Through use of an awl, the surgeon creates tiny fractures in the subchondral bone plate.[9] Blood and bone marrow (which contains stem cells) seep out of the fractures, creating a blood clot that releases cartilage-building cells. The microfractures are treated as an injury by the body, which is why the surgery results in new, replacement cartilage.[10] The procedure is less effective in treating older patients, overweight patients, or a cartilage lesion larger than 2.5 cm.[10] Further on, chances are high that after only 1 or 2 years of the surgery symptoms start to return as the fibrocartilage wears away, forcing the patient to reengage in articular cartilage repair.