In the world of total knee arthroplasty (TKA), the arguments about retaining the posterior cruciate ligament (PCL) versus stabilizing the knee with posterior-stabilized components have raged for more than 30 years. The number of cohort studies and controlled trials attempting to clarify the issue have been too high to count. In the July 5, 2017 issue of The Journal, Vertullo et al. use the power of the Australian national joint registry to add additional important clinical information to the debate.
More than 62,000 TKA cases formed the substrate of this analysis. In a study-design twist, the revision-related outcomes were analyzed on the basis of the preference surgeons had for the two different design options, not on the basis of which prostheses were actually used. Consequently, there was a likelihood that the cohort of patients treated by surgeons who had a preference for posterior-stabilized designs would include some PCL-retained cases, and vice-versa. The authors claim that this “instrumental variable analysis” has “the capacity to remove the confounding by indication or disease severity against posterior-stabilized total knee replacements.” However, as with any registry study, there were still many confounding variables that could have influenced the revision rate, not the least of which is surgeon skill in component alignment and ligament tensioning.
Nevertheless, with selection bias minimized, Vertullo et al. found a real difference in revision rates favoring retention of the PCL. That finding does make biomechanical sense to this non-arthroplasty surgeon, who would expect less stress on the tibial component-bone interface at the extremes of knee motion with the PCL-retaining procedure.
Biomechanics notwithstanding, I think this very large registry-based arthroplasty study will influence the debate going forward, but I doubt it will end the debate or that it will change the TKA practices of many surgeons worldwide. For a more definitive and potentially practice-changing resolution to this clinical conundrum, we’ll need a very large (2,000 to 3,000 patients in each arm) international trial where surgeons and patients accept randomization between these two choices.
Marc Swiontkowski, MD