Trunnionosis and Taper Corrosion

Morse Taper Fundamentals

Taper design principles:

• Morse taper creates interference fit via cold welding
• Taper angle varies between manufacturers (5°44' common)
• Taper length affects contact area and stability
• Impaction force critical for initial fixation

Load transmission:

• Axial loads from body weight
• Torsional loads from gait (internal/external rotation)
• Bending moments from offset/neck length
• Larger heads amplify moment arm at taper

Types of Corrosion at the Taper

Fretting corrosion:

• Cyclic micromotion (less than 100 microns) between surfaces
• Disrupts protective chromium oxide passivation layer
• Exposes reactive base metal
• Each cycle releases metal particles

Crevice corrosion:

• Taper junction creates oxygen-depleted environment
• Differential aeration cell forms
• Local pH drops (becomes acidic)
• Accelerated metal dissolution

Galvanic corrosion:

• Dissimilar metals create electrochemical potential
• CoCr head on Ti stem = galvanic couple
• Ti acts as cathode (protected)
• CoCr acts as anode (corrodes preferentially)

Adverse Local Tissue Reaction (ALTR)

Pathological cascade:

• Metal ions and particles released
• Type IV hypersensitivity reaction in some patients
• Macrophage activation and lymphocytic infiltration
• Cytokine release (IL-6, TNF-alpha)
• Tissue necrosis and pseudotumor formation

ALVAL (Aseptic Lymphocyte-dominated Vasculitis-Associated Lesion):

• Histological pattern of metal hypersensitivity
• Perivascular lymphocytic infiltrates
• Fibrin exudation and tissue necrosis
• Variable clinical correlation

Implant-Related Risk Factors

The worst combination: Large CoCr head (36mm+) on a high-offset titanium stem with extended neck.

Patient-Related Risk Factors

Higher risk patients:

• Male sex (higher activity levels, larger heads used)
• Younger, more active patients
• Higher BMI (increased joint loads)
• Bilateral THA (cumulative exposure)
• Metal hypersensitivity history

Activity-related:

• High-demand activities increase loading cycles
• Impact sports amplify peak loads
• Greater range of motion increases taper micromotion

Intraoperative Considerations

Assembly technique matters:

• Dry taper essential (blood/fluid reduces cold weld)
• Adequate impaction force (studies suggest 4-8 kN)
• Clean taper surfaces (debris interferes with seating)
• Correct orientation (avoid toggle during impaction)

Component positioning:

• Excessive femoral anteversion may increase torsional loads
• Combined version affects loading patterns

Serum Metal Ion Testing

Interpretation pearls:

• Co:Cr ratio greater than 2:1 suggests taper corrosion (rather than bearing wear)
• Levels can fluctuate - trend is important
• Some patients develop ALTR with low ion levels (hypersensitivity)
• Whole blood samples required (not serum alone)

MHRA guidance (UK) - widely adopted:

• Co or Cr greater than 7 ppb = concern, requires imaging
• Annual surveillance for at-risk implants

Metal Artifact Reduction Sequence MRI

Gold standard imaging for ALTR:

• Reduces metal artifact allowing soft tissue visualization
• Detects pseudotumors, fluid collections, muscle atrophy
• Grades severity of tissue destruction

Key findings to report:

• Pseudotumor: Cystic or solid mass, may be large
• Fluid collections: Periprosthetic collections
• Abductor status: Atrophy, fatty infiltration, detachment
• Bone involvement: Osteolysis, cortical erosion
• Muscle necrosis: Signal changes indicating tissue death

Anderson classification of pseudotumors:

• Type 1: Thin-walled cystic
• Type 2a: Thick-walled cystic, atypical contents
• Type 2b: Thick-walled, mixed solid/cystic
• Type 3: Predominantly solid

Higher types correlate with worse tissue damage.

Additional Imaging Modalities

Joint Aspiration

Indications:

• Rule out infection (essential before revision)
• Relieve symptoms from large effusion
• Obtain fluid for analysis

Analysis:

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

Characteristic ALTR fluid: Dark gray or black, metallic-stained, often sterile.

Non-Operative Surveillance

Indications for surveillance (no immediate surgery):

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

Surveillance protocol:

• Annual clinical review
• Annual metal ions (Co and Cr)
• MARS MRI every 1-2 years if ions elevated
• Patient education on warning symptoms

Upgrade to revision if:

• Symptomatic progression
• Rising metal ion trend
• ALTR progression on MRI
• Systemic symptoms develop

When to Revise

Absolute indications:

• Symptomatic ALTR with tissue destruction
• Progressive pseudotumor
• 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 pseudotumor
• Progressive bone loss
• Patient anxiety affecting quality of life

Factors favoring surgery:

• Younger, active patient
• Good bone stock
• Abductors still salvageable
• Minimal systemic symptoms

Surgical Management

Pre-operative planning:

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

Surgical principles:

• Extended approach for adequate exposure
• Complete debridement of necrotic tissue
• Thorough lavage to remove debris
• Assess abductor function intraoperatively
• Component exchange considerations

Component strategy:

• Remove corroded head and trunnion adapter
• Consider modular neck exchange if available
• New ceramic head (eliminates metal bearing)
• If stem well-fixed but taper damaged: ceramic head with Ti adapter sleeve
• If stem loose or severe trunnion damage: stem revision
• Cup revision only if loose or malpositioned

Implant Selection at Revision

Ceramic head advantages:

• No metallic corrosion products
• Excellent wear characteristics
• Hard bearing surface
• Caution: fracture risk (rare with modern ceramics)

Surgical Approach

Extensile approach required:

• Posterior approach with extended capsulotomy
• May need trochanteric osteotomy for access
• Must visualize extent of pseudotumor
• Direct lateral may limit access posteriorly

Key exposure steps:

• Identify and protect sciatic nerve (often displaced by pseudotumor)
• Evacuate pseudotumor contents carefully
• Complete capsulectomy and synovectomy
• Debride all necrotic tissue back to bleeding margins

Soft Tissue Handling

Pseudotumor management:

• Decompress large collections before dislocation
• Send fluid for culture (rule out infection)
• Excise pseudotumor wall completely
• May extend into pelvis - be aware of vascular structures

Abductor assessment:

• Gluteus medius often damaged/necrotic
• Assess remaining viable muscle
• May need augmentation or reconstruction
• Tensor fascia lata advancement possible
• Consider constrained liner if abductors deficient

Histology:

• Send tissue for histopathology
• ALVAL score provides prognostic information
• Rule out occult infection with frozen section

Intraoperative Decision-Making

Stem assessment:

• Check taper condition carefully
• If moderate corrosion but stem well-fixed: Ti adapter sleeve
• If severe corrosion or loose: stem revision
• Document findings for registry reporting

Cup assessment:

• Position and fixation
• Only revise if loose or malpositioned
• If cup is well-fixed MoM: liner exchange to ceramic/poly

Bearing couple selection:

• Default to ceramic-on-polyethylene
• Smaller head (32mm) preferred
• Avoid MoM at all costs