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- Large head size ➔ higher head-neck ratio, PAR, LR, jump distance
- Lubrication ➔ smooths out itself ("run-in" self-polishing phase), hydrodynamic lubrication (fluid film due to polar contact allowing for fluid entrainment)
- Bone preserving ➔ less bone needs to be removed compared to PE lining
- Improved wear characteristics compared to PE
- MOM particles smaller, resulting in reduced osteolysis
- Total wear also less (1-6 μm/year vs 0-200 μm for PE)
- Wear rates of different bearing surfaces
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- Local - Adverse Reactions to Metal Debris (ARMD), which includes a spectrum of 4 conditions:
- Metallosis - metal staining of tissue
- Pseudotumor - granulomatous lesion, neither infective nor neoplastic, that develops near a THR and resembles a tumor
- ALVAL - aseptic, lymphocyte-dominated vasculitis associated lesion (HISTOLOGICAL diagnosis)
- Macrophage-induced osteolysis due to type 4 hypersensitivity
- Systemic - increased metal ions may be carcinogenic or teratogenic (No evidence)
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- Likely multifactorial
- Several studies found association with increased wear due to implant positioning and edge loading
- However, studies also found pseudotumors with minimal wear features, suggesting hypersensitivity as a cause
- Pseudotumor reactions also reported in non-MOM bearings
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- If Stemmed THR (1) - femoral head size > 36mm
- If Hip resurfacing (3) - all females, all DePuy ASR hip, Hip resurfacing < 48mm
- Technical factors - acetabular inclination > 55 degrees
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- For hip resurfacing < 48mm ➔ found to have more impingement and edge loading
- For THR > 36mm ➔ found to have more frictional torque and more volumetric wear
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- First rule out infection
- Rule out extrinsic sources of pain (e.g., referred pain, spine)
- Evaluate based on MHRA guidelines
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- Based on the MHRA guidelines (Medicines & Healthcare products Regulatory Agency)
- Frequency depends on whether patient is High risk or Low risk
- In general, indications for revision would be high METAL (cobalt and chrome) levels > 7ppb and abnormal MRI imaging.
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- High risk groups (regardless of symptoms) and Low risk symptomatic - YEARLY
- Oxford Hip score, Clinical symptoms
- Annual METAL (Co and Cr) levels
- MRI if symptoms or rising metal ion levels
- Low risk asymptomatic - annually for first 5 years, 2 yearly till 10 and 3 yearly thereafter.
- Oxford Hip score, Clinical symptoms
- METAL (Co and Cr) levels
- XR at each visit ➔ if abnormal ➔ MRI
- MRI if symptoms or rising metal ion levels
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- Anderson classification
- A = Normal appearances including seromas
- B = Infection
- C1 = Mild MOM disease with soft tissue mass < 5cm
- C2 = Moderate MOM disease with soft tissue mass > 5cm, muscle atrophy
- C3 = Severe MOM disease with fracture, tendon avulsion, and bone marrow signal change
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- Based on MHRA guidelines, we should consider revision as long as MRI is abnormal and/or raised metal levels
- However, this is still controversial. There is little knowledge about the clinical relevance of silent pseudotumors and their natural course. Revision outcomes can be poor due to incomplete excision or recurrence requiring re-operation.
- Walter et al. in JoA 2013 found that pseudotumor prevalence was 36% among 248 patients. Only 5 patients who were symptomatic with elevated metal ions required revision.
- Walter recommends revision in Grade C3 + metal ion levels > 7ppb. Consider follow-up with MRI every 6 months until lesion stability is confirmed.
Walter 2013.pdf958.2 KiB
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- Presence of "dishwater fluid" with hazy, gray appearance
- Black metal deposits with extensive synovitis
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- Complex Revision - EEERRR mnemonic, will enlist help of experienced hip surgeon
- Concerns: soft tissue integrity leading to instability, bone loss due to destruction
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- Evaluate pseudotumor destruction
- MRI and CT scan to evaluate bone stock
- Pseudotumor - may be destructive to periprosthetic bone and soft tissue. Evaluate bony destruction ➔ Consider dual mobility, constrained implants
- Templating for bone loss, consider long implants to bypass defects
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- Exposure - extensile
- Extract
- Evaluate and debride - will need aggressive debridement of all involved tissue
- Reconstruct - assess requirements
- Replace
- Post Op - rehabilitation
HIP RESURFACING
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- Perceived advantages include better restoration of hip biomechanics, enhanced proprioception, improved wear characteristics with no polyethylene-induced osteolysis, greater range of motion, reduced dislocation risk, and improved femoral bone stock preservation
- Improved wear characteristics compared to polyethylene
- MOM particles are smaller, resulting in reduced osteolysis
- Total wear is also less (1-6 μm/year vs 0-200 μm for polyethylene)
- Wear rates of different bearing surfaces:
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- Ideal patient - young males who remain active in sports or manual labor
- Contraindications:
- Patient factors - severe osteoporosis, females of childbearing age, severe obesity, metal allergy, kidney disease, steroids (conditions that lead to osteoporosis)
- Anatomy - large cysts in femoral neck, narrow femoral neck, females with small head (some consider < 46mm a contraindication), hip dysplasia
- Pathology - AVN is considered a good indication provided there is sufficient healthy bone stock, DDH and Perthes are contraindicated
- Surgeon - inexperience
- Hip resurfacing procedures are rare and should be performed only by specialized surgeons. The decision should be discussed with hip colleagues to reach a consensus before proceeding
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- Femoral neck fracture in 4% of cases (often due to varus malposition)
- Metal-on-metal issues
- Aseptic loosening
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- Patient - poor bone stock, comorbidities like renal failure leading to osteoporosis
- Technical factors - high acetabular inclination > 40° leads to increased wear rates, notching
- Implant - ASR Hip implants (Birmingham hip resurfacing considered acceptable)
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Whoever trusts in his riches will fall, but the righteous will flourish like a green leaf. Proverbs 11:28
