Total Hip ReplacementsTotal Hip Replacements

About 160,000 THR’s About 160,000 THR’s performed every yearperformed every year

90% of all hip 90% of all hip replacements need replacements need some sort of revisionsome sort of revision

No. 1 reason for No. 1 reason for revision is loosening revision is loosening of the implantof the implant

Why Total Joint Replacement???Why Total Joint Replacement???

Cement vs. CementlessCement vs. CementlessCharacteristicsCharacteristics

Bonding Time: Bonding Time: Cemented vs. CementlessCemented 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)

Healing Time:Healing Time:Cemented vs. CementlessCemented vs. Cementless

CementedCemented CementlessCementless

Weight bearingWeight bearing Next day with Next day with crutchescrutches

Next day depending on stabilityNext day depending on stability

Walk (crutches)Walk (crutches) 2 day after surgery2 day after surgery 2 day after surgery2 day after surgery

6-12 weeks depending on 6-12 weeks depending on surgeonsurgeon

Pain FreePain Free 5 days in some cases5 days in some cases Depends on stabilityDepends on stability

Normal WalkNormal Walk 2 weeks (6 weeks with 2 weeks (6 weeks with crutches is crutches is recommended)recommended)

Couple of months to 1 year Couple of months to 1 year depending on ingrowth ratedepending on ingrowth rate

Everything Everything goodgood

Around six months for Around six months for a knee replacementa knee replacement

2 years onward2 years onward

Loosening in Cemented THR’sLoosening in Cemented THR’s

Loosening is caused by several factorsLoosening is caused by several factors Macrophage Phagocytosis of the bone Macrophage Phagocytosis of the bone

cementcement ““Cement Disease” – lytic erosions in boneCement Disease” – lytic erosions in bone Microcrack formation in the cementMicrocrack formation in the cement

All of these result in the spawning of bone All of these result in the spawning of bone cement particlescement particles

Typical Bone Cement CompositionTypical Bone Cement Composition

90% polymethylmethacrylate (PMMA)90% polymethylmethacrylate (PMMA)

10% Barium Sulfate10% Barium Sulfate

Abrasive WearAbrasive Wear

Particles from loose cement cause abrasive Particles from loose cement cause abrasive wear in the acetabular jointwear in the acetabular joint

Bone Cement StabilizationBone Cement Stabilization

Three ways to try to stabilize bone Three ways to try to stabilize bone cementcement

1)1) Replace Barium Sulfate component with a Replace Barium Sulfate component with a more biocompatible materialmore biocompatible material

2)2) Add additional material to the cement to Add additional material to the cement to reinforce itreinforce it

3)3) Mix the cement in a vacuum to reduce voids Mix the cement in a vacuum to reduce voids and stress concentrationsand stress concentrations

Barium Sulfate SubstitutesBarium Sulfate Substitutes

Materials that are expected to promote Materials that are expected to promote bone growth, which assists in reinforcing bone growth, which assists in reinforcing the cementthe cement

ExamplesExamples Hydroxyapetite (HA)Hydroxyapetite (HA)

Same material found in boneSame material found in bone Tricalcium PhosphateTricalcium Phosphate

Good biocompatibilityGood biocompatibility

Reinforcing MaterialsReinforcing Materials

Reinforcing materials give the PMMA Reinforcing materials give the PMMA more attachment points which increases more attachment points which increases the strength of the bone cementthe strength of the bone cement

Some examples includeSome examples include Glass FibersGlass Fibers Carbon Fibers or ParticlesCarbon Fibers or Particles Metal fibersMetal fibers

Vacuum MixingVacuum Mixing

Vacuum mixing decreases voids in the bone Vacuum mixing decreases voids in the bone cement which decreases stress concentrations cement which decreases stress concentrations within the hardened cementwithin the hardened cement

              

        

             

         

             

         

Impaction GraftingImpaction Grafting

Loose implant and Loose implant and cement is cement is removed.removed.

Extremely thin Extremely thin bone is reinforced bone is reinforced with wire mesh.with wire mesh.

Crushed bone Crushed bone chips are impacted chips are impacted into the canal.into the canal.

Improving methods of fixationImproving methods of fixation

Wear factorsWear factors Patient factorsPatient factors Surgical factorsSurgical factors Implant design variablesImplant design variables

Patient & Patient & Surgical FactorsSurgical FactorsWeightWeightHealthHealthAlignmentAlignmentSurgical debrisSurgical debrisInfectionInfectionImportantImportant

Design VariablesDesign VariablesMaterial PropertiesMaterial Properties

Implant GeometryImplant Geometry

Wear DebrisWear Debris

Method of FixationMethod of Fixation

Material ChoiceMaterial ChoiceMetal on MetalMetal on Metal

Ceramic on CeramicCeramic on Ceramic

Metal on UHMWPEMetal on UHMWPE

Metal on MetalMetal on MetalLow wear rateLow wear rate

Self-healingSelf-healing

Stress shieldingStress shielding

Low conformityLow conformity

Titanium, Cobalt-ChromeTitanium, Cobalt-Chrome

Ceramic on CeramicCeramic on CeramicHigh biocompatibilityHigh biocompatibility

Low wearLow wear

Very brittleVery brittle

Abrasive particlesAbrasive particles

Metal on UHMWPEMetal on UHMWPELow coefficient of Low coefficient of frictionfriction

Viscoelastic Viscoelastic

Low wear resistanceLow wear resistance

Most commonMost common