backside wear of mobile and fixed bearing polyethylene tibial ...superiority of long-term...

Backside wear of mobile and fixed bearing polyethylene tibial inserts + 1,2,3,4 Huang, C H; 2,4 Huang, C H; 1,2 Lu, Y C; 1,2,4 Chang, T K, 4 Cheng, C K + 1 Department of Orthopaedic Surgery, Mackay Memorial Hospital (MMH), 2 Biomechanics Research Laboratory, MMH, 3 Mackay Medical College, 4 Institute 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<0.05). Curvilinear scratching with slight burnishing wear pattern was apparently observed on the backside surface of RP group but not seen for the FB group (Fig. 1A). On the other hand, low-grade wear pattern such as abrasion with slight pitting was more common seen in the FB knee (Table 1). Wear scores and patterns were not significantly different between two FB designs. Protrusion of polyethylene into screw holes, imprinting by the product marks of the metal tray were commonly observed in the FB knees (Fig. 1B, C). Delamination was very few on the backside surface for FB and MB groups. DISCUSSION: Another source of polyethylene wear could produce from the additional inferior articulating surface of modular tibial component. MB knees was introduce to have a better wear resistance on the superior articular surface, however, wear patterns such as burnishing, scratching, pitting/third-body embedding were commonly seen on the backside surface. Scratching pattern of polyethylene insert on the backside surface were of curvilinear scratching and predisposed to be caused by rotational motion against the metallic baseplate. As for FB group, although some abrasion, pitting and debris embedding were evident, high-grade wear patterns as scratching and delamination were less seen on the backside surface. Protrusion of polyethylene into the screw holes and imprints by the marks or lettering of the metal tray was visible, supposing there might be slightly relative movements happening to the interface of polyethylene insert and the metallic baseplate. In addition, the embedded debris was seen on articular surface and the backside of polyethylene inserts in FB group. It implicated that the third-body embedding wear played a part of deleterious effect on the wear of UHMWPE tibial inserts. Mobile bearing rotation platform does not provide a locking mechanism to fix the tibial insert on the metallic baseplate. It’s supposed that it potentially allows macromotion for the interface and it may make foreign debris invade into the interface that could increase friction wear and shear stress, leading to high-grad wear patterns of scratching, pitting and third-body embedding debris were predominantly seen on the backside surface of MB in the current study. Many limitations were inherent in this retrieval study. Firstly, the wear scores was graded from a visual scaling method, this method can merely assess wear patterns in a 2-dimentional manner; therefore, volumetric loss of material of a tibial insert was not analyzed in this study. Secondly, the most of the primary TKAs were not implanted at the author’s institute and many were performed many years ago. It’s difficult to acquire the polyethylene manufacturing information related to surface roughness, sterilization technique, shelf life and storage data. Finally, wear patterns observed in this study are limited to three knee designs, they cannot represent all design features of the contemporary knee prostheses. Obviating these limitations, we believe this study provides valuable information about clinical performance of TKAs and help to differentiate wear patterns between the MB and FB designs. Mobile bearing knee was designed to increase wear resistance on the upper surface by providing more highly conformity of the tibiofemoral articular surface and by decoupling the joint and adding mobility between the tibial insert and the baseplate. However, MB could also generate higher incidence of wear patterns as burnishing, pitting/third-body embedding and scratching on the backside surface. Superiority of long-term performance in fixed or mobile bearing knees require further verification from extensive studies Table 1. Mean wear score on the backside surface for each wear pattern. Pros thes is design L ow-grade wear s core H igh-grade wears core Burnishing Abrasion Cold flow Scratching Pitt ing, Metal embedded Delamination Mobile bearing RP 0.7±2.6 (r ange, 0-9) 0.4±1.0 (range, 0-3) -- 8.3±3.2 (range, 5- 14) 5.3±2.3 ( range,2-9) 0.5±0. 8 ( range,0-2) Fixedbearing P CA 0.0 3.5±4.2 (range, 0- 13) -- 0.0 0.7±2.7 (range, 0- 12) 0.0 MG 0.0 5.0±2.8 (range, 0- 12) -- 0.0 0.5±1.0 ( range,0-3) 0.1±0. 4 (range,0-2) RP: rotating platform knee; PCA: Porous-Coated Anatomic knee; MG : Miller-Galante knee; Maximum score is 18 points (A) (B) (C) Figure 1. shows inferior articular surface of (A) mobile bearing RP; (B)fixed bearing PCA, and (C) MG knee prostheses. Poster No. 2185 • 56th Annual Meeting of the Orthopaedic Research Society

backside wear of mobile and fixed bearing polyethylene tibial ...superiority of long-term...

Documents