TRAP-Positive Multinucleated Giant Cells Are Foreign Body Giant Cells Rather Than Osteoclasts: Results From a Split-Mouth Study in Humans

Sinus augmentation has become a reliable surgical procedure to increase the amount of bone in the upper molar region prior to the insertion of dental implants. Alveolar atrophy in edentulous regions, extremely pneumatized sinus maxillaris, or tumor resection can prevent oral rehabilitation with dental implants. Since the first introduction of this approach by Tatum and Boyne,¹  several surgical techniques and augmentation materials have been described. Autologous bone harvested from the iliac crest or different enoral sites is taken as the gold standard for augmentation procedures due to its osteoinductive, osteoconductive and osteogenic potential.²  However, harvesting autologous bone entails numerous risks, such as donor site morbidity, the requirement for general anesthesia, and the risk of additional complications at the donor site.³ 

Therefore, in recent years, biomaterials research in this area has focused on the development and advancement of nonautologous bone grafts with the aim of avoiding the above-mentioned complications. Xenografts of bovine, equine, or porcine origin are processed mechanically, chemically, and thermally, resulting in inorganic bone matrices serving as scaffolds for the ingrowth of bone and progenitor cells in the host tissue.²  The resulting materials have been applied in a variety of augmentation procedures in numerous in vitro, animal, and clinical trials.^4–6  Furthermore, alloplastic grafts of synthetic origin have been developed, manufactured from hydroxyapatite (HA), α- or β-tricalcium-phosphate (α-TCP and β-TCP) or biphasic glasses, and these materials can be individually tailored to the specific indications of the augmentation process.⁶ 

In a number of consecutive animal and clinical trials, the synthetic bone graft NanoBone (NB; Artoss, Rostock, Germany) has been investigated on the basis of its physicochemical structure and integration in animal implantation models as well as in human tissue.^7–12 

In an experimental trial, the bone substitute NB was implanted subcutaneously in Wistar rats and assessed histologically and histomorphometrically. The main focus was to determine the vascularization and biodegradation behavior over 6 months. Within the implantation bed, the vascularization already reached its peak after 10 days with relatively few TRAP-positive and TRAP-negative multinucleated giant cells and macrophages. Mononuclear cells, such as macrophages, were randomly distributed on the bone substitute material.⁷ 

Similar findings were observed after implantation in the muscle tissue of goats, where NB degradation by TRAP-positive and TRAP-negative multinucleated giant cells could be observed.⁸ 

Preliminary histological investigations of the synthetic bone substitute material NB when used for sinus augmentation in humans showed undisturbed integration of the granules into the host tissue 6 months after augmentation, as well as newly formed bone in all parts of the augmented region. Animal studies revealed that the surfaces of the bone substitute particles were populated by multinucleated giant cells.⁹ 

In a subsequent clinical trial, bone formation when using NB in sinus augmentation procedures was compared 3 and 6 months after augmentation by the histological and histomorphometric analysis of extracted bone biopsies. Six months after augmentation, continuous bone formation was observed in all parts of the biopsies, while 3 months after augmentation, new bone formation could be observed in the caudal two-thirds of the biopsies. However, no statistically significant difference was observed in new bone formation between 3 and 6 months.¹⁰  To determine the impact of the amount of new bone formation on implant stability and survival, the inserted implants were investigated after 3 years. Only 1 of 24 implants was lost at the follow-up investigation, indicating that the synthetic bone substitute material NB already contributes to implant stability 3 months after augmentation.¹¹ 

Recently, the clinical application of synthetic (NanoBone) and xenogeneic bone substitute materials (BO; Bio-Oss, Geistlich Biomaterials, Wolhusen, Switzerland) was investigated for the first time in a clinical split-mouth trial in sinus augmentation in former head and neck cancer patients. The histological and histomorphometric analysis revealed significantly greater vascularization and a greater presence of multinucleated giant cells in the NB group compared to the BO group. For this group of tumor patients, both biomaterials achieved comparable results for new bone formation, although the cellular mechanisms of that formation differed: While NB was degraded by the aforementioned multinucleated giant cells and replaced mainly by connective tissue, BO remained in the implantation bed, serving as a placeholder and a scaffold for bone apposition. However, both bone substitute materials formed a sufficient implantation bed, as 2 years after the insertion of dental implants only 1 implant was lost from the NB group.¹² 

