Uncemented Acetabulum

GoalTHR Uncemented

 

Initial mechanical stability

- adequacy of locking between component and bone

 

Need initial press fit for mechanical stability

Long term require biological fixation

 

Advantage

 

Can change liner

- multiple revision options i.e. for dislocation

 

Simple to remove

- Zimmer Xplant

 

Design

 

1.   Smooth

- relied on mechanical interlock for stability and long term fixation

- unacceptable early revision rates

- initial press fit, but no biological fixation

 

2.  Threaded

- universally bad results

- due to small contact area between bone and implant

 

3.  Smooth HA coated

- improved but still inferior results

 

4.  Porous coated

- allows ingrowth

- much better results compared to smooth components

- titanium or HA

 

5.  Hemispherical

- oversized cup

- initial press fit

 

6.  Flattened hemispherical

- rim fit

 

Requirements

 

1.  Material must be biocompatible

- titanium mesh

- cobalt chromium beads

- HA

- all of these materials have been shown to be adequate provided pore size is correct

 

2.  Surface must have optimal pore size

- between 100 and 400um

 

3.  Component must be placed in intimate contact with viable host bone

 

Press fit

- < 0.5 mm gaps 

- require tight peripheral press fit with complete seating

- maximises surface are available for ingrowth

- maximises area for stress transfer

 

Technique

- 1-2 mm underream

- risks are acetabular fracture and underseating

 

Acetabular Underseating

 

4.  Adequate initial stability to allow reliable ingrowth

- micromotion > 40 um generates fibrous tissue 

 

Options

- press fit with supplemental screw fixation

- line to line reaming with supplemental screws

- spikes

- pegs

 

Supplemental screw fixation

- 2 x bicortical screws

- provide similar stability as press fit

 

Uncemented Cup with screws

 

Screw problems

- can get backside wear / fretting

- holes can provide route for particle wear

 

Technique Uncemented Cup

 

Centre reamer in desired hemisphere of acetabulum

- begin 6 - 10 mm below templated size (44)

- medialise initially

- remainder reaming in direction of final component position

- 45o abduction

- 20 - 30o abduction

- increase until contact anterior and posterior

- AP diameter is what determines cup size

- petechial bleeding

- don't take away all subchondral bone

- continually assess posterior / anterior walls - must preserve

 

Can bone graft base and reverse ream

- especially with flattened hemisphere

 

Insert component 1 - 2 mm larger

- ensure seating (remove insertion handle and probe base)

- ensure stability

- add screws if any doubt

 

Screw placement

 

 Acetabulum Wasielewski Safe Zones

 

Wasielewski et al JBJS 1990

- anatomical cadaveric study

- line ASIS to centre acetabulum & ischial tuberosity

- line perpendicular to this 

- four quadrants 

- safe quadrants = 2 posterior quadrants

- posterior screws do not emerge within pelvis

 

Structures at risk

 

AS quadrant

- external iliac vein > artery

- vessels can be within 0.5 cm of the inner cortex of the pelvis

- become closer with increasing age

- in the anterior quadrants 25mm screws often too large

 

AI quadrant

- obturator nerve & vessels

- femoral artery 

 

PS quadrant

- sciatic nerve / superior gluteal nerve & vessels in danger at greater sciatic notch

- aim screw between 2 cortices of ilium

- direct towards SIJ

- can tolerate 85 mm

 

PI quadrant

- internal pudenal vessels

- inferior gluteal nerve & vessels

- maximum screw length is 25 mm

 

Complications

 

Acetabular fracture

 

Increased risk

- > 2 mm underream

- acetabular sizes < 52

- elderly (consider line to line reaming and use of screws, or use cement)

 

Management

- screws

- posterior column plating

- cage

 

THR Uncemented Cup Acetabular Fracture

 

Failure of liner fixation / acetabular fixation

 

Acetabular spin out

- insufficient initial fixation

- failure biological fixation

 

THR Uncemented Cup Spin out

 

Liner spin out

- must ensure fixation method is sufficiently engaged

 

Errant Screw placement

 

Anterior quadrants

- can cause catastrophic haemorrhage

 

Management

- angiogram  / embolism

- laparotomy / pelvic packing

 

Loosening

 

Loosening Uncemented Cup

 

Can be very difficult to identify with uncemented acetabulum