with extension. Tendon inter- section angles were calculated between the tendon as it entered the carpal tunnel and as it exited the tunnel and were 50±65% of the wrist angle (R 2  0.81±0.96). The radius of curvature was smallest (mean  82±127 mm) with an active pinch grip with the wrist splinted at 45° of ¯exion (mean actual wrist angle 37°). Conclusions. The radius of ¯exor tendon curvature is not constant as previously assumed and is larger than previous estimates. The addition of tendon force with the wrist ¯exed acts to reduce the radius of curvature which further increases the contact stress on the median nerve and other wrist structures. The use of MRI to determine the tendon paths has provided new insight into the relationships between the ®nger ¯exor tendons and other structures at the wrist. Relevance These ®ndings provide data for biomechanical models of the carpal tunnel and predict the possible pathophysiology of work- related carpal tunnel syndrome. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: Wrist; Model; MRI; Radius of curvature; Finger ¯exor tendons; Tendons www.elsevier.com/locate/clinbiomech Clinical Biomechanics 14 (1999) 635±645 Glossary of terms r 1;2;3 moment arm for Landsmeer joint model I, II, III q 1;2;3 radius of curvature for Landsmeer joint model I, II, III h joint angle in radians y distance along axis of bone from the joint centre to the end of the curved section of tendon in Landsmeer's model III d distance from the centre of the shaft of the bone to the tendon in Landsmeer's model III (equal to r 3 with straight joint, h  0) h distance from the joint centre to the tendon in middle of curved portion in Landsmeer's model III (equal to r 3 ) F L normal force per unit arc length * Corresponding author. E-mail: pjkeir@yorku.ca 0268-0033/99/$ - see front matter Ó 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 8 - 0 0 3 3 ( 9 9 ) 0 0 0 1 2 - 1