As the role of the inflammatory response induced by non-autologous bone substitute materials within the sinus cavity remains unclear, a split-mouth sinus augmentation study was performed in humans to compare the synthetic and xenogeneic bone substitute materials in terms of their different cellular mechanisms, as well as in terms of the clinical performance of implants in the augmented region.

The aim of the present study was to compare the synthetic HA-based bone substitute material NanoBone to the xenogeneic bovine bone substitute material Bio-Oss in a sinus augmentation split-mouth trial in humans. Further, this study is intended for comparison with a similar study in patients with head and neck cancer. This comparison will provide information about the differences in cellular reactions between a healthy patient group and a group of head and neck cancer patients. The different cellular mechanisms of biomaterial integration and cellular response were evaluated 6 months after augmentation. Histological and histomorphometric methods were used to assess the percentages of newly formed bone, connective tissue and remaining biomaterial, and the degree of vascularization and degree of multinucleated giant cell formation as an expression of a foreign body response.

The present study was approved by the ethics commission of the University of Frankfurt am Main and performed in accordance with the fifth revision of the World Medical Association Declaration of 2000 in Helsinki. All patients gave informed consent prior to the sinus augmentation procedure.

A total of 14 partially or completely edentulous patients (4 male and 10 female) with an average age of 61 years (ranging from 42 to 75 years) with severe atrophy of the maxillary bone were enrolled from the Department for Oral, Cranio-Maxillofacial and Facial Plastic Surgery, Frankfurt am Main. The alveolar height at the prospective implant site was less than 5 mm. Additional inclusion criteria were that the implant site had to be free of infection and that the included patients had to have adequate oral hygiene. Patients excluded had medical or general contraindications for surgical procedures, chronic alcohol abuse, chronic liver or kidney disease, metabolic diseases (eg, diabetes mellitus or osteoporosis), bisphosphonate therapy, or heavy smoking habits of more than 20 cigarettes per day.

According to the CONSORT 2010 Statement, investigators assessing the histological and histomorphometric results were kept blinded to the allocation. After preoperative assessment of the clinical history and surgical planning, patients achieved augmentation of the subantral space with the synthetic nanocrystalline bone substitute NanoBone and the xenogeneic bone substitute Bio-Oss, each randomly allocated to one side of the sinus cavity.

After a mean healing period of 7 months (ranging from 5 months to 9 months), 50 dental implants (CAMLOG ScrewLine, Camlog Biotechnologies, Basle, Switzerland) were placed in the augmented upper molar region. Simultaneous with the insertion of dental implants, cylinder-shaped bone biopsies of the augmented maxillary bone were extracted for histological and histomorphometric examination of the implantation bed.

Bilateral augmentation of the subantral space with either a synthetic or xenogenic bone substitute material in alternate sides of the sinus cavity was performed under general anesthesia as previously described.^9,10,12  A standard crestal incision was performed, and a vestibular-based mucoperiosteal flap was prepared to expose the maxillary bone. Antrostomy was performed using the Piezosurgery device (Mectron, Carasco, Italy) without perforation of the sinus membrane. After enlarging the subantral space by elevating the sinus membrane, the bone substitute materials NB and BO were incorporated after mixing with blood extracted from the surgical site. No additional autologous bone was used in the form of chips or blocks. Afterward, the prepared antrostomy window was covered with a native collagen membrane (Biogide, Gestlich Biomaterials, Wolhusen, Switzerland), and wound margins were adapted with absorbable tension-free single sutures.

