Design of an implant for first metatarsophalangeal hemi-arthroplasty

This study was designed to examine the three-dimensional geometry of the head of the first metatarsal bone of the foot. Ninety-seven adult first metatarsal head (MTH1) bones were scanned using a laser scanner at 400 dpi. A best-fit ellipsoid was obtained from the articular surfaces of MTH1 for each size group using nonlinear unconstrained optimisation. Average root mean square errors between the articulating surfaces and the optimal fit surfaces of the bone specimens were between 0.29 and 0.42 mm. After classification based on sex and size groups, the profile provided a good fit to individual bones. Consideration of the thickness of cartilage overlying the metatarsal head (MTH) may further improve the fit. The proposed approach provides the basis for a design of an MTH hemi-arthroplasty that has good anatomical congruence with the native joint.