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.

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: