arthroplasty
THA Dislocation and Instability
advanced
6 min
15 marks
5 questions
Clinical Scenario
A 68-year-old woman presents to the Emergency Department 3 weeks after primary right THA via posterior approach for osteoarthritis. She reports sudden onset severe right hip pain after bending to tie her shoelaces. Her leg appears shortened, externally rotated, and adducted. She is unable to weight-bear. Neurovascular examination is intact.
AP pelvis radiograph showing posterior dislocation of right THA. Femoral head displaced posteriorly and superiorly. Cup appears steep (55° inclination).
Image source: Open Access medical literature (NIH/PubMed Central) • CC-BY License
Questions
Question 1 (2 marks)
Describe the radiographic findings and clinical significance.
Question 2 (3 marks)
What are the risk factors for THA dislocation? Classify your answer.
Question 3 (3 marks)
Describe your immediate management including reduction technique.
Question 4 (4 marks)
This patient has now experienced three posterior dislocations in 6 months. How would you investigate and what are your surgical options?
Question 5 (3 marks)
Discuss spinopelvic imbalance and its role in THA instability.
Exam Day Cheat Sheet
Must Mention
- •Lewinnek safe zone: inclination 30-50°, anteversion 5-25°
- •Combined anteversion: 25-45° (cup + stem)
- •Dual mobility = best option for recurrent instability (<5% re-dislocation)
- •Posterior approach higher dislocation risk (2-4%) vs anterior (<1%)
- •36mm head reduces dislocation 50% compared to 28mm
- •Always exclude infection in recurrent instability
Common Pitfalls
- •Liner exchange when cup malpositioned (won't work)
- •Constrained liner as first-line (loosening risk)
- •Not getting CT for accurate version measurement
- •Ignoring spinopelvic factors
- •Repeated reduction without addressing cause
Exam Tips
- →Dislocation rate after primary THA: 1-3%
- →Posterior approach: 2-4% dislocation
- →Anterior approach: <1% dislocation
- →Lewinnek inclination: 30-50° (target 40°)
- →Lewinnek anteversion: 5-25° (target 15-20°)
- →Combined anteversion: 25-45°
- →Dual mobility re-dislocation: <5%
- →Spine fusion + THA: 3-4x dislocation risk