Abstract
Some early failures of metal‐on‐metal (MoM) hip replacements associated with elevated wear have caused concerns for the use of this bearing combination. Simulator studies have shown that microseparation and its associated rim contact and edge loading may produce the most severe wear in MoM bearings. It is generally recognized that this high wear can be attributed to the high contact stress of the head on the rim of the cup. In this study, an improved finite element contact model that incorporates an elastic‐perfectly plastic material property for cobalt‐chrome alloy of the metal bearing was developed in an attempt to provide an accurate prediction of the stress and strain for the rim contact. The effects of the microseparation displacement (0.1−2 mm), cup inclination angle (25−65°) and cup rim radius (0.5−4 mm) on the contact stress/strain were investigated. The results show that a translational displacement >0.1 mm under a load >0.5 kN can produce a highly concentrated contact stress at the surface of the cup rim which can lead to plastic deformation. This study also suggests that the magnitude of translational displacement was the major factor that determined the severity of the contact conditions and level of stress and strain under microseparation conditions. Future studies will address the effect of surgical translational and rotational malposition and component design on the magnitude of microseparation, contact stress and strain and severity of wear. © 2014 The Authors. Journal of Biomedical Materials Research Part B: Applied Biomaterials Published by Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 103B: 1312–1319, 2015.
Highlights
IntroductionMetal-on-metal (MoM) bearings have been used in hip joint replacements due to low wear under standard walking conditions, as shown in both laboratory simulator studies and clinical experiences.[1,2,3,4] MoM bearings have been further developed for surface replacement in order to preserve bone on the femoral side[5,6] and for larger head sizes to achieve enhanced range of motion and stability.[7,8] These designs were often targeted at younger, more active patients.[9,10] adverse conditions such as implant malpositioning can lead to edge loading or head-cup rim contact, and produce high contact stress causing elevated wear.[11] Recently, some early failures of MoM prostheses have been increasingly reported and caused a major concern,[12] leading to a medical device alert issued by the Medicines and Healthcare Products Regulatory Agency in the UK.[13]
With rotational malposition of the cup, such as the cup being positioned in steep inclination or with excessive anteversion, the contact patch developed at the bearing surface may intersect the cup rim, and this situation can be compounded by varied bearing designs such as sub-hemispheric cups and lower bearing clearance or by deflection of the cup bearing during fixation.[14]
A typical condition resulting from translational malpositioning is dynamic microseparation[16,17] of hip joints or subluxation, for which a translation of the head relative to the cup occurs leading to contact at the superior lateral rim of the cup.[18,19]
Summary
Metal-on-metal (MoM) bearings have been used in hip joint replacements due to low wear under standard walking conditions, as shown in both laboratory simulator studies and clinical experiences.[1,2,3,4] MoM bearings have been further developed for surface replacement in order to preserve bone on the femoral side[5,6] and for larger head sizes to achieve enhanced range of motion and stability.[7,8] These designs were often targeted at younger, more active patients.[9,10] adverse conditions such as implant malpositioning can lead to edge loading or head-cup rim contact, and produce high contact stress causing elevated wear.[11] Recently, some early failures of MoM prostheses have been increasingly reported and caused a major concern,[12] leading to a medical device alert issued by the Medicines and Healthcare Products Regulatory Agency in the UK.[13]. Laboratory simulator studies have shown that microseparation produces stripe wear at the bearing surfaces (Figure 1)
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