A quantitative assessment of osteoinductivity of human demineralized bone matrix

Donor age and gender effects on osteoinductivity of demineralized bone matrix

Allogeneic demineralized bone matrix (DBM) has been used extensively as a clinical graft material because of its inherent osteoinductive and osteoconductive properties. There is continued debate over the acceptable age range of donors for bone and whether the effectiveness of the tissue as a graft is influenced by gender. Contradictory evidence has been obtained with DBM prepared from both animals and humans. The goal of the present investigation was to evaluate the effect of donor age and gender on the osteoinductivity of DBM prepared from human donors [male (133) and female (115) donors grouped in 10-year age brackets up to 85 years] with a statistically relevant sample size using the athymic rat ectopic bone formation model. Among males, there was a statistically significant linear association between age and osteoinductivity value (p <.001), but not among females (p =.20). The rate of change among males was 0.009 units per year. The biological relevance of such a small change in osteoinductivity is likely to be negligible, as the total variation explained by the regressions was only 8.2%. A two-way ANOVA as related to donor age (only donors < 76 years of age) and gender yielded no significant statistical association of osteoinductivity with age group, gender, and their interaction. The results confirm that properly processed demineralized bone from donors through at least 85 years of age is a viable grafting material.

In Vivo Effectiveness of a Glycerol-Compounded Demineralized Freeze-Dried Bone Xenograft in the Rat Calvarium

BACKGROUND

Demineralized freeze-dried bone (DFDB) is commonly hydrated with sterile water into a paste-like consistency for improved clinical handling or reconstituted with biodegradable barriers, such as glycerol, to promote handling and wound stability following human periodontal surgery. The purpose of this study was to evaluate the in vivo effects of glycerol-compounded human DFDB on bone formation in the rat calvarial critical-sized defect (CSD) model.

METHODS

Forty-eight adult male Harland Sprague-Dawley rats were assigned to one of four treatment groups: glycerol, DFDB, DFDB plus glycerol, or a non-grafted control, and placed into 8 mm calvarial CSDs. DFDB (particle size 0.106 to 0.5 mm), glycerol, and their combination were from identical sources. Calvaria were harvested at 8 weeks postsurgery and evaluated histomorphometrically.

RESULTS

A statistically significant increased percentage of total bone fill was detected in the glycerol plus DFDB group and DFDB group as compared to glycerol group or the control. However, no significant difference was noted between the DFDB plus glycerol group and the DFDB group.

CONCLUSION

The addition of glycerol to DFDB results in comparable osseous regeneration in the rat calvarium defect model versus DFDB alone; however, based upon clinical judgment, handling characteristics of DFDB were greatly improved.

Factors Influencing the Outcome of Regenerative Therapy in Mandibular Class II Furcations: Part I

BACKGROUND

Factors influencing the outcome of regenerative therapy of Class II furcations are incompletely and poorly understood. The purpose of this 24-month prospective study was to examine the relationship of patient-, site-, and treatment-related factors to the clinical closure of randomly selected mandibular Class II furcations. Results of therapy were evaluated at 1 and 2 years postoperatively. One-year outcome data are presented in this report.

METHODS

A total of 43 otherwise healthy individuals with chronic periodontitis (26 male, 17 female), 36 to 70 years of age, completed the 12-month evaluation of the study. Entry criteria included clinical and radiographic evidence of two or more mandibular facial Class II furcation defects (> or = 3 mm horizontal probing depth). Surgical therapy was completed by four periodontists (two each) in either a university clinic or private practice. Each patient contributed two furcation defects that were treated by combination therapy using an expanded polytetrafluoroethylene (ePTFE) membrane and demineralized freeze-dried bone allograft (DFDBA). Clinical measurements included a gingival index, plaque index, mobility, and, referencing an occlusal stent, probing depth (PD), probing attachment level-vertical (PAL-V), and probing attachment level-horizontal (PAL-H). Multiple linear measurements were recorded for each site clinically and after surgical debridement to characterize defect morphology, root configuration, and barrier placement. Defect volume was computed mathematically. Postsurgical maintenance care was provided at 1 to 2, 4, 6, and 8 weeks, and then biweekly until 3 months, with subsequent supportive periodontal maintenance visits at 3-month intervals. The clinical status of the furcation (open or closed), measured by a non-treating periodontist at 1 and 2 years, was the primary outcome measure. The association of patient-related factors (e.g., smoking), site-related factors (e.g., root configuration and defect morphology), and treatment-related factors (e.g., membrane exposure) to clinical status of furcations was assessed using random effects hierarchical logistic regression analysis, controlling for design and demographic variables. Non-parametric analysis was used for specific group comparisons.

RESULTS

Complete clinical closure was achieved in 74% of all sites. Of the residual furcation defects, 68% were reduced to Class I. No defects progressed to Class III. Significant improvements in mean PD and PAL-V were obtained following surgical therapy. Although the proportion of sites demonstrating complete furcation closure was comparable for smokers and non-smokers, the proportion of Class II residual defects was significantly higher among smokers than non-smokers (62.5% versus 14.3%, respectively). Increases in presurgical PAL-H were associated with monotonic decreases in the percentage of sites demonstrating complete clinical closure, with only 53% of lesions > or = 5 mm responding with complete closure. Similarly, significant reductions in the frequency of clinical closure were associated with increases in the distance between the roof of furcation and crest of bone, roof of furcation and base of defect, depth of horizontal defect, and divergence of roots at the crest of bone.

