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The articulating surfaces of the knee (like any other synovial joint) are lined by hyaline cartilage (which is very firmly adherent to the underlying bone). This is a tough, springy (elastic), self-lubricating tissue with a very smooth surface and low coefficient of friction. It is composed of a matrix of complex molecules (collagen and proteogylcans) which are secreted by cartilage cells (chondrocytes). The proteoglycans are hydrophilic molecules that attract a lot of water molecules into the matrix; this is responsible for the physical properties of the cartilage. The cartilage derives its nutrition from the synovial fluid.

Hyaline cartilage has limited ability to heal. Significant injury to the cartilage, if left untreated, results in permanent damage and progressive wear of the joint – eventually leading to degeneration and arthritis.

Disorders of cartilage can occur due to:
  • Injury: The cartilage can get damaged during sports injuries and accidents. Dislocation of the knee cap can result in a portion of the cartilage being broken off. Usually the cartilage is broken off along with a small amount of underlying bone – this is called osteo-chondral fracture. Injury also causes incongruity at the joint which leads to continued wear of the cartilage.
  • Loss of underlying bony support: Osteochondritis dissecans (OCD) is a condition in which a part of the bone gets detached along with the overlying cartilage. Osteonecrosis or avascular necrosis occurs when the underlying bone loses its blood supply and collapses along with the cartilage lining. This is more common in the hip but also occurs in the knee and other joints.
  • Overloading: Bow-leg deformity of the knee causes excess pressure on the inner half of the knee leading to cartilage damage and wear. Knock-knee deformity affects the outer half.
  • Arthritis: This is the most common cause of cartilage damage. This, however, does not come under the purview of sports medicine. Please see the section on arthritis.
  • Pain in the knee, especially on weight-bearing activities
  • Feeling of a catch in the knee, with or without pain
  • Swelling of the joint
  • Locking of the knee when there is a loose fragment of bone or cartilage
  • Rarely a large piece of cartilage or bone can give rise to instability (buckling)



Hyaline cartilage has limited ability to heal. Significant injury to the cartilage, if left untreated, results in permanent damage and progressive wear of the joint – eventually leading to degeneration and arthritis.

Small defects can be left alone if they are not causing any symptoms. The knee should be protected by avoiding high-impact activities and using a knee brace. However, there should be no instability (due to ligament injury) or angular deformity at the knee because these will increase the stress on the cartilage. Any instability or angular deformity has to be corrected surgically to prevent progressive damage.

Larger defects in cartilage require surgical intervention. Thses are described below.


Chondroplasty is a procedure in which the loose cartilage fragments are trimmed by a motorized device called shaver. This alleviates the symptoms of catching, but does not result in repair of the lost cartilage.


Microfracture is advised when the defects are small. This involves making small holes in the exposed bone to stimulate marrow cells. The defect gets covered by fibrous tissue which gives partial relief. However, the fibrous tissue does not have the same properties as hyaline cartilage.


Osteo-chondral autograft transfer system (OATS) or mosaicplasty is a procedure in which a small plugs of hyaline cartilage along with the underlying bone are taken from non-weight-bearing portion of the joint and transferred into the defect. This is recommended for larger defects. Harvesting of cartilage from non-critical portion does not cause major derangement in the joint. However, minor symptoms of pain and occasional swelling can occur.


Autologous Chondrocyte Implantation (ACI) is a procedure in which cartilage cells (chondrocytes) are implanted into the defect. This is a two-stage procedure. In the first operation, a piece of cartilage is harvested from a non-critical area of the joint. This is then processed in the laboratory where the cartilage cells are separated and grown in culture for 4-6 weeks. In the second operation (usually 6 weeks later), these cells are implanted into the cartilage defect. The cells may be secured with a flap of tissue (periosteal flap) or by self-gelling fibrin glue or implanted in a matrix of collagen (stuck with fibrin glue). The advantages are the ability to cover large defects; and minimizing the amount of harvested cartilage. The resultant cartilage that grows back is far superior as compared to fibrous tissue (that grows with microfracture). The disadvantages are: increase in the cost, and the need for two operations.


Stem cell grafting is similar to ACI. Here stem cells are implanted instead of chondrocytes. Many different techniques have been described. Different commercial kits (along with processing devices) are available to harvest the stem cells. Long-term studies are needed to establish the efficacy of this technique.


Osteo-articular allografting is a procedure in which bone and cartilage grafts from a cadaveric source (treated with special cartilage preserving techniques) are implanted to replace the damaged portion of the knee. The bone gets incorporated but the cartilage cells degenerate. This procedure has been largely given up with the advent of newer techniques mentioned above. It is sometimes used for reconstruction of massive defects following resection of bone tumours.


Artificial surface implants (made of metal or ceramic) have also been advocated. They are sometimes used in the shoulder but not in the knee. The main problem is loosening with time – hence long-lasting results are difficult to achieve.

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