Does initial buccal crest thickness affect final buccal crest thickness after flapless immediate implant placement and provisionalization: A prospective cone beam computed tomogram cohort study

Abstract Background Flapless immediate implant placement and provisionalization (FIIPP) in the aesthetic zone is still controversial. Especially, an initial buccal crest thickness (BCT) of ≤1 mm is thought to be disruptive for the final buccal crest stability jeopardizing the aesthetic outcome. Purpose To radiographically assess the BCT and buccal crest height (BCH) after 1 year and to calculate the correlation between initial and final achieved BCT. Materials and Methods The study was designed as a prospective study on FIIPP. Only patients were included in whom one maxillary incisor was considered as lost. In six centers, 100 consecutive patients received FIIPP. Implants were placed in a maximal palatal position of the socket, thereby creating a buccal space of at least 2 mm, which was subsequently filled with a bovine bone substitute. Files of preoperative (T0), peroperative (T1) and 1‐year postoperative (T3) cone beam computed tomogram (CBCT) scans were imported into the Maxillim™ software to analyze the changes in BCT‐BCH over time. Results Preoperatively, 85% of the cases showed a BCT ≤1 mm, in 25% of the patients also a small buccal defect (≤5 mm) was present. Mean BCT at the level of the implant‐shoulder increased from 0.6 mm at baseline to 3.3 mm immediate postoperatively and compacted to 2.4 mm after 1 year. Mean BCH improved from 0.7 to 3.1 mm peroperatively, and resorbed to 1.7 mm after 1 year. The Pearson correlation of 0.38 between initial and final BCT was significant (p = 0.01) and therefore is valued as moderate. If only patients (75%) with an intact alveolus were included in the analysis, still a “moderate correlation” of 0.32 (p = 0.01) was calculated. Conclusions A “moderate correlation” was shown for the hypothesis that “thinner preoperative BCT's deliver thinner BCT's” 1 year after performing FIIPP.


Replacement of maxillary incisors by immediate implant placement
and provisionalization (IIPP) may be a reliable therapy with respect to implant survival and pink aesthetic outcome. [1][2][3] This minimal invasive procedure improves patient comfort, reduces both treatment time and postoperative complaints, as well as costs compared to early or delayed placement protocols. However, due to the natural process of post-extraction bone remodeling, 4-7 the pink aesthetic outcome may vary. In this perspective, thickness of the buccal bone crest is crucial. 8 Dimensional alterations of the facial soft tissues and buccal bone following tooth extraction negatively influence a successful aesthetic outcome in implant therapy. 9 Cone beam computed tomogram (CBCT) analysis showed vertical loss of the buccal crest up to 7.5 mm within 8 weeks after flapless extraction. 10 This bone loss compromises the aesthetic outcome, because it is followed by retraction of the covering soft tissues, resulting in midfacial soft tissue recession, which in the end may lead to exposure of the implant surface. Immediately replacing a root by a dental implant itself does not prevent resorption of the buccal crest. [11][12][13] In order to compensate for bone resorption, bone augmentation procedures in advance of implant installation have been suggested. Ridge preservation procedures, filling the extraction socket with bone or a bone substitute, have proven to be effective in limiting both horizontal and vertical ridge alterations after extraction. [14][15][16][17] In these procedures, applying freeze-dried bovine bone xenografts is favorable, as they preserve more alveolar bone volume compared to the use of autogenous bone. [18][19][20] During immediate implant placement also ridge preservation can be performed, provided that sufficient space is created, which subsequently can be filled with a bone substitute. 21,22 To create such a gap with optimal dimensions, new insights advocate to install the implant in a more palatal position, on condition of the presence of sufficient apical bone volume. Such a gap leads to less crestal bone resorption and minimal midfacial soft-tissue recession. [23][24][25][26][27][28] This buccal gap even allows new bone formation, coronal to the receding buccal bone wall. [29][30][31] To achieve a minimal gap width of at least 2 mm the use of implants with a smaller diameter is advocated. Thickness of the buccal crest itself also plays an important role, as it consists of bundle bone.
As herein only minimal vascularization and regenerative ability are present, thinner buccal alveolar crests will resorb more. 32 To prevent resorption the buccal gap may be filled with a freeze-dried bovine bone xenograft. In combination with immediate provisionalization, an instant support of the papillae and midfacial soft tissue is delivered, thereby reducing marginal bone changes. [33][34][35][36] Only in case of sufficient initial stability provisional restoration is advocated at the time of flapless implant placement, [37][38][39][40] independent of the patient's biotype. 41 CBCT data giving insight into the process of buccal bone remodeling after IIPP are rare. This prospective multicenter CBCT study reveals the process of buccal bone remodeling in the aesthetic zone after FIIPP, while placing the implants in a palatal position and simultaneously performing a ridge preservation procedure.
Aim of the study is to evaluate bone remodeling of the reconstructed buccal wall after FIIPP and to inventory if there is relation between "initial buccal crest thickness" on one hand, and the "final thickness" of the reconstructed buccal crest on the other hand.
It is hypothesized that the thinner the initial crest thickness, the thinner the reconstructed buccal crest will be after performing FIIPP.