After a mean healing period of 7 months (ranging from 5 to 9 months), 50 dental implants (CAMLOG ScrewLine, Camlog) were inserted in the augmented regions through a crestal approach. Simultaneously with the implantation, bone cores were made with trephine burrs (3 mm in diameter) for histological and histomorphometric examination.

In both surgical operations, penicillin antibiotics were started intraoperatively via intravenous application, continuing orally for 10 days postoperatively. A 0.2% chlorhexidine mouth rinse and 400 mg ibuprofen were also prescribed.

After an additional 6-month period, the inserted implants were exposed and restored with removable or fixed prosthetics.

The fully synthetic bone substitute material NanoBone (NB, Artoss) is composed of HA crystallites embedded in a silica gel matrix structure. The HA granules with an average size of 60 μm are manufactured in a sol-gel technique with temperatures below 700°C. Thus, sintering can be avoided, resulting in a nanoporous bone substitute with a large internal surface area of up to 84 m²/g. Within the silica gel, pores can be from 100 to 1000 μm (macropores) or 2–10 μm (micropores) in size with numerous open links that interact in loose connections with HA crystals. Macroscopically, the bone substitute appears as a cone with an average length of 2 mm, an average diameter of 0.6 mm, and a porosity of 60%–80%.^13,14 

The xenogeneic graft material Bio-Oss (Geistlich Biomaterials) is made from deproteinized bovine bone mineral with a granule diameter ranging from 0.25 to 1.0 mm. This material has a porosity of 70%–75%, with pore sizes ranging from a few nanometers to 1.5 μm. Different chemical and mechanical processing and preparation steps are used to remove the organic components of the bovine bone, leaving an inorganic bone matrix that is chemically and physically similar to the human extracellular bone matrix.

In various human clinical trials, BO has been proven to be an effective bone graft matrix.^4,15,16 

Simultaneously with the insertion of dental implants, 28 bone biopsies (1 biopsy per augmented area) were obtained for histological and histomorphometric analysis, of which 24 were useful for analysis after processing and staining. In accordance with previously described methods, bone-biomaterial interactions and tissue reactions were analyzed.

The biopsies were immediately fixed after extraction in 4% neutral buffered formalin for 24 hours and decalcified in 10% Tris-buffered EDTA (Carl Roth, Karlsruhe, Germany) at 37°C for 10 days. Afterward, biopsies were passed through a series of increasing alcohol concentrations followed by Xylol. Subsequently, biopsies were embedded in paraffin and cut with a microtome (Leica, Wetzlar, Germany) in the longitudinal plane in 3–5 μm–thick sections. Each biopsy was stained with 11 different stains as follows:

The first slide was stained with hematoxylin and eosin, and the second with Masson-Goldner's trichrome with Weigert's iron hematoxylin as a counterstain. The third slide was stained for Giemsa, the fourth slide with Movat's pentachrome, the fifth slide with Sirius Red, and the sixth slide with Azan. To determine potential biomaterial degradation and identify foreign body cells, the seventh slide was histochemically stained with tartrate-resistant acid phosphatase (TRAP). The eighth and ninth slides were immunohistochemically stained to detect vessels with the human CD31 antibody and to detect the TRAP5 enzyme, respectively. These slides were deparaffinized with xylene and hydrated in ethanol, followed by demineralized water and heat-induced epitope retrieval for another 30 minutes. Staining with primary antibodies against the TRAP5 enzyme (LIFESPAN BIOSCIENCES, LS-C18195/11703) at a dilution of 1:40 and against CD31 (DAKO, M0823, clone C70A) at a dilution of 1:50 was performed for 30 minutes in a humid chamber. These slides were counterstained with Mayer's hemalaun. The antibodies were detected with a Dako REAL EnVision Detection System Peroxidase/DAB+, Rabbit/Mouse-Kit (Dako, Glostrup, Denmark) and visualized by DAB (3,3′-Diaminobenzidine). Afterward, the slides were dehydrated by increasing the ethanol concentration up to 100% followed by a final treatment with xylene. Finally, the slides were coated with mounting medium and covered with coverslips. Slides 10 and 11 served as negative controls for immunohistochemical staining in the absence of the primary antibody.^6,7,12,17 

