The objectives of our study were to evaluate (1) patellar height changes when the knee axis correction was?<?15 degrees and (2) the clinical effect after open wedge high tibial osteotomy (OWHTO). Sixty-nine patients (69 knees) undergoing OWHTO between January 2016 and June 2017 were enrolled in this prospective study. All patients underwent OWHTO using a three-dimensional (3D)-printed patient-specific instrument. We used X-ray and lower-limb computed tomography scan to measure the osteotomy angle, patellar height, and other patellofemoral joint indices. We used the hospital for special surgery knee (HSS), Western Ontario and McMaster Universities Arthritis Index (WOMAC), and Kujala's scores to evaluate the clinical outcome. All knee axis corrections were less than 15 degrees. The change in posterior tibial slope (PTS) did not show a statistically significant difference. We observed that the patellar height was not altered when we used femur referenced measurement method. The change in modified Miura-Kawamura index (MKI) and Femoral patellar height index (FPHI) did not show a statistically significant difference. The differences in Blackburne-Peel index (BPI) and Caton-Deschamps index (CDI) were statistically significant, decreasing from 0.91?±?0.12 and 1.06?±?0.11 preoperatively to 0.79?±?0.13 and 0.95?±?0.11 postoperatively, respectively. In the axial plane, we did not observe a change in lateral patellar shift (LPS), but we found that lateral patellar tilt (LPT) showed a significant decrease from 8.67?±?2.60 degrees preoperatively to 6.13?±?2.30 postoperatively, respectively. The tuberositas tibae-trochlear groove (TT-TG) distance showed a significant decrease after OWHTO from 14.30?±?4.10?mm preoperatively to 11.52?±?3.63?mm postoperatively. The clinical score showed a significant increase after OWHTO, and all patients were satisfied with the outcome. After OWHTO, the patellar height was not altered when the knee axis correction was?<?15 degrees. It was also found that there was still lateral displacement of the patella. Internal rotation of the distal tibia is an important cause of LPT reduction. After OWHTO, all patients achieved satisfactory clinical outcomes. At 1-year short-term follow-up, patients did not report discomfort in the patellofemoral joint. The Level of Evidence for this study was IV. Thieme Medical Publishers 333 Seventh Avenue, New York, NY 10001, USA.