Articular geometry of the medial tarsometatarsal joint in the foot: comparison of metatarsus primus adductus and metatarsus primus rectus.

The three-dimensional surface geometry of the medial tarsometatarsal joint ("first metatarsocuneiform") of the first ray was analyzed to determine if the shape of the joint is distinct in the medially deviated first metatarsal with metatarsus primus adductus (MPA). Clinical evaluation of 29 cadaver feet identified 13 feet with MPA and 16 with metatarsus primus rectus (MPR). Three-dimensional (3D) coordinates x, y, z of the first metatarsal and medial cuneiform joint facets of the feet were digitized on a Coordinate Measuring Machine (accuracy = 0.01 mm) and the data fitted with B-spline surfaces from which 3D curvature maps were generated. Comparison of means of surface-averaged maximum and minimum principal curvatures and root-mean-square curvatures showed significant (p < .0005) differences between the MPA and MPR subsets, male and female subsets, and metatarsal and cuneiform subsets. These results show that the articular shape of the medial tarsometatarsal joint in feet with MPA is significantly less contoured, or is flatter, than the same joint in normal or MPR feet. Results also showed that the female joints are more curved than male joints, and that metatarsal and cuneiform facets closely conform in shape to each other. These preliminary results may be related to questions concerning the anatomical and functional basis for the first metatarsal deviation, for radiographic presentation of the joint and surgical options in correcting related forefoot deformities.