A comparison between anorganic bone and collagen-preserving bone xenografts for alveolar ridge preservation: systematic review and future perspectives

Bone grafts and substitutes are increasingly used in dental implantology due to the growing need for replacing insufficient alveolar bone before implant placement [1]. One of the primary reasons for bone deficiency is tooth loss due to periodontal disease, tooth fracture/trauma, periapical lesions, or other pathological conditions [2]. Experimental evidence collected through animal [3, 4] and human [5, 6] studies demonstrated that after tooth extraction, the alveolar bone undergoes a remodeling process with consequent resorption of the vestibular cortical bone and gradual loosening of the marrow component of the alveolus. Bone reduction is mainly due to the lack of intraosseous stimulation normally provided by periodontal ligament fibers [1], and it is probably correlated with disruption of the blood supply and osteoclastic activity that occur after tooth extraction [7, 8]. The greatest amount of alveolar socket resorption occurs in the first 3 months after extraction, with a 30% reduction of the alveolar ridge (3.87 mm in width and 1.67 mm in height) [9‐11]. Dimensional changes take place up to 1 year thereafter, with about 50% total reduction (5–7 mm in width) of the alveolar ridge within 12 months post-extraction [8, 12, 13]. Interestingly, alveolar ridge resorption is more severe on the buccal side than on the lingual side [3, 11].

Bone dimensional changes at the post-extraction site influence the subsequent implant treatment plan; this important clinical issue is currently treated by alveolar ridge preservation (ARP) techniques. Also known as “socket preservation”, ARP includes methods of counteracting alveolar bone resorption after tooth extraction by (1) maintaining the soft and hard ridge components, (2) sustaining bone regeneration within the socket, and (3) facilitating prosthetically driven implant placement [10, 14‐16]. Recent systematic reviews with meta-analyses demonstrated that in comparison with unassisted socket healing, ARP procedures reduce alveolar bone dimensional changes and can promote bone regeneration at the post-extraction site [17‐20]. Furthermore, dental implants inserted into ARP-treated sites exhibited a high survival rate [20]. ARP is most commonly achieved by filling the alveolar socket with a bone grafting material immediately after tooth extraction [13]. The ideal properties of bone substitute materials include osteogenic, osteoinductive, and osteoconductive capacities similar to the native bone, as well as high biocompatibility and low immunogenicity [21]. Materials currently being investigated for ARP use include autologous bone, demineralized or mineralized freeze-dried bone allografts, xenogenic bone, alloplastic polymers, bioactive glasses, and composite ceramic substitutes [22, 23]. Among these options, xenografts seem to avoid comorbidity issues, ensuring larger availability from animal rather than human bone and avoiding tissue-banking costs. Furthermore, xenogenic bone shows better resorption and integration capacity with the host tissue than synthetic materials.

Amongst heterologous materials, the use of anorganic bone xenografts (ABXs) for ARP procedures is well supported by scientific literature, with successful outcomes obtained in both animal preclinical studies and human randomized clinical trials [24‐26]. ABXs are produced by exposure to heat and chemical extraction processes to remove immunogenic and organic components and are then prepared as porous grains (0.25–2 mm) [25, 27]. Regardless of the species of origin (i.e., bovine or porcine), ABXs exhibit structures and properties similar to their human counterparts, with clinical evidence demonstrating comparable outcomes among xenografts from different sources [28]. Besides demonstrating good osteoconductive properties, heat-treated ABXs also have poor resorption rates [29‐31].

Another xenogenic biomaterial successfully used for ARP procedures is non-heat treated cortico-cancellous porcine bone (CPB), which is subjected to a collagen-preserving chemical process for immunogenic component removal and is then prepared as micro-porous particles (diameter 0.6–1 mm) [32]. These collagen-containing porcine bone grafts possess excellent osteoconductive properties and do not cause inflammatory infiltration [33, 34]. These biomaterials also show clear signs of resorption/remodeling after socket grafting, with the formation of scalloped lacunae [35, 36].

