Metal-on-Metal Hip Complications

Metal-on-metal (MoM) bearings were introduced as an alternative to metal-on-polyethylene, to reduce wear-related osteolysis and improve longevity, particularly in younger active patients. Adverse local tissue reactions (ALTR), elevated metal ion levels and pseudotumour formation then emerged, use declined significantly, and multiple products were recalled.

The ASR recall. The DePuy ASR Hip Resurfacing System and ASR XL Acetabular System were voluntarily recalled in August 2010 because of higher than expected revision rates. Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) data showing a 6.4% revision rate at 5 years was instrumental in identifying the problem; the failure rate of the recalled ASR devices is quoted at 12-13% at 5 years.

Scale. Use peaked between 2005 and 2010, before the concerns emerged, and is now near zero for primary THA with very limited resurfacing. An estimated 500,000+ patients worldwide carry a MoM hip and every one of them needs lifetime monitoring. The revision burden is a significant healthcare cost and patient morbidity, and major class action settlements followed globally.

Who does badly, who does not. Pseudotumour is symptomatic in 1-4% of patients. Systemic toxicity is rare but documented, with cardiac, neurological and thyroid effects. At the other end, a well-positioned resurfacing has 95%+ survival at 10 years in experienced hands.

The cascade. ALTR is caused by a combination of wear-generated metal debris and corrosion products from the bearing surface and from modular junctions. In susceptible individuals this triggers a Type IV hypersensitivity reaction, leading to soft-tissue destruction, osteolysis and potentially systemic toxicity.

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Tribology. A MoM bearing relies on fluid-film lubrication to separate the articulating surfaces. When that film fails, through edge loading, malposition or high activity, metal contacts metal and wear debris is generated.

Cup position. Inclination greater than 55 degrees produces edge loading, with metal-on-metal contact outside the bearing surface; a shallow cup with a low coverage arc does the same. Combined anteversion matters too, so aim for inclination of 40-45 degrees and anteversion of 15-20 degrees. Malposition is the strongest predictor of failure.

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Design. Recalled designs such as the ASR carry design-specific higher failure rates. Small-head MoM THAs (less than 50 mm) have higher failure rates than resurfacing, and large heads, long necks and a large taper mismatch make modular-junction corrosion (trunnionosis, in the table above) more common.

The biological response. The characteristic histological finding is ALVAL, the aseptic lymphocyte-dominated vasculitis-associated lesion: perivascular lymphocytic infiltrates, endothelial swelling and tissue necrosis. This pattern distinguishes hypersensitivity from a simple wear-debris reaction and is associated with worse soft-tissue destruction. What that reaction does to the hip, locally and systemically, is set out under Complications.

Three systems are in use, and they grade different things: Hart grades the MARS MRI appearance, the MHRA categories stratify the patient by symptoms and metal ions, and the Anderson classification grades what the surgeon finds at revision.

History. The pain is in the groin, lateral hip or thigh and is often different from arthritic pain, and function declines more than expected after an arthroplasty. Ask about swelling, since a large pseudotumour can be a visible or palpable mass, about clicking or clunking that may indicate subluxation or impingement, and about numbness or weakness from nerve compression. The implant details are critical: the date of surgery and the implant type.

Examination. A Trendelenburg gait points to abductor damage, and a palpable mass sits anterolaterally. Range of motion may be preserved despite significant ALTR. Assess leg length for subsidence or dislocation, examine for sciatic or femoral nerve involvement, and look at the skin, where a sinus or discolouration is rare.

Differential diagnosis. A painful MoM hip is not ALTR until the other causes of pain after arthroplasty have been considered, infection above all.

Metal ions. Measure whole blood cobalt and chromium, not serum: serum underestimates the level by 50%. Cobalt is the more clinically relevant of the two for ALTR; chromium follows a similar pattern. How to act on the number is set out in the next section.

Plain radiographs. An AP pelvis and lateral hip, compared with previous films, for component position (inclination and anteversion, judged against the targets under Pathophysiology), signs of loosening and osteolysis. CT is less useful for soft tissue but shows osteolysis.

MARS MRI. Metal artefact reduction sequence MRI is the gold standard for the soft tissues: it detects fluid collections, pseudotumours, muscle atrophy and osteolysis, and the Hart grade it yields guides management. It should be performed if ions are elevated or symptoms are present.

Ultrasound. The alternative when MRI is unavailable or contraindicated. It is operator dependent but can detect fluid and masses, is less sensitive for muscle atrophy and bone change, and is useful for guided aspiration.

Aspiration. Infection must be ruled out before any revision, and aspiration is essential in every painful MoM hip. Send fluid for microbiology culture with extended incubation, cell count and differential, and synovial alpha-defensin if available.

Read the ions as a pattern. Bearing-surface (articular) wear of a MoM bearing releases both cobalt and chromium, classically in a roughly balanced ratio; chromium is partly retained locally as chromium orthophosphate, but both rise in blood. Mechanically-assisted crevice corrosion at the head-neck taper (trunnionosis) preferentially releases cobalt, so a markedly elevated cobalt with relatively normal chromium, a high cobalt:chromium ratio, points to a taper or modular-junction problem rather than the bearing.

ARMD beyond the MoM bearing. ARMD/ALTR also occurs in metal-on-polyethylene THA with a cobalt-chrome head on a corroding taper, especially with large-diameter heads and certain modular-neck stems: an identical pseudotumour/ALVAL reaction driven by taper corrosion, not the bearing. A painful MoP THA with a rising isolated cobalt should prompt the same ARMD workup of ions, MARS MRI and infection exclusion; "it isn't a MoM bearing" does not exclude an adverse metal reaction.

One hip or two. With a unilateral MoM hip, compare the level to population norms. With bilateral MoM hips the levels are additive, so interpret them with caution.

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

Local soft tissue. The pseudotumour is a sterile inflammatory mass, solid or cystic, and benign: it is not a malignancy. Its histology is ALVAL, and it can cause extensive soft-tissue destruction, including of the abductors.

• Metallosis - black staining of the capsule and soft tissues from deposited metal debris
• Soft-tissue necrosis - destruction of capsule and abductors
• Fluid collections and recurrent effusions

Bone and mechanical.

• Osteolysis - bone resorption around the components
• Aseptic loosening of cup or stem
• Trunnionosis (taper corrosion) in stemmed MoM
• Femoral neck fracture or narrowing in resurfacing
• Instability and dislocation from abductor and capsule loss

Regional. A large pseudotumour can compress the sciatic or femoral nerve, and rarely an expanding mass compresses vessels. Trochanteric bursitis and a Trendelenburg gait disturbance complete the picture.

Systemic. Cardiomyopathy is the most serious, reported at very high cobalt levels. Neuro-ocular toxicity takes the form of cognitive changes and peripheral neuropathy (rare), hearing loss and visual loss; hypothyroidism appears in some case reports; cobalt drives erythropoiesis, producing polycythaemia; and renal metal deposition is of uncertain clinical significance. Complications specific to revision surgery are detailed under Surgical Management.

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):

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.

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