Trunnionosis and Taper Corrosion

Trunnionosis is mechanically-assisted crevice corrosion (MACC) at the modular head-neck taper junction of a total hip arthroplasty. The metal debris it releases provokes an adverse local tissue reaction (ALTR). It is distinct from bearing-surface wear in metal-on-metal articulations.

How common. Trunnionosis is reported in 2-4% of modular THA overall, with higher rates, up to 10%, in specific designs. Joint registry data show elevated revision rates for certain stem-head combinations.

How it came to light. Modularity was introduced in the 1980s for surgical flexibility, and attention first fell on bearing-surface failures in metal-on-metal hips. Taper corrosion was recognised as a distinct entity around 2010, recognition increased dramatically after that, and MHRA alerts and recalls highlighted the problem. It now affects metal-on-polyethylene and ceramic-on-polyethylene combinations too.

Implant factors. Head size is the key modifiable risk factor, and combined with a titanium stem the risk is multiplicative.

High-offset (lateral offset) stems increase torsional loads as well as the bending moment.

Taper geometry. Angular mismatch between head and trunnion shifts contact toward one end of the taper, increasing local stress and micromotion instead of sharing load along the interface. Distal and proximal engagement also place the contact patch at different moment arms from the head centre, one reason offset, head size and mismatch amplify bending and fretting.

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Patient factors. Male sex may carry a higher risk, attributed to higher activity levels and the larger heads used in men. The other patient risk factors:

• Younger, more active patients
• Higher BMI (increased joint loads)
• Bilateral THA (cumulative exposure)
• A history of metal hypersensitivity

Activity. High-demand activities increase the number of loading cycles and impact sports amplify the peak loads. A greater range of motion increases taper micromotion.

Assembly technique. How the taper is assembled matters:

• A dry taper is essential; blood or fluid reduces the cold weld
• Adequate impaction force; studies suggest 4-8 kN
• Clean taper surfaces; debris interferes with seating
• Correct orientation, avoiding toggle during impaction

Component position. Excessive femoral anteversion may increase torsional loads, and combined version affects the loading pattern.

The table applies these factors to common clinical scenarios.

The cascade. Fretting disrupts the protective oxide layer, and the crevice environment prevents the surface from repassivating. Metal ions are released, causing local tissue toxicity (ALTR) and possibly systemic toxicity.

Taper MechanicsCorrosion MechanismsTissue Response

The Morse taper. The head is held on the trunnion by an interference fit, created by cold welding, and the impaction force is critical to that initial fixation. The taper angle varies between manufacturers, 5°44' being common, and taper length affects the contact area and stability.

Loads on the junction. Three kinds of load reach the taper in service:

• Axial loads from body weight
• Torsional loads from gait (internal and external rotation)
• Bending moments from offset and neck length

Why Large Heads Increase Risk

Moment = Force × Distance. Larger heads increase the lever arm from hip centre to taper junction. This amplifies bending moments and torsional stresses at the taper, increasing fretting micromotion. A 36mm head generates approximately 30% more moment than a 28mm head.

Fretting corrosion. Cyclic micromotion of less than 100 microns between the surfaces disrupts the protective chromium oxide passivation layer and exposes the reactive base metal. Each cycle releases metal particles.

Crevice corrosion. The taper junction creates an oxygen-depleted environment in which a differential aeration cell forms. The local pH drops, the crevice becomes acidic and metal dissolution accelerates. Fretting and crevice corrosion together are MACC, the pathomechanism of trunnionosis.

Galvanic corrosion. Dissimilar metals create an electrochemical potential, so a cobalt-chromium head on a titanium stem is a galvanic couple. Titanium acts as the cathode and is protected; cobalt-chromium acts as the anode and corrodes preferentially.

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Adverse local tissue reaction. Released metal ions and particles activate macrophages and draw in lymphocytes, with a type IV hypersensitivity reaction in some patients. Cytokines (IL-6, TNF-alpha) are released, and the end result is extensive tissue necrosis, pseudotumour formation, abductor destruction and bone loss.

