Recurrent Lateral Patellar Dislocation/Subluxation Post Patellofemoral Replacement
Begin evaluation with standing anteroposterior, lateral, and axial (tangential) radiographs of the patellofemoral joint to assess patellar tilt, subluxation, component malalignment, and polyethylene wear—these are the essential first-line imaging studies for this complication 1.
Initial Radiographic Evaluation
The axial radiographs are particularly critical as they directly demonstrate the degree of patellar tilt or subluxation 1. Consider weightbearing axial views to better assess patellofemoral kinematics 1. The radiographic workup should include:
- Standing AP and lateral views to evaluate component positioning and wear patterns
- Axial (tangential) views of the patellofemoral joint to quantify patellar tilt and lateral subluxation
- Full-length standing radiographs (hip-to-ankle) to assess overall limb alignment and mechanical axis 1
Look specifically for joint space narrowing, varus or valgus deformity, patellar tilt, and effusion—these findings suggest polyethylene wear which can be subtle 1.
Advanced Imaging When Needed
If component malrotation is suspected based on clinical examination or plain radiographs, proceed to CT scanning 1. CT is the gold standard for assessing axial rotation of the femoral component, though be aware that supine CT underestimates deformity compared to weightbearing radiographs 1.
Management Algorithm
The management approach differs fundamentally from native patellofemoral instability because you're dealing with prosthetic components:
Assess for Component Malposition
- Femoral component malrotation (excessive internal rotation) is a common cause of post-arthroplasty patellar instability
- Patellar component malposition (lateral placement or tilt)
- Overall limb malalignment outside physiologic 5-8° valgus
Determine Severity and Cause
If component malalignment is identified on imaging:
- Revision arthroplasty to correct component positioning is typically required
- Isolated soft tissue procedures rarely succeed when prosthetic malposition is the underlying cause
If components are well-positioned:
- Consider polyethylene wear as the cause (look for joint space narrowing on weightbearing films) 1
- Evaluate for inadequate soft tissue balancing at the time of initial arthroplasty
Key Clinical Pitfalls
⚠️ Critical caveat: Unlike native patellar instability where MPFL reconstruction is the cornerstone of treatment 2, 3, 4, 5, post-arthroplasty instability usually reflects technical issues with component positioning or wear rather than isolated ligamentous insufficiency. Attempting soft tissue procedures without addressing underlying component malposition will fail.
⚠️ Do not rely on standard AP knee radiographs alone—the axial views are essential and often reveal the pathology 1. Annual weightbearing radiographs are recommended for detecting subclinical wear 1.
⚠️ Recognize that radiographic findings can be subtle—compare to immediate postoperative films if available to detect progressive changes in patellar tracking or component position.
Surgical Decision-Making
Revision arthroplasty is indicated when:
- Component malrotation is documented on CT (particularly femoral component internal rotation)
- Progressive polyethylene wear is demonstrated
- Mechanical axis is outside acceptable parameters on full-length standing films
Conservative management may be attempted only when:
- Components are optimally positioned
- No significant wear is present
- Instability is mild and intermittent
However, understand that recurrent instability after patellofemoral replacement typically requires surgical revision rather than conservative measures, as the underlying problem is usually mechanical rather than purely soft tissue-based.