Discoid Meniscus: Advanced Techniques of Meniscal Stabilization and Reconstruction

Discoid meniscus is the most common congenital anomaly of the knee, representing an abnormal variant of the normal crescent-shaped meniscus. The lateral meniscus is almost exclusively involved, although case reports and small series of medial discoid meniscus have emerged. Discoid menisci have increased meniscal width and—in most instances—height, abnormal collagen arrangement, and decreased vascularity. A subset of cases exhibit abnormal meniscotibial or meniscocapsular attachments, leading to meniscal instability. Management of discoid meniscus has evolved, with modern treatment emphasizing recontouring and stabilization of the tissue to maintain or restore the long-term function of the meniscus. This review explores modern surgical techniques in managing discoid meniscus to help surgeons maximize patient outcomes.

Introduction

Considered a normal anatomic variant, the discoid meniscus is the most common congenital difference of the knee and is found almost exclusively in the lateral tibiofemoral compartment. Discoid menisci are, by definition, wider from medial to lateral. There is a spectrum of morphologic differences ranging from widened “incomplete” discoid menisci that retain the characteristic C-shape of the meniscus to “complete” discoid menisci that cover 90%-100% of the lateral tibial plateau. Discoid menisci also have less-organized collagen structure and decreased vascularity, predisposing them to tears. Meniscocapsular and meniscotibial attachments may also be deficient and lead to instability.

The incidence of discoid lateral meniscus (DLM) is estimated at 5% in the United States population. Ethnic variation has been reported, with up to 16.6% prevalence in patients of Japanese heritage. Bilateral discoid lateral menisci have been reported in 5% to 25% of cases, and caregivers should have a low threshold to image the contralateral knee in the setting of even mild contralateral signs or symptoms. The incidence of discoid medial meniscus is much rarer, with an incidence of 0.06% to 0.3%. ^, ^, The true prevalence of discoid meniscus is unknown, however, as they may be asymptomatic in some patients. ^,

Due to the altered shape and increased tissue volume, a discoid meniscus can change the biomechanics and contact area of the lateral tibiofemoral compartment and may increase the risk of chondral damage, given the association between osteochondritis dissecans (OCD) in the lateral femoral condyle and a DLM.

Historically, discoid menisci were empirically treated with partial, sub-total, or total meniscectomy, which led to high rates of arthritis and disappointing long-term outcomes. ^, Management of DLM has evolved to focus on preserving and reshaping meniscus tissue to recreate the normal contact relationships between the tibia, femur, and meniscus. In addition to recontouring the meniscus, repairing tears within the zone of the typical meniscus shape and stabilizing the meniscus in the setting of hypermobility due to inadequate meniscocapsular or meniscotibial attachments have been advocated. This review explores modern operative techniques to manage symptomatic, torn, or unstable discoid menisci and provides strategies to help surgeons optimize patient outcomes.

Classification

Historically, the most commonly used classification for DLM is the Watanabe, which categorizes menisci as complete (type I), incomplete (type II), or the Wrisberg variant (type III) with peripheral instability. However, over the last several decades, research has found that the Type III variant is quite rare and may better be classified as a sub-group of a normal lateral meniscus. Moreover, it has become clear that many Type I and II DLM have abnormal attachments in a variety of presentations and combinations, leading to the development of more recent classification systems that characterize stability as a separate component to be included in the assessment of an individual discoid.

Although the Watanabe classification is straightforward, it also lacks specificity in describing meniscus morphology. Thus, recently, a new classification scheme has been proposed by Meniscus Research Interest Group of the Pediatric Research in Sports Medicine (PRiSM) organization. The PRiSM DLM classification relies on four main arthroscopic features: meniscal width, height/thickness, peripheral stability, and nonvertical meniscus tears (horizontal or degenerative/complex/radial). The PRiSM DLM classification system has been shown to have moderate to substantial interobserver (0.51 – 0.75) and intraobserver (0.49 – 0.82) reliability in practice and ideally will aid in guiding evidence-based treatment of DLM in coming years. Another classification system that may help inform treatment was developed by Yang et al., which classifies DLM based on stability, morphology, and tear location and can help predict the need for surgical meniscus repair preoperatively.

Indications for Surgery

Many patients with a discoid meniscus are asymptomatic, and the DLM may be identified incidentally on MRI during workup of unrelated knee pathologies. In asymptomatic patients, observation is recommended. The younger a patient is at the time of detection of DLM, the more important it may be to follow the long-term natural history of that knee to ensure that subtle symptoms or signs suggesting degeneration of the meniscal tissue do not arise.

