Subluxation of knees in TKA - a neglected fact? An in vitro study

Aims and Objectives: Correct implant alignment is a well known factor for good functional results and increased implant survival after total knee arthroplasty (TKA). However, in present studies only coronal, sagittal and rotational alignment parameters have been considered, while translational parameters, the relative position between the femur and the tibia in medio-lateral, ventro-dorsal and proximo-distal direction are widely neglected. Due to variable mediolateral, proximodistal and ventrodorsal component placement in TKA, we hypothesised changed relative positions between the femoral and tibial bone resulting in a subluxation of knees after TKA. Materials and Methods: In nine Thiel embalmed whole body cadavers without any history of orthopaedic surgeries, fracutres and osteoarthritis, the relative positions between the femoral and tibial bones were measured between 0 and 90° of flexion before and after TKA (DePuy PFC Sigma, CR, fixed bearing) by means of a navigation system (Brainlab 2.6, Feldkirchen). To compare femoral positions before and after TKA, a paired t-test was used. A two-sided p-value of Results: In full extension, the femoral bones shifted 5.7 mm laterally(SD 4.0, p=0.003), and 2.7 proximally (SD 3.1, p=0.030), which decreased up to 90° of flexion to a lateral shift of 3.7 mm (SD 3.5, p=0.006) and proximal shift of 0.9 mm (SD 1.6, p=0.144). The femoral bones were located more dorsal in extension (2.8 mm dorsally (SD 4.8, p=0.113) which changed to a more ventral position in flexion (2.8 mm, SD 4.2, p=0.077). Conclusion: In the present setting, total knee arthroplasty resulted in significant changes of translational positions between the femoral and tibial bones, thus subluxation of knees. Major reasons for that might be a non centered placement of tibial or femoral implants, symmetric implants which do not respect individual anatomy, operative techniques, and large steps between two component sizes. Subluxation might cause an altered tension of the ligaments and consequently change tibiofemoral or patellar kinematics or tensioning of the periarticular soft tissue resulting in pain. However, clinical trials are necessary to evaluate clinical impact of the presented results.