Meniscal tears: what they are, what causes them, and how to deal with them
The knee is a complex hinge joint that undergoes flexion/extension, rotation and lateral movements. It is a synovial joint with articulations between the femur and the tibia and the femur and patella. The menisci are integral to the normal biomechanics of the knee joint; acting as lubricators, stabilisers, shock absorbers and distributors of load within the joint. They are mobile semilunar wedges of fibrocartilage, convex on the femoral surface and flat on the tibial plateau. The woven collagen fibres dissipate compressive forces in the joint, thereby reducing the direct force on the articular cartilage covering the tibio-femoral joint surfaces. Tears in the menisci are therefore associated with progressive wear of the articular cartilage and the development of osteoarthritis. This article summarises the anatomy and pathophysiology of meniscal tears and the options available for their management.
The medial meniscus is semicircular and attached to the medial collateral ligament (medial collateral ligament) of the knee joint. It only moves 2-5 mm within the joint and is hence more prone to tears than the lateral meniscus which is more circular in shape and moves 9-11mm. The lateral meniscus is often injured at the same time as the Anterior Cruciate Ligament (ACL), whereas the medial meniscus is itself more prone to tears in the chronically 'ACL deficient' knee.
Tears cause characteristic symptoms of pain, swelling, locking and giving way. Intermittent sharp pain is localised to that side of the joint and results from part of the tear catching between the articular surfaces and hence pulling on the well innervated synovial joint capsule. Swelling results from inflammation of the synovium and effusion (excess synovial fluid production). The typical patient is very tender on palpation of the joint line and cannot squat down fully. Locking (the inability to straighten the knee) can occur in certain types of tears where the displaced meniscus blocks full extension of the joint. This becomes an urgent surgical problem as the knee must not be left in a fixed flexed position, as flexion contractures can develop. The delayed treatment of a locked knee often prolongs the rehabilitation by several weeks.
Meniscal tears are common and can be traumatic or degenerative. Traumatic tears occur classically during twisting forces on the knee in young active people, are often vertical longitudinal tears and can be associated with ligamentous injuries. Degenerative tears occur as part of progressive wear in the whole joint, most frequently in the over 40's. These tears are usually horizontal cleavage tears or flaps and have minimal healing capacity.
Tears can be described as being complete or incomplete, stable or unstable and of various patterns (Figure 1).
Those described are vertical longitudinal (including 'bucket-handle' tears where the torn fragment can block full extension of the knee joint), oblique/parrot beak /flap, radial or horizontal tears. The majority are either vertical or oblique (80%). The medial meniscus is more commonly affected - 75% as opposed to 25% in the lateral meniscus; 5% of patients will have tears in both. Most meniscal tears lie within the posterior half of the structure due to the nature of the mechanical forces across the joint.
Some types of tear may cause a valve-like action within the substance of the meniscus, and this can lead to a blow-out or meniscal cyst, which is a painful lump on the joint line. This is still treated in the same way by addressing the primary problem in the meniscus.
Management of these tears includes non-operative treatment, partial meniscectomy and repair. The management decision depends on several factors. In the past, total meniscectomy (removal of the greater part of the meniscal cartilage) was carried out for symptomatic meniscal tears but this is now known to lead to progressive degenerative joint disease(1,2), and so only the bare minimum of meniscus is now resected (partial meniscectomy) to leave a stable rim. This is normally carried out arthroscopically under a general anaesthetic and often as a 'day case' procedure. Arthroscopy is the use of a camera attached to a light source placed through a small incision into the joint and specially designed hand-held arthroscopic instruments used via a second portal.
The indications for meniscal repair on the other hand are quite specific - this is therefore carried out less often. These include the position and pattern of the tear, the age of the tear, the age of the patient and their expected compliance with post-operative instructions. The patient's activity levels and occupation are also taken into account. Classically, meniscal repair is reserved for young, compliant patients with acute (< 8 weeks) peripheral longitudinal tears which lie within the vascular zone(3) (the so called red zone as opposed to inner white zone that does not have a good blood supply) of the meniscus and occur in otherwise stable or concomitantly reconstructed knees (Figure 2).
Repair is not indicated if the tear is stable, <10mm in length and <3mm into the joint or if the tear is not within the peripheral vascular zone of the meniscus which lies 3-5mm from the meniscosynovial junction.
There are many techniques for meniscal repair and these will depend on the location of the tear. The techniques include an open procedure (following arthroscopic examination of the joint) or the arthroscopic 'inside-out', 'outside-in' and 'all-inside' procedures. The open technique has been advocated for vertical tears of the posterior horn of the lateral and medial menisci within 1-2 mm of the meniscosynovial junction, where visualisation with the arthroscope is difficult. In all cases the torn surfaces of the meniscus are debrided of scar tissue and fibrin clot can be placed in situ(4) before the sutures are tied to enhance healing.
Repaired meniscal tears heal if there is adequate blood supply and tissue stability. A stable knee is therefore important and increased healing rates have been seen with repairs done at same time as ACL reconstruction(5,6) - possibly as other factors are liberated at the time of the operation.
Post-operative protocols vary but the majority of patients will be asked to follow a strict physiotherapy regime and avoid contact sports for six months, as opposed to partial meniscectomy when patients can resume normal activities after two weeks.
Meniscal tears are common and can be part of degenerative change within the knee joint or secondary to trauma. They can cause symptoms that affect the function of the joint and require surgical intervention. The majority of symptomatic tears require arthroscopic partial meniscectomy but in a few select cases the tear may be amenable to repair done as an open or arthroscopic procedure.
Kate Owers and Fares Haddad
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6. Tenuta, J.J. and Arciero, R.A. Arthroscopic Evaluation of Meniscal Repairs - Factors that effect healing. Am J Sports Med. 22:6 797-802 1994