I am sure many people have come across the layman expression 'I have torn my cartilage'. In medical terms, such an expression is incorrect. What is generally labelled as 'cartilage' is in medical terms called the meniscus. There are two menisci in each knee. They are of crescent shape structure with triangular cross section and sandwiched between femur and tibia. In principle they function as shock absorbers of the knee joint but also augment joint stability (see ANATOMY & BIOMECHANICS).

The diagram demonstrates the vulnerability of menisci when rotatory or shear forces are applied to the knee, especially if the knee is load bearing (compressed) at the same time.

Menisci are extremely important for physiological knee function. They do not only provide shock absorption but also increase the contact area between femur and tibia and henceforth distribute pressure more evenly across the joint (Baratz et al. 'Meniscal tears' Am J Sports Med 1986;14:270-275).

The left image demonstrates the ability of the meniscus to transfer load evenly from the femur onto the tibia, by utilising its full width. On the right, the meniscus has been removed creating point loading between femur and tibia.

The importance of the meniscus was undervalued in the past, which led surgeons to liberally remove them when torn (Fairbank 'Knee joint changes after menisectomy' J Bone Joint Surg [Br] 1948;30:664-670). Consequently, most patients developed osteoarthritis within a few years after meniscectomy (95% OA at 20 years post meniscectomy). Although menisci are commonly referred to as 'the cartilage', this is a misnomer as they are mainly consisting of fibrous and collagen tissue. The real cartilage or surface cartilage on the other hand is a much stiffer and represents the shiny layer covering opposing surfaces of femur and tibia within the joint, just like the shiny area at the end of a chicken bone. Together with the viscous joint fluid, the surface cartilage allows for smooth joint movements and very little friction.

Schematic drawing of various meniscal tears according to Hans Schaer in 1944.

Menisci usually tear when simultaneously exposed to compression and shear forces, like when twisting the knee whilst skiing downhill. However, under certain circumstances a compression or shear force on its own, especially in the somewhat fragile older meniscus can cause disruption or tear.

Most body tissues slowly deteriorateover time, a process which is called degeneration. One has to imagine that meniscal tissue behaves somewhat like a rubber band, which if exposed to sunlight, will get porous and brittle, tearing easily if stretched. Once the meniscus has developed a certain degree of degeneration, it will be vulnerable to tear even when low or moderate forces act on the knee. Patients often recall having experienced discomfort or pain after a prolonged period of squatting or kneeling or after having performed some home refurbishment. This discomfort settles with rest but re-occurs during exercise. It is often distinct and very localised either around the inner or outer aspect of the knee. Such meniscal tears are generally labelled 'degenerative meniscal tears' compared to acute tears which usually occur in younger patients after significant twisting injuries.