Evaluation of Crestal Bone Resorption around Dental Implants in Flapped and Flapless Surgical Techniques Depending on Cone Beam CT Scan (Comparative Study)

Background: The long term survival of dental implants is evaluated by the amount of crestal bone loss around the implants. Some initial loss of bone around dental implants is generally expected. There is reason to believe that reflecting a mucoperiosteal flap promotes crestal bone loss in the initial phase after an implant has been inserted. The surgical placement of a dental implant fixture is constantly changing and in recent years, there has been some interest in developing techniques that minimize the invasive nature of the procedure, with flapless implant surgery being advocated. The purpose of this study was to compare the radiographic level of the periimplant bone after implant placement between traditional flapped surgery and flapless approach depending on CBCT during 24 weeks healing


INTRODUCTION
Osseointegrated dental implants are usually placed with a flap approach, which is based on soft tissue flap reflection and repositioning with suture after implant placement. This traditional approach has several drawbacks: decreased supraperiosteal blood supply due to flap elevation procedures, which can theoretically lead to bone loss; patient discomfort, which includes pain, bleeding, edema, and a longer surgical time (1,2) .
The objective of modern implant treatment involves not only the successful osseointegration of the dental implant but furthermore an esthetic and functional restoration. The implant is required to be surrounded by stable peri-implant tissue levels that are in harmony with the existing dentition. Stable bone levels at or close to the implant margin (shoulder) are among the factors used to consider implant treatment successful (3,4) . Successful prosthetic reconstruction by dental implant mainly depends on the preservation of peri-implant bone (5,6) . In recent years, flapless surgery is increasingly becoming a routine clinical procedure, and has been reported to have a predictable outcome with high success rate. 1 This is facilitated by modern radiographic technologies and dental implant treatment planning software to perform three-dimensional evaluation of bone volume at anticipated implant sites (7) . Flapless surgery has several theoretical advantages. From a patient point of view, it shortens the surgical time, decreases discomfort by reducing swelling and pain and accelerates postsurgical healing. Moreover, the blood vessels of hard and soft tissues around the implant site are preserved. Some studies state that this less traumatic surgery implies reduced bone resorption (8) .
Nevertheless, flapless surgery has some risks due to the limitations of exposure in the surgical site. A reported risk is buccal or lingual cortical plate perforation. Therefore, the technique requires advanced clinical experience, and surgical judgement for appropriate case selection (patients with sufficient alveolar three dimensional bone and adequate keratinized gingiva) (9) .
Several studies report on bone resorption and ridge alterations after implant placement in humans (10) and animals (11,12) . Many studies in animals assess that leaving the periosteum in place with flapless implant surgery clearly decreases the bony resorption rate (13,14) , and according to some authors, the flapless procedure is superior to flap implant procedures for maintaining original mucosal shape surrounding implants (15) .

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Therefore, it can be speculated that similar events may occur in humans after implant surgery.
Imaging may aid in evaluating implant therapy outcomes, such as peri implant bone defects and level and bone to implant interface (16,17) .
Recently, cone beam computed tomography (CBCT) has been heavily marketed for implantbased oral rehabilitation procedures, mainly for treatment planning (18) . Nevertheless, some studies have used the method for assessment of the marginal peri-implant bone level and thickness, primarily of the buccal bone, and outcome of regenerative procedures (19,20,21) . This study aimed to compare the radiographic level of the peri-implant bone after implant placement between traditional flapped surgery and flapless approach depending on CBCT during 24 weeks healing period. The working hypothesis is that using flapless technique decrease crestal bone resoption around dental implant.

MATERIALS AND METHODS
This clinical randomized prospective study was conducted from December 2017 to November 2018, it included 25 Iraqi patients who received 46 implants with an age range of 20-60 years. The patients were randomly divided into two groups: a control group which involved 27 implants inserted by conventional flapped surgical approach and study group which involved 19 implants inserted by flapless surgical approach. Preoperative clinical and radiographic assessment were done for all patients. One implant system was utilized in the study (Nucleoss Co., Turkey).

