Purpose: Manufacturers of total knee arthroplasty (TKA) have introduced narrower femurs to improve bone‐implant fit. However, few studies have reported the clinical consequences of mediolateral oversizing. Our hypothesis was that component oversizing negatively influences the results after TKA.Methods: One hundred and twelve prospectively followed patients with 114 consecutive TKA (64 females and 50 males) were retrospectively assessed. The mean age of the patients was 72 years (range, 56 to 85 years). The dimensions of the femur and tibia were measured on a preoperative CT‐scan and were compared with those of the implanted TKA. The influence of size variation on the clinical outcomes 1 year after surgery was assessed.Results: Mediolateral overhang was observed in at least one area in 66 % of the femurs (84 % in females and 54 % in males) and 61 % of the tibia (81 % in females and 40 % in males). Twenty‐two patients presented no overhang in any area and 16 had overhang in all studied zones. The increase in the Pain and KOOS scores were 43 ± 21 and 36 ± 18 in the patients without overhang and 31 ± 19 and 25 ± 13 in patients with overhang (p = 0.033; p = 0.032). Knee flexion was 127° ± 7 and 121° ± 11, respectively. Regression and latent class analysis showed a significant negative correlation between overall oversizing and overall outcome.Conclusions: This study confirms that oversizing may lead to worse clinical results in TKA. The clinical consequences are that surgeons should pay attention not to oversize implants during implantation nd that oversizing should be ruled out in case of so called unexplained pain.Level of evidence: IV.

Acetabular offset (AO) is the distance between the centre of the femoral head and the true floor of the acetabulum. We quantified the AO in normal hips and compared the displacement of the centre of rotation of the hip (CRH) after conventional and anatomical cup implantation during THA. 100 CT-scans of normal hips were analysed before and after simulating implantation of the acetabular component. Mean AO was 30.8 mm ± 3. The medial shift of the CRH was 1.6 mm ± 1.2 with the anatomical and 4.8 mm ± 1.9 with the conventional technique (p<0.0001). Medialisation was greater than 5 mm in 44% of the cases when the conventional technique was used, but occurred in no case when using the anatomical technique. Differences between men and women were significant: 5.6 mm ± 1.6 and 3.5 mm ± 1.7 with the conventional technique; 2.0 mm ± 1.1 and 0.9 mm ± 0.9 with the anatomical technique (p<0.0001 for both measurements). The concept of hip offset cannot be limited to that of the femoral offset. AO widely varies and cannot be neglected. In patients with significant AO, surgeons should pay close attention to the preparation of the acetabulum. This should be done conservatively so that the acetabular cup can be placed anatomically in order to restore the native hip biomechanics.

Background: During total hip arthroplasty (THA), medialisation of the cup decreases the lever arm of body weight and is considered to be beneficial. Theoretically it should be compensated by an equivalent increase of the femoral offset in order to maintain global offset. Goal of the study: We investigated via a numerical model the forces on each bundle of the abductor muscles and the loading forces at the head-cup interface. We compared THA with and without medialisation of the cup and with and without restoration of the global offset. Method: A digital finite element model of the hip joint with THA was constructed. Monopodal weight-bearing was simulated with the cup positioned so that the hip centre of rotation was left unchanged. Forces in the abductors and pressure on the prosthetic head were measured and then compared with three other configurations: 1. Medialisation of the cup by 5 to 15mm with reproduction of the native femoral offset or; 2. with an increased femoral offset compensating for the cup medialisation and; 3. no medialisation of the cup with a femoral offset increased. Results: Medialisation of the cup decreased stresses on the head-cup interface and on the abductor muscles, even when the global offset was not restored. Anatomical placement of the cup did not give the best results. The optimal outcome in terms of stress was observed when the cup was medialised but global offset restored. Any increase of global offset significantly increases the length and the tension in the anterior bundles of the abductor muscles.