What is a meniscus tear?The knee joint is formed by two fibrocartilaginous structures called meniscus that act as shock-absorbants. The meniscus is C-shaped, with a triangular part and placed between the femur and tibia. It provides a wider surface for contact between the surfaces of the joints, increasing their congruence. One meniscus is medial (i.e. inside the knee – medial meniscus),  the other is lateral (i.e. outside the knee - lateral meniscus).A meniscus tear can occur in any age group. It commonly occurs when the knee joint is bent and the knee is then twisted with the foot blocked on the ground, or it can occur after flexion at maximum degrees under weight-bearing stress, followed by suddenly standing up again. When the meniscus tears, luxation can occur and the torn piece displace itself between the joint surfaces, impinging the joint mechanism and causing a “meniscal lock” (inability to extend the knee).A part from traumatic, meniscus tears can also be degenerative “degenerative meniscopathies”, where alterations of the fibrocartilaginous tissue are a result of repeated microtraumas, without a single specific trauma.How do you make diagnosis?Final diagnosis of torn meniscus is predominantly clinical, based on a correct physical examination done by a specialist, who cannot be replaced by instrumental examinations alone. Clinical history is very important, because the patient reports onset of symptoms. When the lesion is acute the onset of pain is usually a result of knee distortion; at the same time, the patient complains the perception of a “clunk”. In chronic lesions patients usually do not refer a specific distortion, but generally complain of recurrent pain and tenderness.On physical examination a limited range of motion may be seen, sometimes associated to effusion (swelling due to liquid inside the joint). Pain may be present at palpation on the medial and lateral compartment, together with other clinical signs of meniscus tear (McMurray Test, Appley Test, etc.).Arthocentesis (fluid drainage from the knee using a syringe) is rarely necessary and it may detect the presence of serum and blood in the liquid.Which are the most common instrumental examinations?X-rays of the knee in the two standard projections antero-posterior (AP) and latero-posterior (LL) are mandatory to exclude other bone or osteochondral lesions. For meniscus tears CT scan with contrast dye may be used, but the diagnostic gold standard is MRI (the most reliable for diagnosis of meniscus tears).Surgery: which options?Surgery of the meniscus today is done in arthroscopy, a mini-invasive technique that uses a camera and special instruments without opening the joint capsule.“Total meniscectomy” (surgical removal of the entire meniscus) was once the treatment of choice for meniscus tears, but numerous clinical studies have shown that total meniscectomy of the knee leads to a premature degenerative arthritis, especially in young athletes. Meniscectomy thus is reserved in cases when suture of the meniscus is not technically possible, as for example in complicated lesions or when the tear is not sufficiently close to the outer edge. However, whenever possible it should always be selective and not extended to the entire meniscus.That in the long term meniscectomy may evolve in arthrosis of the knee is renown since 1948, thanks to a famous scientific paper (T. J. Fairbanks. Knee joint changes after meniscectomy. J.B.J.S. Br. 1948).According to a more recent study conducted in 1996 by the French Society of Arthroscopy, meniscectomy performed when the anterior cruciate ligament is perfectly healthy, at 10 years from surgery presents radiologic evidence of arthrosis of the knee in 20-40% of cases, not necessarily accompanied by clinical pain. If meniscectomy is performed in presence of a complete lesion of the anterior cruciate ligament not reconstructed, at 25 years knee arthrosis is radiologically seen in 95% of patients.In young patients under 45 years of age with modern orthopaedic surgery torn meniscus can be replaced when lesions are too vast and complex to perform a total or partial meniscectomy. Replacement is performed with a homologous (from donor) meniscus replacement or with bovine collagen meniscus implant.How is post-operative rehabilitation done?Rehabilitation of a knee operated in arthroscopy is more simple than that for other surgery about the knee. Following medial meniscectomy the patient can walk with crutches 24 hours from surgery and recover complete joint movement in about two weeks. Often a few sessions of physiotherapy are needed: electrostimulation, stretching and proprioceptive rehabilitation of the knee may speed recovery. Lateral meniscectomy requires a longer recovery. On the contrary, suture of the meniscus with autologous graft and a meniscus implant need a different rehabilitation: a knee brace for 15-20 days that holds the knee fully extended while walking with no weight-bearing for 30 days to allow the meniscus suture or the graft to scar and stabilize. Two to three weeks after meniscus arthroscopy repair an athlete can return to running and after an average of 20 – 30 days from arthroscopy the patient can play soccer or other contact sports.Other useful information?The average time of hospitalization is one night, but surgery can also be performed in day hospital. Two to seven days are needed to return to a sedentary job, a heavy job requires about 3-4 weeks. The suture stitches are removed after 12-14 days from surgery. Usually the scar is limited to the arthroscopic accesses, 2 or 3 incisions about 1 cm long anterior to the knee. The scar must be kept dry for 15-20 days. During this time it is recommended not to go swimming or take a bath. Driving is allowed 12-14 days from surgery.Edited byMaurilio Marcacci MD, 1st Orthopaedic and Traumatologic Clinic; Stefano Zaffagnini MD, Giulio Maria Marcheggiani Muccioli MD, General Orthopaedics (Rizzoli-Sicilia Department).Istituto Ortopedico Rizzoli IRCCS, Bologna