The metatarsophalangeal joint–metatarsal ratio (MTPJ-MTR): An anatomic parameter for obtaining an extracapsular metatarsal osteotomy in fourth-generation minimally invasive hallux valgus correction

Background: Fourth-generation operative techniques for hallux valgus correction utilize a transverse extracapsular osteotomy to maximize correction while minimizing stiffness and risk of intra-articular debris. Previous studies have not provided anatomically based methods of ensuring an extracapsular osteotomy. Purpose: This study aimed to provide a practical, anatomically based reference for surgeons to utilize to perform an extracapsular metatarsal osteotomy while validating previous reports of capsular insertion anatomy. Study Design: Cadaveric study Methods: Twenty-one cadaveric specimens (13 female, 8 male) from 13 cadavers were dissected and used for this study. Mean age of specimens was 78.1 years. Capsular origin at the hallux metatarsophalangeal joint was measured from the central point of the metatarsal head in the dorsal, medial, lateral, plantar-medial, and plantar-lateral aspects. Ratios of capsular length to total metatarsal length in the dorsal and plantar dimensions were calculated to generate a length-independent reference unaffected by parallax or fluoroscopic magnification. Results: The mean distances from the central hallux metatarsal head to the metatarsophalangeal joint proximal capsular origin were 14.1 mm dorsal, 7.7 mm lateral, 6.3 mm medial, 16.8 mm plantar-medial, and 17.5 mm plantar-lateral. The mean total length of the metatarsal was 64.2 mm dorsally and 67.1 mm plantarly. The dorsal capsular ratio was 0.22, plantar-medial capsular ratio was 0.25, and plantar-lateral capsular ratio was 0.26. Conclusion: The metatarsophalangeal joint–metatarsal ratio provides an anatomically based reference to identify the proximal capsular insertion independent of individual metatarsal size, fluoroscopic magnification, or parallax. This study validates previous descriptions of capsular insertion variation.