Meniscus Tears

We have two menisci in the knee: one meniscus that sits on the inside of the knee called the medial meniscus and one that sits on the outside part of the knee called the lateral meniscus. The medial meniscus has a C-shape and is firmly attached to the joint capsular lining as well as to the deep portion of the medial collateral ligament (MCL). The lateral meniscus, on the other hand, is more O-shaped and is more mobile and not as firmly attached as the medial meniscus. This is because the lateral meniscus does not have any attachments to the lateral collateral ligament and actually has a space for the popliteus tendon to enter the joint (popliteal hiatus). Thus overall, the lateral meniscus is more mobile and not as firmly attached as the medial meniscus is.

When looking down on the overall structure of the meniscus, each menisci have anterior and posterior horns, a central body, and root attachments onto the tibial plateau surface. These roots are essential to stabilize the meniscus and are essential for their shock absorption function. Furthermore, both menisci are attached to the tibia with meniscofemoral and meniscotibial ligaments that help to stabilize the menisci during motion. The menisci are composed of type 1 collagen, other extracellular matrix proteins, and chondrocyte cells (meniscus image inserted here).

The menisci have two types of fibers or cables that form it:

1. Circumferential fibers: These fibers run the entire length of the meniscus and help to resist hoop stresses as we bear weight. This helps disperse the load from weight bearing, standing, or squatting across the meniscus and into the bone. Think of these fibers as a kind of electrical grounding pad for the knee distributing the electricity of weightbearing out safely through the circumferential fibers into the root attachments
2. Radial fibers: The radial fibers in the meniscus prevent longitudinal splitting of the meniscus. Think about these fibers as spokes of a wheel which prevent collapse of the meniscus on itself during weightbearing.

The vascularity of the meniscus is also crucial to think about. We divide the menisci into thirds with an outer third, middle third, and inner thirds:

1. The outer third is deemed a “red-red” zone with excellent vascularity as this portion of the meniscus has blood supply coming in from small vessels that also supply the joint capsule.
2. The middle third is thought of as a “red-white” zone and has less vascularity than the outer third
3. The inner third is termed the “white-white” zone which is predominantly without its own true blood supply as it receiving its nutrition from the synovial fluid that bathes it

Understanding the vascularity of the meniscus is important because in counseling patients, not all meniscus tears are equal. Tears that occur in the outer third of the meniscus are much more likely to heal with a repair than tears that involve the inner third. Additionally, the menisci in terms of pain sensation are primarily innervated in the outer third in that red red zone and this is clinically relevant as these can be a source of pain when torn in this outer zone.