الفهرس 
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Abstract
This in-vitro study was designed to evaluate the accuracy of 4 intraoral scanners regarding different span lengths.
Hence the accuracy of intraoral scanners are described according to ISO 12836 as trueness and precision. the deviation of measurements between the reference model and the intraoral scan model is termed the ‘trueness’ of impression technique, and the deviation of measurements between digital models of the same intraoral scanner is ‘precision’ of impression technique.
The aim of the study was to evaluate the effect of different span lengths and different convergence angle of axial walls in teeth preparations on the accuracy of four intraoral scanners.
The preparations were done on a typodont acrylic model (Nissen, Kyoto, Japan), with the aid of computer numerical control machine (CNC machine).
Four intra oral scanners took part in our study the CEREC AC Omnicam (Sirona Dental, GmBH, Germany), the Trios (3Shape, Copenhagen, Denmark), the Medit I500 Dent-Core (Seoul,s.korea) and the CEREC Primescan (Dentsply Sirona, Bensheim,Germany).
For the trueness measurement all the prepared casts are scanned with a desktop scanner inEos X5 (Dentsply, Sirona) to get a reference model.
All the preparations were scanned with the intraoral scanners 7 times (n=7) with each scanner and then with the help of a reverse engineering 3D analysis software used to superimpose the scans and calculate the amount of deviation between them.
For the precision measurement the calculations are performed intra division which means every scan was considered the reference model once and all the remaining scans were superimposed on it.
A color-difference map (qualitative data) and a report (quantitative data) of the projections were obtained for every single superimposition process.
Data was recorded, tabulated and analyzed. Statistical work was done using t test.
Our results showed that:
1- Significant difference in the trueness of different types of scanners (p<0.001). The highest value was found in Omnicam (35.70±8.35), followed by Medit i500 (32.58±13.08), then 3Shape TRIOS (32.33±12.19), while the lowest value was found in Primescan (32.26±9.57).
2- A statistically significant difference in the trueness for different span lengths (p<0.001). The highest value was found in second molars (47.42±3.94), followed by second premolars (28.42±3.78), while the lowest value was found in canines (23.80±3.85).
3- A statistically significant difference in the trueness for different convergence angles (p<0.001). 6° (35.45±10.88) had a significantly higher value than 10° (30.98±10.62).
4- There was a significant difference in the precision of different types of scanners (p<0.001). The highest value was found in Omnicam (29.84±3.89), followed by Medit i500 (28.04±2.94), then 3Shape TRIOS (25.64±3.11), while the lowest value was found in Primescan (24.69±5.25).
5- A statistically significant difference in the precision for different span lengths (p<0.001). The highest value was found in second molars (28.97±5.27), followed by second premolars (27.59±3.97), while the lowest value was found in canines (24.60±2.04).
6- A statistically significant difference in the precision for different convergence angles (p<0.001). 6° (29.13±3.43) had a significantly higher value than 10° (24.97±4.22).