to the peripheral border. These fibers blend the ligamentous connections of the meniscal horns to the tibial articular surface (Figure 3).10,27,49,156 In the most superficial region of the menisci, the type I fibers are oriented in a more radial direction. Radially oriented tie fibers are also present in the deep zone and are interspersed or woven between the circumferential fibers to provide structural integrity (Figure 3).# There is lipid debris and calcified bodies in the ECM of human menisci.54 The calcified bodies contain long, slender crystals of phosphorous, calcium, and magnesium on electron-probe roentgenographic analysis.54 The function of these crystals in not completely understood, but it is believed that they may play a role in acute joint inflammation and destructive arthropathies. Noncollagenous matrix proteins, such as fibronectin, contribute 8% to 13% of the organic dry weight. Fibronectin is involved in many cellular processes, including tissue repair, embryogenesis, blood clotting, and cell migration/adhesion. Elastin forms less than 0.6% of the meniscus dry weight; its ultrastructural localization is not clear. It likely interacts directly with collagen to provide resiliency to the tissue.**
Proteoglycans Located within a fine meshwork of collagen fibrils, proteoglycans are large, negatively charged hydrophilic molecules, contributing 1% to 2% of dry weight.58 They are formed by a core protein with 1 or more covalently attached glycosaminoglycan chains (Figure 4).122 The size of these molecules is further increased by specific interaction with
hyaluronic acid.67,72 The amount of proteoglycans in the meniscus is one-eighth that of articular cartilage,2,3 and there may be considerable variation depending on the site of the sample and the age of the patient.49 By virtue of their specialized structure, high fixed-charge density, and charge-charge repulsion forces, proteoglycans in the ECM are responsible for hydration and provide the tissue with a high capacity to resist compressive loads. The glycosaminoglycan profile of the normal adult human meniscus consists of chondroitin-6-sulfate (40%), chondroitin-4-sulfate (10% to 20%), dermatan sulfate (20% to 30%), and keratin sulfate (15%; Figure 4).65,77,99,159 The highest glycosaminoglycan concentrations are found in the meniscal horns and the inner half of the menisci in the primary weightbearing areas.58,77 Aggrecan is the major proteoglycan found in the human menisci and is largely responsible for their viscoelastic compressive properties (Figure 5). Smaller proteoglycans, such as decorin, biglycan, and fibromodulin, are found in smaller amounts.124,151 Hexosamine contributes 1% to the dry weight of ECM.57,74 The precise functions of each of these small proteoglycans on the meniscus have yet to be fully elucidated.
Matrix Glycoproteins Meniscal cartilage contains a range of matrix glycoproteins, the identities and functions of which have yet to be determined. Electrophoresis and subsequent staining of the polyacrylamide gels reveals bands with molecular weights varying from a few kilodaltons to more than 200 kDa.112 These matrix molecules include the link proteins that stabilize proteoglycan hyaluronic acid aggregates and a 116-kDa protein of unknown function.46 This protein resides in the matrix in the form of disulfide-bonded complex of high molecular weight.46 Immunolocalization studies suggest that it is predominantly located around the collagen bundles in the interterritorial matrix.47 The adhesive glycoproteins constitute a subgroup of the matrix glycoproteins. These macromolecules are partly responsible for binding with other matrix molecules and/or cells. Such intermolecular adhesion molecules are therefore important components in the supramolecular organization of the extracellular molecules of the meniscus.150 Three molecules have been identified within the meniscus: type VI collagen, fibronectin, and thrombospondin.112,118,181
VAscUlAr AnAtomy The meniscus is a relatively avascular structure with a limited peripheral blood supply. The medial, lateral, and middle geniculate arteries (which branch off the popliteal artery) provide the major vascularization to the inferior and superior aspects of each meniscus (Figure 5).9,12,33-35,148 The middle geniculate artery is a small posterior branch that perforates the oblique popliteal ligament at the posteromedial corner of
Figure
