Retentive force of telescopic crowns combining fiber-reinforced composite and zirconia

Objectives: CAD/CAM technology might facilitate manufacture of double crowns. Zirconia primary crowns were already successfully applied in vivo. Recently, fiber-reinforced composite (FRC) has been promoted as a digitally fabricated alternative for secondary crowns (SCs). Inherent FRC resiliency might be problematic for telescopic retention. Therefore, retention behavior of zirconia primary with FRC SCs was studied before and after mechanical aging.
Methods:
Zirconia primary crowns were produced with a nominal convergence angle of 0°. Forty-eight SCs were milled from FRC and divided into three study groups (n=16/group) differing in (i) milling parameters and (ii) post-milling adjustment. In group 1 milling parameters were adjusted for tight initial fit, in groups 2 and 3 for improved initial fit (offset -20µm in contrast to group 1). Whereas inner surfaces of the SCs were polished with diamond paste in group 1 and 2, silicon points were used for group 3. In a universal testing machine, SCs were fitted with different force magnitudes (25-100N) and retentive forces were measured during removal from the primary crowns. These test were repeated after an aging simulation (10,000 insertion/removal cycles). Test data were compared using ANOVA and Tukey post-hoc tests.
Results: It was possible to realize intial retentive forces of approximately 10N (valid range from 5 to 15N): 9.4±3.1N (group 1), 7.5±1.8N (group 2), 9.2±2.5N (group 3). Tight initial fit (group 1) required most post-milling adjustment time (mean 13.1min/crown). Use of silicon points with improved initial fit (group 3) resulted in least adjustment time (mean 8.9min/crown). There was a significant change in retention after aging in all groups. Retention decreased in group 3 but tended to increase in group 1.
Conclusions: Adequate initial retentive force can be achieved with telescopic crowns combining zirconia and FRC. For acceptable long-term retention, however, milling parameters and post-milling procedures need to be finetuned.