The avascular nature of broken cartilage, especially articular cartilage of the joint, such as the knee, does not heal readily because it does not have blood vessels to provide the required nutrients, oxygen, and repairing cells to the area of injury. The only cells in the cartilage are called chondrocytes and are ensnared in a thick extracellular matrix that constrains their movement, growth, and the production of the matrix, which creates a low intrinsic repair process. Partial-thickness defects do not cause bone marrow signals and, thus, result in progressive degeneration but not regeneration, whereas full-thickness injuries can develop inferior fibrocartilage scar tissue of the marrow stem cells, which is less durable and strong (in biomechanics) than the original hyaline cartilage. This inefficient healing exposes joints to osteoarthritis because mechanical stress atrophies tissue around them with time. There is also the lack of vigorous inflammation, which further reduces the release of growth factors and vascular invasion required to conduct effective repair, as is the case with vascular tissues like the skin or muscle. Poor metabolism and low cell turnover contribute to the problems, and it is not common to recover naturally unless an intervention is done. Figuring out cartilage areas and ineffective healing reactions in defects.
MBH/AB