Clinical evaluation of different alveolar ridge preservation techniques after tooth extraction: a randomized clinical trial

The aim of the present randomized controlled trial (RCT) was to evaluate the efficacy of different alveolar ridge preservation (ARP) techniques on dimensional alterations after tooth extraction, based on clinical measurements. Alveolar ridge preservation (ARP) is a common procedure in every day clinical practice, when dental implants are involved in treatment planning. In ARP procedures, a bone grafting material is combined with a socket sealing (SS) material in order to compensate the alveolar ridge dimensional alterations after tooth extraction. Xenograft and allograft are the most frequently used bone grafts in ARP, while free gingival graft (FGG), collagen membrane, and collagen sponge (CS) usually applied as SS materials. The evidence comparing xenograft and allograft directly in ARP procedure is scarce. In addition, FGG is usually combined with xenograft as SS material, while the evidence combing allograft with FGG is absent. Moreover, CS could probably be an alternative choice in ARP as SS material, since it has been used in previous studies but more clinical trials are required to evaluate its effectiveness. Forty-one patients were randomly assigned in four treatment groups: (A) freeze-dried bone allograft (FDBA) covered with collagen sponge (CS), (B) FDBA covered with free gingival graft (FGG), (C) demineralized bovine bone mineral xenograft (DBBM) covered with FGG, and (D) FGG alone. Clinical measurements were performed immediately after tooth extraction and 4 months later. The related outcomes pertained to both vertical and horizontal assessment of bone loss. Overall, groups A, B, and C presented significantly less vertical and horizontal bone resorption compared to group D. No statistically significant difference was observed between allograft and xenograft, except for the vertical bone resorption at the buccal central site, where xenograft showed marginally statistically significantly reduced bone loss compared to allograft (group C vs group B: adjusted β coef: 1.07 mm; 95%CI: 0.01, 2.10; p = 0.05). No significant differences were observed in hard tissue dimensions when CS and FGG were applied over FDBA. No differences between FDBA and DBBM could practically be confirmed. In addition, CS and FGG were equally effective socket sealing materials when combined with FDBA, regarding bone resorption. More RCTs are needed to compare the histological differences between FDBA and DBBM and the effect of CS and FGG on soft tissue dimensional changes. Xenograft and allograft were equally efficient in ARP 4 months after tooth extraction in horizontal level. Xenograft maintained the mid-buccal site of the socket marginally better than the allograft, in vertical level. FGG and CS were equally efficient as SS materials regarding the hard tissue dimensional alterations. Clinical trial registration Number: NCT 04934813 (clinicaltrials.gov)


Introduction
Tooth extraction is followed by a sequel of events that lead to alterations in the alveolar ridge dimensions, such as the reduction of the width and height of the alveolar crest [1].
Extended author information available on the last page of the article 1 3 The most significant changes take place in the first 6 months after tooth extraction with an average horizontal and vertical bone resorption of 3.79 mm and 1.24 mm, respectively [2]. The implications of the decrease in osseous volume are profound in implant dentistry, since complex and arduous bone grafting techniques may be required, while esthetics may be compromised, especially in the maxillary anterior zone.
In order to compensate the aforementioned alterations, an array of alveolar ridge preservation (ARP) procedures has been proposed. In general, the basis of these techniques includes the placement of a bone graft into the socket, immediately after tooth extraction, and the sealing of the socket with a barrier [3]. The effectiveness of these interventions has been evaluated in several studies, and so far, it is well documented that ARP procedures result in significantly reduced bone resorption compared to management without ARP [4]. Indeed, the 15th European Workshop of Periodontology concluded that ARP may prevent 1.5-2.4 mm of horizontal, 1-2.5 mm of vertical mid-buccal, and 0.8-1.5 mm of vertical mid-lingual bone reduction as compared to tooth extraction alone [5].
Various bone grafting materials have been tested in socket preservation procedures with allografts and xenografts being the most prevalent and well-documented. The results from randomized controlled trials (RCTs) demonstrate that both types of bone grafts reduce alveolar bone resorption and contribute to a significant maintenance of the immediate post-extraction volume and contour of the alveolar crest [6][7][8][9]. However, it is not yet possible to identify the most superior bone graft, both clinically and histologically, since there is currently very scarce evidence comparing allografts and xenografts directly [5,10,11].
Another critical component in ARP procedures is the sealing of the socket with a barrier. This enhances mainly blood clot stabilization, resulting in increased effectiveness of the osseous graft and in some cases in soft tissue conditioning [12][13][14]. Several socket sealing (SS) materials have been utilized in ARP procedures. In cases where soft tissue conditioning is needed, the use of autologous free gingival grafts (FGG; soft tissue punch) from the palate has been proposed [15]. Τhis technique not only protects the grafting material and clot, but also increases the soft tissue thickness and the zone of the keratinized tissue. However, such grafting procedures are technique-sensitive, and therefore other materials, such as collagen membranes and collagen sponges, have also been evaluated. To date, weak evidence exists to support the superiority of one SS material over the others, in relation to hard tissue changes [5].
Therefore, in order to better understand the effectiveness of different materials in ARP procedures, the aim of the present study was to evaluate the efficacy of different alveolar ridge preservation techniques, based on clinical measurements.

