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.

Aims: To prospectively study the optimum fixation of the tibial component in patients younger than 65 years, where the mode of fixation is randomized. Methods: 35 patients (mean age 56 years, range 29 to 64) were operated with the Profix (Smith& Nephew) TKA due to gonarthrosis grade III to V. The patients were randomly allocated to fixation of the tibial component with cement (Group C) (n=6), uncemented fixation with hydroxyapatite coating (HA) without screws (Group HA−) (n=14), or uncemented fixation with HA coating and with screws (Group HA+) (n=15). Radiostereometry (RSA) was performed postop., 3, 12 and 24 months postop. Results: There were no complications or revisions during the follow-up. Up to 3 months the cemented implants migrated the least and the HA- group the most (P = 0.009 – 0.036). From 3 to 24 months however, the implants in all three groups displayed very small migration, magnitudes well below the detection limit of RSA, and there were no differences between the three groups. Conclusions: The uncemented tibial component displays relatively large migration within the first 3 months compared to the cemented implant, and uncemented fixation without screws have larger migration than when screws are used. This larger initial migration for the uncemented fixation probably is due to “setting-in” of the prostheses. However, if the uncemented HA-coated implant “survives” this early period, the results of the present study indicate a good long term prognosis, even in designs where no screws are used for additional stability.

Aims: To determine the in vivo movements between the polyethylene and the metal tibial tray in modular fixed bearing TKA. Methods: 16 patients (median age 72) operated with the NexGen TKA were studied. The metal tray of the tibial component was equipped with 5 tantalum markers, and the polyethylene insert with 6 markers. Radiostereometric (RSA) investigation was performed within one week postop., and at 12 months. Change in position of the poly insert in relation to the metal tray between the postop and the 12 months investigation was analyzed. Insert motion index (IMI) = √AP² +ML2 was also calculated where AP is the largest AP-translation and ML the largest ML-translation. Results: The polyethylene component rotated a median 0.55° (0.09° to 1.21°) in relation to the metal tray in the horizontal plane, mostly externally, corresponding to a median IMI of 415 μm (19 to 920 μm). Movements of the polyethylene out of the plane of the tibial tray were very small and generally below the precision of the RSA method. Conclusions: This study shows for the first time that movements do occur in vivo between the polyethylene insert and the metal tray in modular fixed bearing TKA. Almost all movements occur in the horizontal plane of the metal tray and very little movements are detected in directions out of this plane. The magnitudes of IMI found in vivo study are equivalent to those found in vitro in studies of explanted components.