Conformity Materials Fixation Gender

Conformity

 

Definition

 

Geometric measure of the closeness of the fit of the knee articulation

 

Concepts

 

The ideal TKA maximises articular conformity while minimising axial constraint

 

Highly conforming / Fully congruent

- constant sagittal femoral radius

- large contact areas

- theoretical limitation of flexion to 120o due to posterior impingement of the tibial component

- low wear rates

- poor ROM

- may be improved by mobile bearing

 

Low conforming / Partially congruent

- large contact areas in the first 20o of flexion

- contact area decreases with flexion due to a decreasing sagittal radius

- improves flexion range

- good ROM

- higher wear rates

 

Tibia all poly v metal backed

 

All poly tibia

 

TKR All Poly Tibia APTKR All Poly Tibia Lateral

 

Advantage

- cheaper (by $2000)

- good in elderly patients

- eliminates backside wear

 

Disadvantage

- lack of modularity

- difficult to insert if PS

 

Metal backed

 

Advantage

- improved load distribution

- modularity improves revision options (i.e. poly exchange)

- modularity enables increasing trialling before definitive implants

 

Disadvantage

- problems with locking mechanism

- most have some micromotion

- potential for backside wear / increased particulate debris

 

TKR Dislocated Fixed Bearing Poly

 

Fixation

 

Best technique controversial

 

Options

 

1. Cemented

2. Press-fit with porous ingrowth

3. Hybrid

 

Cemented

 

Current standard

- longest follow up

- cement "Seal" to particle migration

- easier to revise if infected

 

Uncemented

 

Initial results comparable

 

Risks

- very difficult to remove well fixed uncemented infected TKR

 

Advantage

- faster operation

 

Results

 

Baker et al JBJS Br 2007

- RCT of PFC cemented v uncemented

- 15 year follow-up

- 80% 15 year survival in cemented

- 75% 15 year survival in uncemented

 

Australian Joint Registry 2010

 

9 year survival

- cemented TKR      5.0%

- uncemented TKR  5.2%

- hybrid TKR           4.9%

 

Materials

 

Cobalt chronium femur

- can be covered with oxidised layer

- S&N oxinium

 

Cobalt chronium tibial base plate

- can be porous and HA coated

 

Gender

 

Anatomic differences

 

Female Femur

1.  Narrower medial lateral width for same AP diameter

2.  Variety of subtle PFJ anatomical and biomechanical differences

 

Evidence

1.  No evidence showing increased failure rates in men compared to women

2.  No evidence of improved outcome for women using gender specific knees