Uncemented

GoalTHR Uncemented

 

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

 

Dorr A Femur

 

Type B 

- between 0.5 and 0.75 

- thinning of post cortex on lateral

- intermediate

 

Dorr B Femur

 

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

 

Femur Fully Coated Proximal Shielding

 

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

 

THR Spot Weld

 

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

 

THR Subsidence Uncemented Component

 

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

 

Uncemented Femur Intraoperative Fracture

 

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

 

THR Proximal Stress Shielding

 

Osteolysis