| Study population
In total, 100 consecutive patients were included in this prospective clinical study between 2014 and 2017. In all patients, one upper tooth was in danger of being lost. This study was approved by the Ethics Committee of the Radboud University Medical Center Nijmegen (2014/157) and registered in the Dutch Trial Register (NTR) on 20 October 2015 (NTR5583/NL4170). Written informed consent to participate in this study, as well as for use and publication of the data, was derived from all participants. This manuscript was written conform the Strengthening the Reporting of Observational studies in Epidemiology (STROBE) guidelines. These guidelines were created to aid the author in ensuring high-quality presentation of the conducted observational study.
Prerequisites were that the one failing single maxillary incisor was surrounded by two healthy teeth, that the extraction socket was intact and that sufficient occlusal support was present in the absence of periodontal disease and bruxism. Furthermore, to allow for primary implant stability, sufficient apical bone volume had to be present in the apical region.
Besides intact sockets, also sockets showing solely a periapical bone defect or a bone crest defect ≤5 mm, defined as EDS-2 or EDS-3, 42 were included.
Patients suffering from the following habits or diseases were excluded: smoking more than 10 units a day, drug or alcohol abuse, uncontrolled diabetes, pregnancy, or when disturbed bone healing could be expected, such as in case of local or systemic disease, severe osteoporosis, Paget's disease, renal osteodystrophy, radiation in the head-neck region, immune-suppression or corticosteroids treatment in the recent past. Finally, the aesthetic expectations had to be achievable.

| Multicenter
In total, six centers for oral implant therapy participated; one university, one hospital, and four referral dental clinics. In the two first centers, an oral maxillofacial surgeon installed the implants, while the restorative procedure was performed by a separate restorative dentist. In the remaining four centers, the complete IIPP procedure was performed by a dentist trained in oral implantology.

| Preoperative measures
Patients were instructed to take 2 g amoxicillin 1 h before surgery, and 500 mg/3Â day for 5 days starting in the morning after surgery. If patients were allergic to amoxicillin 600 mg clindamycin 1 h before surgery, and 300 mg/4Â day for 5 days starting in the morning after surgery, was advised. In addition, patients had to rinse with 0.12% chlorhexidine solution twice a day for 14 days, starting the day before surgery. Also, it was instructed to take 1 g paracetamol or 600 mg ibuprofen 1 h before surgery.

| Operative procedure
After atraumatic tooth removal, an osteotomy was conducted in the palatal wall of the socket in a more palato-apically direction compared to the original apex ( Figure 1A). Subsequently, the last used drill remained in the preparation to prevent bone substitute plugging it ( Figure 1B). Hereafter, the socket was filled with a mixture of blood and bovine bone (Bio Oss™ S 0.25-1 mm, Geistlich Biomaterials, Wolhusen, Switzerland) after which the drill was removed carefully by turning it anti-clockwise, creating a clean corridor to install the implant (NobelActive Conical Connection™ NobelBiocare, Washington, DC) ( Figure 1C). The seat of the implant was placed 3 mm apically from the buccal gingival margin and at least 2 mm palatal of the buccal bone plate ( Figure 1D) to create the recommended space buccally and to allow a suitable emergence angle of <30 . 43 To evaluate the implant position a low dose small field CBCT scan was made; in case of inaccuracies still corrections could be conducted.
Hereafter, a titanium temporary customized platform-switch Procera™ abutment (Nobel Biocare, Washington, DC) was placed ( Figure 2A) allowing fabrication of a composite screw-retained provisional restoration. Care was taken to prevent contact with the antagonistic dentition in occlusion or articulation. After implant placement (3-9 months), the final impression was taken to fabricate either an individualized, screw-retained, zirconium-oxide porcelain veneered crown, or an individualized zirconium-oxide abutment (Procera™, NobelBiocare, Washington, DC) with a resin cemented porcelain facing ( Figure 2B). 44

| Radiographic procedure measurements
To minimize the effective dose, only small field-of-view scans (6 Â 6 cm) were applied. For analysis, the preoperative, peroperative, and CBCT data after 12 months were imported as Digital Imaging and Between midfacial, 1 mm to the mesial or 1 mm to the distal side, no difference in measurements for thickness or height is present, 36 therefore, solely midfacial measurements were conducted.