Qualitative histopathological evaluation was performed according to previously described methods to assess the tissue-bone substitute interaction within the implantation bed and the peri-implant tissue.^7,9,18,19  The presence of neutrophils, plasma cells, lymphocytes, multinucleated giant cells, macrophages, and TRAP-positive osteoclast-like cells—as well as the level of vascularization—were evaluated to analyze the inflammatory response and the potential foreign body reaction induced by the biomaterials. Photomicrographs were recorded using a Nikon DS-Fi1 digital camera and a digital sight control unit (Nikon, Tokyo, Japan).

In addition to the qualitative histological analysis, quantitative histomorphometric analysis of stained slides was performed with a research scanning microscope in combination with NIS-Elements software (Nikon).

As previously published, images of the total implantation beds (“total scans,” see Figure 1), that is, a complete image of the biopsy including the bone substitutes and the peri-implant sinus tissue, were digitized with a DS-Fi1 digital camera connected to an Eclipse 80i histological microscope (Nikon), equipped with an automatic scanning table (Prior, Rockland, Md).^10,12,19 

The NIS-Elements software was used to determine the total implant area, the amount of newly formed bone and connective tissue, and the amount of remaining bone substitute material. The areas of the various tissue components were then calculated as a percentage of the total implant area. The software was used to measure the number and the area (in μm²) of vessels within the implantation beds by manually marking the vessels within the digitized scans. Thus, the total number of vessels (in vessels/mm²) as well as the percentage vessel area (as a fraction of the total implant area in %) could be determined.

Additionally, the amounts of material-associated multinucleated giant cells and their subforms—that is, TRAP-negative and TRAP-positive multinucleated giant cells—were also counted manually using the NIS-Elements “count” tool. The total number of these cells and the number of each cell subform were calculated with respect to the total implant area on the slides (cell number/mm²).

Quantitative data are reported as the mean ± standard deviation (SD), and a one-way univariate analysis of variance (ANOVA) accompanied by LSD post hoc assessment were used to compare groups using SPSS 16.0.1 software (SPSS Inc, Chicago, Ill). Differences were considered significant (*) at P < .05, and highly significant (**) at P < .01. GraphPad Prism version 6.0 (GraphPad Software, La Jolla, Calif) was used to prepare graphs.

The qualitative histological analysis of extracted bone biopsies revealed the integration of NB particles into the host tissue 6 months after augmentation. Bone substitute particles were surrounded by newly formed trabecular bone tissue to some extent (Figure 2a1 and 2). Portions of the intergranular space were filled with granulation tissue—including fibroblasts, lymphocytes, monocytes, and macrophages—as well as newly invaded blood vessels. Multinucleated giant cells were visible on the surface of remaining NB granules. As multinucleated giant cells mainly express the TRAP enzyme (Figure 3b1 and 2, red arrows), histochemical TRAP staining showed that these cells could be divided into osteoclast-like TRAP-positive and TRAP-negative multinucleated giant cells. The distribution of multinucleated giant cells within the 2 groups showed that the giant cells are present in greater numbers in the NanoBone (Figure 3b1 and 2) group.

Greater proportions of the biomaterial remained in the NanoBone group, leading to a slight decrease in bone and tissue formation when compared to the Bio-Oss group (Figure 4b1 and 2).

Analysis of the Bio-Oss implantation bed revealed good integration within the surrounding host tissue (Figure 2a1 and 2). The particle surfaces were mainly integrated in a network of newly formed bone tissue and well-vascularized connective tissue (Figure 4a1 and 2). Additionally, fibroblasts, monocytes, and macrophages—as well as a very low number of lymphocytes—could be observed within the surrounding tissue. New bone formation seemed to originate from active osteoblasts on the surface of the bone substitute granules. Compared to the synthetic bone substitute NB, the granule surface area was less populated by multinucleated giant cells (Figure 3a1 and 2, red arrows) and mainly consisted of mononuclear cells. The TRAP enzyme staining showed that most of the multinucleated giant cells were TRAP-negative.

