When implants are placed in the maxillary anterior area, aesthetics is the most considerable factor. But it is quite difficult to get an aesthetic result in the maxillary anterior area.

The primary reasons are not only the lack of alveolar bone of maxillary anterior portion and gingiva but also the histological and morphological difference of implant with natural tooth.1,2 In order to overcome these problems, both soft / hard tissue augmentation and abutment development, like as customized abutment and ceramic abutment, are being considered, which is contributed to esthetic enhancement.3-8

In terms of aesthetics, there are still problems such as gingival recession by bone resorption after implant placement or alveolar bone resorption. Many papers were already reported on gingival recession and bone resorption after implant placement or abutment and crown connection.9-15 Under the actual clinical situations, just after implant placement or prosthodontic restoration some parts of implant abutment or implant itself might be occasionally exposed due to alveolus or gingival recession.

Oh et al.15 reported that trauma during the treatment, occlusal force, peri-implantitis, microgap, biological width and crestal module are responsible for the bone resorption. Hermann et al.16 emphasized the microgap around restoration margins, which allowed germs and other bacteria to infiltrate implant screw damaging tissues surrounding implants.17,18 Broggini et al.19 also notified that inflammation lasted in the contact part between fixture and abutment. Others also informed biologic width was the major factor inducing bone resorption.20-22 Lindquist et al.9 mentioned smoking and bad oral hygiene. According to many results, bone resorption is induced not because of specific factors but because of comprehensive causes.

Level of bone resorption varies to some extent after implant second surgery or abutment connection. It is about 1.5 - 2.0 mm and never exceeds 2 mm in 1 year. Under 0.2 mm a year is regarded as an implant success.23

Radiographic results were used for the evaluation - same radiography in comparison with several pictures and standard measurement of a specific site on implant or using computer program.24,25 These methods determined mesiodistal bone resorption surrounding implants. This is because general radiography did not show the buccolingual resorption. Most clinical reports just introduced gingival recession and periodontal indexes not a bone resorption. Some histological results on animal experiments were reported to evaluate labial bone recession when it comes to aesthetics. But there is not any papers reported on the change of labial bone when implants placed measured by cone beam computed tomography (CBCT).

The purpose of this study is to evaluate the change, the amount of resorption and thickness of labial bone in anterior maxillary implants using CBCT.

Among 26 implants placed in maxillary anterior part, only 17 had a labial bone on the top 1/3 of implants. The nine implants showed severe labial bone resorption to apex of implant or perforation of labial bone. The mean value of resorption of labial bone (from the platform of implant to residual bone) in 17 cases, was 1.32 ± 0.86 mm. The least level was 0.08 mm, and the most severe level was 2.47 mm. In cases that residual bone existed on the top 1/3 of implant, the average thickness of labial bone was 1.91 ± 0.45 mm, the thinnest 1.27 mm, the thickest 2.66 mm.

CBCT was used to measure the labial bone resorption and level of residual alveolar bone. CBCT has advantages such as less exposure time than general CT, less X-ray radiation and the accuracy of radiographic images.26 For these reasons CBCT is more popular in particularly detection of diseases or lesions on orofacial part, the evaluation of pre/post fracture surgery, evaluation of temporomandibular joint (TMJ) and primary diagnostic criteria of implant treatment plan in dental fields.26

Generally, CBCT has a good reputation to reduce the artifact induced by metal using revising algorithms. However, it also has an artifact as well. Due to this reason, no attempt was done to evaluate the buccolingual bone of the implant placement site. Draenert et al.27 reported that CBCT had more artifacts around implants than multidetector MDCT did. The main causes were wide cone angle and lack of revising algorithms. Nevertheless, the images by MDCT had less sharpness and artifact compared to latest CBCT images. According to his report precise distinctions were possible more than in 90%. But images taken by MDCT were almost similar to those taken by CBCT and Ondemand3D program. That's because the latest CBCT and revising algorithms were more advanced. When the artifacts were severe, the evaluation of labial bone was undetectable. However, the more advanced the techniques were, the less artifacts prominently produced. Considering that the artifact by implants had tendency of vertically radial shape, labial bone and clear continuity of labial bone with adjacent tooth became the baseline. It was estimated that actual residual bone would exist closer to the apex than the baseline.

Only 17 implants were selected to evaluate the change of labial bone of implant which showed clinically successful or acceptable results. The other 9 were considered as a failure. They showed the apical perforation or severe resorption in labial bone. Among 26 implants placed in maxillary anterior part, 17 had a labial bone on the top 1/3 of implants, which meant approximately 65.38%. There was a considerable difference of general implants success rate which was already known as more than 90%.28,29 The causes were like higher failure rate of maxillary anterior implant placement and more technique-sensitive procedure. Also in most cases of no labial bone on top 1/3, they had a tendency of continuous narrow bone loss like craters from platform to apex. But they were rarely detected only by clinical examinations and the standard radiographic images. The overall success rate included all cases like this, which led to higher success rate. On applying to clinical examinations and standard radiographic images, the success rate reached 96%. This had no difference of typical success rate.

