1. Kieu-My Nguyen OMS-III – University of New England College of Osteopathic Medicine
2. Kimberly Drago OMS-III – University of New England College of Osteopathic Medicine
3. L. Ryan Smart MD – Syracuse Orthopedic Specialists

Abstract

Introduction
The discoid meniscus of the knee is a rare anatomic variant, often found in the lateral meniscus with an incidence of 1.2-5.2%. A discoid medial meniscus is rarer, with an incidence of only 0.12%.

Case Presentation
A 15-year-old male soccer player presented for evaluation of medial right, sharp knee pain with a locking and popping sensation with quick flexion. Radiographs were unremarkable, and magnetic resonance imaging resulted in the diagnosis of a tear in the right medial meniscus. Arthroscopy showed a thickened plica band and an abnormally thickened meniscus, particularly on the posterior horn, which encompassed the entire half of the anterior to posterior diameter of the medial compartment. A partial medial meniscectomy with saucerization and plica band excision were performed. The patient has made a successful recovery with return to activity.

Discussion
Medial discoid meniscus predisposes individuals to meniscus tears. Currently, most symptomatic discoid menisci are treated with partial meniscectomies, preserving as much of the meniscus as possible to decrease the rate of degenerative changes. Although, more recent studies of pediatric patients with discoid lateral meniscal tears were treated with saucerization. The success with saucerization has been extrapolated to medial discoid menisci, with positive results thus far.

Conclusion
This case exemplifies the clinical manifestations of the discoid medial meniscus and how treatment options can be extrapolated from the more common conventional medial meniscus injury as well as the discoid lateral meniscus.

Keywords: discoid medial meniscus, meniscectomy, saucerization, pediatric knee pain

Manuscript

Introduction
The discoid meniscus is a rare anatomic variant resulting in a thicker and wider meniscus. There are two main ideologies in the current literature describing the pathogenesis of the discoid meniscus. The discoid shape is an intermediate stage during fetal development of the meniscus. Simillie describes the pathogenesis of the discoid meniscus as a failure of absorption and persistence into adulthood (1). However, Kaplan described the pathogenesis involving mechanical factors such as posterior segment hypermobility, as some anatomic studies have shown the meniscus never has a discoid shape in embryonic development (1). Kaplan only described this for a lateral discoid meniscus (1). The discoid lateral meniscus is more common with an incidence of 1.2-5.2% (2). Discoid medial meniscus (DMD) is a rare diagnosis, representing only about 3% of all discoid menisci (3). Even rarer is the incidence of bilateral discoid menisci, at 0.012% (2).

The most common symptoms of DMD are medial knee pain and effusion and locking in flexion, however the pain and effusion are more likely due to a tear rather than the discoid shape itself (1). Locking may be attributed to the discoid shape, as the thick central region must move over the medial femoral condyle (1). A palpable snap may indicate the tear is more unstable. This is usually related to the more unstable Wrisberg type (5). Radiographic findings of the discoid lateral meniscus reported are widened joint space up to 11 mm, squaring of the medial condyle, and cupping of the lateral tibial plateau, all which may be extrapolated to the medial variant (6). MRI is the best modality to confirm diagnosis, which requires three or more 5 mm sagittal images with meniscal continuity (6).

Surgery is indicated only if patients are symptomatic. For symptomatic lesions, arthroscopic saucerization has been shown to be superior over total meniscectomy (7). Total meniscectomy increases the probability of osteoarthritis (OA) in the future compared to saucerization (7). The saucerization technique of partial meniscectomy involves respecting the central portion of the discoid meniscus to recreate the shape of a normal meniscus. Short- and long-term outcomes for surgical treatment of a discoid meniscus have shown to be successful with symptom relief and return to everyday function.

