Imaging Gallery

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Metal-on-Metal Hip Arthroplasty Complications

• Cobalt (Co): ~60-65%. Base metal. Hardness.
• Chromium (Cr): ~27-30%. Passivation (Corrosion resistance).
• Molybdenum (Mo): ~5-7%. Hardness and Grain refinement.
• Nickel (Ni): less than 1% (Trace). Use with caution in severe allergy.
• Carbon: Form carbides (M₂₃C₆). Hard ceramic-like particles that improve wear resistance but reduce ductility.

Process:

1. Cast (ASTM F75): Molten metal poured into mould. Used for complex shapes (e.g., Femoral Condyles). Large grains. Carbides provide wear resistance.
2. Wrought (ASTM F1537): Forged (Hot worked). Used for Femoral Heads. Smaller grains = Stronger and tougher.

• Young's Modulus: 220-240 GPa.
  • Very Stiff.
  • Would cause massive Stress Shielding if used as a femoral stem (hence Ti stems are preferred, or CoCr stems are cemented).
• Fatigue Strength: High.
• Wear Resistance: Superior to Stainless Steel and Ti. Best metal for articulation against Polyethylene.
• Corrosion: Very resistant (Passivation layer - Chromium Oxide).

Cobalt-chromium alloys (Co-Cr-Mo) are the hardest and most wear-resistant orthopaedic metals, primarily used for bearing surfaces in joint arthroplasty—TKA femoral components and THA heads. Composition includes cobalt (60-65%), chromium (27-30%) for passivation, and molybdenum (5-7%) for hardness. Cast (ASTM F75) manufacturing is used for complex shapes like femoral condyles, while wrought/forged produces stronger hip heads. CoCr has poor osseointegration and requires titanium/HA porous coating on uncemented components. Major concerns include metal ion toxicity (cobaltism) and ALVAL (aseptic lymphocytic vasculitis-associated lesions) causing pseudotumours, particularly problematic in metal-on-metal articulations.

Cobalt-chromium-molybdenum (Co-Cr-Mo) alloys represent the gold standard bearing surface material in orthopaedic joint replacement surgery. First introduced in the 1930s as "Vitallium," these alloys have evolved through decades of metallurgical refinement to become indispensable in modern arthroplasty.

Key Applications:

• Total Knee Arthroplasty: Femoral component (articulates with polyethylene tibial insert)
• Total Hip Arthroplasty: Femoral heads (28-36mm diameter, articulating with poly/ceramic liner)
• Metal-on-Metal bearings: Historical use in hip resurfacing (now largely abandoned due to ARMD)
• Modular junctions: Femoral head-neck tapers

Why CoCr for Bearings? The exceptional hardness (Vickers 300-400) and wear resistance make CoCr ideal for articulating surfaces. When highly polished (Ra less than 0.05 μm), CoCr produces minimal polyethylene wear debris compared to other metals. The chromium content creates a self-healing passive oxide layer (Cr₂O₃) providing excellent corrosion resistance in the physiological environment.

Critical Limitation: CoCr does NOT osseointegrate. Bone cannot grow directly onto polished CoCr surfaces. Therefore, uncemented CoCr implants require additional surface treatments (porous coating, plasma-sprayed titanium, or hydroxyapatite) to achieve biological fixation.

Metal Debris:

• CoCr wear generates ions (Cr³⁺, Co²⁺) and nanoparticles.
• ALVAL (Aseptic Lymphocytic Vasculitis-associated Lesions): Type IV Hypersensitivity reaction to metal ions. Leads to "Pseudotumours" and soft tissue necrosis.
• Systemic Toxicity: Cobaltism (Cardiomyopathy, Hypothyroidism, Neuropathy).

Biocompatibility:

• Poor osseointegration.
• Uncemented CoCr implants usually have a Porous Coating (beads or wire mesh) or a Plasma Spray (Titanium/HA) to encourage bone ingrowth.

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Crystal Structure: CoCr alloys exist primarily in two crystallographic phases:

• Face-Centered Cubic (FCC): High-temperature stable phase, more ductile
• Hexagonal Close-Packed (HCP): Low-temperature stable phase, harder but more brittle

The transformation between phases during cooling affects mechanical properties. Controlled processing maintains optimal phase balance.

