Method
Changes of the removal torque occurred when the abutments were screwed in with CHX gel and loaded with 10,000 cycles in the test carrier filled with saliva substitute. The test setup was consisted of three different examination groups (dry, Sialin-Sigma, Sialin-Sigma + CHX-gel), in which the abutments were all loaded with 250N for 10,000 cycles in the testing machine (Zwick/Roell Z010 TN, Zwick GmbH & Co. KG, Ulm, Germany). By embedding the implants in cold-curing polymer (Combipress N/LM, Merz Dental, Lütjenburg, Germany) with the aid of a drill chuck, it was tried to place them as perpendicularly and reproducibly as possible. In order to achieve a perpendicular rotation of the abutment screws, the torque-measuring device with the hex wrench (Ratchet, BEGO Implant Systems, Bremen, Germany) fixed therein was clamped in a drill stand to turn the abutment in and out.
The abutments were loaded vertically. The setting of the tests was edited using the test software testXpert II (Zwick GmbH & Co. KG, Ulm, Germany). Due to technical problems, two tests could not be evaluated because the test machine aborted several times. The technical problem was the result of a loosened screw of the protective door. Within group 3, an abutment screw clamped with CHX gel using the hex wrench. One point of criticism of this experimental design was that all implants were embedded vertically in prosthesis articulation and the abutments were loaded perpendicularly without a prosthetic restoration by a flat plate. In comparison to other studies, such as the in vitro study by Zipprich et al. in which titanium balls of 4 mm diameter were cemented on the abutments [3].
In the study by Koutouzis et al., the dynamic load was performed by a masticatory simulator and transferred to the abutments at a 30°-angle through a stainless steel stylus [15]. The angle of 30° was intended to reflect the cusp inclination of the molars and thus transfer the stress as similar as possible to force transmission in the oral cavity. Regarding the study design, it is also to be criticized that only implant-abutment connection with an internal hexagonal compound was tested in the examination groups.
The results showed that the removal torque is always less than the insertion torque. In the two-group comparison using the Mann–Whitney U test, the results were not significant. To illustrate the results, the mean values of the three study groups were determined (Fig: 4). The control group had the smallest range of values. The maximum-value in this group was 23 Ncm and the minimum-value 18 Ncm. The maximum-value of group 2 was 24 Ncm and of group 3 23 Ncm. The minimum-value of group 2 and 3 were 14 Ncm and 12 Ncm, respectively. Butignon et al. and Khraisat et al. showed in their in-vitro study that the removal torque significantly decreased after cyclic loading[16, 17]. By scanning with an electron microscope, structural changes of the abutments, such as wear and structural losses were compared before and after the cyclic loading [16].
The flattening of microscopic unevenness leaded to a circumvention of the friction. The relaxation of the contact surfaces was also described as a "settling-effect" and resulted in a reduction of the preload, since part of the applied torque was used to circumvent the friction [13, 18].
Due to the great influence of the implant-abutment connection on the long-term success of implants, the clinical importance of this potential weak spot is high. The unavoidable micro leakage between the implant and the abutment allows penetration of periodontal-pathogenic bacteria, which could lead to an inflammatory reaction of the surrounding tissue and in worst-case cause the loss of the implant [19]. Ghannad et al. showed in their study that the application of 1% CHX gel into the abutment screw hole in morse-taper implant systems leads to a significant reduction of bacteria over a period of 110 days [20].