General Flashcards
THR implant fixation wear and osteolysis catastrophic wear prosthetic joint infections
1
Q
What does the cement in cemented THR do?
What are the indications for its use?
A
- Interlocking fit
- provides mechanical interlock of methylmethacrylate to the inctercises of bone ( acts as grout)
- this is a static interface with limited remodelling potential
- bone-cement is stronger in osteporotic/ irradiated bone ( as doesn;t rely on ingrowth)
2
Q
How do the acetabular compotents of cemented THR fail?
A
- Higher shear and tensional forces
3
Q
Describe the 1st generation cementing technique?
A
- Hand mixed cement
- finger packed cement
- no canal preparation or cement restrictor
4
Q
Describe the 2nd generation cementing technique?
A
- Cement restrictor placement
- Cement gun
-
Femoral canal preparation
- brush and dry
5
Q
Describe the 3rd generation cementing technique?
A
- Vacuum mixing to reduce cement porosity
- cement pressurisation
-
femoral canal preparation
- pulsatile lavage
6
Q
What is the criteria to optimise cement fixation of prothesis?
A
- Limit gaps
- Limit porosity of cement
- Create cement mantle at least 2mm thick
- increase risk of matle fx if <2mm thick
- for small canals in which 2mm mantle is impracticle, 2/3 of the canal should be filled by the femoral stem and 1/3 by cement
- use stiffer femoral stem
- increase Young’s modulus
- Use smooth femoral stem
- no sharp corners
-
Avoid mantle defects
- defined as any area where the prosthesis touches cortical bone with no cement between
- creates an area of _higher concentrated stress _& is assoc with higher loosening rates
7
Q
What is bioloigic interdigitation?
A
- Is a dynamic interface and there is potential for bone remodelling and a life last bond
8
Q
Decribe the methods of bioloigcal interdigitation?
A
-
Porous coated metallic surfaces
- implant covered in porous covered surface that allows bone ingrowth fixation
- extent of porous coating
-
Proximal coating only
- adv in less distal shear stress
-
Extensively coated stem
- disav is stress sheilding of proximal bone
- used in tx of periprosethtic fx
-
Proximal coating only
-
Grit blasted
- implant is grit blasted creating a roughed surface that allow bony ongrowth
- surface roughness directly proportinal to Interface shear strength
- roughness= av distance from peak to valley
- disadv = requires larger area of grit- blasted metal relative to porous coated metal
-
hydroxyapatite (HA)
- an osteoconductive agent used as an adjunct to porous - coated and grit blasted surfaces
- promotes more rapid closure of gaps
- disav -> potential to delaminte from surface coating
- success depends on
- high crytsallinity
- thickness 50um
9
Q
What are the mechanism for porous coating?
A
-
Titanium plasma sprayed
- often used to create pores
- then covered with HA to supplement
- Tricalcium phosphate
- Ha coating
10
Q
Can you descibe the different extent of porous coating?
A
-
Complete vs incomplete
- Both proximal and distal fixation important
- trade off for fixation vs stress shielding
-
Proximal only
- Proximal loading of bone
- minimise proximal stress shielding
- more common
-
Extensively
- improve likelihood of solid fixation
- Distal loading of bone
- > proximal stress shielding
- mainly diaphyseal spot welding
11
Q
What are the optimal characteristics for porous coating implants?
A
Optimal values
- Pore size 50-300um
- Porosity 40-50% ( greater -> shear off metal)
- Gap <50um
( between bone and prosthesiss - Micromotion <150um
- anthing > 150 -> fibrous ingrowth
12
Q
write the formula of hydroxyapetite?
A
- Ca 10 (PO4)6 (OH)2
13
Q
What is hydroxyapetite?
A
- An osteoconductive agent used as an adjunct to porous coated and grit blasted surfaces
14
Q
What is the key to uncemented fixation?
How is this achieved?
A
-
Obtain rigid fixation
- micromotion kept <150um
- otheriwse firbous tissue will develop-> unstable implant
Methods to obtain this
-
Press- fit technique- non porous implant
- implant slightly larger than surrounding bone 1-2 mm
- Bone expands around prosthesis
- Generate hoop stresses around prothesis minimise micromotion
-
Line to line contact
-
mechanical strength of biological interdigitation s stronger if implant is seated against cortical bone
- in acetabulum obtain contact between cup and cortical rim
-
mechanical strength of biological interdigitation s stronger if implant is seated against cortical bone
15
Q
What is stress shielding?
A
- Stress shielding is the redistribution of load (and consequently stress onthe bone) that occurs when the femoral head is replaced by the femoral component of a total hip replacement;
- as a result there is a reduction in density of bone often at proximal femur due to removal of normal stress of the bone by an implant ( femoral component of hip replacement)
- This is because Woolfe’s law: bone in healthy person /animal will remodel in response to the loads placed upon it.
- So if the load decreases the bone will become weaker and less dense as no stimulus for it to remodel to maintain normal bone density
- stress on proximal 10 cm of femoral cortex is reduced, because much of the load bypasses this region and is carried in the metal stem to the isthmus of the femur
16
Q
Where is stress shielding most common?
A
- Proximal femur
17
Q
What factors increase stress shielding?
A
- larger diameter stems
-
stiffer stems
- radius
- young’s modulus
- geometric shape
- extensively porous coated stem
- greater preoperative osteopenia
19
Q
How is the extent of stress shielding evaluated?
A
- Spot welding ( increase density) at tip of extensive porous coated stem
22
Q
What are the signs of uncemented implant failure?
A
- Implant migration
- subsides and varus tilt
- Progressive lucency on serial xrays
- development of inferior pedestal
30
Q
Describe macrophage activated ostoegensis and osteolysis?
A
- MAcrophage activation
- wear debris particles at bone/cement interface results in macrophage activation and futher macrophage recruitement
- macrophage release proinflammatory mediators and proteolytic enzymes e.g cytokines, osteolytic factors
- TNF-aplha
- TGF beta
- oseteoclast activation factor
- oxide radicals
- hydrogen peroxide
- IL1/6
- prostaglandins
- Increase TNF alpha increases RANK
- Increase in VEGF with UHMWPE enhances RANK and RANKL activation
- RANKL, TNF-alpha, IL-1, IL-6, IL-17, and M-CSF mediate the differentiation of myeloid precursor cells into multinucleated osteoclasts, which release cathepsin K and acid and cause resorption lacunae;
- Mesenchymal cells (prosthesis-loosening fibroblasts) present at the bone surface contribute actively to bone resorption.
- Osteolysis around prothesis -> micromotion
- micromotion-> > particle wear -> loosening
40
Q
How can PE machining effect catastophic failure?
A
- Cutting tools can disrupt chemcial bonds of PE
- Cause of failure
- machining shear forces cause Subsurface region (1-2mm) stretching of PE chains
- esp in amorphois regions>crystalline regions
- leads to subsurface delamination and fatigue cracking
- can show classic white band of oxidation in subsurface 1-2mm below articular surface
- machining shear forces cause Subsurface region (1-2mm) stretching of PE chains
- Solution
-
Use direct-compression moulding of PE
- preform by moulding directly from PE powder to desired product
- results in less fatigue crack formation and propagation compared to ram bar extrusion
-
Use direct-compression moulding of PE
