What is the best imaging modality, MRI (Magnetic Resonance Imaging) or CT (Computed Tomography), to evaluate for recurrent dislocation of hip arthroplasty?

CT Without IV Contrast is the Preferred Imaging Modality for Recurrent Hip Arthroplasty Dislocation

CT without IV contrast is the recommended imaging modality to evaluate recurrent dislocation of hip arthroplasty, as it provides superior assessment of joint congruence, component positioning, intraarticular fragments, and acetabular fractures that may contribute to instability. 1

Algorithmic Approach to Imaging

Step 1: Initial Radiographic Assessment

• Begin with standard hip/pelvis radiographs to assess component position, orientation, and obvious mechanical causes of instability 1
• Radiographs alone are insufficient for comprehensive evaluation but are essential for initial screening 1

Step 2: CT Without IV Contrast as Primary Advanced Imaging

• CT without IV contrast should be obtained when radiographs are negative or indeterminate but clinical suspicion for mechanical instability persists 1
• CT excels at evaluating:
  • Joint congruence and component positioning 1
  • Intraarticular fracture fragments that may prevent stable reduction 1
  • Acetabular wall fractures requiring surgical management to prevent repeat dislocation 1
  • Component malposition or impingement mechanisms 2

Step 3: MRI Without IV Contrast for Soft Tissue Evaluation

• MRI without IV contrast is indicated when soft tissue pathology is suspected as the cause of instability 1
• MRI is superior for evaluating:
  • Abductor tendon tears or muscle injuries compromising hip stability 1
  • Ligamentous damage 1
  • Soft tissue envelope integrity 3

Evidence-Based Rationale

Why CT is Preferred

• The American College of Radiology specifically recommends CT without IV contrast for post-reduction evaluation to detect complications that increase risk of recurrent dislocation 1
• CT has 87.3% sensitivity for detecting intraarticular fracture fragments, though arthroscopy may detect additional small fragments in 43.3% of cases with negative CT 1
• Advanced CT modeling techniques can identify impingement mechanisms responsible for instability, allowing surgeons to develop targeted treatment plans 2

Why Contrast is Unnecessary

• There is no evidence supporting CT with IV contrast or CT without and with IV contrast for evaluating hip dislocation 1
• Contrast adds cost and risk without diagnostic benefit for mechanical causes of instability 1

When MRI Becomes Relevant

• MRI is the next useful examination specifically for tendon, muscle, or ligament injury evaluation 1
• MRI without and with IV contrast has no supported role in this clinical scenario 1
• For non-metal-on-metal prostheses, MRI has insufficient evidence as initial imaging but may be useful for soft tissue complications 1

Critical Pitfalls to Avoid

Delayed Advanced Imaging

• Obtaining CT promptly reduces morbidity by detecting complications sooner, decreasing immobility time and preventing secondary displacement 1
• Early identification of mechanical causes allows appropriate treatment planning to improve outcomes 1

Limitations of CT

• CT may miss small intraarticular fragments; 78% of patients with negative radiographs and CT had intraarticular fragments at arthroscopy 1
• If CT is negative but instability persists, consider that soft tissue causes may require MRI evaluation 1

Component-Specific Considerations

• For ceramic acetabular liner fractures, CT may be more sensitive than radiographs 1
• Prominent anterior inferior iliac spine can limit hip range of motion and contribute to instability, identifiable on CT 2

Clinical Decision Framework

Use CT without IV contrast when:

• Evaluating mechanical causes of recurrent dislocation 1
• Assessing component position and orientation 1, 2
• Detecting intraarticular fragments or acetabular fractures 1
• Planning revision surgery for instability 2

Use MRI without IV contrast when:

• Soft tissue pathology is suspected (abductor tears, muscle injury) 1
• CT is negative but pain and instability persist 1
• Evaluating the integrity of the soft tissue envelope around the prosthesis 3