CONCLUSIONS

The successful clinical closure of Class II furcations was achievable at 1 year following combination therapy with an ePTFE membrane and DFDBA. The highest frequency of clinical furcation closure was observed in early Class II defects. Furcations with vertical or horizontal bone loss of 5 mm or greater responded with the lowest frequency of complete clinical closure. Nevertheless, complete furcation closure was achievable in 50% of molars with extensive bone loss. Also, 15 out of 22 (68%) of all residual defects were reduced to Class I and only seven (8%) failed to improve, demonstrating that successful clinical resolution of advanced defects remains an attainable goal.

Quantitative and sensitive in vitro assay for osteoinductive activity of demineralized bone matrix

A sensitive, rapid, reliable and quantitative method to check the bone forming potential of demineralized bone matrix (DBM) has been developed. The osteoinductivity of the bone morphogenetic proteins (BMPs), present in DBM, can be measured in vitro using a pluripotent myoblast C2C12 cell line. Alkaline phosphatase activity induced by co-incubation of DBM with C2C12 cells was dose-responsive and corresponds to the amount of active BMPs in DBM. Bone forming potential was simultaneously tested in vivo by implanting DBM intra-muscularly in nude rats. ALP activity induced in C2C12 cells, correlated with bone formation in vivo (r=0.88), determined by alkaline phosphatase activity, mineralization density and histomorphology of the DBM explants. Results from DBM batches, originating from five established Bone Banks, showed good consistency between in vitro and in vivo assays. However, DBM activity varied widely from bank to bank as well as from batch to batch within the same bank.

Restoration of large bone defects using a hard-setting, injectable putty containing demineralized bone particles compared to cancellous autograft bone

An injectable, hard-setting, calcium sulfate-based putty containing demineralized bone matrix particles (AlloMatrix II, Wright Medical Technology, Inc, Arlington, Tenn) was compared to autogenous cancellous bone graft to evaluate healing in a canine model. Area fraction of new bone, modulus of elasticity, and compressive strength of new bone were evaluated, as was radiographic and histologic healing. Bilateral defects were created in the proximal humeri, and each defect was implanted with either the putty or autogenous bone according to a randomized schedule. Dogs were euthanized at 6, 13, and 26 weeks. The area fraction, modulus of elasticity, and compressive strength of newly formed bone was not significantly different between the putty and autogenous bone at 6, 13, or 26 weeks. The putty had excellent handling characteristics, was biocompatible, and was as effective as autograft bone in achieving near complete bony restoration of a large, critical-sized defect.

Comparative evaluation of the osteoinductivity of two formulations of human demineralized bone matrix

In the United States, demineralized bone matrix (DBM) is considered a transplantable tissue and therefore is regulated primarily by the American Association of Tissue Banks. Even though DBM is not subjected to the same regulations relative to performance claims as medical devices are, one would expect different processing methods might yield DBM preparations of different osteoinductive potential. The purpose of this study was to use an established athymic rat model to compare the osteoinductive properties of two commercially available human DBMs prepared using different methods but having essentially identical product claims. Sixteen female athymic rats were used to test equivalent volumes of two lots each of Grafton Putty (Osteotech, Inc., Eatontown, NJ), Osteofil (Regeneration Technologies, Inc., Alachua, FL), and rat DBM. At 28 days after implantation, qualitative and semiquantitative microscopy showed no significant differences in bone formation between the two lots from each source, but rat DBM produced significantly more bone than Grafton, which produced significantly more bone than Osteofil. Our results suggest that methods of graft processing may represent a greater source of variability than do differences among individual donors. Whether these differences relate to methods of demineralization, carrier, dose of DBM per volume, or to some other factor remains to be determined.

In vitro evaluation of the mitogenic effect of platelet-derived growth factor-BB on human periodontal ligament cells cultured with various bone allografts

BACKGROUND

Several studies have documented the role of growth factors in periodontal regeneration. It has been shown that platelet-derived growth factor (PDGF) is a potent stimulator of human periodontal ligament (PDL) cells. A variety of bone graft materials are used to treat osseous defects caused by periodontal disease. We evaluated the mitogenic effect of PDGF on human PDL cells cultured with different allografts to determine which of the allografts with or without PDGF promoted periodontal regeneration.

METHODS

Two human demineralized freeze-dried allografts of cortical (DFDBA) and cancellous (DFBA) bone and a non-demineralized freeze-dried allograft (FBA) from cancellous bone were used alone or supplemented with PDGF-BB. Human PDL cultures were derived from the mid-root of 2 maxillary premolars extracted for orthodontic reasons. Cells were grown separately in 24-well dishes with or without 20 mg of each bone allograft. On day 2 of quiescence, new medium was added with 10 ng/ml of PDGF-BB. DNA synthesis was estimated by measuring [3H] thymidine incorporation to determine the effects of the test agents on cell proliferation. Cells were processed and subjected to scintillation counting after 48 hours of incubation. Counts per minute (cpm/well) were determined for each sample.

