Why I Use Metal on Metal

1. High-Yield Summary

• Historical Rationale & The Dislocation Paradox: Second- and third-generation metal-on-metal (MoM) hip bearings (high-carbon CoCrMo alloy) were adopted in the late 1990s and 2000s to eliminate polyethylene wear-induced periprosthetic osteolysis and reduce dislocation in active patients through large femoral heads (36 mm to > 48 mm) and hip resurfacing. While large diameters successfully lowered primary dislocation rates below 1%, increased frictional torque and modular taper corrosion triggered catastrophic failure rates.
• Tribological Failure & Trunnionosis: Under ideal fluid-film lubrication, MoM bearings achieve low steady-state volumetric wear. However, cup steepness (inclination > 45°), microseparation, and edge-loading transition the interface to high-wear boundary lubrication. In modular stemmed total hip arthroplasty (THA), mechanically assisted crevice corrosion (MACC) and fretting at the head-neck taper junction (trunnionosis) generate massive amounts of soluble cobalt and chromium ions and toxic particulate debris.
• Pathological Sequelae: Particulate metal debris and toxic divalent/trivalent ions provoke adverse local tissue reactions (ALTR), adverse reactions to metal debris (ARMD), aseptic lymphocytic vasculitis-associated lesions (ALVAL), extensive abductor muscle and tendon necrosis, and cystic or solid pseudotumors.
• Diagnostic Benchmarks: Surveillance requires plain standing radiographs to assess cup inclination and anteversion; Multi-Acquisition Variable-Resonance Image Combination (MAVRIC) or Slice Encoding for Metal Artifact Reduction (SEMAC) metal-artifact reduction sequence (MARS) MRI to detect subclinical soft-tissue destruction; and whole blood cobalt (Co) and chromium (Cr) ion quantification, where concentrations exceeding 7 ppb (7 µg/L) or a Co:Cr ratio greater than 2:1 indicate severe wear or trunnion corrosion.
• Modern Clinical Consensus & Revision Strategy: Stemmed large-head MoM THA has been universally abandoned worldwide due to high midterm failure rates. Symptomatic ALTR, expanding pseudotumors, progressive osteolysis, or abductor compromise require prompt revision surgery comprising radical synovectomy, pseudotumor debridement, trunnion inspection, and conversion to ceramic-on-highly cross-linked polyethylene or dual-mobility bearings.

2. Anatomy

The hip joint is a constrained enarthrodial (ball-and-socket) articulation formed by the hemispherical femoral head and the acetabular socket of the pelvis. Normal adult acetabular orientation demonstrates 40° ± 10° of radiographic inclination (abduction) and 15° to 20° of anteversion, providing a stable ceiling for weight-bearing force transmission. In total hip arthroplasty, orientation within this anatomical envelope is critical; excessive cup inclination (> 45° to 50°) or inadequate anteversion reduces the functional contact patch, shifting joint loading to the cup rim and precipitating edge-loading.

The fibrous hip capsule is reinforced by three primary capsuloligamentous structures: the anterior iliofemoral ligament (ligament of Bigelow), the anteroinferior pubofemoral ligament, and the posterior ischiofemoral ligament. Dynamic pelvic stability in single-leg stance depends entirely upon the abductor muscle complex, consisting of the gluteus medius, gluteus minimus, and tensor fasciae latae. The gluteus medius inserts via an anterior lateral tendon and a posterior-superior lateral tendon onto the greater trochanter, while the gluteus minimus inserts onto the anterior trochanteric facet. Because these muscles lie in immediate proximity to the anterosuperior, lateral, and posterior capsules, they represent the primary soft-tissue target destroyed by expanding metallosis and adverse local tissue reactions.

Internervous Planes & Danger Zones

Revision surgery for failed metal-on-metal hip arthroplasty requires precise knowledge of surgical approaches, internervous planes, and regional neurovascular anatomy:

