Anatomical study of the radial side of thumb metacarpophalangeal joint: the joint capsule and flexor pollicis brevis muscle.

INTRODUCTION

Radial collateral ligament reconstruction for thumb metacarpophalangeal joint instability can result in residual pain and limited motion. While the flexor pollicis brevis, located on the radial side of the metacarpophalangeal joint, is believed to contribute to dynamic stability, the radial collateral ligament provides static stability. However, the structural relationship between the flexor pollicis brevis, radial collateral ligament and joint capsule remains unclear. This study aimed to clarify the anatomical and functional relationship between flexor pollicis brevis, radial collateral ligament and joint capsule of the thumb metacarpophalangeal joint, and to validate these findings using in vivo imaging.

METHODS

Eighteen cadaveric thumbs were examined using micro-computed tomography (micro-CT), macroscopic dissection, and histological analysis. Axial CT images were reconstructed to assess cortical bone thickness. Ultrasonographic imaging was performed on 10 thumbs from healthy volunteers to verify the anatomical findings.

RESULTS

Macroscopic analysis showed that the flexor pollicis brevis intramuscular aponeurosis merges with the joint capsule to form a 'capsuloaponeurotic membrane' that incorporates the radial collateral ligament. Histology confirmed this structural integration. Cortical bone was thickened at the radial collateral ligament attachment sites, consistent with mechanical loading. Ultrasonography demonstrated continuity between the flexor pollicis brevis aponeurosis and the joint capsule in living subjects, supporting cadaveric findings.

CONCLUSION

The flexor pollicis brevis muscle contributes to dynamic stabilization of the thumb metacarpophalangeal joint via its aponeurosis, which integrates with the joint capsule and radial collateral ligament. These findings provide new insight into metacarpophalangeal joint stability and may improve surgical and rehabilitative approaches for radial-sided thumb instability.