Effect of rapidly resorbable bone substitute materials on the temporal expression of the osteoblastic phenotype in vitro

Effect of a synthetic hydroxyapatite-based bone grafting material compared to established bone substitute materials on regeneration of critical-size bone defects in the ovine scapula

Lately, the potential risk of disease transmission due to the use of bovine-derived bone substitutes has become obvious, demonstrating the urgent need for a synthetic grafting material with comparable bioactive behaviour and properties. Therefore, the effect of a synthetic hydroxyapatite (HA) (Osbone®) bone grafting material on bone regeneration was evaluated 2 weeks, 1 month, and 3, 6, 12 and 18 months after implantation in critical-size bone defects in the ovine scapula and compared to that of a bovine-derived HA (Bio-Oss®) and β-tricalcium phosphate (TCP) (Cerasorb® M). New bone formation and the biodegradability of the bone substitutes were assessed histomorphometrically. Hard tissue histology and immunohistochemical analysis were employed to characterize collagen type I, alkaline phosphatase, osteocalcin, as well as bone sialoprotein expression in the various cell and matrix components of the bone tissue to evaluate the bioactive properties of the bone grafting materials. No inflammatory tissue response was detected with any of the bone substitute materials studied. After 3 and 6 months, β-TCP (Cerasorb® M) showed superior bone formation when compared to both HA-based materials (3 months: β-TCP 55.65 ± 2.03% vs. SHA 49.05 ± 3.84% and BHA 47.59 ± 1.97%; p ≤ 0.03; 6 months: β-TCP 62.03 ± 1.58%; SHA: 55.83 ± 2.59%; BHA: 53.44 ± 0.78%; p ≤ 0.04). Further, after 12 and 18 months, a similar degree of bone formation and bone-particle contact was noted for all three bone substitute materials without any significant differences. The synthetic HA supported new bone formation, osteogenic marker expression, matrix mineralization and good bone-bonding behaviour to an equal and even slightly superior degree compared to the bovine-derived HA. As a result, synthetic HA can be regarded as a valuable alternative to the bovine-derived HA without the potential risk of disease transmission.

Osteogenic Effect of a Bioactive Calcium Alkali Phosphate Bone Substitute in Humans

(1) Background: The desire to avoid autograft harvesting in implant dentistry has prompted an ever-increasing quest for bioceramic bone substitutes, which stimulate osteogenesis while resorbing in a timely fashion. Consequently, a highly bioactive silicon containing calcium alkali orthophosphate (Si-CAP) material was created, which previously was shown to induce greater bone cell maturation and bone neo-formation than β-tricalcium phosphate (β-TCP) in vivo as well as in vitro. Our study tested the hypothesis that the enhanced effect on bone cell function in vitro and in sheep in vivo would lead to more copious bone neoformation in patients following sinus floor augmentation (SFA) employing Si-CAP when compared to β-TCP. (2) Methods: The effects of Si-CAP on osteogenesis and Si-CAP resorbability were evaluated in biopsies harvested from 38 patients six months after SFA in comparison to β-TCP employing undecalcified histology, histomorphometry, and immunohistochemical analysis of osteogenic marker expression. (3) Results: Si-CAP as well as β-TCP supported matrix mineralization and bone formation. Apically furthest away from the original bone tissue, Si-CAP induced significantly higher bone formation, bone-bonding (bone-bioceramic contact), and granule resorption than β-TCP. This was in conjunction with a higher expression of osteogenic markers. (4) Conclusions: Si-CAP induced higher and more advanced bone formation and resorbability than β-TCP, while β-TCP's remarkable osteoconductivity has been widely demonstrated. Hence, Si-CAP constitutes a well-suited bioactive graft choice for SFA in the clinical arena.

A tissue engineered 3D printed calcium alkali phosphate bioceramic bone graft enables vascularization and regeneration of critical-size discontinuity bony defects in vivo

