Concept
THR in dysplastic hips has a higher failure rate
- due to anatomic abnormalities
- due to generally younger age
Aim
Restore normal biomechanics and preserve bone stock
Issues
Soft tissues
Sciatic nerve in abnormal position / danger
Hamstring / adductors / RF tight
Horizontal abductors - function less efficiently
Thick hourglass capsule
Thickened psoas tendon
Acetabulum
Low subluxation
- shallow with wide opening
- small
- deficient anterior / lateral / superior
- better bone stock posteriorly
High dislocation
- small pelvis
- thin & soft acetabular wall
- gross anteversion
Femur
Increased anteversion
- valgus neck shaft angle
Narrow tapered femoral canal
- tight isthmus
- AP diameter > ML
Posterior displacement of the greater tuberosity
LLD
Can be very short
- maximum sciatic nerve can be lengthened is 4 cm
Crowe Classification
Based on extent of proximal migration of femoral head compared to the height of the undeformed femoral head
- femoral head is 20% height of pelvis
- measure the vertical distance between the inter-teardrop distance and the head neck junction
- this distance as a ratio of the femoral head
Crowe I: Proximal displacement < 50% femoral head (10% pelvis)
Crowe II: Proximal displacement femoral head 50-75%
Crowe III: Proximal displacement femoral head 75 - 100%
Crowe IV: Proximal displacement femoral head >100% (20% pelvis)
Efekhar Classification
A Elongated dysplastic acetabulum
B Intermediate acetabulum
C High false acetabulum
D High but no false acetabulum
Examination
Pelvic tilt
Lumbosacral flexibility
Fixed hip deformities
Real and apparent LLD
Previous scars
Operative Management
Aim
Restore hip centre
Acetabular bony coverage
Restore LLD
Technical Factors
Soft tissue release
- capsule / psoas / adductors / abductors
- abductor slide or release from ilium
- protect sciatic nerve
Acetabulum
- need small components
- restore centre of rotation / bring down to true floor
- may need to augment superolateral acetabulum
Femur
- small components
- correct femoral anteversion
- restore offset as best able
- may require trochanteric slide
LLD
- > 4 cm need femoral osteotomy
Acetabular component
Options
1. Restore normal hip centre
2. High hip centre
3. Medialise cup
Restore normal hip centre
A. Recreate centre of rotation
Place in true acetabulum
- transverse ligament is anatomical landmark
Template hip centre
- inter-tear drop line is inferior margin
- ilio-ischial line is medial margin
- superior edge acetabulum lateral margin
B. Need for augmentation
Superior defect must be < 30%
Options
- bulk femoral head autograft
- mesh + impaction bone graft
- reinforcement rings / cages
- augmented cups
Acetabulum Reconstruction
Bulk Femoral Head Autograft
Advantage
- restore hip centre
- improve bone stock for revisions
Technique
- fashion femoral head into 7 graft
- screw into place with 2 x 6.5 mm cancellous screws
- ream into inferior aspect of graft
Spangehl et al JBJS Am 2001
- 44 hips followed up for 7.5 years
- femoral autograft with uncemented cup
- 4 revisions / 10%
- acceptable early results
Harris JBJS 1997
- 55 autogenous bulk autograft + cemented acetabulum
- average follow up 16.5 years
- average age of patient at time of surgery 42 years
- average size of acetabulum 40mm
- average coverage of cup by graft 49%
- 29% (16/55) revised and further 31% (17/55) radiographically loose
- those grafts 30% or less of cup coverage were well fixed at 16 years
- the greater the coverage of bone graft initially, the greater the rate of late revision
- most hips did well for initial 5 - 10 years
Mesh + Impaction Bone Grafting
Reinforcement ring / cage + bone graft
Technique
- morcellised bone graft
- support with acetabular reinforcement ring
- usually screwed into ilium and ischium
- cement acetabular component into ring
Muller JBJS 1998
- 87 hips, majority Crowe 3
- Muller acetabular roof reinforcement ring
- autograft and cemented polyethylene cup
- 10% revision at 9.4 years
Ganz J Arthroplasty 2005
- 33 cases at 10.8 years
- 3 revisions (9%)
- 2 of the revisions had structural allograft
DDH augmented cups
High Hip Centre
Advantage
- allows coverage by native bone
- decreases need for femoral shortening
Disadvantages
- very small acetabular component
- very thin poly
- abnormal hip biomechanics
- risk of bony impingement
- may lateralise hip centre
Results
Kaneuji et al J Arthroplasty 2009
- 30 hips followed up for 15 years
- mild superior hip centre compared to contralateral normal hip (13 mm average)
- 1/30 revised
Socket medialisation / acetabuloplasty / medial protrusio technique
Technique
- controlled medialisation with deliberated over-reaming
- can deliberately fracture medial wall
Advantage
- improves lateral coverage
- decreases JRF through medialisation
Disadvantage
- loss of medial bone stock compromising future revision
- risk of early catastrophic component migration medially into pelvis
Results
Femur
Issues
Small and narrow
Excess anteversion
Management
Small components
Uncemented
Need to be modular
A. Abnormal shape of proximal femur
- difficult to obtain press fit / risk fracture
- diaphyseal press fit
- small modular metaphyseal component
B. Need modularity to adjust anteversion
- SROM prosthesis
- dial in version
Cemented DDH prothesis
Design
- smaller with minimal metaphyseal flare
- this allows stem to be orientated independently of patients anteversion
LLD / Abductor Tension
Issues
Only lengthen sciatic nerve 4cm
Abductors very tight and prevent lengthening
Difficulty reducing hip
Difficulty reducing hip
1. Psoas release
2. Subtrochanteric osteotomy
3. GT osteotomy
Tight abductors
Trochanteric slide allows
- acetabular exposure
- retensioning abductors
- reposition abductor insertion to correct anteversion
LLD
Subtrochanteric osteotomy
Advantage
- acetabular exposure (lift up)
- correction anteversion
- shortening femur
Technique
- mark rotation with 2 x small drill holes
- make osteotomy
- transverse osteotomy allows rotational adjustment
- step cut more difficult but gives rotational stability
- insert trial femur proximally
- reduce hip joint
- calculate resection based on overlap of proximal and distal femoral segments
- uncemented or cemented stem
- use bone resected as onlay
Management Algorithm
Acetabulum
Crowe I
Mildly dysplastic
- minimal deformity, good bone stock
- small standard cup medialised for coverage
- < 30% uncovering allowed
- small femoral stem
Crowe II / III
Usually very deficient laterally
- due to femoral head eroding acetabulum
- restore hip centre by reaming medially
- then need to provide superolateral coverage
- autograft + mesh / allograft / DDH cup / tantalum
Crowe IV
Usually good bone stock in true acetabulum
- femoral head has not eroded bone
- recreate acetabulum and place small component
- use teardrop and fovea as landmarks
Femur
Crowe I/II
Minimal LLD
- sess femoral shortening required
- avoid excessive anteversion based on abnormal femoral neck
- otherwise get anterior instability and loss ER
Crowe III/IV
If greater than 4cm LLD
- need to shorten femur