The Journal of Bone and Joint Surgery

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Commentary & Perspective

Commentary & Perspective on
"Factors Affecting the Severity of Backside Wear of Modular Tibial Inserts"
by Michael A. Conditt, PhD, et al.

Commentary & Perspective by
Gerard A. Engh, MD, and Matthew B. Collier, MS*,
Anderson Orthopaedic Institute, Alexandria, Virginia

Conditt et al. have reported on backside surface damage of 124 revision-retrieved, fixed-bearing tibial polyethylene inserts encompassing twelve designs. The study features a roughly equal distribution of posterior cruciate-retaining and cruciate-substituting inserts, most of which had been mated with a titanium base-plate containing screw-holes. The investigators examined the statistical association of patient and implant factors on semiquantitative surface damage scores (e.g., burnishing) and the relative height differentials between the polyethylene surfaces that extend into or surround screw-holes of the tibial base-plate.

An important limitation of this study and prior studies^1-4 that graded backside wear of retrieved polyethylene inserts is that the volume of polyethylene lost from the backside was not assessed. In terms of risk factors for periprosthetic osteolysis, how bad the backside surface looks or what percentage of the surface is burnished is clearly not as important as how much material has been displaced from the surface.

We agree with the authors' contention that projections of polyethylene extending into screw-holes of the base-plate or around the base-plate are mainly the result of loss of material. Comparing an insert with obvious polyethylene projections ("severe" per the authors' definition) to a less worn or unimplanted component of the same size consistently demonstrates that these projections continue to lie within the original envelope of the insert (rather than protrude outside the envelope). In some instances, one can appreciate that the plane of the backside surface (that surrounding the screw-hole projections for a base-plate with screw-holes) has been refashioned and is no longer parallel to the flat nonarticular reference surfaces on the opposite side of the insert (or no longer perpendicular to the vertical sides of the insert). Other evidence that an appreciable volume of polyethylene has been lost from an insert's backside surface includes polyethylene projecting into a central pinhole or other recessed region of the base-plate, erosion of product identification markings pressed or etched into the polyethylene, and erosion or deformation of a dovetail machined into the insert. Relating these characteristic changes seen on the insert undersurface to the original geometry (or to nonarticular regions) of the insert is one means by which investigators may be able to estimate the volume of material that has been lost from the surface.

Analysis of retrievals offers preliminary clues as to why fixed-bearing polyethylene inserts sometimes lose large volumes of polyethylene from the undersurface. The combination of an active patient and a rough (often titanium) base-plate surface appears to be a prerequisite. Increased tibiofemoral articular constraint and unintended engagement between the cam and the post of cruciate-substituting designs (e.g., at the anterior aspect of the post with relative hyperextension between the femoral and tibial components) seem to raise the risk of backside wear even further.

At revision surgery, when faced with obvious volumetric loss on the backside of the insert, the surgeon should appreciate that a simple polyethylene exchange is unlikely to halt the generation of particulate debris. Therefore, strong consideration should be given to complete revision of an otherwise well-fixed base-plate, replacing it with a base-plate that has a smoother surface finish and a more durable locking mechanism or with a one-piece component. Consideration should also be given to the possibility that the overall alignment of the knee or relative malalignment between the tibial and femoral components may be contributory. The potential of encountering an insert with clear volumetric loss from its undersurface should be weighed prior to revision and entered into the preoperative plan regardless of the symptoms that prompt revision. Close inspection of the insert during surgery is important. Prospectively, we believe it wise to implant either a base-plate with a polished superior surface or a one-piece tibial component at the time of primary total-knee arthroplasty.

We encourage the authors and others to perform single-design studies that assess the relative contributions of different factors (patient characteristics, surgical technique, and type of base-plate) to material loss at the insert undersurface as well as at its articular surface. We believe that the lessons learned from these studies will not only assist surgeons in providing better care to patients who present with tibial polyethylene wear and osteolysis but also will provide important clues to limiting the future prevalence of wear and osteolysis.

*The authors did not receive grants or outside funding in support of research or preparation of this manuscript. The authors did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

References

1. Wasielewski RC, Parks N, Williams I, Surprenant H, Collier JP, Engh G. Tibial insert undersurface as a contributing source of polyethylene wear debris. Clin Orthop. 1997;345:53-9.
2. Gabriel SM, Dennis DA, Honey MJ, Scott RD. Polyethylene wear on the distal tibial insert surface in total knee arthroplasty. The Knee. 1998;5:221-8.
3. Furman BD, Schmieg JJ, Bhattacharyya S, Li S. Assessment of backside polyethylene wear in three different metal backed total knee designs. Trans Orthop Res Soc. 1999;24:149.
4. Rao AR, Engh GA, Collier MB, Lounici S. Tibial interface wear in retrieved total knee components and correlations with modular insert motion. J Bone Joint Surg Am. 2002; 84:1849-55.