Retentive Force of Zirconia Copings Luted on Zirconia Abutment Analogs

Method: We manufactured 240 zirconia implant abutmen-analogs with 4-taper-, 240 with 8-taper-degree, and 2400 zirconia copings. Subgroups of the copings were inside air-abraded with 50µm, or 120µm grain size, or had an inner surface as machined. Copings were luted using zinc-oxide-phosphate, glass-ionomere, self-adhesive-composite, eugenol-free provisional, and composite cement with plastifizer according to a standardized protocol. 8 specimens per group were axially pulled off (1mm/min) after 24h or thermal-cycling (6,000 x 5°C/55°C). Mean and SD of the retentive forces were calculated; Bonferroni post hoc tests were performed (α=0.05).

Result: The highest retentive forces were achieved with a 4-taper degree for zinc-oxide-phosphates (400N), self-adhesive composite cements (310N) or glass-ionomers (260N). Provisional cements (90N) and composite cements containing a plastifier (45N) showed significantly lower retentive forces. Thermal cycling further reduced (down to zero for provisional cements) the retentive forces. A rougher inner surface did not significantly improve the retention forces of all investigated luting agents.

Conclusion: Only cements setting as a solid body showed a clear relationship between retentive force and taper degree after 24 water storage and after thermal-cycling. Higher roughness did not significantly increase retention. Provisional (viscous, elastic) luting agents were highly affected by thermal cycling, and can completely loose their retentiveness.