Abstract
Soft tissue balancing during Total Knee Arthroplasty (TKA) is a step every surgeon takes during surgery. Coronal and transverse plane mechanical alignment is another parameter that surgeons address during surgery in an attempt to decrease wear and increase longevity. To date, a correlation between laxity, component wear patterns, and alignment of the tibial and femoral implant components has not been established. Theoretically, suboptimal alignment and poor soft tissue balancing should increase polyethylene wear and decrease implant survivorship, contributing to implant loosening and costly revision surgeries. This study utilizes a retrieval program of functioning TKAs obtained at the time of necropsy. By utilizing CT scans, mechanical laxity testing, and polyethylene damage scores, we aimed to determine if any correlation between proper alignment and ligament balancing to polyethylene damage scores exists.
Methods:
Computed Tomography (CT) scans were performed on 17 cadaveric knees containing TKAs obtained from the Medical Education and Research Institute (Memphis TN) using a GE Brightspeed scanning system with a 1.25 mm slice thickness. Transverse slices from these scans were used to calculate the femoral and tibial component rotation for each specimen. Component rotation was determined by utilizing previously published methods (Berger et al), and component mismatch was defined as the difference in rotation angles of the femoral and tibial components. After removal of skin, subcutaneous and muscle tissue, the tibia and femur of each leg was cut transversely, and the specimens were mounted in a custom knee testing machine (Little Rock, AR). Specimens were subjected to a 10 Nm varus and valgus torque and a 1.5 Nm internal and external rotation torque. Data was continuously recorded, and the angle or displacement at each torque or force was noted. Each test was performed at full extension and 30, 60, and 90 degrees of flexion. TKA components were then removed from the cadaveric knees, cleaned of PMMA, and visually inspected for wear using a grading system with 10 wear areas on the articulating surface of the polyethylene tibial insert (Hood et al). Scores were assigned based on severity of 7 different degradation characteristics, and were separated based on medial or lateral compartment. The maximum possible total score was 210 for each knee.
Results:
The average length of TKA implantation was 10 years. The coronal angle at +10 Nm (varus) moment ranged from 5 to 12 degrees, while the angle under a −10 Nm (valgus) moment ranged from 7 to 11 degrees across 10 specimens. The average component rotational mismatch was 20.5 degrees. The average overall medial wear score was 8.8, while the lateral average was 9.6. Wear scores showed a higher correlation to laxity in the medial compartment than the lateral side (Figures 1 and 2).
Discussion:
TKA polyethylene wear shows a relationship to ligament laxity in these specimens. Component mismatch shows a negative correlation to wear scores. If surgeons are able to test for these parameters sooner, it may be possible to prevent unnecessary TKA wear and thus many revision surgeries.
