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Revista Romn de Anatomie funcional i clinic, macro- i microscopic i de Antropologie

Vol. XIII Nr. 3 2014 UPDATES

THE ANATOMY OF THE MENISCI OF THE KNEE JOINT REVIEW OF THE

LITERATURE

D.E. Costin1, Al.T. Ispas2, Laura Stroic2 , V. Ardeleanu3

1. MD, Emergency Hospital of AlexandriaRomanian Handball Federation

2. University of Medicine and Pharmacy Carol Davila BucharestDiscipline of Anatomy

3. Lower Danube University of GalatiFaculty of Medicine and Pharmacy

THE ANATOMY OF THE MENISCI OF THE KNEE JOINT REVIEW OF THE LITERATURE (Abstract): The menisci are fibrocartilaginous, semilunar shaped structures, located in the knee joint, that provide congruent articular surfaces of the tibia and femur. The shape of the lateral meniscus is more like the letter O, and of the medial meniscus like the letter C. The me-nisci are described as having three parts: two extremities called horns - anterior and posterior - and one body. The horns area attached by insertional ligaments to the tibial plateau. The posterior horn of the lateral meniscus is attached to the medial condyle of the femur through the anterior and posterior meniscofemural ligaments. This peculiarity of the lateral meniscus anatomy explains why, during the rotation movement, the motions of the meniscus and the femoral condyle are coupled. The lateral condyle is loosely attached through its external border to the articular capsule. Thus, the lateral meniscus is more mobile. For the medial meniscus, the posterior horn is wider than the anterior. The attachment areas for the anterior horn is larger than that for the posterior horn. The dimensions of the insertional areas may be responsible for a stronger attachment, and a lesser probability for meniscal tear, but in alliance with collagene composition and type and insertional angle (usually a larger area means a more oblique angle of insertion). The knowledge of the vascular and nerve supply of the menisci is very important not only for scientifical, but also for clinical reasons. The vascular distribution is strongly correlated with the capacity of regen-eration and healing. Meniscal tears that occur in the external one third of the menisci - which have a greater vascularity are more likely to be healed. The nervous distribution may explain the extero and proprioception at the level of the kneee joint, as well as the vasomotor reactions. Key-words: MENISCI, HORN, INSERTIONAL LIGAMENTS, KNEE

The menisci are fibrocartilaginous, semilu-nar shaped structures, located in the knee joint, that provide congruent articular surfaces of the tibia and femur.

EMBRYOLOGYThe skeleton of the human embryo has a

continuous structure with no spaces or joints separating the main components. But as the mesenchimal skeleton suffers the condrification process, the second stage of bone development, an interzone appears at the future level of the

joints. This zone had a three layers structure: two chondrogenic layers which represent the articular surfaces, and an intermediate layer, from which the connection structures of a joint develop.

The menisci develop from mesenchymal tis-sue and can be distinctively observed by week 8 of intrauterine life. Clark and Ogden (1, 2) observed that at the end of the forth gesta-tional month, the shape of the menisci is al-ready settled. At birth, the menisci have a very high cellular density, and also a rich vascular

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network. At the age of nine weeks, the menisci of the newborn are mostly alike the adult one, excepting the structure. Until adolescence, a decrease of cellularity and vascularity can be observed, and in the same time an increase in collagen content of the meniscus. These newly acquired collagen fibers are oriented in order to bear the body weight, right after the child begins to walk. Fukazawa (3) reported that the medial and lateral menisci development is dif-ferent, as the layered structure can be observed earlier in the lateral than the medial meniscus. Clark and Ogden (1,2) calculated the ratio between the meniscus surface ant the tibial pla-teau surface, in order to show the amount of coverage of the articular surface of tibia. For the medial meniscus, the ratio was between 51 and 74%, with a mean of 64%, and for the lateral meniscus, the same ratio was 84%. Al-though, the surfaces of the two menisci are almost equal. The ratio didnt show a signifi-cant change between prenatal and postnatal life, neither a significant variation during the two periods. The main anomaly of the menisci de-velopmnet is the discoid meniscus, with a fre-quency of 1.5-4.6% for the lateral meniscus and only 0.3% for the medial one (4).

GROSS ANATOMY

The menisci measure approximately 35 mm in diameter (5) and the length of the rim that attaches the meniscus to the articular capsule is 110 mm (6). The shape of the lateral menis-cus is more like the letter O, and of the medial meniscus like the letter C. In cross-section, they are triangular in shape, as the external heigth is 5 mm, and the inner border is only a thin edge. They are firmly attached to the anterior and posterior aspects of the tibial plateau by the root ligaments. In the rabbit, these ligaments are easy to differentiate from the meniscal tissue by their stiffness on palpa-tion. The attachment ligaments are very impor-tant in distributing the load at the level of knee joint (7).

