Injuries to the menisci are recognized as a cause of significant musculoskeletal morbidity. The unique and complex structure of menisci makes treatment and repair challenging for the patient, surgeon, and physical therapist. Furthermore, long-term damage may lead to degenerative joint changes such as osteophyte formation, articular cartilage degeneration, joint space narrowing, and symptomatic osteoarthritis. Hominids exhibit similar anatomic and functional characteristics, including a bicondylar distal femur, intra-articular cruciate ligaments, menisci, and asymmetrical collateral.
In the primate lineage leading to humans, hominids evolved to bipedal stance approximately 3 to 4 million years ago, and by 1. Unique in Homo sapiens is the presence of 2 tibial insertions—1 anterior and 1 posterior—indicating a habitual practice of full extension movements of the knee joint during the stance and swing phases of bipedal walking.
The characteristic shape of the lateral and medial menisci is attained between the 8th and 10th week of gestation. Gross examination of the knee menisci reveals a smooth, lubricated tissue Figure 1. They are crescent-shaped wedges of fibrocartilage located on the medial and lateral aspects of the knee joint Figure 2A.
The peripheral, vascular border also known as the red zone of each meniscus is thick, convex, and attached to the joint capsule. The innermost border also known as the white zone tapers to a thin free edge.
The superior surfaces of menisci are concave, enabling effective articulation with their respective convex femoral condyles. The inferior surfaces are flat to accommodate the tibial plateau Figure 1. Gross photograph of human tibial plateau demonstrating the relative size and attachments of the medial and lateral menisci. The medial and lateral menisci left side and right side of image, respectively are connected by a transverse ligament TL.
Reprinted with permission from Messner and Gao. A Anatomy of the meniscus viewed from above adapted image reprinted with permission from Greis et al 63 ; original from Pagnani et al B Axial view of a right tibial plateau showing sections of the meniscus and their relationship to the cruciate ligaments. Reprinted with permission from Johnson et al. The semicircular medial meniscus measures approximately 35 mm in diameter anterior to posterior and is significantly broader posteriorly than it is anteriorly.
There is significant variability in the attachment location of the anterior horn of the medial meniscus. Johnson et al reexamined the tibial insertion sites of the menisci and their topographic relationships to surrounding anatomic landmarks of the knee.
The area of the anterior horn insertion site of the medial meniscus was the largest overall, measuring 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.
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. 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. The cells of the menisci are referred to as fibrochondrocytes because they appear to be a mixture of fibroblasts and chondrocytes. Both cell types contain abundant endoplasmic reticulum and Golgi complex.
Mitochondria are only occasionally visualized, suggesting that the major pathway for energy production of fibrochondrocytes in their avascular milieu is probably anaerobic glycolysis. 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.
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. The collagens are heavily cross-linked by hydroxylpyridinium aldehydes. These fibers blend the ligamentous connections of the meniscal horns to the tibial articular surface Figure 3. There is lipid debris and calcified bodies in the ECM of human menisci.
Schematic diagram demonstrating the collagen fiber ultrastructure and orientation within the meniscus: 1, superficial network; 2, lamellar layer; 3, central main layer. Arrowheads, radial interwoven fibers; arrow, loose connective tissue. Reprinted with permission from Petersen and Tillmann. Elastin forms less than 0. It likely interacts directly with collagen to provide resiliency to the tissue. Extracellular matrix. A Electron micrograph of an aggrecan aggregate shadowed by platinum.
Many free aggrecan molecules are also seen. B Schematic drawing of an aggrecan aggregate shown in part A. Reprinted with permission from Alberts et al. 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. 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. Confluence of geniculate arteries anterior view. Reprinted with permission from Brindle et al. 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 kDa.
The adhesive glycoproteins constitute a subgroup of the matrix glycoproteins. Such intermolecular adhesion molecules are therefore important components in the supramolecular organization of the extracellular molecules of the meniscus. 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.