• Posterior (Moore / Southern) Approach: Utilizes no true internervous plane; the gluteus maximus (inferior gluteal nerve) is split along the direction of its fibers, and the short external rotators (piriformis, superior gemellus, obturator internus, inferior gemellus, quadratus femoris) are tagged and detached from their trochanteric insertions. Danger Zone: The sciatic nerve exits the greater sciatic notch beneath the piriformis and descends immediately posterior to the short external rotators and posterior acetabular wall. In metallosis revisions, the sciatic nerve is frequently encased in dense fibrotic scar, displaced by posterior pseudotumors, or adhered directly to the posterior capsule. The nerve must be formally identified, palpated, and protected prior to capsular resection.
• Direct Lateral (Hardinge) Approach: Develops an interval within the abductor muscle mass by splitting the anterior one-third of the gluteus medius and vastus lateralis. Both segments of the split gluteus medius are innervated by the superior gluteal nerve. Danger Zone: The superior gluteal nerve branches into the deep surface of the gluteus medius and minimus approximately 3 to 5 cm proximal to the tip of the greater trochanter. Splitting the abductor muscle belly greater than 3 cm proximal to the trochanteric tip risks denervating the anterior gluteus medius and tensor fasciae latae, resulting in permanent postoperative abductor lurch.
• Direct Anterior (Smith-Petersen) Approach: Exploits the true internervous plane between the tensor fasciae latae (superior gluteal nerve) and the rectus femoris and sartorius (femoral nerve). Danger Zone: The lateral femoral cutaneous nerve (LFCN) crosses the surgical interval superficial to the tensor fasciae latae fascia; the femoral neurovascular bundle lies medial to the rectus femoris and iliopsoas sheath, susceptible to compression from massive anterior pseudotumors tracking into the iliopsoas bursa or direct injury from anterior acetabular retractors.
• Medial Pelvic Danger Zone: The medial acetabular wall separates the joint space from the retroperitoneal pelvic basin. Medial cup migration, medial screw placement, or intrapelvic pseudotumor extension directly threatens the obturator neurovascular bundle, the internal iliac vessels, and the external iliac vein and artery. Blind debridement through medial acetabular wall perforations is strictly contraindicated.

3. Pathogenesis

The failure of modern metal-on-metal hip arthroplasty results from an interplay of tribological breakdown, mechanically assisted crevice corrosion at modular junctions, and a destructive cell-mediated host biological response.

Tribology & Lubrication Regimes: Modern MoM articulations are fabricated from high-carbon (> 0.20% carbon) wrought or cast cobalt-chromium-molybdenum (CoCrMo) alloy (ASTM F75 or ASTM F1537) with dispersed carbide microstructures. Friction and wear between the articulating surfaces adhere to the classic Stribeck curve. Under optimal conditions (clearance 100–200 µm, low surface roughness Ra < 0.01 µm, and sufficient entrainment velocity during steady walking), synovial fluid generates mixed-to-hydrodynamic fluid-film lubrication. In this regime, fluid separation minimizes direct asperity contact, yielding an initial “bed-in” or running-in wear phase (< 1–2 million cycles) followed by a steady-state volumetric wear rate of 0.1 to 1.0 mm³ per year, which is 20- to 100-fold lower than conventional ultra-high-molecular-weight polyethylene.

However, fluid-film lubrication requires strict mathematical congruency and complete fluid entrainment. When the acetabular component is placed at a steep inclination angle (> 45° to 50°) or abnormal anteversion, or when microseparation occurs during the swing phase, the contact patch shifts to the sharp acetabular rim. This phenomenon, termed edge-loading, collapses the fluid film, concentrating multi-gigapascal contact stresses and transitioning the bearing into severe abrasive and adhesive boundary lubrication. The resulting stripe wear exponentially accelerates metallic nanoparticle generation.

Mechanically Assisted Crevice Corrosion (MACC) & Trunnionosis: In modular stemmed total hip arthroplasty, metallic debris originates not only from the bearing surface but predominantly from the modular head-neck Morse taper junction (trunnionosis). The junction between a titanium alloy (Ti-6Al-4V) stem trunnion and a large-diameter CoCrMo femoral head forms a galvanic couple within a microscopic, fluid-filled crevice. Under cyclic gait loads, mechanical micromotion disrupts the protective passive metal oxide layers (TiO2 and Cr2O3), a process termed fretting. Within the constrained crevice, rapid consumption of dissolved oxygen prevents repassivation. Hydrolysis of dissolved metal ions leads to accumulation of hydrogen ions and chloride ions, driving crevice fluid pH below 2.0. This highly acidic, hypoxic microenvironment produces relentless electrochemical dissolution (crevice corrosion). Because large femoral heads (≥ 36 mm) exert a significantly larger frictional torque and bending moment arm across the taper junction, they dramatically magnify taper micromotion and accelerate MACC.

