422 THE JOURNAL OF BONE AND JOINT SURGERY We report a prospective, stratified study of 60 PCA-cups and 60 RM-polyethylene cups which have been followed for a median time of 90 months, with annual radiography. The radiological migration of cups was measured by the computer-assisted EBRA method. A number of threshold migration rates from 1 mm in the first year to 1 mm in five years have been assessed and related to clinically determined revision rates. A total of 28 cups showed a total migration of 1 mm or more within the first two years; 13 of these cups have required revision and been exchanged. The survival curves of cups which had previously shown early migration were considerably different from those without early migration. For cups with a migration of less than 1 mm within the first two years the mean survival at 96 months was 0.96 ± 0.02; for migrating cups, it was 0.63 ± 0.11 (log-rank test, p = 0.0001; chi-square value = 39.4). Early migration is a good predictor for late loosening of hip sockets. J Bone Joint Surg [Br] 1996;78-B:422-6. Received 31 March 1995; Accepted after revision 21 November 1995 One of the main problems in clinical outcome studies of hip replacement is that a follow-up of about ten years is required to obtain reliable information on revision rates. By the time that results are available the prosthesis may no longer be in use or may have been modified considerably. Before such evaluation is complete, many patients may have been given a prosthesis which has a risk of a poor outcome. There is a need to predict the results from a new prosthesis as early as possible. Freeman and Plante-Bordeneuve (1994) have shown the predictive value of vertical migration of femoral stems and several other authors have studied the migration of hip sock- ets by roentgen stereophotogrammetry (RSA) (Baldursson et al 1980; Mjöberg et al 1986; Snorrason and Kärrholm 1990; Mjöberg 1991; Onsten et al 1994). These studies sug- gest that hip sockets which migrate rapidly in the first two years are more likely to fail than those which do not. We have investigated the predictive value of socket migration on survival curves and revision rates. PATIENTS AND METHODS Between 4 January 1984 and 10 December 1985 in a pro- spective, randomised trial we implanted 120 hip prostheses in patients aged between 50 and 65 years with primary or secondary osteoarthritis, excluding cases of rheumatoid arthritis and osteonecrosis. They were stratified with regard to surgeon and implant, so that two experienced surgeons performed 60 operations, each implanting 30 Robert Mathys (RM) cups and 30 Porous-Coated Anatomic (PCA) cups according to a random-number protocol. The uncemented all-polyethylene RM cups (Morscher type; Robert Mathys Co, Bettlach, Switzerland) were combined with ME Müller straight stems (Protek AG, Berne, Switzerland) and the PCA cups with PCA stems (Howmedica, Rutherford, New Jersey). The hip arthroplasties were performed in an ultraclean laminar-flow operating room. All 60 PCA prostheses were implanted unilaterally in different patients. One patient had bilateral RM cups, and this group therefore included 59 patients. For this report we studied only the cups, and all data refer to these. The patients had annual follow-up examinations includ- ing hip radiography and completion of a questionnaire which since 1991 has used the Standard System of Termi- nology for Reporting Results (Johnston et al 1990). The median follow-up time is now 97 months (interquartile range 78 to 110). In this period 14 patients have died and 20 have had a revision operation for aseptic loosening of the cup. Excluding these 34 cups, the median follow-up is 104 months (interquartile range 91 to 112). For 60 of these cups EARLY MIGRATION PREDICTS LATE ASEPTIC FAILURE OF HIP SOCKETS M. KRISMER, B. STÖCKL, M. FISCHER, R. BAUER, P. MAYRHOFER, M. OGON From the University Orthopaedic Hospital, Innsbruck, Austria M. Krismer, MD, Senior Registrar B. Stöckl, MD, Registrar M. Fischer, MD, Registrar R. Bauer, MD, Professor of Orthopaedics M. Ogon, MD, Senior Registrar University of Innsbruck, Anichstrasse 35, A-6020 Innsbruck, Austria. P. Mayrhofer, PhD, Assistant Professor Department of Geometry, Institute of Mathematics and Geometry, Univer- sity of Innsbruck, Technikerstrasse 13, A-6020 Innsbruck, Austria. Correspondence should be sent to Dr M. Krismer at the Orthopädische Uni- versitätsklinik, Anichstrasse 35, A-6020 Innsbruck, Austria. ©1996 British Editorial Society of Bone and Joint Surgery 0301-620X/96/31168 $2.00