Bearing Surfaces in Hip Arthroplasty (MoM, MoP, CoC, CoP)

The choice of bearing surface in total hip arthroplasty (THA) determines the tribological performance (friction and wear characteristics), the longevity of the implant, and the specific failure modes to which the patient is exposed. The four main bearing couples in contemporary THA are metal-on-polyethylene (MoP), ceramic-on-polyethylene (CoP), ceramic-on-ceramic (CoC), and metal-on-metal (MoM). Each combination has distinct advantages and disadvantages. Metal-on-metal bearings, once widely used for their low volumetric wear and large head size, have been largely abandoned following the recognition of adverse local tissue reactions (ALTR) driven by metallic corrosion products. Highly cross-linked polyethylene (HXLPE) has transformed the outcomes of MoP and CoP bearings, making these the dominant global choices.

• Tribology principles: wear in arthroplasty is governed by hardness mismatch (harder materials scratch softer), surface finish (rougher surfaces generate more wear), lubrication (hydrodynamic film from synovial fluid reduces contact stress), and contact stress (load per unit area — larger femoral heads reduce contact stress on the PE liner but increase volumetric wear due to larger swept area); wear mode: adhesive (direct material transfer between surfaces), abrasive (third body particles or surface roughness scoring the bearing), and fatigue (cyclic delamination) — the dominant mechanism varies by bearing couple
• Femoral head size: historically 22–28 mm (Charnley used 22.25 mm — small head, low frictional torque, but high dislocation risk); modern heads typically 32–36 mm (large head increases jump distance — the head must sublux further before dislocation — improving stability); with HXLPE liners, larger heads (36–40 mm) are used with MoP and CoP bearings; MoM bearings allowed very large heads (44–56 mm — resurfacing) due to their low volumetric wear at large diameters — this advantage is negated by ALTR risk

• The MoM crisis (2010–2012): the DePuy ASR (Articular Surface Replacement) and several other large-head MoM THA designs were recalled or subjected to enhanced surveillance after NJR data showed revision rates of 12–15% at 5 years — far exceeding any other bearing; the Medicines and Healthcare Products Regulatory Agency (MHRA) issued Medical Device Alerts (MDAs) mandating annual blood metal ion monitoring (serum cobalt and chromium) and MARS MRI for all patients with MoM THA; the ASR recall alone affected approximately 93,000 patients worldwide; the crisis permanently damaged confidence in MoM THA
• MHRA thresholds for investigation: serum cobalt OR chromium >7 µg/L (ppb) → enhanced surveillance + MARS MRI + consider revision; the threshold was lowered to 4 µg/L in 2017 for unilateral MoM THA; elevated metal ions + pseudotumour on MARS MRI + symptoms = strong indication for revision; revision of failed MoM THA is technically complex — significant soft tissue damage (ALTR) may have occurred, requiring debridement of necrotic tissue and reconstruction
• Hip resurfacing — remaining indications: metal-on-metal hip resurfacing (Birmingham Hip Resurfacing — BHR; Cormet; Conserve Plus) resurfaces the femoral head rather than replacing it; preserves femoral head and neck bone stock; allows large head sizes (equivalent to native femoral head — 44–54 mm) with very high jump distance and very low dislocation rates; the BHR has 10-year survival of ~95% in young active males in high-volume specialist centres; indications — young (<55 years), male (female sex is a risk factor for ALTR — smaller femoral head sizes, thinner acetabular component, higher ion release), high activity level, good femoral head anatomy (no AVN, cyst <1 cm); the BHR retains regulatory approval and NJR-acceptable revision rates in this specific demographic

• Ceramic fracture management: if a ceramic head or liner fractures in vivo, the joint must be thoroughly lavaged (multiple litres of saline) at revision surgery to remove all ceramic fragments; all bearing components (head AND liner) must be exchanged; residual ceramic fragments act as third-body abrasives and will rapidly destroy any new bearing surface; a metal-on-HXLPE bearing is typically used after ceramic fracture revision (ceramic-on-ceramic should NOT be used after ceramic fracture — risk of residual particle damage to new ceramic components); the revision acetabular shell must be assessed — ceramic liner fragments may have damaged the locking mechanism
• Squeaking in CoC THA: audible squeaking from CoC bearings occurs in 0.5–2% of cases; caused by `stripe wear` (edge loading of the ceramic head on the ceramic liner rim during activities at extremes of ROM — walking upstairs, rising from a chair); once stripe wear occurs, the smooth ceramic surface is disrupted and the lubrication film breaks down (mixed lubrication → boundary lubrication → stick-slip → acoustic emission = squeak); management — modification of activities; revision for severe or distressing squeaking (to CoP or MoP bearing); squeaking is more common with cup malposition (steep abduction angle >50° increases edge loading)

• Most widely used global bearing: Metal-on-HXLPE; dramatic wear reduction (50–90%); <0.05 mm/year; allows large heads 32–40 mm; excellent 15–20 year survivorship; first choice for most THA patients
• CoP: ceramic head reduces PE abrasion; ZTA (Biolox delta) ceramic — excellent scratch resistance; low fracture risk (~0.004%); if fracture → thorough lavage + exchange ALL components → MoP not CoC after fracture
• CoC: lowest wear rate (<0.001 mm/year); biocompatible particles; best for young active patients; complications — squeaking (0.5–2%; stripe wear from edge loading), liner fracture (rare with modern ZTA), chipping during implantation
• CoC squeaking: edge loading → stripe wear → boundary lubrication breakdown → stick-slip → acoustic emission; more common with steep cup abduction (>50°); activity modification; revision to CoP/MoP for severe cases
• MoM: low volumetric wear; large heads possible; ABANDONED for most THA due to ALTR (pseudotumour, ALVAL, soft tissue necrosis, elevated Co/Cr ions — cardiomyopathy, neuropathy, thyroid disease)
• MHRA: Co/Cr >7 µg/L (or >4 µg/L unilateral) → enhanced surveillance + MARS MRI + consider revision; ASR recall 2010 — 93,000 patients worldwide; annual blood metal ion monitoring mandatory for all MoM THA
• Hip resurfacing (BHR): remaining MoM indication; young (<55) active males; large head = low dislocation; preserves femoral bone stock; 10-year survival ~95% in specialist centres; avoid in females (smaller head → more ions → ALTR risk)
• Ceramic fracture rule: thorough lavage (litres of saline); exchange head AND liner; do NOT use CoC after ceramic fracture; use MoP-HXLPE; check acetabular shell locking mechanism for damage
• HXLPE generations: 1st — irradiation + remelting (reduces fracture toughness); 2nd — irradiation + annealing ± Vitamin E antioxidant (optimises wear resistance AND fatigue properties); 2nd generation now standard