Review of discoid meniscus

Abstract

Discoid lateral meniscus is an intra‐articular knee disorder that typically presents in children and adolescents. The natural history depends on the type of anomaly and the nature and presence of symptoms. Management of this disorder should be directed toward resolution of the symptoms while preserving meniscal tissue and function. Modern surgical techniques make suturing and preservation of meniscal tissue feasible. In the present article, the clinical manifestations, diagnostic criteria and practical management considerations are reviewed.

Introduction

Discoid lateral meniscus (DLM), a common anatomical anomaly of the lateral meniscus, was first described in cadavers by Young in 1889 ¹ . Descriptions of this anomaly, its natural history and clinical presentation were published, followed by classification of its various forms and establishment of treatment protocols. This article elaborates current knowledge concerning this meniscus anomaly.

Anatomy and etiology

The menisci have been confirmed to have the roles of transmitting the intra‐articular load and absorbing intra‐articular shock in order to protect the knee. From gestation to birth, the menisci are completely vascular and are not of discoid shape ² . However, the central third of the menisci is avascular by 9 months of age. By 10 years of age only the peripheral third has a blood supply, the inner two‐thirds being sustained by secretion of intra‐articular fluid ³ .

The medial meniscus is C‐shaped and covers 50% of the medial tibial plateau. An average lateral meniscus is circular, 12 mm wide and 4 mm high ⁴ . It covers 70% of the lateral tibial plateau and connects firmly to several joint attachments (the anterior and posterior meniscofemoral ligaments), whereas it lacks a lateral attachment (no attachment to the lateral collateral ligament), which means the lateral meniscus moves further than the medial one (10 mm vs. 3 mm) during extension and flexion. This anatomical feature protects the lateral meniscus against the occurrence of tears ⁵ .

More variants of the lateral than the medial meniscus have been reported. A discoid meniscus has abnormal morphology. It is thicker and covers more of the tibial plateau. The incidence of lateral discoid meniscus is 0.4% to 17% ⁵ , compared to 0.06% to 0.3%for the medial one ⁶ . Twenty percent of DLM are bilateral, whereas bilateral discoid medial meniscus is rare ⁷ . There is a greater incidence of discoid meniscus in Asian countries. Reported incidences include 16.6% by Ikeuchi and 13% in Japan by Fukuta et al., 10.6% in Korea by Kim et al. and 5.8% in India by Rao et al. ⁶ , ⁸ , ⁹ , ¹⁰ Its association with some other anatomical knee variants has been reported, including high fibular head, fibular muscular defects, hypoplasia of the lateral femoral condyle ¹¹ , ¹² , osteochondritis dissecans of the lateral femoral condyle ¹³ , hypoplasia of the lateral tibial spine ¹⁴ and cupping of the lateral tibial plateau ¹⁵ .

The precise cause of discoid meniscus is unknown, though there are some theories. Smillie suggested that the discoid shape is an intermediate stage during the fetal state, and that occurrence of the final discoid morphology is caused by absorption failure of the central part of the menisci (“congenital discoid meniscus”) ¹⁶ . However, his proposal is weak because such a stage has not been identified by anatomical study in either human or animal embryos ² , ³ . Kaplan suggested that deficiency of the posterior meniscal attachments causes meniscal hypermobility, leading to a high incidence of repetitive microtrauma that results in morphological changes ² . Kaplan's theory, however, makes no sense in cases with normal posterior attachments. The congenital theory is supported by reports of familial transmission and of the anomaly occurring in twins. Gebhardt and Rosenthal first described bilateral discoid menisci in identical female twins in 1979 ¹⁷ . De Lambilly et al. reported three bilateral symptomatic cases of lateral discoid menisci in a family of three brothers and sisters; their parents had never experienced any knee symptoms ¹⁸ . Similar cases of this bilateral abnormality were reported by Nami Komatsu et al. in 2006 ¹⁹ . Wen et al. reported 12 arthroscopically confirmed cases of the disease in two consecutive generations in six families ²⁰ . It is worthy of note that the six cases in the first generation were all women. These scattered case reports indicate there may be a hereditary predisposition to discoid meniscus.

Classification

Smillie first classified discoid meniscus into three types: the primitive type that affects the whole disc, the intermediate type that is smaller and less complete, and the infantile type, which differs in that the middle segment has greatly increased breadth ¹⁶ . In 1969, Watanabe et al. published a three‐type classification based on the arthroscopic appearance: complete, incomplete and Wrisberg‐ligament types ²¹ . In the Wrisberg‐ligament type the posterior meniscotibial attachment is absent, resulting in a hypermobile meniscus and the classical “snapping knee” syndrome. This, the most frequently used classification system, was expanded by Monllau et al. with a fourth type in which there is a ring‐shaped meniscus with a normal peripheral attachment, however the value of this additional type in clinical treatment decision‐making is doubtful ²² . In 1996, Jordan classified discoid meniscus according to its peripheral rim stability ⁴ . Both incomplete and complete menisci are included in the stable type, which was further divided by the presence or absence of symptoms and tears. Because both present similar symptoms and require similar treatments, unstable discoid meniscus and unstable normal meniscus were both classified as the unstable type, in which there is greater excursion because of the lack of a posterior‐tibial attachment ⁴ .

