## 2 Materials and Methods  
A retrospective study was designed with a consecutive series of patients who had undergone a primary total knee arthroplasty using conventional instrumentation over one year. The inclusion criteria were patients between 50 and 85 years old with primary knee osteoarthritis (all patients were classified as grades III (34%) and IV (66%) on the Kellgren-Lawrence scale [16]). The exclusion criteria were active or suspected sepsis, a tumour around the knee, a previous TKA, the presence of hardware that could leave artefacts, any extra-articular deformity that required osteotomy around the knee in conjunction with TKA and other rheumatic disease. Patients who were diagnosed with osteoporosis and patients who needed a tibial or femoral coronal recut during surgery were excluded because both conditioning elements might suppose an added factor for imprecision. Four senior surgeons from the same institution performed all the procedures following the same pre, intra- and post-operative protocols. Four different implants were used:the Triathlon Knee System(r) (Stryker, Mahwah, NJ, USA), the U2 Knee System TKA(r) (United Orthopedic Corporation, New Taipei City, Taiwan), the NexGen LPS Knee System(r) (Zimmer, Irvine, CA, USA) and the Genutech Knee System(r) (Surgival, Paterna, VC, Spain). The surgeons decided to use postero-stabilized (PS) system or cruciate-retaining (CR) system, depending on the posterior cruciate ligament integrity. The surgical technique was standardized for all surgeons. For the procedures, a tourniquet and intradural anaesthesia were used with all patients. Femoral and sciatic blocks were used for pain control during the postoperative period. Prior to all procedures, antibiotic prophylaxis and tranexamic acid were administered to the patients. A standard midline anterior skin incision and a medial parapatellar deep approach were performed. In terms of the bone cutting, an intramedullary guide was used for the distal femoral cut, and an extramedullary guide was used for the proximal tibial cut in all surgeries. The femoral intramedullary guide was always placed beyond the femoral isthmus, and the tibial extramedullary guide was always placed with the same reference at the mid-malleolar point. The surgeons aimed at 90\({}^{\circ}\) for the tibial cut and a variable degree for the coronal femoral cut in all procedures, depending on the alignment or ligament balance encountered in each patient. The distal femoral cut was planned based on long leg X-rays while considering the angle between the mechanical and anatomic femoral axis; however, the surgeon always had the option to change the femoral distal cut angle up to three degrees depending on the ligament balance (gap-balancing technique). The femoral and tibial coronal cuts were made with a 1.27 mm thick sawblade. All implants, femoral and tibial, were cemented only on the surface of the component in all patients and were well impacted until no cement exuded from the sides of the prosthesis. Patellar replacement was performed on all the patients. The operation used to calculate the main variable was always calculated by subtracting the real angle measured in the X-ray from the desired angle. All cutting angles were recorded in a surgical report. Two independent observers measured the anatomical lateral distal femoral angle (aLDFA) and mechanical medial proximal tibial angle (mMPTA) in the six-month follow-up using long leg X-rays. Patient positioning, in terms of rotation and extension for the X-ray projection, followed strict guidelines (patient standing, with knees fully extended and patellae facing forward. The patella needed to always be centred and aligned with the femoral condyles. Otherwise, the technician modified the rotation of the limb until the desired position was achieved.) [17]. The Picture Archiving and Communication System (PACS) was used by both observers to perform measure collection, all to one decimal point. The final analysis was performed with the average measure of both observers.