17 cases reported residual labial bone existed had it that the average length was 1.32 ± 0.86 mm, in 3.1 years after placement. Lindquist et al.9 carried out the prospective study on bone loss of initial implant placement for 15 years. They reported 0.5 mm of resorption in the first year and 0.05 mm every year in the condition of mandible edentulous state. There is a difference between the result of us and that of Lindquist. This can be attributed to two factors, one is the difference of position of implant placement and the other is that of measurement site of bone loss. Their research evaluated the amount of bone loss at the mesial and distal of implant in full edentulous mandible with hybrid denture. However, the two considerations, which one is that they reported two times more resorptive pattern in lower anterior part than lower posterior one and they evaluated only mesiodistal site of implants, and the other is that timing of our research was in mean 3.1 years after implant placement, could make such a difference acceptable.

Esposito et al.30 reported that in case of single implant, bone resorption 3 years after implant placement had a difference of 0.97 mm when compared between pre-operation and crown setting, and 0.32 mm for 3 years after crown setting. This paper also showed more resorptive pattern in anterior part than posterior one, specifically 1.01 mm of bone loss from pre-operation to crown setting. Although this figure was measured from the mesiodistal of implant to the CEJ of adjacent teeth, just considering bone resorption the result was 1.29 - 1.33 mm. This was approximately similar to the result of this paper. Particularly Esposito's experiment was similar to method using external connection system.30 Internal connection system presented marginal bone resorption as well and it happened at early stage after implant surgery.22,31 Hänggi et al.31 using internal hexa connection reported that causes of initial bone resoption were the biologic width and the microgap between implant and abutment. And most resorption took place without occlusal force at an initial state. Concerning this experiment and previous experiment reported, buccolingual resorption would happen similar to mesiodistal pattern and the similar loss level as well.

Average bone thickness measured by OnDemand3D program was 1.91 ± 0.45 mm. This showed a considerable difference from that of the labial bone of natural tooth by CBCT, which was estimated less than 1 mm. This data would be quite practical to determine whether the bone augmentation would be done or not and to establish the amount of labial bone necessary for placement of implant in anterior region. The existing labial bone means that if bone level maintained, labial bone resorption and changes would not happen. There was not any report on the thickness of labial bone 3 years after implant placement. However, more studies showed the implant failure and reaction of labial bone on implant placement. Spray et al.32 observed the change of labial bone at second surgery after measuring thickness of labial bone as implant placed. According to the paper, average thickness of labial bone was 1.7 mm at implant placement. When second surgery was done, mean bone resorption was 0.7 mm. There was no statistically significant difference in the rate of implant failure, comparing less than 1 mm and 1 - 2 mm of labial bone thickness, however, significantly less implant failure when more than 2 mm of labial bone thickness. Resorptive pattern of labial bone was as followed: the most prominent resorptive pattern in less than 1 - 1.4 mm of labial bone thickness, reduction of resorption level in 1.4 - 1.7 mm, remarkable reduction of resorption or no change in more than 1.8 mm and also had a possibility of bone formation. They suggested that the critical thickness should be 2 mm for the reduction of labial bone resorption and the frequency of bone loss. 2 mm of the critical value that they suggested is similar to the result of this experiment. However, there is a difference in the amount of labial bone resoprtion between Spray's result 32 and ours one. The mean bone loss of this experiment was 1.32 mm and Spray's, 0.7 mm. This difference compared to Spray's experiments32 came from those factors such as different sites of implant placement - anterior part, measurement time - not at the time of second surgery but of 2 years after final restoration. We had a similar result with more than 1.91 mm of labial bone thickness, 0.88 mm bone loss occurred and with less than 1.91 mm thickness, 1.64 mm bone loss was evident. This meant the clinical thickness was 1.91, which presented similarity.

The minimum bone width for the implant placement used to be more than 1 mm and recently the width became at least more than 1.5 mm than the diameter of implant. Considering the result of this study and Spray's one, however, it is suggested that the minimum change of marginal bone may happened only if labial bone existed more than 2 mm not than the diameter of implant but to the labial side of implant.

The purpose of this study was to evaluate the amount of resorption and thickness of labial bone in anterior maxillary implant (average 3.1years, 2003 - 2007) using Con Beam Computed Tomography. 21 patients with 26 implants were followed up and among 26 implants, 17 cases reported residual labial bone existed on top 1/3 were measured for the resorption and the thickness of labial bone. The results were as followed:

- the average bone resorption of maxillary anterior implant was 1.32 ± 0.86 (0.08 - 2.47) mm and the average thickness of residual labial bone was 1.91 ± 0.45 (1.27 - 2.66) mm.

The result would contribute to determine the thickness of labial bone on implant placement. According to result, approximately more than 2 mm minimized the change and the resorption of labial bone in maxillary anterior implants.