Study design
The present study is a prospective randomized, triple-blind controlled trial. The study was conducted in full accordance with the World Medical Association's Declaration of Helsinki, as revised in 2008. The study protocol was approved by the Committee of Research and Ethics of the Dental School of the National and Kapodistrian University of Athens, Greece, and registered in a publicly accessible database (clinicaltrials.gov/NCT 04934813). Written informed consent was provided by all study participants. The CONSORT statement was followed for the reporting of the present work [16].

Participants
Subjects were recruited from the patients proceeding to the post-graduate clinic of Periodontology of the Dental School of the National and Kapodistrian University of Athens, in Greece.
All candidates required extraction of at least one nonrestorable single-rooted tooth in the maxilla which would be replaced by an implant as part of their ongoing treatment plan. Further inclusion criteria were systematically healthy patients, non-smokers, and no use of antibiotics for the last 3 months. Patients were excluded when a bony wall loss of more than 50% was verified for the socket after the extraction.

Randomization, allocation concealment, and blinding process
Randomization was performed by an independent dentist (YB), using a computer software (randomizer.org). The results were concealed in envelopes and opened by the therapist (IES) immediately after tooth extraction and only if the socket met the inclusion criteria. If a patient had 2 or more teeth extracted, the respective number of envelopes were opened. The patients were blinded regarding the applied bone graft, while the periodontist who measured the alveolar ridge dimensions at re-entry (EO) and the data analyst (KD) were all blinded regarding the treatment performed.
Each individual was assigned to one of the following four groups: Group A: cancellous bone allograft with granules size 0.2-1.6 mm (Phoenix® cancellous bone powder, TBF) was placed to the bone crest and sealed with a collagen sponge (Jason® fleece, Botiss). Group B: cancellous bone allograft with granules size 0.2-1.6 mm (Phoenix® cancellous bone powder, TBF) was placed to the bone crest and sealed with an autogenous FGG (soft tissue punch). Group C: demineralized bovine bone mineral xenograft (Cerabone® bone mineral, Botiss) was placed to the bone crest and sealed with an autogenous FGG (soft tissue punch). Group D: the socket was sealed with an autogenous FGG (soft tissue punch), while no bone graft was used.

