In vivo evidence of lumbricals incursion into the carpal tunnel in healthy hands: An ultrasonographic cross sectional study.

Introduction: During finger flexion, the tendons of flexor digitorum profundus migrate proximally, along with their attached lumbrical muscles. This incursion was suggested to extend into the Carpal Tunnel. Ultrasonographic imaging can be used to assess in vivo soft tissue behavior and incursion.

Purpose Of The Study: To clinically quantify the lumbrical muscles incursion in different finger positions.

Study Design: Cross sectional, observational study.

Methods: The lumbricals of 20 healthy adults with no history of hand injuries were evaluated with neuromuscular ultrasound imaging (n = 160 lumbricals). The lumbrical muscles migration was measured as the participants actively moved their fingers from full extension to 50% flexion, and 100% flexion.

Results: Of the 160 lumbricals measures, the incursion occurred at 18.1% of fingers at 50% finger flexion, and increased to 79.4% during full finger flexion. The lumbricals migrated a total of 2.99 cm after full finger flexion, and ended up 0.76 cm (SD = 0.86 cm) inside the Carpal Tunnel. The metacarpophalangeal joint range of motion of the index finger at the point where the lumbricals entered the distal border of the Transverse Carpal Ligament was 84.4° (SD = 6.8°). The Carpal Tunnel cross-sectional area during finger extension was 1.68 (0.35) cm, and increased to 1.81 (0.33) cm after full finger flexion.

Conclusion: This study showed direct evidence of lumbrical incursion into the Carpal Tunnel during finger flexion. The cross-sectional area of the Carpal Tunnel increased during full finger flexion in comparison to full finger extension, supplementing the evidence of increase content within the Carpal Tunnel. The findings of this study have significant clinical implications for the conservative treatment of the Capral Tunnel Syndrome.