The Significance of Adverse 3rd-Body Wear Damage in the Failure of Metal-on-Metal Bearings used in resurfacing and Total-Hip Arthroplasty

Abstract

Large diameter, metal-on-metal (MOM) bearings used in total hip arthroplasty (THA) have been abandoned due to adverse wear. Commonly cited risks included excessive wear in large diameter CoCr bearings, edge-loading in steeply-inclined cups, and corrosion under modular CoCr heads. An alternative failure mechanism was suggested to be due to hip-impingement whereby the cup rim habitually creates pitting and microgroove damage in the opposing femoral head. It is to be noted that an RSA hip at impingement will risk a CoCr cup impinging on the tissues of the natural neck. In contrast, THA impingement risks a CoCr cup impinging on the metal femoral-neck. For this MOM retrieval study, we matched 12 each retrievals of total hip arthroplasty (THA) with modular CoCr heads and resurfacing arthroplasty (RSA) for study of microscopic wear damage. Anatomic RSA and THA models were developed to compare head:neck ratios, range of hip motion, and location of microgroove damage sites at impingement. The salient finding we report for the 1st time was that all RSA bearings featured arrays of large pits, typically 40 - 100 um or larger. The likely source of large metal fragments in RSA cases is suggested to be the CoCr cup impinging/subluxing on the femoral head, thereby creating microgroove damage. The circulating metal fragments becoming trapped under the cup created the basal and polar microgrooves, typically 40 - 100 um wide, and aligned under the rim profile of the cup. The abrasive power of such large circulating metal fragments was evident in the microscopic analyses in these 24 MOM retrievals. CoCr debris previously analyzed in MOM simulator and retrieval studies was typically characterized with particles sized less than 50 nm, i.e. 2,000 times smaller. The evidence of pitting and micro-groove morphology proved similar in RSA and THA regardless of design differences. Our study is the 1st confirmation of the MOM wear/impingement mechanics offered in a prior study of fixed-head McKee-Farrar THA, i.e. hip impingement produces fatigue wear in CoCr components with release of large CoCr fragments, consistent with sliding/impaction mechanisms and adverse wear in MOM. With continued function these large fragments will decompose into CoCr particulates and also ionize into solution.

Source: https://actascientific.com/ASOR/pdf/ASOR-01-0003.pdf. Abstract reproduced under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/); formatting normalized.