Tooth extraction and grafting procedure
Teeth were extracted in an atraumatic manner, using periotomes, thin luxators, and forceps in order to maintain the anatomy of the buccal and lingual bony plate. Then, the granulation tissue was thoroughly removed, and the integrity of the bony walls was evaluated. Clinical measurements were performed by means of a surgical stent, as described below, and patients were randomly assigned in one of the aforementioned four treatment groups.
When bone graft was applied into the socket, the allograft or xenograft was first placed in sterile saline for at least 5-10 min, as indicated by the manufacturer. Then, the bone graft was condensed into the socket and covered with the corresponding socket barrier. In case of a FGG sealing, a graft of 2-3 mm thickness was harvested from the patient's palate. Subsequently, the soft tissue margin of the socket was de-epithelialized using a diamond bur under copious water irrigation, and the tissue punch was stabilized with 6-8 single interrupted monofilament non-resorbable sutures (Seralon® polyamide 6/0), as previously described [15]. In cases where a collagen sponge was used, the material was stabilized with a " Figure 8" suture technique, using the same suture material (Seralon® polyamide 6/0) (Figs. 1 and 2).
Patients were instructed to rinse twice a day for 2 weeks with 0.2% chlorhexidine (Chlorhexil® 0,2%, Intermed), and nonsteroidal anti-inflammatory drugs (ibuprofen 400 mg, tid) and antibiotics (amoxicillin 500 mg, qid) were prescribed for 5 days. Patients were reevaluated at 10-14 days, had their sutures removed, and then were followed-up until complete soft tissue healing.
Four months after tooth extraction, a full-thickness flap was elevated at the site of extraction, and prior to implant placement, the alveolar ridge dimensions were measured.

Clinical measurements of the alveolar ridge dimensions
The measurements of the alveolar ridge dimensions were evaluated immediately after tooth extraction and 4 months later. In order to ensure standardization of the clinical measurements, surgical stents were fabricated for each site as previously described [17,18]. In brief, prior to tooth extraction, study casts of the patients were fabricated. Subsequently, the surgical stents were constructed using a 1.5-mm thick hard celluloid shells which adapted at least on 2 adjacent teeth to ensure stability during the measurements. Four holes were prepared at the occlusal aspect of the stent, 1 central-buccal, 1 mesial, 1 distal,

Fig. 1 Treatment group A. a
The socket immediately after tooth extraction. b The bone graft (allograft) placed and condensed into the socket. c The collagen sponge placed over the bone graft (allograft) for socket sealing. d Stabilization of the collagen sponge with 6/0 monofilament sutures and 1 central-lingual above the post-extraction socket and served as reference points in order to measure the vertical dimensional alterations after the extraction. In addition, 2 holes were created at the buccal and 2 at the lingual aspect of the alveolar ridge, 2 mm and 4 mm apical to the bone margin of the socket, in order to measure the horizontal dimensional changes of the alveolar ridge (Fig. 3).
The distance between the above mentioned reference points and the alveolar crest was noted with an endo file, which had a stopper on it, and measured with a digital caliper (Logilink digital caliper WZ0031). For the horizontal measurements, a modified digital caliper with sharp edges was used to measure the ridge width. The edges passed through the soft tissue (bone sounding), in order to avoid flap elevation. All distances were measured twice to ascertain the reproducibility of the process. The initial measurements were performed by a periodontist (IES), while the measurements at reentry were performed by a different periodontist (EO) who was blinded on group allocation of the patients. Both periodontists were calibrated prior to the initiation of the trial and the inter-rater agreement was assessed through Bland-Altman plot, with the respective limits of agreement (LoA). Mean difference between interrater measurements were at the level of − 0.01 (95% confidence interval, CI: − 0.08, 0.05), with LoA ranging from − 0.19 to 0.16. The Pitman's test of difference in variance was non-significant (p = 0.29), as well.

