Ceramic Bearing in Total Hip Replacement

Factors Influencing the Rate of Head Fractures

  1. Quality of Impaction
    • Performed along the axis of the neck of the prosthesis on a clean Morse taper.
    • Persistent tissue debris or a damaged Morse taper surface (e.g., scratches) can reduce static fracture load to 20% of its normal value.
  2. Cleanliness of the Morse Taper During Impaction
    • A clean, undamaged taper is essential to prevent fracture.
  3. Diameter of the Femoral Head
    • Advantages of Larger Head Diameters:
      • Reduced risk of impingement between the neck and the rim of the ceramic liner or metal cup.
      • Increased range of motion.
      • Decreased risk of dislocation.
    • Problems with Larger Head Diameters (>36 mm):
      • Positive offset develops as the head center moves outside the cup center, reducing head coverage angle.
      • Edge loading occurs during micro-separation phases (inevitable during walking).
      • Increased friction between bearing couple elements leads to noise (squeaking).
      • Potential ceramic surface modification, though the impact on fracture rates remains unknown.

Liner Fracture

Contributing Factors:

  1. Implantation Techniques
  2. Cup Design
  3. Functional “Limits” During Edge Loading

Specific Issues with Cup and Liner:

  1. Thin Liners:
    • Liners that are too thin are more prone to fractures, especially in 50-54 mm diameter cups.
  2. Cup Design:
    • Cup diameter and thickness indirectly affect liner impaction quality.
    • Narrow Morse taper angles (<10°) complicate insertion, increasing peripheral stresses.
    • Recent cups with longer Morse tapers (9 mm or 11 mm) aim to improve liner stability but present challenges with:
      • Tissue interposition.
      • Difficulties in cleaning the taper during minimally invasive procedures.
      • Deformation of perforated, non-impermeable metal cups during impaction.
  3. Insertion Issues:
    • Misaligned impaction can result in peripheral stress and chipped edges.
    • Ceramic liners are rigid and cannot adapt to socket deformations, unlike polyethylene liners.

National Joint Registry (NJR) Data: 13-Year Revision Rates in THR

Fixation Type Bearing Type Revision Rate (<55 years) Revision Rate (>55 years)
Cemented MoP 10 4.5
Cemented CoP 3.8 4.5
Cemented CoC - -
Uncemented MoP 7 6
Uncemented CoP 4.5 4.5
Uncemented CoC 6.5 5.5

Key Recommendations

  1. Fixation Preferences:
    • Most patients choose cemented fixation with hard-on-soft bearings.
  2. Bearing Type Preferences:
    • Ceramic bearings are preferred over metal.
  3. Patients Under 55 Years:
    • Consider discussing dynamic fixation using uncemented components.
  4. Cup Design Considerations:
    • Ensure appropriate liner thickness and proper Morse taper alignment to minimize risks of fracture and edge loading.
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