Detection of meniscal extrusion: comparison of standing computed tomography to non-loaded magnetic resonance imaging

      Purpose: Meniscal extrusion is associated with knee pain and rapid progression of OA, but is not visualized on radiographs, the most common knee joint imaging modality. While MRI is the standard for non-invasive visualization of menisci, absence of standing limits evaluation of the functional configuration of the menisci (i.e. when the menisci and cartilage are compressed due to weight bearing and muscle contractile forces). Standing CT (SCT) has the capability to quickly image the knees in bipedal weight-bearing stance. Given the increased meniscal extrusion visualized during loading, SCT has the potential to improve sensitivity to meniscal extrusion beyond that of MRI. The objectives of this study were to investigate a) the additional diagnostic value of SCT for detecting meniscal extrusion in comparison with non-loaded MRI and b) the relationship between knee pain and meniscal extrusion detected on SCT and non-loaded MRI.
      Methods: Participants from the Multicenter Osteoarthritis Study (MOST) with KL grade ≥1 on prior knee radiographs were imaged with both 1.5T MRI and SCT (N = 56) at the 144-month MOST visit. One musculoskeletal radiologist read Whole-Organ Magnetic Resonance Imaging Score (WORMS) for extrusion of the meniscal body on MRI coronal STIR sequences and a second radiologist read SCT for meniscal extrusion. Original SCT images were reconstructed in two ways, (i) SCT-sharp: with a sharp kernel reformatted into the coronal plane with 0.3 mm slices and (ii) SCT-smooth: with a smooth kernel intended for soft tissue with 2.96 mm slice thickness. Extrusion of the medial and lateral meniscal bodies was assessed on coronal images in the section in which the medial tibial spine had the greatest volume for both of the SCT renderings and also on MRI. The edge of the tibial plateaus (excluding osteophytes) was used as the reference for measuring extrusion of the meniscal bodies. For each modality, medial and lateral meniscal extrusions were assigned grade 0–2 (grade 0, no extrusion; grade 1, extrusion ≤ 50% of the body; grade 2, extrusion > 50% of the body). Participants completed the Western Ontario and McMaster University Osteoarthritis Index (WOMAC) to assess pain in the index limb over the past 30-days. Frequencies of meniscal extrusion scores were calculated for each modality (SCT-sharp, SCT-smooth, MRI). Spearman correlation coefficients were calculated to determine the associations between meniscal extrusion score (0/1/2) on each of the imaging modalities and WOMAC pain scores for the knees studied.
      Results: Of the 56 participants included (40 women), 30 contributed left knees and 26 contributed right knees. Mean ± SD age was 72.7 ± 7.1 years, BMI was 29.2 ± 4.1 kg/m2, and WOMAC pain score was 1.6 ± 2.2 (median 0.5, IQR 0–2.5 points). As presented in Table 1 below, MRI did not detect medial meniscal extrusion in 36% of cases that were detected on SCT-sharp and SCT-sharp demonstrated greater medial meniscal extrusion score than MRI in 66% of cases (30 with 1 grade higher and 7 with 2 grades higher). For lateral meniscal extrusion, 10.7% demonstrated a higher score on SCT-sharp than on MRI.
      Table 1Comparison of Medial Meniscal Extrusion Scores from MRI and SCT-sharp
      SCT-sharp Meniscal Extrusion ScoreMRI Meniscal Extrusion ScoreTotals
      113 (65%)9 (35%)3 (30%)25 (45%)
      27 (35%)17 (65%)7 (70%)31 (55%)
      Totals20 (36%)26 (46%)10 (18%)56 (100%)
      After reconstruction with a smooth tissue kernel and 2.96 mm image slices, 2 participants' SCT-smooth images were unreadable (N = 54). MRI did not detect medial meniscal extrusion in 28% of cases that were detected on SCT-smooth and SCT-smooth demonstrated greater medial meniscal extrusion score than MRI in 41% of cases (19 with 1 grade higher and 3 with 2 grades higher). For lateral meniscal extrusion, 2% demonstrated a higher score on SCT-smooth than on MRI. Correlations between self-reported pain in the knee studied and meniscal extrusion scores on each of the modalities are presented in Table 2 below.
      Table 2Spearman Correlation Coefficients between Knee Pain and Meniscal Extrusion Score
      Correlation with Knee Pain0.15−0.08−0.11−0.020.02−0.07
      Conclusions: These findings support the capability of SCT to detect meniscal extrusions that are not detected by MRI. Knee pain was not significantly associated with meniscal extrusion scores for either MRI or SCT. Additional research is required to optimize the reconstruction settings of SCT images for detection of meniscal extrusion and also to determine the clinical relevance of the meniscal extrusions that are identified on SCT, but not on MRI.