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Backside wear of mobile and fixed bearing polyethylene tibial inserts
+1,2,3,4Huang, C H; 2,4Huang, C H; 1,2Lu, Y C; 1,2,4Chang, T K, 4Cheng, C K +1Department of Orthopaedic Surgery, Mackay Memorial Hospital (MMH), 2Biomechanics Research Laboratory, MMH,
3Mackay Medical College, 4Institute of Biomedical Engineering, National Yang Ming University, Taipei, Taiwan [email protected]
INTRODUCTION: Modularity of tibial component allows the surgeon to choose the optimum thickness and design of the tibial insert during surgery. However, a possible disadvantage of modularity is that the interface between the backside of the tibial insert and the tibial baseplate is a potential site of polyethylene wear and generation of debris. The contemporary tibial component can be either a fixed bearing (FB) or a mobile bearing (MB) design. The major design differences between FB and MB are the level of conformity on the tibiofemoral articular surface and the locking mechanism between tibial insert and baseplate (allow mobility of tibial insert or not). The aim of this study was to analyze the wear patterns and wear scores on the backside surface of tibial insert in retrieved FB and MB rotation platform total knee prostheses. METHODS: Seventy-three retrieved polyethylene tibial inserts were subdivided into mobile bearing (MB) rotation platform group and fixed bearing (FB) groups. The MB group consisted of 15 knees with the Low Contact Stress Rotating Platform prosthesis (LCS RP, DePuy, Warsaw, Ind). The fixed bearing group consisted of 22 knees with a Porous-Coated Anatomic prosthesis (PCA I, Howmedica, Rutherford, New Jersey) and 36 with a Miller-Galante prosthesis (MG I, Zimmer, Warsaw, Ind). The average length of implantation time was 121 ± 38.2 months (range, 48-162) for MB knees, 93.6 ± 39.6 (48-180) months for PCA knees and 109 ± 31.3 (range, 50 - 156) months for MG knees, respectively. The average body weight was 72.8 ± 7.3 kg (range, 60-87) for the patients with RP knee, 73.5±6.4 kg (range, 64-84) for MG and 67.2 ±13.8 kg for PCA. (range, 45-92). There was no significant difference in body weight between MB and FB groups. Wear patterns and scores (scaling method was modified from a well-reported method of Hood RW et al.) on the backside surface were analyzed for each retrieved polyethylene inserts. We divided the wear patterns into two groups according to their wear mechanisms: low-grade wear (burnishing, abrasion, and cold flow) and high-grade wear (scratching, pitting, metal embedding, and delamination). The wear scores of the RP and FB groups were analyzed by regression analysis and chi-square distribution using commercial software (SPSS version 11.5; SPSS Inc., Chicago, IL, USA). Results were considered statistically significant when p ≤ 0.05. All data are shown as mean ± standard deviation. RESULTS: The major wear patterns on the backside surface of MB knee were burnishing, scratching and pitting/third body embedding. For burnishing scores was 0.7±2.6 points (range, 0-9), scratching score was 8.3±3.2 points (range, 5-14), and pitting/embedding was 5.3±2.3 points (range, 2-9) (Table 1). These scores were significantly higher than the counterpart of the FB group (p