The presentation of DLM is variable. Symptoms can include chronic, achy lateral knee pain or the acute onset of sharp pain and effusion, often suggesting a tear. Mechanical symptoms may be the most common, including locking, snapping, popping, clicking, clunking, or a lack of terminal extension. The presence of a torn discoid meniscus warrants surgery, as such tears are unlikely to heal spontaneously and may propagate or deteriorate, adversely affecting a young patient’s long-term prognosis. Even without a tear, surgery is recommended to treat mechanical symptoms, as non-operative interventions such as physical therapy will not correct the underlying mechanical anomaly causing these symptoms. Even patients with mild symptoms, such as painless snapping, clicking, or clunking, have a discoid meniscus that is at risk of progressing to tearing, and studies have shown a high rate (up to 39%) of conversion to surgery with attempted non-operative treatment. ^,

When surgery is pursued, the primary goal is preserving meniscal function, with tissue that remains as close to normal in width, height, and peripheral stability as possible while eliminating mechanical symptoms. The general categories of surgical techniques applied to a discoid meniscus are saucerization/partial meniscectomy, stabilization/repair, and reconstruction/meniscoplasty, each of which will be explored below.

Concomitant Conditions

In managing DLM, surgeons should also consider the potential for concomitant conditions such as OCD of the lateral femoral condyle and lower extremity valgus alignment. OCD of the lateral femoral condyle is known to co-occur in up to 14.5% of patients and is thought to develop due to the abnormal biomechanics of the lateral compartment. ^, OCD lesions have also been reported to occur following DLM saucerization or meniscectomy. Studies have reported that younger age at surgery and subtotal meniscectomy are risk factors for the development of postoperative OCD, which further reinforces the importance of meniscus preservation in the initial management of DLM. ^,

Regarding lower limb alignment, some authors note that lower extremity valgus may be a risk factor for poor postoperative outcomes following DLM surgery. ^, Wang et al. also demonstrated that even partial meniscectomy of a DLM can shift a patient’s mechanical axis laterally. Therefore, evaluation of lower extremity alignment is important both pre-and postoperatively.

Surgical Checklist

In addition to a standard knee arthroscopic setup, the following list of items is a suggested “discoid meniscus” set to facilitate smooth surgery and anticipate challenges:

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Small hand-held shaver, straight and curved (3.5mm or smaller if available).
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Small scalpels utilized for arthroscopy, such as ‘banana blades’ or long-handled ‘beaver blades’ (for anterior meniscus saucerization).
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Outside-in meniscus repair tools for anterior horn tears or instability (18g spinal needle, small suture passers, and monofilament suture for passing stitches).
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Inside-out meniscus repair tools for mid-body and posterior horn tears (zone-specific cannulas or commercially available needle delivery devices with double-armed sutures).
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Spoon-like retractors or a speculum to aid in posterolateral protection of neurovascular structures with inside-out meniscus repair of the posterior horn or posterior horn-body junction.
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All-inside meniscus repair devices for the posterior horn (which may obviate the need for more invasive posterolateral dissection associated with the inside-out technique or used in conjunction with it as a ‘hybrid’ technique).
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Meniscal rasp (to abrade/prepare meniscal tissue for repair).
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Microfracture picks or small k-wires (to enhance biological healing environment with notch microfracture or microperforation, designed to allow stem cell-rich cancellous bone bleeding that theoretically may coat the meniscal repair site).

Surgical Setup

In the most common cases of DLM, the patient will be placed supine, with a fold-away or removable lateral posts, as the majority of discoid surgery is performed with the lower extremity in the figure-four position.

Access to the knee is via standard anteromedial and anterolateral portals. However, an accessory anteromedial portal, placed 1-2cm more proximal and more medial than the standard peripatellar position, should be considered to optimize access into the anterior aspect of the lateral compartment. (In the setting of a medial discoid meniscus, a similarly adjusted anterolateral portal would be utilized). A diagnostic arthroscopy is performed to thoroughly understand the morphology, stability, and tearing of the meniscus. Assessment of anterior pathology may be enhanced by viewing through the anteromedial portal, and the authors recommend this as a standard portion of the diagnostic arthroscopy.

Surgical Technique: Saucerization/Partial Meniscectomy

The mainstay of discoid meniscus surgery is saucerization, which involves removing and recontouring the central meniscal tissue to create a shape that will restore the biomechanical relationship between the femoral condyle and tibial plateau and more effectively distribute hoop stresses during loading ( Fig. 1 ). Reestablishing a more physiologic force distribution within the tibiofemoral joint will ideally preserve the longevity of articular cartilage within the compartment and eliminate mechanical symptoms during motion.