Carbide Formation: Carbon (0.05-0.35%) combines with chromium and molybdenum to form carbides:

• M₂₃C₆ carbides: Primary strengthening phase, distributed at grain boundaries
• High carbon alloys: More carbides = better wear resistance but reduced ductility
• Low carbon alloys: Fewer carbides = better fatigue strength, preferred for high-stress applications

Grain Structure:

• Cast alloys: Large, dendritic grains with interdentritic carbides
• Wrought alloys: Fine, equiaxed grains from thermomechanical processing
• Smaller grain size = higher strength (Hall-Petch relationship)

Passive Layer: The 2-5 nm thick chromium oxide (Cr₂O₃) passive layer forms spontaneously in air/physiological fluids. This layer:

• Self-repairs if scratched (repassivation within milliseconds)
• Provides corrosion resistance in chloride-rich body fluids
• Can be disrupted by fretting, leading to ion release

When to Suspect Metal-Related Complications:

Clinical assessment focuses on detecting adverse reactions to metal debris (ARMD), particularly relevant for:

• Metal-on-Metal (MoM) hip articulations
• Large-diameter metal heads (greater than 36mm)
• Modular neck-stem junctions (mechanically-assisted crevice corrosion)

History:

• Pain: Groin pain (even with well-fixed implant), thigh pain
• Systemic symptoms: Fatigue, cognitive changes, cardiac symptoms (cobaltism)
• Timing: Symptoms may develop years after implantation

Examination:

• Hip: Limited ROM, positive impingement signs, palpable mass (pseudotumour)
• Skin: Rash or dermatitis (metal hypersensitivity)
• Neurological: Check for peripheral neuropathy

Red Flags for Metal Toxicity:

• Unexplained pain in well-functioning arthroplasty
• Elevated serum cobalt or chromium (greater than 7 μg/L)
• Fluid collections on imaging
• Systemic symptoms: visual changes, hearing loss, cardiomyopathy, thyroid dysfunction

Laboratory Testing:

Imaging:

• Radiographs: Assess implant position, loosening, osteolysis
• MARS-MRI (Metal Artifact Reduction Sequence): Gold standard for soft tissue assessment
  • Detects pseudotumours, fluid collections, muscle atrophy
  • Requires specialized sequences to reduce metal artifact
• Ultrasound: Alternative if MRI unavailable; operator-dependent
• CT with metal subtraction: Assesses bone stock, osteolysis

MARS-MRI Classification (Anderson):

Material Testing (Laboratory/Research):

• Wear simulation testing (hip simulator)
• Surface roughness measurement (Ra values)
• Corrosion testing (electrochemical methods)
• Metallographic analysis (grain structure, carbides)

Metal Ion Toxicity (Cobaltism): Systemic cobalt toxicity can occur with elevated serum levels (typically greater than 20 μg/L):

• Cardiac: Cardiomyopathy, heart failure
• Neurological: Peripheral neuropathy, cognitive impairment, hearing/visual changes
• Thyroid: Hypothyroidism
• Haematological: Polycythaemia

ALVAL (Aseptic Lymphocyte-dominated Vasculitis-Associated Lesion): Type IV hypersensitivity reaction to metal debris:

• Perivascular lymphocytic infiltration
• Pseudotumour formation (cystic or solid masses)
• Soft tissue necrosis and bone destruction
• May occur with normal or elevated ion levels

Corrosion-Related Complications:

• Taper corrosion: Mechanically-assisted crevice corrosion at modular junctions
• Fretting: Micromotion between components damages passive layer
• Galvanic corrosion: When CoCr contacts dissimilar metals (Ti stem)

Wear-Related Complications:

• Polyethylene wear (third-body particles if CoCr surface damaged)
• Metal debris generation (MoM articulations)
• Osteolysis from particle-induced inflammation

Material Failure:

• Fatigue fracture (rare with modern alloys)
• Casting defects (porosity as stress concentrators)
• Implant fracture at stress risers

