CORR Insights^®: Polyethylene Damage Increases With Varus Implant Alignment in Posterior-stabilized and Constrained Condylar Knee Arthroplasty

Where Are We Now?

In the current study, Li and colleagues found increased surface damage to the posterior-cruciate-ligament-substituting polyethylene tibial bearing in knees that had been surgically reconstructed into a more varus limb alignment. Yet with polyethylene performance not expected to limit the lifetime of many knee arthroplasties that were done then and since that era, the 2018 reader of this publication stands to be more concerned about how coronal plane limb alignment impacts arthroplasty survival, the durability of tibial component fixation (perhaps even more so given how the arthroplasties of this retrieval study failed), and various aspects of knee function. While many orthopaedists have already reached a level of comfort with their own particular approach to restoring limb alignment during knee arthroplasty, those looking for evidence-based guidance on how to best align the limb in the coronal plane will seldom find a consensus. As one example, some studies confirm the biomechanically-rooted assumption that excess varus limb alignment of a cemented TKA increases the risk of tibial loosening [3, 5] whereas others do not [1, 4]. The relative dearth of instructive data available to the contemporary orthopaedist stems mostly from the fact that few research groups possess the comprehensive resources needed to conduct these clinical investigations, which require larger case sizes, precisely controlled full-limb weight-bearing postoperative imaging, quantification of functional parameters beyond those ordinarily assessed during routine examinations, and lengthier clinical and radiographic followup. Additionally, orthopaedists have traditionally been averse to participating in, or disseminating results from, research studies that stand to show that how they perform the procedure does at times adversely affect the result experienced by the patient.

Where Do We Need To Go?

The development and deployment of novel technologies such as computer-guided placement of implants, bone-surface-anatomy-matched cutting guides, image-guided robotic milling of bone fixation surfaces, patient-customized implant geometries, and instrumented trial tibial bearings that provide objective feedback during knee balancing has sparked a renewed interest in coronal plane limb alignment during knee arthroplasty. These technologies offer increased control and quantification of certain surgical technique factors that orthopaedists have, in the past, gauged subjectively and less accurately. Researchers who collect this newly available data and impart it into studies designed to identify how such parameters influence various knee arthroplasty outcomes have the potential to generate evidence-based guidance on how knees should be reconstructed in the modern era.

Regarding coronal plane limb alignment, a pressing question orthopaedists should ask is: “What should be the limb-alignment target for this knee scheduled to undergo arthroplasty?” Until more clinical data become available, one should naturally be skeptical as to whether any device whose use improves accuracy of limb alignment could consistently extend knee arthroplasty survival or improve knee function.

How Do We Get There?

Researchers looking to generate data that would assist the practicing orthopaedist in setting suitable targets for coronal-plane limb alignment should consider prospectively following a series of knee arthroplasties in a study that emphasizes (1) more-accurately measuring coronal plane limb alignment (2), quantifying knee function objectively, (3) and identifying radiographic signs of cemented tibial component loosening. Randomized comparisons of traditional mechanical-axis-biased approaches (where the coronal plane tibial and femoral cuts are chosen to produce a desired limb alignment value) to more kinematic-biased approaches (where, for example, the coronal plane cut angles are dictated by the flexion-extension axis of the femur and those two resections collectively determine the limb alignment) seem a natural step in that direction [2]. Yet even as attention shifts more to the particular approach or philosophy used to realign the limb, the shorter- to longer-term clinical consequences of coronal limb alignment remain important to track within mechanically-aligned series (where the limb alignment target could even be randomized within a safer band of mechanical varus/valgus) and within kinematically-aligned series (where limbs are sometimes placed in amounts of mechanical varus or mechanical valgus that are avoided by users of mechanical alignment techniques). With the operative alignment of the limb being such an integral part of these studies, acquisition of precisely controlled full-limb weight-bearing anteroposterior radiographs in the early followup period seems imperative, while three-dimensional imaging of the loaded limb may perhaps yield even better-quality assessments.

The kinematic and anatomical differences that exist between mechanically varus knees with osteoarthritis (OA) and mechanically valgus knees with OA suggest that the role of limb alignment should be examined independently for each preoperative diagnosis. In order to better understand the origins of tibial component loosening and the role that limb alignment may play in that result, future studies should include multivariate statistical analysis of other potential risk factors such as intraoperative collateral ligament balance measurements, translational and rotational positioning of the femoral component relative to the tibial component, tibiofemoral articular constraint level, translational and angular positioning of the tibial component relative to the tibia, and objectively-determined patient activity assessments. And were polyethylene performance to emerge as more consequential than many now assume, relevant bearing material parameters such as crosslink density, free radical concentration, antioxidant inclusion, would make for logical risk factors to enter into studies of arthroplasty survival, tibial component loosening, and polyethylene tibial bearing surface damage or thickness loss.