The ideal meniscus width or coverage of the plateau is difficult to generalize, as this depends on the size of each patient and the morphology of their tibiofemoral articulation. Ideal widths reported in the literature range from 5mm to 16mm. For treating DLM, some authors recommend using the mid-body width of the ipsilateral medial meniscus as an internal reference for estimating the width of the preserved lateral meniscus. Current evidence suggests that surgeons should aim to preserve 8-10 mm of meniscus width after saucerization.

Multiple instruments can be helpful when performing saucerization, including arthroscopic basket biters, punchers, scissors, straight or curved shavers, radiofrequency ablation, and a small scalpel. The goal of resection should be to preserve at least an 8mm rim of peripheral meniscal tissue; thus, using a probe with a known length can be helpful when measuring the resection. The inner border of the meniscus should also be smooth when possible, without loose flaps or frayed edges, as these can be risk factors for retears or propagation of new tears. Following saucerization, the tibiofemoral joint should be ranged and inspected to ensure that no obvious impingement is occurring and that any preoperative mechanical symptoms have been resolved.

In severely degenerative tears, particularly with radial flaps that cause mechanical symptoms, simple debridement or subtotal lateral meniscectomy may play a role. As described below, there may be novel reconstruction techniques designed to salvage segments of the meniscus. When subtotal or total lateral meniscectomy is necessary, patients should be monitored over time as candidates for a lateral meniscus transplant. While there is some debate regarding the timing of performing such a surgery, any signs and symptoms of lateral compartment overload in the setting of meniscal deficiency may be an indication.

Surgical Technique: Meniscus Repair for Tears in the Normal Vascular Zones

Meniscus tears have been reported to occur in as high as 76% of patients with symptomatic DLM, with 11% of patients having multiple tears. Most tears occur in the posterior horn (41%) or meniscus body (34%), with 25% tears in the anterior horn. As with tear management in the typical meniscus, partial meniscectomy should be performed for tears in the avascular white-white zone or the ‘saucerization zone’ of central tissue. Generally, a repair should be performed when good quality meniscus tissue is present and in the more vascularized peripheral tissue, i.e. the approximate red-white and red-red zones that one would consider typical of a non-discoid meniscus. Meniscus repair strategies may include all-inside repair (most commonly in the posterior horn), inside-out repair using zone-specific cannulas (commonly used in any of the zones), and outside-in repair (usually reserved for tears of the anterior horn). Surgeons can also consider enhancing the biological environment for healing via abrasion of tear surfaces that will be approximated with rasps or shavers and notch microfracture.

Complex and horizontal tears are the most common patterns in pediatric patients with DLM. ^, Horizontal tears may appear as two large surface-based (superior and inferior) white-appearing lamina, separated by more degenerative-appearing, yellowish-colored interposed tissue, or simply frank detachment of the two leaflets. The optimal technique for this common intra-operative presentation has not been well-studied and includes one of three approaches. If there is asymmetry in the height of the two leaflets, such that one leaflet may appear more normal in height, this one can be preserved, while the other leaflet can be resected completely to recreate a near-normal meniscus height. Alternatively, the inferior leaflet can be partially resected to create a wedge-like contour resembling a typical meniscus. However, the degree to which the two unrepaired residual leaflets have any micromotion relative to each other, generate symptoms, or degenerate in the future is unknown. Direct repair of the two leaflets may be performed in cases with near-normal meniscus height and peripheral extension of the tear. All-inside suture-passing devices allow for precisely placed all-inside sutures that can be tied under direct visualization. This may leave low-profile knots above or below the repaired discoid leaflets, but such a ‘sandwich repair’ may be the best way to preserve the function of the meniscus. When a horizontal tear is present with peripheral rim instability, a circumferential stitch, or “hay-bale” repair, may compress the tear leaflets and secure the meniscus to the capsule to restore peripheral stability.

Surgical Technique: Meniscal Repair Stabilization for Peripheral Instability/Hypermobility

Peripheral rim instability has been reported in up to 28-48.5% of DLM and may affect any portion of the meniscus. ^, ^, Again, thoroughly interrogating the entire meniscus, especially the anterior horn, is essential. Peripheral instability should be addressed before saucerization to perform a more accurate saucerization and preservation of meniscal width.

Capsule-based stabilization of the meniscus is similar to other meniscus repair techniques and can involve all-inside, inside-out, or outside-in strategies ( Fig. 2 , Fig. 3 ). Peripherally unstable menisci may also exhibit a deficiency in the meniscotibial ligaments. While meniscotibial attachments can be restored using anchors into the proximal tibial plateau with sutures through the peripheral meniscus, this is not typically performed, as access can be difficult or require an arthrotomy, and there is currently no compelling data that restoring meniscotibial attachments in addition to the capsule improves outcomes.