Current Concepts: Evaluation and Treatment of Discoid Meniscus in the Pediatric Athlete
Dr. Nicholson and Dr. Petit, are orthopaedic surgery residents, Yale School of Medicine, New Haven, Connecticut. Dr. Egger is an orthopaedic surgery resident, Cleveland Clinic, Cleveland, Ohio. Dr. Saluan is an orthopaedic surgeon, Cleveland Clinic, Cleveland, Ohio. Dr. Carter is an orthopaedic surgeon, NYU School of Medicine, New York, New York.
Authors’ Disclosure Statement: The authors report no actual or potential conflict of interest in relation to this article.
Address correspondence to: Cordelia W. Carter MD, NYU-Langone Medical Center, Department of Orthopaedic Surgery, 301 East 17th Street, NY, NY 10003 (tel, 212 598-6000; email, firstname.lastname@example.org).
Am J Orthop. 2018;47(12). Copyright Frontline Medical Communications Inc. 2018. All rights reserved.
Discoid meniscus is a rare anatomical variant with altered morphology and structure that can sometimes present symptomatically, typically in the pediatric population. The discoid meniscus is usually in the lateral compartment of the knee and is characterized by a partial or complete filling-in of central meniscal tissue, increased meniscal thickness, disorganization of longitudinal collagen fibers, and sometimes lack of peripheral attachments. These changes to both the macro- and micro-structure of the meniscus predispose affected patients to increased rates of both meniscal tears and mechanical symptoms. Surgical management of symptomatic discoid meniscus is directed toward symptom resolution while preserving sufficient functional meniscal tissue to delay or prevent the development of osteoarthritis. Modern surgical techniques consist of arthroscopic saucerization of the discoid meniscus with repair of associated meniscal tears and stabilization of peripheral attachments. Although long-term outcome data are lacking, short- and mid-term outcomes for patients treated with arthroscopic partial meniscectomy and meniscal repair and/or stabilization as needed are generally good.
The discoid meniscus is an uncommon anatomical meniscal variant that may present with pain, snapping, motion loss, swelling, and locking of the affected knee, typically during childhood or adolescence.1 Although the etiology of discoid meniscus is not completely understood, it is considered a congenital anomaly with a possible genetic component.2, 3 Incidence is estimated at 0.4% to 5.2% in the Western European population but is reportedly much higher (roughly 17%) in Asian populations.4-6, Discoid menisci, when present, are almost always in the lateral compartment, although cases of medial discoid meniscus have also been reported. The overall incidence of medial discoid meniscus has been estimated to be between 0.06% and 0.3% while that of lateral discoid meniscus, as detailed above, is significantly higher.7-11
ANATOMY AND MORPHOLOGY
The menisci differentiate from mesenchymal tissue early during fetal development with a clear definition by 8 weeks and a mature anatomical shape by 14 weeks in utero.12 Interestingly, menisci never possess a discoid shape during the normal course of development.13,14 The meniscus is fully vascularized at birth, with the central one-third becoming avascular by 9 months as the vascular supply regresses.14 By 10 years, only the peripheral one-third maintains its vascular supply, and the menisci have adult-like characteristics with tissue composed mainly of circumferential collagen fibers.4,14-16
A normal meniscus is wedge-shaped in the coronal plane and crescent-shaped in the axial plane. The medial meniscus is shaped like the letter “C,” is connected firmly to the joint capsule, and covers approximately 50% of the medial tibial plateau. The normal lateral meniscus covers 70% of the lateral tibial plateau and has firm anterior and posterior attachments while the lateral portion is less securely tethered.17 A discoid meniscus is characterized by an atypical shape—the center being partially or completely filled in (or “disc-like”)—often associated with anomalous attachments to the tibia, femur, and surrounding joint capsule. In addition to an atypical morphology, the discoid meniscus may also be much thicker than normal, resulting in a “block-like” appearance. The increased thickness may be responsible for the pathognomonic snapping of the knee as it is brought passively into flexion or extension (hence, the term “snapping knee” was traditionally used to describe a symptomatic discoid lateral meniscus). While the discoid shape is relatively straightforward to address, the increased meniscal thickness may be quite difficult to correct surgically.4-6,18
In addition to the macromorphological differences characteristic of discoid menisci, histological differences have also been described, including differences in collagen density and disorganization of the circumferential collagen network.19-21 Taken together, these differences may compromise the ability of the discoid meniscus to withstand normal forces placed across the knee and predispose it to tear. This is important because the normal meniscus plays a critical role in facilitating load distribution across the knee joint, in addition to assisting with functions including shock absorption, proprioception, and stabilization of the knee. Torn and/or unstable discoid menisci are unable to perform these functions adequately, which may ultimately result in degenerative joint disease and progress to end-stage osteoarthritis (OA).
