Edge-Scraping Wear in Metal-on-Metal Total Hips + 1 Elkins, JM; 1 Pedersen, DR; 1,2 Callaghan, JJ; 1 Brown, TD + 1 Department of Orthopaedics and Rehabilitation, University of Iowa, Iowa City, IA, 2 Veterans Affairs Medical Center, Iowa City, IA. [email protected]@uiowa.edu INTRODUCTION: Owing to increased demand for hip replacement in younger and more active patients, metal-on-metal (MoM) bearings have experienced renewed interest in the past decade. However, concerns have heightened regarding the deleterious consequences of malpositioned implants, especially regarding immunologic reactions. A particular concern in metal-on-metal (MoM) bearings is the occurrence of edge loading, where near line-contact between the femoral head and the acetabular cup lip results in a very narrow region of engagement, and therefore highly concentrated stress. This deleterious situation interferes with the precise articulation conditions necessary for fluid-film lubrication, resulting in increased friction and accelerated wear. Of particular interest is the occurrence of edge-loading due to femoral head subluxation for deep- flexion activities. While cup orientation is the most commonly cited surgical factor influencing edge loading in hard bearings [1], there is little consensus as to which positions are at highest risk for edge- scraping wear. METHODS: An established [2-3] finite element model of THR impingement/ dislocation was applied to investigate edge-loading during subluxation. The model consisted of the hip capsule and a contemporary 36-mm implant. Cups were positioned (radiographic definition) in various values of acetabular inclination (30° to 65°, in 5° increments), and acetabular anteversion (10° to 40°, in 5° increments). Three stem anteversion angles (0°, 12.5° and 25°) were considered. Four distinct posterior-subluxation challenges were investigated: sit-to-stand from a normal height, sit-to-stand from a low height, squatting and stooping, as well as an anterior dislocation challenge (Fig. 1). Posterior scraping wear (Fig. 2a), which occurred during subluxation due to interaction of surface contact stress and relative surface sliding/scraping distances, was computed using an Archard-Lancaster wear formulation. Scraping wear on the femoral head and cup liner were averaged for each of the 840 distinct FE simulations utilized in this investigation. Figure 1: Edge-loading arises at extremes in joint range of motion, resulting in damage to the posterior region of the cup (a, flexion and endorotation) and the superior region (b, extension and exorotation). RESULTS: Scraping wear was found to be highly sensitive to cup positioning (Fig. 3), and was greatest at extremes of both cup anteversion and cup inclination. Figure 2: (a) During extremes in joint range of motion, THA instability, as measured in terms of head subluxation, generated edge-scraping. A similar damage feature is apparent on a femoral head component from an implant that was revised due to dislocation. Scraping wear severity was lowest for intermediate-values of cup tilt and cup anteversion for the posterior edge-loading scenarios (Fig. 3a), while the superior-anterior edge-loading challenges demonstrated lowest severity for decreased values of cup inclination and cup anteversion (Fig. 3b). When arithmetically averaged, the single best cup orientation as regards scraping wear severity minimization was at 40° inclination and approximately 23° of cup anteversion. Figure 3: Average scraping severity score for the 672 distinct FE simulations of posterior edge-loading (a) and 168 distinct simulations of superior edge-loading (b). When simultaneously considering both edge- loading mechanisms, a small region centered at approx. 40° inclination and 23° anteversion demonstrated best scraping wear minimization. Figure 4: Values of cup inclination (left axis) and anteversion (right axis) which minimized scraping wear severity were monotonically correlated with femoral stem anteversion. DISCUSSION: The exact mechanism(s) for the unusually high failure rate of some MoM THRs are still unknown. We hypothesized that edge-loading as occurs during extremes of both hip flexion and hip extension/exorotation may be a contributing factor. The present data suggest that the lowest scraping wear occurs for cups in 20-25° of anteversion. While posterior scraping wear was shown to be influenced by cup inclination, dependency upon cup orientation was much greater as regards cup anteversion. Conversely, superiorly-directed scraping wear was influenced approximately equally by both cup inclination and anteversion. Interestingly, the optimized values for cup anteversion which minimized scraping wear severity increased for increased values of femoral anteversion. While the present study has already determined scraping wear (especially posteriorly-directed) to be exquisitely sensitive to cup anteversion, this interesting finding warrants further investigation into the role of femoral anteversion in reducing edge-loading wear. SIGNIFICANCE: Edge loading-associated wear is a serious concern with metal THA bearings. This computational study identified cup orientation as a major determinant of metal edge wear. REFERENCES: [1] Williams S et al. J Bone Joint Surg Am. 2008 [2] Elkins JM et al. J Orthop Res. 2011 [3] Elkins JM et al. Clin Orthop Relat Res. 2011 ACKNOWLEDGEMENTS: Support provided by the VAMC and the NIH AR46601 and AR53553 Poster No. 2008 • ORS 2012 Annual Meeting
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Edge-Scraping Wear in Metal-on-Metal Total Hips
+1Elkins, JM; 1Pedersen, DR; 1,2Callaghan, JJ; 1Brown, TD +1Department of Orthopaedics and Rehabilitation, University of Iowa, Iowa City, IA, 2Veterans Affairs Medical Center, Iowa City, IA.
