Acetabular Protrusio in Patients With Osteogenesis Imperfecta: Risk Factors and Progression
Authors: Ahn Junho , Carter Erin , Raggio Cathleen L. , Green Daniel W. 
Published: July 2017

Background:
Osteogenesis imperfecta (OI) is a genetic disorder commonly associated with osteopenia, osteoporosis, bone fractures, bone deformities, and other clinical features. A frequent radiologic finding with OI is acetabular protrusio (AP). We hypothesized that AP develops in patients with OI over time. In addition, we hypothesized that AP also develops in patients with OI without radiographic evidence of AP on initial examination.

Methods:
Medical records and radiographs of 55 patients (109 hips) diagnosed with OI evaluated at our institution were retrospectively reviewed. Previously established radiographic criteria using the center-edge (CE) angle of Wiberg, position of the acetabulum relative to the iliopectineal line, crossing of the acetabulum across the ilioischial (Kohler) line, and position of the teardrop figure relative to the ilioischial (Kohler) line were utilized to assess AP severity. In addition, pharmacological treatments and patient factors including body mass index (BMI) were recorded. Radiographs of patients with OI that were taken ≥2 years apart were analyzed utilizing AP radiographic criteria to assess for changes. The changes in AP-related measurements were standardized by distance or degree per year. In addition, patient factors were evaluated for associations with AP development.

Results:
In this series of 109 hips (55 patients), incidence of AP in earliest radiographs was 45% (49/109). Patients with OI type I and III demonstrated the highest incidence of AP (65%). Among the hips that did not meet the criteria for AP in their early radiographs, 24 (40%) were positive for AP by their latest radiograph. In the hips that initially presented with AP, 42% showed increased CE angles on later radiographs. Twenty-six hips (24%) showed either no observable changes or reduced CE angles. Risk factors that were significantly associated with greater odds of developing AP included (1) an age under 12; (2) a BMI>25; (3) presence of AP of the contralateral hip; and (4) female sex. Bisphosphonates, vitamin D, physical therapy, and other drugs related to treatment of OI reduced the risk of developing AP but did not achieve statistical significance.

Conclusions:
AP is a common finding in OI patients (54%). Among hips of OI patients that met criteria for AP in early radiographs, 42% (20/48) demonstrated greater CE angles in their latest radiographs. Similar changes were observed in OI patients who did not initially meet criteria for diagnosis for AP. However, CE angle measurements between the 2 groups did not significantly differ (P=0.71). In terms of Kohler line crossing, patients that met criteria for AP in early radiographs had significantly greater change per year than those that did not have AP criteria (P<0.05). The findings suggest AP may develop over time in patients with OI and may be influenced by patient factors such as age, sex, and BMI. In addition, unilateral AP may have a significant impact on the development of AP of the contralateral hip.

Level of Evidence:
Level IV—retrospective case series.