arthroplasty
TKA Instability
advanced
6 min
15 marks
5 questions
Clinical Scenario
A 68-year-old woman presents 6 months after primary PS TKA for osteoarthritis. She reports recurrent giving way episodes, particularly on stairs and uneven ground. She feels unstable and has had several near-falls. There has been no trauma. On examination, there is valgus alignment with palpable lateral thrust during gait.
AP and lateral radiographs of TKA showing valgus alignment with lateral tibial translation. Assess component position, alignment, polyethylene thickness, and cam-post engagement on lateral view.
Image source: Open Access medical literature (NIH/PubMed Central) • CC-BY License
Questions
Question 1 (3 marks)
What are the types and causes of TKA instability?
Question 2 (3 marks)
How do you assess TKA instability clinically?
Question 3 (2 marks)
Describe the radiographic findings and likely diagnosis in this case.
Question 4 (4 marks)
What are the surgical options and how do you select the appropriate constraint level?
Question 5 (3 marks)
What preoperative investigations are essential before revision for instability?
Exam Day Cheat Sheet
Must Mention
- •Types: extension, flexion, mid-flexion, global instability
- •Varus/valgus stress testing at 0° AND 30°
- •Constraint ladder: CR → PS → VVC → Hinge
- •Use MINIMUM constraint necessary (loosening risk)
- •CT rotation protocol for malrotation
- •ALWAYS exclude infection before revision
Common Pitfalls
- •Not identifying instability direction
- •Over-constraining (loosening risk)
- •Poly exchange when components malpositioned
- •Missing component malrotation (need CT)
- •Forgetting to exclude infection
Exam Tips
- →Joint line elevation >8mm = mid-flexion instability
- →Femoral internal rotation >3° = revision indication
- →Stress test at 0° and 30° flexion
- →VVC loosening rate higher than PS (use only when needed)