A randomized RSA study concentrating especially on continuous migration

Abstract

Background and purpose - There is a general call for phased introduction of new implants, and one step in the introduction is an early evaluation of micromotion. We compared the micromotion in the Triathlon and its predecessor, the Duracon total knee prosthesis, concentrating especially on continuous migration over 5 years of follow-up. Patients and methods - 60 patients were randomized to receive either a cemented Triathlon total knee prosthesis or a cemented Duracon total knee prosthesis. 3-D tibial component migration was measured by radiostereometric analysis (RSA) at 3 months and at 1, 2, and 5 years. Results - There was no statistically significant difference in maximum total point motion (MTPM) between the 2 groups (p = 0.1). The mean MTPM at 5 years for the Duracon was 1.10 (SD 1.21) mm and for the Triathlon it was 0.66 (SD 0.38) mm. The numbers of continuously migrating prostheses were similar in the groups at the fifth year of follow-up; 6 of 21 prostheses in the Duracon group and 3 of 21 in the Triathlon group had migrated more than 0.3 mm between the second year and the fifth year of follow-up (p = 0.2). Interpretation - The Triathlon has a micromotion pattern similar to that of the Duracon total knee system at both short-term and medium-term follow-up, and may therefore, over time, show the same good long-term mechanical stability.

Figure 1.

Consort fl ow chart

Figure 2.

Box plots of median MTPM of the Triathlon group (left) and the Duracon group (right) at 1 year, 2 years, and 5 years. The horizontal hatched line denotes 0.5 mm. The central red line denotes the median value. The box denotes the second and third quartiles. The whiskers denote the range, excluding outliers. Dots denote outliers.

Figure 3.

The numbers of patients who were classifi ed as stable at 1, 2, and 5 years were 21, 31, and 34, respectively, depending on the choice of method: MTPM <0.5 mm at 1 year according to Pijls et al., MTPM <0.2 mm between 1 and 2 years according to Ryd et al., or MTPM <0.3 mm between 2 and 5 years according to Ryd et al. 42 RSA assessments (n = 60) were complete at all 3 follow-ups. 16 baseplates were stable and 3 were continuously migrating at all 3 follow-ups. 18 were excluded because of missing data at any of the follow-ups or all of them. Fisher’s exact test: p-values <0.05 lead to rejection of the null hypothesis. There was no difference in prediction whether made at 2 years or 5 years.

Figure 4.

The numbers of patients classifi ed as having continuously migrating prostheses at 1, 2, and 5 years were 21, 11, and 8, respectively, depending on the choice of method: MTPM <0.5 mm at 1 year according to Pijls et al., MTPM <0.2 mm between 1 and 2 years according to Ryd et al., or MTPM <0.3 mm between 2 and 5 years according to Ryd et al. 42 RSA assessments (n = 60) were complete at all 3 follow-ups. 16 baseplates were stable and 3 were continuously migrating at all 3 follow-ups. 18 were excluded because of missing data at any of the follow-ups or all of them. Fisher’s exact test: p-values <0.05 lead to rejection of the null hypothesis. There was no difference in prediction whether made at 2 or 5 years.