![]() Knowing that MoM hips can potentially lead to very severe complications, how should surgeons approach these patients, and when should they intervene? Should all MoM THA be revised? Its believed that significant ARMD occurs in 1% of cases within 5 years. Not all patients respond to the same levels of metal ions, each patient has a different threshold before they begin to demonstrate symptoms of metal hypersensitivity. Cobalt is certainly the more toxic of the two metals. This film provides continuous lubrication and dramatically reduces wear (hip simulators showed encouraging wear rates 36 mm. The success of metal-on-metal devices depends on a fluid-film layer that develops between components when the space is between 100-200 μm. The end result is that many designs were recalled and MoM has fallen out of favor. The terms ARMD (Adverse Reaction to Metal Debris) or ALTR (adverse local tissue reaction) are similar broad terms to encapsulate these soft tissue lesions. Metallosis is defined as macroscopic staining, necrosis/fibrosis of periprosthetic tissue, and is associated with solid soft tissue masses, aseptic cysts, and significant soft tissue necrosis. While the overall wear rates are lower than metal-on-poly, the metal debris is considerably more reactive, and can lead to macroscopic muscle necrosis (abductor injury is particularly concerning), significant osteolysis, and large sterile cystic masses (called “pseudotumors”). These failures were correlated with significant adverse tissue reaction from the metal particles generated by wear. Registry data (out of Europe) started showing a 2-3x higher failure rates compared to metal-on-poly. ![]() MoM quickly became incredibly popular in the USA, with over 50% of THA using this combination as recently as 2006. Improved technology in biologic fixation and implant manufacturing suggested a promising low-wear alternative to metal-on-poly implants, with the hope of improving wear rates and overcoming the barrier to long-term survivorship. The promise of these findings (the potential for superior longevity with MoM) at a time when poly implants were failing because osteolysis (a result of poor quality PE) ushered in a new era of MoM. ![]() In fact, 110 of these hips were reexamined in 1990s and showed no signs of loosening. Yet these early MoM implants showed binary results: either failure due to early loosening or excellent long-term survivorship > 20 years. The history of Metal-on-Metal (MoM) implants dates pre-Charnley, with the McKee-Farrar implants, which fell out of favor due to early loosening and subsequently paved the road for Charnley’s metal-poly design. ![]()
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