Contemporary femoral designs in total knee arthroplasty: effects on the patello-femoral congruence

The objective of this study was to evaluate the radiological and clinical correlations between implant design and patellar positioning in patients who underwent TKA utilizing femoral implants with modern designs. Thirty consecutive PFC PS Sigma TKAs, characterized by a new prolonged anterior flange and a "smoother" trochlea, were prospectively reviewed. All patellae were replaced. All patients were evaluated pre-operatively and prospectively at two years follow-up both clinically according to the Knee Society score as well as radiographically. This included computed tomography (CT); patellar tilt, patellar conformity angle, patellar lateralization, and femoral component external-rotation in relation to the clinical trans-epicondylar axis. Average patellar tilt at follow-up was 3A degrees (+/- 7.5A degrees) with respect to a pre-operative 18.5A degrees (+/- 8.5A degrees). Average patellar congruence angle at follow-up was -3A degrees (range, -11A degrees to +9A degrees) with respect to a pre-operative 10.3A degrees (range, + 1.5A degrees to 25.5A degrees). Average lateralization index at follow-up was 2.7 mm (range, -3.4 mm to +7.1 mm) with respect to a pre-operative 12.2 mm (+/- 4.8 mm). Femoral component positioning related to the trans-epicondylar axis showed an external rotation of 2.80A degrees (+/- 2.10A degrees) at follow-up with respect to 5.7A degrees (+/- 1.80A degrees). Clinically, two (6.6%) patients reported patello-femoral complications related to imperfections in the surgical technique more than the implant's design. This study highlighted that modern femoral designs in TKA allow for a correct reproducibility of a normal patello-femoral conformity. Strict surgical principles are paramount to avoid patello-femoral complications even when modern implants are used.