After tooth extraction, the surrounding hard and soft tissues undergo physiological and structural changes that may jeopardize their integrity and volume, which is critical for any subsequent implant rehabilitation. The peri-implant mucosa needs to be supported by an adequate three-dimensional osseous volume to achieve functional and esthetic success [16].
According to Araujo et al. (2013), the extent of bone loss following tooth extraction depends on factors such as the bone wall thickness, position and angulation of the tooth, presence of surgical trauma, the flap elevation, a lack of functional stimulus and of the periodontal ligament as well. Regarding the bone thickness, animal and human studies have shown that the facial bone wall, especially in anterior maxilla sites, can be less than 1 mm in 90% of cases and less than 0.5 mm in almost 50% of cases. Thus, the reason for the significant loss after extraction observed in some cases may be because thin bone walls mainly consist of bundle bone, a lamellar bone structure with a thickness of 0.2–0.4 mm depending on the presence of the tooth and the blood supply from the periodontal ligament [17,18,19].
By contrast, thick facial bone wall and the palatal/lingual plates of teeth undergo minor changes. Although molar sites show larger ridge reductions, it is more critical to treat the anterior maxillary region because it has a larger impact on esthetics and the quality of the bone is lower [20, 21].
To counteract physiological bone loss, alveolar ridge preservation techniques that use autologous bone or biomaterial grafting and an additional absorbable membrane have been widely applied. According to recent clinical trials and systematic reviews, although socket grafting cannot prevent the resorption of facial and palatal bone wall, it helps to preserve the volume of the bone and leads to better clinical and histological outcomes [22,23,24].
A recent clinical study by Lee et al. (2021) tested the feasibility of implant placement in periodontally compromised patients treated either with ARP or with SH: the results showed that ARP can ease the implant placement and reduce the quantity of bone augmentation required [25].
For some important indications, alveolar ridge preservation procedures might be a valid alternative not only to spontaneous healing but also to other procedures such as the immediate implant placement or type 1 implants, according to the consensus statement by Tonetti et al. [11]: younger or systemically compromised patients, post-extractive sites in which it is not possible to achieve primary stability, and proximity to important anatomical structures (i.e., maxillary sinus, inferior alveolar nerve). Furthermore, ARP may be a predictable treatment choice for patients who are unable to undergo implant placement and desire to wait for longer periods than usual to preserve the architecture in an esthetic area and to avoid more invasive procedures in the subsequent phases, such as bone augmentation or sinus-lifting procedures [26].
The present study compared the dimensional changes at 6 months post-treatment between an extraction site treated with ARP and an alveolus treated with unassisted healing: the null hypothesis of no difference in the alveolar ridge and soft tissue changes between spontaneous healing and ARP was rejected and the results obtained using a bovine xenograft covered with an absorbable bovine pericardium membrane were comparable with those of Jung et al., who used xenografts [15]. In that study, the group treated with demineralized bovine bone mineral with either a soft tissue graft or a collagen matrix showed less vertical and horizontal change than the test group treated with beta-tricalcium phosphate and the control group who underwent spontaneous healing.
The ARP in our study was performed both flapless and with the elevation of a muco-periosteal flap, which was carried out only when required due to the complexity of the extraction. The influence of flap design on improving healing is much debated. According to more recent clinical studies, the surgical trauma, which includes raising a muco-periosteal flap with vertical incision and the detachment of periosteum, may lead to an extra amount of bone loss from the external part of the socket in addition to the bone resorption determined by the loss of bundle bone from the inside [27,28,29,30].
The systematic review by Vignoletti et al. [31] identified the wound closure technique as the most important factor influencing the outcome of ARP. In contrast, a more recent systematic review by Lee et al. [32] investigated the effects of different ARP procedures and found no significant differences in alveolar ridge height and width between the technique performed with the flap, the flapless technique with the application of a membrane, and the adjunctive use of a free gingival graft. Notably, compared with secondary closure, primary wound closure did not have a positive effect on preservation. It had a negative effect because it can result in greater shrinkage of the keratinized gingiva.
The optimal type of graft remains unclear. Bone grafting materials used for alveolar ridge preservation can be autogenous, allografts, xenografts, or alloplasts. Each biomaterial has specific features such as the resorption rate, which can be very slow for alloplasts and xenografts but faster for autogenous bone and allografts [33].
