Metallosis is the body's response to tiny particles of metal released by a hip implant. In its strict sense it refers to the dark grey staining of joint tissue that can result; in everyday clinical use it sits inside a broader family of problems known as Adverse Reaction to Metal Debris, or ARMD. The same family includes ALVAL, a lymphocyte-rich inflammatory pattern, and pseudotumour, a fluid-filled or solid mass that can develop in the soft tissues around the joint.
The mechanism is straightforward. Where two metal surfaces articulate against each other inside a hip, or where modular metal junctions corrode, cobalt and chromium ions are released into the local tissues and the bloodstream. In some patients the body mounts an inflammatory reaction; in a small number, blood levels rise high enough to cause symptoms elsewhere in the body. The condition is reported predominantly in metal-on-metal hip replacements and resurfacings, and at much lower rates in some modular ceramic and dual-mobility constructs from taper corrosion.
Most patients with a well-functioning metal-on-metal hip never develop a clinical problem from metallosis. The point of UK surveillance is to identify the small number who do, early, while the tissues are healthy and revision is straightforward. If you have a metal-on-metal hip and you are reading this because something feels different, the practical next step is to check whether your symptoms warrant urgent review, and to confirm that your annual surveillance is up to date.
Every artificial joint releases some particles. In a metal-on-metal bearing, those particles are predominantly cobalt and chromium, and they are extremely small: small enough to be taken up by local cells, transported through the lymphatic system, and to enter the bloodstream. At very low levels this is biologically uneventful. At higher levels, two patterns of problem can emerge.
The first is local. Soft tissues around the joint, particularly the capsule and the abductor muscles, may react with chronic inflammation, fluid accumulation, or in some cases the formation of a pseudotumour. The amount of debris released depends on the bearing diameter, the position of the components, the design of the implant, and the activity of the patient. Smaller bearing diameters and sub-optimally positioned cups tend to produce more wear.
The second is systemic, and uncommon. When cobalt blood levels rise to very high concentrations over a long period, the metal can affect organs distant from the hip: the heart, the thyroid, the nervous system, and occasionally vision or hearing. Cases are rare and almost always associated with severely failing implants, but the possibility is the reason that blood ion monitoring sits at the centre of UK surveillance.
Most metallosis is identified through routine surveillance before symptoms appear. When symptoms do develop, they tend to fall into two groups: those that warrant prompt review, and those that can wait until the next scheduled appointment.
If any of the prompt-review symptoms apply, contact the hospital where your hip surgery was done, or arrange a private review. If you are uncertain which side the symptoms apply to, mention this when you call. Annual surveillance is designed to catch the slower changes; the urgent list above is designed to catch the faster ones.
Diagnosis is a step-by-step process rather than a single test. Each step adds information, and most patients only need the early ones.
A focused conversation about symptoms, an examination of the hip's range of motion and the strength of the abductor muscles, and a check of your walking pattern. Pre-existing scores such as the Oxford Hip Score may be repeated for comparison with earlier visits.
An anteroposterior pelvis and a lateral view of the affected hip allow component position to be checked, and any loosening or osteolysis to be identified. These are part of routine annual review for most patients.
A simple blood test, taken through a plastic cannula to avoid contamination from a metal needle. The UK Medicines and Healthcare products Regulatory Agency (MHRA) sets an action level of 7 µg/L for either ion: above that threshold, the test is repeated at three months and cross-sectional imaging is arranged. The direction of travel matters as much as the absolute number; rising levels in serial tests warrant earlier review even when each individual result is below the threshold.
Conventional MRI is degraded by the metal in an implant. A specialised sequence called MARS-MRI (Metal Artifact Reduction Sequence) reduces this and is the cross-sectional test of choice for soft-tissue assessment around metal-on-metal hips. Ultrasound, performed by a musculoskeletal radiologist, can be a useful alternative. Either modality looks for fluid collections, pseudotumours, and damage to the abductor muscles.
If infection cannot be excluded clinically, a single needle is passed into the hip under ultrasound or fluoroscopic guidance, with local anaesthetic, and a sample of joint fluid is sent for cell count and prolonged culture. The procedure takes around half an hour including preparation, and most patients are sore for a day or two afterwards. The aspirate may be dark grey or blood-stained in metallosis, which itself provides useful information.