Introduction: Recently, efforts towards the development of patient-specific 3D printed scaffolds for bone tissue engineering from bioactive ceramics have continuously intensified. For reconstruction of segmental defects after subtotal mandibulectomy a suitable tissue engineered bioceramic bone graft needs to be endowed with homogenously distributed osteoblasts in order to mimic the advantageous features of vascularized autologous fibula grafts, which represent the standard of care, contain osteogenic cells and are transplanted with the respective blood vessel. Consequently, inducing vascularization early on is pivotal for bone tissue engineering. The current study explored an advanced bone tissue engineering approach combining an advanced 3D printing technique for bioactive resorbable ceramic scaffolds with a perfusion cell culture technique for pre-colonization with mesenchymal stem cells, and with an intrinsic angiogenesis technique for regenerating critical size, segmental discontinuity defects in vivo applying a rat model. To this end, the effect of differing Si-CAOP (silica containing calcium alkali orthophosphate) scaffold microarchitecture arising from 3D powder bed printing (RP) or the Schwarzwalder Somers (SSM) replica fabrication technique on vascularization and bone regeneration was analyzed in vivo. In 80 rats 6-mm segmental discontinuity defects were created in the left femur. Methods: Embryonic mesenchymal stem cells were cultured on RP and SSM scaffolds for 7d under perfusion to create Si-CAOP grafts with terminally differentiated osteoblasts and mineralizing bone matrix. These scaffolds were implanted into the segmental defects in combination with an arteriovenous bundle (AVB). Native scaffolds without cells or AVB served as controls. After 3 and 6 months, femurs were processed for angio-µCT or hard tissue histology, histomorphometric and immunohistochemical analysis of angiogenic and osteogenic marker expression. Results: At 3 and 6 months, defects reconstructed with RP scaffolds, cells and AVB displayed a statistically significant higher bone area fraction, blood vessel volume%, blood vessel surface/volume, blood vessel thickness, density and linear density than defects treated with the other scaffold configurations. Discussion: Taken together, this study demonstrated that the AVB technique is well suited for inducing adequate vascularization of the tissue engineered scaffold graft in segmental defects after 3 and 6 months, and that our tissue engineering approach employing 3D powder bed printed scaffolds facilitated segmental defect repair.

Calcium Phosphate Modified with Silicon vs. Bovine Hydroxyapatite for Alveolar Ridge Preservation: Densitometric Evaluation, Morphological Changes and Histomorphometric Study

After tooth extraction, the alveolar bone undergoes a physiological resorption that may compromise the future placement of the implant in its ideal position. This study evaluated bone density, morphological changes, and histomorphometric results undergone by alveolar bone after applying a new biomaterial composed of calcium phosphate modified with silicon (CAPO-Si) compared with hydroxyapatite of bovine origin (BHA). Alveolar ridge preservation (ARP) was performed in 24 alveoli, divided into a test group filled with CAPO-Si and a control group filled with BHA. Three months later, the mineral bone density obtained by the biomaterials, horizontal and vertical bone loss, the degree of alveolar corticalization, and histomorphometric results were evaluated. Both biomaterials presented similar behavior in terms of densitometric results, vertical bone loss, and degree of alveolar corticalization. Alveoli treated with CAPO-Si showed less horizontal bone loss in comparison with alveoli treated with BHA (0.99 ± 0.2 mm vs. 1.3 ± 0.3 mm), with statistically significant difference (p = 0.017). Histomorphometric results showed greater bone neoformation in the test group than the control group (23 ± 15% vs. 11 ± 7%) (p = 0.039) and less residual biomaterial (5 ± 10% vs. 17 ± 13%) (p = 0.043) with statistically significant differences. In conclusion, the ARP technique obtains better results with CAPO-Si than with BHA.

β-tricalcium phosphate for bone substitution: Synthesis and properties

β-tricalcium phosphate (β-TCP) is one the most used and potent synthetic bone graft substitute. It is not only osteoconductive, but also osteoinductive. These properties, combined with its cell-mediated resorption, allow full bone defects regeneration. Its clinical outcome is sometimes considered to be "unpredictable", possibly due to a poor understanding of β-TCP physico-chemical properties: β-TCP crystallographic structure is not fully uncovered; recent results suggest that sintered β-TCP is coated with a Ca-rich alkaline phase; β-TCP apatite-forming ability and osteoinductivity may be enhanced by a hydrothermal treatment; β-TCP grain size and porosity are strongly modified by the presence of minute amounts of β-calcium pyrophosphate or hydroxyapatite impurities. The aim of the present article is to provide a critical, but still rather comprehensive review of the current state of knowledge on β-TCP, with a strong focus on its synthesis and physico-chemical properties, and their link to the in vivo response. STATEMENT OF SIGNIFICANCE: The present review documents the richness, breadth, and interest of the research devoted to β-tricalcium phosphate (β-TCP). β-TCP is synthetic, osteoconductive, osteoinductive, and its resorption is cell-mediated, thus making it one of the most potent bone graft substitutes. This comprehensive review reveals that there are a number of aspects, such as surface chemistry, crystallography, or stoichiometry deviations, that are still poorly understood. As such, β-TCP is still an exciting scientific playground despite a 50 year long history and > 200 yearly publications.