The outer borders of the menisci are convex and attached to the articular capsule, and the inner borders are concave and free. The supe-rior surfaces of the menisci are concave, to match the surface of the femoral condyles. The inferior surface are flatter to match the tibial condyles.

The menisci are described as having three parts: two extremities called horns (anterior and posterior) and one body.

The anterior horns of the lateral and medial menisci are attached to each other through the transverse ligament. The insertional ligament for the anterior horn attaches to the anterior intercondillar eminence of the tibia, just behind the ACL. A part of its fibers blend with the fibers of ACL. The posterior horn of the lat-eral meniscus is attached to the medial condyle of the femur through the anterior and posterior meniscofemural ligaments, also known as Hum-phrey and Wrisberg ligaments. This peculiarity of the lateral meniscus anatomy explains why, during the rotation movement, the motions of the meniscus and the femoral condyle are cou-pled. The attaching ligament of the posterior horn is inserted on the tibial plateau, posterior to the lateral intercondyllar eminence and an-terior to the insertion of medial meniscus.

The lateral condyle is loosely attached through its external border to the articular cap-sule. Because of this looseness of the capsular attachment, and the fact that it is not attached to the lateral colateral ligament, the lateral me-niscus is more mobile. Between the capsule and the lateral meniscus a meniscocapsular tunnel is delimited. This tunnel is crossed by the pop-liteus tendon. In case of flexion and internal rotation, the tendon retracts the posterior horn, explaining why the lateral meniscus is more rarely injured than the medial one.

There were described discoid lateral me-nisci, with complete absence of the tibial at-tachment of the posterior horn, so the poste-rior horn is attached only to the femoral condyle via the Wrisberg ligament (8).

For the medial meniscus, the posterior horn is wider than the anterior. The anterior horn is firmly attached to the tibia, at 6-7mm anterior to the anterior cruciate ligament (ACL) (6, 9). The attaching ligament is flat and fan-shaped. A part of its fibers blend and participate to the transverse ligament. The posterior horn is at-tached in front of the attachment of the poste-rior cruciate ligament (PCL) and posterior to the insertion of the lateral meniscal ligament (6). The attachemt areas for the anterior horn is larger than that for the posterior horn. Re-garding these areas of the lateral meniscus, there are controversies between the specialists

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(6, 9). The dimensions of the insertional areas may be responsible for a stronger attachment, and a lesser pbobability for meniscal tear, but in alliance with collagene composition and type and insertional angle (usually a larger area means a more oblique angle of insertion) (10, 11).

The external border of the medial meniscus is attached to the articular capsule and the me-niscotibial and meniscofemural ligaments at-tach the meniscus to the two bones, and are known as the deep medial collateral ligaments.

MICROANATOMY

The microanatomy is very important for un-derstanding the injury pattern. There are sev-eral types of collagen fibers dispositions. The main pattern is a network of circumferential fibers, which allow the dispersion of the com-pression loads. Another pattern is that of ra-dial fibers, which prevent the excessive move-ment between the circumferential fibers and consequently the longitudinal tear of the me-nisci. At the surface of the menisci, a specific pattern cant be emphasized, the fibers being disposed randomly (10).

FUNCTIONAL ANATOMY

The incongruency between the convex ar-ticular surfaces of the femoral condyles and the flat articular surface of the tibial condyles is compensated by the meniscal fibrocartilage, with its concave superior surface directed to-wards the femur, and the inferior flat surface directed towards the tibia. Thus, the menisci significantly increase the contact surface and the weight stresses on the tibia decrease. Dur-ing in vitro experiments, the authors observed that 70% and 50% of the loads in the knee joints were transmitted through the lateral and me-dial menisci (11, 12, 13, 14 ).

These different pergentages are related to the different amount of coverage of the articu-lar surface of the tibia by the two menisci. The menisci distribute forces throughout underlying articular cartilage, thus minimizing point con-tact. They bear 40 to 50% of the total load transmitted across joint in extension and 85% of the compressive load is transmitted through the menisci at 90 degrees of flexion (15). After removal of the menisci, the contact areas be-tween the femoral condyles and the tibial pla-teau is reduced and the sterss forces on the

tibial cartilace considerably increase. This role of distributing load forces is possible because of the firm anterior and posterior attachment of the meniscal horns to the tibia. This prevent the menisci from escaping from the knee joint during axial loads. The load forces tension the insertional ligaments and the circumferential fibers of the menisci, thus transforming the axial load into hoop stress.

This can be explained by the fact that the circumferential collagen fibers of the meniscal body are continuous into the horns and even the insertional ligaments. The continuity as-sures for a strong bony attachment and enables the transformation of the axial loads into hoop stresses (11). In the rabbitt knee, the anterior insertional ligaments were stronger than the posterior, and the ligaments of the lateral me-niscus were stronger than the medial. Based on the idea that the amount of calcification of the under articular cartilage cortical bone depends on the loads that act upon it, Benjamin et al (16) observed a thicker calcified bone in the anterior insertional area of the lateral meniscus than the similar zone of the medial meniscus. The authors explain their discovery by the fact that the fibers of the lateral anterior insertional ligament blend with the ACL fibers and thus the higher forces are transmitted through this insertional area. Other specialists add that it also means that the lateral meniscus bears high-er load forces than the medial, thus its higher force and the stronger insertional ligaments.