Statistical analysis
Based on previous data [5,19] and assuming that a difference of 2.0 mm could be considered clinically significant (with common SD = 1.5) and with an alpha error rate 5% and power = 0.8, we calculated that a sample size of 10 patients/group were necessary. Data distribution was checked for normality of residuals visually through qqplots and statistically through Shapiro-Wilk tests. As normality could not be confirmed, non-parametric descriptive statistics were used. For the regression model and due to the small sample size, standard errors were derived using the bootstrap method with 1000 replications.
Likelihood ratio tests were applied between multilevel and original models to test evidence of clustering effects within the sample. As this was not confirmed, we proceeded with the original model. In essence, a multivariate linear regression model was fitted with derived standard errors (SE) through the bootstrap method with 1000 replications, for the effect of intervention, age, and tooth type on the six correlated outcomes, namely average bone change in the vertical mesial region, bone change in the vertical buccal central, bone change in the vertical palatal central, and average bone change in the vertical distal as well as bone change in the horizontal region at 2 mm and 4 mm, within the socket of extracted teeth. The tissue punch prepared for socket sealing. c The bone graft (allograft) placed and condensed into the socket. d The soft tissue punch stabilized with 6/0 monofilament sutures Box plots were constructed to allow for an assessment of alveolar bone changes, by intervention category and across examined regions of the socket.
The level of statistical significance was pre-specified at p < 0.05 (two-sided). Statistical analyses were performed with STATA version 15.1 software (Stata Corporation, College Station, TX, USA).

Study population
Forty-one patients participated in the study with 41 singlerooted maxillary teeth extracted. Overall, 20 males and 21 females were included and patients' age ranged from 22 to 67 years with a median value of 52 years. Table 1 presents the demographic information of the patients as well as the distribution of the extracted teeth based on their type. The flowchart of the study is shown in Fig. 4.

Dimensional changes of the alveolar crest
Due to the non-normal distribution of the sample, it was not possible to calculate the mean value of the dimensional changes and the standard deviation. Therefore, the median values as well as the extreme values of the vertical and horizontal alterations are presented (Tables 2 and 3). The descriptive statistics are summarized and visualized with box plots in Fig. 5.
With regard to vertical measurements, a common observation across all groups was that greater bone resorption took place at the central buccal region of the alveolar ridge. The median value of bone resorption at the central buccal site was 0.85 mm for group A (interquartile range, IQR:   (Table 3).

Inter-group comparisons
The results of the multivariate regression analysis are shown in detail in Table 4. Group B was considered the reference group for all inter-group comparisons. At the vertical level, there was strong evidence of association between bone loss and type of intervention provided at the buccal central   (Table 4). At the horizontal level, bone resorption was more pronounced at the 2 mm level in all groups. Sockets at group D presented significantly greater bone loss compared to group B at the 2 mm level (adjusted β = − 2.10; 95% CI: − 2.77, − 1.43; p < 0.001), as well as at the 4 mm level (adjusted β = − 0.80; 95% CI: − 1.23, − 0.38; p < 0.001). No significant differences were detected at the horizontal level, between group B and the remaining groups ( Table 4).
The effect of age and tooth type was non-significant for all associations under examination (Table 4).

