Uncemented acetabulum

Acetabular UnderseatingUncemented Cup with screws

 

Concept

 

1. Initial press fit with mechanical stability

2. Osteoconductive surface to allow osteointegration

3. Contact viable host bone

 

Advantage over cemented cups

 

Liner options

- different thickness for head sizes

- different bearing surfaces

- elevated liners

 

Press fit

 

Goal

- tight peripheral press fit with complete seating

- < 0.5 mm gaps

- < 150 um of micromotion to limit fibrous ingrowth

 

Reaming

- underream by 1 mm in normal bone

- underream by 2 - 3 mm in osteoporotic bone

 

Cup Design

 

Trident Stryker

Stryker Trident Titanium and HA coated

 

Zimmer

Zimmer Trabecular Metal Tantalum Cup

 

Material

- titanium - similar modulus of elasticity to bone, most common

- tantalum - ? superior osteointegration, used often in revision

 

Porous coatings

- titanium / hydroxyapatite coating

- high coefficient of friction for initial rigid fixation

- pores allow bony ingrowth long term

 

Shape

- designed to engage in the outer periphery of acetabulum

- hemispherical / nonhemispherical / elliptical

 

Results

 

Australian Joint Registry 2023 Revision rates by fixation (400,000 THA)

 

  Cemented Uncemented Hybrid
5 year 2.6 3.0 2.6
10 year 3.8 4.3 3.9
15 year 5.1 5.9 5.3
20 year   7.0 6.7

 

15 year revision rate by age

 

FixationAJR

64 - 74 year                                                                                  > 75 years

 

  Cemented Uncemented Hybrid
< 55   6.4 7.2
55 - 64 6.2 5.5 6.1
65 - 74 5.4 5.5 5.2
> 75 3.3 6.8 4.7

 

Screw fixation

 

Uncemented Cup with screws

 

Goal

 

Transacetabular screw fixation to augment initial cup stability

 

Safe zones

 

Acetabulum Wasielewski Safe Zones

 

Wasielewski et al JBJS 1990

- anatomical cadaveric study

- line ASIS to to ischial tuberosity through center of acetabulum

- line perpendicular to this creates four quadrants 

- safe quadrants are the posterior quadrants

- anterior screws can emerge within pelvis

 

Structures at risk

 

AS quadrant

- external iliac vein and artery

 

AI quadrant

- obturator nerve and vessels, femoral artery 

 

PI quadrant

- internal pudenal vessels, inferior gluteal nerve & vessels

- screws < 25 mm

 

PS quadrant

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

- aim screw between 2 cortices of ilium

- direct towards sacro-iliac joint

- can tolerate 85 mm screws

 

Results

 

Miura et al Medicine 2022

- meta-analysis of 4 RCTs

- no effect of supplementary screw fixation

 

Shengui et al J Orthop Traumatol 2022

- systematic review of 19 studies and 4000 patients

- screw fixation did not alter migration / revision / wear

 

Technique Uncemented Cup

 

Centre reamer in desired hemisphere of acetabulum

- begin 6 - 10 mm below templated size

- medialize initially

- remainder reaming in direction of final component position

- 45o abduction, 20 - 30o anteversion

- 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 into posteriosuperior quadrant

 

Complications

 

Acetabular fracture

 

Increased risk

- small acetabulum

- under-reaming

- elderly / osteoporotic bone

 

Management

- screws

- posterior column plating

- cage

 

THR Uncemented Cup Acetabular FractureIntra-op acetabular fracture

 

Failure of initial acetabular press fit

 

Acetabular spin out

 

THR Uncemented Cup Spin out

 

Errant Screw placement

 

Anterior quadrants

- can cause catastrophic haemorrhage

 

Management

- angiogram  / embolism

- laparotomy / pelvic packing

 

Malposition

 

Cup mal 1Cup mal 2Cup mal 3

 

Loosening

 

Loose cup