Introduction:
The term menisci comes from the Greek word meniskos which means crescent. As the name implies, the menisci are crescent-shaped structures forming a concavity in the knee. They act in harmony with knee movements. They are fibrocartilaginous in nature, accommodate the femoral condyle, and act as shock-absorbers (rubbery cushioning effect), avoiding friction and dispersing the compressive stress and load between the femur (thigh bone) and the tibia (shin bone). The menisci are two in number,
- Medial Meniscus: Present on the inner surface of the knee
- Lateral Meniscus: Present on the outer surface of the knee.
What Is a Discoid Meniscus?
The normal anatomical structure of the discoid is altered in some people, making them thicker and disc-shaped. It may not be symptomatic in all, but in a few cases, this anatomical variation can cause pain and a clicking sensation inside the knee. The condition is also commonly referred to as popping knee syndrome. The presentation is often noticed in the young population during their growing phase.
What Causes Discoid Meniscus?
Discoid meniscus is considered to be congenital in origin. The discoid meniscus is more vulnerable to irritation and injury. The lateral meniscus is more commonly involved than the medial one. But almost 25 % of cases will have bilateral involvement. Microscopically, the discoid meniscus has many deranged collagen fibers when compared to normal meniscus.
What Are the Types of Discoid Meniscus?
Studies have described three types of discoid meniscus. It is known as the Watanabe classification. They are,
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Type I - Complete: The meniscus is typically disc-shaped and covers the tibia completely. Meniscotibial attachments are fine.
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Type II - Incomplete: The meniscus thickness is slightly greater than normal and does not envelope the tibia completely - less than 80 % is only covered. This variation also presents with normal meniscotibial attachments.
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Type III - Incomplete Wrisberg Variant: The shape of the meniscus is almost normal. There is a devoid ligament that attaches the meniscus to the femur and the tibia. Only the Wrisberg ligament is present at the back for stability. This type is more severe than the others because the meniscus is more prone to get stuck in the knee joint and tearing.
How Does Discoid Meniscus Develop?
The change in the shape of the meniscus may be congenital, and there is an organizational defect in the collagen fibrils. The micro defect in the collagen, vascular malformation, and increased tissue thickness predispose the meniscus to injury. The integrity of the tissue is lost, resulting in mucoid degeneration and meniscal tears. A normal meniscus tear occurs in a vertical pattern, but the tear in the discoid meniscus happens in the horizontal direction.
What Are the Symptoms of Discoid Meniscus?
Patients with discoid meniscus often complaints of:
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Anterior or lateral knee pain.
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A clicking or popping sound is heard while bending the knees. Because of this feature, the discoid meniscus is also referred to as the snapping knee syndrome.
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Knee joint instability.
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Reduced range of motion.
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Accumulation of fluid in the joints (effusion).
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Catching or locking sensation, especially during knee movements.
How to Diagnose Discoid Meniscus?
The discoid meniscus is evaluated from the patient’s description of pain and other mechanical symptoms. The type and severity of the condition can be investigated through imaging tools.
1) X-ray:
Plain X-rays can show distinct features in a well-established discoid meniscus. Knee joint space widening and changes in the femur condyle and tibial plateau are the few findings noted in normal radiographs.
2) Magnetic Resonance Imaging (MRI):
MRI scan is standard and effective for diagnosing discoid meniscus since MRI images will accurately display increased meniscus thickness on sagittal layers (side plane) and the meniscus coverage on the coronal view (front plane). Meniscus layer tears can be seen in advanced radiographic techniques.
3) Bow-tie sign:
The sagittal images of the MRI show meniscus continuity between the bones of the knee joint, which looks like a bow tie. Such bow-tie appearance is indicative of discoid meniscus.
How to Manage Discoid Meniscus?
The asymptomatic discoid meniscus can be left as such unless it causes any trouble. But if there is pain and restricted movements associated with the discoid meniscus, the definitive treatment would be surgery.
Knee Arthroscopy -
Arthroscopy is a minimally invasive procedure that uses a tiny camera and fine instruments that are inserted through holes in the skin. The procedure is guided by seeing on the monitor. The following are the ways in which discoid meniscus can be managed surgically.
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Saucerization: The process involves reshaping the meniscus to attain its anatomical crescent form to improve its functions. This is the most preferred procedure since the meniscus stability can be maintained.
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Partial Meniscectomy: If the meniscus between the bones is torn, then the fragment is removed or trimmed away.
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Meniscal Fixation: Meniscus fixation refers to the procedure of attaching the meniscus to the joint lining. This may be needed in the case of type III - incomplete Wrisberg variant of the discoid meniscus, where they present with instability and hypermobility.
The associated issues with the related structures are also restored during the knee arthroscopy. The surgery is followed by an extensive program of rehabilitation with physical therapy to strengthen the muscles surrounding the knee joint and regain the complete range of motion.
What Are the Differential Diagnosis and Complications of Discoid Meniscus?
The symptoms of discoid meniscus like pain, stiffness, locking and clicking sensations in the knee joint coincides with other medical conditions like knee joint ligament tears, inflammation, injury or fracture.
The treatment of discoid meniscus with partial meniscectomy has an excellent outcome, with studies showing most patients remaining asymptomatic on a five-year follow-up. However, a possible complication after this surgery is osteochondritis dissecans (affecting bones and cartilage in the joint due to lack of blood supply).
Conclusion:
Discoid meniscus is technically a structural variant of naturally occurring in meniscus (a shock absorber of the knee joint). An interprofessional team of doctors will evaluate and treat the discoid meniscus. The treatment for discoid meniscus is indicated only in the presence of symptoms. Painful discoid meniscus will definitely need surgical intervention.