Discussion
The purpose of the present RCT was to compare the effectiveness of four different ARP techniques to preserve the dimensions of the alveolus 4 months after tooth extraction. A large number of studies have assessed the effect of various bone grafts in ARP [4,20]. In the present study, we opted to evaluate a xenograft and an allograft, since a recent systematic review suggested that these materials are the most effective and commonly used in everyday clinical practice [21].
The results of our study demonstrate that the addition of a bone grafting material into the socket leads to significantly less horizontal and vertical mid-buccal bone resorption, irrespective of the SS material used.
These findings are in overall agreement with the evidence provided by a series of systematic reviews/meta-analyses [4,22,23]. The clinically important results of bone grafting as an ARP strategy may be attributed to the properties of the bone grafts to maintain space, to prevent soft tissue collapse, and to secure favorable conditions for the formation of new bone [24,25].
Interestingly, in the present study, the favorable outcomes derived from the use of bone grafts involved the preservation of the alveolar ridge both at the horizontal (2 mm and 4 mm from the bone margin) and the vertical (mid-buccal site) level. These findings corroborates with the existing evidence [4]. In addition, our study demonstrated that bone resorption was more pronounced at the mid-buccal site compared to the mid-palatal, mesial, and distal sites at the vertical level, which is also in agreement with previous studies [26][27][28].
Regarding the differences in the efficacy between the two bone grafting materials used, our results demonstrated that the allograft presented a marginal increased bone loss compared to the xenograft only at the central buccal site vertically. No significant differences were observed at any of the remaining sites, horizontally or vertically. Similarly, other reports have failed to show any significant clinical advantage of one of these bone grafts over the other [10,11]. In a recent RCT involving twenty patients, Serrano-Mendez and co-workers did not report any meaningful clinical difference between the two grafts 6 months after ARP [11]. Moreover, in a systematic review of RCTs, socket grafting with xenografts was associated with a bone loss of 1.3 mm horizontally and 0.57 mm at the mid-buccal site vertically 12 weeks after extraction, while the corresponding bone loss for allografts was 1.63 mm and 0.58 mm, respectively. This difference was not statistically significant, and thus the authors concluded that both grafts were comparable in terms of efficacy [21]. It is worth mentioning that the extent of bone resorption described by the above systematic review is similar to that found in our study. The marginally significant clinical advantage of the xenograft group found at the midbuccal site may be related to the graft's slower resorption rate compared to the allograft, within the evaluation time frame of the study [29].
Another critical open question associated with ARPs is the clinical effect of various barriers used to cover the postextraction socket. A resent systematic review demonstrated that the application of a socket sealing material provides better results compared to no coverage, regarding the dimensional alterations of the alveolar ridge [14]. In addition, several studies have evaluated the effect of the combination of a xenograft with FGG and clearly suggest clinical benefits in reducing post-extraction bone resorption [12,15,30]. On the contrary, the combination of an allograft with FGG has been assessed only to a very limited extent, and the evidence regarding its effectiveness is inconclusive [31]. To our knowledge, no prior study has compared the combination of these two types of bone grafts with FGG in ARP. Our findings demonstrate, for the first time, that the combination of an allograft with FGG may provide clinical results that are comparable to those derived by the combination of a xenograft with the FGG.
Another alternative to the FGG is the use of a collagen sponge. Despite its rapid resorption rate, the collagen sponge contributes to the stabilization of the underlying bone grafting material and of the blood clot during the first crucial days of healing, without the discomfort caused by   [32]. Nevertheless, research regarding the use of this material in ARP remains limited [8,17,33,34], while no comparison has been made with FGG. Data from our study reveal that the collagen sponge is equivalent to FGG regarding socket preservation when covering an allograft. Therefore, it could replace the FGG in order to avoid the additional discomfort caused at the donor site.
In the present study, we also examined the efficacy of the FGG without bone grafting, since some authors have suggested, that by sole sealing of the socket with a FGG may lead to less bone resorption compared to unintentional healing [12,35]. Although this may be true, our findings suggest that this modality is probably inferior compared to ARP with xenograft or allograft and FGG. Such results highlight the importance of using a bone graft in order to achieve more favorable clinical outcomes.
Finally, it should be mentioned that this study is not free of limitations. As the differences between the clinical outcomes of the different ARP modalities may be in the order of tenths of a millimeter, a larger sample may be needed in order define them more precisely. Also, another limitation may be the fact that clinical factors such as the thickness of the buccal bone plate and the soft tissue phenotype were not taken under consideration. Future studies could assess their role in post-extraction bone resorption and further evaluate the dimensional changes of the soft tissues after ARP.

Conclusions
Within the limitations of our study we may conclude that in ARP: i) Allografts and xenografts were equally effective in minimizing both vertical and horizontal bone resorption, when a FGG was also applied. ii) FGG alone was inferior compared to FGG plus bone grafting iii) CS and FGG were equally effective in minimizing bone resorption when used with a bone graft.
Author contribution Iosif El-Sioufi and Elias Oikonomou conceived the idea, collected the data, and wrote the main manuscript as well as configured the photos. Despina Koletsi analyzed the data and led the results presentation. Yiorgos Bobetsis, Spyridon Vassilopoulos, and Phoebus Madianos led the writing and corrected the manuscript.