ALVAL. The aseptic lymphocyte-dominated vasculitis-associated lesion is the histological pattern of metal hypersensitivity: perivascular lymphocytic infiltrates with fibrin exudation and tissue necrosis. Its clinical correlation is variable.

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Local symptoms. Patients may present with groin pain, which may be insidious in onset, or hip pain with activity. Patients may also describe clicking, squeaking or grinding, or swelling from a pseudotumour. Abductor destruction causes progressive weakness; patients may also report a sense of instability or recurrent dislocation.

Systemic symptoms. The patient may report fatigue and malaise, cognitive change (memory and concentration), peripheral neuropathy, visual or hearing change, palpitations or dyspnoea from cardiomyopathy, symptoms of hypothyroidism, or depression.

Clinical stages. The disease runs from silent ion release to systemic toxicity.

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Ions screen; imaging decides. Cobalt and chromium levels guide management, but imaging (MARS MRI) determines the extent of tissue damage and drives the surgical decision.

Metal ions. Take whole-blood samples; serum alone is not enough.

Reading the result. A Co:Cr ratio greater than 2:1 suggests taper corrosion rather than bearing wear. Levels can fluctuate, so the trend is important.

MHRA guidance. The UK guidance, widely adopted, treats Co or Cr greater than 7 ppb as a concern requiring imaging and recommends annual surveillance for at-risk implants.

MARS MRI. Metal artefact reduction sequence MRI is the gold-standard imaging for ALTR. It reduces metal artefact enough to show the soft tissues, detects pseudotumours, fluid collections and muscle atrophy, and grades the severity of tissue destruction. The report should cover:

• Pseudotumour: cystic or solid mass, which may be large
• Fluid collections around the prosthesis
• Abductor status: atrophy, fatty infiltration, detachment
• Bone involvement: osteolysis, cortical erosion
• Muscle necrosis: signal change indicating tissue death

The Anderson classification grades pseudotumours, and higher types correlate with worse tissue damage:

• Type 1: thin-walled cystic
• Type 2a: thick-walled cystic, atypical contents
• Type 2b: thick-walled, mixed solid and cystic
• Type 3: predominantly solid

A pseudotumour is not in itself an indication for surgery. In Hart's case-control study (Evidence Base) pseudotumours were common even in well-functioning metal-on-metal hips, with no significant difference in prevalence from painful ones, so the decision integrates symptoms, ion trend and lesion type.

Other imaging. The other modalities are summarised below.

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Aspiration. Aspirate to rule out infection, which is essential before revision; it also relieves symptoms from a large effusion and provides fluid for analysis. Characteristic ALTR fluid is dark grey or black, metallic-stained and often sterile. Send it for:

• Cell count and differential
• Gram stain and culture
• Metal ion levels in fluid (research use)
• Crystal analysis

Metal ions with MARS MRI distinguish trunnionosis from the other causes of a painful THA.

Surveillance. Surveillance without immediate revision suits:

• The asymptomatic patient with an at-risk implant
• Low-grade metal ion elevation (Co less than 7 ppb)
• No significant ALTR on MARS MRI
• Well-fixed, well-positioned components

The protocol is annual clinical review and annual metal ions (Co and Cr), with MARS MRI every 1-2 years if the ions are elevated, and patient education on the warning symptoms. Upgrade to revision on symptomatic progression, a rising metal ion trend, ALTR progression on MRI or the development of systemic symptoms.

Absolute indications for revision

• Symptomatic ALTR with tissue destruction
• Progressive pseudotumour
• Systemic cobalt toxicity (cobaltism)
• Component loosening
• Recurrent dislocation due to abductor failure

Relative indications

• Significantly elevated metal ions (Co greater than 10-15 ppb)
• Asymptomatic but enlarging pseudotumour
• Progressive bone loss
• Patient anxiety affecting quality of life

Surgery is favoured in a younger, active patient with good bone stock, abductors that are still salvageable and minimal systemic symptoms.