Physiochemical properties of copper doped calcium sulfate in vitro and angiogenesis in vivo

Bone cements were prepared by mixing calcium sulfate and copper sulfate in various proportions. We examined physical and physicochemical properties of the copper doped calcium sulfates and the effects of the cements on angiogenesis in vivo. Rod shaped calcium sulfate crystals were visible by scanning electron microscopy in the cement that contained no copper sulfate, whereas plate-like crystals covered the surface of the cement with high copper content. After immersion of the cements in simulated body fluid (SBF) for 1 day, X-ray diffractometric analysis showed that gypsum precipitates had formed in the copper doped calcium sulfate. The compressive strength of the cements increased from 3.3 MPa for pure calcium sulfate to 6.4 MPa for samples with copper sulfate added. Calcium ion release was greatest from pure calcium sulfate, and the rate of copper ion release was higher for cements containing the most copper. We found that 6 weeks after implantation, more blood vessels had formed around the high copper cement than for the copper-free cement. Copper doped calcium sulfate appears to be useful for application to regenerative medicine including wound healing and bone tissue engineering.

Osteoclastic Bioresorption of Biomaterials: Two- and Three-Dimensional Imaging and Quantification

Purpose

Bioresorbable materials have been developed in the hope that the body will replace them with newly formed tissue. The first step of this remodeling process in bone is the bioresorption of the material by osteoclasts. The aim of this study was to analyze osteoclastic resorption of biomaterials in vitro using the commonly used two-dimensional methods of light-microscopy (LM) and scanning electron microscopy (SEM) in comparison with infinite focus microscopy (IFM), a recently developed imaging method allowing for three-dimensional surface analysis.

Methods

Human hematopoietic stem cells were cultivated in the presence of the cytokines M-CSF and RANK-L for 4 weeks directly on dentin and a calcium phosphate cement. Osteoclast development was surveyed with standard techniques. After removal of the cells, resorption was characterized and quantified by LM, SEM and IFM.

Results

Osteoclast cultures on the biomaterials presented the typical osteoclast-specific markers. On dentin samples LM, SEM as well as IFM allowed for discrimination of resorption. Quantification of the resorbed area showed a linear correlation between the results (LM vs. SEM: r=0.996, p=0.004; SEM vs. IFM: r=0.989, p=0.011; IFM vs. LM: r=0.995). It was not possible to demarcate resorption pits on GB14 using LM or SEM. With IFM, resorption on GB14 could be visualized and quantified two- and three-dimensionally.

Conclusions

In this paper we introduce IFM as a technology for three-dimensional visualization and quantification of resorption of biomaterials. Better understanding of the bioresorption of biomaterials may help in the design of better materials and might therefore constitute an important step on the avenue to the development of artificial bone.

Mechanical and biological characterization of alkaline substituted orthophosphate bone substitutes containing meta- and diphosphates

Despite the growing knowledge on the mechanisms of fracture healing, bone defects often do not heal in a timely manner. Clinically, tricalcium phosphate (TCP) bone substitutes are used to fill bone defects and promote bone healing. However, the degradation rate of these implants is often too slow for sufficient bone replacement. The use of calcium phosphate material with the crystalline phase Ca₁₀[K/Na](PO₄)₇ containing different amounts of di- and metaphosphates may overcome this problem, because these materials show an accelerated degradation. Therefore, we generated alkaline substituted Ca-P scaffolds with different amounts of ortho-, di- and metaphosphates. The degradation of these materials was analyzed in TRIS-HCl buffer solution in vitro. Moreover, we measured the compressive strength and porosity of the scaffolds by micro-CT analysis. The biocompatibility of the scaffolds was evaluated in vivo in the mouse dorsal skinfold chamber by means of intravital fluorescence microscopy and histology. We found that higher amounts of incorporated di- and metaphosphates increase the degradation rate and compressive strength of the scaffolds without inducing a stronger leukocytic inflammatory host tissue reaction after implantation. Histological analyses confirmed the good biocompatibility of the scaffolds containing di- and metaphosphates. In summary, this study demonstrates that the compressive strength and degradation rate of Ca-P scaffolds can be improved by incorporation of di- and metaphosphates without affecting their good biocompatibility. Hence, this material modification may be particularly beneficial for the treatment of metaphyseal bone defects in weight bearing locations.