Apart from histological analysis, biopsies were investigated histomorphometrically to determine the formation of newly developed bone, connective tissue, and remaining bone substitute material.

NanoBone biopsies consisted of 44.20 ± 3.97% connective tissue, 19.02 ± 7.28% newly formed bone, and 36.77 ± 4.83% remaining bone substitute.

In contrast, biopsies from the Bio-Oss group consisted of 48.73 ± 13.43% connective tissue, 23.66 ± 7.97% newly formed bone and 27.61 ± 11.71% remaining bone substitute.

Within all three tissue fractions, no significant difference could be observed between the two study groups (Figure 4).

Within the implantation bed, the total number of multinucleated giant cells was determined and expressed as number of cells/mm².

The histomorphometric analysis of the total number of multinucleated giant cells revealed 47.61 ± 14.23 multinucleated giant cells/mm² in the NB augmentation sites and 8.36 ± 3.25 multinucleated giant cells/mm² in the BO augmentation sites, a statistically significant difference (**P < .01) (Figure 5a).

The numbers and distributions of biomaterial-associated TRAP-positive and TRAP-negative multinucleated giant cells were determined for both bone substitutes investigated here by expressing the number of cells per 1 mm² (number of cells/mm²) in the implantation bed.

The histomorphometric analysis of the occurrence of TRAP-positive multinucleated giant cells within the implantation beds of the bone substitutes showed that the numbers of that cell type were significantly higher in the NanoBone group (41.97 ± 15.75 TRAP[+] giant cells/mm²) as compared to the numbers of biomaterial-associated TRAP-positive multinucleated giant cells in the Bio-Oss group (5.60 ± 2.81 TRAP[+] giant cells/mm²) (**P < .01 ) (Figure 5b).

The histomorphometric analyses of the presence of TRAP-negative multinucleated giant cells also showed significantly higher numbers of this cell type in the NanoBone group (5.63 ± 2.02 TRAP[-] giant cells/mm²) as compared to the Bio-Oss group (2.76 ± 1.98 TRAP[-]) giant cells/mm²) (*P < .05) (Figure 5c).

The histomorphometric analyses also revealed that significantly more vessels were present in the NanoBone study group (28.69 ± 10.54 vessels/mm²) compared to the Bio-Oss implantation beds (10.42 ± 3.74 vessels/mm²) (***P < .001) (Figure 6a).

Furthermore, the analyses also revealed significantly higher vessel fractions within the NanoBone implantation beds (2.13 ± 0.82%) as compared to the Bio-Oss group (0.86 ± 0.33%) (**P < .01) (Figure 6b).

The histological and histomorphometric analysis of the NanoBone and Bio-Oss bone substitutes augmented in the sinus cavities of humans revealed differences in the cellular responses to these biomaterials and different mechanisms of integration. Histological analysis of the implantation bed showed that NB particles were surrounded by newly formed bone and connective tissue, with mainly TRAP-positive and TRAP-negative multinucleated giant cells populating the granule surfaces.

In contrast, BO particles were mainly integrated into newly formed bone and well-vascularized connective tissue. Only a few multinucleated giant cells were visible on the surfaces of the BO granules, with mononuclear cells as the principal cell type.

The histomorphometric investigation analyzed the ratio of newly formed bone, connective tissue, and remaining bone substitute material within the augmentation bed. Furthermore, the total numbers of multinucleated giant cells, TRAP-positive multinucleated giant cells, and TRAP-negative multinucleated giant cells were analyzed.

Comparing the histomorphometric results of the tissue distribution in the NB and BO groups, no significant differences in new bone formation, remaining bone substitute material, or connective tissue formation could be shown.

In contrast to the tissue distribution, significantly higher values were observed in the NB group in the total number of multinucleated giant cells, and the number of TRAP-positive and TRAP-negative multinucleated giant cells.

