Development of a Radiographic Scoring System to Estimate Acetabular Protrusion Risk in Patients with Osteolytic Periacetabular Metastases

BACKGROUND: For patients with periacetabular metastases, protrusio acetabuli is a severely painful and mobility-impairing complication that requires subsequent open joint surgery. We aimed to identify specific structural changes that are associated with progression to protrusio acetabuli and to create a scoring system to guide risk stratification. METHODS: In this single-institution cohort study, we identified all patients who underwent primary surgical stabilization for periacetabular metastases with osteolytic or mixed osteolytic-osteoblastic characteristics from October 2017 through January 2025. Cases of protrusio acetabuli prior to surgical intervention were identified. Pain and ambulatory functional scores and treatment history were recorded. Locations of bone destruction were evaluated using coronal-cut computed tomography (CT) scans obtained within 3 months before clinical presentation (and earlier, as available). Trabecular and subchondral cortical bone mass of the periacetabular weight-bearing portions were indirectly assessed via Hounsfield unit ratio comparisons across scans. Univariable analysis of each feature was performed. The highest-scoring features were used to create a scoring system and analyzed using a receiver operating characteristic (ROC) curve. Finite element analysis was performed for biomechanical validation. RESULTS: Eighty-seven patients (67 non-protrusio [mean age of 65.5 ± 13.0 years; 37 female]; 20 protrusio [mean age of 72.9 ± 10.1 years; 11 female]) were included. Locationally, bone defects, thinning, or linear fractures in the middle-third (apex) alongside contiguous involvement of either the medial- or lateral-third of the weight-bearing dome were highly predictive of protrusio. A >50% cortical bone-mass decrease of the acetabular weight-bearing dome was associated with protrusio (p < 0.05). A radiographic risk scoring system was then constructed using a grading system from low- to high-risk features. ROC analysis showed a score of ≥3.0 as 95.0% sensitive and 91.0% specific for progression to protrusio. Finite element analysis further showed that cortical bone loss of the middle-third (apex) of the weight-bearing dome was critical. CONCLUSIONS: We propose the use of clinical and radiographic risk predictors to stratify patients with periacetabular metastases on the basis of the risk of protrusio. Anatomically, surgical stabilization of the middle-third (apex) of the weight-bearing dome is critical to preventing or delaying progression to protrusio. LEVEL OF EVIDENCE: Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.