Host Biological Response: ALTR, ARMD, and ALVAL: Wear and corrosion release billions of nanometer-sized (20–50 nm) metallic particles and soluble metal ions (divalent cobalt, Co²⁺; trivalent chromium, Cr³⁺; and hexavalent chromium, Cr⁶⁺). The biological response exhibits two distinct pathways:

• Cytotoxic Macrophage Response: High concentrations of ionic cobalt and chromium exert direct cellular toxicity, inducing mitochondrial oxidative stress, DNA damage, and apoptotic or necrotic cell death in local macrophages, fibroblasts, and osteoblasts. This manifests macroscopically as extensive tissue necrosis, gray/black metallosis staining, and synovial sludge.
• Cell-Mediated Hypersensitivity (ALVAL): Soluble metal ions act as haptens, binding to endogenous tissue proteins to form immunogenic complexes. These complexes are processed by antigen-presenting dendritic cells and presented to T lymphocytes, initiating a Type IV delayed-type hypersensitivity reaction. Histologically, this produces aseptic lymphocytic vasculitis-associated lesions (ALVAL), characterized by dense perivascular lymphocytic cuffing (CD4+ and CD8+ T cells), postcapillary high endothelial venules, fibrinoid synovial surface necrosis, and a marked paucity of neutrophils.
• Pseudotumor Formation & Soft-Tissue Necrosis: The combination of persistent ALVAL and high-volume tissue necrosis results in pseudotumors (aseptic solid, semi-solid, or cystic inflammatory masses). These collections are not true neoplasms; they represent granulomatous and fluid-filled lesions that expand aggressively through paths of least resistance, eroding the joint capsule, invading the iliopsoas and trochanteric bursae, and directly necrotizing the gluteus medius and minimus abductor tendon insertions.

4. Natural History

To understand the natural history of metal-on-metal arthroplasty, one must evaluate its historical evolution, initial clinical triumphs, and subsequent global regulatory withdrawal.

Historical Evolution: Early first-generation MoM hip implants developed in the 1960s (e.g., McKee-Farrar, Ring) exhibited variable manufacturing tolerances, equatorial binding, and elevated loosening rates, leading to their abandonment in favor of Charnley’s low-friction metal-on-polyethylene (MoP) arthroplasty. However, as conventional ultra-high-molecular-weight polyethylene (UHMWPE) wear debris caused widespread periprosthetic osteolysis and aseptic loosening in young active patients, interest in MoM was rekindled. In the late 1990s, improved metallurgy and precision engineering yielded second-generation MoM implants (e.g., Metasul) and modern hip resurfacing arthroplasty (HRA) designs, such as the Birmingham Hip Resurfacing (BHR). HRA demonstrated that anatomical femoral head sizes offered exceptional stability and eliminated the risk of wear-induced osteolysis.

Emboldened by resurfacing success, the orthopaedic reconstructive community sought to apply the benefits of large femoral head diameters (≥ 36 mm, 40 mm, 44 mm, and larger) to stemmed modular total hip arthroplasty. Large heads markedly increased the “jump distance” (the vertical displacement required for the femoral head center to clear the acetabular rim) and substantially widened the impingement-free range of motion. Early clinical series confirmed that large-diameter MoM THA virtually eradicated early postoperative dislocation (0% to 0.5% dislocation rates). Between 2003 and 2008, large-head MoM captured more than one-third of the primary total hip arthroplasty market in the United States and worldwide.

Catastrophic Registry Failure & Regulatory Recalls: By 2008 to 2010, worldwide joint registries began identifying an alarming divergence in survivorship. The National Joint Registry (NJR) of England and Wales, the Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR), and European registries documented unacceptably high midterm revision rates for stemmed MoM total hips. Specifically, the NJR revealed 5-year failure rates of 6.2% for stemmed MoM THA compared to 1.7% for conventional metal-on-polyethylene, with failure rates rising steeply with increasing femoral head size (hazard ratio 1.02 per 1 mm increase in head diameter). Certain designs, most notably the DePuy ASR (Articular Surface Replacement) XL acetabular system, featured a sub-hemispheric arc of cover (160° vs. standard 180°), which led to catastrophic rim edge-loading, severe metallosis, and 5-year revision rates exceeding 12% to 40%.

In August 2010, DePuy issued a worldwide voluntary recall of the ASR system. In 2010 and 2012, the United Kingdom Medicines and Healthcare products Regulatory Agency (MHRA) issued urgent Medical Device Alerts mandating lifelong clinical, biochemical, and cross-sectional imaging surveillance for all patients with MoM implants. The United States Food and Drug Administration (FDA) followed with safety communications, expert advisory panel reviews, and proposed reclassification of MoM total hip systems to Class III (premarket approval). Today, stemmed large-diameter MoM THA is globally recognized as an obsolete, failed technological iteration.

Unmonitored Disease Progression: If left untreated in a failing MoM articulation, continuing edge wear and taper corrosion produce relentless destruction of periprosthetic soft tissue and bone. The biological cascade leads to complete detachment and liquefactive necrosis of the abductor tendon complex (gluteus medius and minimus), massive pelvic and femoral osteolysis, pathologic periprosthetic fractures, and permanent neuromuscular impairment.