Inclusion Criteria
1. Patients had to be at least 18 years of age 2. Implants to be placed at least 6 months after teeth extraction (healed sites). 3. Only those patients who did not need any soft or hard tissue augmentation. 4. A minimum distance of 2 mm to adjacent anatomical structures (mandibular canal, maxillary sinus, mental foramen, and adjacent teeth). 5. The Presence of attached non-mobile soft tissue at least 1.5 mm in thickness above the crest of bone in the area receiving the implant, as measured by reamer and stopper. 6. The presence of adequate bone width at least 5 mm, 3 mm apical to crest measured by bone caliper after measurement of soft tissue above the crest, no need for bone augmentation procedures. 7. Patients with good oral hygiene.

Exclusion Criteria
1. Insufficient keratinized tissue above the implant site (less than 2 mm above the ridge crest). 2. Insufficient bone width (less than 5 mm, 3 mm apical to crest). 3. Presence of any pathological condition adjacent to proposed implant site or at the implant zone. 4. Any medical conditions that could not withstand implant surgery or conditions that would interfere with normal healing mechanism including uncontrolled diabetes, osteoporosis, psychosis, current pregnancy at the time of surgical procedures. 5. Heavy smokers (more than 20 cigarette per day) 6. Patients with history of head and neck radiotherapy or chemotherapy over the past 5 years. 7. History or clinical evidence of para-functional habits such as bruxism and clenching. 8. Patients with active periodontitis. 9. Any dehiscence or fenestration of alveolar bone happened during the operation of implant placement.

Surgical Procedures Flap Procedure (Control Group)
After anaesthetization with Lidocaine 2% by infiltration technique, a three sided flap (extensive flap design) or two sided flap was made initiated via paracrestal incision with palatal bias for better visibility, preserving a wider band of keratinized attached gingiva for more solid wound closure and avoiding wound dehiscence as seen in Fig (1).
Full thickness mucoperiosteal flap reflection to expose crestal and buccal alveolar bone using periosteal elevator. The implant bed was prepared by the conventional drilling procedure. The predetermined dental implant size installed in its position. Soft tissue closure was achieved with 3/0 black silk non absorbable suture (simple interrupted technique).   followed by sequential stepped drilling until reaching the appropriate final drill size according to manufacturer instructions. The predetermined dental implant size installed in its position, Fig. (5), and the margin of implant checked by periodontal probe to ensure that it was with the level of the surrounding bone followed by subjoining the cover screw into the fixture, Fig. (6). Then a single 3/0 black silk suture was used to preserve blood clot above the implant area.    Data were collected first at the seventh day after surgery (first follow up). Determination of bone level immediately that represents as base line data for each implant in both group on two sides, buccal side and palata/lingual side, firstly a vertical line was drawn at the center with the long axis of the dental implant passing through the notch of the cover screw which represent a reference point, then another horizontal line was drawn at the top of implant to determine its level with the crestal bone, also another two vertical lines along the buccal and palatal/lingual sides of implant were drawn where from these lines about 1mm the bone level was measured on each side of implant if the bone thickness was enough (such as implants placed in posterior area) or about 0.5 mm if the bone was thin (such as implants placed in anterior region), Fig. (9 A) All the patients were investigated by CBCT (time 2) after 24 weeks post-operatively with the same relations of (time 1), also assessment of osseointegration, survival and early failure rate with the aid of Alberktson criteria of success (implant immobility, no peri-implant radiolucency, asymptomatic), and checking for any complication that may have occurred.
Determination of crestal bone level after 24 weeks was done with the same method in (time 1) and the difference was calculated which represented bone loss that was shown as a shadow in the area, Fig. (9 B). In order to view the same section in the CBCT and to avoid any error that may affect the result in reading the section, the notch of the cover screw is used as a reference point.

