Dislocation

 

 

Incidence

 

van Erp et al Arch Orthop Trauma Surg 2022

- systematic review of 174 studies and 5 million THA

- overall incidence of dislocation 1.7%

- reducing over time

- dislocation rate 2010 - 2020 0.7%

 

Factors

 

Kunutsor et al Lancet Rheum 2019

- meta-analysis of risk factors for dislocation

- 125 studies and 4.5 million THA

Patient factors	Surgical factors
AVN / RA / neck of femur fractures	Surgeon experience
Revision THA	Surgical approach
Females Elevated BMI Medical co-morbidities (ASA 3) Neurological conditions Psychiatric disorders	Implant factors - component position - femoral head size - elevated liners - dual mobility
	Soft tissue balancing - offset and LLD
	Impingement

 

Surgical factors

 

1.  Surgeon Experience

 

2.  Approach

 

Koster et al J Orthop Trauma Rehab 2023

- meta-analysis of 11 studies and 2,000 patients undergoing THA

- overall dislocation rates 0.8%

- posterior 1.4% / anterior 0.4% / lateral 0%

 

Steenbergen et al Hip Int 2023

- Dutch registry of 270,000 THA

- posterior 1.4% / anterior 0.4% / lateral 0.6%

 

3.  Component position

 

 

Acetabular safe zones

- inclination 40 +/- 10^o

- anteversion 15 - 30^o

 

Abdel et al CORR 2016

- 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

 

Zijlstra et al Acta Orthop 2017

- Dutch Registry of 166,000 hips

- dislocation rates higher with 22 - 28 mm heads compared to 32 mm heads

- 36 mm heads reduced dislocation rates with posterolateral approach

 

B. Acetabular liners

- posteriorly elevated profiles

- i.e. neutral liners v 10^o elevated rim liners

 

C.  Dual mobility

 

Romagnoli et al Int Orthop 2019

- systematic review of 15 studies and 2400 patients

- comparison of dual mobility with standard THA

- reduced risk of dislocation with dual mobility

 

5.  Soft tissue tension

 

A.  Restore LLD and offset

- reduced offset associated with increased dislocation

- reduces soft tissue tension

- increases risk of impingement

 

B. 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

 

6. Impingement

- remove osteophytes

- restore LLD and offset

- always put hip through ROM

- ensure in full extension and ER, no posterior impingement between neck an

- ensure in flexion 90^o and IR, no anterior impingement

 

 Positions

 

Posterior dislocation

- hip flexed, adducted, internally rotated

- 80%

 

Anterior dislocation

- hip extended, adducted, externally rotation

 

Anterior dislocation

 

Timing

 

Early < 3 - 6 months

- most common

- excessive hip position by patient

- before adequate muscle control & soft tissue healing

 

Secondary 6 months - 5 years

- represents majority of recurrent dislocations

- component malposition / soft tissue tensioning

 

Late dislocations > 5 years

- typically due to polyethylene wear

 

Natural History

 

Cnudde et al J Clin Med 2024

- 136,000 THA in Swedish Registry

- 30 day revision rate 0.9%

- 50% had re-dislocation within first year

- 10% required revision for dislocation

 

Hip precautions

 

Post operative

- abduction pillow

- no driving, high chairs, low cars 6 weeks

- no crossing legs

 

Korfitsen et al Acta Orthop 2023

- systematic review of 8,800 patients

- no evidence that hip precautions (flexion / adduction / internal rotation) decrease dislocation rates

 

Management

 

Early 

 

MUA 

- re-educated

- mobilise as tolerated

 

Recurrent

 

Evaluate for causes

Revision

 

X-ray evaluation

 

1.  Acetabular Inclination

 

AP Xray

 

 

 

2.  Acetabular Anteversion

 

Xray

 

Methods for measuring cup anteversion on xray PDF

 

 

 

CT scan

 

Altered by pelvic tilt

 

 

3. Femoral anteversion

 

CT scan

- line prosthesis neck

- line posterior femoral condyles

 

4.  Offset

 

Xray

- difficult to evaluate on Xray

- affected by femoral anteversion

 

 

5.  Soft tissue tension

 

Increased dislocation with GT fractures

 

 

6. Eccentric liner wear

 

 

7. Loosening

 

 

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

 

Dislocated constrained liner

- difficult to reduce

- may require open reduction

 

 

Reduction

 

 

 

B. Inhibit ROM and transmit significant forces, which may contribute to early loosening

 

2.  Failure or Unreliable patients 

 

Options

- bipolar hemiarthroplasty

- girdlestones