From among the 81 patients who underwent MIS-TKA at our center between December 2018 and February 2019, we recruited 55 patients for this randomized clinical trial. Patients were eligible for inclusion if they 1) had osteoarthritis, with Kellgren–Lawrence (K-L) grade 3-4; 2) were undergoing primary TKA; and 3) had varus or neutral knee. Patients were excluded if they had 1) rheumatoid arthritis, 2) history of surgery or infection of the knee, 3) ligament injury of the knee, 4) valgus deformity, or 5) severe bone defects that might require insertion of pads. A total of 26/81 patients were excluded: 16 patients with K-L grade 2 severity, 3 with rheumatoid arthritis, 2 with severe varus knees and obvious bone defect, and 5 with valgus deformity. The remaining 55 patients were randomly separated into two groups: one group was assigned to receive patellar reduction (group A; n = 27) and the other to receive patellar subluxation (group B; n = 28) during surgery.
This study was approved by the hospital ethics committee, and written informed consent was obtained from each patient. Operations were performed by the same senior surgeon. The patients and the staff who assessed postoperative efficacy were blinded to the intervention.
General preoperative and postoperative measures
Anteroposterior long-leg standing radiograph was taken to guide the selection of the valgus angle of the femoral component. Computed tomography (CT) scan of the knee was also acquired preoperatively. An antibiotic (second-generation cephalosporin) was routinely administered intravenously 20 minutes before surgery. The postoperative protocol for pain relief was the same for all patients. There was no drainage after surgery. The long-leg standing radiograph and CT of the operated knee were repeated at 5-7 days after operation. All patients received the same postoperative rehabilitation.
A gap-balance technique was used in the operation . All patients received a PS prosthesis (GENESIS II; Smith & Nephew, USA). The patient was placed in the supine position, and an air tourniquet applied at 250 mm Hg; the tourniquet was loosened immediately after prosthesis implantation. Midline skin incision and the midvastus approach were used to expose the knee without everting the patella. The anterior cruciate ligament and meniscus were excised, and the distal femur was exposed. According to the preoperative long-leg standing radiograph, the valgus angle of the femoral component (5°-7°) was selected. An intramedullary guide was used to perform distal femoral osteotomy. After adequate exposure, all osteophytes and menisci were removed. Then, the tibia was subluxated anteriorly. Osteotomy was performed using an extramedullary guide to ensure that each cut was perpendicular to the mechanical axis of the tibia in the coronal plane. The posterior inclination along the sagittal plane and the rotational alignment were checked. After the osteotomy, the gap balance was assessed with a conventional spacer block. The assessment was performed after reduction of the patella in group A patients, and with the patella in the subluxated state in group B patients. If there was asymmetry, appropriate release of soft tissues was performed until a balanced extension gap was obtained in the coronal plane.
The femoral sizing guide was closely applied to the osteotomy surface of the distal femur, and the size of the osteotomy module was determined according to the anteroposterior diameter of the femur. A 3.2-mm drill bit was used to drill through the nail hole on the femoral sizing guide, which was then removed. Next, the external osteotomy angle of the femoral posterior condyle was redetermined by the gravity method, with an assistant lifting the patient’s thigh to make the tibia sag naturally. In group A, the patella was maintained in the reduced position while the knee was flexed to 90°. In group B, the patella was first subluxated and then the knee was flexed to 90°. Then the matching rectangle was used to record the line (Figure 1). The positioning hole of the osteotomy module was adjusted according to the scribe line. Then, the anteroposterior cutting block was placed on the osteotomy surface of the distal femur, fixed in place with three nails. A complete femoral osteotomy was performed after confirming that the flexion gap was satisfactory. A femoral trial prosthesis was installed. The matching (left or right) offset-type tensor was selected and fixed to the proximal tibia, and then fitted to the femoral trial prosthesis. The joint distraction force was set at 40 lb in each patient. Gap length and varus ligament imbalance were measured at 0° and 90° of knee flexion. In group A patients the measurements were only made with the patella reduced, whereas in group B patients the measurements were made with the patella laterally shifted and then, again, with the patella reduced.
The following were recorded:
- Basic information (sex, age, height, weight, left or right knee)
- Femoral rotational alignment and femoral prothesis rotation angle. On the CT scan of the knee, the angle between the surgical transepicondylar axis and the posterior condyle line of the femur was the femoral rotational alignment, and the angle between the surgical transepicondylar axis and posterior condyle line of the prosthesis was femoral prosthesis rotation angle (Figure 2). External rotation was indicated with the “plus” sign, and internal rotation with the “minus” sign. The surgical transepicondylar axis represents the axis of rotation of the original femur of the knee joint. The posterior condyle line of the prosthesis can be regarded as the axis of rotation of the prosthesis. The femoral prothesis rotation angle reflects the coincidence of the axis of rotation of the prosthesis with the axis of rotation of the original femur.
- Mechanical femoral axis–to–tibial axis angle (mFTA). The angle between the mechanical axis of the tibia and the mechanical axis of the femur before and after surgery were measured on the long-leg standing radiographs; the target angle after surgery was 0° ± 3°.
- Offset-Repo-Tensor measurement of joint gap and varus angle (Figure 3). As recommended in previous studies , the joint gap length and varus angle were measured with the knee in full extension and 90° of flexion at. A torque wrench was used to ensure that the joint distraction force was set at 40 lb (Figure 3)
- Knee Society Score. KSS score was assessed before surgery and at 1 month, 3 months, and 6 months after surgery. The KSS (maximum possible score 100) takes into consideration pain, activity, and stability and reflects knee function.
- Pain assessment. A visual analog scale (VAS; ranging from 0 to 10) was used to evaluate knee pain before surgery and at and 1 month, 3 months, and 6 months after surgery.
- Range of flexion. Maximum flexion achievable in the knee was recorded before surgery and at 1 month, 3 months, and 6 months after surgery.
SPSS 25.0 (IBM Corp., Armonk, NY, USA) was used for statistical analysis. Differences between groups in age, body mass index (BMI), femoral rotational alignment, femoral prosthesis rotational alignment, joint gap, varus angle, KSS, and VAS were compared using the independent-samples t test. The joint gap and varus angle in group B before and after reduction were compared using the paired sample t test. Pearson chi-square test was used to compare categorical variables (sex distribution and side affected). Statistical significance was at P ≤ 0.05.