What metallosis looks like at the time of revision
Contains an intraoperative clinical photograph. Hidden by default.
At revision, metallosis is recognisable on sight. Published intraoperative descriptions converge on the same vocabulary: a grey-black or tarry discolouration of the joint capsule and pseudocapsule, dull and friable abductor muscles, and where a fluid pseudotumour is present, dark grey or sooty-brown fluid sometimes likened to engine oil. The histology underneath shows necrotic and haemorrhagic tissue with a lymphocyte-rich infiltrate, with metal debris visible inside macrophages.For clinical illustration only. Not representative of any individual patient.
Every patient with a metal-on-metal hip in the UK should be under lifelong follow-up under the framework set by the MHRA in 2017. The framework risk-stratifies implants and patients: higher-risk implants (such as the DePuy ASR family and large-head stemmed metal-on-metal designs) and any symptomatic patient require closer attention. A separate 2025 alert covers Profemur cobalt-chrome modular-neck stems, with around 2,000 UK patients being invited for review.
In practical terms, the schedule normally looks like this:
Surveillance is not a treatment in itself. It is a way of catching change early, while the surrounding tissues are still healthy and any intervention is more straightforward.
Decisions about treatment follow a ladder. Where on the ladder a patient sits depends on symptoms, the trend of blood ions over time, the appearance on cross-sectional imaging, and the type of implant.
Monitor
Asymptomatic, blood ions below 7 µg/L, no progression on imaging. Annual surveillance continues.
Image
Symptoms, blood ions at or above 7 µg/L, rising ion trend even below 7, or component malposition on plain films. MARS-MRI or ultrasound, and a clinical review with a revision-experienced surgeon.
Revise
Progressive symptoms, progressive pseudotumour on serial imaging, persistently elevated ions in a higher-risk implant, abductor damage on MARS-MRI, or any solid pseudotumour. Revision surgery is planned.
The contemporary trend is towards earlier revision than was practised a decade ago. Published series show that outcomes are better when the operation is undertaken before significant soft-tissue or bony damage has developed, and the threshold for offering revision has shifted accordingly.
Revision for metallosis is a different operation from a primary hip replacement. The failed metal bearing is removed, any pseudotumour and necrotic soft tissue is debrided, and the joint is reconstructed with a non-metal bearing, typically a ceramic head on a highly cross-linked polyethylene liner. Where soft-tissue damage is significant, a dual-mobility cup may be used to reduce the risk of dislocation; where there is bone loss, augments or custom components may be required. Recovery is generally a longer process than a first-time replacement, with hospital stay, mobilisation and return to function each taking somewhat longer.
Published series of revision for adverse reaction to metal debris report patient survival in the order of 96% at five years, with a re-revision rate around 7 to 8%. Outcomes are improving over time, an effect attributed to earlier intervention through surveillance.
A full description of the revision pathway, including imaging, anaesthetic, the operation itself and rehabilitation, sits on the main practice site.
Revision Hip Surgery (main site)The story of metallosis is largely the story of metal-on-metal hip bearings. Modern ceramic-on-ceramic resurfacings, such as the ReCerf and the H1, do not release cobalt or chromium at the bearing surface and so cannot give rise to the same problem. For new patients today, the question of metallosis is mainly a question of which bearing is chosen and on what evidence.
Adverse reaction to metal debris is, however, not exclusively a problem of historical metal bearings. A small number of case reports describe similar reactions in ceramic-on-ceramic and dual-mobility constructs from corrosion at modular metal junctions. The implication for practice is not to abandon modern designs, but to recognise that vigilance, surveillance, and a measured approach to any new hip symptom remain appropriate at any stage after any hip replacement.
For a broader explanation of hip resurfacing today, including the role of ceramic bearings, see Hip Resurfacing, or the guide to hip resurfacing complications for how metallosis sits among the operation's other risks.
If you have an existing metal-on-metal hip and would like a private review, or a second opinion on surveillance results, a consultation can be arranged at any of the Birmingham practice locations. Bring your most recent imaging and any prior blood ion results.
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