Incidence
2-3% of cases
- doubles with infrequent operator
- second most common reason for revision after loosening
Australian Joint Registry
- dislocation accounts for 14.8% of revisions
Positions
Posterior dislocation
- hip flexed, adducted, IR
- 80%
- usually getting out of chair
Anterior dislocation
- hip extended, adducted, ER
Timing
Early < 6/ 52
- majority of single dislocations
- usually excessive hip position by patient
- before adequate muscle control & soft tissue healing
- after six weeks strong pseudocapsule forms about hip
- adds to stability +++
Chance of recurrence 40%
Late > 6 weeks
- represents majority of recurrent dislocations
- sually due to increase in ROM & activity
- manifests unrecognised impingement / malposition
Chance of recurrence 60%
Factors
Surgeon factors
- experience
- approach
- component position
- component design
- soft tissue balance
- impingment
Patient factors
- soft tissue
- cognitive disorders
- NM disorders
- NOF fracture
- revision
Surgeon Factors
1. Surgeon Experience
- < 30 THR per year
- 2 x dislocation rate
2. Approach
A. Increased with posterior approach
- common in early papers
- reduced with short ER repair / use of large heads / component position
- now equivalent rates to anterolateral
B. Transtrochanteric
- increased with trochanteric non union x6
Koster et al J Orthop Trauma Rehab 2023
- meta-analysis of 11 studies and 2,000 patients
- dislocation rates
- posterolateral 1.4% / anterior 0.4% / anterior 0%
3. Component position
A. Acetabular safe zones
- abduction 40 +/- 10o
- anteversion 15 - 30o
B. Excessive femoral anteversion
- especially when combined with excessive acetabular anteversion
- predisposes to anterior dislocation
C. Ranawat concept of combined anteversion
- acetabular + femoral anterversion
- 25 - 35o for men
- 35 - 45o for women
D. May wish to increase anteversion in posterior approach and reduce it in the anterior approach
- 9800 hips
- evaluated Lewinnek safe zone
- cup inclination 40+/-10
- cup anteversion 15+/-10
- 58% of dislocations had cup position within safe zone
4. Component design
A. Increased head size
Increased size increases head-neck ratio
- reduces impingement / increases arc of motion
Increased jump distance
- seated deeper in acetabulum
- decreases jump distance
- greater translation before dislocation
B. Liner profile
- posteriorly elevated profiles
- i.e. neutral liners v 10o elevated rim liners
- theoretically more stable
- reduces dislocation rates early, but not late
- can cause impingement in extension and ER
- this may lead to dislocation and increased wear
- can put hood in variety of positions
- usually postero-superior
5. Soft tissue tension
Capsular Management
Kobayashi et al Arch Orthop Trauma Surg 2023
- systematic review of capsular resection versus capsular repair
- lower dislocation and better HHS with capsular repair
A. Restore LLD and offset
- reduced offset associated with increased dislocation
- reduces ST tension
- increases risk of impingement
B. Dislocations reduced with careful capsular and soft tissue repair
- reduces dislocation rate in posterior approach
6. Impingement
- when two non articular surfaces come into contact during joint ROM
- decrease by increasing head neck ratio
- may be liner / osteophyte / excessive capsule
- always put hip through ROM
- ensure in full extension and ER, no posterior impingment
- ensure in flexion 90o and IR, no anterior impingment
Patient Factors
1. Soft tissue function
- previous hip surgery
- revision THR
- weak abductors
2. Cognitive disorders
- dementia / delerium
- alcoholism
3. NM disorders
4. Women
5. Post THR for Neck of Femur fracture
- no stabilising capsular hypertrophy / fibrosis seen in OA
6. Revision
- dislocation rates higher in revision setting
Prevention
Pre-operative education
- avoid dislocation in first 6 weeks
Template
- restore offset and leg length
Approach
Posterior approach
- careful short ER repair
Trochanteric osteotomy
- large flat surface
- strong repair
- protected WB /52
Component positioning
A. Extrapelvic Landmarks
- careful patient positioning
- patient stable
- ASIS perpendicular to floor
- use guides on acetabular insertion jigs
B. Intrapelvic Landmarks
- transverse acetabular ligament
- anterior and posterior acetabular walls if no osteophytes
Component design
- liner lip posterosuperior
Prevent impingement
- remove wall osteophytes
- restore offset
- anterior capsule can cause impingement
- avoid excess cement
Large head neck ratio
Trial reduction
- flex to 90o, IR 45o, adduct 20o
- full extension, ER 45o
- ensure stability
- restore offset
- check LLD
Post operative
- avoid extremes of position
- abduction pillow
- knee immobiliser in confused patients / limits hip flexion
- post-op education
- no driving, high chairs, low cars 6 weeks
- no crossing legs ever
Management
Early
- MUA
- re-educated
- mobilise as tolerated
Late
1st episode
- treat with reduction
Recurrence
- treat with abduction brace
- 20° flexion / Abudction / ER
- for 6/52
Repeated
- revision
X-ray evaluation
A. Component malposition
1. Acetabular Abduction
Easy to assess on AP
2. Acetabular Anteversion
Much more difficult to assess
- compare ellipse of acetabulum on AP pelvis and AP hip
Concept
- AP pelvis the beam is centred over the pelvis
- AP hip the beam is centred over the hip
If cup anteverted
- looks flat on AP pelvis
- looks elliptical on AP hip
If cup retroverted
- looks elliptical on AP pelvis
- looks flat on AP hip
B. Other
Eccentric liner wear
- draw lines on paper, compare each side
- thickness not equal both sides with wear
Loosening
Insufficient offset
Surgical Revision
Need to decide cause of problem
- preoperative and intraoperative
- malposition / impingement / soft tissue
- have options available to address each problem
Initial
1. Impingement
- removal of osteophytes or cement
- exchange components to improve head neck ratio
- may need to adjust component malposition
2. Malposition
- assess stem + cup on CT
Options
- change for Augmented polyethylene lining (if uncemented cup)
- revise component positioning
- larger head technology
3. Incorrect tissue tension
- longer neck / correct offset
4. Worn liner
- exchange liner
5. Abductor insufficiency
- trochanteric advancement
- increase femoral offset (modular head, lateralised liner)
Salvage
1. Constrained cups
Concept
- an acetabular component that uses a mechanism to restrain the femoral head within the cup
- can be implanted denovo or cemented into well fixed cup
- usually has a metal locking ring
Indication
- deficient soft tissues
- paralysed abductors
- GT non union
Varieties
A. Cup and monopolar liner with locking ring
B. Bipolar constrained liner with locking ring
Problems
A. A constrained cup may still dislocate
- usually require surgery to relocate / require revision
B. Inhibit ROM and transmit significant forces, which may contribute to early loosening
2. Failure or Unreliable patients
Options
- bipolar hemiarthroplasty
- girdlestones