Impact of First Metatarsal Shortening on Forefoot Loading Pattern: A Finite Element Model Study

Backgrounds There has long been agreement that shortening of the first metatarsal during hallux valgus reconstruction could lead to postoperative transfer metatarsalgia. But appropriate shortening is sometimes beneficial for correcting severe deformities or relieving stiff joints. So this study is to investigate, from the biomechanical perspective, whether and how much shortening of the first metatarsal could be allowed. Methods A finite element model of the human foot simulating the push-off phase of the gait was established and validated by subject-specific plantar pressure data. Shortening of the first metatarsal for 2mm, 4mm, 6mm, 8mm were sequentially applied to the model, and the corresponding instant forefoot loading pattern during push-off phase, especially the loading ratio at the central rays, was respectively calculated. The effect of depressing the first metatarsal head was also observed at last. Results With increasing shortening level of first metatarsal, the plantar pressure of the first ray decreased, while the lateral rays continued to rise. When the shortening reaches 6 mm, the load ratio of the central rays exceeds a critical threshold of 55%, which was considered risky. But it could still be manipulated to normal if the distal end of the first metatarsal displaced plantarly by 3 mm. Conclusions During the first metatarsal osteotomy, a maximum of 6 mm of shortening is considered to be within the safe range. Whenever a higher level of shortening is necessary, pushing down the distal metatarsal segment could be a compensatory procedure to maintain normal plantar force distributions.