Worldwide, millions of total hip and knee replacements are inserted every year and new designs are introduced continuously, sometimes with catastrophic results. In 1995, Henrik Malchau (Malchau 1995) presented an algorithm for optimal introduction of new implants. The algorithm was based on 15 years of clinical research including the Swedish Hip Arthroplasty project and the development of radiostereometric analysis (RSA) to evaluate implant fixation. Bengt Mjöberg, one of the pioneers of RSA, used implant migration as detected by RSA to define loosening (Mjöberg 1986, 1991). He compared contrast and radionuclide arthrography, bone scintimetry, and RSA and concluded that RSA was the most sensitive method for detection of loosening. He also noted that implant migration over time was more sensitive for detection of loosening than prevocational investigations with RSA. He suggested that mechanical loosening should be defined as early migration after the operation. 10 years later, sufficiently large patient materials were available to study the clinical relevance of early implant migration measured with RSA (Kärrholm et al. 1994, Ryd et al. 1995). These 2 studies evaluated the predictive value of early migration for the risk of subsequent revision of a cemented matte stem and different designs of tibial components. The hypothesis that early migration could be used as a surrogate variable for clinically relevant loosening was confirmed. These findings were supported by observations of a high degree of early migration for certain implant designs, which after longer follow-up showed high clinical failure rates (Mogensen et al. 1982, Snorrason and Kärrholm 1990, Thanner et al. 1995).