Objective To investigate the accuracy of split three-dimensional (3D) printing patient-specific instrumentation (PSI) in medial open-wedge high tibial osteotomy (MOWHTO) and its effectiveness in treating medial knee osteoarthritis.MethodsClinical data of 14 patients with medial knee osteoarthritis and treated with split 3D printing PSI-assisted MOWHTO between August 2019 and August 2020 were retrospectively analyzed. There were 5 males and 9 females with an average age of 61 years (range, 43-68 years). The disease duration ranged from 1 to 16 years, with an average of 4.7 years. Preoperative Kellgren-Lawrence grading of knee osteoarthritis included grade Ⅰ in 2 cases, grade Ⅱ in 6 cases, and grade Ⅲ in 6 cases. The Hospital for Special Surgery (HSS) score was 59.1±4.9. The weight bearing line ratio (WBL), hip-knee-ankle angle (HKA), medial proximal tibial angle (MPTA), posterior tibial slope angle (PTSA), and actual correction angle of the lower limbs were measured on postoperative imaging data, and compared with the preoperative measurements and the designed target values to evaluate the accuracy of the PSI-assisted surgery. The patients’ knee function were evaluated with the HSS score at 3 and 6 months postoperatively, and at last follow-up.ResultsOne patient suffered from an incision exudation at 2 weeks postoperatively, and the incision healed after symptomatic treatment. The incisions of other patients healed by first intention. All patients were followed up 7-19 months (mean, 14.8 months). There was no neural injuries, hinge fracture, plate or screw fractures, loosening, or other complications. The WBL was maintained at the postoperative level according to the X-ray examination during the follow-up period. The WBL, HKA, MPTA, and PTSA were all within a satisfactory range after operation. The WBL, HKA, and MPTA were significantly improved when compared with the preoperative measurements (P<0.05). There was no significant difference between preoperative and postoperative PTSA (P>0.05). The differences in postoperative WBL, HKA, MPTA, and correction angle compared with the preoperative designed target values were not significant (P>0.05). The HSS scores were 69.2±4.7, 77.7±4.3, and 88.1±5.4 at 3 and 6 months postoperatively, and last follow-up, respectively. The differences between time points were significant (P<0.05).ConclusionFor patients with medial knee osteoarthritis, the split 3D printing PSI can assist the surgeon in MOWHTO with accurate osteotomy orthopedics and achieve favorable effectiveness.