The traditional Watanabe classification of discoid meniscus consists of 3 variants: type I, a complete discoid shape; type II, an incomplete discoid shape; and type III, the Wrisberg variant.22 The Wrisberg variant is described as a more normal-appearing meniscus but lacks normal peripheral attachments.5, 6, 15 This classification system was expanded by Monllau and colleagues, 23 who described a ring-type meniscus variant with normal posterior tibial attachments. Although the Watanabe classification is commonly used, its utility in assisting with clinical decision-making may be limited.
Several other classification schemes exist. Jordan and colleagues24 described a classification scheme defining a meniscal type as complete or incomplete, also noting the presence of symptoms, tearing, and peripheral rim instability. They grouped stable types together, regardless of morphology, and then further classified them based on the presence of symptoms and tears. Similarly, the unstable types were grouped together and then subclassified in the same manner.17,24 Klingele and colleagues25 also described a contemporary classification scheme of discoid meniscus evaluating peripheral stability patterns that may be more clinically and surgically relevant. This classification is based on the type of discoid morphology (complete vs incomplete), the presence or absence of peripheral rim stability (stable vs unstable), and the presence or absence of a meniscal tear (torn vs untorn).5,25
A stable discoid meniscus is often an incidental finding, seen either on advanced diagnostic imaging performed for another reason or at the time of arthroscopy to address another problem. Younger children with discoid meniscus tend to present with symptoms such as popping and snapping related to instability and the abnormal morphology of the discoid meniscus. Older patients tend to present with symptoms related to acute tears through the abnormal meniscal tissue. Although discoid menisci can become acutely symptomatic in the presence of a tear, the onset of symptoms may occur in the absence of a discrete traumatic event.1 Alternatively, some patients will report a clear history of injury, often a noncontact, rotational injury mechanism related to an athletic activity. Patients with torn discoid menisci may report pain, catching, locking, and/or giving way of the knee, and on examination may have limited extension, snapping, effusion, quadriceps atrophy, and joint line tenderness. Eponymous meniscal compression tests including the McMurray, Apley, and Thessaly tests, may also be performed when meniscal tear is suspected, although this may be tricky for younger children.1
Considering the high association of meniscal tears with ligamentous injuries, examination of knee stability is important. Plain radiographs of the knee should be taken, although the results will often be negative for osseous injury in the case of an isolated meniscal tear. Radiographs of a discoid knee may show subtle differences compared with radiographs of a non-discoid knee. A recent comparison of children with symptomatic lateral discoid menisci with age-matched controls found statistically significant increased lateral joint space, elevated fibular head, increased height of the lateral tibial spine, and increased obliquity of the tibial plateau.26 They did not find statistically significant increased squaring of the lateral femoral condyle or cupping of the lateral tibial plateau. Radiographic signs can be subtle and may not all be present in a patient with a discoid meniscus.
Magnetic resonance imaging (MRI) is the assessment technique of choice for the diagnosis of discoid meniscus, although MRI may not reliably identify a Wrisberg variant or incomplete discoid menisci (Figure 1).
