Clean Run - February 2012 - 28

ends of the joint. While both types of cartilage effectively disperse load, fibrocartilage is very resistant to the forces that try to pull it apart whereas hyaline cartilage is not. The medial meniscus is C-shaped and the lateral meniscus is more O-shaped when looking from above (Figure 3). Both are triangular in cross-section (Figure 2 and Figure 5). The tibial surface of the meniscus is flat and the femoral surface is concave to match the femoral condyle’s curved shape. The surface of the meniscus is glistening white and made of crosshatched collagen fibers. This arrangement
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of collagen makes the surface extremely resistant to shear forces. In contrast, much of the substance of the meniscus is formed by bundles of concentric collagen fibers (Figure 4). This array of fibers enables the meniscus to resist the tension forces that regularly try to tear it apart. Radially oriented fibers (“tie fibers”) also support the meniscus architecture and resist the forces that try to split the meniscus longitudinally (that is, along the length of the meniscus) (Figure 5). The ends (horns) of each meniscus are firmly anchored to the bony plateau and help create a construct that resists the significant tension forces experienced by each fibrocartilage crescent. At the outside edge (peripheral margin) of each meniscus is a collection (plexus) of small vessels (Figure 2). This perimeniscal capillary plexus provides the blood supply to the outer third of each meniscus and is substantial enough to allow tears in that area to heal if the tear is stable or operatively stabilized.

Anterior Cruciate Ligament

Medial Meniscus

Mechanics
Lateral Meniscus Posterior Cruciate Ligament

The tibial plateau from above showing the C-shaped medial meniscus and O-shaped lateral meniscus. 4

When considering the function of the meniscus it is convenient to represent it as a two-phase structure: one phase is fluid and one phase is a solid fiber matrix. Picture a wet sponge with elastic bands running through it. Fluid moving through the solid phase (squeezing the sponge) allows it to act as a very efficient shock absorber. As such, it can effectively disperse compressive forces that cross the knee. The collagen fiber architecture of the meniscus (the elastic band) makes it very resistant to the forces that are applied to it. One of the main functions of the meniscus is to transmit and disperse load. By increasing the contact area, the menisci decrease the load per unit area and thereby decrease the forces seen by the condyles of the femur and the plateau of the tibia. We have already reviewed how the sponge-like nature of the menisci makes them very efficient shock absorbers. The shock-absorbing capacity of the joint has been shown to significantly decrease by removal of the meniscus. For that reason alone, significant effort is made to preserve as much meniscus tissue as possible. The shape and position of the menisci allow those structures to contribute to the stability of the knee. When the cruciate
Clean Run | February 12

Photomicrograph of concentric, longitudinal collagen fibers of the meniscus. Special thanks to Steven P. Arnoczky, DVM, Dipl. ACVS and Professor of Veterinary Surgery, College of Veterinary Medicine, Michigan State University.

Radial “Tie” Fiber
Cross-section of the meniscus illustrating the radial “tie” fibers. Special thanks to Steven P. Arnoczky, DVM, Dipl. ACVS and Professor of Veterinary Surgery, College of Veterinary Medicine, Michigan State University. 28

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Clean Run - February 2012

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