Does increased topside conformity in modular total knee arthroplasty lead to increased backside wear?

BACKGROUND: Modular metal-backed tibia components allow surgeons intraoperative flexibility. Although it is known that modular tibia components introduce the possibility for backside wear resulting from relative motion between the polyethylene insert and the tibial baseplate, it is not known to what degree variability in the conformity of the tibial polyethylene liner itself might contribute to backside wear. QUESTIONS/PURPOSES: The purpose of this study was to determine whether a flat, cruciate-retaining tibial polyethylene bearing generates less backside wear than a more conforming (curved) tibial polyethylene bearing in an analysis of specimens explanted during revision surgery. METHODS: The study included 70 total knee inserts explanted at revision surgery, all implanted and explanted by the same surgeon. Two different cruciate-retaining insert options in an otherwise similar knee system were used: one with a curved-on-flat (17) articular geometry and one with a highly conforming curved-on-curved design (53); both groups were sequential cohorts. The composite backside wear depth for the insert as well as the volume of backside wear was measured and compared between groups. RESULTS: The median linear backside-normalized wear for the posterior lipped inserts was 0.0063 mm/year (range, 0-0.085 mm/year), which was lower than for the curved inserts at 0.05 mm/year (range, 0.00003-0.14 mm/year) (p<0.001). The median calculated volumetric backside-normalized wear for the posterior lipped inserts was 14.2 mm3/year (range, 0-282.8 mm3/year) compared with 117 mm3/year (range, 2.1-312 mm3/year) for the curved inserts (p<0.001). CONCLUSIONS: In this retrieval study, more conforming tibial inserts demonstrated more backside-normalized wear than the flatter designs. This suggests that in this modular total knee arthroplasty design, higher articular conformity to address the issues of high bearing contact stress comes at a price: increased torque transmitted to the backside insert-to-tray interface. We suggest further work be undertaken to examine newer insert designs to evaluate if our conclusions hold true with the newer generation locking mechanism, tibial tray finish and polyethylene designs, as more highly conforming tibial inserts are introduced into the market. LEVEL OF EVIDENCE: Level III, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.

Equal channel angular extrusion of ultra-high molecular weight polyethylene.

Ultra-high molecular weight polyethylene (UHMWPE), a common bearing surface in total joint arthroplasty, is subject to material property tradeoffs associated with conventional processing techniques. For orthopaedic applications, radiation-induced cross-linking is used to enhance the wear resistance of the material, but cross-linking also restricts relative chain movement in the amorphous regions and hence decreases toughness. Equal Channel Angular Extrusion (ECAE) is proposed as a novel mechanism by which entanglements can be introduced to the polymer bulk during consolidation, with the aim of imparting the same tribological benefits of conventional processing without complete inhibition of chain motion. ECAE processing at temperatures near the crystalline melt for UHMWPE produces (1) increased entanglements compared to control materials; (2) increasing entanglements with increasing temperature; and (3) mechanical properties between values for untreated polyethylene and for cross-linked polyethylene. These results support additional research in ECAE-processed UHMWPE for joint arthroplasty applications.

Synovial fluid differential cell count in wear debris synovitis after total knee replacement.

BACKGROUND: Determining the cause of synovitis following total knee arthroplasty (TKA) can be challenging. The differential diagnoses include infection, hemarthrosis, instability, crystalline disease, wear debris or idiopathic causes. Wear particle synovitis can mimic periprosthetic infection with symptoms of pain and effusion. Radiographs and physical exam are often inconclusive in differentiating the two. Synovial fluid analysis is routinely used in evaluating periprosthetic infections. We examined the association between synovial white blood cell count and differentials, and polyethylene wear and osteolysis, to see if fluid analysis can aid in establishing the diagnosis of wear particle synovitis. METHODS: A cell count and differential was obtained from synovial fluid samples from 54 TKAs undergoing revision for aseptic failure. Explanted polyethylene inserts were analyzed for linear and volumetric wear, oxidation (ketone peak height), and damage features. Analysis was performed to assess the relationship between cell counts and polyethylene wear indicators as well as severity of intra-operative and radiographic osteolysis. RESULTS: Total and percent mononuclear (monocyte and lymphocyte) cell counts were found to be elevated in the presence of documented wear debris synovitis and an association was suggested between their levels and maximum ketone levels. CONCLUSION: The present study implies that the differential cell count of knee fluid can help distinguish wear debris from infection as a source of synovitis following TKA and identifies the value of the mononuclear cell count as a possible tool to assess abnormal wear rates of the polyethylene insert. Further research into identifying the exact role of monocytes in the wear debris synovitis and osteolytic pathways is warranted. LEVEL OF EVIDENCE: Level II, diagnostic study.