Geometric analysis of indications for minimally invasive distal metatarsal osteotomy in treatment of hallux valgus

BACKGROUND Minimally invasive distal metatarsal osteotomy (MIDMO) is to be indicated for all patients with angles of IMA <20° and HV <40°, but many authors doubt whether this procedure is capable of correcting all types of hallux valgus deformities. The aims of this study were to perform a geometric analysis of MIDMO indications and to show which preoperative radiological parameters are necessary to achieve sufficient contact between bone fragments and sufficient correction with this operative technique. METHODS A geometric mathematical model in AP and lateral radiographic plane was created based on preoperative measurements of the intermetatarsal angle (IMA), subcapital metatarsal width, medial bunion eminence, and metatarsal length. MIDMO was simulated with possible dorsal/plantar fragment displacement in order to assess postoperative contact between fragments (either 4-5 mm or half of the metatarsal width) and sufficient correction (postoperative IMA 8°). RESULTS The metatarsal neck should be at least 8 mm wider from the bunion eminence to achieve the minimally required contact between fragments. For sufficient correction, the metatarsal head translation should be at least 0.018 of the metatarsal length for every degree of IMA reduction. The medial bunion eminence, in comparison with metatarsal width/length size, determines whether MIDMO is a suitable procedure for a given patient. CONCLUSIONS MIDMO cannot sufficiently correct all deformations within the boundaries of IMA angle <20° and HV angle <40°. In patients with large eminences and narrow metatarsals, complications related to insufficient postoperative fragment contact can be expected, while sufficient hallux valgus correction in patients with small eminences and long metatarsals is questionable.