We assessed the migration and movement between the half-bearings of the Interax prosthesis in 18 patients using roentgen stereophotogrammetric analysis (RSA). This study showed movements of the tibial component even in successful knee arthroplasty: 17 prostheses migrated only initially and remained stable after 1 year, whereas 1 prosthesis migrated consistently throughout 3-year follow-up. RSA revealed no or minimal displacement between the half-bearings until the 4-year follow-up, confirming that their fixation to the baseplate was adequate. In the case of loosening, a rotation about the longitudinal axis of 1.3 degrees and a medial-lateral translation of 0.5 mm was observed between the half-bearings. Cold flow was revealed by RSA in the posterior region of the medial half-bearing.