Allogenic human serum, a clinical grade serum supplement for promoting human periodontal ligament stem cell expansion

Exposing human periodontal ligament stem cells (hPDLSCs) to animal proteins during cell expansion would compromise quality and safety of the hPDLSCs for clinical applications. The current study aimed to evaluate the replacement of animal-based serum by human serum for the expansion of hPDLSCs. hPDLSCs were cultured in culture media supplemented with four types of serums: Group A: fetal bovine serum (FBS); Group B: allogeneic human male AB serum (HS); Group C: in-house autologous (Auto-HS); and Group D: in-house allogeneic human serums (Allo-HS). Exhibitions of mesenchymal stem cell characteristics of hPDLSCs were examined. Then, growth and osteogenic (OS) differentiation potential of hPDLSCs in FBS and HS at passages 5 and 15 were compared to investigate the effects of serum supplements on growth and expansion stability of the expanded hPDLSCs. After that, growth and OS differentiation of hPDLSCs in Auto- and Allo-HS were investigated. Flow cytometrical analyses, functional differentiations, cell growth kinetic, cytogenetic analysis, alkaline phosphatase and calcium content assays, and oil red O and von Kossa staining were performed. Results showed that at passage 5, HS promoted growth and OS differentiation of hPDLSCs and extensive cell expansion, decreased growth and differentiation potential of the expanded hPDLSCs, particularly in HS. Growth and OS differentiation of hPDLSCs in Auto-HS and Allo-HS were not different. In summary, allogeneic human serum could be a replacement to FBS for hPDLSC expansion. In vitro cell expansion of hPDLSCs should be minimal to ensure optimal cell quality. Copyright © 2016 John Wiley & Sons, Ltd.

Development of a synthetic tissue engineered three-dimensional printed bioceramic-based bone graft with homogenously distributed osteoblasts and mineralizing bone matrix in vitro

Over the last decade there have been increasing efforts to develop three-dimensional (3D) scaffolds for bone tissue engineering from bioactive ceramics with 3D printing emerging as a promising technology. The overall objective of the present study was to generate a tissue engineered synthetic bone graft with homogenously distributed osteoblasts and mineralizing bone matrix in vitro, thereby mimicking the advantageous properties of autogenous bone grafts and facilitating usage for reconstructing segmental discontinuity defects in vivo. To this end, 3D scaffolds were developed from a silica-containing calcium alkali orthophosphate, using, first, a replica technique - the Schwartzwalder-Somers method - and, second, 3D printing, (i.e. rapid prototyping). The mechanical and physical scaffold properties and their potential to facilitate homogenous colonization by osteogenic cells and extracellular bone matrix formation throughout the porous scaffold architecture were examined. Osteoblastic cells were dynamically cultured for 7 days on both scaffold types with two different concentrations of 1.5 and 3 × 10⁹ cells/l. The amount of cells and bone matrix formed and osteogenic marker expression were evaluated using hard tissue histology, immunohistochemical and histomorphometric analysis. 3D-printed scaffolds (RPS) exhibited more micropores, greater compressive strength and silica release. RPS seeded with 3 × 10⁹ cells/l displayed greatest cell and extracellular matrix formation, mineralization and osteocalcin expression. In conclusion, RPS displayed superior mechanical and biological properties and facilitated generating a tissue engineered synthetic bone graft in vitro, which mimics the advantageous properties of autogenous bone grafts, by containing homogenously distributed terminally differentiated osteoblasts and mineralizing bone matrix and therefore is suitable for subsequent in vivo implantation for regenerating segmental discontinuity bone defects. Copyright © 2016 John Wiley & Sons, Ltd.

Cytocompatibility of the selected calcium phosphate based bone cements: comparative study in human cell culture

Calcium phosphate cements (CPC) are valuable bone fillers. Recently they have been also considered as the basis for drug-, growth factors- or cells-delivery systems. Broad possibilities to manipulate CPC composition provide a unique opportunity to obtain materials with a wide range of physicochemical properties. In this study we show that CPC composition significantly influences cell response. Human bone derived cells were exposed to the several well-characterized different cements based on calcium phosphates, magnesium phosphates and calcium sulfate hemihydrate (CSH). Cell viability assays, live/dead staining and real-time observation of cells in contact with the materials (time-laps) were performed. Although all the investigated materials have successfully passed a standard cytocompatibility assay, cell behavior in a direct contact with the materials varied depending on the material and the experimental system. The most recommended were the α-TCP-based materials which proved suitable as a support for cells in a direct contact. The materials which caused a decrease of calcium ions concentration in culture induced the negative cell response, however this effect might be expected efficiently compensated in vivo. All the materials consisting of CSH had negative impact on the cells. The obtained results strongly support running series of cytocompatibility studies for preclinical evaluation of bone cements.

