## 2 Materials and Methods  
### Study Design  
This is a prospective, comparative, single-centre study. The inclusion criteria were a primary TKA for medial femorotibial osteoarthritis without previous osteotomy. The exclusion criteria were a revision of TKA, lateral femorotibial osteoarthritis or isolated  
Figure 1: Navigation setup with software and sensors. patellofemoral osteoarthritis, and previous osteotomy surgery. The accelerometer-based navigation system was used according to the system's availability and systematically when a patient can be included. The patients in the accelerometer group were operated on with the Perseus system for the tibial cut and the conventional ancillary for the femoral cuts. Twenty patients (20 TKA) were included in this study group. Patients in the accelerometer group were matched 2:1 on age, body mass index (BMI), and preoperative deformity (HKA angle: Hip Knee Ankle angle) with previous primary TKA to form the control group. The patients in the control group were operated on with conventional ancillary for all the cuts. Forty patients (40 TKA) were included in this control group. Demographic data are presented in Table 1. There was no significant difference between the two groups. The average age was 70 years-old in both groups. BMI was 30.4 and 30.2, there was 50% and 45% of females in the Perseus and Control group respectively. Pre-operative HKA was 172.5\({}^{\circ}\) in the Perseus group and 173.1 in the Control group. Patients were included between 2020 and 2021. The mean follow-up was 12 months. No patients were lost to follow-up. The same single primary prosthesis was used for all patients: a postero-stabilized TKA with a third condyle, rotational tibial plateau, and with patellar resurfacing (HLS Kneetec, CORIN(r), Tampa, FL, USA). All surgeries were performed with a thigh root tourniquet inflated to 300 mmHg. All implants were cemented with high viscosity cement. A trans-quadricipital medial approach was used for all patients. No femoral rotation was applied; the distal femoral cut was made with a 7\({}^{\circ}\) valgus intramedullary guide. The tibial cut was performed in the control group using a mechanical instrumentation device with intra-medullary and extra-medullary guides. In the accelerometer group, the tibial cut was performed with an accelerometer-based navigation system, the Perseus system (ORTHOKEY(r), Nasetved, Denmark). This system was based on sensors connected via Bluetooth to a tablet available on the operating field in sterile packaging. Two sensors (single-use, sterile-packed) per surgery were required, one on the cutting guide and the other attached to the ankle with a dedicated elastic band (Figure 1). The targets recorded on the software were a mechanical alignment with a tibial cut at 90\({}^{\circ}\) of the tibial mechanical axis in the frontal plane and with a slope of 90\({}^{\circ}\). The calibration and the acquisition of the good positioning of the cutting guide were obtained by two series of knee movements in the sagittal and frontal planes. After adjusting the cutting guide in the frontal plane (Varus/Valgus) and the sagittal plane (tibial slope) according to the indications on the software, the cutting guide was fixed with dedicated pins. To confirm the position of the cutting guide before the tibial cut, we performed two acquisitions (Figure 2). Once the tibial cut was made using the accelerometer sensors, the rest of the surgical technique was similar to the control group. All patients had a patellar resurfacing, using an on-lay symmetrical spherical dome-shaped polyethylene patellar button. The patellar resurfacing cut was performed such that a minimal thickness of 14 mm of patellar bone remained. A lateral release was performed to improve patellar tracking in cases of patellar subluxation during flexion. A vacuum drain was used only in case an early curative anticoagulation was necessary. \begin{table}