Dynamics of the meniscus underweight-loading conditions: an investigation using upright magnetic resonance imaging

OBJECTIVES: Meniscal morphology underweight-loading conditions remains unknown, because weight loading is rarely performed during magnetic resonance imaging (MRI). We focused on the weight-loading condition and developed an upright MRI. This study aimed to investigate the shift of the medial and lateral meniscus in healthy adults under both fully weight loaded and unloaded conditions, using both supine and upright MRI. METHODS: Eighteen volunteers (13 men and 5 women) without previous knee complaints underwent an MRI of the right knee. The mean age of the subjects was 21.8±3.1 years. The examination was performed in three positions: supine, double-leg upright (DLU), and single-leg upright (SLU). The knees were fully extended in each position. For both the supine and upright examinations, we used a special MRI apparatus (Gravity MRI, Hitachi, Japan) that is capable of imaging in any position with a static magnetic field strength of 0.4 T (Figure). T1-weighted images were obtained in both coronal and sagittal planes with a slice thickness of 2 mm. The medial or lateral, anterior and posterior shifts of both the medial and lateral meniscus were measured, against the tibial wedge. The medial and lateral meniscal shift rates were measured as assessment items under each condition. The shift rate was defined as the proportion of the measured meniscal shift compared to the overall width of the medial and lateral meniscus. The MRI reference section on the sagittal plane was the midpoint of the medial or lateral femoral condyle. On the coronal plane, the reference point was the midpoint of the medial and lateral tibial condyle. The measurement items were compared in all three imaging positions using analysis of variance and multiple comparison tests, with statistical significance set at P < 0.05. RESULTS: We show the results from the analysis of the dynamics of the medial and lateral meniscus in Table. The difference among the three conditions for the medial shift rate of the medial meniscus was significant. The medial shift rate was significantly greater in the DLU and SLU positions than in the supine position (supine: 7.3±5.8% vs. DLU: 20.0±8.8 % (P<0.01), supine vs. SLU: 21.5±7.6% (P<0.01), DLU vs. SLU (P=0.7)). No significant differences were observed for the anterior and posterior shift rates of the medial meniscus (anterior: P=0.6, posterior: P=0.4). In the lateral meniscus, the lateral shift rate was significantly greater in the SLU position than in the supine position, but no significant difference was observed in the DLU position (supine: -1.2±11.8% vs. DLU: 2.7±11.1 % (P=0.2), supine vs. SLU: 4.5±10.8% (P=0.04), DLU vs. SLU: P=0.7). Both anterior and posterior shift rates in the lateral meniscus were significantly greater in DLU and SLU positions than in the supine position (anterior/ supine: -20.6±14.3% vs. DLU: -14.7±12.2% (P<0.01), supine vs. SLU: -8.9±14.9% (P<0.01), DLU vs. SLU: P=0.3, posterior/ supine: -78.0±19.6% vs. DLU: -63.7±18.7 % (P<0.01), supine vs. SLU: -57.8±19.2% (P<0.01), DLU vs. SLU: P=0.4). CONCLUSIONS: Under upright weight-loading conditions, the medial meniscus showed only medial shift, whilst the lateral meniscus was seen to shift in lateral, anterior, and posterior directions. No significant difference was observed between the DLU and SLU shift rates in both the medial and lateral meniscus in healthy adults. We believe that this result will be useful when considering loading and rehabilitation following meniscal repair.