Advantages of cone beam computed tomography for evaluation of subchondral insufficiency fractures of the knee compared with MRI

Abstract Purpose To determine the diagnostic yield of cone beam computed tomography (CBCT) compared with 3T magnetic resonance imaging (MRI) for the evaluation of subchondral insufficiency fractures of the knee. Methods Consecutive patients with subchondral insufficiency fractures of the knee examined by 3T MRI and CBCT of the femoral condyles were reviewed. Two experienced raters graded the lesion severity on 3T MRI and CBCT images: grade 1: no signs of a subchondral bone lesion; grade 2: subchondral trabecular fracture or cystic changes, but without infraction of the subchondral bone plate; grade 3: collapse of the subchondral bone plate. Ratings were repeated after six weeks to determine reliability. In addition, the bone lesion size was measured as elliptical area (mm 2 ) and compared between CBCT and T1-weighted MRI sequences. Results Among 30 patients included (43.3% women; mean age: 60.9 ± 12.8 years; body mass index (BMI) 29.0 ± 12.8 kg/m 2 ), the medial femoral condyle was affected in 21/30 patients (70%). The grading of subchondral lesions between MRI and CBCT did not match in twelve cases (40%). Based on MRI images, an underestimation (i.e., undergrading) compared with CBCT was observed in nine cases (30%), whereas overgrading occurred in three cases (10%). Compared to CBCT, routine T1-weighted 3T sequences significantly overestimated osseus defect zones in sagittal (84.7 ± 68.9 mm 2 vs. 35.9 ± 38.2 mm 2 , p < 0.01) and coronal orientation (53.1 ± 24.0 mm 2 vs. 22.0 ± 15.2 mm 2 , p < 0.01). The reproducibility of the grading determined by intra- and inter-rater agreement was very high in MRI (intra-class correlation coefficient (ICC) 0.78 and 0.90, respectively) and CBCT (ICC 0.96 and 0.96, respectively). Conclusion In patients with subchondral insufficiency fractures of the knee, the use of CBCT revealed discrepancies in lesion grading compared with MRI. These findings are clinically relevant, as precise determination of subchondral bone plate integrity may influence the decision about conservative or surgical treatment. CBCT represents our imaging modality of choice for grading the lesion and assessing subchondral bone plate integrity. MRI remains the gold standard modality to detect especially early stages.