For severe recurrent patellar dislocation with poor patellar maltracking, derotation osteotomy is an effective clinical treatment. However, derotation osteotomy requires very high requirements for the surgeon. It is necessary to determine the osteotomy surface, control the axis and complete internal fixation in three dimensions. The deviation of the osteotomy surface may cause deformities such as postoperative knee valgus, knee hyperextension, or restricted extension. The hypothesis of this study is to design a 3D osteotomy template for derotation osteotomy with the aid of computer-assisted simulated surgery. Compared with traditional osteotomy, it can effectively improve the treatment effect of recurrent patellar dislocation due to poor patellar maltracking. The incidence of postoperative knee valgus shortens the operation time and reduces the number of intraoperative fluoroscopy. This study intends to adopt a randomized controlled study, and the selection criteria are adolescent patients with recurrent patellar dislocation, aged >14 years, with a positive J sign. The experimental group used mimics 20.0 software to reconstruct the three-dimensional model of the patient based on the full-length images of the lower limbs in the weight-bearing position and the CT of the hip, knee and ankle before the operation. The osteotomy template was designed and 3D printing technology was used to make the osteotomy template for intraoperative osteotomy. The control group used traditional de-rotation techniques. The knee joint range of motion, patella stability, residual rate of J-sign, knee valgus angle, femoral anteversion angle, gait analysis, etc. were compared between the two groups after surgery, and the differences between the two surgical techniques were evaluated

Inclusion Criteria:

• patients with recurrent patellar dislocation
• >14 years old
• positive preoperative J-sign
• agree to participate the study

Exclusion Criteria:

• Combined with other ligament injuries of knee joint
• Skin and soft tissue conditions do not permit surgery