Metal-on-Metal Implants and the Body Jewellery Lesson
Key Takeaways:
» Metal-on-metal (MoM) hip implants used a cobalt-chromium ball riding in a cobalt-chromium socket, and wear plus corrosion at that bearing released metal ions and particles into surrounding tissue
» The clinical consequence, adverse reaction to metal debris (ARMD), includes soft-tissue damage, pain, implant loosening, pseudotumours, and the need for revision surgery
» Tribocorrosion, the combined action of mechanical wear and electrochemical corrosion, is the mechanism that generated the debris; it applies to any metal rubbing against metal in a biological environment
» In body jewellery, metal-on-metal contact occurs in threaded jewellery connections, hinged pieces, and layered rings, and the same ion-release physics applies at a smaller scale
» Cobalt and chromium are the ions regulators and clinicians monitor; for sensitised individuals, even low-level release from jewellery can drive allergic contact dermatitis
» The implant lesson is why material selection matters: a single-metal or polymer-on-metal interface, and implant-grade alloys, reduce the debris load compared with mismatched metals in motion
1. What Metal-on-Metal Implants Were Supposed to Do
Earlier metal-on-polyethylene designs wore over time, shedding plastic debris that triggered osteolysis, the progressive destruction of bone around the implant.
A MoM hip implant uses a cobalt-chromium-molybdenum alloy ball that articulates directly inside a cobalt-chromium-molybdenum socket, with no polymer liner in between. The bearing surfaces are engineered to be extremely hard and smooth, and the design relies on a thin film of joint fluid to separate the two metal surfaces during normal motion.
Hundreds of thousands of patients received MoM hips worldwide before the failure pattern became undeniable. By the early 2010s, regulators including the FDA, the UK MHRA, and Health Canada had issued alerts, and many MoM systems were withdrawn from the market.
2. The Failure Mechanism: Wear, Corrosion, and Ions
The failures were driven by tribocorrosion: the combined action of mechanical wear and electrochemical corrosion on the same surface. In the body, the cobalt-chromium bearing is immersed in saline joint fluid, a corrosive electrolyte. Every step applies cyclic load that rubs the two metal surfaces together. The rubbing strips the protective oxide film from the metal surface, and the exposed metal corrodes, releasing ions and generating particles. The corrosion products repassivate the surface, only to be stripped again at the next cycle.
This wear-corrosion cycle produced two distinct hazards. The first was metal particles and ions entering the bloodstream: cobalt and chromium were measured at elevated levels in the blood of many MoM patients. The second was a local soft-tissue reaction now called adverse reaction to metal debris, or ARMD. In some patients the body mounted an immune response to the metal debris that produced pain, swelling, fluid collections, and pseudotumours, non-cancerous but destructive masses of inflamed tissue around the joint. The FDA describes the cascade plainly: soft tissue damage may lead to pain, implant loosening, device failure, and the need for revision surgery, in which the old device is removed and replaced.
Regulators responded with monitoring protocols. The UK MHRA issued medical device alerts recommending blood tests for cobalt and chromium and imaging for patients with painful MoM implants. Patients with progressing adverse local tissue reactions were advised to consider earlier revision to prevent extensive damage to bone, muscle, and nerves.
3. What This Means for Body Jewellery
Body jewellery operates at a much smaller scale than a hip implant, but it is the same physics. The earring back rubbing against the post, a hinged ring closing and opening, two stacked rings sliding against each other, a threaded ball seating against the barbell end: every one of those is a metal-on-metal interface in a saline biological environment. Tribocorrosion does not switch off at smaller dimensions; it just generates proportionally smaller debris loads.
For most people, the debris from jewellery-scale contact is negligible and clinically silent. But the implant record matters for two groups. The second is anyone with a healing piercing, where the tissue is already inflamed and more permeable to anything the jewellery releases.
| Contact type | Material pairing | Debris risk | Clinical relevance |
|---|---|---|---|
| Threaded barbell end | Titanium on titanium | Low | Minimal for non-sensitised wearers |
| Hinged ring | Steel on steel | Moderate | Can irritate a healing piercing |
| Stacked rings | Steel on gold | Higher | Galvanic plus wear effects; risk for sensitised skin |
| Implant ball joint | Co-Cr on Co-Cr | High (orthopaedic) | The MoM lesson: avoid metal-on-metal in load-bearing sites |
This is why the professional standard for initial piercings is a single-material implant-grade piece, and why polymer jewellery exists. A polymer or single-metal interface removes the metal-on-metal contact entirely. Our metallurgy guide on 316L versus titanium walks through which alloys leach least, and the titanium versus surgical steel comparison explains the practical differences for daily wear.
4. Why Material Science Matters More Than the Label
The MoM implant episode is a useful corrective because it shows what happens when a material is chosen for hardness and marketed on promises while the electrochemical reality of the body is ignored. The cobalt-chromium alloy in those hips is an excellent metal: hard, corrosion-resistant in isolation, and perfectly fine as a single material. It failed as a bearing because articulation destroyed the surface and corrosion followed.
Body jewellery manufacturers who understand tribocorrosion design around it: they use implant-grade titanium for initial piercings, single-metal construction, and polymer options where flexibility or non-conductive properties matter. They avoid unnecessary metal-on-metal contact in high-friction locations. The materials story is not about avoiding metal; it is about understanding which metal, in which configuration, against which counterface, and for which wearer.
