Commentary & Perspective on
"Osteolysis Associated with a Cemented Modular Posterior-Cruciate-Substituting Total Knee Design. Five to Eight-Year Follow-up"
by Michael R. O'Rourke, MD, et al.

Commentary & Perspective by
James A. Rand, MD*,
Orthopaedic Department, Mayo Clinic Scottsdale, Scottsdale, AZ

Osteolysis was an infrequent problem in patients who underwent total knee arthroplasty with use of posterior-cruciate-substituting implants that included an all-polyethylene or monolithic factory-molded metal-backed tibial component. Recently, however, Mikulak et al. reported that sixteen (2.9%) of 557 knees treated with a modular posterior-cruciate-substituting design were revised at fifty-six months because of loosening and osteolysis¹. O'Rourke et al. have now reported osteolysis in association with seventeen (16%) of 105 knees treated with a different modular posterior-cruciate-substituting tibial design. Why did such a small change in design result in a major change in results?

Factors in polyethylene wear include the base resin, the manufacturing process, the mode of sterilization, the length of shelf life prior to implantation, prosthetic design, the alignment of the implant, the stability and kinematics of the knee, polyethylene thickness, any interposed third-body particles, and the material and surface finish of the metal tray of the tibial component². Patient characteristics such as activity level and weight also affect wear.

The modern era of posterior-cruciate-sacrificing design began with the Total Condylar Knee, a prosthesis that included a one-piece all-polyethylene tibial component. The long-term results of total knee arthroplasty with use of the original total condylar prosthesis have been excellent. With revision as the end point, survivorship of the implant has been reported to be between 89% and 96% at fifteen to twenty years^3,4.

In vitro studies have shown decreased stresses in the cancellous bone and cement of the proximal tibia when a metal-backed tibial component was used. Thus, the addition of the metal backing in the design of the tibial component was expected to decrease the prevalence of mechanical loosening. Yet, with use of the total condylar prosthesis, there were no significant differences in the rates of implant survivorship between an all-polyethylene and a metal-backed tibial component.

A metal backing in the tibial component also introduced several advantages in regard to the surgical technique: a modular polyethylene can be used, allowing the surgeon to adjust soft-tissue tension once the implants are fixed to bone and prior to placement of the polyethylene insert; acrylic bone cement can more easily be removed from the back of the joint to avoid entrapment of cement particles and resultant third-body wear; and the conformity of the polyethylene can be varied to compensate for ligament laxity or deficiency. The disadvantages of modular polyethylene designs are related to the need for a secure locking mechanism between the polyethylene and the metal tibial tray and, because of the thickness of the metal tray, the necessity of using a thinner polyethylene. The locking mechanism must allow ease of insertion of the polyethylene at the time of implantation but must also be secure enough to prevent dislodgment and macromotion of the polyethylene in vivo. A variety of locking mechanisms have been used including snap-fit, pins, clips, and combinations of these mechanisms. Motion between the backside of the polyethylene that articulates against the metal tray has been recognized as a source of wear debris, which has been associated with osteolysis and failure of some implants. Therefore, the use of a modular polyethylene tibial implant has had some unanticipated adverse consequences.

Should modular polyethylene tibial components be abandoned in favor of either all-polyethylene or nonmodular metal-backed tibial components? The answer is difficult to determine as some designs with a modular polyethylene tibial component have had excellent results. As new designs are introduced, it is essential that both laboratory testing and clinical trials be conducted to document the performance of these modular designs, including those with all-polyethylene and those with metal-backed tibial trays.

When posterior-cruciate-substituting designs were first considered, increased stress on the bone-cement interface that might result in loosening of the tibial component was a concern. Yet, aseptic loosening was an infrequent problem with the initial monolithic posterior-cruciate-substituting tibial component designs. In modular posterior-cruciate-substituting designs, these same stresses affect the interface between the modular polyethylene insert and the metal-backed tibial component. In relatively flat posterior-cruciate-retaining designs, the locking mechanism between the polyethylene and the metal-backed tibial component receives less stress than in the same design with a posterior-cruciate-substituting all-polyethylene tibial component. Post wear is an additional problem associated with the posterior-cruciate-substituting design⁵. Hyperextension of the knee or a higher degree of posterior slope to the tibial cut may allow impingement between the anterior aspect of the peg and the femoral component in a posterior-cruciate-substituting design. This impingement creates local wear and increases stresses on the locking mechanism between the polyethylene insert and the tibial tray, which increases backside wear.

Will the use of highly cross-linked polyethylene or mobile-bearing designs solve the problem of polyethylene wear and osteolysis? In vitro studies demonstrate lower wear rates for highly cross-linked polyethylenes in comparison with conventional polyethylene when used in highly conforming articulations such as a total hip, but their long-term performance in the less conforming articulations of a total knee is not yet known. Furthermore, cross-linking is associated with a decrease in fatigue strength, which might present additional problems in locking the polyethylene to a metal tray. The use of mobile bearings may increase the conformity of the articulation and so reduce wear. However, retrieval studies have shown a higher rate of particle generation adjacent to mobile-bearing knee implants in comparison with fixed-bearing designs⁶. Therefore, mobile-bearing designs may also be associated with the development of osteolysis.

The surgeon must carefully evaluate the design characteristics of each implant system. Surgical technique is different for total knee arthroplasty with posterior-cruciate-substituting and posterior-cruciate-retaining designs. Locking mechanisms between the polyethylene insert and the tibial component need improvement. The use of all-polyethylene and nonmodular polyethylene metal-backed tibial components should be considered. Use of mobile bearings holds the promise of decreased wear, but identification of the optimum design will require long-term clinical studies of the many new designs.

*The author did not receive grants or outside funding in support of the research or preparation of this manuscript. He received payments or other benefits or a commitment or agreement to provide such benefits from Smith and Nephew. 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 author is affiliated or associated.

References

1. Mikulak SA, Mahoney OM, dela Rosa MA, Schmalzried TP. Loosening and osteolysis with the press-fit condylar posterior-cruciate-substituting total knee replacement. J Bone Joint Surg Am. 2001;83:398-403.
2. Schmalzried TP, Callaghan JJ. Wear in total hip and knee replacements. J Bone Joint Surg Am. 1999;81:115-36.
3. Ranawat CS, Flynn WF Jr, Deshmukh RG. Impact of modern technique on long-term results of total condylar knee arthroplasty. Clin Orthop. 1994;309:131-5.
4. Gill GS, Joshi AB. Performance of posterior cruciate retaining knee arthroplasty:15 years and beyond. Presented at the Annual Meeting of the American Academy of Orthopaedic Surgeons; 28 Feb 2001; San Francisco, CA.
5. Puloski SK, McCalden RW, MacDonald SJ, Rorabeck CH, Bourne RB. Tibial post wear in posterior stabilized total knee arthroplasty. An unrecognized source of polyethylene debris. J Bone Joint Surg Am. 2001;83:390-7.
6. Hirakawa K, Bauer TW, Stulberg BN, Wilde AH, Borden LS. Characterization of debris adjacent to failed knee implants of 3 different designs. Clin Orthop. 1996;331:151-8.