5. Clinical Presentation

History

Patients typically present after an asymptomatic “honeymoon” period lasting 2 to 7 years following index arthroplasty. Key historical features include:

• Pain Characteristics: Insidious onset of dull, aching groin discomfort, anterior thigh pain, or lateral trochanteric tenderness. Discomfort is aggravated by weight-bearing, active hip flexion, and rising from a seated position.
• Mechanical Symptoms: Audible clicking, popping, grinding, or a deep “clunking” sensation during terminal gait phases. Unlike the benign squeaking occasionally heard in ceramic-on-ceramic bearings, clunking in MoM often reflects bearing microseparation, subluxation, or component loosening.
• Compressive Neurovascular Symptoms: Rapidly expanding pseudotumors can cause compressive neuropathy. Anterior iliopsoas pseudotumors compress the femoral nerve, producing quadriceps weakness and anterior thigh numbness. Posterior pseudotumors compress the sciatic nerve, causing radicular pain, foot drop, or dysesthesias. Extrapelvic venous compression of the femoral or iliac vein can present as unilateral lower-extremity edema.
• The Silent Failure Phenomenon: Up to 30% of patients with extensive ALTR, large fluid collections, and partial abductor muscle necrosis are completely asymptomatic. Absence of pain does not guarantee an intact bearing or an uncomplicated articulation; asymptomatic patients with high-risk implants require rigorous proactive surveillance.

Physical Exam

A systematic physical examination must be performed on every patient presenting with a metal-on-metal hip:

• Trendelenburg Stance & Gait Assessment: Evaluates abductor integrity. A positive Trendelenburg sign (pelvic drop of the unsupported limb during single-leg stance) or an overt abductor lurch indicates functional insufficiency or structural disruption of the gluteus medius/minimus complex [Sensitivity 75%, Specificity 82%].
• Palpation for Pseudotumor / Soft-Tissue Mass: Careful deep palpation of the anterior groin (femoral triangle), lateral peritrochanteric region, and posterior buttock. A firm, fixed, or fluctuant palpable mass represents a large pseudotumor extending outside the capsule [Sensitivity 40%, Specificity 90%].
• Resisted Hip Abduction Test: The patient lies in lateral decubitus and actively abducts against manual resistance. Inability to hold resistance or sharp lateral trochanteric pain indicates acute abductor tendon tearing or avulsion [Sensitivity 76%, Specificity 84%].
• Passive Range of Motion & Provocative Stress (FADIR): Passive flexion to 90°, adduction, and internal rotation reproduces deep anterior groin pain, indicating active synovitis, capsular distension, or anterior impingement [Sensitivity 88%, Specificity 55%].
• Log-Roll Test: Passive internal and external rotation of the supine extended limb demonstrates deep joint irritability and active inflammatory synovitis [Sensitivity 65%, Specificity 74%].
• Neurovascular Examination: Motor testing of hip flexors (L2), quadriceps (L3–L4), tibialis anterior (L4), extensor hallucis longus (L5), and gastrocsoleus (S1); sensory mapping over the LFCN, femoral, saphenous, peroneal, and tibial distributions; and assessment of distal peripheral pulses.

6. Imaging and Diagnostic Studies

Evaluation of metal-on-metal hip articulations mandates a multi-tiered diagnostic protocol integrating plain radiographs, cross-sectional metal-suppressed magnetic resonance imaging, laboratory metal ion levels, and joint aspiration.

