Abstract
Background: RSA cannot discern whether a single prosthesis is fixed or migrating below the detection level. Samples of patients usually show migration values that appear to be continuously distributed. Is there a dichotomy between stable and migrating prostheses?
Methods: We analysed the migration of 147 cemented acetabular cups of 7 different designs, by use of a new set of algoritms for frequency distribution analysis called Rmix. The migration vector lengths were assumed to be a compound of log-normal distributions. The algoritm then calculated if the observed frequency distribution is best explained by one or more log-normal distributions.
Results: After 2 years there was a significant dichotomy (p=0.006) between 2 lognormal subgroups within the sample. Neither cup design, sex or operating department could explain the dichotomy into two groups, which appears to reflect the existence of two different types of behaviour. The migration along the 3 axes in space, showed a similar dichotomy. During the second year, around 80 % of the patients belonged to a distinct, normally distributed subgroup with a mean not different from 0 mm and a small variation, corresponding to the measuring error. The remainder differed significantly from this subgroup and showed migration.
Interpretation: The majority of the cups belonged to a subpopulation that appeared completely stableduring the second year. For a single type of prosthesis, the relative size of the stable subgroup might be a good index of the expected performance.
Correspondence should be addressed to EORS Secretariat Mag. Gerlinde M. Jahn, c/o Vienna Medical Academy, Alserstrasse 4, 1090 Vienna, Austria. Fax: +43-1-4078274. Email: eors@medacad.org
