Can you use stem cells to repair knee cartilage?

It's Possible. Check out Regenexx.Com. The have the single largest registry in the country of patients with degenerative joint disease that have used their stem cell procedures with good outcomes. Obviously not everyone gets better, but a majority have found the ability to forego knee surgery.
Yes. Similar technology can be used for small areas of cartilage damage, but very few patients are good candidates for this kind of treatment.
Yes. Both bone marrow derived cd34 stem cells and mixed population stem cells acquired from fat (fat derived mesenchymal stem cells, stromal vascular fraction stem cells, mult names) have been used successfully to repair knee cartilage in dogs, horses, and yes, humans.
Can Help. Use of adult mesenchymal stem-stromal cells (mscs) are currently in clinical trial in us/internationally. Trend is toward use of fat derived mscs, havested by liposuction, isolated and concentrated in tissue culture. Early reports suggest significant improvement in neurological, autoimmune and organ functions. Many more mscs in fat than bone marrow making ad-msc the center of most research now.
No. Not at this time. Autologous chondrocyte implantation (aci) has been around for 25 years and involves harvesting cartilage cells, cloning them, then reinserting them back into the knee to produce new articular cartilage. This has been studied in labs, done on patients, and shown to work. There are no well done studies demosntrating the effectiveness of stem cells to repair knee cartilage.