Quantification of meniscal volume by segmentation of 3T magnetic resonance images

Abstract

Meniscal injuries place the knee at risk for early osteoarthritis (OA) because they disrupt their load-bearing capabilities. Partial resection is routinely performed to alleviate symptomatic meniscal tears. While the removal of meniscal tissue may not be the only factor associated with partial meniscectomy outcome, the amount removed certainly contributes to functional loss. It is unknown, however, whether there is a critical amount of meniscal tissue that can be removed without diminishing the structure's chondroprotective role. In order to examine the existence of such a threshold, it is necessary to accurately quantify meniscal volume both before and after partial meniscectomy to determine the amount of meniscal tissue removed. Therefore, our goal was to develop and validate an MR-based method for assessing meniscal volume. The specific aims were: (1) to evaluate the feasibility of the MR-based segmentation method; (2) to determine the method's reliability for repeated measurements; and (3) to validate its accuracy in situ. MR images were obtained on a 3T magnet, and each scan was segmented using a biplanar approach. The MR-based volumes for each specimen were compared to those measured by water displacement. The results indicate that the biplanar approach of measuring meniscal volumes is accurate and reliable. The calculated volumes of the menisci were within 5% of the true values, the coefficients of variation were 4%, and the intraclass correlation coefficients were greater than 0.96. These data demonstrate that this method could be used to measure the amount of meniscal tissue excised during partial meniscectomy to within 125.7 mm(3).

Figure 1

The user interface of the commercial segmentation software (Mimics 9.11; Materialise, Ann Arbor, MI) used for this study. The top panes show coronal (left) and axial (right) views; the bottom panes show the sagittal view (left) and 3-D meniscal reconstructions (right).

Figure 2

Mean (+1 standard deviation) MR-based and water displacement volumes for right and left medial and lateral menisci of specimens used in Aim 3. Water volumes of medial and lateral menisci for each pair of knees showed mean coefficients of variation (CV) of 5.78% and 4.95%, respectively. MR-based volumes of medial and lateral menisci for each pair showed mean CVs of 6.45% and 4.32%, respectively. Mean MR-based volumes of the medial menisci of the right and left knees were 3102mm³ and 2969mm³, respectively. Mean MR-based volumes of the lateral menisci of the right and left knees were 2913mm³ and 2859mm³, respectively.

Figure 3

Mean (+1 standard deviation) meniscal volumes for surgical and contralateral control knees using MR-based segmentation and water displacement. 565mm³ (range 325–1000mm³; water displacement-based) were removed from the medial meniscus of the surgical* knee. The mean surgical* pre-op water volume was 3020 ± 987mm³ (the sum of the post-op water volume and the volume of meniscus removed during meniscectomy). Pre-op water displacement volume equaled post-op water volume for medial control and all lateral menisci.