Case Description
The patient is a 15 year old male who presented for evaluation of right knee pain. He had right knee pain for 3 years, which worsened approximately one-and-a half months prior to this appointment, when he started playing soccer again. It is localized to the medial and lateral aspects of his right knee, radiating down the shin. He rates the pain a 6/10 when it occurs and describes it as sharp. He experiences a painful locking and popping sensation in his knee with quick flexion or when he ambulates. Playing soccer worsens the pain and the pain is relieved with rest. He also reports pain in the knee after sitting for an extended period of time or when he is swimming. He has been wearing a knee brace while playing sports. He has sustained some injuries to his right knee during sports, but nothing severe or requiring surgery.

On a physical exam of the right knee, there is no obvious effusion or warmth. There is tenderness medially and over the medial femoral condyle. Range of motion revealed full extension, flexion to approximately 140 degrees, and pain with deep flexion. He was ligamentously stable to varus and valgus stress tests. The Lachman test was negative and the McMurray test was mildly positive.

AP and tunnel radiographs from his primary care physician of the right knee showed no lytic or blastic changes, no fracture or dislocation. Given the chronic history and physical exam findings with evidence of plica band syndrome, an MRI was performed which demonstrated a right medial meniscus tear.

The patient underwent arthroscopy of the right knee under general anesthesia with local injection of 60 mL 0.25% Marcaine with epinephrine. A thickened plica band was found medially rubbing against the medial femoral condyle. The medial compartment showed an abnormal thickened meniscus especially on the posterior horn which encompassed the entire half of the anterior to posterior diameter of the medial compartment. This was diagnosed as a DMD with an extensive horizontal cleavage tear in the mid-section of the posterior horn. The meniscus was considered to be unrepairable, so saucerization was performed. The meniscus was trimmed with biters leaving root and peripheral attachments intact. A plica band excision was also performed.

At the post-operative visit 1 week later, the patient reported slow improvement with stiffness and soreness, and some swelling. Ice, ibuprofen, and physical therapy were recommended. The patient has since made a complete recovery with full return to all pre-operative activities.

Figure 1: Right knee MRI demonstrating a right medial meniscus tear. A) Sagittal view. B) Coronal view.

Figure 2: Arthroscopic images. A) Discoid medial meniscus. B) Discoid medial meniscus tear in the posterior horn. C) Medial meniscus after saucerization.

Discussion
The discoid meniscus is a rare abnormality of the knee with low prevalence. While a discoid lateral meniscus is both a documented and well-studied entity, its medial counterpart is less understood and reported (8). There are less than 100 cases of the presence of DMD reported in the current literature. An 1889 anatomical study by Young et al revealed the first lateral discoid meniscus (9). It was not until 1941 that Cave reported the medial space of the knee joint can also have a discoid meniscus (10). This low prevalence is simply attributable to the fact that patients do not present to a provider unless they are symptomatic. Discoid menisci, both lateral and medial, can also present bilaterally as reported by multiple studies (2, 11). Unless patients present with bilateral knee pain, the unaffected knee is never imaged and therefore can contribute to the low prevalence.

Discoid menisci findings are described as an abnormally wide and thick meniscus that covers more of the articular surface of the tibial plateau, leaving it more vulnerable to damage as a result. The discoid menisci differs in its structure, including the organization of the collagen fibers. This ultrastructure difference of discoid meniscus, including its thickness and stress redistribution, are factors leaving it vulnerable to injuries and tearing (4). In terms of characteristics of discoid menisci, histologic studies have only been performed on discoid lateral menisci, and an anatomic classification exists based on the lateral variant. Watanabe et al, classified it as complete, incomplete, or Wrisberg depending on the degree of coverage of the lateral tibial plateau and presence of posterior attachment (1). However, three variants have been described by Flouzat-Lachaniette et al for DMD that are based on position of anterior horn insertion: normal, deficient insertion of the anterior horn onto the tibia with continuity of the anterior horn and anterior intermeniscal ligament over the anterior cruciate ligament (ACL), and anterior horn in continuity with ACL (1).

The most common symptoms of a discoid lateral meniscus tends to be hypermobility with snapping in childhood. These symptoms can manifest on their own without any trauma or tearing. In contrast, DMD is typically asymptomatic in childhood, with symptoms only beginning with injury and tearing (8). Most patients with this anomaly do not have any clinical symptoms, leaving the true incidence of DMD to likely be greater than estimated (2).

