Mean tensile strength of the PCL in TKA depends on the preservation of the tibial insertion site.

PURPOSE: The tibial insertion of the posterior cruciate ligament (PCL) frequently becomes damaged when performing a tibial cut in a PCL-retaining total knee replacement (TKA). The aim of this study was to quantify the functional effect of this structural damage on the tensile strength and failure load. METHODS: Six paired knees from fresh-frozen cadaver specimens were used. All soft tissues but the PCL were removed. In the left-sided specimens, a classic tibial cut at a depth of 9 mm with 3° of posterior slope was made, while in the right-sided specimens, a bone block was left in front of the tibial PCL insertion. After cementing a tibial tray, the specimens were mounted in a loading frame in 60° of flexion. The femur was translated anteriorly at a constant velocity rate of 0.5 mm/s. Tensions in the PCL were measured continuously until failure occurred. RESULTS: In one specimen, the tibial PCL insertion was completely removed by the tibial cut. In the other five paired specimens, the mean tensile strength of the PCL was 380.6 ± 154.7 N in the left-sided knees. In the right-sided knees, the mean tensile strength was 738.4 ± 166.7. The average right-to-left ratio was 2.2 ± 0.7 (p = 0.006). CONCLUSION: The results of this study indicate that the conventional technique for tibial preparation in cruciate-retaining total knee arthroplasty can result in a significant decrease in tensile strength of the PCL, rendering it susceptible to failure and subsequent midflexion instability. Therefore, we recommend leaving the posterior tibial cortex anterior to the PCL insertion intact when performing a cruciate-retaining TKA.

Post-cam mechanics and tibiofemoral kinematics: a dynamic in vitro analysis of eight posterior-stabilized total knee designs.

PURPOSE: Posterior cruciate ligament (PCL)-substituting total knee arthroplasty (TKA) designs were introduced to avoid paradoxical roll forward of the femur and to optimize knee kinematics. The aim of this in vitro study was to investigate post-cam function and contact mechanics and relate it to knee kinematics during squatting in eight contemporary posterior-stabilized TKA designs. METHODS: All prostheses were fixed on custom-designed metal fixtures and mounted in a knee rig and five sequential-loaded squats were performed between 30° and 130° of flexion. Contact pressure and contact area were measured using pressure-sensitive Tekscan sensors on the posterior face of the post. Kinematics was recorded with reflective markers and infrared light-capturing cameras. RESULTS: The post-cam mechanisms analyzed in this study are very variable in terms of design features. This leads to large variations in terms of the flexion angle at which the post and cam engage maximal contact force, contact pressure and contact area. We found that more functional post-cam mechanisms, which engage at lower flexion angle and have a similar behavior as normal PCL function, generally show more normal rollback and tibial rotation at the expense of higher contact forces and pressures. All designs show high contact forces. A positive correlation was found between contact force and initial contact angle. CONCLUSION: Post-cam contact mechanics and kinematics were documented in a standardized setting. Post-cam contact mechanics are correlated with post-cam function. Outcomes of this study can help to develop more functional designs in future. Nevertheless, a compromise will always be made between functional requirements and risk of failure. We assume that more normal knee kinematics leads to more patient satisfaction because of better mobility. Understanding of the post-cam mechanism, and knowing how this system really works, is maybe the clue in further development of new total knee designs.

The influence of malrotation and femoral component material on patellofemoral wear during gait.

Complications involving the patellofemoral joint, caused by malrotation of the femoral component during total knee replacement, are an important cause of persistent pain and failure leading to revision surgery. The aim of this study was to determine and quantify the influence of femoral component malrotation on patellofemoral wear, and to determine whether or not there is a difference in the rate of wear of the patellar component when articulated against oxidised zirconium (OxZr) and cobalt-chrome (CoCr) components. An in vitro method was used to simulate patellar maltracking for both materials. Both rates of wear and changes in height on the patellar articular surface were measured. The mean rates of wear measured were very small compared to standard tibiofemoral wear rates. When data for each femoral component material were pooled, the mean rate of wear was 0.19 mm3/Mcycle (sd 0.21) for OxZr and 0.34 mm3/Mcycle (sd 0.335) for CoCr. The largest change in height on each patella varied from -0.05 mm to -0.33 mm over the different configurations. The results suggest that patellar maltracking due to an internally rotated femoral component leads to an increased mean patellar wear. Although not statistically significant, the mean wear production may be lower for OxZr than for CoCr components.

Femoral component loosening in high-flexion total knee replacement: an in vitro comparison of high-flexion versus conventional designs.

High-flexion total knee replacement (TKR) designs have been introduced to improve flexion after TKR. Although the early results of such designs were promising, recent literature has raised concerns about the incidence of early loosening of the femoral component. We compared the minimum force required to cause femoral component loosening for six high-flexion and six conventional TKR designs in a laboratory experiment. Each TKR design was implanted in a femoral bone model and placed in a loading frame in 135° of flexion. Loosening of the femoral component was induced by moving the tibial component at a constant rate of displacement while maintaining the same angle of flexion. A stereophotogrammetric system registered the relative movement between the femoral component and the underlying bone until loosening occurred. Compared with high-flexion designs, conventional TKR designs required a significantly higher force before loosening occurred (p < 0.001). High-flexion designs with closed box geometry required significantly higher loosening forces than high-flexion designs with open box geometry (p = 0.0478). The presence of pegs further contributed to the fixation strength of components. We conclude that high-flexion designs have a greater risk for femoral component loosening than conventional TKR designs. We believe this is attributable to the absence of femoral load sharing between the prosthetic component and the condylar bone during flexion.

The influence of malrotation of the femoral component in total knee replacement on the mechanics of patellofemoral contact during gait: an in vitro biomechanical study.

Malrotation of the femoral component is a cause of patellofemoral maltracking after total knee arthroplasty. Its precise effect on the patellofemoral mechanics has not been well quantified. We have developed an in vitro method to measure the influence of patellar maltracking on contact. Maltracking was induced by progressively rotating the femoral component either internally or externally. The contact mechanics were analysed using Tekscan. The results showed that excessive malrotation of the femoral component, both internally and externally, had a significant influence on the mechanics of contact. The contact area decreased with progressive maltracking, with a concomitant increase in contact pressure. The amount of contact area that carries more than the yield stress of ultra-high molecular weight polyethylene significantly increases with progressive maltracking. It is likely that the elevated pressures noted in malrotation could cause accelerated and excessive wear of the patellar button.

Iatrogenic surface damage during femoral component impaction in total knee arthroplasty.

Application of prosthesis components during knee arthroplasty surgery involves impacting the femoral component using an impaction device and a heavy mallet. This could damage the component and may therefore be of concern to knee surgeons. Using a drop tower with a set-up that mimics the impaction generated clinically when a surgeon hits the femoral component, we investigated the possible surface damage to the femoral component. Three parameters were obtained and compared with a contact profilometer to characterize the roughness: R(a), R(pk) and R(z). The effect of the impacts on the contour of the femoral components was also investigated. After 3 series of impactions, no difference in surface roughness of the femoral component important enough to increase the wear rate could be detected neither for Cobalt Chrome or Oxidized Zirconium components. Our study therefore indicates that impacting the femoral component during TKA does not alter the component's surface roughness.