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

Background: Carpal tunnel syndrome is a disorder caused by increased pressure in the carpal tunnel associated with repetitive, stereotypical finger actions. Little is known about in vivo geometrical changes in the carpal tunnel caused by motion at the finger joints and exerting a fingertip force. Methods: The hands and forearms of five subjects were scanned using a 3.0 T magnetic resonance imaging scanner. The metacarpophalangeal joint of the index finger was placed in: flexion, neutral and extension. For each joint posture subjects either produced no active force (passive condition) or exerted a flexion force to resist a load (~ 4.0 N) at the fingertip (active condition). Changes in the radii of curvature, position and transverse plane area of the flexor digitorum profundus tendons at the carpal tunnel level were measured. Results: The radius of curvature of the flexor digitorum profundus tendons, at the carpal tunnel level, was significantly affected by posture of the index finger metacarpophalangeal joint (P < 0.05) and the radii was significantly different between fingers (P < 0.05). Actively producing force caused a significant shift (P < 0.05) in the flexor digitorum profundus tendons in the ventral (palmar) direction. No significant change in the area of an ellipse containing the flexor digitorum profundus tendons was observed between conditions. Interpretation: The results show that relatively small changes in the posture and force production of a single finger can lead to significant changes in the geometry of all the flexor digitorum profundus tendons in the carpal tunnel. Additionally, voluntary force production at the fingertip increases the moment arm of the FDP tendons about the wrist joint.