Meniscal Pathology: Meniscus Anatomy

Introduction

In the United States a meniscal tear is the most common diagnosis among patients undergoing knee arthroscopy, with incidence rates reported to exceed 50 people per 100,000. Patients with meniscal tears or meniscal deficiency have been shown over time to progress to early degenerative joint disease and osteoarthritis, demonstrating the essential chondroprotective role of this structure in the human knee joint. The meniscus optimises axial load transmission across the knee joint by increasing congruity between the round femoral condyles and the dish-shaped medial tibial condyle and the convex-shaped lateral tibial condyle. Increasing joint congruity increases the joint contact area between the femoral and tibial condyles, which decreases point loading on the articular cartilage that covers the joint. Surgical resection of as little as 10% of the meniscus has been reported to increase the incidence of full-thickness chondral injuries and results in lower subjective and objective clinical outcome measures. Due to the fact that meniscal tissue is more elastic than articular cartilage and absorbs stress caused by impact loading, the menisci serve as important shock absorbers in the knee joint. , ^{, ,} The menisci also help stabilise the knee joint, with the medial and lateral menisci functioning as secondary stabilisers for anteroposterior translation and rotatory motion, respective-ly. ^{, , ,} For all of the above reasons, it is important to repair a torn meniscus when possible. After injury to the meniscus, patients may initially experience symptoms of joint line pain, swelling and mechanical symptoms, such as locking, catching and giving way. However, over time, loss of meniscal function as a result of injury or surgery may start a cascade of events that leads to the development of degenerative joint disease and osteoarthritis of the knee joint. ^{, , , ,} Therefore preserving the meniscus is crucial for maintaining the long-term health of the knee joint. ^{, , , , , , ,}

The goal of this chapter is to help the reader better understand the anatomy of the menisci, with an emphasis on the location of the clinically relevant peripheral soft tissue and bony root attachments that can aid the surgeon in restoring normal anatomy when performing meniscal surgery. The gross anatomy of the menisci, the meniscal root attachment sites, the ligaments which play a key role in stabilising the meniscus on the tibial condyles, and the fine structure of the meniscus will be covered. Finally, the vascular anatomy of the meniscus will be covered because current algorithms for repair of the menisci are largely based on the location of the tear relative to the location of the vascular zone (i.e., red-red, red-white and white-white).

Gross Anatomy

The menisci are crescent-shaped wedges of fibrocartilage interposed between the femoral and tibial condyle of the knee. In cross-section the menisci have a triangular shape. The outer margin of the meniscus is convex and attached to the joint capsule of the knee joint. The meniscus tapers from the thicker outer margin to a thin inner margin. The superior surface of the meniscus is concave to allow a congruent articulation with the convex femoral condyles. The inferior surface of the menisci is flat to accommodate the relatively flat surfaces of the tibial plateaus ( Fig. 13.1 ). In anatomical studies the medial meniscus has been reported to cover 51% to 74% of the medial tibial plateau and the lateral meniscus covers 75% to 93% of the lateral tibial plateau ( Fig. 13.2 ). Using three-dimensional magnetic resonance imaging (MRI) analysis, the lateral meniscus was found to cover 59% of the lateral tibial plateau and the medial meniscus and 50% of the medial tibial plateau.

Medial Meniscus

Seven morphological shapes of the menisci have been described. Fig. 13.3 . A crescent-shaped (semilunar) meniscus has thin anterior and posterior horns and a thin body. A sickle-shaped meniscus has thin anterior and posterior horns, a thick body and a wide gap between the anterior and posterior horns. In the C-shaped type, the widths of the anterior and posterior horns and the body are similar. In the U-shaped type, the widths of the anterior and posterior horns and the body are similar and the tips are rounded and the gap between the horns is wide. In the V-shaped type, the meniscus has a shape resembling the letter V. A complete discoid meniscus covers the tibial plateau circularly and has a defect at the centre of the meniscus and no gap between the horns. An incomplete discoid meniscus is a discoid meniscus with an open area at the centre of the meniscus (see Fig. 13.3 ).

