Metal-on-Metal Hip Complications

Metal-on-metal (MoM) hip bearings were introduced as an alternative to metal-on-polyethylene to reduce wear-related osteolysis and improve longevity, particularly in younger active patients. However, concerns about adverse local tissue reactions (ALTR), elevated metal ion levels, and pseudotumor formation have led to a significant decline in their use and multiple product recalls.

Tribological Failure

Metal-on-metal bearings rely on fluid film lubrication to separate the articulating surfaces. When this lubrication fails (due to edge loading, malposition, or high activity), metal-on-metal contact occurs, generating wear debris.

Biological Response

Differential Diagnosis

The complications of a MoM bearing arise from chronic metal-ion and particulate release. They are best grouped as local (the great majority), regional/mechanical, and systemic (rare).

MoM management remains an area of genuine debate. Examiners reward candidates who can articulate the uncertainty rather than quoting cut-offs as absolutes.

Global Epidemiology:

An estimated one million MoM bearings were implanted worldwide during the peak period of roughly 2003 to 2010, the majority in younger, more active patients. Use has since collapsed to near zero for primary arthroplasty. Large national registries were decisive in exposing the problem before manufacturer surveillance, and remain the backbone of outcome data.

Registry Evidence (Side by Side):

Guidelines Compared (Side by Side):

Recalled and Withdrawn Devices (Global):

• DePuy ASR (recalled 2010): ASR Hip Resurfacing System and ASR XL Acetabular System
• DePuy Pinnacle MoM liner option: progressively withdrawn
• Multiple large-head stemmed MoM and other resurfacing systems voluntarily withdrawn worldwide

High- vs Limited-Resource Practice Variation:

• Well-resourced settings: structured recall registries, routine whole-blood ICP-MS metal-ion assays, and ready MARS MRI access enable proactive surveillance and earlier revision.
• Limited-resource settings: ion assays and MARS MRI may be unavailable, so practice relies more on symptoms, plain radiographs, and ultrasound; patient tracing after recalls is harder. The principle remains identical - any symptomatic MoM hip warrants infection exclusion and cross-sectional imaging where obtainable, and lifelong follow-up.

Documentation Principle:

Whatever the health system, document surveillance, counselling, and informed consent in writing, and give patients a written surveillance plan - MoM hips require lifelong monitoring with no defined endpoint.