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




Can change liner

- multiple revision options i.e. for dislocation


Simple to remove

- Zimmer Xplant




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




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



- 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 



- 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




Acetabular fracture


Increased risk

- > 2 mm underream

- acetabular sizes < 52

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



- 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



- angiogram  / embolism

- laparotomy / pelvic packing




Loosening Uncemented Cup


Can be very difficult to identify with uncemented acetabulum