Assessment of the Dorsal Tangent of the Metacarpal Neck as an Intraoperative Reference for Metacarpophalangeal Joint Arthroplasty.

Objectives: In metacarpophalangeal (MCP) joint arthroplasty, some implants require an oblique osteotomy of the metacarpal neck. If the osteotomy is not aligned with the rotational axis, the osteotomy surface itself can tilt ulnarly, leading to unintended ulnar deviation of the finger. This study aimed to evaluate whether the dorsal tangent angle (DTA) of the metacarpal neck can serve as a reliable intraoperative reference for rotational alignment during MCP joint arthroplasty.

Methods: This cross-sectional study analysed CT data from 44 hands of 44 Japanese patients (15 hands from 15 RA patients and 29 hands from 29 non-RA patients). Three-dimensional CT models of the 2nd to 5th metacarpals were generated, and the DTA was measured as the angle between the dorsal tangent of the metacarpal neck and the rotational axis of the metacarpal head. Differences in DTA among the metacarpals and gender differences were analysed.

Results: The median DTA increased progressively from the 2nd to the 5th metacarpals (77.5°, 82.0°, 87.8°, and 94.6°, respectively). A significant gender difference was observed in the 3rd metacarpal, with a greater DTA in women than in men (84.4° versus 78.7°). No significant differences in DTA were found between the RA and non-RA groups, both in the overall population and when the analysis was limited to women.

Conclusions: The dorsal tangent of the metacarpal neck can serve as a reliable intraoperative reference for rotational alignment in MCP joint arthroplasty. Supination is recommended for the 2nd and 3rd metacarpals, though a slightly reduced degree of supination is preferable for the 3rd metacarpal, particularly in women.