• Plain Radiographs:
  • Standard Views: Standing anteroposterior (AP) pelvis, dedicated AP hip, and true cross-table lateral (or 40° Dunn lateral).
  • Acetabular Cup Inclination: Measured between the interteardrop line and the face of the acetabular component. Inclination exceeding 45° (and especially > 50° to 55°) markedly increases contact stress and edge-loading.
  • Acetabular Anteversion: Calculated using the Pradhan or Widmer method on AP radiographs, or measured directly on cross-table lateral views (normal 15°–20°). Retroversion or excessive anteversion predisposes to edge wear.
  • Periprosthetic Lucencies & Osteolysis: Evaluated across DeLee and Charnley zones (I–III) on the acetabulum and Gruen zones (1–7) on the femur. Progressive radiolucency > 2 mm, bone scalloping, or calcar resorption indicates severe wear-debris osteolysis.
  • The “Bubble Sign” & “Cloud Sign”: Subtle curvi-linear or amorphous radiopaque metallic densities drifting within the periarticular soft tissues, pathognomonic for gross metallosis.
• Cross-Sectional Imaging (MARS-MRI):
  • Standard MRI is non-diagnostic due to severe susceptibility artifacts generated by metallic implants. Multi-Acquisition Variable-Resonance Image Combination (MAVRIC) and Slice Encoding for Metal Artifact Reduction (SEMAC) sequences are the gold standard for soft-tissue evaluation.
  • MARS-MRI visualizes capsular synovial hypertrophy, capsular distension (> 3 mm thickness is abnormal), cystic or solid pseudotumor morphology, periprosthetic fluid volume, and neurovascular compression.
  • Critical assessment of the abductor tendon complex: Identifies gluteus medius/minimus partial or complete detachment, tendon retraction, and muscle belly fatty degeneration (graded according to Goutallier criteria).
• Blood Metal Ion Quantification (Co and Cr):
  • Whole blood or serum cobalt and chromium concentrations are quantified using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) collected in specialized royal-blue trace-element collection tubes.
  • Threshold of Concern: Worldwide consensus (AAHKS, AAOS, The Hip Society, and UK MHRA) establishes 7 parts per billion (ppb; equivalent to 7 µg/L or 119 nmol/L for Co and 134.5 nmol/L for Cr) as the threshold indicating abnormal bearing wear or severe taper corrosion. In bilateral MoM implants, the threshold rises to approximately 10 to 12 ppb.
  • Cobalt-to-Chromium (Co:Cr) Ratio: Pure bearing surface wear generates cobalt and chromium in roughly equimolar or proportional amounts (Co:Cr ratio near 1:1). Conversely, mechanically assisted crevice corrosion at modular trunnions releases preferentially soluble cobalt ions into systemic circulation while chromium precipitates locally as insoluble chromium orthophosphate. Thus, a Co:Cr ratio > 2:1 to 3:1 strongly implicates trunnionosis rather than bearing surface failure.
• Joint Aspiration & Infection Exclusion:
  • Aspiration under sterile fluoroscopic or ultrasound guidance is mandatory prior to revision surgery to differentiate ALTR from periprosthetic joint infection (PJI).
  • Synovial Fluid Characteristics: Fluid in ALTR is characteristically discolored, ranging from murky yellow to gray, brownish, or opaque “engine oil” metallosis fluid.
  • Analytical Pitfalls: Metallic particulate debris interferes with automated Coulter cell counters, generating falsely elevated leukocyte counts. Synovial fluid total nucleated cell count (TNCC) must be performed manually using a hemocytometer. A true neutrophil differential > 70% to 80% suggests concurrent infection, whereas ALTR characteristically shows a predominance of mononuclear cells and histiocytes. Alpha-defensin assays and leukocyte esterase test strips can yield false-positive results in the presence of heavy metallic debris.
  • Synovial cultures (aerobic, anaerobic, and fungal) must be incubated for a full 14 days to identify low-virulence pathogens such as Cutibacterium acnes.

7. Classification

Two validated classification systems govern the evaluation and staging of metal-on-metal adverse reactions: the Hart classification for cross-sectional MRI pseudotumors and the ALVAL histological scoring system.

8. Treatment

Medical / Surveillance

Nonoperative management in metal-on-metal hip arthroplasty does not reverse local tissue destruction; it is strictly an algorithm of risk-stratified surveillance based on the consensus guidelines of the American Association of Hip and Knee Surgeons (AAHKS), American Academy of Orthopaedic Surgeons (AAOS), and The Hip Society (Kwon et al., 2014):

• Low-Risk Stratum: Asymptomatic patients with well-positioned components (cup inclination < 45°, normal anteversion), whole blood cobalt and chromium < 7 ppb, and no evidence of fluid collection on baseline imaging. Management consists of annual comprehensive clinical evaluation and serial plain radiography.
• Moderate-Risk Stratum: Asymptomatic patients with elevated metal ion concentrations (> 7 ppb) OR sub-optimal component alignment (inclination > 45°) OR small thin-walled cystic fluid collections (Hart Type 1) without muscle invasion. Management mandates cross-sectional MARS-MRI and repeat blood metal ion testing every 6 to 12 months. Any progressive enlargement of fluid collections or rising ion velocity mandates operative intervention.
• High-Risk Stratum: All symptomatic patients with pain, mechanical clunking, or abductor weakness, regardless of ion levels; OR asymptomatic patients possessing thick-walled or solid pseudotumors (Hart Type 2 or 3); OR patients exhibiting metal ions > 15 to 20 ppb; OR documented abductor tendon disruption. Nonoperative observation is contraindicated; prompt surgical revision is indicated.