A premeniscal capillary network arising from the branches of these arteries originates within the synovial and capsular tissues of the knee along the periphery of the menisci. Perimeniscal capillary plexus PCP can be seen penetrating the peripheral border of the medial meniscus. F, femur; T, tibia. Reprinted with permission from Arnoczky and Warren. Bird and Sweet examined the menisci of animals and humans using scanning electron and light microscopy.
These canals may play a role in the transport of fluid within the meniscus and may carry nutrients from the synovial fluid and blood vessels to the avascular sections of the meniscus. The knee joint is innervated by the posterior articular branch of the posterior tibial nerve and the terminal branches of the obturator and femoral nerves.
The lateral portion of the capsule is innervated by the recurrent peroneal branch of the common peroneal nerve. These nerve fibers penetrate the capsule and follow the vascular supply to the peripheral portion of the menisci and the anterior and posterior horns, where most of the nerve fibers are concentrated. The mechanoreceptors within the menisci function as transducers, converting the physical stimulus of tension and compression into a specific electrical nerve impulse.
Studies of human menisci have identified 3 morphologically distinct mechanoreceptors: Ruffini endings, Pacinian corpuscles, and Golgi tendon organs. Type II Pacinian mechanoreceptors are low threshold and fast adapting to tension changes. These neural elements were found in greater concentration in the meniscal horns, particularly the posterior horn.
The asymmetrical components of the knee act in concert as a type of biological transmission that accepts, transfers, and dissipates loads along the femur, tibia, patella, and femur. Several studies have reported that various intra-articular components of the knee are sensate, capable of generating neurosensory signals that reach spinal, cerebellar, and higher central nervous system levels. The biomechanical function of the meniscus is a reflection of the gross and ultrastructural anatomy and of its relationship to the surrounding intra-articular and extra-articular structures.
The menisci serve many important biomechanical functions. In a study on ligamentous function, Brantigan and Voshell reported the medial meniscus to move an average 2 mm, while the lateral meniscus was markedly more mobile with approximately 10 mm of anterior-posterior displacement during flexion.
The anterior and posterior horn lateral meniscus ratio is smaller and indicates that the meniscus moves more as a single unit. Thompson et al found that the area of least meniscal motion is the posterior medial corner, where the meniscus is constrained by its attachment to the tibial plateau by the meniscotibial portion of the posterior oblique ligament, which has been reported to be more prone to injury.
The greater differential between anterior and posterior horn excursion may place the medial meniscus at a greater risk of injury. Diagrams showing the mean movement mm in each meniscus during flexion shaded and extension hashed.
Reproduced with permission from Thomspon et al. The differential of anterior horn to posterior horn motion allows the menisci to assume a decreasing radius with flexion, which correlates to the decreased radius of curvature of the posterior femoral condyles.
The function of the menisci has been clinically inferred by the degenerative changes that accompany its removal. Fairbank described the increased incidence and predictable degenerative changes of the articular surfaces in completely meniscectomized knees. Firm attachments by the anterior and posterior insertional ligaments prevent the meniscus from extruding peripherally during load bearing. Free body diagram of forces acting on the meniscus during loading.
As the femur presses down on the meniscus during normal loading, the meniscus deforms radially but is anchored by its anterior and posterior horns F ant and F post. During loading, tensile, compressive, and shear forces are generated. A tensile hoop stress F cir results from the radial deformation, while vertical and horizontal forces F v and F h result from the femur pressing on the curved superior surface of the tissue.
A radial reaction force F rad balances the femoral horizontal force F h. Reprinted with permission from Athanasiou and Sanchez-Adams. The menisci play a vital role in attenuating the intermittent shock waves generated by impulse loading of the knee with normal gait. The geometric structure of the menisci provides an important role in maintaining joint congruity and stability. The superior surface of each meniscus is concave, enabling effective articulation between the convex femoral condyles and flat tibial plateau.
