Mechanical and kinematic alignment in total knee arthroplasty: A comparative study on sizing discrepancies

Kinematic alignment (KA) in total knee arthroplasty (TKA) aims to restore natural knee kinematics by aligning components to the patient's pre-arthritic anatomy, unlike mechanical alignment (MA), which prioritizes the mechanical axis. Proper prosthesis sizing is essential for optimizing knee kinematics, yet most systems are designed around a MA philosophy, potentially affecting sizing when utilizing a KA philosophy. This study investigated whether intrinsic differences between kinematic and mechanical alignment techniques lead to discrepancies in femoral and tibial prosthesis sizing in primary TKA.

Demographic and operative data, including femoral and tibial component sizes, were collected on 324 patients undergoing primary TKA. Patients were categorized into mechanical and kinematic alignment groups. Predicted component sizes were calculated using established equations incorporating patient variables. Discrepancies between predicted and implanted sizes were categorized and analyzed.

Kinematic alignment was associated with a higher incidence of femoral components being smaller than tibial components (10.7 vs. 1.4 %, P < 0.001) and equal-sized femoral and tibial components (51.1 vs. 28.8 %, p < 0.001). Conversely, MA resulted in a greater proportion of femoral components larger than tibial components (69.9 vs. 38.2 %, P < 0.001). Predicted versus actual size analysis indicated KA led to smaller femoral implants, with a higher incidence of predicted femoral sizes exceeding actual sizes (68.7 vs. 53.6 %, P = 0.012).

Femoral sizing distributions vary significantly between kinematic and mechanical techniques, underscoring the need to consider alignment methods in component design. As KA adoption increases, recognizing these differences is essential for optimizing implant selection and improving outcomes.