1. Introduction
Nowadays, implantation in dentistry has become one of the predictable treatments [1, 2, 3, 4]. It is not possible to achieve an absolute passive fit between the implant and the prosthesis; however, the first step to minimizing this misfit is an accurate impression [8, 9 ,2]. Impression accuracy is affected by various factors [4, 10, 11, 12]. Various studies have investigated the effect of implant positioning angle on impression accuracy. Farronato showed that implant angulation is the important parameter for the impression accuracy [13]. Increasing the implant angulation reduces the impression accuracy [14]. Implant positioning angle up to 15 degrees has no significant effect on impression accuracy [16]. The exact placement of impression coping for implantation is very important. Therefore, this study aims to investigate the effect of implant angulation on the accuracy of placing impression coping on implants with internal hex.
2. Materials and Methods
In this experimental study, a gypsum model with four implants mounted at different angles was first prepared. To prepare the model, first three layers of base plate wax were glued together. Long needles were placed at four points axially to reconstruct the zero-degree angle. In the first point, the implant was placed parallel to the first needle and at a zero-degree angle. In the second, third and fourth points, angles of 15, 25 and 35 degrees were reconstructed respectively. Four wax circles with a diameter of 10 mm were placed on the implants. The box walls were reconstructed using wax and poured with dental stone. The wax was then removed from the implants and replaced with a gingival analog.
During a training session, 40 final-year students were taught how to properly place the impression coping on the implant. They were asked to place four closed-tray impressions on four implants at different mesiodistal angles. After removal of gingival analog, the correct or wrong placement of impression coping was evaluated by a specialist in prosthetics. The collected data were statistically analyzed using Cochran Q test and Wilcoxon test.
3. Results
The number of correct or incorrect cases of impression coping placement on the implant is shown in Table 1.


Based on the results of Cochrane Q test, there was a statistically significant difference in performance among four angles (P=0.001). Based on the results of pairwise comparison, no significant difference was observed between the two groups of 0 and 15 degrees (P=0.16), while there was a significant difference between other groups (P<0.05).
4. Discussion
As mentioned before, the first step to achieve a passive fit between the implant and the prosthesis is the accurate impression [6, 10, 17, 18] Angled implants reduces the impression accuracy [4 ,17, 19] There is no general principle regarding the protocol for impression of angled implants [18].
The results of the present study showed that with the increase of implant angle, the probability of error in accurately placing the impression coping increases. Other studies have shown that the implant angulation up to 15 degrees has no effect on impression accuracy [8, 12, 20, 21]. According to the results of the present study, mesiodistal placement angle up to 15 degrees had no significant effect on the accuracy of impression coping placement on the implant. Shim investigated the effect of mesiodistal and buccolingual angles of the implant on dimensional accuracy of the impression and stated that the mesiodistal angle up to 15 degrees had no significant effect on the dimensional accuracy of impression, but the buccolingual angle of 15° reduced the dimensional accuracy of impression [12]. Increasing the mesiodistal angle of the implant by more than 15 degrees reduced the accuracy of the impression coping placement and had a negative effect on the dimensional accuracy of impression. There was also an inverse relationship between the depth of implant placement and the accuracy of impression coping placement [22].
5. Conclusion
The implant angulation has a significant effect on the accuracy of impression coping placement on the implants with internal hex; by increasing the implant angulation, the accurate placement of impression coping decreases. 

Ethical Considerations
Compliance with ethical guidelines
This article is approved by the Ethics Committee of Arak University of Medical Sciences (Ethics ID: IR.ARAKMU.REC.1397.283).

Funding
Research Vice-Chancellor of Arak University of Medical Sciences sponsored this study.

Authors' contributions
All authors contributed equally in preparing all parts of the research

Conflicts of interest
The authors declared no conflict of interest.

Acknowledgements
The authors are grateful to Arak University of Medical Sciences for the technical support of this research study.


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