Abstract
Introduction
When performing total knee arthroplasty (TKA), surgeons often utilize a posterior-stabilized (PS) design which compensates for the loss of the posterior cruciate ligament (PCL). These designs attempt to replicate normal knee kinematics and loading using a cam and post to provide posterior restraint of the tibia during flexion. However, these designs may not be able to compensate for the increase in flexion space or the inherent loss of coronal stability after PCL release compared to a cruciate retaining (CR) design. This study aimed to compare stability of PS and CR TKA designs by assessing laxity in three planes.
Methods
The specimens utilized in this study were lower extremities from fresh cadavers of donors who had previously undergone a total knee replacement (Medical Education and Research Institute (Memphis, TN) and Restore Life USA (Johnson City, TN)). IRB approval was obtained prior to performing the study. Twenty-three knee specimens (8 left, 15 right) were retrieved and all skin, subcutaneous tissue and muscle was removed. The femur and tibia were cut transversely 180 mm superior and inferior to the knee joint line, respectively, and specimens were mounted in a custom knee testing machine. Specimens were tested with the knee joint at full extension and at 30, 60, and 90 degrees of flexion. Laxity was assessed at 1.5 Nm of internal and external torque and 10 Nm varus and valgus torque, as well as a 35 N anterior and posterior force. Laxity was expressed as degrees of tibial displacement in the coronal plane under a varus/valgus torque and degrees of displacement in the transverse plane under an internal/external torque, as well as mm of anterior or posterior displacement. TKA components were retrieved to determine PS or CR design and grouped accordingly.
Results
Of the 23 implants, 10 were PS designs and 13 were CR. PS posterior laxity was 1 mm greater in full extension (p = 0.02, Figure 1), and PS varus laxity increased by 6 degrees at 90 degrees of flexion over CR laxity (p = 0.04, Figure 3). Varus to valgus laxity range of PS knees was greater than CR knees for all flexion angles. PS external rotational laxity at 90 degrees of flexion was greater than that of CR laxity by 7 degrees (p = 0.02, Figure 2).
Discussion
Results indicate significant laxity differences between PS and CR designs in both full extension and 90 degrees of flexion. PS designs have decreased coronal stability compared to CR, but appear to mimic AP constraint in midflexion and flexion. Mihalko et al. (2000) showed that loss of the PCL during TKA leads to a decrease in coronal stability, which is confirmed here. The post and cam mechanism of the PS designs restores AP stability during flexion but does not restore this coronal stability. These results may be limited by variations in implant design.