Gans and colleagues27 examined preoperative MRI and clinical examination compared with pathology found during arthroscopy. Although they found that MRI and clinical examination had excellent diagnostic accuracy of 92.7% and 95.3%, respectively, the most common missed pathology on MRI later found on diagnostic arthroscopy was the presence of a lateral discoid meniscus, which occurred in 26.7% of missed diagnoses. Adult diagnostic criteria of discoid meniscus include ≥3 contiguous 5-mm sagittal cuts showing continuity between the anterior and posterior horns of the meniscus. Other criteria include a minimal meniscal width >15 mm on the coronal view or a minimum meniscal width that is >20% of the width of the maximal tibial width.28 These criteria are often applied to children as well. Additionally, if >50% of the lateral joint space is covered by meniscal tissue, a diagnosis of discoid meniscus should be considered.6
Management of symptomatic discoid meniscus is directed toward resolving symptoms while preserving meniscal tissue and preventing the development of OA. Incidentally found discoid menisci in asymptomatic patients should not be treated surgically and can have periodic follow-ups for detection of any functional deterioration or symptom development. Surgical treatment may be beneficial for patients with symptoms related to their discoid menisci such as pain, recurrent effusion, limited knee motion, mechanical symptoms (e.g., locking and catching), and activity restrictions.
Traditionally, surgical treatment of a symptomatic discoid meniscus consisted of subtotal (meaning nearly complete, or <3mm of peripheral rim remaining) or complete meniscectomy, often performed in an open fashion. Surgical techniques have evolved; current surgical approaches typically include diagnostic arthroscopy followed by arthroscopic saucerization (also called partial meniscectomy) of the central portion of the “disc,” leaving a peripheral rim of 6 mm to 8 mm to approximate that of the normal meniscus (Figure 2).29,30 Saucerization removes the redundant central meniscal tissue in an attempt to create a more “normal” C-shaped morphology, although it does not address the increased thickness characteristic of many discoid menisci. It may be particularly difficult to “debulk” an abnormally thick discoid meniscus in the coronal plane, and there is little in the outcomes literature to support this approach. Following partial meniscectomy, the remaining meniscal tissue should be inspected thoroughly both for the presence of instability and for residual tears. Meniscal tears should be repaired in standard fashion; commonly, this may be a combination of all-inside sutures placed into the posterior horn, inside-out sutures placed into the midbody, and outside-in sutures placed into the anterior horn. Peripheral rim instability—identified, for example, by the ability to translate the posterior horn of the meniscus fully onto the anterior tibial plateau—should be addressed by suturing the meniscus to the adjacent capsule (Figure 2).25, 31 Menisci that have residual complex tears that are not amenable to repair may be treated by subtotal meniscectomy. These patients may be candidates for subsequent meniscal allograft transplantation.32,33
Performing a partial meniscectomy has consistently been shown in the laboratory to increase contact stresses proportionally to the percentage of tissue removed.34 It follows logically that performing a subtotal or complete meniscectomy in a young patient would yield increased, abnormal contact stresses throughout the knee with resultant degenerative changes and progression to OA over time. While long-term outcome studies of subtotal or complete meniscectomy have shown somewhat variable results, the majority report the development of pain, instability, poor function, osteoarthritic changes and even the development of osteochondritis desiccans.5,35
In an early investigation of long-term outcomes associated with surgical resection of a discoid lateral meniscus, Räber and colleagues36 retrospectively examined 17 knees that had undergone total meniscectomy for the lateral discoid meniscus. At a mean follow-up of 19.8 years, these authors reported that patients accounting for 10 of 17 operative knees had developed symptoms of osteoarthrosis such as pain. In addition, two-thirds of the knees that had follow-up radiographs performed (10 of 15 knees) had visible osteoarthritic changes present.
In 2011, Stilli and colleagues37 conducted a mid-term follow-up study examining 104 knees over an average follow-up of 8.5 years, with an average age of 8 years at the time of surgery. Younger patients, 2 to 7 years, underwent subtotal meniscectomy whereas patients, 8 to 14 years, underwent arthroscopic partial meniscectomy. Patients with a Wrisberg variant underwent removal of the posterior horn. The authors of this study reported that younger patients who had undergone subtotal meniscectomy had the best outcomes (defined by self-reported questionnaire responses and clinical evaluation). However, these results should be interpreted with caution: at the time of follow-up, patients in this study would still have been in adolescence and may not yet have developed the symptomatic degenerative changes in the knee joint that are strongly associated with meniscectomy.
