Purpose: The objectives of this in vitro study were to evaluate changes in the retention force of the indexed conometric system subjected to repeated insertion-removal cycles and to examine the elements (matrix and patrix) that cause retention changes in the system as a result of repeated cycles.
Materials and methods: Monolithic zirconia crowns were cemented on 24 matrixes and 24 patrixes fixed on the implant analog. The pull-out test was performed with 12 matrixes and 12 patrixes attached, and the initial retention forces were recorded. Six of them were subjected to five insertion-removal cycles (5-cycle group), and the other six samples to 15 cycles (15-cycle group), and the final retention forces were calculated. Then, in both groups, an element of each pair exposed to the cycle was matched by an exchanging with new pieces: 5 cycles of matrix-new patrix (5M/0P), 5 cycles of patrix-new matrix (5P/0M), 15 cycles of matrix-new patrix (15M/0P), and 15 cycles of patrix-new matrix (15P/0M); thus, subgroups were created. A pull-out test was applied to these subgroups, and retention forces were recorded. Data were subjected to a paired-samples t test (α = .05). Two specimens from the 15-cycle group were randomly selected and analyzed by scanning electron microscopy.
Results: In the 5-cycle group, the mean initial retention force was 153.13 ± 7.08 N, and the mean final retention force was 111.59 ± 18.30 N. In the 15-cycle group, the mean initial retention force was 143.30 ± 15.46 N, and the mean final retention force was 78.55 ± 17.03 N. There was a statistically significant loss of retention in both groups (P < .05). The change in retention in the 5P/0M and 15P/0M subgroups was not statistically significant (P > .05). However, significant loss of retention was found in the 5M/0P and 15M/0P subgroups (P < .05).
Conclusion: There was a significant loss of the retention force after insertion-removal cycles. The increase in these cycles caused a further decrease in retention force. The decrease in the retention force of this system mainly resulted from the changes in the surface characteristic of the frictional surfaces of the matrix.