Toward smart implant synthesis: bonding bioceramics of different resorbability to match bone growth rates

Craniofacial reconstructive surgery requires a bioactive bone implant capable to provide a gradual resorbability and to adjust to the kinetics of new bone formation during healing. Biomaterials made of calcium phosphate or bioactive glasses are currently available, mainly as bone defect fillers, but it is still required a versatile processing technique to fabricate composition-gradient bioceramics for application as controlled resorption implants. Here it is reported the application of rapid prototyping based on laser cladding to produce three-dimensional bioceramic implants comprising of a calcium phosphate inner core, with moderate in vitro degradation at physiological pH, surrounded by a bioactive glass outer layer of higher degradability. Each component of the implant is validated in terms of chemical and physical properties, and absence of toxicity. Pre-osteoblastic cell adhesion and proliferation assays reveal the adherence and growth of new bone cells on the material. This technique affords implants with gradual-resorbability for restoration of low-load-bearing bone.

Nanostructured platforms for the sustained and local delivery of antibiotics in the treatment of osteomyelitis

This article provides a critical view of the current state of the development of nanoparticulate and other solid-state carriers for the local delivery of antibiotics in the treatment of osteomyelitis. Mentioned are the downsides of traditional means for treating bone infection, which involve systemic administration of antibiotics and surgical debridement, along with the rather imperfect local delivery options currently available in the clinic. Envisaged are more sophisticated carriers for the local and sustained delivery of antimicrobials, including bioresorbable polymeric, collagenous, liquid crystalline, and bioglass- and nanotube-based carriers, as well as those composed of calcium phosphate, the mineral component of bone and teeth. A special emphasis is placed on composite multifunctional antibiotic carriers of a nanoparticulate nature and on their ability to induce osteogenesis of hard tissues demineralized due to disease. An ideal carrier of this type would prevent the long-term, repetitive, and systemic administration of antibiotics and either minimize or completely eliminate the need for surgical debridement of necrotic tissue. Potential problems faced by even hypothetically "perfect" antibiotic delivery vehicles are mentioned too, including (i) intracellular bacterial colonies involved in recurrent, chronic osteomyelitis; (ii) the need for mechanical and release properties to be adjusted to the area of surgical placement; (iii) different environments in which in vitro and in vivo testings are carried out; (iv) unpredictable synergies between drug delivery system components; and (v) experimental sensitivity issues entailing the increasing subtlety of the design of nanoplatforms for the controlled delivery of therapeutics.

A new printable and durable N,O-carboxymethyl chitosan–Ca2+–polyphosphate complex with morphogenetic activity

In the absence of Ca²⁺ the polymers N,O-carboxymethyl chitosan, together with Na-polyphosphate and alginate, form random-coiled structures. Addition of Ca²⁺ transforms these polymers to durable implants.

Osteoinductivity of calcium phosphate mediated by connexin 43

Recent reports have alluded to the osteoinductive properties of calcium phosphate, yet the cellular processes behind this are not well understood. To gain insight into the molecular mechanisms of this phenomenon, we have conducted a series of in vitro and in vivo experiments using a scaffoldless three dimensional (3D) dental pulp cell (DPC) construct as a physiologically relevant model. We demonstrate that amorphous calcium phosphate (ACP) alters cellular functions and 3D spatial tissue differentiation patterns by increasing local calcium concentration, which modulates connexin 43 (Cx43)-mediated gap junctions. These observations indicate a chemical mechanism for osteoinductivity of calcium phosphates. These results provide new insights for possible roles of mineral phases in bone formation and remodeling. This study also emphasizes the strong effect of scaffold materials on cellular functions and is expected to advance the design of future tissue engineering materials.