There are few long-term outcome studies for arthroscopic saucerization. In 2014, Ahn and colleagues38 examined patient-reported outcomes and radiographic changes for 48 knees in pediatric patients treated arthroscopically for a symptomatic discoid meniscus. Patients were treated with partial meniscectomy (22 knees), partial meniscectomy with meniscal repair (18 knees), or subtotal meniscectomy (8 knees). Out of 48 knees evaluated, 45 knees (94%) were rated as “excellent” or “good” at an average 10-year follow-up. However, a significant percentage of patients had evidence of degenerative changes on follow-up radiographs, specifically: 88% of the subtotal meniscectomy group, 39% of the partial meniscectomy with repair group, and 23% in the partial meniscectomy-alone group. This finding suggests that the early appearance of radiographic changes suggestive of OA does not necessarily correlate with the development of knee symptoms in this cohort, although longer-term follow-up of these patients into adulthood and middle age is clearly needed.
Recently, short-term outcomes have been reported for contemporary arthroscopic saucerization of discoid menisci with peripheral rim stabilization performed as deemed necessary at the time of surgery. In 2012, Carter and colleagues39 examined 57 knees that underwent saucerization alone or saucerization with peripheral rim stabilization. At an average follow-up of 15 months, both patient populations had equivalent self-reported outcomes and clinical examination findings. The authors concluded that peripheral stabilization does not negatively affect short-term outcomes if instability is recognized and treated.
Yoo and colleagues40 found equivalent results between patients undergoing saucerization, saucerization with rim stabilization, and subtotal meniscectomy for 100 pediatric knees with an average follow-up of 4.7 years. Subtotal meniscectomy was defined as <3 mm of peripheral rim remaining and was performed when there was severe degeneration or complex tearing of the remnant rim following arthroscopic saucerization. Taken together, these data seem to suggest that short- and mid-term outcomes for pediatric patients treated surgically are generally good in terms of reported clinical function and development of OA, although long-term follow-up studies will be essential for understanding the true impact of surgical interventions.
Shieh and colleagues41,42 investigated risk factors for failed meniscal surgery in 324 menisci from 2008 to 2012, including in 46 discoid saucerization procedures with or without stabilization. At a mean of 40 months, 15% of the discoid saucerization cohort required a revision procedure, with increased odds of revision surgeries in patients who had undergone meniscal tear repair. The most frequent indication of revision surgery was sustaining a meniscal tear during intense physical activity in the first year after the index procedure, and patients underwent either debridement or repair for their revision procedure.
Discoid meniscus is a rare anatomical meniscal variant characterized by an abnormal “O-like” shape, increased meniscal thickness, disorganization of collagen fibers, and variable absence of peripheral attachments. These morphological changes predispose patients with discoid menisci to increased rates of symptomatic meniscal instability and tearing. An MRI remains the most sensitive imaging modality for the diagnosis of a discoid meniscus, with ≥3 contiguous 5-mm sagittal cuts showing continuity between the anterior and posterior horns indicative of a discoid variant. Surgical treatment for symptomatic discoid menisci has evolved from subtotal meniscectomy to contemporary arthroscopic saucerization techniques with repair of meniscal tears and peripheral stabilization performed as needed. Long-term outcome studies for patients who undergo subtotal or complete meniscectomy reveal an association with osteoarthritic changes of the knee. Short- and mid-term outcome studies for patients who undergo arthroscopic saucerization with or without repair and/or peripheral stabilization are generally good, although reoperation rates have been estimated at 15% in the first 3 to 4 years. Longer-term follow-up, with the inclusion of validated functional outcomes measures, will be essential for understanding the true impact of various surgical interventions over time.