Goal
Initial press fit
- implant geometry fits the cortical bone in the proximal femur
- good initial mechanical stability
Biological fixation for success
- good press fit
- minimal micromotion
- bony or fibrous tissue ingrowth or ongrowth
Longetivity
- avoidance adverse stem bone stiffness ratios
- fixation surface that provides a transitional stress transfer from the proximal femur to the diaphysis
- avoid extreme stress shielding or excessive rigidity
Press fit
True Press-Fit in Bone
Bone is a viscoelastic material
- implies that its elastic recoil will become less with time
- the amount that bone will "creep" or undergo stress-relaxation depends on its density
- cortical bone has less viscoelastic behavior than cancellous bone
- the fact that bone will relax and lose elasticity over time limits the amount of time over which a true press-fit can be maintained in bone
- once the initial press-fit dissipates,a prosthesis may move under load in the bone and either re-establish a press-fit or become loose
Non porous coated uncemented implants are commonly referred to as press fit implants
Design
Proximal metaphyseal filling
- curved, anatomic stem
- most common
- tight proximal fit
Distal isthmus filling
- straight stem
- used more commonly in revision
Techniques of Initial Fixation
Definition Rigid Fixation
- micromotion <150 microns
- ideal 50-100
A. 'Press fit' (1-2mm undersized) technique
- bone expands around prosthesis
- generates hoop stresses
- femur and acetabulum
B. Line to line fit
- bone is prepared to same size as implant
- extensive porous coating with stem
Contraindications
Stove pipe femurs (Dorr < 0.75)
Poor bone stock
Proximal femoral geometry / Dorr calcar-to-canal ratio
Important if considering uncemented prosthesis
3 types - 501's, Stove pipe, Flares
- measure canal at LT & 10cm below
- inner diameter at midportion of LT divided by diameter 10 cm distal
- must be <75% for uncemented prosthesis
Type A
- ratio < 0.5
- cortices seen on both AP & lat
- most amenable to uncemented component
Type B
- between 0.5 and 0.75
- thinning of post cortex on lateral
- intermediate
Type C
- > 0.75
- thinning of cortices on both views
- "stovepipe" femur
- favours use of cemented stem
Biologic fixation
Two types
1. Ingrowth
- porous coating
- HA coated
- combinations
2. Ongrowth
- grit blasted
- increases roughness
- typically needs to be entire surface
Ingrowth
Pore size
- optimum pore size 50-350 microns (ideal 50-150)
Porosity
- 40-50%
Pore depth
- deeper pores better
- increased shear strength with loading
Mechanism of porous coating
Titanium plasma sprayed
- often used to create pores
- then covered with HA to supplement
Tricalcium phosphate
- also used
HA coating
- sprayed on as a porous coating
- osteoconductive
- surface dissolution to Ca and Phosphate
- stimulates osteoblasts
Extent of Porous coating
Complete / incomplete
- both proximal and distal fixation are important
- is a trade off between fixation and shielding
Extensively coated implants
- improve likelihood of solid fixation
- distal loading of bone
- get mainly diaphyseal spot welding
- increase proximal stress shielding
- same problem with cemented implants
Proximal porous coating
- proximal loading of bone
- minimises proximal shielding
- more common
Materials
Rigidity
Want less rigidity to minimise stress shielding
Stiffness related to
- modulus
- fourth power of the stem radius
- solid v slotted / fluted stems
Young's modulus of Elasticity
Bone 12
Titanium 117
Cobalt-chromium 210
Minimise rigidity
1. Titanium alloy v cobalt chromium
- less structural rigidity
- lower modulus of elasticity
- 2 - 3 x less stiff
2. Implant size
- as size increases, rigidity increases
3. Design
- some stems have a coronal slot to decrease rigidity
Osteointegration
Engh et al categories
1. Osseointegration
2. Stable Fibrous ingrowth
3. Unstable fixation
A. Signs of osteo-integration
Take 1 year to see
1. Spot welds
- densification of endosteal bone
- usually in the region of termination of the porous coating on the implant
2. Absence of any radiodense reactive lines
- may occur around the smooth portion of the implant
- this is where bone ingrowth is not expected to occur
- they should not be present adjacent to the porous coating
3. Calcar atrophy
- this change is sometimes subtle
4. Increased cancellous density / cortical hypertrophy distal to the coated region
B. Failed bone ingrowth / successful stabilization by fibrous tissue ingrowth
1. Parallel Sclerotic lines
- remodelling signs around the porous surface
2. Less atrophy of the medial femoral neck
2. No progressive migration
3. No local cortical hypertrophy / spot welding
C. Signs of frank implant instability
1. Component migration
- usually by subsidence and varus tilt
2. Progressive luceny on serial radiographs
3. Development of inferior pedestal
Complications
Fracture
- slow careful insertion / make sure is advancing with each blow
- can prevent or treat with cerclage wire
- assess stability
- revert to cemented stem if unable to obtain stability with press fit
Thigh pain
Causes
1. Initial instability (lack of press fit)
2. Failed bony ingrowth / Late instability
3. Micromotion at distal stem
- disadvantage of proximal coating
- will usually resolve over 2 years
- only 1% severe pain
4. Mismatch modulus of elasticity
- lower with titanium
- tend to have lower incidence of thigh pain
- smaller stems
5. Osteoporotic bone
Treatment
- can cerclage wire cortical strut grafts
- improve bony rigidity over distal stem
Stress shielding
Most common with distal press fit / fixation
Osteolysis