In neonatal specimens, sickle shape (36.4% in one study and 46.2% in another) and V shape (23.6% and 22.7%) were reported to be the most common shapes for the medial meniscus. ^, The incidence of a discoid medial meniscus was 0% in both studies. Crescent shaped (54.6%) and V shaped (34.6%) were the most common shapes for the medial meniscus in the adult knee, and no discoid medial meniscus were reported. The width of the medial meniscus increases as one moves from the anterior horn to the posterior horn. The width of the medial meniscus has been reported to range from 7.6 to 9.0 mm in the anterior third, 9.3 to 12.2 mm in the middle third and 12.6 to 17.37 mm in the posterior third. ^, The thickness of the medial meniscus in the anterior third has been reported to range from 5.4 to 6.4 mm, 5.6 to 6.93 mm in the middle third and 5.18 to 6.72 mm in the posterior third. ^, Rashmi et al have reported that the anterior and posterior thirds of the medial meniscus are significantly thicker than the corresponding areas in the lateral meniscus. The anterior and posterior horns of the medial meniscus are anchored by strong ligament-like structures, the anterior and posterior medial meniscal roots. ^, The distance between the anterior and posterior horns of the medial meniscus has been reported to be 28.87 mm. The outer margin of the medial meniscus is anchored along its entire length to the medial tibial plateau by the meniscotibial ligament (coronary ligament). The medial meniscus also has strong attachments to the posterior oblique ligament (POL) and the posteromedial capsule. A detailed description of the medial meniscus root attachments and stabilising ligaments anchoring the medial meniscus to the medial tibial condyle will be covered in a separate section of this chapter.

Śmigielski et al. have proposed division of the medial meniscus into five anatomical zones: zone 1, the anterior root; zone 2, the anteromedial zone (subdivided into zones 2a and 2b); zone 3, the region of the superficial medial collateral ligament; zone 4, the posterior horn; and zone 5, the posterior root ( Fig. 13.4 ). In contrast to previous descriptions, this zonal division is based on the different anatomical characteristics of the medial meniscus in each zone.

Zone 1 is the attachment site of the medial meniscus anterior root. In this zone the medial meniscus is attached directly to the bone of the medial tibial condyle. The anterior root of the medial meniscus inserts along the anterior intercondylar crest of the anterior slope of the tibia. ^, In an anatomical study of 48 cadaveric knees, Berlet et al. described four types of bony attachments of the anterior root of the medial meniscus: Type I (59% of all cases) was located on the flat intercondylar region of the tibial plateau, also called the cristae area intercondylaris anterior ; type II (24%) was located on the downward slope from the medial articular plateau to the intercondylar region; type III (15%) was located on the anterior slope of the tibial plateau; and in type IV (3%) there is no solid fixation of the meniscus to the medial tibial condyle. According to Rainio et al., in 1% of cases there is an atypical insertion of the anterior root of the medial meniscus representing either complete absence or hypermobility of the anterior medial meniscus root attachment. LaPrade et al. have reported that the area of the anterior root attachment of the medial meniscus is 110.4 mm.

Zone 2 starts at the posterior border of the anterior medial meniscus root attachment site and ends at the anterior border of the superficial medial collateral ligament (sMCL). Zone 2 may be further subdivided by the meniscal attachment of the anterior intermeniscal ligament into two subzones, zones 2a and 2b. Zone 2a starts at the posterior border of the anterior root of the medial meniscus and ends at the attachment site of the anterior intermeniscal ligament. The attachment site of the anterior intermeniscal ligament marks the start of zone 2b, which ends at the anterior border of the sMCL. In zones 2a and 2b, the inferior margin of the medial meniscus is attached to the medial tibial condyle by the meniscotibial ligament, also called the coronary ligament . ^{, ,} The superior margin of the medial meniscus within zone 2a shows no attachment to the medial joint capsule. However, in zone 2b the superior margin of the periphery of the medial meniscus is attached to synovial tissue ( Fig. 13.5 ).

Zone 3 starts at the anterior border of the sMCL and ends at the posterior border of the sMCL. Zone 3 is the only zone in which the superior and inferior margins of the medial meniscus are attached to the medial joint capsule. In this zone there is a thickening of the medial joint capsule known as the deep medial collateral ligament (dMCL). In zone 3 the inferior margin of the medial meniscus is attached to the medial tibial condyle by the meniscotibial ligament and the superior margin is attached to the medial femoral condyle by the meniscofemoral ligament ^, ( Fig. 13.6 ).

