1. Total Hip Replacements About 160,000 THRs 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. Cementless Characteristics

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 CementedCementless Weight bearing Next day with crutches Next day depending on stability Walk (crutches) 2 day after surgery 6-12 weeks depending on surgeon Pain Free 5 days in some cases Depends on stability Normal Walk 2 weeks (6 weeks with crutches is recommended) Couple of months to 1 year depending on ingrowth rate Everything good Around six months for a knee replacement 2 years onward

6. Loosening in Cemented THRs Loosening is caused by several factors Macrophage Phagocytosis of the bone cement Macrophage Phagocytosis of the bone cement Cement Disease – lytic erosions in bone Cement Disease – lytic erosions in bone Microcrack formation in the cement 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 1) Replace Barium Sulfate component with a more biocompatible material 2) Add additional material to the cement to reinforce it 3) 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) Hydroxyapetite (HA) Same material found in bone Tricalcium Phosphate 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 Glass Fibers Carbon Fibers or Particles Carbon Fibers or Particles Metal fibers 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 Patient factors Surgical factors Surgical factors Implant design variables Implant design variables

15. Patient & Surgical Factors WeightHealthAlignment Surgical debris InfectionImportant

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