The most important finding of this study was that we found a new type of morphological abnormality of the medial meniscus. Although there are some reports on the embryology of the discoid meniscus [20,21,22,23], this remains largely unclear. Evidence suggests that the meniscus becomes a clearly defined structure by the 8th week of fetal development. The meniscus assumes its relationship with the rest of the knee by the 14th week . Fukazawa et al. reported that the discoid shape is established postnatally due to hypertrophy of the meniscus following lesions caused by detachment of the posterior portion of the meniscus from the tibial plateau . This report presents a new incomplete type of discoid medial meniscus with an oversize posterior segment, which has never been reported previously.
Significantly larger sizes characterized the abnormal morphology of the oversized posterior segment medial meniscus at the mid-body and posterior segment, without a significant difference at the anterior segment compared to the normal group. Previous reports of a discoid medial meniscus pointed out “cupping” [26, 27] of the medial tibial plateau on AP radiographs and the central indentation of the medial tibial plateau on sagittal MRI as characteristic imaging findings . Similar to previous reports, all patients with discoid and oversized posterior segment medial menisci in our study indicated indentation of the medial tibial plateau on lateral radiographs and sagittal MRI. Moreover, we found a difference in the location of the indentation between patients with discoid and oversized posterior menisci. In patients with discoid menisci, indentations were observed near the center of the medial plateau, whereas they were observed in the posterior area in the oversized posterior segment of the medial menisci (Fig. 2b–d).
The medial meniscus is mainly C-shaped, and various shapes of the medial meniscus have been reported, and morphometry of the medial meniscus is also reported [22, 23]. Bloecker et al. reported morphometric differences between the medial and lateral menisci by three-dimensional analysis using MRI . These results were similar in width to the anterior, mid-body, and posterior segments, measured as a normal group at our hospital. In our study, if the AP length of the posterior segment of the medial meniscus exceeded 19.9 mm, the patients were defined as those with an oversized posterior segment. The ML widths of the mid-body were < 14 mm in the normal group, 16–22 mm in the oversize group, and > 25 mm in the discoid group. A significant difference was observed between the normal and oversize groups. In contrast to the posterior and mid-body segments, there were no differences in the anterior segment between the oversize and normal groups (< 14 mm in both groups). Therefore, the oversize posterior segment medial meniscus could be defined with an abnormally oversize posterior segment (≥ 20 mm) and normal anterior segment size (< 14 mm). In addition to the normal size of the anterior segment, the lower ML width of the mid-body could be a possible parameter for distinguishing the oversized posterior segment from the discoid medial meniscus. However, the size of the meniscus is likely to change with sex, age, height, and weight, etc. One report stated that the meniscus size correlated more strongly with the size of the tibial plateau . Therefore, we calculated the AP length ratio of the posterior segment relative to the medial tibial plateau and found a significant difference in the AP length ratio between the oversize group and the normal group (Table 2) (0.48 vs 0.30, P < 0.001).
For the classification of discoid lateral menisci, complete, incomplete, and Wrisberg types are often used [7, 31]. Regarding the shape of the discoid lateral meniscus, the central part of the meniscus was smaller and thinner in incomplete discoid lateral menisci than in complete discoid menisci. Notably, medial menisci with oversized posterior segments in our study had larger middle to posterior segments than those with normal medial menisci. The shapes appeared different from the incomplete type, but resembled the Wrisberg-type lateral discoid menisci. Although the oversized posterior segment medial meniscus could be regarded as an “incomplete type of discoid medial meniscus,” we specifically named it as “oversize posterior segment” to clarify the characteristics of this new morphological abnormality in the medial meniscus. Moreover, none of the patients in our study had meniscal anomalies of “abnormal anterior horn insertion on the ACL” [32,33,34] and “meniscal cyst,” which were reported as coexisting abnormalities with the discoid medial meniscus . Therefore, none of the patients with oversized posterior segment had a discoid medial meniscus characteristic in this study. To the best of our knowledge, there have been no previous reports of incomplete discoid medial menisci and/or oversized posterior segments. Therefore, this report is the first to identify a new type of abnormality of the medial meniscus as an incomplete discoid medial meniscus. We also identified the clinical features of oversized posterior-segment medial menisci. All the patients started experiencing medial knee pain without obvious trauma or strenuous sports, as often observed in patients with discoid lateral and discoid medial menisci . Therefore, if clinicians encounter relatively younger patients who begin having medial pain without any cause and have a wide, severe horizontal tear in the posterior segment, an oversized-posterior-segment medial meniscus can be considered a possible etiology.
This study has some limitations. First, the number of cases with oversized posterior segments was small. As there were four cases, there is a possibility that measurement errors may have increased, and there may be less reliability of the clinical symptoms and imaging findings. Therefore, it is necessary to conduct future studies with a larger number of cases. Second, there is a possibility that there may have been measurement errors because we used MRI for measurements. Thirdly, there is a potential for bias in the measurement of meniscus sizes. Therefore, the measurements were taken twice by two examiners. The intra-class correlation coefficients (ICC) were excellent for all measurements, showing that the measure was reliable (intra: 0.92, inter: 0.90). Finally, although an oversize posterior segment characterized this new type of incomplete discoid medial meniscus as the morphological abnormality, we could not find clearly clinical significancy in this study. However, the incomplete discoid medial meniscus with an oversize posterior segment did not have the morphological features that should be in the discoid. In particular, the incomplete discoid medial meniscus was characterized by the normal anterior segment. We believe that this clearly distinguishes it from the discoid meniscus. Therefore, further research would be required to characterize better the epidemiology of this new type of incomplete discoid meniscus, establish diagnostic methods other than MRI, and provide specific surgical treatment.