Comparative evaluation of different calcium phosphate-based bone graft granules - an in vitro study with osteoblast-like cells

OBJECTIVE

Granule-shaped calcium phosphate-based bone graft materials are often required for bone regeneration especially in implant dentistry. Two newly developed bone graft materials are Ceracell(®) , an open-celled highly porous bioceramic from β-tricalcium phosphate (β-TCP) under addition of bioglass and Osseolive(®) , an open porous glass ceramic with the general formula Ca2 KNa(PO4 )2 . The goal of this study was to characterize different modifications of the two bone graft materials in vitro in comparison to already established ceramic bone grafts Cerasorb M(®) , NanoBone(®) and BONIT Matrix(®) .

MATERIALS AND METHODS

Adhesion and proliferation of SaOS-2 osteoblast-like cells were evaluated quantitatively by determining DNA content and lactate dehydrogenase (LDH) activity and qualitatively by scanning electron microscopy (SEM). In addition, MTT cell-vitality staining was applied to confirm the attachment of viable cells to the different materials. Osteogenic differentiation was evaluated by measurement of alkaline phosphatase (ALP) activity as well as gene expression analysis of osteogenic markers using reverse transcriptase PCR.

RESULTS

DNA content and LDH activity revealed good cell attachment and proliferation for Ceracell and Cerasorb M. When pre-incubated with cell-culture medium, also Osseolive showed good cell attachment and proliferation. Attachment and proliferation of osteoblast-like cells on NanoBone and BONIT Matrix was very low, even after pre-incubation with cell-culture medium. Specific ALP activity on Ceracell(®) , Osseolive (®) and Cerasorb M(®) increased with time and expression of bone-related genes ALP, osteonectin, osteopontin and bone sialoprotein II was demonstrated.

CONCLUSIONS

Ceracell as well as Osseolive granules support proliferation and osteogenic differentiation in vitro and may be promising candidates for in vivo applications.

Calcium Alkaline Phosphate Scaffolds for Bone Regeneration 3D-Fabricated by Additive Manufacturing

Calcium alkaline phosphate granulates can be used for substitution of several bone defects but for the reconstruction of large skeletal parts in the maxillofacial and orthopaedic fields fitted scaffolds are preferable. Within the additive manufacturing methods, the 3D printing process offers exciting opportunities to generate defined porous scaffolds. We used a R1 printer from ProMetal Company, USA, for producing scaffolds directly from a ceramic powder. For this direct free form fabrication technology the powder has to possess a lot of specific properties both for the generation of a stable green body and also for the subsequent sintering preparation. For this printing process we prepared different granules in a fluidized bed process containing Ca2KNa(PO4)2as main crystalline phase. Granules were characterized by different methods and several sieve fractions were used for preparing disc like and cylindrical parts. The suitability of granules for this printing process was determined by porosity and strength of produced bodies. Next to granules’ performance both of these properties can be directly influenced by 3D printing process parameters. With knowledge of suitable process parameters scaffolds with different porosity in a respective desired design can be created. In this study, cylindrical scaffolds with graded porosity were produced for bone regeneration of segmental defects in maxillofacial surgery and dental implantology by tissue engineering.

Volumetric analysis of osteoclastic bioresorption of calcium phosphate ceramics with different solubilities

Commonly, to determine osteoclastic resorption of biomaterials only the resorbed area is measured. The depth of the resorption pit, however, may also be important for the performance of a material. To generate such data we used two calcium phosphate ceramics (Ca(10) and Ca(2)). The solubility of the materials was determined according to DIN EN ISO 10993-14. They were scanned three-dimensionally using infinite focus microscopy and subsequently cultivated for 4 weeks in simulated body fluid without (control) or with human osteoclasts. After this cultivation period osteoclasts number was determined and surface changes were evaluated two- and three-dimensionally. Ca(10) and Ca(2) showed solubilities of 11.0+/-0.5 and 23.0+/-2.2 mgg(-1), respectively. Both materials induced a significant increase in osteoclast number. While Ca(10) did not show osteoclastic resorption, Ca(2) showed an increased pit area and pit volume due to osteoclastic action. This was caused by an increased average pit depth and an increased number of pits, while the average area of single pits did not change significantly. The deduced volumetric osteoclastic resorption rate (vORR) of Ca(2) (0.01-0.02 microm(3)microm(-2)day(-1)) was lower than the remodelling speed observed in vivo (0.08 microm(3)microm(-2)day(-1)), which is in line with the observation that implanted resorbable materials remain in the body longer than originally expected. Determination of volumetric indices of osteoclastic resorption might be valuable in obtaining additional information about cellular resorption of bone substitute materials. This may help facilitate the development of novel materials for bone substitution.