Because of the vague symptomatic presentation, the discoid meniscus is primarily diagnosed via MRI or arthroscopy. Currently, utilizing MRI to diagnose a discoid meniscus is the same for both lateral and medial. The diagnosis can be made using the “bowknot” or “bow-tie” sign (6). This sign is depicted in the sagittal view as the anterior and posterior corners of the meniscus form a “bow-tie”. This meniscal continuity of the anterior and posterior horn should be seen in at least 3 or more continuous layers at 5-mm thickness intervals to make this diagnosis (2). Our patient did not have this characteristic sign and the discoid nature of the meniscus was not appreciated until the arthroscopy, which is why it remains the “gold standard” for diagnosing a discoid meniscus.

Surgical indications are the same for both medial and lateral discoid menisci, with only symptomatic lesions being operated on (8). In the past, total meniscectomy was widely indicated and used for a symptomatic discoid lateral meniscus. Multiple studies revealed positive results with this treatment, including Washington et al in 1995 concluding it may offer the best prognosis (12). Although, in 1998 Raber et al reported increased osteoarthritic changes of the knee after a total meniscectomy in contrast to the patient’s untreated knee, when retrospectively comparing long-term results (13). Ultimately, total meniscectomy was the recommended treatment for a symptomatic discoid lateral meniscus to completely remove the cause of discomfort and prevent risk of relapse; although, this treatment comes with the risk of progressive OA, leading to a poor prognosis in the long term (8, 14). Currently, a partial meniscectomy with saucerization is the recommended treatment of choice. The goal of this treatment is to leave as much of the meniscus as possible to decrease instability and the degeneration of the cartilage seen with a total meniscectomy (8). The literature recommends at least 6-8 mm from the periphery should remain, as larger meniscus remnants after saucerization have been associated with increased retear rates (5).

The success of saucerization in treating discoid lateral menisci has been extrapolated to the medial variant, with positive results thus far. Multiple cases in pediatric male patients who underwent saucerization for DMD were asymptomatic and successfully returned to pre-injury level activity at one and two years postoperatively (15). A systematic review looking at long-term surgical outcomes of the discoid lateral meniscus have reported positive outcomes overall, with minimal progression of degenerative changes (16). Perkins et al looked at patients under 18 treated with saucerization for the lateral variant and found 89% of patients returned to the same or higher level of activity postoperatively and low rates of revision surgery (17). These patients had a minimum follow-up period of 24 months.

The concern of the development of OA is still present; although, saucerization with partial meniscectomy has shown good results. There have yet to be long-term follow up studies following adolescent patients into adulthood, which is when the typical symptoms of OA begin to present. When specifically referring to DMD, even less long-term data exists, as the literature has only reported a follow-up time of maximum of 2 years postoperatively. In those of the discoid lateral meniscus, the significant risk factors leading to the development of OA radiographically, at a minimum follow-up of 5 years, includes the presence of a horizontal tear, prolonged symptom duration, and increased relative meniscus thickness (18). When thinking about the relationship between DMD and OA, OA is more common in the medial compartment as 60% of the load while ambulating goes through the medial knee (19). Therefore, the question that is posed now is if the risk of development of OA is higher in those who underwent repair of a DMD compared to the lateral counterpart. Further longitudinal follow-up studies must be completed following the course of those who underwent DMD saucerization and partial meniscectomy.

Conclusion
In conclusion, DMD is a rare abnormality. This case report discusses an adolescent male athlete with DMD and successful treatment with partial meniscectomy and saucerization. DMD should be considered on the differential diagnosis if a patient presents with medial knee pain with locking or clicking in flexion. MRI is the diagnostic imaging modality of choice. Treatment should be limited to symptomatic patients and the success of partial meniscectomy and saucerization in the lateral variant can be applied to DMD, as patients have successfully returned to a high level of activity.

Figure 1 | Figure 2

References

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