KNEE MENISCUS ANATOMY
Knee Meniscus Meniscal Anatomy
The menisci are two crescent shaped wedges in the knee joint. The menisci cover two thirds of the plateau at the top of the tibia. Each meniscus is cartilaginous and tough centrally where it is compressed between the tibia and femur. The meniscus is ligamentous and pliable at its thick peripheral attachments to the joint capsule. The meniscus' anterior and posterior tips or horns are connected to the underlying tibia. Knee Meniscal Anatomy.
The outer third of each meniscus has a good blood supply from the joint capsule and synovium, allowing the possibility of healing of tears in this region. The outer quarter of each meniscus has a nerve supply that may contribute to the awareness of joint position.
The menisci move and change shape as the larger rounded femoral condyles glide and roll over the smaller, flatter tibial plateau. The menisci assist joint stability via their wedge shape and mechanical spacer effect. They increase the contact surface area between the tibia and femur by one third, and have a slight meniscal shock-absorbing effect, leading to less force per unit area of the bone’s articular cartilage. The menisci also promote synovial fluid distribution within the knee joint, assisting lubrication and nutrition of the bone’s cartilage surface. Knee Meniscal Anatomy.
The horns of the medial meniscus are attached further apart, it sits in a slight hollow on the tibial surface and its middle section is firmly attached to the medial collateral ligament. This makes the medial meniscus less mobile, and more susceptible to tearing during violent knee twisting. Knee Meniscal Anatomy.
The blood supply of the menisci is dervied from the terminal branches of the superior and inferior medial and lateral geniculate arteries. These vessels supply the connective tissue adjacent to the periphery of the meniscus. The collateral branches perforate the outer 20 to 30% of the meniscus. This leaves an inner 70 to 80% which is the avascular segment. The central meniscus is more vascular than the superior or inferior surfaces. The anterior and posterior horns are enveloped by synovial tissue, and are therefore more highly vascularized than the middle segment.
The meniscus is composed of cells which are surrounded by an extracellular matrix. The basic cell of the meniscus is the fibrochondrocyte. Two distinct types of chondrocytes have been observed, and are identified by their round or oval shape. The meniscal surface chondrocytes are usually oval, while those of the deeper layers tend to be more rounded. Both contain few mitochondria, suggesting that anerobic glycolysis is their main respiratory pathway. The extracellular matrix is composed of collagen, proteoglycans, matrix gycoproteins and elastin. The matrix is composed mainly of circumferentially arranged collagen fibres. These provide tensile strength and, to a lesser extent, radially arranged cross-links which provide shear strength. The radially-orientated fibres are more densely packed in the meniscal surface layer, which is 30 to 120 mm thick. Collagen makes up about 60 to 70% of the dry weight of the meniscus. The collagen fibres are arranged in bundles which are 50 to 150 mcm in diameter.
Proteoglycans are hydrophilic negatively charged micromolecules held together by collagen fibrils. They provide the meniscus with a high capacity to resist large compressive loads. However, they do not contribute significantly to its tensile strength.
The lack of congruence of the articular surfaces of the tibio-femoral joint is corrected by the interposition of the menisci or semi-lunar cartilages. The menisci are triangular in cross-section. The superior surface which is in contact with the femoral condyles, is concave. The peripheral surface which is adherent to the deep surface of the joint capsule, is cylindrical. The inferior surface, which rests on the edges of the medial and lateral tibial condyles, is almost plane. The meniscal rings are incomplete in the regions of the intercondylar tubercles of the tibia so ....