Standard CoCr Implants (Metal-on-Poly):

• Routine arthroplasty follow-up
• No specific metal ion monitoring required
• Standard radiographic surveillance for loosening/wear

Metal-on-Metal Hip Patients: MHRA (UK) and TGA (Australia) guidelines recommend:

• Annual review: Clinical assessment + serum Co/Cr
• MARS-MRI: If symptomatic or ions greater than 7 μg/L
• Increased frequency: If abnormalities detected

Modular Junction Concerns: Large-diameter heads on tapered stems warrant attention:

• Monitor for groin pain (may indicate taper corrosion)
• Consider ion testing if symptomatic
• Lower threshold for imaging in high-risk combinations

Patient Education:

• Report new or unexplained hip/thigh pain
• Systemic symptoms to report: fatigue, visual/hearing changes, palpitations
• Importance of attending follow-up appointments
• MoM patients need lifelong surveillance

After Revision for ARMD:

• Serial ion levels (should decline post-revision)
• Monitor for resolution of systemic symptoms
• Imaging to confirm tissue healing

CoCr on Polyethylene (Standard Bearing):

• Excellent long-term survivorship (greater than 95% at 15 years)
• AOANJRR data supports CoCr heads on HXLPE as gold standard
• Wear rates: 0.05-0.1 mm/year with modern HXLPE
• Minimal osteolysis with highly cross-linked polyethylene

Metal-on-Metal (Historical):

• Initial enthusiasm for large diameter heads (greater range of motion, stability)
• Registry data revealed higher failure rates than expected
• Cumulative revision rate 15-20% at 10 years for some designs
• ASR hip recall (2010) highlighted widespread problems
• Now limited to specific hip resurfacing indications in young active males

Ceramic vs CoCr Heads:

Revision for ARMD:

• Outcomes depend on severity of tissue damage at revision
• Mild ARMD: Good outcomes expected
• Severe muscle/bone destruction: Higher dislocation rates, functional limitations
• Early revision (before extensive damage) associated with better outcomes

Key Registry Data:

• AOANJRR (Australian): Demonstrates superior survivorship of ceramic/metal heads on HXLPE vs MoM
• NJR (UK): Identified higher failure rates of MoM leading to regulatory action
• SHAR (Swedish): Long-term data on CoCr performance over decades

Landmark Studies:

Current Evidence Consensus:

• CoCr-on-HXLPE remains excellent bearing choice
• MoM articulations reserved for specific hip resurfacing cases
• Ion monitoring essential for MoM patients
• Threshold of 7 μg/L for concern (MHRA guidance)

Ongoing Research Areas:

• Optimal taper design to minimize corrosion
• Role of titanium sleeves to prevent taper corrosion
• Alternative bearing surfaces (oxidized zirconium, vitamin E HXLPE)
• 3D-printed CoCr implant performance data

AOANJRR (Australian Orthopaedic Association National Joint Replacement Registry):

• World's largest joint registry with comprehensive bearing surface data
• Demonstrates CoCr head on HXLPE as low revision rate bearing
• Provides surgeon and hospital benchmarking
• Annual reports inform implant selection decisions

TGA (Therapeutic Goods Administration) Guidance:

• Issued MoM hip surveillance recommendations
• Recall of high-risk MoM devices (e.g., ASR)
• Requires post-market surveillance for arthroplasty devices
• Monitors adverse event reports

Australian Surveillance Recommendations:

• Annual clinical review for MoM hip patients
• Serum Co/Cr levels annually
• MARS-MRI if symptomatic or elevated ions
• Lifelong follow-up recommended

Australian Practice Patterns:

• MoM hip use has dramatically declined since 2010
• Ceramic-on-HXLPE increasingly popular for younger patients
• CoCr-on-HXLPE remains common, particularly in TKA
• Registry participation is mandatory for prostheses

1. Martell JM, et al. The effect of femoral head size on polyethylene wear in total hip arthroplasty. JBJS Am. 2003.
2. Jacobs JJ, et al. Metal release and metabolism from total joint replacement. Instr Course Lect. 1998.