Clinical presentation

Stable discoid meniscus is usually detected incidentally in asymptomatic patients. Symptoms may occur because of a tear, most commonly a horizontal cleavage ²³ , ²⁴ . The patient may present with pain, swelling, tenderness in the joint line, locking, limited extension and the classical “snapping knee” syndrome. However, the snapping sound relates to the type of the discoid meniscus and the presence of a tear ² , ²⁵ , ²⁶ . It is commonly associated with an unstable discoid meniscus and, in the absence of a traumatic cause, occurs mainly in children and adolescents. On physical examination the patient may present joint line tenderness, effusion, anterolateral bulge, and even a positive McMurray test ⁴ , ²⁵ , ²⁷ . However, compared to arthroscopic diagnosis, the accuracy of physical examination ranges from 29% to 93%, depending on the examiner's experience and knowledge of this abnormality ²⁸ , ²⁹ , ³⁰ , ³¹ , ³² , ³³ .

Imaging

Radiography

Plain radiography contributes much to the diagnosis of DLM when MRI is not routinely used. Classical findings, sometimes as a whole set, including a widened lateral joint space, cupping of the lateral tibial plateau, squaring of the lateral femoral condyle, hypoplasia of the lateral tibial eminence and elevation of the fibular head, are present in some cases ¹⁰ .

Magnetic resonance imaging

With arthroscopy, it is difficult to outline the entire configuration because the discoid meniscus has a thick and deformed shape. MRI, which reveals the shape, dimensions and signal changes within the meniscus, has become a viable alternative to invasive arthroscopy in the setting of discoid meniscus and tear. Some atypical discoid menisci may be misdiagnosed because there are currently no consistent MRI diagnostic criteria. One possible criterion is the meniscal width on the coronal slice, 14 mm having been proposed by Samoto et al. ³⁴ and 15 mm by Araki et al. ³⁵ Samoto et al. recommended diagnostic criteria comprising a ratio of the minimal meniscal width to maximal tibial width (on the coronal slice) of more than 20% (Fig. 1a) and a ratio of the sum of the widths of both lateral horns to the meniscal diameter (on the sagittal slice showing the maximal meniscal diameter) of more than 75% (Fig. 1b). Both ratios have a sensitivity and specificity of 95% and 97%, respectively, even when tears are present ³⁴ .

Figure 1.

Diagrams of the measurements obtained via MRI. (a) Coronal slice. Meniscal width: D mm. Ratio of the meniscus to the tibia = D/E * 100%. (b) Sagittal slice. Percent coverage of the meniscus = (A + B)/C * 100%.

MRI is valuable for detecting degeneration of the inner meniscus. Stone and Miller reported 21 cases of symptomatic discoid meniscus with internal injury or degenerative changes shown in MRI, of which only three were diagnosed arthroscopically ³⁶ . MRI serves as a road map for operative conditions and can prevent repeated arthroscopic surgery, which is mainly caused by neglected insidious horizontal tears ³⁷ .

Treatment

Treatment of discoid meniscus depends on its type, concomitant symptom(s), duration of the symptom(s) and the patients' age. Snapping knee without symptoms (e.g. pain, locking, and swelling) should simply be followed‐up, surgery being considered only when symptoms develop. Meanwhile, for asymptomatic patients who are detected incidentally and do not have a tear, it is generally believed that treatment is not required.

For the unstable type (the Wrisberg‐ligament type), which is frequently observed in children and adolescents, total meniscectomy used to be performed. Nowadays, many surgeons suggest repair of the posterior tibial detachment ⁴ , ⁶ , ²⁶ , ²⁷ , which eliminates the risk of osteoarthritis consequent on total resection of the meniscus ²⁷ .

For the symptomatic, stable type, conventional total resection was once the first choice since some authors believed that there were intrinsic abnormalities in any meniscal tissue left ²³ . However, many more recent studies have shown that total removal leads to a high subsequent risk of degenerative osteoarthritis ³⁸ , ³⁹ , ⁴⁰ , ⁴¹ , ⁴² , ⁴³ compared with partial meniscectomy with preservation of a stable peripheral rim, which has an ideal short‐term outcome ⁴⁴ . In partial meniscectomy, the width of the remaining peripheral rim should be between 5 mm and 8 mm to prevent instability of the remnant, as such instability may cause a secondary meniscal tear ⁴⁵ , ⁴⁶ . If a peripheral tear is present, or happens to be in the vascular zone, arthroscopic suture repair of the tear can be effective ²⁷ , ⁴⁷ .

Treatment outcome

Many studies have demonstrated that the patients' sex and type of tear have no correlation with the treatment outcome on long‐term follow‐up ⁴⁸ , ⁴⁹ . Younger patients are more likely to achieve better results on long‐term follow‐up ⁵⁰ , ⁵¹ . However, the amount of meniscus removed, which is proportional to the increase in local contact pressure, is correlated with the risk of subsequent cartilage degeneration of the knee caused by overloading ⁴⁸ . If such degeneration produces enough repetitive microtrauma over time, osteochondritis dissecans of the lateral femoral condyle can develop in some cases ⁵² , ⁵³ . After meniscectomy, the incidence of signs of osteoarthritis on plain radiographs ranges from 48% to 89% ⁴⁹ , ⁵⁴ . Clinically, as better arthroscopic techniques have developed, traditional meniscectomy is being challenged by saucerization with repair of the tear and stabilization of the menisci. The results of partial meniscectomy are reportedly better than those of total meniscectomy on long‐term follow‐up, both clinically and radiologically ⁴³ , ⁵⁵ . However, the effect of partial meniscectomy on the long‐term prognosis needs further observation.

Disclosure

The authors did not receive any outside funding or grants in support of the research for, or preparation of, this work. Neither they nor a member of their immediate families received payments or other benefits, or a commitment or agreement to provide such benefits, from a commercial entity.