In contrast, Roentgen Stereophotogrammetric Analysis(RSA)can detect the presence or absence of mobility between intervertebral segments (i.e. functional stability of the fusion) with a high degree of accuracy. RSA is a radiographic technique that uses small tantalum balls implanted into the patient's bone to measure micromotion (<1 mm) at the bone - implant interface, something which is not possible to do reliably using regular x-ray techniques. RSA provides a method for detecting inferior implant designs or cement formulations using only a small number of patients before release of these products into large clinical trials.