The main finding of the current study is the description of the histologic features of discoid lateral meniscus in a group composed only by children and adolescents (aged 9–18) without a meniscal tear. The main feature observed is severe disorganization of the collagen fibers, irrespectively of the age of the patients. Furthermore, we found no vessels even in the inner part of discoid lateral meniscus in all patients, but the 18-year-old.
In particular, the findings of the current study provide some insights on the vascularity of discoid lateral meniscus. Vascularity of normal meniscus has already been described by Clark et al. (Clark & Ogden, 1983) who reported on a great number of cells and blood vessels during the fetal development of the normal meniscus, that are progressively lost after birth. Even though the meniscus reaches its adult structure between the ages of 10 and 11, also in patients of these ages, blood vessels still could be found throughout the inner parts. In a previous study, Inoue et al. (Inoue et al., 2016) reported on a great concentration of blood vessels in the intercondylar region of complete discoid lateral meniscus in a small group of 6 patients (aged 8 to 17 years). Our results are in disagreement with Clark et al. (Clark & Ogden, 1983) and Inoue et al. (Inoue et al, 2016), in fact we were not able to find any blood vessel in samples from our youngest patients (aged 9 and 10), but only from the 18-year old one.
To the best of our knowledge, there have been only 2 cases of regrowth of a discoid lateral meniscus after surgery, in a 5-year-old girl (Stein et al., 2013) and in an 11-year-old boy (Bisicchia & Tudisco, 2013). In both cases, the authors did not perform histology. However, Bisicchia et al. (Bisicchia & Tudisco, 2013) speculated that the vasculature in discoid menisci should be the same as in normal ones and suggested the blood vessels in the inner part of the meniscus as a possible explanation of the re-growth after surgery. The results of the current study do not support their theory, and question how the re-growth was possible. In fact, we found blood vessels only in the oldest patient, and not in the youngest ones.
To the best of our knowledge, there are only few papers in literature about the histology of discoid meniscus (Atay et al., 2007; Papadopoulos et al., 2009; Cui & Min, 2007; Choi et al., 2017; Inoue et al., 2016). The main limitation of those studies is the inclusion of a great percentage of adults and meniscal tears. Atay et al. (Atay et al., 2007) reported collagen fibers to be fewer and to have a heterogeneous course. Most of their patients were adults, with a mean age of 29 years (range, 5–50 years). All these features were found in all patients, therefore also in the two pediatric ones (5 and 9 years, respectively). This is in agreement with our data showing that the same severe disorganization of the collagen fibers is present in all patients, irrespectively of their age.
Cui et al. (Cui & Min, 2007) reported the femoral surface to be covered by dense and well-arranged thick fibrils, while the tibial surface to show an irregular orientation of fibers. We did not analyze the whole meniscus as Cui et al. (Cui & Min, 2007), but we observed the tibial surface to be flat and smooth while the femoral side to be undulated and often with clefts. We could speculate that the slipping pressure of the femur could easily fray the anomalous tissue observed in the discoid meniscus.
Subsequently, Papadopoulos et al. (Papadopoulos et al., 2009) confirmed the significant disorganization of the circular collagen network in the discoid meniscus with a heterogeneous course of the circumferentially arranged collagen fibers, compared with the normal meniscus with areas of degeneration or osseous metaplasia. The results of their study (Papadopoulos et al., 2009) are biased by the age of their patients. In fact, all of their patients were adults, with a mean age of 30.22 ± 7.61 years (range, 17 to 39 years). Also, in our study, the significant disorganization of the circular collagen network in the discoid meniscus with a heterogeneous course of the circumferentially arranged collagen fibers was observed. We did not find any areas of metaplasia in our younger patients and we speculate that the presence of areas of metaplasia in our oldest patient were due to the progressive meniscal degeneration with ageing and not related to discoid shape itself.
Choi et al. (Choi et al., 2017) in their ultrastructural study found that intact discoid lateral menisci had a more homogenous collagen pattern and greater collagen fibrils number, compared with their torn counterparts. Even in this study, most of their patients were adults (30 subjects included, aged 15 to 58). The results of our study disagree with the findings reported by Chioi et al. (Choi et al., 2017), in fact we observed severe disorganization of the collagen fibers also in intact discoid lateral meniscus.
In agreement with the findings of previous studies (Atay et al., 2007; Papadopoulos et al., 2009; Cui & Min, 2007; Choi et al., 2017; Inoue et al., 2016) we found irregularly oriented collagen fibers in discoid lateral meniscus. On the other hand, in contrast to the results reported by Clark et al. (Clark & Ogden, 1983), there were no blood vessels in the inner part of discoid lateral meniscus in our youngest patients (9 and 10 years old).
Limitations should be acknowledged for this study. A limited number of patients were included due to selection only of children and adolescents. In fact, most of the published studied included also adults (Atay et al., 2007; Papadopoulos et al., 2009; Cui & Min, 2007; Choi et al., 2017). Only Inoue et al. (Inoue et al, 2016) included only children and adolescents and their sample size in comparable to the current study. Our aim was to report on this particular population of patients that may challenge the orthopedic surgeon. We believe that inclusion of patients of all ages to increase the sample size of our study may significantly bias the results and lead to incorrect conclusions.
Histologic analysis was performed only on the resected fragment instead on the whole meniscus. In a previous study, Hamada et al. (Hamada, 1994) resected discoid lateral menisci “en-block” and analyzed the entire meniscus. Given the importance of the meniscus in load distribution, lubrication, and cushioning, the current trend in meniscal preservation surgery, and the long-term outcomes in favor of saucerization over total meniscectomy (Smuin et al., 2017), we believe it is not ethical to resect “en block” an entire meniscus in children and adolescents. Lack of electron microscopy analysis is another limitation of this study, but this microscope is not available at the author’s Institution.