Abstract
Purpose To identify the anatomical basis of entrapment of radial sesamoid bone of thumb into intercondylar notch of first metacarpal head and to reproduce radiographic “overlapping sign” on sesamoid subluxation. Methods Ten human cadaveric hands were dissected to measure the anthropometric parameters of radial sesamoid bone and intercondylar notch of first metacarpal head. Radiographs were taken to identify and to reproduce “overlapping sign” on sesamoid subluxation into intercondylar notch. Results Intercondylar notch of first metacarpal head was consistently larger than radial sesamoid bone, which allowed the entrapment. Radiographic “overlapping sign” was reproducible when such subluxation occurred. Conclusion This anatomical study identifies sesamoid subluxation into intercondylar notch as the cause of first metacarpophalangeal joint rotary subluxation. Radiographic “overlapping sign” is diagnostic of this clinical entity and is an important consideration for open reduction.
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