## 4 Discussion
The changes of the alveolar bone dimensions can be measured manually with a periodontal probe or radiographically [45; 46; 47; 48]. Some previous studies used simple periapical radiography with custom made film holders, while other CBCT studies also used the same measurement points as we have [49; 50; 51]. Both methods were used for the analyses, and the same tendencies were observed after alveolar preservation compared to the control groups. CBCT was used in the present study for evaluation due to its low radiation exposure compared to its high resolution that allowed accurate measurements. Previous study used the same anatomical lines as reference at CBCT images [49]. The results of the vertical bone loss are nearly identical with the present study ([PERSON]: control--2.6 mm, graft--1.12 mm; present study: control--2.54 mm, graft--1.37 mm). Horizontal bone changes differ which can be the consequence that simple tooth extractions were performed contrary to surgical third molar removals. Several studies observed the same tendency: the main effect of alveolar preservation can be seen at the buccal site of the alveolar ridge, and it has a slight effect on horizontal ridge dimensions [45; 48; 49; 50; 51]. The exact result data might vary due to the used material and the location of the extracted tooth. The present study has justified previous results, and this fact might have significance for third molar surgeries when the tooth is deeply impacted. The most significant result of this study is the MBL gain at the distal site of the second molar when bone grafting was applied. CBCT measurements are reliable; they correlate with the manual periodontal probe measurement results [52]. The most obvious result is that bone grafting maintained the MBL. The interesting part was to observe if other factors had an impact on postoperative MBL or not. According to our result, in case of more than 3 mm loss on the distal site of the second molar or direct contact between the second and third molar without persisted interalveolar septum bone grafting should be considered, otherwise it is not required. It needs to be clear that more than 3 mm preoperative MBL loss can be measured only on radiographic image, but not by periodontal probing. The presence of impacted third molar hinders the real bone loss. The significance of preoperative measured MBL loss can be revealed during surgery. When the alveolar was left empty, the MBL remained at almost the same level behind the second molar. In the control group, eight patients had more than 3 mm MBL loss preoperatively, meaning high risk of postoperative pocket formation. Periodontal pocket formed in five patients at the one-year follow-up, meaning 62.5% pocket formation rate without alveolar preservation when preoperatively MBL loss was more than 3 mm. Previous study examined the effect of preoperative periodontal status and oral hygiene [17]. The MBL loss at the distal site of second molar was 5.4 \(\pm\) 0.8 mm 6-36 months after third molar surgical removal without alveolar preservation. Mesioangular third molar position, preoperative radiolucency around the tooth and inadequate plaque control increased MBL loss. This value is remarkably higher than the control data of present study, further emphasizing the importance of bone augmentation when preoperative bone loss occurs. In the present study third molar position did not show significant correlation with MBL loss, which might be the result of low sample size. When the alveolar was filled with bone graft, the mean MBL loss was reduced to 1.24 \(\pm\) 0.55 mm from 4.66 \(\pm\) 2.3 mm. preoperatively, eight patients had more than 3 mm MBL loss, equal to the control group. At the 1-year follow-up none of them had pocket at the distal site of second molar. Other study claimed bone grafting did not have effect on MBL loss after two years, however, preoperative MBL loss was less than 2 mm both in control and grafted groups. [20]. It may enforce the observation that alveolar preservation is not necessary when preoperative MBL loss is less than 3 mm.