Meniscus: Blood Supply, Healing Potential, and Clinical Implications

What is the blood supply of the meniscus?

The meniscus, a crucial fibrocartilaginous structure in the knee, possesses a unique and limited blood supply primarily concentrated in its outer third, a characteristic that profoundly impacts its capacity for healing after injury.

Understanding the Meniscus

The menisci (plural for meniscus) are two C-shaped wedges of fibrocartilage located between the femur (thigh bone) and the tibia (shin bone) in each knee joint. There are two menisci in each knee: the medial (inner) meniscus and the lateral (outer) meniscus. These vital structures serve several critical functions:

• Shock Absorption: They act as cushions, absorbing impact forces across the knee joint.
• Load Distribution: They distribute compressive forces evenly across the articular cartilage, preventing excessive stress on specific areas.
• Joint Stability: They deepen the articular surface of the tibia, enhancing the stability of the knee joint.
• Lubrication and Nutrition: They assist in the spread of synovial fluid, which lubricates the joint and provides nutrition to the articular cartilage.

The Unique Vascularization of the Meniscus

The blood supply to the meniscus is not uniform, which is a key factor in its healing potential. It is typically divided into three distinct zones based on vascularity:

• The Peripheral (Red-Red) Zone:

  • This is the outermost 10-30% of the meniscus.
  • It receives a direct and relatively rich blood supply from the surrounding synovial membrane and joint capsule.
  • This zone is often referred to as the "red-red" zone due to its vascularity, indicating a higher potential for healing if torn.
  • Capillary loops extend from the perimeniscal plexus into this region.

• The Middle (Red-White) Zone:

  • This is the transitional zone located between the vascularized periphery and the avascular center.
  • It has a limited and diminishing blood supply, with some capillaries extending inwards from the red zone.
  • Healing potential in this zone is moderate but less predictable than in the red-red zone.

• The Central (White-White) Zone:

  • This is the innermost 60-75% of the meniscus.
  • It is largely avascular, meaning it has no direct blood supply.
  • Nutrition to this zone primarily occurs through diffusion of synovial fluid and mechanical loading (compression and decompression during movement), which helps to pump nutrients into the tissue.
  • This zone is often referred to as the "white-white" zone because of its lack of blood supply, signifying a very poor or non-existent healing capacity.

Arterial Supply to the Meniscus

The primary blood supply to the perimeniscal capillary plexus, which then feeds the outer meniscal zones, originates from branches of the popliteal artery. Specifically, these include:

• The superior and inferior medial genicular arteries.
• The superior and inferior lateral genicular arteries.

These arteries form an extensive network around the menisci, with small branches penetrating the meniscal tissue from the periphery.

Clinical Implications of Meniscal Blood Supply

The unique vascularization pattern of the meniscus has profound clinical implications, particularly concerning injury and repair:

• Healing Potential: Tears that occur in the vascularized "red-red" zone have a significantly better chance of healing, either spontaneously or with surgical repair, because of the presence of blood flow that can deliver healing cells and nutrients. In contrast, tears in the avascular "white-white" zone typically do not heal and often require surgical removal of the torn portion (meniscectomy) if symptomatic.
• Age-Related Changes: With increasing age, the vascularity of the meniscus tends to decrease, further reducing the healing potential even in the outer regions. This contributes to the higher incidence of degenerative meniscal tears in older populations.
• Nutrient Diffusion: For the avascular central region, joint movement is crucial. The cyclical compression and decompression of the meniscus during activity help to pump synovial fluid into the tissue, facilitating nutrient delivery and waste removal. Prolonged immobilization can hinder this process.

Factors Influencing Meniscal Healing

Beyond the inherent blood supply, several other factors can influence the success of meniscal healing:

• Location of the Tear: As discussed, tears in the peripheral, vascularized zone heal better.
• Type of Tear: Certain tear patterns (e.g., longitudinal tears) may have better healing potential than others (e.g., radial or complex tears).
• Patient Age: Younger patients generally have better healing outcomes due to more robust vascularity and biological healing responses.
• Associated Injuries: The presence of other knee injuries, such as an anterior cruciate ligament (ACL) tear, can sometimes influence meniscal healing, as an ACL reconstruction can enhance the biological environment of the joint.
• Surgical Technique: For tears amenable to repair, modern surgical techniques aim to preserve meniscal tissue and optimize healing conditions.

Conclusion

The blood supply of the meniscus is a critical anatomical feature that dictates its ability to heal after injury. With its limited vascularity primarily confined to the outer red zone, understanding this unique characteristic is paramount for diagnosing, treating, and predicting outcomes for meniscal tears. This knowledge underscores why tears in the outer portion of the meniscus have a much better prognosis for repair compared to those in the avascular inner regions, influencing surgical decisions and rehabilitation strategies for optimal knee health.