Operative Strategy

The primary goals of revision surgery for failed MoM arthroplasty are: (1) radical debridement of necrotic tissues and pseudotumors, (2) complete eradication of all active metallic wear and corrosion interfaces, (3) restoration of joint biomechanics, and (4) preservation or reconstruction of abductor stability.

• Exposure & Radical Synovectomy: A generous posterior or anterolateral approach is utilized. Pseudotumor pseudocapsules and necrotic tissues must be excised completely while meticulously neurolysing the sciatic nerve and safeguarding the femoral neurovascular bundle. The joint capsule is typically thickened, gray-black, and friable; a complete 360° capsulectomy and synovectomy is performed to clear metallic debris and reduce local inflammatory burden.
• Evaluation and Management of the Femoral Trunnion:
  • Following removal of the metal femoral head, the stem trunnion must be inspected under magnification and cleared of debris. Trunnion damage is graded according to the modified Goldberg scale (Grade 1: minimal coloration; Grade 2: mild discoloration/fretting; Grade 3: moderate material loss; Grade 4: severe loss of taper ridges and structural asymmetry).
  • Pristine or Mild Taper Damage (Goldberg Grade 1): If the femoral stem is well-fixed and the taper exhibits no structural loss, a titanium adaptor sleeve (e.g., Biolox Option) paired with a modern ceramic femoral head may be utilized. Placing a new ceramic or metal head directly onto a corroded trunnion without a sleeve invites catastrophic ceramic fracture or recurrent taper failure.
  • Moderate to Severe Taper Damage (Goldberg Grade 2–4) or Loose Stem: The femoral stem must be explanted. Extended trochanteric osteotomy (ETO) is frequently employed to safely remove well-fixed cylindrical or tapered stems without fracturing fragile, osteolytic proximal femurs. Reconstruction is performed using a fluted, tapered modular or monoblock revision titanium stem.
• Acetabular Revision & Bearing Selection:
  • Monoblock metal-on-metal cups (e.g., DePuy ASR, Zimmer Durom, Wright Conserve) must be completely explanted. Explantation is executed using curved acetabular extraction blades (e.g., Explant system) to preserve native host pelvic bone.
  • For modular cups with well-fixed titanium shells where only the metal liner failed (e.g., Pinnacle Ultamet), the shell may be retained only if perfectly positioned (inclination < 45°, anteversion 15°–20°), pristine locking mechanism, and no retroacetabular osteolysis. A highly cross-linked polyethylene liner is inserted.
  • Reconstruction Modality: The acetabulum is reconstructed using a porous titanium or trabecular metal shell augmented with multi-hole dome screw fixation. In cases of extensive abductor muscle deficiency, dual-mobility acetabular constructs (either modular dual-mobility liners or cemented dual-mobility shells into metal augment/cage constructs) represent the standard of care to counter post-revision instability.
• Abductor Mechanism Repair: Partial tears of the gluteus medius or minimus are repaired directly to the greater trochanter using transosseous non-absorbable heavy sutures or suture anchors. In catastrophic cases with complete abductor avulsion or muscle liquefaction, options include gluteus maximus / tensor fasciae latae muscle transfers or synthetic mesh augmentation, though functional recovery remains guarded.

Indications & Contraindications

Definitive Indications for Revision: Persistent hip pain or functional decline matching ALTR; cross-sectional imaging demonstrating Hart Type 2 or 3 pseudotumors; progressive abductor tendon tearing or fatty degeneration; whole blood cobalt or chromium > 7 ppb with progressive soft-tissue abnormalities; progressive periprosthetic osteolysis; component loosening or mechanical failure.
Contraindications to Surveillance (Immediate Surgery Required): Impending neurovascular compromise from mass effect; spontaneous abductor avulsion; acute prosthetic dislocation; progressive pelvic osteolysis threatening column continuity.
Contraindications to Isolated Bearing Exchange: Severe trunnion damage (Goldberg Grade ≥ 2); acetabular shell malposition (inclination > 45° or version outside 10°–25°); shell loosening; extensive retroacetabular osteolysis.

9. Outcome

The clinical outcomes of revision surgery for metal-on-metal failure are substantially inferior to those of routine aseptic revision total hip arthroplasty, primarily driven by underlying soft-tissue and abductor destruction.