Planning the revision. Revision is complex, and the preparation reflects it:

• MARS MRI to map tissue destruction
• CT for bone stock assessment
• Cardiac evaluation if cobalt is very elevated
• Infection work-up (aspiration, bloods)
• Blood products available; the surgery is often bloody

Approach. An extensile approach is required, one that shows the full extent of the pseudotumour: posterior, with an extended capsulotomy, and a trochanteric osteotomy if needed for access. A direct lateral approach may limit access posteriorly.

Exposure and debridement. The key steps:

• Identify and protect the sciatic nerve, often displaced by the pseudotumour
• Decompress large collections before dislocation, sending fluid for culture to rule out infection
• Evacuate the pseudotumour contents carefully and excise its wall completely; it may extend into the pelvis, so be aware of the vascular structures
• Complete capsulectomy and synovectomy
• Debride all necrotic tissue back to bleeding margins
• Lavage thoroughly to remove debris

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Abductors. Gluteus medius is often damaged or necrotic, so assess how much viable muscle remains. The abductors may need augmentation or reconstruction, and tensor fascia lata advancement is possible. Consider a constrained liner if they are deficient.

Histology. Send tissue for histopathology. The ALVAL score provides prognostic information, and frozen section helps rule out occult infection.

The stem. Remove the corroded head and trunnion adapter, then check the taper condition carefully and document the findings for registry reporting. A well-fixed stem with moderate corrosion can be kept, with a titanium adapter sleeve under a new ceramic head; severe trunnion corrosion or a loose stem needs stem revision. The Goldberg score (below) grades the taper for this decision. With a modular-neck stem, consider modular neck exchange if available.

The cup. Assess its position and fixation, and revise it only if loose or malpositioned. A well-fixed metal-on-metal cup can have its liner exchanged to ceramic-on-polyethylene.

The bearing. Avoid metal-on-metal at all costs; the table sets out the default at each level.

Ceramic heads. A ceramic head produces no metallic corrosion products and is a hard bearing surface with excellent wear characteristics. The caution is fracture, rare with modern ceramics.

After revision for ALTR. Revision for ALTR has higher complication rates than routine revision THA. The dislocation rate reflects abductor deficiency, and the patient should be counselled that constrained components, a brace or assistive devices may be needed long-term.

Prognostic factors. A better prognosis goes with:

• Early diagnosis before extensive tissue damage
• Intact or recoverable abductors
• Good bone stock
• Low metal ion levels
• Complete surgical debridement achieved

A worse prognosis goes with:

• Delayed diagnosis with extensive destruction
• Abductor loss or atrophy
• Significant bone loss
• Very high metal ion levels (cobaltism)
• Systemic toxicity (cardiac, neurological)
• Multiple prior revisions

Global Epidemiology

• Trunnionosis-related ALTR is reported across all major modular THA designs and bearing couples, including conventional metal-on-polyethylene, not only MoM
• True incidence is uncertain because many cases are subclinical; symptomatic revision for ALTR remains a small but rising proportion of all revisions
• Large-head MoM THA (36mm and over) and large-diameter resurfacings concentrated the early failures that drew attention to the modular junction
• Recognition surged after 2010 following retrieval studies and regulatory alerts; conventional MoP taper corrosion is now an established differential for the painful THA worldwide

Side-by-Side Guidance

Registry Evidence

• MoM bearings carry the highest revision rate of any bearing couple across the AOANJRR (Australia), NJR (UK) and AJRR (US)
• Large-head MoM THA performs worse than resurfacing, and ceramic-on-polyethylene / ceramic-on-ceramic show the lowest revision rates
• Registries flagged specific titanium-stem plus large-CoCr-head combinations for surveillance and informed device withdrawals
• Registry data underpins the global shift toward smaller ceramic heads on polyethylene at primary and revision surgery