| Statistical methods
For all measurements, the range, median, mean and standard deviation were calculated. Differences in BCT and BCH were tested with a paired sample t-test. Statistics were calculated for all clinical parameters using F I G U R E 1 (A) The osteotomy was conducted in the palatal wall, after which (B) the last drill used was placed into the socket. After filling the socket with (C) a mixture of blood and Bio-Oss™, the drill was removed and (D) the implant installed The inter-observer performance was analyzed using a paired sample t-test. For this purpose in total 36 measurements were repeated.
The reliability was calculated as Pearson's correlation, and the random error was calculated as the standard deviation of the difference between observers, divided by √2.
To assess if the two methods (BCT vs OBC minus IBC) led to a different outcome also a paired sample t-test was conducted.
To inventory if there was a correlation between the "initial BCT" and "final BCT after 1 year," the Pearson's correlation coefficient was calculated.
Both mean BCT and BCH per time point are depicted in Table 1. With respect to BCT and BCH, differences between T1 versus T0, T3 versus T0, and T3 versus T1 were statistically significant (all p = 0.003) ( Table 2).
To assess how the initial bone crest thickness (BCT-T0) is related to the final bone crest thickness (BCT-T3), the Pearson's correlation was calculated for all 81 patients ( Figure 5).  Considering the need of filling the gap, Naji et al. suggested that solely a thick bone crest with a BCT of ≥1 mm, allows a stabilized formed coagulum without the need for regenerative materials. 47 Others stated that in case of an initial buccal bone plate width of <1 mm or with fenestration, gap grafting, and regeneration are recommended to enhance bone filling and reduce the bone reduction. [50][51][52] CBCT is a useful tool that has been successfully used for reproducibility and accuracy of bone crest level measurements, 53,54 as corroborated in the present study showing a mean difference of 0.069 mm between both observers (p = 0.192).
The initial mean width BCT in our study was 0.6 mm (SD 0.5) which is in accordance with previous studies using CBCT scans to measure bone width around maxillary anterior teeth. 8 Horizontal reduction of BCT also can be clarified by condensation of the bone substitute particles in time.
In 20 (25%) of the patients preoperatively, a small buccal bone defect (≤5 mm) was present. If this group was included in the statistical analysis, the Pearson correlation between initial and final BCT was 0.38. If this group was excluded from analysis, still a significant Pearson's correlation of 0.32 remained. As such there is indeed a moderate correlation indicating that thin buccal plates will result also in thinner buccal plates after reconstruction. Of course, also other factors will influence the end result after reconstruction, such as age, general condition, and width of the created gap.
To our knowledge, this is the first prospective study assessing the relation between the initial BCT and the BCT after 1 year. A 'moderate correlation' was shown for the hypothesis that 'thinner preoperative BCT's deliver thinner BCT's' one year after performing FIIPP.
Nevertheless, independent of the initial BCT, 1 year after following the presented FIIPP protocol, both bone crest thickness and height were in 90% of the cases still substantial, meaning that more than the required minimal BCT of 1 mm was present, thereby creating a stable base for the soft tissues.
The lesson to be learned is that FIIPP may be successful in cases of a bone defect at the implant shoulder (BCT-T0 = 0); however, it is not a panacea. In total, 20 of such cases (25%) were included: 14 were successful (BCT-T3 ≥1 mm) and 6 cases (7.5%) failed, meaning that the bone crest both in thickness and height scored zero 1 year postoperatively.
Long-term prospective studies need to be performed to prove if both BCT and BCH will also be stable over time. Retrospective CBCTdata after 7 years showed already promising results. 59

CONFLICT OF INTEREST
The authors declare no conflict of interest.

AUTHOR CONTRIBUTIONS
Tristan Ariaan Staas conceptualized the project idea, conducted the literature search, collected and analyzed the data, and drafted the manuscript. Edith Groenendijk conceptualized the project idea, collected and analyzed data, and made corrections to the drafted manuscript. Gerry Max Raghoebar contributed to the data analysis.
Ewald Bronkhorst conducted the statistical analysis. Gerry Max Raghoebar and Gert Jacobus Meijer critically reviewed and revised the manuscript.

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.