Morphometrics of the human thumb metacarpal bone: interest for developing an osseointegrated prosthesis.

INTRODUCTION: Amputation of the thumb presents a serious insult to the hand and diminished quality of life for a patient physically, vocationally, and possibly psychologically. The aim of this study was to define the geometry of the thumb metacarpal in order to help create a standardized set of transcutaneous osseointegrated prostheses to treat patients who have suffered amputation of the thumb at the level of the metacarpophalangeal joint.

MATERIALS AND METHODS: A total of 80 metacarpals from 46 cadavers were studied. All soft tissues were removed and the thumb metacarpals were imaged using computed tomography. Three-dimensional models were constructed using images from the coronal, sagittal, and axial planes. Using HyperMesh™ CAD software, the bones were analyzed for overall length, radius of curvature, medullary canal diameter, cortical thickness, and distance to the isthmus, defined as the narrowest portion of the intramedullary canal.

RESULTS: The average length of the first metacarpal was 47.6 mm (±3.3 mm, 39.2-56.9 mm). The average radius of curvature was 55.5 mm (±10.7 mm, 33-78.9 mm). Inner bone diameter, measured in two axes, was 10.5 mm (±1.3 mm, 5.4-18.7 mm) for the major axis and 7.7 mm (±0.9 mm, 4.3-17.8 mm) for the minor axis. The average cortical thickness was 1.4 mm (±0.3 mm, 0.7-3.1 mm). The distance to the center of the isthmus from the distal end had an average length of 21.3 mm (±1.9 mm, 17-25 mm).

CONCLUSIONS: Using these findings a standardized set of intramedullary stems can be developed as a base for a transcutaneous osseointegrated prosthesis, helping to create a reliable method for treating patients with amputated thumbs.