Abstract
Purpose:
Knee pain and instability during high demand activities such as stair descent are reported by patients after TKA. Previous studies theorized that this pain is from increased demand on the quadriceps required to stabilize the femur on the tibia. In this study we explore the relationship between implant design, the posterior cruciate ligament (PCL), and AP stability of the knee during stair descent.
Methods:
CTs of 6 fresh-frozen human cadaveric knees (average age: 61 ± 6.5 years) with functioning cruciates were prepared. All specimens were mounted in a computer controlled, 6 DOF simulator programed to apply physiologic muscle loads and flexion/extension moments simulating the highest demand phase of stair descent (terminal swing to initial contact). A contemporary design of TKA was implanted in each specimen by an experienced surgeon. Testing was repeated after implantation of tibial inserts of the CR, CS with and without a PCL and PS designs.
Results:
In the initial contact position, quad contraction in the intact knee displaced the tibia 5.53 mm anteriorly, compared to 2.01 mm with the CR implant, 2.31 mm with the CS, and 1.07 mm with the PS component. Loss of the PCL in the presence of the CS implant caused tibial displacement to increase from 2.31 mm to 8.21 mm. At initial contact without quad contraction, only the CS knee with a deficient PCL shifted more than the intact (3.87 ± 3.25 mm posterior) and could not restore kinematics back to the intact state (p = 0.0005). Only the CR was unable to restore the tibio-femoral relationship back to its terminal swing position (posterior shift 3.36 ± 1.36 mm) in the CR knee compared with the intact (p = 0.0037). The PS had the least amount of tibial shift with and without quad contraction (with quad: 1.08 ± 0.35 mm anterior shift; without quad: 1.11 ± 0.38 mm posterior shift).
Conclusion:
In the intact state, quad contraction does stabilize the tibia in stair descent. Although the CR displayed similar kinematics to the intact state prior to quad contraction, a functioning quad was not able to properly stabilize the CR compared to the intact and thus would result in a more overall unstable knee. In the absence of the PCL, the CS design was unable to provide A/P stability. Both the CS with the PCL present and the PS designs provided A/P stability equal or greater to the intact state in this high demand activity.
Significance: To identify variables that affect knee instability for possible predictors of knee pain during high demand activities.
