 RSA is a highly accurate radiographic technique developed in Lund, Sweden and has for some time been the gold standard in Europe for assessing the stability of implants within bone Selvik 1990, Ryd 1992, Catani et al. 1998, Alfaro-Adrian et al. 1999. It is at least ten times more accurate than conventional radiography Ryd et al. 1986. RSA involves the insertion of small tantalum markers 0.8mm into the bone and prosthesis at the time of surgery Ryd 1986, Ryd et al. 1986, Ryd et al. 1989, Selvik 1990, Ryd 1992. By using a calibrated cage with additional markers and simultaneous bi-planar x-rays, micromotion at the prosthesis-bone interface can be very precisely determined Selvik et al. 1983. Two types of micromotion can be investigated: migration, i.e. gradual motion over time, and inducible displacement, i.e. instant motion in response to external forces Ryd et al. 1986. The pattern of micromotion exhibited by various prostheses and fixation techniques within the first post-operative year has been shown to be predictive of the long-term fixation of the component to bone, especially when considering the tibial prosthesis-bone interface Ryd 1986, Ryd et al. 1986, Ryd 1992, Ryd et al. 1995. The value of this precise radiographic technique is that new technologies and techniques can be introduced in vivo while limiting their exposure to as few patients as possible. RSA is a proven, safe and invaluable method for assessing methods of tibial component fixation as well as different design concepts Ryd et al. 1987, Nilsson et al. 1991, Ryd et al. 1992, Ryd et al. 1993, Hilding et al. 1995, Onsten et al. 1998, Nilsson et al. 1999. 