Background  The rotation alignment of the femoral prosthesis is very important in total knee arthroplasty (TKA). There are many methods to determine the external rotation osteotomy of the distal femur, each has its own advantages and disadvantages, without any unified standards.

  Objective  To evaluate the application value of personalized distal femoral external rotation osteotomy in TKA in the treatment of knee osteoarthritis.

  Methods  Ten adult knee joint specimens were taken for three-dimensional CT reconstruction. The angle between the posterior condylar line (PCL) and the surgical transepicondylar axis (sTEA) was measured, that was defined as the posterior condylar angle (PCA). The cadaver specimens of the knee joint were dissected, and then TKA was simulated, the four points determining the sTEA and PCL were identified by the naked eyes and were marked by round head of pins. Then three-dimensional CT reconstruction of knee joint was performed. According to the marked points, PCL and sTEA were determined, and the angle between them was measured, which was defined as PCA’. The difference of angle between the two groups was compared. From December 2019 to August 2020, 101 patients with knee osteoarthritis were randomly divided into observation group and control group according to different methods of distal femoral rotation osteotomy. Depending on the PCA which was measured preoperatively, individualized external rotation osteotomy of the distal femur was performed in the observation group to complete TKA, while conventional 3° external rotation osteotomy of distal femur was applied in the control group. The knee varus angle, medial proximal tibial angle (MPTA), pain score, range of motion (ROM) and KSS score were measured before operation. During the operation, the soft tissue balance of 90° knee flexion and whether the patellofemoral track was good or not were recorded. The rotation error of femoral prosthesis and knee varus angle were measured after operation, and the knee varus angle, pain score, ROM and KSS were recorded at 3 months and 1 year after operation, and the clinical effect was evaluated.

  Results  In anatomical research, PCA was 4.2°±1.3° and PCA 'was 4.3°±0.9°, without significant difference between the two groups (P > 0.05). In clinical research, there were 51 cases in the observation group, including 8 males and 43 females, aged 50-84 (66.9±7.1) years, and 50 patients in the control group, including 8 males and 42 females, aged 56-83 (67.8±6.4) years. All of them were followed up successfully. The PCA of the two groups was 4.13°±1.19°, which was higher than 3° (P＜0.05). The operation time and perioperative blood loss of the two groups were similar. In the aspect of soft tissue balance of knee flexion 90°, only 6 cases in the observation group had initial flexion gap imbalance, which was better than 15 cases in the control group (P＜0.05). The medians of postoperative knee varus angle in both the observation group and the control group are 1(IQR: 1, 2)°, which were significantly better than that those before operation (MdIQR: 73, 14°, 83, 14°) (P＜0.05), but no significant difference was found between the two groups (P＞0.05). The rotation error of femoral prosthesis in the observation group (MdIQR: 00, 3°) was lower than that in the control group (MdIQR: 11, 4°) (P＜0.05). The knee joint pain score, ROM and KSS of the two groups improved significantly at 3 months and 1 year after operation (P＜0.025), and these indexes in observation group were better than that of the control group (P＜0.05).

  Conclusion  The method of measuring PCA with three-dimensional CT reconstruction is reliable. PCA is different in different people. Individualized external rotation osteotomy of the distal femur can achieve better balance of flexion space, reduce the incidence of postoperative pain, and improve the range of motion and clinical effect of TKA.