Goals of a bearing
An ideal bearing would provide:
- low friction
- very low wear
- resistance to scratching and fracture
- biocompatibility
- large safe range of motion
- predictable long-term performance
No bearing achieves every goal perfectly.
Ceramic-on-polyethylene
A common modern combination.
Advantages:
- low wear with highly cross-linked polyethylene
- ceramic head resists scratching
- avoids metal-on-metal articulation
A ceramic head offers:
- high hardness
- resistance to scratching
- excellent surface finish
Combined with highly cross-linked polyethylene, it provides a low-wear bearing while avoiding ceramic liner complications.
Metal-on-polyethylene
Long clinical history and broad implant compatibility. Modern highly cross-linked polyethylene has markedly improved wear performance compared with older conventional polyethylene.
This remains widely used.
Advantages:
- long clinical experience
- broad implant compatibility
- modern highly cross-linked polyethylene has low wear
Potential problems include:
- polyethylene wear
- liner damage
- third-body abrasion
- osteolysis
Ceramic-on-ceramic
Advantages:
- very low wear
- excellent surface hardness and wettability
Limitations:
- brittleness and rare fracture
- liner damage from mal-seating
- edge loading
- noise/squeaking in some hips
- technically sensitive insertion
When revising a fractured ceramic bearing, retained ceramic debris can damage subsequent surfaces; meticulous synovectomy and bearing selection are important.
Advantages:
- extremely low wear
- inert wear debris
- hard smooth surfaces
Limitations:
- brittle material
- rare fracture
- liner chipping
- noise/squeaking
- sensitivity to malposition and edge loading
If a ceramic component fractures, retained ceramic particles can act as highly abrasive third-body debris. Revision requires meticulous debridement/synovectomy and an appropriate new bearing choice.
Head size
Larger heads can increase jump distance and range before impingement, but choice is constrained by liner thickness, bearing material and implant design.
Larger heads:
- increase jump distance
- improve range before neck-liner impingement
Trade-offs include:
- reduced polyethylene thickness within a fixed shell
- increased taper forces with very large heads
- bearing-specific design constraints
Tribology
Wear depends on more than material. Important factors include:
- lubrication
- clearance
- surface roughness
- component position
- impingement
- third-body particles
- edge loading
Bearing selection should therefore be integrated with implant position and soft-tissue reconstruction.
Tribological goals
A bearing should minimise:
- friction
- wear
- corrosion
- particle generation
- fracture risk
- adverse biological reaction
At the same time it must allow adequate component geometry and stability.
Metal-on-metal
Large-diameter metal-on-metal bearings were associated with:
- metal ion release
- wear/corrosion
- pseudotumour and soft-tissue necrosis
- adverse reaction to metal debris
These complications can occur even without dramatic radiographic loosening.
Component position
A theoretically excellent bearing can fail if cup position causes:
- edge loading
- impingement
- instability
- increased wear
FRCS synthesis
Bearing selection should be explained as a balance of wear, fracture risk, head size, patient demand and component design, not simply as a ranking of materials.