Messner and Gao (11) observed that the struc-ture of the anterior and posterior insertional ligaments differs significantly. The anterior liga-ment has a typlical ligamentous structure, and the posterior ligament has a fibrocartilaginous structure, resembling to the meniscus. The ex-planation resides in the anatomical position of these ligaments: the anterior ligament is attached in front of the joint, and is probably affected only by tension forces, wchich explain its liga-mentous structure; the posterior ligament is placed near the centre of the joint, being af-fected by both traction and compression forces.

Paletta (17) observed that transection of the anterior and posterior insertional ligaments has the same result over the load distribution as the total meniscectomy. Gao and Messner (8) ob-served, in a rabbit model, that 6-12 weeks after transection of anterior or posterior insertional

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ligaments, the changes in bone and cartilage structure of the tibia are similar to those that result from a complete meniscal resection. In contrast, after the resection of the meniscal body, with the horns and their attacments left untouched, the menisci continue to transmit load forces (18). There is a linear correlation between the surface of meniscal body removed and the increase of the load exercised on the tibial articular surfaces.

During flexion and extension, the lateral meniscus moves about 10 mm, twice as much as the medial. Meniscal motion allows maximal congruency during knee flexion and helps to protect the mensici from injury (15). In his study, Vedi (19) studied meniscal movement using a dynamic MRI. The conclusions were that the anterior horn of medial meniscus moved through a mean of 7.1 mm and posterior horn through 3.9 mm, and there was 3.6 mm of mediolateral radial displacement; the anterior horn of the lateral meniscus moves 9.5 mm and the posterior horn moves 5.6 mm, and there was 3.7 mm of radial displacement.

The authors felt that the relative immobility of the posterior horn of the medial mensicus may account for its propensity for injury (15) Because of the greater displacement of the an-terior horn, the shapes of the menisci also modify during the knee movements (20)

BLOOD SUPPLY AND INNERVATION OF THE MENISCIThe arteries emerge from the lateral and

medial superior and inferior genicular arteries, which are branches of the popliteal artery. They reach the periphery of the meniscus through the synovial membrane that covers the attachment ligaments of the meniscal horns and form a perimeniscal capillary network, from which radial branches emerge (11). In fetal life, the menisci have many blood vessels throughout their entire substance. After birth, a significant decrease in blood vessels density can be ob-served, begining from the internal to the exter-nal borders. In the second year of life, an avas-cular zone in the inner part of the menisci can be seen. Thats why the adult meniscus is avas-cular in the inner two thirds (this region is called white zone) and has a more visible vas-cularity in the outer one third and the adjacent

capsular ligaments (this region is called red zone). Another avascular zone is situated in the external part of the lateral meniscus, adjacent to the popliteus muscle tendon. In this area, the popliteus tendon is placed between the lateral meniscus and the lateral inferior genicular ar-tery. The arterial penetration is between 10 and 30% of the width for the medial meniscus and between 10 and 25% for the lateral meniscus. The horns are more vascularized than the body (11, 21, 22).

The diference in vascularity between the fetal and adult meniscuis could explain the bet-ter regenerative and repair power of the devel-oping meniscus.

As for the vascularity, the perimeniscal and peripheral meniscal zones are well innervated. There are the larger nerve fibers, which have a circumferential course. Most of these fibers, but not all of them are accompanied by vessels. The smaller nerve fibers have a radial direction, towards the external one third of the menisci. Their pattern distribution is similar and close-ly associated with the vascular distribution. Some single axons can be seen in the perimenis-cal zone and in the outer one third of the me-nisci without vessels to accompany them. Ex-actly like the vessels, no nerve can be observed in the inner two-thirds of the menisci (22). The innervation pattern is similar for the medial and lateral meniscus. The innervation of the horns is greater than in the body of the meniscus.

The knowledge of the vascular and nerve supply of the menisci is very importnant not only for scientifical, but also for clinical rea-sons. The vascular distribution is strongly cor-related with the capacity of regeneration and healing, although there are several authors that describe healing processes taking place in the avascular zones. Meniscal tears that occur in the external one third of the menisci - which have a greater vascularity are more likely to be healed. The nervous distribution may ex-plain the extero and proprioception at the level of the kneee joint, as well as the vasomotor reactions. Some authors claim that the menisci innervation is also responsible for complex ac-tions, such as postural reflexes (22), and that some of the pain that accompanies a meniscal tear may originate from the meniscus itself, espacially the peripheral lesions (23).

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