The Meniscus




© Springer International Publishing Switzerland 2017
E. Carlos Rodríguez-Merchán and Alexander D. Liddle (eds.)Joint Preservation in the Adult Knee10.1007/978-3-319-41808-7_7


7. The Meniscus



Luke D. Jones  and Sean O’Leary2, 3  


(1)
Nuffield Orthopaedic Centre, Oxford, UK

(2)
Royal Berkshire Hospital, Reading, UK

(3)
UK National Ligament Registry, Reading, UK

 



 

Luke D. Jones (Corresponding author)



 

Sean O’Leary



Abstract

Damage to the menisci of the knee represents the commonest indication for orthopaedic surgical intervention. Their complex role within the knee must be appreciated by all surgeons who aim to preserve tibiofemoral chondral surfaces. This narrative review considers the anatomy and microscopic structure of the menisci and relates this to function. Mechanisms of injury are considered, and strategies for managing patients in both the short and long term are presented in light of current evidence.


Keywords
KneeMeniscal injuriesDiagnosisTreatment



7.1 Anatomy of the Meniscus



7.1.1 Morphology


The menisci, historically known as the semilunar cartilages, were originally described by anatomists as vestigial structures of limited significance. Found in all mammals, in the human knee they are formed of fibrocartilage and are crescent-shaped in the axial plane and triangular in cross section. The menisci are attached at their ends (horns) to the intercondylar area of the tibia and peripherally they are attached to the fibrous capsule of the knee joint.

The medial meniscus is C-shaped and occupies approximately 60 % of the articular contact area of the medial compartment. The posterior horn of the medial meniscus is significantly wider than the anterior horn. The medial meniscus is wider in the anterior posterior plane than the medio lateral plane. The anterior horn can have a variable site of attachment but the commonest is into intercondylar region of the tibial plateau anterior to the anterior cruciate ligament (ACL). The posterior horn attaches to the tibia just anterior to the attachment of the posterior cruciate ligament (PCL). Peripherally, the coronary ligament attaches the meniscus to the upper tibia.

The lateral meniscus, in contrast, is smaller, almost circular and much more mobile. It covers a greater proportion of the articular surface, approximately 80 %, and in a common variation known as discoid meniscus can cover the articular surface in its entirety. The anterior horn of the lateral meniscus is attached to the intercondylar fossa next to the broad foot print of the ACL and posteriorly, the posterior menisco femoral ligament (of Wrisberg) and the anterior menisco femoral ligament (of Humphrey) attach the posterior horn to the PCL and the medial femoral condyle [1].


7.1.2 Blood Supply


The menisci are relatively avascular structures that obtain their blood supply from the periphery via the medial and lateral inferior and middle geniculate arteries that are themselves branches of the popliteal artery. Radial branches from a perimeniscal plexus enter the meniscus with the anterior and posterior horns having the greatest blood supply. Endoligamentous vessels proceed into the body of the meniscus from the horns forming terminal loops that provide potential for vascularisation [2]. The remainder of the meniscus receives nourishment via synovial diffusion or mechanical motion. As a result of this, the lateral meniscus is vascularized in the peripheral 10–25 % and the medial meniscus in the peripheral 10–30 % only. This has led to the concept of vascularization zones within the menisci that are used to guide treatment algorithms. The peripheral 10–20 % of the meniscus is described as the red–red zone and is fully vascularized [3]. The border of the vascular area is described as the red–white zone and the white–white zone is found within the avascular area of the meniscus.


7.1.3 Microscopic Structure


The menisci are composed of cells (fibroblast-like and chondroblast-like cells) and extra cellular matrix (ECM). The dense ECM is composed of water (approximately 75 %), type I collagen (20 %) and 5 % of other substances including proteoglycans, glycosaminoglycan, elastin and type II collagen with the precise makeup of the ECM varying with age, injury and presence of pathological conditions. Collagen is the main fibrillary component of the meniscus. Different collagen types exist in various quantities in each region of meniscus. In the red–red zone, Type I collagen is predominant (80 % composition in dry weight), but other collagen variants (e.g., type II, III, IV, VI, and XVIII) are present at less than 1 %. In the white–white zone, collagen makes up to 70 % dry weight, of which 60 % is type II collagen and 40 % is type I collagen [4]. Collagen fibres appear to be organized peripherally in a highly aligned circumferential direction, with a woven, less organized and aligned structure in the inner meniscus. This arrangement acts to convert compressive forces into radially directed forces that are distributed throughout the meniscus and subsequently resisted as hoop stresses.


7.2 Meniscal Function


Meniscal function is closely related to its anatomical and microscopic structure. The lateral meniscus is considerably more mobile than the medial meniscus with twice the extrusion of the medial side during range of movement and rotation [5]. The key biomechanical functions of the meniscus include load transmission, shock absorption, stability, nutrition, joint lubrication and proprioception. In addition, they act to decrease contact stresses and increase congruency of the knee particularly in the lateral compartment where the relatively convex tibial plateau and narrow femoral condyle predispose to high contact stresses. The surgeon, when considering their role in preserving or reconstructing the menisci, does so with the distribution of contact stresses and prevention of osteoarthritis (OA) foremost in their mind.

