horn is attached to the posterior intercondylar fossa of the tibia between the lateral meniscus and the posterior cruciate ligament (PCL; Figures 1 and 2B). Johnson et al reexamined the tibial insertion sites of the menisci and their topographic relationships to surrounding anatomic landmarks of the knee.82 They found that the anterior and posterior horn insertion sites of the medial meniscus were larger than those of the lateral meniscus. The area of the anterior horn insertion site of the medial meniscus was the largest overall, measuring 61.4 mm2, whereas the posterior horn of the lateral meniscus was the smallest, at 28.5 mm2.82 The tibial portion of the capsular attachment is the coronary ligament. At its midpoint, the medial meniscus is more firmly attached to the femur through a condensation in the joint capsule known as the deep medial collateral ligament.175 The transverse, or intermeniscal, ligament is a fibrous band of tissue that connects the anterior horn of the medial meniscus to the anterior horn of the lateral meniscus (Figures 1 and 2A).
Lateral meniscus. The lateral meniscus is almost circular, with an approximately uniform width from anterior to posterior (Figures 1 and 2A). It occupies a larger portion (~80%) of the articular surface than the medial meniscus (~60%) and is more mobile.10,31,165 Both horns of the lateral meniscus are attached to the tibia. The insertion of the anterior horn of the lateral meniscus lies anterior to the intercondylar eminence and adjacent to the broad attachment site of the ACL (Figure 2B).9,83 The posterior horn of the lateral meniscus inserts posterior to the lateral tibial spine and just anterior to the insertion of the posterior horn of the medial meniscus (Figure 2B).83 The lateral meniscus is loosely attached to the capsular ligament; however, these fibers do not attach to the lateral collateral ligament. The posterior horn of the lateral meniscus attaches to the inner aspect of the medial femoral condyle via the anterior and posterior meniscofemoral ligaments of Humphrey and Wrisberg, respectively, which originate near the origin of the PCL (Figures 1 and 2).75
Meniscofemoral ligaments. The literature reports significant inconsistencies in the presence and size of meniscofemoral ligaments of the lateral meniscus. There may be none, 1, 2, or 4.|| When present, these accessory ligaments transverse from the posterior horn of the lateral meniscus to the lateral aspect of the medial femoral condyle. They insert immediately adjacent to the femoral attachment of the PCL (Figures 1 and 2). In a series of studies, Harner et al measured the crosssectional area of the ligaments and found that the meniscofemoral ligament averaged 20% of the size of the PCL (range, 7%-35%).69,70 However, the size of the insertional area alone without knowledge of the insertional angle or collagen density does not indicate their relative strength.115 The function of these ligaments remains unknown; they may pull the posterior horn of the lateral meniscus in an anterior direction
to increase the congruity of the meniscotibial fossa and the lateral femoral condyle.75
UltrAstrUctUre And Biochemistry Extracellular Matrix The meniscus is a dense extracellular matrix (ECM) composed primarily of water (72%) and collagen (22%), interposed with cells.9,55,56,77 Proteoglycans, noncollagenous proteins, and glycoproteins account for the remaining dry weight. Meniscal cells synthesize and maintain the ECM, which determines the material properties of the tissue. The cells of the menisci are referred to as fibrochondrocytes because they appear to be a mixture of fibroblasts and chondrocytes.111,177 The cells in the more superficial layer of the menisci are fusiform or spindle shaped (more fibroblastic), whereas the cells located deeper in the meniscus are ovoid or polygonal (more chondrocytic).55,56,178 Cell morphology does not differ between the peripheral and central locations in the menisci.56 Both cell types contain abundant endoplasmic reticulum and Golgi complex. Mitochondria are loccasionally visualized, suggesting that the major pathway for energy production of fibrochondrocytes in their avascular milieu is probably anaerobic glycolysis.112
Water In normal, healthy menisci, tissue fluid represents 65% to 70% of the total weight. Most of the water is retained within the tissue in the solvent domains of proteoglycans. The water content of meniscal tissue is higher in the posterior areas than in the central or anterior areas; tissue samples from surface and deeper layers had similar contents.135 Large hydraulic pressures are required to overcome the drag of frictional resistance of forcing fluid flow through meniscal tissue. Thus, interactions between water and the matrix macromolecular framework significantly influence the viscoelastic properties of the tissue.
Collagens Collagens are primarily responsible for the tensile strength of menisci; they contribute up to 75% of the dry weight of the ECM.77 The ECM is composed primarily of type I collagen (90% dry weight) with variable amounts of types II, III, V, and VI.43,44,80,112,181 The predominance of type I collagen distinguishes the fibrocartilage of menisci from articular (hyaline) cartilage. The collagens are heavily cross-linked by hydroxylpyridinium aldehydes.44 The collagen fiber arrangement is ideal for transferring a vertical compressive load into circumferential hoop stresses (Figure 3).57 Type I collagen fibers are oriented circumferentially in the deeper layers of the meniscus, par