Further, histomorphometric analysis of the vascularization in the augmentation beds of both bone substitute materials revealed significantly more vessels in the NB augmentation bed. Similar results were observed for the vessel fractions in the augmentation beds, with a significantly higher ratio in the NB group than in the BO group.

In contrast to the present study, a similar study performed previously in a group of head and neck cancer patients revealed significant differences in tissue distribution between implant types.¹²  While the fraction of newly formed bone was not statistically significantly different (as in the present study), the amount of remaining bone substitute material was significantly lower and the amount of connective tissue was significantly higher in the NB group. Similar to the present study, the study in cancer patients found significantly higher numbers of multinucleated giant cells, TRAP-positive and -negative cells, and a significantly higher degree of vascularization in the NB group. The fact that the presence of significantly more multinucleated giant cells led to a significantly lower amount of remaining bone substitute in the head and neck cancer patient study raises the question of the actual function of these cells, which seemed to be able to degrade the HA-based NB to a certain degree. Thus, the question is whether these cells exert an osteoclastic function or act as foreign body cells. Osteoclastic cells not only degrade bone but also positively influence the deposition of new bone. As the multinucleated giant cells did not lead to an increase in new bone formation, these multinucleated giant cells can more likely be considered foreign body giant cells than osteoclastic cells.

The aim of the present study was to determine whether the observed biomaterial degradation initiated by multinucleated giant cells is specific to head and neck cancer patients. Therefore, we compared the tissue reaction to the synthetic HA-based NB and the synthetic HA-based NB in head and neck cancer patients with the tissue reaction in healthy patients.

In accordance with the study in head and neck cancer patients, the number of multinucleated giant cells and the degree of vascularization reached significantly higher values in the NB group. Moreover, similar nonsignificant differences were observed between the groups in new bone formation. In contrast to the head and neck cancer group, the present study did not reveal significant differences in the amount of remaining bone substitutes.

In conclusion, the results of the tissue distribution of multinucleated giant cell formation and vascularization indicate that multinucleated giant cells are foreign body giant cells without osteoclastic function. Their presence is associated with greater vascularization, which does not affect tissue regeneration or new bone formation, as a certain degree of vascularization seems to be enough to induce sufficient tissue and bone regeneration. Greater vascularization does not necessarily lead to better or faster tissue/bone regeneration. The in vivo studies and clinical trials of xenogenic and synthetic bone substitute materials allow us to postulate that a certain degree of vascularization is enough to induce sufficient regeneration. Greater vascularization is not necessary, especially if it is caused by a foreign body reaction.

Prior to the initiation of clinical trials in a healthy group and in a group of head and neck cancer patients, our group has thoroughly investigated the tissue reaction to nanocrystalline NB in vivo as well as clinically.

In the subcutaneous implantation model in Wistar rats and muscle pouches of goats, the augmented bone substitute material NB demonstrated early vascularization and degradation initiated by TRAP-positive and TRAP-negative multinucleated giant cells together with macrophages and lymphocytes.^7,8  In additional clinical trials, NB was used for sinus augmentation in a group of healthy patients and in a group of head and neck cancer patients to form a sufficient implantation bed for the insertion of dental implants. In all studies, NB led to sufficient formation of new bone in the augmented region after 6 months, with 1 study observing bone formation only 3 months after augmentation. As observed in the animal trials, the augmentation bed was well vascularized, and multinucleated giant cells were attracted to the site. These cellular reactions seem to be part of an inflammatory response to the synthetic biomaterial, which is treated as a foreign body by the human organism. In the special patient group of head and neck cancer patients, the synthetic NB was compared to the xenogeneic BO, which induced almost no foreign body response within the implantation bed. Interestingly, although the vascularization—and therefore the supply of progenitor cells for new bone formation—was significantly higher in the NB group, with no significant difference in the formation of new bone observed within the augmented region.^9,10,12 