• Survivorship & Re-Revision Rates: Midterm survivorship free of re-revision following revision of stemmed MoM THA ranges between 75% and 85% at 5 years. Multicenter registry data demonstrate that MoM revisions carry a 2- to 3-fold higher hazard ratio for subsequent re-revision compared to revisions performed for conventional polyethylene wear or aseptic loosening.
• Instability & Dislocation: Dislocation is historically the most frequent complication following MoM revision, occurring in 8% to 15% of standard single-bearing reconstructions due to extensive capsulectomy and abductor muscle loss. The adoption of dual-mobility bearings has transformed this outcome, reducing post-revision dislocation rates to below 2% to 3%.
• Postoperative Metal Ion Clearance: Following complete explantation of MoM bearings and corroded trunnions, circulating serum and whole blood cobalt levels decline rapidly, exhibiting an initial clearance half-life of 1 to 2 weeks, with over 80% to 90% cleared within 3 to 6 months. Chromium clearance is significantly slower due to cellular intracellular trapping and tissue binding, with modest residual elevation persisting up to 1 to 2 years postoperatively. Failure of metal ions to drop post-revision indicates either retained metallosis debris, hidden corrosion at another modular junction, or renal impairment.
• Functional Outcomes & Patient-Reported Measures: Harris Hip Scores (HHS) and Hip Disability and Osteoarthritis Outcome Scores (HOOS) show meaningful postoperative improvements from preoperative baselines. However, patients with severe preoperative abductor necrosis and high ALVAL histological scores achieve significantly lower final functional scores, with up to 25% exhibiting a persistent Trendelenburg limp.

10. Complications

11. Pearls and Pitfalls

• The Automated Cell Count Trap: Never trust an automated laboratory Coulter cell count on synovial fluid aspirated from a metal-on-metal hip. Microscopic metal nanoparticles are registered by automated counters as leukocytes, yielding falsely elevated total white cell counts exceeding 50,000 cells/µL. Mandatory Pearl: Always order a manual hemocytometer cell count with visual differential. A true infection exhibits > 70% polymorphonuclear neutrophils, whereas ALTR reveals predominant mononuclear histiocytes and necrotic debris.
• The Corroded Trunnion Pitfall: Never impact a new ceramic or cobalt-chromium femoral head directly onto a retained, previously corroded femoral stem trunnion. Even microscopic fretting damage distorts taper contact geometry, concentrating hoop stresses that shatter brittle ceramic heads or accelerate galvanic corrosion with metallic heads. Mandatory Pearl: If the stem is well-fixed and taper damage is minimal (Goldberg Grade 1), use a manufacturer-approved titanium adaptor sleeve (e.g., Biolox Option). If structural taper loss or asymmetry is visible, the stem must be explanted.
• Abductor-Driven Constraint Selection: Do not rely on standard large-diameter single bearings during MoM revision. The abductor musculature is compromised in over 60% of these hips, leaving the joint biomechanically unstable. Mandatory Pearl: Have a low threshold to utilize a dual-mobility construct or constrained liner when abductor tears involve > 50% of the gluteus medius insertion.
• Pelvic Pseudotumor Dissection Rule: When dissecting pseudotumors extending anteriorly into the iliopsoas bursa or medially through acetabular floor defects, never dissect bluntly outside the pseudocapsule. Stay strictly intraluminal within the pseudotumor cavity, curetting and debriding the necrotic lining from within to protect the adjacent femoral vessels, obturator nerve, and external iliac system.
• The Co:Cr Ratio Pearl: When evaluating a patient with a modular stemmed total hip replacement, pay close attention to the ratio of cobalt to chromium. In pure bearing surface wear, both ions rise in parallel. When cobalt is markedly elevated (e.g., Co 18 ppb) while chromium is relatively low (e.g., Cr 3 ppb), resulting in a Co:Cr ratio > 3:1, the primary culprit is trunnion taper crevice corrosion.

12. Controversy

While stemmed large-diameter metal-on-metal total hip arthroplasty has been universally condemned and discarded, significant debate persists regarding the modern role of hip resurfacing arthroplasty (HRA) and the management of systemic cobaltism.

Hip Resurfacing Arthroplasty: A Specific Niche vs. Complete Abandonment: International registries and long-term cohort studies demonstrate a stark dichotomy between stemmed MoM THA and monoblock hip resurfacing. Stemmed MoM THA failed due to the additive pathology of trunnion taper corrosion and sub-hemispheric cup geometry. In contrast, when modern hip resurfacing (such as the Birmingham Hip Resurfacing) is performed by high-volume surgeons in strictly selected candidates, long-term survivorship exceeds 90% to 95% at 15 to 20 years. Current international consensus restricts hip resurfacing to: (1) young (< 50–55 years), active male patients, (2) diagnosis of primary osteoarthritis, (3) femoral head diameter ≥ 50 mm, and (4) normal proximal femoral anatomy. Conversely, resurfacing is strictly contraindicated in females (who exhibit 3- to 4-fold higher failure rates due to smaller head sizes, edge-loading, and higher rates of metal hypersensitivity), small anatomy (< 48 mm), renal insufficiency, hip dysplasia, or proximal femoral cystic changes. Proponents argue that HRA preserves femoral bone stock, facilitates high-impact athletic return, and yields near-zero dislocation. Opponents maintain that modern ceramic-on-highly cross-linked polyethylene THA matches or exceeds HRA survivorship without subjecting patients to lifelong metal ion exposure.