The use of a barrier membrane has also been investigated, and recent studies have found that the combination of a biomaterial and an absorbable membrane can succeed in decreasing both horizontal and vertical ridge shrinkage [34].
The review by Jung et al. (2018) examined ARP procedures for soft tissue and hard tissue preservation. For soft tissue preservation, the options available are autogenous connective tissue graft from tuberosity or palate or soft-tissue substitutes in order to reduce postoperative discomfort. For hard tissue preservation, the most documented procedure was the combination of a bone substitute material covered with an absorbable membrane. Although various materials have been used for ARP procedures, no one material has been found to be better than the others [35].
The systematic review by Avila et al. (2019) evaluated the effect of different ARP treatment modalities by analyzing clinical, radiographic, and patient-reported outcomes. The authors concluded that ARP procedures could prevent horizontal, vertical mid-buccal, and vertical mid-lingual bone resorption, but it was not possible to assess the superiority of one procedure over another [36].
A clinical trial by Papace et al. (2021) compared the use of autologous connective tissue graft with a collagen matrix for soft tissue management in ARP: they resulted to be comparable in terms of gingival thickness and peri-implant health [37].
The biomaterial used in our study was a bone substitute of bovine origin processed at low temperatures and able to create an environment favorable to osteoblast proliferation and bone regeneration. The main advantage of this biomaterial is its osteoinductive property due to the preservation of most of the bone cell matrix proteins. These proteins can induce specific bone markers involved in bone regeneration, such as osteopontin, osteocalcin, osteonectin, and type 1 collagen, as has been demonstrated in in vitro experiments. Moreover, its microporosity allows vessels and cells to colonize the graft so that its resorption time is reduced.
Regarding the width and height of the alveolar ridge, our findings are in line with those of previous studies that found that ARP techniques cannot fully preserve them. A certain amount of bone resorption should be expected when implementing ARP procedures, although there will be less resorption than if the alveolus was left to heal spontaneously [4].
The bone loss that occurred at the extraction sites in the control group was similar to that reported in the systematic review by Tan et al. [38]. That review stated that after tooth extraction, the percentage of vertical buccal bone resorption was 11–22% (0.8–1.5 mm, weighted average 1.24 mm at 6 months), and the percentage of horizontal buccal bone resorption was 29–63% (2.46–4.56 mm, weighted mean 3.79 mm at 6 months), with vertical resorption being less pronounced than horizontal resorption at 6 months.
The anterior superior esthetic zone is considered to be an area at high risk of alteration following tooth extraction and can experience marked changes in the surrounding soft tissue. Adequately supporting the soft tissue during ARP procedures is crucial for esthetics, and it may decrease the need for further soft tissue grafting in the future [39].
This study used digital 3D models of the dental arches to analyze the soft tissue independent of bone quantification.
The mean buccal soft tissue loss over a 6-month period after tooth extraction was 62.66 mm3 ± 17.50 mm3 for the ARP group and 106.41 ± 24 mm3 for the SH group, which was a significant difference (P = 0.004).
The results of the present study indicate that a reduction in the buccal soft tissue volume is expected following tooth extraction in which the alveolus is left to heal spontaneously. The interpretation of these data necessarily requires caution because the soft tissue volume loss was measured independently of the volume of the underlying bone resorption, and its clinical significance requires a different interpretation in relation to the linear measurements reported in millimeters.
Because of the high heterogeneity among studies in terms of morphology, biomaterials, surgical techniques, and healing periods, caution is needed when comparing our results with those of previous studies. Our study was a retrospective analysis of a small sample who were treated without a standardized protocol. Moreover, our study did not utilize randomization or histological analysis, which could have helped to evaluate the quality of the vital regenerated bone and its proportions. Nevertheless, the comparison of the ARP and SH groups offers an analysis of many factors that might positively influence ARP procedures, such as the preoperative evaluation of FSTT, analyzing the buccal bone thickness with CBCT, assessing the effect of a flapless approach when possible, and examining the combination of a xenograft plus an absorbable membrane. Moreover, the results were evaluated using linear measurements of CBCT and volumetric measurements of the changes in the region of interest to analyze the soft tissue profile. Finally, patient-reported outcomes and professional esthetic analysis with the Pink Esthetic Score were reported at 1 year post-treatment.