Cartilage can be focally damaged, producing a “pot hole” in the joint surface, when the knee ligaments are injured. For instance, if the anterior cruciate ligament were to rupture, the tibia can slide forward (subluxate) and impact the femoral condyle (a so-called kissing contusion). This impact can create a chondral defect (as shown in the figures below).

Articular cartilage is responsible for providing a smooth, low-friction gliding of surface within a joint and, perhaps needless to say, a pot hole is neither smooth nor low-friction. Indeed, the presence of such a defect may spur the appearance of post-traumatic arthritis.

The goal of treatment is to attempt to make the joint as normal as possible, but there are biological constraints not only on the human spirit (as Mel Konner put it) but also on the human articular surface. (And these constraints are not imposed on humans alone, which allows, of course, for translating laboratory findings to clinical practice.)

If surgeons find a defect with a cartilage flap, they could try to attach the flap to the underlying bone. This fixation is generally more successful in patients with open growth plates, though this may be attempted in somewhat older patients.

The surgeon can also try to fill the defect. This has three variations.

• The surgeon can drill into the underlying bone and stimulate bleeding. This allows access of mesenchymal stem cells of marrow to enter the defect and form a scar. This scar is made of fibrocartilage, not articular cartilage. Fibrocartilage is not as smooth as articular cartilage, and it is not as resilient. Nonetheless, stimulation of scar formation is the easiest of the three procedures and can be effective in small lesions, especially those of the femur. Note that fibrocartilage and articular cartilage differ in terms of their collagen type and other standardized-test-relevant minutiae. For our purposes here, focus on the fact that fibrocartilage is not organized like articular cartilage (and therefore neither as resilient nor as smooth) and thus is an imperfect replacement.
• For tissue grafting (Figure 4), plugs of cartilage-covered bone can be harvested from the patient’s own body (ideally from areas less important functionally than the area with the defect) or from a cadaver. These are then impacted into sockets drilled in the defect, with the bone plug healing to the adjacent bone.
• Implanting autologous chondrocytes, in a two stage operation. Pieces of cartilage are first harvested from the patient’s own body and then sent to a commercial laboratory. In the lab, the cells are encouraged to multiply. This larger number of cells is then implanted into the defect under a soft tissue flap inserted by the surgeon. This is thought to regenerate normal, or at least closer-to-normal cartilage. This is expensive, but may be the closest to actually healing the defect.

If the interventions described previously are not an option, debridement (smoothing the edges and removing debris) can be chosen, primarily to minimize mechanical symptoms. This is unlikely to be successful in the long term, however, as the protective layer of the articular cartilage is gone permanently.

Note also that in the case of a patellar dislocation, cartilage can be damaged either as the bone goes out of place or when it is reduced (returns to the trochlea). For reasons not entirely clear, the surgical options for a defect in the patella are more limited and less effective.