The study demonstrates the probability of intrusion of each root of the maxillary molar teeth into the MS in a sample of the Egyptian population that was not involved in any previous study. This alerts the operator to the probability of undesirable complications of oroantral communication allowing him to take all the appropriate precautions before carrying out any invasive procedure in the maxillary posterior region.
Patients were selected to have at least two normally erupted upper molars on each side to include the largest number of roots within the smallest acceptable sample size. Only patients older than 23 years were included to ensure the complete eruption and formation of both the upper molars and MS [31, 32].
Premolars were not involved in the current study since many studies performed during the last decade agreed that the premolars roots are rarely in contact with the MSF and they represent the lowest risk of oro-antral communication [14, 15, 18, 23, 25, 29, 33,34,35,36]. Roots with chronic periapical lesions and root abnormalities were excluded to avoid any confusion in the observation. Supernumerary and supplemental teeth were excluded because of their confusing abnormal shapes and positions.
Among all radiographic techniques, CBCT was the imaging technique used in this study because of its tomographic nature, high contrast and accuracy in evaluating the sinus floor without any superimposition, and magnification or blurring in comparison to conventional imaging [7, 10, 27]. The i-CAT Next Generation scanner with its proprietary software was used in image acquisition and observation in this study following the track of many other studies, because of its availability, high image resolution, and ease of manipulation [19, 27, 29, 30, 37].
The relation between the MSF and the root tips of the maxillary posterior teeth was categorized by many studies [8,9,10,11,12,13,14,15,16,17,18,19,20,21, 23, 24]. The classification used in the current study was previously documented by many authors in different studies [16,17,18,19,20,21]. It is represented in class 1: root tip is outside the sinus, class 2: root tip is against the sinus wall, and class 3: root tip is in the sinus. This scoring system was implemented in this study due to its simplicity.
A popular and more complicated classification proposed by Kwak et al.  and implemented by Razumova et al.  and Ali et al.  is represented as type I: the inferior wall of the MS is located above the root apex of the buccal and palatal roots, type II: the inferior wall of the MS is located below the level connecting the buccal and palatal root apices without an apical protrusion over the MS, type III: an apical protrusion of the buccal root apex is observed over the inferior wall of the MS, type IV: an apical protrusion of the palatal root apex is observed over the inferior wall of the MS, and type V: apical protrusions of the buccal and palatal root apices are observed over the inferior wall of the MS.
The interrater reliability between the observers upon the images was high because of the high image quality, standardization of all exposure factors, and the good experience of the three observers in the field of oral radiology especially CBCT images.
The result of the present study revealed that, from a total of 342 maxillary molar roots of the Egyptian population, 35.3% was located outside the sinus, 23.4% contacted the sinus floor, and 41.2% protruded the sinus. While Estrela et al. , who implemented a closely related scoring system in studying the Brazilian population also using the i-CAT Vision CBCT software (Imaging Sciences International, USA), reported that 25.16% out of 600 maxillary molar were located outside the sinus, 40.67% contacted the sinus floor, and 34.17% protruded the sinus. This difference in the results is related to difference in populations and sample size.
Pagin et al.  performed another study on the Brazilian population to assess the sinus-root relation also using i-CAT Vision CBCT software (Imaging Sciences International, USA). They reported that from a total of 601 maxillary tooth root 130 roots (21.6%) were in close contact with the MSF without sinus floor elevation which is considered a very close percentage to that of the current study that was 23.4%. While 86 roots (14.3%) were protruded into the sinus producing an elevation of the bony cortex and that was far away from the percentage of the current study that was 41.2%. This difference may be related to the difference in populations and sample size.
Haghanifar et al.  used on Demand CBCT software (Cybermed, Korea) to categorize the sinus-root relation in the Iranian population. They reported that from a total of 419 maxillary molar, 23.9% were located outside the sinus, while the current study reported that 35.3% of the roots were outside the sinus. They reported that 66.6% contacted the sinus floor which is considered a very high percentage compared to that of the current study represented in 23.4%. Only 9.5% protruded the sinus and this is a very low percentage compared to that of the current study represented in 41.2%. This obvious discrepancy between the two studies is related to the difference in populations, sample size and CBCT software used in observation.
Zhang et al.  studied the sinus-root relation in the Chinese population on a total of 800 right and left first and second molars using CBCT images without mentioning the employed software. They nearly agreed with the current study in the percenatge of the roots contacting the sinus floor where they reported 21.75% and this study reported 23.4%. The major difference between both studies was represented in the percentage of roots located outside the sinus and intruding the sinus. They reported that 57.1% of these teeth were placed outside the sinus and 21.23% protruded the sinus, while the percentages of the current study was 35.3% and 41.2% respectively. The discripancy between both studies in reporting the sinus-root relation may be due to differences in populations, manipulated softwares, and sample size. Moreover, they involved only the first and second molars while the current study included the three maxillary molars.
A study implemented by Mattar et al.  that involved panoramic images of 266 premolar and molar tooth from a sample of the Saudi Arabian population reported that 37.6% of the teeth were outside the sinus, 20.3% contacted the sinus floor, and 42.1% of the teeth protruded the sinus. Despite the difference in sample size, technique used, and the involvement of the premolars, these results are considered the closest of all previously mentioned ones to ours. This high degree of agreement between the two studies may be related to the similarity of both populations being both Arabian and exhibiting similar circumstances.
The current study reported that—in a sample of the Egyptian population—gender is an influencing factor to the intrusion of roots into the sinus where the involved probability is significantly higher in males than in females. The result of this study agreed with Shokri et al.  and Haghanifar et al.  who reported that males are more susceptible to root protrusion into the sinus than females of the Iranian population. Also Kilic  reported the same result; however, he did not specify a certain population. While Gu Yehen et al.  and Pei et al.  who evaluated the relationship between the maxillary posterior teeth and MSF in the chinese population reported the little influence of gender on this relation.
Despite approaching the traditional threshold of statistical significance, we cannot assert that the left molars are of higher probability of intrusion into the sinus in relation to the right ones in the involved sample of the Egyptian population. A number of studies reached the same result including Shokri et al. , Kilic , and Pei et al. .
Concerning the probability of intrusion of each molar as a whole, the p value approached the threshold of statistical significance but did not reach it. Therefore, we also can not assert that the second molar has the highest probability of intrusion into the sinus in the involved sample of the Egyptian population although it exceeded the percentage of the third and first molars by 3% and 15% respectively. Georgiev et al.  also reported that the maxillary second molar was the most common teeth to project into the sinus in patients from the Varna region in Bulgaria.
The influence of the type of root on its probability of intrusion into the sinus is very limited as the three types (mesiobuccal, distobuccal, and palatal) had very close intrusion probabilities and the p value was far away from the threshold of statistical significance (p = 0.869).
As for the individual root with the highest probability of intrusion, the current study reported that the mesio-buccal root of the right third molar is the most frequent root to be inside the sinus and the mesio-buccal root of the right first molar is the least frequent. However, many studies recently carried out agreed that the mesiobuccal root of the maxillary second molar is the closest root to the MSF among all maxillary teeth roots [8, 9, 15, 19, 23, 28, 30, 34, 36, 38, 39].
While other studies agreed that the distobuccal root of the maxillary second molar is the closest to the MSF with the highest risk of oro-antral communication [25, 26, 33, 40, 41]. However, Kaushik et al.  and Didilescu et al.  concluded that the roots of the first maxillary molar are the nearest to the MSF among all posteriors. Didilescu et al.  specified the palatal root of the involved tooth with this close relation.