Is Femoral Head Bone Marrow Edema of Unknown Etiology Associated with Acetabular Overcoverage? A CT-Based Three-Dimensional Study

<b>Background:</b> This study aimed to investigate the association between femoroacetabular impingement (FAI) morphology and femoral head bone marrow edema of unknown etiology on hip magnetic resonance imaging (MRI), and to assess the added value of computed tomography-based three-dimensional maximum intensity projection (CT-MIP) measurements in identifying a predisposition to acetabular overcoverage. <b>Methods:</b> Hip MRI examinations performed between January 2007 and 2025 were retrospectively reviewed. Cases with bone marrow edema attributable to identifiable etiologies were excluded. Twenty-six patients with available hip or pelvis computed tomography (CT) examinations obtained within one year were included, along with an age- and sex-matched control group imaged for indications unrelated to hip pain. A total of 104 hip joints were evaluated. Alpha angles were measured on axial oblique CT reformations. Virtual pelvic radiographs generated from CT-based three-dimensional reconstructions were used for lateral center-edge angle (LCEA) measurements, and acetabular coverage was quantified using the acetabular coverage index derived from CT-MIP images. Appropriate statistical analyses were performed, with <i>p</i> < 0.05 considered statistically significant. <b>Results:</b> FAI was identified in 82.7% of cases with bone marrow edema of unknown etiology on MRI (<i>p</i> < 0.001), with pincer-type morphology being the most prevalent subtype (55.8%). Bone marrow edema was significantly more common in pincer-type FAI compared with other subtypes (<i>p</i> < 0.001) and predominantly involved the posterolateral femoral head. Mean alpha angle, LCEA, and acetabular coverage index values were significantly higher in the case group than in controls (<i>p</i> < 0.001). For the detection of pincer-type FAI, CT-MIP-based acetabular coverage index demonstrated superior diagnostic performance compared with LCEA (AUC, 0.917 vs. 0.855; <i>p</i> = 0.017), with an optimal cutoff value of 0.93 yielding high specificity and accuracy. All measurements showed excellent intraobserver and interobserver reliability. <b>Conclusions:</b> Femoral head bone marrow edema of unknown etiology may serve as a radiologic clue to underlying pincer-type FAI, while CT-MIP-based analyses may provide incremental value beyond conventional angular measurements in characterizing acetabular overcoverage.