Wear damage in mobile-bearing TKA is as severe as that in fixed-bearing TKA

Abstract

Background: Mobile-bearing TKAs reportedly have no clinical superiority over fixed-bearing TKAs, but a potential benefit is improved polyethylene wear behavior.

Questions/purposes: We asked whether extent of damage and wear patterns would be less severe on retrieved mobile-bearing TKAs than on fixed-bearing TKAs and if correlations with patient demographics could explain differences in extent or locations of damage.

Methods: We performed damage grading and mapping of 48 mobile-bearing TKAs retrieved due to osteolysis/loosening, infection, stiffness, instability or malpositioning. Visual grading used stereomicroscopy to identify damage, and a grade was assigned based on extent and severity. Each damage mode was then mapped onto a photograph of the implant surface, and the area affected was calculated.

Results: Marked wear damage occurred on both surfaces, with burnishing, scratching, and pitting the dominant modes. Damage occurred over a large portion of both surfaces, exceeding the available articular borders in nearly 30% of implants. Wear of mobile-bearing surfaces included marked third-body debris. Damage on tibiofemoral and mobile-bearing surfaces was not correlated with patient BMI or component alignment. Damage on mobile-bearing surfaces was positively correlated with length of implantation and was greater in implants removed for osteolysis or instability than in those removed for stiffness or infection.

Conclusions: Each bearing surface in mobile-bearing implants was damaged to an extent similar to that in fixed-bearing implants, making the combined damage score higher than that for fixed-bearing implants. Mobile-bearing TKAs did not improve wear damage, providing another argument against the superiority of these implants over fixed-bearing implants.

Fig. 1A–B

(A) The tibiofemoral articular surface and (B) the mobile-bearing surface of the components were each divided into 10 sections. Wear damage grading scores were assigned for each section using light stereoscopic examination and a subjective grading score.

Fig. 2A–D

Damage mapping was performed from digital photographs of (A) the tibiofemoral articular surface and (B) the mobile-bearing surface as a means of determining the amount of the available (C) tibiofemoral and (D) mobile-bearing surfaces experiencing wear damage (by the mode of damage observed). The color code corresponds to burnishing (gray), scratching (blue), third-body debris (yellow), abrasion (purple), pitting (orange), delamination (green), and surface deformation (red). Black areas did not show wear damage. This implant was revised for infection in an 82-year-old man (with a BMI of 29) 3 years after it had been implanted. The damage score for the tibiofemoral surface was 39; the score for the mobile-bearing surface was also 39.

Fig. 3A–B

(A) A SEM photomicrograph (left) of the mobile-bearing surface shows embedded PMMA debris, and EDS analysis (right) of the PMMA particle at the top of the photomicrograph shows large amounts of zirconium, reflecting the zirconia used to render the PMMA radiopaque. (B) Another SEM photomicrograph (left) shows metallic beads embedded into the polyethylene mobile-bearing surface, and EDS (right) confirmed the presence of titanium and aluminum, consistent with the titanium alloy beads used for biologic fixation of the metallic component interfaces.

Fig. 4

A photograph of the bearing surface on the superior metallic tibial tray shows severe scratching of the originally polished surface, with the scratches forming concentric rings around the center of the tray (seen in the upper part of the photograph).

Fig. 5A–B

Histograms of the percentage of the possible bearing surface demonstrating wear damage for (A) the tibiofemoral and (B) the mobile-bearing surfaces of the retrieved tibial components show damage extended over large portions of the surface, even beyond the intended tibiofemoral bearing surface for six of the components.

Fig. 6A–B

(A) AP and (B) lateral radiographs show an implant revised after 3.8 years for tibial loosening and subsidence due to osteolysis.