Successful ARP outcomes were recently achieved by grafting the post-extraction socket with enzyme-deantigenic equine bone (EDEB), which also consists of a mixture of cancellous and cortical bone granules (diameter 0.25–1 mm) made non-antigenic with digestive enzymes [37, 38]. In addition to ARP procedures, EDEB was used with satisfactory results in peri-apical cyst-removal management [39], horizontal/vertical ridge and sinus augmentation [40‐42], and orthopedic applications [43‐45].

Unlike ABXs, CPB and EDEB are collagen-preserving bone xenografts (CBXs) manufactured by chemical (CPB) or enzymatic (EDEB) treatment that maintains type I bone collagen in its native state. This may offer important advantages in terms of stimulation of the regenerative process, integration with the host tissue, and graft resorption rate [38, 46, 47].

There is scant evidence in the literature about which of these two classes of xenogenic bone substitutes—ABXs or CBXs—is better for preserving post-extraction sockets. To the best of our knowledge, only three clinical trials have compared the dimensional and histomorphometric outcomes of ABXs and CBXs, with one suggesting that CBX might produce a better healing pattern, and one demonstrating that collagen-preserving material obtained by enzymatic treatment ensures better bone regeneration and graft resorption [31, 36, 38]. This systematic review was performed to (1) compare bone dimensional changes after tooth extraction and ARP by ABXs or CBXs and (2) analyze and compare histologic and histomorphometric outcomes for post-extraction sites grafted with the two types of bone substitutes.

The present review was designed and conducted according to PRISMA (Preferred Reporting Items Systematic review and Meta-Analyses) guidelines [48, 49].

The effects of ridge preservation with the use of different biomaterials have been thoroughly investigated, and filling of post-extraction sites with bone xenografts was clinically demonstrated to significantly reduce ridge changes in comparison with spontaneously healed sockets [91]. Ridge preservation treatment also reduced the need for further bone augmentation at the time of implant placement, ameliorating the aesthetic outcome of implant rehabilitation [34, 81]. Xenogenic material currently used for ridge preservation is predominantly anorganic bovine/porcine bone made from the inorganic portion of animal bone tissue. The manufacturing process to produce ABXs is based on high-temperature treatment (> 300 °C), which removes cells and xenogenic antigens to avoid potential immunologic reactions. This method also eliminates all organic components and proteins, while HA with enhanced crystallinity is maintained as the main graft constituent [60, 92, 93]. Deproteinized xenografts were demonstrated to have good physico-chemical and osteoconductive properties in ridge preservation strategies. Nevertheless, suboptimal biointegration and bioabsorption characteristics of heat-treated materials suggest that the processing protocol for xenograft bone substitutes may greatly affect the biomaterial behavior in situ regarding the regenerative potential and quality of NFB [93]. To overcome these limitations, bone xenografts fabricated with less aggressive treatment to remove xenogenic antigens were proposed to preserve the collagen component of the animal bone, ultimately improve the bioactive properties of the final product [38, 94]. The preservation of type I collagen in bone substitutes can improve socket healing in ARP procedures by a series of processes, including (1) enhanced stimulation by endogenous growth factors; (2) longer duration of regenerative stimuli; (3) physiological modulation of bone metabolism and remodeling; and (4) increased osteoblast adhesion, proliferation, and differentiation [95‐98]. Indeed, this might have contributed to the successful clinical outcomes with CBXs use reported for different oral surgery procedures including sinus lift bone grafting [42, 99‐102], ridge augmentation [103‐105], and peri-implant-guided bone regeneration [106‐108]. However, direct clinical comparisons between anorganic and CBXs for socket preservation were only reported in three clinical trials [31, 38, 82], so the superiority of one biomaterial over another has not been established yet. In this work, clinical research testing ABXs or CBXs for ridge preservation was systematically reviewed to perform a preliminary comparison in terms of the biomaterials’ dimensional and histomorphometric outcomes. Table 6 summarizes the collected results, presenting minimum and maximum average values and standard deviations recorded for horizontal/vertical ridge resorption, as well as the percentage of NFB, connective tissue, and residual graft particles at the grafted sites.

