- Open Access
Discoid lateral meniscus in children and adolescents: a histological study
© The Author(s). 2018
- Received: 5 May 2018
- Accepted: 13 September 2018
- Published: 24 September 2018
Discoid lateral meniscus is the most frequent variant of the meniscus. Although the histology of normal menisci in children and in adults has been well described, few studies have focused on the histology of discoid menisci. Furthermore, most of the patients in those studies were adults. The aim of the present study was to report the histological findings of discoid lateral meniscus in a group of children and adolescents, aged between 9 and 18, after arthroscopic partial resection, focusing on cellularity, arrangement of collagen fibers, and vascularity of the excised fragments. Furthermore, to report on MRI findings compared to the histological findings in the same region.
Six patients (one female and five males) aged 9, 10, 13, 15, 17, and 18, were arthroscopically operated on partial meniscectomy (saucerization) of a discoid lateral meniscus, and the specimens were histologically examined.
The extracellular matrix showed a different distribution and characteristics depending on the different side of the meniscus. Irregularly oriented collagen fibers in discoid lateral meniscus were found. There were no blood vessels in the inner part of discoid lateral meniscus in all patients but the 18-year old (in which we observed also endothelials cells, edematous tissue and leaking of erythrocytes in the extracellular matrix). In the discoid lateral menisci analyzed, irregularly oriented collagen fibers with blood vessels were found only in the presence of degenerating tissue.
Discoid lateral meniscus is different from a normal meniscus in terms of vascularity and disorganization of collagen fibers.
- Discoid lateral meniscus
Discoid lateral meniscus is the most frequent variant of the meniscus and it is bilateral in up to 20% of cases (Yaniv & Blumberg, 2007). Although the histology of normal menisci in children and in adults has been well described (Clark & Ogden, 1983), few studies have focused on the histology of discoid menisci. Furthermore, most of the patients in those studies were adults (Atay et al., 2007; Papadopoulos et al., 2009; Cui & Min, 2007; Choi et al., 2017; Inoue et al., 2016). For these reasons, a description of the histology of discoid lateral meniscus in children and adolescents is warrant. As shown in a previous study, symptomatic discoid lateral meniscus has tear or degeneration, though no visible tear is observable at arthroscopy. Intra-meniscal degeneration can be detected at MRI examination (Hamada, 1994).
The aim of the present study was to report the histological findings of discoid lateral meniscus in a group of children and adolescents, aged between 9 and 18, after arthroscopic partial resection, focusing on cellularity, arrangement of collagen fibers, and vascularity of the excised fragments. Furthermore, to report on MRI findings compared to the histological findings in the same region.
Group I: linear, horizontal region of high signal intensity in the substance of the meniscus (4 cases);
Group II: flattening of the meniscus in addition to regions of high signal intensity in the substance (2 cases);
Group III: regions of high signal intensity in the anterior segment of the meniscus alone (none).
During each intervention, partial meniscectomy (saucerization) with preservation of a stable peripheral rim was performed to obtain the shape of a normal meniscus. The resected tissue, the central part of the discoid meniscus, was excised with a one-piece technique, anonimyzed, processed in EDTA buffered for 2 h to soften the tissue and fixed in 4% buffered formaldehyde for 24 h. Before proceeding to dehydration each surgical tissue was cut into two fragments and then placed in a series of graded ethanol solutions. Each fragment was then embedded in paraffin with different orientation, either parallel or perpendicular to the articular horizontal plane. From the paraffin blocks, 3-μm thick sections were cut and stained with hematoxylin and eosine, and Masson trichromic. The sections were examined with an Axioskop microscope (Carl Zeiss Light Microscopy, Göttingen, Germany) and photographed with a digital camera (Rt Slider, Diagnostic Instrument Inc., Germany).
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.
The findings of our study suggest that discoid lateral meniscus is different from a normal meniscus in terms of vascularity and disorganization of collagen fibers.
SB was involved in interpretation of data and drafting of the manuscript. FB prepared and analyzed all histological sections and was involved in the draft of the manuscript. CT was involved in research design and revised the manuscript critically. All the authors have given final approval of the version to be published.
Ethics approval and consent to participate
Institutional Review Board approval was waived for this study at the author’s Institution. Parents of the young patients signed a written informed consent prior to surgery, to authorize histological examination of the specimens.
Consent for publication
Written informed consent was obtained from the parents of the patient for publication of this study and any accompanying images. A copy of the written consent is available for review by the Editor of this journal.
The authors declare that they have no competing interests.
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