The menisci are two crescentic wedges of fibro-cartilage positioned between the tibia and the femur in the medial and lateral compartments of the knee. They used to be considered the vestigial remnants of a muscle within the knee, but it is now recognised that they have various important functions.
The principal functions of the meniscus are load transmission and shock absorption, based on the meniscal collagen architecture, the biochemical fluid composition, and the proteoglucan-collagen meshwork. The mobile menisci transmit 50-90% of load over the knee joint, depending on knee flexion angle, femoral translation and rotation. The meniscus contributes to knee joint proprioception and probably also to joint stability. Late consequences of total and partial meniscectomy are radiographic osteoarthritis, with a varying percentage of these patients having symptoms. Malalignment, concomitant articular cartilage lesions, and ligament instability are absolute risk factors, while age, lateral compartment, and continued sport activity are relative risk factors.
Incidence of tears
Meniscal tears are the most common injury of the knee, with an incidence of meniscal injury resulting in meniscectomy of 61 per 100 000 population per year. They may occur in acute knee injuries in younger patients, or as part of a degenerative process in older individuals. Medial meniscal tears occur more frequently than tears of the lateral meniscus, at a ratio of approximately 2:1
Acute meniscal tears may be radial, vertical-circumferential or horizontal-cleavage in orientation. The central portion of a circumferential tear may be unstable, and can displace inwards within the relevant compartment of the knee. It is then known as a bucket-handle tear, and frequently causes mechanical locking of the knee. Degenerative tears tend to be complex in morphology and to occur mostly in the posterior horn.
Consequence of meniscectomy
Partial meniscectomy will preserve some of the load distribution function of the meniscus only when the meniscal body entity is preserved. There are studies that have illustrated individuals who have had resection of the entire meniscus are potentially vulnerable to arthritis and the arthritic changes become evident within 5-10 years of the resection. Fairbanks was the first to relate meniscectomy to ensuing degenerative changes, with these clinical observations in the late 1940’s being supported by extensive research since that time. Clinical and animal studies have confirmed the increased potential for osteoarthritis after meniscectomy, with a near-certainty predictability of developing osteoarthritis over time following a total meniscectomy.
Scheller, Sobau and Bulow reviewed 75 patients five to 15 years after arthroscopic partial lateral meniscectomy in an otherwise normal knee. Despite a high percentage of radiological changes, there was no significant correlation between these and the subjective symptoms or functional outcome.
Various authors have also found differences in outcome after partial meniscectomy, depending on the site of the lesion within the meniscus. Hede et al observed lower knee scores after partial meniscectomy for posterior horn tears, compared with those with either anterior horn or bucket-handle tears. They also noted that preservation of the peripheral rim of the meniscus is essential to obtain the best long-term results. Similarly, Chatain et al noted that resection of the posterior one-third or the wall of the meniscus predisposed to a poor radiological outcome.