Statistical Analysis
The data were analyzed using Statistical Package for Social Sciences (SPSS) version 25. The data presented as mean, standard deviation and ranges. Categorical data presented by frequencies and percentages. Independent t-test and Analysis of variances (ANOVA) (two tailed) was used to compare the continuous variables among study groups accordingly. Z-test was used to evaluate the difference in proportions between flapped and flapless cases regarding survival rate. A level of P value less than 0.05 was considered significant.

RESULTS
There was no significant difference between study (flapless) and control (flapped) groups in the mean of total crestal bone resorption for buccal and palatal sides after 24 weeks from implant placement (P= 0.393 for buccal side and P= 0.214 for palatal side), Table (1). There was highly significant differences between buccal and palatal sides regarding crestal bone loss around implants measured by CBCT after 24 weeks from implants placement for both flapped and flapless surgical techniques (P = 0.001), table (2).

DISCUSSION
According to our data flapless surgery cannot fully avoid bone resoption this was true with all implants placed by flapless technique in this study as they were evaluated by CBCT. This agree with a study done by Nickenig et al. (2010) (22) , they found that the mean crestal bone loss of (0. 5 mm) for implants placed with flapless surgery after a healing period determined radiographicaly using digital panoramic radiographs.
So it possible to say that gentle flap raising does not hamper bone healing around dental implants in humans. Based on the results obtained from this study, the choice of type of surgical approach does not affect peri-implant bone resorption.
Our data on bone resoption are in line with the majority of data in the literature, Lin et al. (2014) (23) reported no significant reduction of marginal bone resorption with flapless technique, also studies showed that crestal bone loss was comparable among implants placed either using flapped or flapless surgical technique (24,25) . The findings of this study disagree with In this study, there was significant difference between total mean of buccal and palatal aspects (P = 0.001) for both flapped and flapless group. There was obvious that bone resoption in buccal side much more than palatal / lingual side in both groups. Also bone resoption in buccal side was a little higher in maxilla than mandible and anteriorly more than posteriorly for the flapped group, while for flapless group was nearly the same. The data from this study can be accounted for the following reasons: According to anterior maxilla the bone loss in buccal/facial side was higher than palatal side in both flapped and flapless groups, this may be related to the thinner facial plate that leading to more bone resoption after implant placement. This was supported by EL Nahass & Naiem. (2015) (27) , who reported that in the incisor region, the buccal bone plate around a tooth was thinner than 1 mm in 86% of the cases as demonstrated by Computerized tomography. For posterior maxilla and mandible in flapless group also the bone loss was higher in buccal side than palatal/lingual side, there was no clear interpretation for this result but after evaluation of the buccal plates for implants placed with flapless procedures, we noticed that most of them were thinner than palatal/lingual plates, this may be either they were initially thin or implant were deviated slightly towards buccal side during placement which made them more thin. It however noteworthy that thin buccal plates lost more bone than thick buccal plates.
The deviation of implants supported by a study done by Van de Velde et al. (2008) (28) performed an in vitro model study to analyze deviations in the position and inclination of implants placed with flapless surgery compared with the ideal, virtual planned position and they concluded that location of implants installed with a flapless approach differed significantly from the ideal position. The findings were supported or in line with a hypothesis reported by (Teughels et al., 2009; Merheb et al., 2015) (29) (30) , stated that a thin buccal plate is less resistant to the different types of trauma an implant can endure and would therefore be more prone to resoption and buccal implant exposition. According to posterior maxilla and mandible in flapped group the buccal bone loss was higher than palatal/lingual bone, this may be related to the flap elevation during surgical procedure on the buccal side and subsequent trauma that occurred more buccally rather than palatal/ lingual sides where there was no flap elevation. This was supported and in accordance with the fundamental studies reported by Merheb et al. (2015) (30) , who stated that flap elevation lead to a bone resoption from the surgical trauma of up to 0.4 mm. Conclusions: Bone resoption around dental implants placed with conventional flap surgery compared to flapless surgery does not seem to be influenced during the healing period before implant loading. Bone resoption in buccal side facing dental implants compared to palatal side seems to be influenced in conventional flap and flapless surgery.