Compare concentrations of MSC's in patients with pre collapse, post collapse osteonecrosis and "normal" hips. Patient-derived MSC will be quantified at the different stages of disease and assays will be performed in order to assess their viability. A control group will be selected in patients undergoing hip arthroscopy for femoral/acetabular impingement (FAI). Since FAI does not affect the intraarticular environment of the hip, theoretically these patients would have a "normal" concentration of synovial stem cells.