Effect of Artificial Aging and Cyclic Loading on Reverse Torque Values and Survival of Zirconia Implants with Carbon Fiber-Reinforced PEEK Polymer Screw

Abstract

Statement of problem: The introduction of carbon fiber-reinforced PEEK polymer screw in two-piece zirconia implants, facilitated the development of “metal-free" implant solutions. However, the literature lacks studies on the performance of such a screw joint assembly, particularly regarding reverse torque values. Purpose: To evaluate the effect of artificial aging and cyclic loading on reverse torque values and survival rates of carbon fiber-reinforced PEEK polymer screws in two-piece zirconia. Materials and Methods: A total of 20 implants were divided into two main groups. The testing group utilized 10 zirconia implants (NobelPearl Tapered RP 4.2 x 10 mm, Nobel BioCare, Kloten, Switzerland) with carbon fiber-reinforced polymer screws (VICARBO®, NobelPearl, Nobel BioCare, Kloten, Switzerland) to retain the zirconia crown cemented on zirconia abutments (NobelPearl abutment straight IX RP 1). The control group used 10 titanium implants (NobelReplaceTM Conical Connection, RP 4.3 x 10 mm, Nobel BioCare, Kloten, Switzerland) with titanium alloy screws (Clinical Screw Conical Connection, Nobel BioCare, Kloten, Switzerland) to retain zirconia crowns cemented on titanium abutments (Snappy abutment 5.5 CC RP 1.5, Nobel BioCare, Kloten, Switzerland). Implants were cemented into twenty glass cloth reinforced epoxy resin cylinders (G10, National Electrical Manufacturers Association, Rosslyn, Virginia, USA) using a dual-cured resin (Rock Core, Zest dental solutions, California, USA). All zirconia crowns were digitally designed to an identical shape maxillary central incisors, milled from zirconia blanks (IPS e.max ZirCAD Prime 25 x 98.5mm, Ivoclar, Vivadent USA), sintered, and then cemented with resin cement (Panavia V5, Kuraray Noritake, Okayama, Japan). A total of 60 screws were used in 6 groups. In the first two groups (T24 and Z24) a torque limiting wrench (NobelPearl Manual Torque Wrench, Nobel BioCare, Kloten, Switzerland) was used to deliver the manufacture's recommended torque values to all the samples, followed by another torque after 20 minutes. After 24 hours, a digital torque gauge (Chatillon Model DFS2-R-ND, Ametek, Largo, FL) was used to record the reverse torque values for all the samples. A new set of screws was then used for the next two groups (Tth and Zth). The same torque protocol was followed, and samples were subjected to thermocycling in 5 C, 55C distilled water baths for 10,000 cycles, and a dwelling time of 2 minutes. The reverse torque values were recorded again using the same digital torque gauge. The last two groups (Tcl and Zcl) received another new set of screws, and the same torque protocol was followed. After that, the samples were mounted at a 30° to the cyclic loading machine to be subjected to one million cycles of 200N load at a rate of (2 cycles/second (Hz)). When a failure was detected, the number of cycles at the time of discovery and failure mode were recorded. Preload efficiency values were calculated to standardize the reverse torque values of groups (T24,Z24,Tth and Zth). To compare the means of preload efficiency, a two-way analysis of variance was done followed by Tukey-Kramer post hoc analysis. The difference in number of cycles to failure between groups (Zcl and Tcl) was evaluated using the LIFEREG procedure of SAS. Results: Preload efficiency was 74.96% for T24, 72.17% for Z24, 76.4% for Tth and 63.96% for Zth. Results of the 2-way analysis of preload efficiency showed that the interaction was statistically significant (P = 0.0010), therefore, the pairwise comparisons between the 4 subgroups (T24, Z24, Tth and Zth) were adjusted by Tukey testing, which indicated that the preload efficiency of the zirconia group after thermocycling (Zth) was statistically significantly different from all other groups. The LIFEREG procedure of SAS did not detect a statistically significant difference in the number of cycles to failure between the groups (Tcl and Zcl). Furthermore, three zirconia implants, eight zirconia abutments, and two PEEK screws fractured during cyclic loading. One titanium implant exhibited minor deformation, while four titanium abutments and five titanium screws fractured. Conclusion. The preload efficiency of zirconia implants with PEEK screws was comparable to the titanium group after 24-hours of initial torque. However, after artificial aging, zirconia implants with PEEK screws exhibited a statistically significant decrease in preload efficiency compared to all other groups. No statistically significant difference was found in the number of cycles to failure between the zirconia and titanium systems.