One-Year Outcomes of First Metacarpal Osteotomy Using a Precontoured Locking Plate (Shamoji Plate) for Thumb...

Metacarpal osteotomy is an effective surgical option for thumb carpometacarpal (CMC) osteoarthritis with expanding indications, even in advanced stages. Recently, locking plate fixation has been reported to provide more stable fixation and to allow earlier postoperative mobilization with fewer complications; however, dedicated implants were not previously available, necessitating the intraoperative modification of generic plates. Therefore, we developed a precontoured plate (the 'shamoji plate') specifically designed for the first metacarpal based on 3D CT. This study aimed to determine whether the use of the novel anatomically contoured shamoji plate in first metacarpal abduction-opposition osteotomy would result in significant improvement in clinical findings and radiographic outcomes.

In this retrospective case series, we included 22 thumbs with Eaton stage 2 or 3 disease that underwent abduction-opposition osteotomy with subsequent fixation using the shamoji plate between January 2022 and May 2024 and had at least 12 months of follow-up. Early range-of-motion exercises were initiated on postoperative days two and three. Clinical and radiographic outcomes were evaluated preoperatively and at the final follow-up.

Almost all clinical metrics significantly improved, including pain, grip strength, lateral pinch strength, thumb range of motion, and the Disabilities of the Arm, Shoulder, and Hand (DASH) scores. Bone union and radiographic realignment (radial and dorsal subluxation correction) were achieved.

The shamoji plate enables safe, early mobilization and reproducible correction without the need for intraoperative plate bending.