Planning tool for first metatarsal length in hallux valgus surgery

•Shortening of the first ray occurs during distal hallux valgus surgery.•Alteration of the osteotomy angle leads to excessive shortening and risks transfer metatarsalgia.•A new planning guide helps to predict postoperative first ray length in distal hallux valgus surgery. The precise planning of metatarsal (MT) I length in hallux valgus surgery is important. However, currently no tool exists which allows the surgeon to reliably predict this parameter. 30 virtual 3-dimensional hallux valgus surgeries were performed on varied deformation models based on cadaveric feet scans. The shortening of the first ray during distal metatarsal I osteotomy for different osteotomy angles were measured. An algebraic 2-dimensional calculation was done and compared to the results obtained from the 3-dimensional models. Inadvertent shortening of the first metatarsal bone can be as much as 8 mm depending on the amount of intermetatarsal angle (IMA) correction and osteotomy angle. Comparison of the 3 dimensional simulations and the 2 dimensional model resulted in a very strong correlation (R > 0.99 p < 0.00001). Based on our findings an anterior pointing osteotomy of approximately 10° is necessary to restore the length in distal metatarsal I hallux valgus surgery. A slight misdirection of the osteotomy plane in distal hallux valgus surgery may result in relevant unwanted alterations in first metatarsal bone length and triangulation by eye is insufficient in this complex geometrical situation without appropriate planning. The present study provides surgeons a practical tool to plan and control the change of first metatarsal length during hallux valgus procedure through exact orientation of the osteotomy angle. If no alteration of length is intended, it may be generalized that an anterior direction of the cut relative to the second metatarsal bone will preserve the length of the first metatarsal bone.