[email protected]@uiowa.edu INTRODUCTION: Owing to increased demand for hip replacement in younger and more active patients, metal-on-metal (MoM) bearings have experienced renewed interest in the past decade. However, concerns have heightened regarding the deleterious consequences of malpositioned implants, especially regarding immunologic reactions. A particular concern in metal-on-metal (MoM) bearings is the occurrence of edge loading, where near line-contact between the femoral head and the acetabular cup lip results in a very narrow region of engagement, and therefore highly concentrated stress. This deleterious situation interferes with the precise articulation conditions necessary for fluid-film lubrication, resulting in increased friction and accelerated wear. Of particular interest is the occurrence of edge-loading due to femoral head subluxation for deep-flexion activities. While cup orientation is the most commonly cited surgical factor influencing edge loading in hard bearings [1], there is little consensus as to which positions are at highest risk for edge-scraping wear.
METHODS: An established [2-3] finite element model of THR impingement/ dislocation was applied to investigate edge-loading during subluxation. The model consisted of the hip capsule and a contemporary 36-mm implant. Cups were positioned (radiographic definition) in various values of acetabular inclination (30° to 65°, in 5° increments), and acetabular anteversion (10° to 40°, in 5° increments). Three stem anteversion angles (0°, 12.5° and 25°) were considered. Four distinct posterior-subluxation challenges were investigated: sit-to-stand from a normal height, sit-to-stand from a low height, squatting and stooping, as well as an anterior dislocation challenge (Fig. 1). Posterior scraping wear (Fig. 2a), which occurred during subluxation due to interaction of surface contact stress and relative surface sliding/scraping distances, was computed using an Archard-Lancaster wear formulation. Scraping wear on the femoral head and cup liner were averaged for each of the 840 distinct FE simulations utilized in this investigation.
Figure 1: Edge-loading arises at extremes in joint range of motion, resulting in damage to the posterior region of the cup (a, flexion and endorotation) and the superior region (b, extension and exorotation).
RESULTS: Scraping wear was found to be highly sensitive to cup positioning (Fig. 3), and was greatest at extremes of both cup anteversion and cup inclination.
Figure 2: (a) During extremes in joint range of motion, THA instability, as measured in terms of head subluxation, generated edge-scraping. A similar damage feature is apparent on a femoral head component from an implant that was revised due to dislocation.
Scraping wear severity was lowest for intermediate-values of cup tilt and cup anteversion for the posterior edge-loading scenarios (Fig. 3a), while the superior-anterior edge-loading challenges demonstrated lowest severity for decreased values of cup inclination and cup anteversion (Fig. 3b). When arithmetically averaged, the single best cup orientation as regards scraping wear severity minimization was at 40° inclination and approximately 23° of cup anteversion.
Figure 3: Average scraping severity score for the 672 distinct FE simulations of posterior edge-loading (a) and 168 distinct simulations of superior edge-loading (b). When simultaneously considering both edge-loading mechanisms, a small region centered at approx. 40° inclination and 23° anteversion demonstrated best scraping wear minimization.
Figure 4: Values of cup inclination (left axis) and anteversion (right axis) which minimized scraping wear severity were monotonically correlated with femoral stem anteversion. DISCUSSION: The exact mechanism(s) for the unusually high failure rate of some MoM THRs are still unknown. We hypothesized that edge-loading as occurs during extremes of both hip flexion and hip extension/exorotation may be a contributing factor. The present data suggest that the lowest scraping wear occurs for cups in 20-25° of anteversion. While posterior scraping wear was shown to be influenced by cup inclination, dependency upon cup orientation was much greater as regards cup anteversion. Conversely, superiorly-directed scraping wear was influenced approximately equally by both cup inclination and anteversion. Interestingly, the optimized values for cup anteversion which minimized scraping wear severity increased for increased values of femoral anteversion. While the present study has already determined scraping wear (especially posteriorly-directed) to be exquisitely sensitive to cup anteversion, this interesting finding warrants further investigation into the role of femoral anteversion in reducing edge-loading wear. SIGNIFICANCE: Edge loading-associated wear is a serious concern with metal THA bearings. This computational study identified cup orientation as a major determinant of metal edge wear. REFERENCES: [1] Williams S et al. J Bone Joint Surg Am. 2008 [2] Elkins JM et al. J Orthop Res. 2011 [3] Elkins JM et al. Clin Orthop Relat Res. 2011 ACKNOWLEDGEMENTS: Support provided by the VAMC and the NIH AR46601 and AR53553
Poster No. 2008 • ORS 2012 Annual Meeting
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