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Orthopaedic Materials / Bearing Surfaces

 

synonyms:

Polyethylene

  • Low wear, lowest cost, high oxidative resistance
  • Highly cross-linked polyethylene wear rate <0.1mm/year.
  • Sterilization in air = oxidation = increased poly fracture and wear.
  • High dose radiation to polyethylene (10 Mrad) leads to higher cross-linking and decreased adhesive and abrasive wear.  However, fatigue wear and creep (deformation without wear)   increases when polyethylene is melted (to remove free radicals) during the cross-linking process.
  • Heat pressed ultra-high molecular weight polyethylene has been associated with poor clinical performance.

Metal on Metal Bearing Surfaces

  • Lower wear than metal on poly, high cost
  • Initial high wear stage last for @1yr followed by steady state wear of a few microns/year.
  • Metal ions are found in blood and urine at 5-10 times normal levels.
  • Theoretical cancer risk. 28yr Nordic follow-up demonstrated cancer incidence in Metal-on-metal patients to be consistent with general poplulation. (Visuri TI, Proc Inst Mech Eng [H] 2006;220:399).
  • Relative contraindications: female of childbearing age, kidney disorder, known metal allergy.
  • For MOM articulations: Polar contact should exceed equatorial contact.  High carbon content decreases wear.
  • Radial mismatch of the femoral head to the acetabular component:  too small leads to seizing of the implants.  too large increases contact stresses leading to exceptionally high wear.Ideal radial mismatch is 50 microns.
  • Debris particles generated by metal-on-metal articulations in total hip arthroplasty are several orders of magnitude smaller and 100 times more numerous than those for metal-on-polyethylene articulations.

Metal-on-metal Total Hip Arthroplasty (MOM)
Metal-onmetal articulation: excellent wear rates in vitro. 
Local soft-tissue reactions, pseudotumors, and potential systemic reactions: renal failure, cardiomyopathy, carcinogenesis
Histology of metal-on-metal (MOM) hip arthroplasties / pseudotumor: predominantly tissue necrosis with infiltration of lymphocytes and plasma cells.
Relatively contraindicated in women of child-bearing age; potential teratogenesis (metal ions may transfer across the placental barrier)
Painful metal-on-metal THA: ESR, CRP, and serum cobalt and chromium ion levels, MARS MRI(fluid collections, pseudotumors, and abductor mechanism destruction).  Hip arthrocentesis indicated if CRP is elevated to eval for concomitant infection.  Manual cell count and differential required do to potential falsely elevated cell counts with automated systems.
Elevated serum metal ions and abductor dysfunction, suggests poor bearing function and adverse local tissue reaction (ALTR). 
Cobalt disproportionately higher than chromium suggests corrosive changes at the prosthetic femoral neck (trunnionosis).
Instability is the most common complication following revision of failed metal-on-metal hip replacements.
Generally do not remove a stable cementless femoral stem.  If revising for metalosis or there is some damage to the taper use a titanium taper sleeve with a ceramic ball. Use of a ceramic head on a previously used trunnion risks fracture of the ceramic head.  

Ceramics

  • Lowest wear, higly biocompatible, high cost, potential squeaking and fracture
  • Alumina components subject to fracture risk due to brittleness.
  • Potential for stipe weah if improperly positioned.
  • Squeking 1-3%, etiology unknown. (Walter WL, J Arthroplasty 2007;22:496).
  • Zirconia = yttrium-stabilized zironia: theoretical increased toughness and lower wear, but clinical studies indicate increase osteolysis likely due to phase transition in vivo. (Hernigou P, JBJS 2003;85Br:504).
  • Future alternatives: Alumina (75%)-Zirconia(25%) composites, oxidized zirconium, ceramic on metal articulations.

Young’s modulus of elasticity (E)

  • is a measure of the stiffness of a material and its ability to resist deformation.
  • In the elastic region of the stress-stain curve, E = stress/strain.
  • Alumina ceramic  E = 380 Gigapascals (GPa)
  • Cobalt-chromium = 210 GPa
  • Stainless steel = 190 GPa
  • Titanium = 116 GPa
  • PMMA = 1.1 to 4.1 GPa.