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Total Hip Replacements

Total Hip Replacements. About 160,000 THR’s performed every year 90% of all hip replacements need some sort of revision No. 1 reason for revision is loosening of the implant. Why Total Joint Replacement???. Cement vs. Cementless Characteristics. Bonding Time: Cemented vs. Cementless.

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Total Hip Replacements

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  1. Total Hip Replacements • About 160,000 THR’s performed every year • 90% of all hip replacements need some sort of revision • No. 1 reason for revision is loosening of the implant

  2. Why Total Joint Replacement???

  3. Cement vs. CementlessCharacteristics

  4. Bonding Time: Cemented vs. Cementless • Cemented • Approximately 10min • Cementless • One year for good ingrowth • Up to two years for 100% of ingrowth bone (30% of the total porous surface) • Cemented • Approximately 10min • Cementless • One year for good ingrowth • Up to two years for 100% of ingrowth bone (30% of the total porous surface)

  5. Healing Time:Cemented vs. Cementless

  6. Loosening in Cemented THR’s • Loosening is caused by several factors • Macrophage Phagocytosis of the bone cement • “Cement Disease” – lytic erosions in bone • Microcrack formation in the cement • All of these result in the spawning of bone cement particles

  7. Typical Bone Cement Composition • 90% polymethylmethacrylate (PMMA) • 10% Barium Sulfate

  8. Abrasive Wear • Particles from loose cement cause abrasive wear in the acetabular joint

  9. Bone Cement Stabilization • Three ways to try to stabilize bone cement • Replace Barium Sulfate component with a more biocompatible material • Add additional material to the cement to reinforce it • Mix the cement in a vacuum to reduce voids and stress concentrations

  10. Barium Sulfate Substitutes • Materials that are expected to promote bone growth, which assists in reinforcing the cement • Examples • Hydroxyapetite (HA) • Same material found in bone • Tricalcium Phosphate • Good biocompatibility

  11. Reinforcing Materials • Reinforcing materials give the PMMA more attachment points which increases the strength of the bone cement • Some examples include • Glass Fibers • Carbon Fibers or Particles • Metal fibers

  12. Vacuum Mixing • Vacuum mixing decreases voids in the bone cement which decreases stress concentrations within the hardened cement

  13. Impaction Grafting • Loose implant and cement is removed. • Extremely thin bone is reinforced with wire mesh. • Crushed bone chips are impacted into the canal.

  14. Improving methods of fixation • Wear factors • Patient factors • Surgical factors • Implant design variables

  15. Patient & Surgical Factors • Weight • Health • Alignment • Surgical debris • Infection • Important

  16. Design Variables • Material Properties • Implant Geometry • Wear Debris • Method of Fixation

  17. Material Choice • Metal on Metal • Ceramic on Ceramic • Metal on UHMWPE

  18. Metal on Metal • Low wear rate • Self-healing • Stress shielding • Low conformity • Titanium, Cobalt-Chrome

  19. Ceramic on Ceramic • High biocompatibility • Low wear • Very brittle • Abrasive particles

  20. Metal on UHMWPE • Low coefficient of friction • Viscoelastic • Low wear resistance • Most common

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