First Metacarpal Extension Osteotomy by Use of a New Anatomical Shamoji Plate for Base of Thumb Arthritis: Short-Term Results

<b>Background:</b> The first metacarpal extension osteotomy (FMEO) is an effective surgical procedure for thumb carpometacarpal (CMC) arthritis, but a dedicated fixation implant was not yet available. The Shamoji plate is a newly developed anatomical locking implant for FMEO. We report the short-term postoperative results of FMEO using the Shamoji plate in patients with early to moderate stage of CMC arthritis of the thumb. <b>Methods:</b> We reviewed 17 thumbs in 17 patients who underwent FMEO using the Shamoji plate. At the initial time of diagnosis, all cases were classified as Eaton stage 2 of CMC joint arthritis. The postoperative follow-up period ranged from 5 to 18 months, with an average of 10.1 months. An L-sized Shamoji plate (30 mm, six locking screws and one cortical screw) with a 30° bend was used most. Postoperatively, active range-of-motion exercises started the following day after surgery. Clinical outcomes and realignment of CMC joint on plain radiograph and computed tomography were evaluated. <b>Results:</b> An average operation time was 52 minutes. Plain radiographs showed early bone union in all patients with an average time of 3.3 months. Visual analogue scale, tip pinch strength and Quick Disabilities of the Arm, Shoulder and Hand (QuickDASH) score were significantly improved postoperatively. The flexion angle of the metacarpophalangeal joint decreased approximately 10°. The metacarpals were dorsally realigned 23°-25° after FMEO. The dorsal subluxation angle and % subluxation were significantly improved, indicating a significant improvement in malalignment of the CMC joint. <b>Conclusions:</b> The Shamoji plate is a sufficient internal fixation implant with good short-term results in terms of osteotomy angle, bone healing, pain relief, functional recovery and realignment of the CMC joint. FMEO by use of Shamoji plate is expected to provide reliable clinical outcomes for patients with the CMC joint arthritis in Eaton stage 2. <b>Level of Evidence:</b> Level IV (Therapeutic).