The role of the meniscus in the pathogenesis of osteoarthritis of the knee has been understood for some time. Loss of meniscal function, whether due to trauma, degeneration or iatrogenic causes, increases peak stresses and contact pressures at the articular surface by up to nine times their physiological normal [6]. Similar biomechanical studies have demonstrated that the greater the loss of meniscal volume, the greater the increase in articular contact pressures [7]. In human studies, total meniscectomy [8] has been demonstrated to lead to increased rates of cartilage volume loss compared to controls (6.9 % greater loss per year). Meniscal tears and damage in those with existing OA of the knee has been demonstrated to lead to increased cartilage loss over 2 years compared to those without [9, 10]. Other retrospective studies of knees with previously normal cartilage surfaces followed for the development of OA over time [11, 12] have established correlation between a history of meniscal injury and the development of OA. What is not clear from these studies, which tend to be small and retrospective, is whether the underlying injury that leads to the meniscal damage played a role in the development of the OA, or whether it is the loss of meniscal function itself that leads to the disease. A key step in this debate was the demonstration that defunctioning the medial meniscus leads to the development of medial compartment OA in animal models [13]. In this study on ten mice, lesions reliably developed on the central weight-bearing portions of the medial tibial plateau and the medial femoral condyle at 4 weeks post-surgery. Subsequent prospective studies in larger groups of patients with no history of OA who undergo medial meniscectomy have confirmed these findings in humans [14, 15]. The development of cartilage degeneration and osteoarthritis following trauma or resective surgery to both the medial and lateral meniscus is a well-established phenomenon.


7.3 Meniscal Injury


Meniscal injury can be classified as acute or degenerative.

Degenerative meniscal tears occur in older patients in the presence of osteoarthritis. These tears tend to be complex in nature, and irreparable. The incidence of degenerative meniscal tears detected on magnetic resonance imaging (MRI) in patients with asymptomatic knees is as high as 33 % [16]. The presence of underlying osteoarthritis can commonly lead to symptoms such as swelling, pain and catching sensations that can be misinterpreted as arising from the degenerative meniscal injury. Management of these patients requires careful thought and consideration of the underlying states of the chondral surfaces as arthroscopic debridement of the degenerative meniscus has been shown to be no better than sham procedures in blinded randomized trials [17, 18].

Acute meniscal injury is a very common occurrence with over a million surgical procedures performed in the USA per year for this problem [19], and an estimated 35 surgeries to the meniscus per 100,000 people in the United Kingdom NHS [20]. As these numbers refer to only those who required surgery for the injury, the true number of injuries is likely to be greater. Overall, males are four times more likely to sustain a meniscal injury than women, with a peak incidence occurring between 21 and 30 years of age in men and 11 and 20 years of age in women [21]. Medial meniscal injuries are more common than lateral tears with lateral tears seen more frequently in association with ACL tear.

The mechanism of meniscal injury tends to be due to a combination of axial loading and rotational forces that lead to a shear load on the meniscus. These shear forces exceed the elastic limit of the collagen fibres resulting in lesions with the meniscus that can propagate to the surface of the meniscus, manifesting as a tear.

Symptomatically, the patient will typically experience an immediate onset of pain and may describe a tearing or grinding sensation within the knee at the time of injury. Swelling secondary to a haemarthrosis typically occurs after 1–2 h in contrast to the immediate massive swelling of an ACL injury. Many patients will describe being able to complete the game or activity they were undertaking with loss of function occurring typically 1–2 days later. Following the acute injury, patients will complain of mild to moderate swelling, pain over the medial or lateral compartment of the knee as appropriate, and mechanical symptoms such as catching, locking and giving way. On examination, wasting of the vastus medialis is often present, sometimes as soon as 5 days post injury. The typical quartet of effusion, joint line tenderness, pain on full flexion and pain on full extension are found.

As with many aspects of knee surgery, classification of acute meniscal injuries is useful as it guides not only treatment but prognosis. The International Society of Arthroscopy, Knee Surgery and Orthopaedic Sports medicine have produced a validated classification system for meniscal injuries [22] that evaluates tears based on the following criteria:


  1. 1.


    Tear depth: partial or complete.

     

  2. 2.


    Rim width: Zone 1 tears are tears with a rim of less than 3 mm, Zone 2 tears have a rim width of 3≤5 mm, Zone 3 tears have a rim width of greater than 5 mm.

     

  3. 3.


    Radial location: posterior, mid body or anterior in location.

     

  4. 4.


    Lateral meniscal tears that extend partially or completely in front of the popliteal hiatus should be graded as central to the popliteal hiatus.

     

  5. 5.


    Tear pattern: longitudinal-vertical (the extension of which is the bucket handle tear), horizontal, radial, vertical flap, horizontal flap or complex.

     

  6. 6.


    Quality of the tissue: degenerative characteristics include cavitation, multiple tear patterns, softened meniscal tissue, fibrillation or other degenerative changes

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Nov 17, 2017 | Posted by in MUSCULOSKELETAL MEDICINE | Comments Off on The Meniscus

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