When the meniscus is intact, axial loading of the knee has a multidirectional stabilizing function, limiting excess motion in all directions. Markolf and colleagues have addressed the effect of meniscectomy on anterior-posterior and rotational knee laxity. Recently, Musahl et al reported that the lateral meniscus plays a role in anterior tibial translation during the pivot-shift maneuver.
The menisci may also play a role in the nutrition and lubrication of the knee joint. The mechanics of this lubrication remains unknown; the menisci may compress synovial fluid into the articular cartilage, which reduces frictional forces during weightbearing.
There is a system of microcanals within the meniscus located close to the blood vessels, which communicates with the synovial cavity; these may provide fluid transport for nutrition and joint lubrication. The perception of joint motion and position proprioception is mediated by mechanoreceptors that transduce mechanical deformation into electric neural signals. Mechanoreceptors have been identified in the anterior and posterior horns of the menisci.
The microanatomy of the meniscus is complex and certainly demonstrates senescent changes. With advancing age, the meniscus becomes stiffer, loses elasticity, and becomes yellow. Shear between these layers may cause pain. The torn meniscus may directly injure the overlying articular cartilage. Ghosh and Taylor found that collagen concentration increased from birth to 30 years and remained constant until 80 years of age, after which a decline occurred.
Peters and Smillie observed an increase in hexosamine and uronic acid with age. McNicol and Roughley studied the variation of meniscal proteoglycans in aging ; small differences in extractability and hydrodynamic size were observed. The proportions of keratin sulfate relative to chondroitinsulfate increased with aging.
Petersen and Tillmann immunohistochemically investigated human menisci ranging from 22 weeks of gestation to 80 years , observing the differentiation of blood vessels and lymphatics in 20 human cadavers. At the time of birth, nearly the entire meniscus was vascularized. In the second year of life, an avascular area developed in the inner circumference.
In the second decade, blood vessels were present in the peripheral third. After 50 years of age, only the peripheral quarter of the meniscal base was vascularized. The dense connective tissue of the insertion was vascularized but not the fibrocartilage of the insertion.
Blood vessels were accompanied by lymphatics in all areas. Arnoczky suggested that body weight and knee joint motion may eliminate blood vessels in the inner and middle aspects of the menisci. A requirement for nutrition via diffusion is the intermittent loading and release on the articular surfaces, stressed by body weight and muscle forces. Magnetic resonance imaging MRI is a noninvasive diagnostic tool used in the evaluation, diagnosis, and monitoring of the menisci.
MRI is widely accepted as the optimal imaging modality because of superior soft tissue contrast. On cross-sectional MRI, the normal meniscus appears as a uniform low-signal dark triangular structure Figure 9. A meniscal tear is identified by the presence of an increased intrameniscal signal that extends to the surface of this structure. A sagittal magnetic resonance proton-density image of a healthy knee depicting the medial menisci arrows. The concave superior meniscal surface improves contact with the femoral epicondyles, and a flat undersurface improves contact with the tibial plateau.
The periphery is thicker than the central portion, allowing for firm attachment to the joint capsule. Several studies have evaluated the clinical utility of MRI for meniscal tears. In general, MRI is highly sensitive and specific for tears of the meniscus.
There have been discrepancies between MRI diagnoses and the pathology identified during arthroscopic examination. Increased signal intensity in the anterior horn does not necessarily indicate a clinically significant lesion. The menisci of the knee joint are crescent-shaped wedges of fibrocartilage that provide increased stability to the femorotibial articulation, distribute axial load, absorb shock, and provide lubrication to the knee joint.
Preservation of the menisci is highly dependent on maintaining its distinctive composition and organization. We wish to thank Tom Cichonski for his assistance in the formatting of this manuscript. References 7 , 25 , 51 , , , , , National Center for Biotechnology Information , U.
