## 2 Materials and Methods
### Study Methodology and Hopkins Criteria  
We evaluated the absolute reliability using the Hopkins criteria (a minimum of thirty cases, at least six blinded observers as evaluators, and at least three evaluations per observer separated by at least two weeks) [36]. Reliability can be understood as the reproducibility of the values of a test, trial, or other measurement in repeated trials with the same individuals. Higher reliability means a higher precision in individual measurements. Systematic changes in the measurement means between successive trials represent effects such as learning,motivation, or fatigue, and they should be removed from estimates of within-subject variations. The Hopkins criteria ensured that these factors were eliminated. The study was conducted with eight independent evaluators. One was an orthopaedic specialist dedicated to knee surgeries and with routine (daily practice) measurements of the study values (we considered him an expert). Two observers occasionally performed angle measurements of the lower limbs with LLRs and were considered semi-experts. The remaining five observers needed to gain experience in measuring the study values and were considered novices. Several briefing sessions were held before completing the measurements, with comprehensive information on the survey and training on the software used, with images selected for training practices other than those of the study. We used audiovisual media for training in.pdf and.mp4 formats. For each image, the observers determined the centre of the femoral head, assisted semi-automatically by the software, and the centre of the femoral intercondylar notch to trace the mechanical femoral axis. The centre of the femoral head was determined graphically. First, the area of the femoral head was zoomed in on to facilitate the measurement, and the brightness and contrast were adjusted if necessary. The software displays a horizontal line across the screen's full width that follows the cursor's movement. By clicking with the mouse, the observer fixes the line at the most proximal position of the femoral head. The software then displays a circle with the cursor's position determining its horizontal position and radius and its top point anchored to the previously defined line. The operator matches the position and radius of the circle to those of the femoral head and clicks the mouse. The observers also determined the centre of the notch of the tibial spines (marking the centre of the distance between the tips of both spines) and the centre of the ankle joint to trace the mechanical tibial axis in each image. We decided that the anatomical landmark of the ankle should be the midpoint of a straight line between the most prominent points of the medial and lateral malleoli cortex close to the joint line [11]. The software calculates the ankle centre's anatomical reference by dividing the distance between the two marks by two. On postoperative X-rays, the observers marked the anatomical landmarks at the neo-joints (the centres of the intercondylar notches and the tibial metal trays). To simplify our analysis of accuracy and absolute reliability but still strictly adhere to the [PERSON] criteria of absolute reliability [36], we decided to use the observers' preoperative mHKA angles (the angles between the mechanical axes of the femurs and tibias in the coronal planes) of the 52 LLRs in the three-spaced sessions (i.e., 1248 values). We decided not to use the lateral distal femoral and medial proximal tibial angles, also measured by the observers, in this study. Osteoarthritic knees show enormous variation in the overall coronal alignments of the limbs, as well as in the coronal femoral and tibial alignments [41; 42]. Since osteoarthritis knees present no mirror image of one knee over the other for each patient, we could directly consider each knee as an independent case (n = 52) if the object of the analysis was to compare the angular values.