Clinical outcomes for alveolar ridge dimensional changes showed successful socket preservation when using both ABXs and CBXs in comparison with spontaneous healing, with ABXs yielding better results than untreated control and largely similar to bone allografts and synthetic materials. Horizontal ridge resorption was calculated to range from 0.065 to 2.8 mm for ABXs and from 0.93 to 3.5 mm for CBXs, with standard deviations ranging from 0.14 to 3.34 mm and from 0.55 to 1.3 mm, respectively. Thus, lower minimum and maximum values of horizontal bone loss were observed for ABXs, but the standard deviations showed a broader value range compared with CBXs (Table 6).

Vertical ridge reduction was found to be between 0.1 and 2.92 mm for ABXs and between 0.2 and 1.1 mm for CBXs, with standard deviations ranging from 0.2 to 3.6 mm and from 0.5 to 1.54 mm, respectively. In this case, ABXs showed a lower minimum change but higher maximum alteration of vertical ridge dimensions with respect to CBXs, but the value range for standard deviation was still broader for the heat-treated bone substitute (Table 6).

Histomorphometric evaluations after ARP of the post-extraction sockets produced less obvious results for the superiority of both anorganic bone substitutes and CBXs over spontaneous healing or other treatments, since significant differences in terms of new bone formation were less frequently reported by clinicians. However, high biocompatibility and capacity to promote bone regeneration were observed for both xenografts. Remarkably, Di Stefano and co-workers [38] provided the first evidence of significantly better histological performance for CBXs rather than ABXs, supporting the hypothesis that maintaining type I collagen in its native conformation may improve the biological effects of the graft and promote faster remodeling of the heterologous material [109].

In summary, despite the much larger number of clinical trials for ABXs rather than CBXs, the two types of xenografts seem to provide overlapping dimensional/histological outcomes with large measurement dispersion, underscoring the need of comparative clinical studies that may demonstrate the superiority of one material over the other at a statistically significant level.

Regarding histomorphometrical measurements, NFB was between 5.3 and 37.68% for ABXs and between 22.5 and 45.12% for CBXs, with standard deviations ranging from 4.32 to 26.51% and from 3.9 to 20.6%, respectively. Based on that, higher amount of NFB and lower variability were registered for CBXs versus ABXs. This trend was also confirmed for data concerning residual graft particles, which overall exhibited better results for CBXs (lower range values, 10.92–29.2%) compared to ABXs (higher range values, 8.89–52.03%), with less variability for the collagen-preserving biomaterials (10.91–29.2% for CBXs and 8.89–52.03% for ABXs) (Table 6). As shown in Table 6, the amount of NFB was on average higher for CBXs rather than ABXs, with the minimum value being much greater (> 17.2%) for CBXs with respect to ABXs. On the other hand, the average amount of residual graft particles was lower for CBXs, which had a clearly inferior maximum value and standard deviation range with respect to ABXs. Regarding connective tissue evaluation, lower measurement dispersion was observed for CBXs in comparison with ABXs. Although these trends need to be verified in controlled clinical studies, they are in line with evidence collected by recent trials that compared ABXs and CBXs and demonstrated better histomorphometric outcomes for CBXs in both ARP [38] and sinus augmentation [42] procedures.

Concerning dimensional outcomes, some possible trends might be hypothesized based on collected data regarding horizontal ridge resorption, which seems to be more limited by anorganic bone grafting, albeit with a larger measurement dispersion (maximum standard deviation for ABXs is about three times higher than for CBXs). Conversely, vertical ridge preservation seems to be well achieved by CBXs, with maximum resorption measures more than halved compared to ABXs (Table 6).

One topic meriting discussion is data variability, which appears high for all the endpoints of interest, both among different studies and within each study included in this review. Variability among studies may be explained by the different surgical techniques and various methods to measure the same endpoints. Endpoints describing dimensions varied: vertical or horizontal width, buccal versus lingual plates, measurements performed at the crestal level or at different vertical levels apically from the crest. In addition, no standard methods for histomorphometric measurements were considered, which also contributed to histological outcome variability.