Zone 4 extends from the posterior border of the sMCL to the posterior medial meniscus root attachment. This is a very important zone because it is the most common site of medial meniscal tears and is also the zone where meniscus repairs are most commonly performed. ^{, ,} In this zone the posteromedial joint capsule does not attach to the superior edge of the medial meniscus but attaches just below the medial joint line ^, (Fig. 13.7) . The inferior edge of the medial meniscus is attached to the medial tibial condyle 6 mm below the medial joint line by the meniscotibial ligament. The meniscotibial ligament and the posteromedial joint capsule and fibres of the posterior oblique ligament (POL) join together, creating a space, the posteromedial femoral recess, posterior to the superior margin of the medial meniscus ^, (see Fig. 13.7 ). The posteromedial femoral recess can be visualised during knee arthroscopy by passing the arthroscope between the medial border of the posterior cruciate ligament (PCL) and the lateral border of the medial femoral condyle. The posteromedial femoral recess is the area where ramp lesions occur ( Fig. 13.8 ).

Zone 5 is the posterior root attachment of the medial meniscus. The posterior root attachment of the medial meniscus is located posterior to the apex of the medial tibial eminence (MTE) and lateral to the articular cartilage inflection point of the medial tibial condyle. ^,

Lateral Meniscus

In neonatal specimens a discoid lateral meniscus (77.2%) is the most common shape and C-shaped (9.09%) and crescent-shaped lateral menisci (13.6%) are much less common. In adults the lateral meniscus is most commonly C shaped (56.4%) or crescent shaped (41.6%), and only 2% had a discoid shape. The width of the lateral meniscus has been reported to be 10 to 11.9 mm in the anterior third, 10 to 12.5 mm in the middle third and 9.8 to 12 mm in the posterior thirds. ^{, ,} Within a given specimen, the width of the lateral meniscus is relatively constant in the anterior, middle and posterior thirds. The thickness of the lateral meniscus has been reported to range from 3.8 to 4.73 mm in the anterior third, 5.9 to 6.5 mm in the middle third and 5.3 to 6.2 mm in the posterior third. , , The anterior and posterior horns of the lateral meniscus are anchored to the lateral tibial condyle by strong ligament-like structures, the anterior and posterior meniscal roots. ^, The distance between the anterior and posterior horns of the lateral meniscus has been reported to be 12.6 mm compared with 28.9 mm for the medial meniscus ( Fig. 13.9 ). Due to the shorter distance between the anterior and posterior lateral meniscus root attachments sites, a bone bridge technique is commonly used when performing a lateral meniscus transplant. The outer margin of the lateral meniscus is loosely attached to the margin of the lateral tibial plateau by the meniscotibial ligament except in the area where it is crossed by the popliteus tendon. The lateral meniscotibial ligament is thinner and more elastic than that on the medial side of the knee. This fact and the lack of a capsular attachment in the region of the popliteus tendon allows for a larger amount of medial-lateral and anteroposterior translation of the lateral meniscus compared with the medial meniscus.

The lateral meniscus begins at the lateral meniscus anterior root attachment and ends with the lateral meniscus posterior root attachment. Similar to the medial meniscus, the lateral meniscus can be divided into zones. The lateral meniscus is divided into six zones: zone 1, the anterior root; zone 2, the anterolateral zone (subdivided into zones 2a and 2b); zone 3, the popliteus hiatus; zone 4, the posteroinferior popliteomeniscal fascicle; zone 5, the ligamentous zone; and zone 6, the posterior root ( Fig. 13.10 ).

Zone 1 is the lateral meniscus anterior root attachment. The anterior root of the lateral meniscus inserts on the tibia deeply beneath the tibial attachment of the anterior cruciate ligament (ACL). ^, The tibial attachment of the ACL commonly forms a C-shaped insertion in the middle of which there is the centre point of the anterior lateral meniscus root attachment. The ACL tibial attachment site is also commonly referred to as a duck-foot, with the ACL forming a ‘tent’ over the anterior root attachment of the lateral meniscus. The clinical significance of the intimate relationship between the anterior lateral meniscus root and the ACL is that the anterior root of the lateral meniscal root can be injured during drilling of the ACL tibial tunnel.

