success. A couple of physicians are promoting an alternative treatment implanting autologous mesenchymal stem cells directly into the cartilage defect, without having to penetrate the subchondral bone. Microfracture reports Studies have shown that microfracture techniques do not fill in the chondral defect fully, forming fibrocartilage rather than hyaline cartilage. Fibrocartilage is not as mechanically sound as hyaline cartilage; it is much denser and unable to withstand the demands of everyday activities as well as the original cartilage and is thus at higher risk of breaking down. The blood clot is very delicate after surgery and needs to be protected. In terms of time, the clot takes about 8 to 15 weeks for conversion to fibrous tissue and is usually fibrocartilage by about four months post surgery, holding implications for the rehabilitation. Chondrocyte implantation procedures (CCI), a cell-based articular cartilage repair procedure that aims to provide complete hyaline repair tissues for articular cartilage repair, have been posed by some as an alternative to microfracture surgery. In February 2008, Saris et al. published a large-scale study claiming that CCI results in better structural repair for symptomatic cartilage defects of the knee than microfracture surgery. According to the study, one year after treatment, the regenerated tissue associated with CCI is of better quality than that of microfracture surgery. Use in professional sports Many notable professional athletes have undergone the procedure. Partially because of the high level of stress placed on the knees by these athletes, the surgery is not a panacea and results have been mixed. Many players' careers effectively end despite the surgery. However, some players such as Jason Kidd, Steve Yzerman, John Stockton, Kenyon Martin and Zach Randolph have been able to return at or near their pre-surgery form while players Ron Harper, Brian Grant, Chris Webber, Allan Houston, Penny Hardaway, and the late Derek Smith never regained their old form. Others such as Jamal Mashburn and Terrell Brandon never recovered and retired. Portland Trail Blazers rookie Greg Oden underwent the procedure on his right knee in early September 2007 and missed the entire 2007–2008 NBA season. At only 19 at the time of the surgery, doctors were confident that he would return to at or near his full strength by the 2008–2009 season; he had a second microfracture surgery, this time on his left knee, in November 2010. Subsequently, Oden did not play in the NBA for over four years,