Subchondral Cyst in Acetabulum
Begin with plain radiographs (AP pelvis and lateral hip views) as your initial imaging, and if the cyst is confirmed, proceed directly to MRI without contrast to assess cartilage integrity and bone marrow edema—this determines whether the patient is a candidate for joint preservation versus arthroplasty. 1
Initial Imaging Algorithm
Plain radiography serves as the excellent screening tool and should include both AP pelvis and proximal femur views, not just the affected hip 1. This bilateral comparison is critical because:
- Radiographs guide selection of additional imaging and provide baseline for comparison
- For osteoarthritis-related cysts (the most common scenario), radiographs combined with physical examination may have better diagnostic utility than MRI alone 1
MRI Evaluation - The Critical Decision Point
Once a subchondral acetabular cyst is identified on radiographs, obtain MRI to stratify surgical candidacy. The presence or absence of bone marrow edema fundamentally changes prognosis:
MRI Findings That Predict Poor Outcomes:
- Subchondral edema with cystic change correlates with 91% incidence of full-thickness (grade 4) cartilage lesions 2
- These patients have significantly inferior outcomes after hip arthroscopy (67% success rate versus 85% in controls, P=0.04) 2
- Mean functional scores are substantially lower (mHHS 79.9 vs 86.6, P=0.03) 2
MRI Findings That Permit Joint Preservation:
- Isolated cysts without significant edema or with maintained joint space may still be candidates for arthroscopic intervention 3
- A 2026 multicenter study showed patients with acetabular cysts achieved 82.93% MCID and outcomes similar to matched controls when appropriately selected 3
Management Algorithm
For Patients WITHOUT Significant Edema or Advanced Arthritis:
Hip arthroscopy with concurrent subchondroplasty (calcium phosphate injection) is the preferred joint-preserving approach:
- Address underlying femoroacetabular impingement and labral pathology arthroscopically
- Inject calcium phosphate bone substitute directly into the cyst under fluoroscopic guidance 4, 5
- This technique shows no perioperative complications, no material migration, and 82.4% return to sport at 3-year follow-up 4
- Critical technical point: Use fluoroscopy during injection to detect intra-articular extravasation immediately 5
Alternative for larger, symptomatic isolated cysts:
- Open surgical excision with curettage and cancellous allograft provides rapid symptom relief and functional recovery 6
- Consider this when the cyst is large enough to cause mechanical symptoms but arthritis is minimal
For Patients WITH Significant Edema/Advanced Arthritis:
Total hip arthroplasty is the definitive treatment:
- In patients over 65-70 years with displaced fractures or end-stage arthritis, proceed directly to THA 7
- Use cemented femoral stems (strong recommendation from 2022 AAOS guidelines) 7
- Cysts left untreated during THA spontaneously consolidate in most cases—71 of 88 cysts (81%) disappeared at 6.3-year follow-up with no loosening 8
- Persistent cysts decrease in size and do not compromise implant survival 8
Common Pitfalls to Avoid
Do not perform arthroscopy on hips with significant subchondral edema and cystic change without counseling patients about 33% failure rate 2
Do not obtain MR arthrography initially—non-contrast MRI at 3T is sufficient for evaluating both labrum and subchondral bone 1
Do not fill acetabular cysts with bone graft during THA—they consolidate spontaneously and grafting adds no benefit 8
Do not ignore underlying FAI when treating the cyst—the cyst is typically secondary to abnormal mechanics that must be corrected 3, 4
Key Prognostic Factors
The size and location of the cyst matter less than the presence of bone marrow edema 2. Even large cysts (up to 57 mm²) can be successfully managed if edema is absent 8. Cysts in Charnley zone I tend to persist longer but remain asymptomatic 8.