Data variability was also present at the single-study level, highlighting how bone regeneration and dimensional resorption are multifactorial processes. That is, histomorphometric and dimensional outcomes are expected to be influenced by a number of variables that might act as confounders when investigating if the two types of xenografts have any differential effects when used for ARP.

Among such confounders, the time from surgery when dimensional and histomorphometric assessment are performed might play a pivotal role. In fact, differences in the bone-formation rate might be more evident and statistically significant if clinical evaluations are performed at earlier rather than later timepoints. This hypothesis is supported by the retrospective clinical study by Di Stefano and collaborators [100], demonstrating that when CBX was used for sinus augmentation, no significant differences in NFB and residual graft material were detected between samples evaluated at different times from grafting (i.e., 3–5 months, 6–8 months, 9–12 months). These data suggest that new bone formation with CBXs occurred soon after the grafting surgery. Remarkably, early bone deposition is consistent with the significant difference detected in the amount of NFB provided by CBXs rather than ABXs in studies of ARP and sinus augmentation [38, 42]. In this regard, the clinical trials included in this systematic review also showed certain variability for the time of analysis, suggesting that the influence of this confounding factor on detecting significant differences among experimental groups remains to be clarified with appropriate studies.

Concerning the amounts of NFB that might be achieved with the two types of xenografts, one might speculate that there is an upper limit. Indeed, recent evidence showed that post-natal intramembranous bone regeneration mirrors the intramembranous ossification that occurs during embryonic bone development, with several molecular and cellular actors involved in both scenarios [110]. Because of this, the upper limit to NFB might be equal to the physiological amount of bone that patient has at the position of the arch where regeneration will occur. This might be a reasonable assumption, at least when osteoconductive grafts are used and one does not use recombinant growth factors or other drugs capable of altering bone metabolism in a relevant way. If this is the case, another factor affecting the dimensional and histomorphometric outcomes of ARP might be the position within the two arches. Indeed, a recent retrospective assessment of 6060 bone density measurements performed in 2048 patients across the two arches showed that bone density (i.e., the amount of bone by volume unit) at each position within the upper or the lower jaw exhibits significant interindividual variation, and the same patient may display significantly different densities at various positions [111]. Thus, the amount of bone growth expected should vary according to the location of the grafted site.

Finally, within the limits of the present systematic review, it is worth pointing out that the addition of a collagen carrier to ABXs did not improve dimensional and histomorphometric results compared to ABXs alone, remaining merely a technical option that allows easier biomaterial handling and application.

Thus, although the trends described in the present study suggest that ABXs and CBXs may provide different dimensional and histomorphometric outcomes when used for ARP, whether they actually do remain an open question. Answering it will require appropriate RCTs with adequate sample sizes and an experimental design carefully conceived to eliminate or at least limit the effects of several confounding factors. Possibly, studies should focus on more homogeneous patient subgroups as far as bone density is concerned (as opposed to the general population who might be subjected to ARP). Researchers should also compare xenografts grafted in symmetric or adjacent positions within the same jaw; biopsies for histomorphometric assessment should be taken soon after procedures to detect if bone formation kinetics vary between the two types of xenografts. Furthermore, the effect of carriers should be carefully investigated. While collagen added to ABXs does not seem to provide any advantage, except for better handling, it (and other carriers) might still act as a confounder, so in our opinion, studies should first compare xenografts (i.e., bone granules) with no carrier added. Finally, should any difference in histomorphometric outcomes ever be observed between ABXs and CBXs when used in ARP, future studies should investigate if this correlates with dimensional preservation of the ridge, as this point still seems unclear. Well-designed studies comparing ABXs and CBXs for ARP procedures may also allow to minimize data variability and study heterogeneity; those of data collected and discussed in the present review were indeed too high to perform any meaningful statistical analysis. This is an important limitation of the present work.