High- vs Limited-Resource Practice

Related pages: Metal-on-Metal Complications covers the bearing-driven adverse reaction to metal debris that trunnionosis is constantly confused with - the distinction matters because the taper produces ALTR in hips with no metal bearing at all; Corrosion Mechanisms sets out mechanically-assisted crevice corrosion, fretting and galvanic coupling in full, and Cobalt-Chrome Alloys with Titanium Alloys explain why a mixed-alloy couple corrodes more than a matched one; Tribology and Wear and Wear Mechanisms hold the mechanics behind the lever-arm argument, with THA Bearing Surfaces, Ceramics and Polyethylene UHMWPE and XLPE covering the ceramic-head-on-titanium-sleeve construct that is the reference revision; THA Wear and Osteolysis and THA Aseptic Loosening are the particle-driven differentials for the painful arthroplasty, and Periprosthetic Joint Infection with THA Complications: Infection must be excluded before any metal debris diagnosis is made, since ALTR and infection can raise the same inflammatory markers; Revision THA covers the reconstruction, and THA Complications: Dislocation with THA Dual Mobility address the instability that follows abductor destruction and drives re-revision after ARMD surgery.

Metal ion thresholds. The 7 micrograms/L trigger is pragmatic, not absolute. Some patients develop florid ALTR at near-normal ions (hypersensitivity), while others tolerate very high levels, so the trend and the Co:Cr ratio matter more than any single number. Cooper's dual-taper series makes the point: mean serum cobalt was 6.0 ng/mL in hips that all needed revision.

The asymptomatic patient. When to revise an asymptomatic patient with a rising ion level or an enlarging pseudotumour is unresolved. Prophylactic revision must be balanced against the high complication rate of ALTR revision surgery.

Titanium sleeve or stem revision. Retaining a well-fixed stem with a ceramic head and titanium sleeve avoids morbidity but leaves the original, damaged trunnion in situ. The long-term durability of the sleeve construct against full stem revision remains debated.

Chelation and systemic cobaltism. Evidence for chelation therapy in arthroprosthetic cobaltism is weak and largely anecdotal. Source control by revision to cobalt-free implants is the mainstay, and the reversibility of cardiac and neurological injury is variable.

A second junction. In a dual-taper (modular-neck) stem the surgeon selects a separate modular neck, adjusting version, offset and length independently of stem fixation. That adds a neck-body (neck-stem) junction, distinct from the head-neck taper. It sits closer to the fixed stem, carries large cantilever bending moments and is a corrosion site in its own right, the mechanism behind the withdrawal of several dual-taper designs (for example the Rejuvenate and ABG II modular-neck stems).

Why it fails. The classic pairing is a cobalt-chromium modular neck on a titanium-alloy stem body, a mixed-alloy (galvanic) couple. The long lever arm and cyclic bending drive fretting that disrupts passivation, then crevice corrosion in the oxygen-depleted junction: the same MACC process as at the head-neck taper. Because the junction is proximal and load-bearing, failure can occur early, within the first post-operative year, and rarely the modular neck can also fatigue-fracture.

Presentation mirrors head-neck trunnionosis: new groin or thigh pain, effusion, pseudotumour and ALTR on MARS MRI.

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Why grade the taper. Keeping the stem with a titanium sleeve for moderate corrosion, or revising it for severe corrosion, needs an objective way to grade the taper. The reference tool is the Goldberg semiquantitative fretting-and-corrosion score, a four-point visual scale applied to each taper surface (the femoral head bore and the stem trunnion), read by quadrant.

What the retrieval study showed. The landmark study that defined the score also produced the mechanistic evidence behind the risk factors in this topic. Mixed-alloy couples corrode more than matched couples, heads tend to corrode more than necks, and neck flexural rigidity and implantation time predict damage: the mechanical basis for the head-size, offset and titanium-stem warnings.

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