Zone 2 starts at the lateral border of the anterior lateral meniscal root attachment and ends at the anterior border of the popliteal hiatus. Because of a thin, elastic meniscotibial ligament, zone 2 is a very mobile part of the lateral meniscus, translating anteriorly and posteriorly over the lateral tibial plateau as the knee moves through flexion and extension. Similar to the medial meniscus, zone 2 of the lateral meniscus can be further subdivided by the meniscal attachment of the anterior intermeniscal ligament into two subzones: zones 2a and 2b. Zone 2a covers the area from the lateral border of the anterior root of the lateral meniscus to the attachment site of the anterior intermeniscal ligament; zone 2b runs from the attachment site of the anterior intermeniscal ligament to the anterior border of the popliteus hiatus.

Zone 3 starts at the anterior border of the popliteus hiatus and ends at the start of the posterosuperior popliteomeniscal fascicle. This zone contains the popliteal hiatus and the popliteus tendon, the anteroinferior popliteomeniscal fascicle and the meniscofibular ligament (MFL). ^{, , ,} The area along the lateral margin of the lateral meniscus in which the popliteus tendon travels from an extracapsular location through the posterolateral capsule to enter into the intraarticular region of the knee is called the popliteal hiatus . The length of the popliteal hiatus has been reported to be 12.1 mm. Evolutionary and developmental anatomy is the key to understanding the complex anatomy of the lateral meniscus. Approximately 360 million years ago in vertebrates the fibula articulated with the femur. As the vertebrate knee evolved, the fibula and the attached lateral part of the knee joint capsule moved distally, resulting in the popliteal hiatus and an intraarticular popliteus tendon. ^,

The popliteal hiatus is triangular in cross-section and has three borders and three openings. The lateral border is formed by the popliteus tendon, the superior border or roof is formed by the posterosuperior popliteomeniscal fascicle and the inferior border or floor is formed by the anteroinferior popliteomeniscal fascicle ^, ( Fig. 13.11 ) . The openings of the popliteal hiatus include the posterior, superior and inferior openings. The posterior opening is where the popliteus tendon passes through the posterior capsule and enters into the knee joint. This opening cannot be visualised arthroscopically. The popliteus tendon enters the intraarticular environment of the knee joint and the popliteal hiatus through the inferior opening. The inferior opening lies beneath the inferior surface of the meniscus and is where the tendon crosses the lateral tibial plateau. The lateral meniscus lacks a capsular attachment in this region. The inferior opening can be visualised arthroscopically by passing the arthroscope under the inferior surface of the lateral meniscus. The superior opening is located at the junction of the posterior and middle segments of the lateral meniscus. In this area the superior edge of the lateral meniscus lacks an attachment to the lateral capsule. The superior opening is easily visualised arthroscopically. The lateral meniscus is very mobile in zone 3 as a result of the lack of capsular attachments.

Zone 4 starts at the posteroinferior popliteomeniscal fascicle and ends at the start of the lateral meniscotibial ligament. Zone 4 contains the posteroinferior popliteomeniscal fascicle. Zone 5 starts from the lateral meniscotibial ligament and ends at the start of the posterior lateral meniscus root. This region can be referred to as the ligamentous zone because it contains the lateral meniscotibial ligament and the anterior and posterior meniscofemoral ligaments.

Zone 6 is the posterior lateral meniscus root attachment. The posterior root attachment of the lateral meniscus is a flat structure with a mean insertion site size between 28.5 and 115.0 mm. ^, The centre of the posterior lateral meniscus root attachment is posteromedial from the apex of the lateral tibial eminence (LTE), and medial to the lateral articular cartilage edge. You et al. evaluated 105 knees in a 3.0 Tesla MRI and found three different types of posterior root attachment of the lateral meniscus. In 76% of cases, two insertion sites were identified, with the majority of fibres attaching to the intertubercular area and the minority of fibres attaching to the posterior slope of the lateral tibial eminence. In the remaining 24% of cases the posterior root of the lateral meniscus presented an isolated insertion site to either the intertubercular area or the posterior slope of the lateral eminence, respectively.