THE EFFECT OF COMPONENT AND LOWER LIMB ALIGNMENT ON TKA JOINT MECHANICS

Introduction The current standard for alignment in total knee arthroplasty (TKA) is neutral mechanical axis within 3° of varus or valgus deviation [1]. This configuration has been shown to reduce wear and optimally distribute load on the polyethylene insert [2]. Two key factors (patient-specific hip-knee-ankle (HKA) angle and surgical component alignment) influence load distribution, kinematics and soft-tissue strains across the tibiofemoral (TF) joint. Improvements in wear characteristics of TKA materials have facilitated a trend for restoring the anatomic joint line [3]. While anatomic component alignment may aid in restoring more natural kinematics, the influence on joint loads and soft-tissue strains should be evaluated. The purpose of the current study was to determine the effect of varus component alignment in combination with a variety of HKA limb alignments on joint kinematics, loads and soft-tissue strain. Methods A dynamic three-dimensional finite element model of the lower limb of a TKA patient was developed. Detailed description of the model has been previously published [4]. The model included femur, tibia and patella bones, TF ligaments, patellar tendon, quadriceps and hamstrings, and was virtually implanted with contemporary cruciate-retaining fixed-bearing TKA components. The model was initially aligned in ideal mechanical alignment with neutral HKA limb alignment. A design-of-experiments (DOE) study was performed whereby component placement was altered from neutral to 3° and 7° varus alignment, and HKA angle was altered from neutral to ±3° and ±7° (valgus and varus) (Figure 1). Results HKA angle has a greater influence on kinematics, particularly PF medial-lateral (M-L) translation in early flexion and TF internal-external (I-E) rotation; at 60° flexion change in TF I-E rotation due to HKA angle was 12.4° compared to change due to component V-V alignment of 2.3° (Figure 1). Component alignment was the main factor in overall TF loads; varus component alignment increased the medial force, external torque and valgus torque acting on the insert. Shear force at the bone-implant interface increased by 15% (∼90N) with varus component rotation of 7°. Varus component alignment increased forces in the lateral structures and reduced forces in the medial structures (Figure 2). Both valgus HKA angle and varus component alignment altered M-L load distribution by reducing medial forces and increasing lateral forces (Figure 3). Discussion Placement of TKA components in anatomic alignment has potential to better integrate the implants with the soft-tissues of the joint and may better reproduce natural kinematics. However, varus component alignment in conjunction with valgus HKA limb alignment substantially alters M-L distribution of load across the condyles, increasing the load on the lateral condyle. Varus component alignment will result in load distributions which are different from their mechanically aligned counterparts. As such, pre-clinical evaluation of components used in varus alignment should ensure that components are robust to loading conditions which will be encountered across the range of TKA patient HKA alignments.