Systemic Cobaltism & Carcinogenic Risk: Massive release of ionic cobalt (blood levels exceeding 50 to 100+ ppb, often secondary to fractured ceramic heads revised to cobalt-chromium heads, or catastrophic trunnion wear) can induce systemic cobalt toxicity. Clinical manifestations include sensory-neural hearing loss, optic atrophy, cognitive decline, peripheral sensorimotor neuropathy, severe dilated cardiomyopathy, and primary hypothyroidism. Although rare in standard MoM articulations, subclinical cardiac dysfunction (assessed by echocardiographic strain imaging) has been documented in cohorts with chronic ion elevations > 10 ppb. Furthermore, while extensive national registry linkage studies have found no overall increased incidence of systemic cancer or hematologic malignancy in MoM patients compared to age-matched populations, persistent uncertainty remains regarding long-term local sarcomatous transformation, with isolated case reports describing high-grade osteosarcoma and malignant fibrous histiocytoma developing within chronic metallosis capsules.

13. Evidence-Based Synthesis

Contemporary clinical guidelines and surgical decision-making in metal-on-metal hip arthroplasty are established upon seminal multicenter national registry analyses, inter-society consensus risk-stratification algorithms, and pivotal long-term cohort studies.

14. References

• Smith AJ, Dieppe P, Vernon K, Porter M, Blom AW; National Joint Registry of England and Wales. Failure rates of stemmed metal-on-metal hip replacements: analysis of data from the National Joint Registry of England and Wales. Lancet. 2012;379(9822):1199-1204. doi:10.1016/S0140-6736(12)60353-5 [Level II: Prognostic Multicenter Registry Cohort Study]
• Kwon YM, Lombardi AV, Jacobs JJ, Fehring TK, Lewis CG, Cabanela ME. Risk stratification algorithm for management of patients with metal-on-metal hip arthroplasty: consensus statement of the American Association of Hip and Knee Surgeons, the American Academy of Orthopaedic Surgeons, and the Hip Society. J Bone Joint Surg Am. 2014;96(1):e4. doi:10.2106/JBJS.M.00160 [Level V: Clinical Practice Consensus Guideline]
• Lombardi AV Jr, Berend KR, Adams JB, Sesselmann S. Large-diameter metal-on-metal total hip arthroplasty: dislocation infrequent but survivorship poor. Clin Orthop Relat Res. 2015;473(2):474-481. doi:10.1007/s11999-014-3976-0 [Level IV: Therapeutic Case Series]
• Lombardi AV Jr, Mallory TH, Alexiades MM, et al. Metal-on-metal total hip arthroplasty with large heads may prevent early dislocation. Clin Orthop Relat Res. 2005;(441):106-112. doi:10.1097/01.blo.0000193810.23706.73 [Level IV: Therapeutic Case Series]
• Pandit H, Glyn-Jones S, McLardy-Smith P, et al. Pseudotumours associated with metal-on-metal hip resurfacings. J Bone Joint Surg Br. 2008;90(7):847-851. doi:10.1302/0301-620X.90B7.20213 [Level IV: Prognostic Case Series]

15. Master Class Pro-Tip

“When tackling a revision for metal-on-metal failure or severe trunnionosis, the most critical error is underestimating the degree of abductor devastation hidden beneath intact deep fascia. Do not commit to a standard bearing reconstruction until you have directly palpated and visualized the gluteus medius and minimus insertions onto the greater trochanter. If more than 50% of the abductor tendon complex has undergone liquefactive necrosis or detachment, implanting a standard large-head single bearing invites recurrent dislocation. In such scenarios, prepare immediately for a dual-mobility acetabular construct. Furthermore, never seat a new ceramic or metal head directly onto a scarred or corroded stem trunnion; if retaining a well-fixed stem with mild surface fretting, a titanium transition sleeve is non-negotiable to protect against catastrophic ceramic fracture or persistent galvanic crevice corrosion.”

Master Class Surgical Pearl: Revision of Metal-on-Metal Arthroplasty & Trunnionosis