Congenital Meniscal Abnormalities: Agenesis, Hypoplasia, Discoid Meniscus, Symptoms, and Management

Can you be born without a meniscus?

While exceptionally rare, it is indeed possible for an individual to be born without a meniscus, a condition known as meniscal agenesis or aplasia. More commonly, congenital abnormalities involve an abnormally shaped or underdeveloped meniscus, such as a discoid meniscus.

Understanding the Meniscus

The knee joint, a marvel of biomechanical engineering, relies on several critical structures for its stability, mobility, and shock absorption. Among these, the menisci play a paramount role. There are two menisci in each knee: the medial meniscus (on the inner side of the knee) and the lateral meniscus (on the outer side). Both are C-shaped (or semi-lunar) wedges of fibrocartilage.

Their primary functions include:

• Shock Absorption: Distributing compressive forces across the joint, reducing peak stress on the articular cartilage.
• Load Transmission: Spreading the load over a larger surface area of the tibia, protecting the articular cartilage.
• Joint Stability: Deepening the tibial plateau, which helps to stabilize the femur on the tibia.
• Lubrication and Nutrition: Assisting in the distribution of synovial fluid.
• Proprioception: Contributing to the body's sense of position and movement.

Without properly functioning menisci, the knee joint is significantly more vulnerable to stress, wear, and tear.

Congenital Meniscal Abnormalities

While meniscal injury due to trauma is common, some individuals are born with meniscal anomalies.

• Meniscal Agenesis (Aplasia): This is the condition of being born entirely without a meniscus. It is an extremely rare congenital anomaly, resulting from a failure of the meniscus to develop during fetal embryogenesis. Due to its rarity, much of the understanding comes from case reports. Individuals with meniscal agenesis often face a higher risk of early-onset osteoarthritis and chronic knee pain due due to the absence of the crucial shock-absorbing and load-distributing functions of the meniscus.
• Meniscal Hypoplasia: Less severe than agenesis, hypoplasia refers to an underdeveloped or abnormally small meniscus. While present, its reduced size means it cannot perform its biomechanical functions as effectively, leading to similar, albeit potentially less severe, long-term issues as agenesis.
• Discoid Meniscus: This is the most common congenital anomaly of the meniscus, particularly affecting the lateral meniscus. Instead of the typical C-shape, a discoid meniscus is abnormally thick, oval, or disc-shaped, covering a larger portion of the tibial plateau.
  • Types of Discoid Meniscus: It can be classified into complete, incomplete, and Wrisberg ligament types, based on its shape and attachment.
  • Clinical Presentation: Many individuals with a discoid meniscus remain asymptomatic throughout their lives. However, due to its abnormal shape and often less stable attachments, a discoid meniscus is more prone to tearing, snapping (a characteristic "clunk" or "pop" when the knee is extended), pain, and effusion (swelling) compared to a normal meniscus.

Causes of Congenital Meniscal Anomalies

The exact causes of meniscal agenesis, hypoplasia, and discoid meniscus are not always clear. They are believed to stem from errors in knee joint development during the embryonic stage. Genetic factors may play a role in some cases, but often these conditions appear sporadically without a clear familial pattern.

Symptoms and Diagnosis

The symptoms associated with congenital meniscal anomalies vary widely depending on the specific condition and its severity.

• Agenesis/Hypoplasia: May be asymptomatic in early life, with symptoms developing as chronic stress leads to articular cartilage damage. Symptoms can include chronic knee pain, instability, swelling, and a feeling of "giving way."
• Discoid Meniscus: Commonly presents in childhood or adolescence with a characteristic "snapping knee" sensation, especially during knee extension. Pain, swelling, and locking or catching of the knee are also common, particularly after an injury or increased activity.

Diagnosis typically involves:

• Thorough Clinical Examination: Assessing range of motion, stability, and palpating for tenderness.
• Imaging Studies: Magnetic Resonance Imaging (MRI) is the gold standard for visualizing the menisci and accurately diagnosing congenital anomalies, including agenesis, hypoplasia, and the specific type of discoid meniscus. X-rays may show secondary signs of early arthritis but cannot visualize the meniscus itself.

Management and Implications

The management of congenital meniscal anomalies depends on the specific condition, the presence and severity of symptoms, and the patient's age and activity level.

• Meniscal Agenesis/Hypoplasia:
  • Conservative Management: If asymptomatic, treatment focuses on joint preservation through activity modification, strengthening exercises (especially quadriceps and hamstrings), and maintaining a healthy weight.
  • Surgical Options: In symptomatic cases, particularly with significant pain and early degenerative changes, meniscal transplantation may be considered. This is a complex procedure with specific criteria, involving transplanting a cadaveric meniscus. The goal is to restore some of the knee's natural biomechanics and potentially delay the onset or progression of osteoarthritis.
• Discoid Meniscus:
  • Conservative Management: Asymptomatic discoid menisci do not require treatment.
  • Surgical Intervention: If symptomatic (pain, snapping, locking, tearing), arthroscopic surgery is often recommended.
    • Saucerization: Reshaping the discoid meniscus into a more normal C-shape, while preserving as much functional tissue as possible.
    • Partial Meniscectomy: Removing the unstable or torn portion of the meniscus if saucerization is not feasible or sufficient.
    • Total Meniscectomy: Rarely performed due to the high risk of early osteoarthritis, but may be necessary in cases of severe, irreparable tears or instability.

Long-term implications for all congenital meniscal anomalies often involve an increased risk of early-onset osteoarthritis due to altered biomechanics and increased stress on the articular cartilage over time. Therefore, careful monitoring and proactive management are crucial for preserving knee health.

Conclusion

While being born without a meniscus (agenesis) is an extremely rare occurrence, it highlights the critical role these cartilaginous structures play in knee joint health. More commonly, individuals may have an abnormally shaped discoid meniscus. Understanding these congenital anomalies is vital for proper diagnosis and management, aiming to mitigate symptoms, preserve joint function, and delay the onset of degenerative changes. For individuals with such conditions, a tailored approach involving conservative strategies and, when necessary, precise surgical intervention, is key to maintaining long-term knee health and function.