Journal List Sports Health v. Sports Health. Alice J. Scott A. Author information Copyright and License information Disclaimer. This article has been cited by other articles in PMC. Abstract Context: Information regarding the structure, composition, and function of the knee menisci has been scattered across multiple sources and fields. Results: This study highlights the structural, compositional, and functional characteristics of the menisci, which may be relevant to clinical presentations, diagnosis, and surgical repairs.
Conclusions: An understanding of the normal anatomy and biomechanics of the menisci is a necessary prerequisite to understanding the pathogenesis of disorders involving the knee. Keywords: knee, meniscus, anatomy, function. Meniscal Phylogeny and Comparative Anatomy Hominids exhibit similar anatomic and functional characteristics, including a bicondylar distal femur, intra-articular cruciate ligaments, menisci, and asymmetrical collateral.
Embryology and Development The characteristic shape of the lateral and medial menisci is attained between the 8th and 10th week of gestation. Gross Anatomy Gross examination of the knee menisci reveals a smooth, lubricated tissue Figure 1.
Open in a separate window. Figure 1. Figure 2. Medial meniscus The semicircular medial meniscus measures approximately 35 mm in diameter anterior to posterior and is significantly broader posteriorly than it is anteriorly. Meniscofemoral ligaments The literature reports significant inconsistencies in the presence and size of meniscofemoral ligaments of the lateral meniscus. Figure 3. Figure 4. Figure 5. Matrix Glycoproteins Meniscal cartilage contains a range of matrix glycoproteins, the identities and functions of which have yet to be determined.
Vascular Anatomy The meniscus is a relatively avascular structure with a limited peripheral blood supply. Figure 6. Neuroanatomy The knee joint is innervated by the posterior articular branch of the posterior tibial nerve and the terminal branches of the obturator and femoral nerves.
Biomechanical Function The biomechanical function of the meniscus is a reflection of the gross and ultrastructural anatomy and of its relationship to the surrounding intra-articular and extra-articular structures. Figure 7. Load Transmission The function of the menisci has been clinically inferred by the degenerative changes that accompany its removal. Figure 8. Shock Absorption The menisci play a vital role in attenuating the intermittent shock waves generated by impulse loading of the knee with normal gait.
Joint Stability The geometric structure of the menisci provides an important role in maintaining joint congruity and stability. Joint Nutrition and Lubrication The menisci may also play a role in the nutrition and lubrication of the knee joint. Proprioception The perception of joint motion and position proprioception is mediated by mechanoreceptors that transduce mechanical deformation into electric neural signals. Maturation and Aging of The Meniscus The microanatomy of the meniscus is complex and certainly demonstrates senescent changes.
Magnetic Resonance Imaging of The Meniscus Magnetic resonance imaging MRI is a noninvasive diagnostic tool used in the evaluation, diagnosis, and monitoring of the menisci. Figure 9. Conclusions The menisci of the knee joint are crescent-shaped wedges of fibrocartilage that provide increased stability to the femorotibial articulation, distribute axial load, absorb shock, and provide lubrication to the knee joint.
Acknowledgments We wish to thank Tom Cichonski for his assistance in the formatting of this manuscript. References 1. The extracellular matrix of the meniscus. Knee Meniscus: Basic and Clinical Foundations. Isolation and characterization of high-buoyant-density proteoglycans from semilunar menisci. J Bone Joint Surg Am.
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Orthop Trans. Collagen of fibrocartilage: a distinctive molecular phenotype in bovine meniscus. FEBS Lett. Vessels supplying the body are limited to the peripheral one-third, except in the fetus. There is an avascular area adjacent to the popliteus tendon. The perimeniscal tissue is richly innervated. Most nerves are associated with vessels. Smaller nerves and axons run radially in convoluted patterns. Single axons course through the perimeniscal tissue, and many nerves are seen in the interstitial tissue of the outer one-third of the meniscus and in the anterior and posterior horns.
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