Numerous investigations of the xenogeneic, porcine-derived bone substitute Bio-Oss have demonstrated its biocompatibility and suitability for different augmentation methods in humans. The total absence of a foreign body reaction or adverse effects in combination with clinical performance and ability to form new bone highlight Bio-Oss as one of the most reliable and frequently used biomaterials in oral, maxillofacial, and implant surgery. A long-term follow-up investigation to analyze the stability of BO augmented in the human sinus cavity revealed no significant change in the proportion of remaining BO, while the proportion of bone marrow and lamellar bone increased significantly.²⁰  Histomorphometric analysis of bone cores extracted 9 years after BO sinus augmentation supports these findings, as the tissue consisted of 16.0% BO.²¹ 

The previous trials investigating NB have shown that the induction of multinucleated giant cells within the implantation bed might be related to the synthetic origin of the biomaterial. The presence of multinucleated giant cells—and especially TRAP-positive multinucleated giant cells—can be interpreted as a manifestation of an ongoing foreign body reaction to the nonbiological origin of the material.

In contrast, the porcine-derived Bio-Oss induced negligible amounts of multinucleated giant cells. The absence of these cells might be attributed to the biologic origin of the biomaterial, avoiding the development of a foreign body response in the augmentation site. Additionally, the absence of a foreign body reaction seems to be a reliable indicator of the purity of Bio-Oss, which has recently been demonstrated.²² 

Although in recent years, biomaterial and tissue engineering research has focused on the search for an ideal bone substitute material, a gold standard has yet to be found. Allogeneic, xenogeneic, and alloplastic biomaterials have been tested with the aim of meeting the ideal criteria for bone regeneration: the induction and conduction of new bone in the augmentation site. In addition, biomaterial scientists and clinicians have begun to investigate the tissue reactions to the various biomaterials according to their origin and physicochemical structure.

Thus, biomaterial scientists need to know what type of cellular reaction is desirable in the human organism induced by bone substitute materials. Future biomaterial research must clarify the influence of a foreign body reaction in the long-term outcome of augmentation procedures. In this context, vascularization and the formation of multinucleated giant cells are critical. Researchers should investigate whether the induction of multinucleated giant cells, a greater degree of vascularization, and the formation of connective tissue lead to encapsulation or to the degradation of the augmented bone substitute material. That said, it is important to consider whether the extent of vascularization correlates positively with the formation of new bone or whether a saturation occurs. Another research approach is to use a synthetic bone substitute material that interacts within the human organism in a similar way as biological bone substitute materials without inducing a foreign body reaction, a material that cannot cause immunological responses because of its origin.

This study examined the material-specific tissue response to a xenogeneic, bovine-based and a fully synthetic, HA-based bone substitute material. Both bone substitute materials were augmented in the human sinus cavity to increase the amount of local bone in the upper molar region for the subsequent insertion of dental implants. Histological, histomorphometric, and immunohistochemical investigation of extracted biopsies were used to determine the formation of bone and connective tissue and the amount of remaining bone substitute, as well as the vascularization and the induction of a foreign body reaction. Both bone substitute materials supported new bone formation to a similar degree; however, the cellular reactions differed significantly. While the synthetic bone substitute material induced high vascularization and led to the presence of multinucleated giant cells within the implantation bed—indicative of a foreign body reaction—the xenogeneic bone substitute material was incorporated with a comparably lower degree of vascularization and multinucleated giant cell induction. The present study showed that depending on their origin, processing, physicochemical structures and biomaterials provoke different cellular reactions within the human organism. Therefore, knowledge of the biomaterial-specific cellular response is essential not only for biomaterial scientists but also for clinicians to ensure reliable and predictable clinical results.

Abbreviations

BO

Bio-Oss

HA

hydroxyapatite

NB

NanoBone

TRAP

tartrate-resistant acid phosphatase

α-TCP

α-tricalcium-phosphate

β-TCP

β-tricalcium-phosphate

This study was supported by a grant from the Camlog Foundation, Basel, Switzerland. The authors would like to thank Mrs. Ulrike Hilbig for her excellent technical assistance.