Alice J. S. Fox, MSc,* Asheesh Bedi, MD, and Scott A. Rodeo, MD
Context: Information regarding the structure, composition, and function of the knee menisci has been scattered across multiple sources and fields. This review contains a concise, detailed description of the knee menisciincluding anatomy, etymology, phylogeny, ultrastructure and biochemistry, vascular anatomy and neuroanatomy, biomechanical function, maturation and aging, and imaging modalities. Evidence Acquisition: A literature search was performed by a review of PubMed and OVID articles published from 1858 to 2011. 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
Once described as a functionless embryonic remnant,162 the menisci are now known to be vital for the normal function and long-term health of the knee joint. The menisci increase stability for femorotibial articulation, distribute axial load, absorb shock, and provide lubrication and nutrition to the knee joint.4,91,152,153 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, longterm damage may lead to degenerative joint changes such as osteophyte formation, articular cartilage degeneration, joint space narrowing, and symptomatic osteoarthritis.36,45,92 Preservation of the menisci depends on maintaining their distinctive composition and organization.
AnAtomy of menisci Meniscal Etymology The word meniscus comes from the Greek word me -niskos, meaning crescent, diminutive of me -ne -, meaning moon.
Meniscal Phylogeny and Comparative Anatomy Hominids exhibit similar anatomic and functional characteristics, including a bicondylar distal femur, intraarticular cruciate ligaments, menisci, and asymmetrical collateral.40,66 These similar morphologic characteristics reflect a shared genetic lineage that can be traced back more than 300 million years.40,66,119 In the primate lineage leading to humans, hominids evolved to bipedal stance approximately 3 to 4 million years ago, and by 1.3 million years ago, the modern patellofemoral joint was established (with a longer lateral patellar facet and matching lateral femoral trochlea).164 Tardieu investigated the transition from occasional bipedalism to permanent bipedalism and observed that primates contain a medial and lateral fibrocartilaginous meniscus, with the medial meniscus being morphologically similar in all primates (crescent shaped with 2 tibial insertions).163 By contrast, the lateral meniscus was observed to be more variable in shape. Unique in Homo sapiens is the presence of 2 tibial insertions1 anterior and 1 posteriorindicating a habitual practice of full extension movements of the knee joint during the stance and swing phases of bipedal walking.20,134,142,163,168
Embryology and Development The characteristic shape of the lateral and medial menisci is attained between the 8th and 10th week of gestation.53,60 They arise from a condensation of the intermediate layer of mesenchymal tissue to form attachments to the surrounding joint capsule.31,87,110 The developing menisci are highly cellular and vascular, with the blood supply entering from the periphery and extending through the entire width of the menisci.31 As the fetus continues to develop, there is a gradual decrease in the cellularity of the menisci with a concomitant increase in the collagen content in a circumferential arrangement.30,31 Joint motion and the postnatal stress of weightbearing are important factors in determining the orientation of collagen fibers. By adulthood, only the peripheral 10% to 30% have a blood supply.12,31 Despite these histologic changes, the proportion of tibial plateau covered by the corresponding meniscus is relatively constant throughout fetal development, with the medial and lateral menisci covering approximately 60% and 80% of the surface areas, respectively.31
Gross Anatomy 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).28,175
Medial meniscus. The semicircular medial meniscus measures approximately 35 mm in diameter (anterior to posterior) and is significantly broader posteriorly than it is anteriorly.175 The anterior horn is attached to the tibia plateau near the intercondylar fossa anterior to the anterior cruciate ligament (ACL). There is significant variability in the attachment location of the anterior horn of the medial meniscus. The posterior