RESULTS

There was no statistically significant difference (P<0.05) on PDL cell proliferation when the allografts were used alone. PDL cells exhibited significantly greater proliferative responses to the 2 demineralized bone allografts, DFDBA and DFBA, when combined with PDGF-BB. A statistically significant difference on DNA synthesis was noticed when PDGF-BB was added to PDL cells cultured with FBA. PDL cells displayed no significant increase in mitogenic activity when cultured with PDGF-BB alone.

CONCLUSIONS

The findings of this study demonstrate the beneficial role of DFDBA, DFBA, and FBA as synergic agents with PDGF-BB to periodontal regeneration. The significant ability of the 2 decalcified bone allografts, DFDBA and DFBA, combined with PDGF to stimulate PDL cell proliferation might be a useful adjunct in the treatment of periodontal defects.

Assessment of the value of anorganic bone additives in sinus floor augmentation: a review of clinical reports

In order to objectively assess the value of anorganic bone additives in sinus floor augmentation, a review of the English literature till May 2002 was carried out. The studies or parts of studies had to include at the least two patients, with a follow-up of at least 3 months, whilst histomorphometric data ought to be available of biopsies taken from the core of the graft. Only 12 studies fulfilled these criteria. Firm conclusions could not be drawn because there were substantial differences in histomorphometric techniques, healing periods and ratio autogenous bone anorganic additives, but it seemed that autogenous bone without anorganic additives resulted in the highest amount of bone after a 4-6 months healing period, while hydroxyapatite and bovine bone mineral, used as a bone substitute, gave the lowest amount of bone. No correlation between bone volume and time of graft healing could be found. The consequences, however, for implant placement and survival are at present, not fully understood. A plea is made for systematic prospective clinical and experimental studies to assess the various bone additives and ratios of autogenous bone to bone additives in composite grafts analysed with an adequate histomorphometric technique. Part of this study was presented at the 15th ICOMS, Durban, South Africa, 19-24 May 2001.

Characterization of bone defect repair in young and aged rat femur induced by xenogenic demineralized bone matrix

BACKGROUND

The osteoinductive effect of some biomaterials could be affected by those systemic conditions typical of old age. The aim of the present paper was to assess the effects of age on the healing of bone defects treated with demineralized bone matrix (DBM).

METHODS

The study was conducted in young (3 month old) and aged (18 month old) rats to assess the efficacy of DBM in the treatment of osseous defects in bone with limited repair capacities. A standard bone defect was created in the distal femoral condyles of male Wistar rats: the left condyle was filled with rabbit DBM granules, while the right condyle was left empty (control). Histological and microhardness analyses were performed at 30 and 45 days after implant surgery.

RESULTS

After implantation of xenogenic DBM, bone healing areas of the aged and young groups showed a significant increase in the formation of newly mineralized bone relative to controls. Measurements of trabecular thickness on day 45 revealed no differences between newly formed and preexisting bone in the young group, while control values were lower. Microhardness measurements demonstrated that newly mineralized bone, either induced by DBM or not, and preexisting bone were comparable in terms of trabecular hardness after 45 days.

CONCLUSIONS

In conclusion, xenogenic DBM seems to be effective in bone defect healing, since it increases mineralized tissue volume. In both DBM-filled and empty sites, age seems to have a detrimental effect on the volume of new bone formation but no influence on bone maturation.

The modulation of androgen metabolism by estradiol, minocycline, and indomethacin in a cell culture model

BACKGROUND

This investigation attempts to clarify the proanabolic effects of minocycline and indomethacin by studying their effects on androgen metabolism and mediation by estradiol. A cell culture model was used with androgen substrates because of the proanabolic effects of androgen metabolites.

METHODS

Monolayer cultures of human gingival fibroblasts (HGF) derived from 6 patients were incubated in duplicate with 14C- testosterone or 14C-4-androstenedione as substrates and optimal concentrations of estradiol (E1,3 microgram/ml) and minocycline (M25 microgram/ml) or indomethacin (I, 1 microgram/ml) alone and in combination (E1,3+11 or E1,3+M25 microgram/ml); similar experiments were carried out with human oral periosteal fibroblasts (HPF), M, I, E, and the combinations. At the end of a 24-hour incubation period in Eagle's MEM, the medium was solvent extracted with ethyl acetate and the metabolites were separated by TLC in a benzene:acetone solvent system (4:1 v/v). The separated metabolites were quantified using a radioisotope scanner.

RESULTS

Both androgens were metabolized to 5alpha-dihydrotestosterone (DHT) and 4-androstenedione (4-A) or testosterone (T) at baseline and in response to the agents tested, by HGF and HPF. With HGF, there were significant increases in the yields of DHT and 4-A or T in response to M, E, and M+E, resulting in 50% to 2.4-fold increases in these metabolites over control incubations (n = 6; P<0.01). The responses to I and combinations of I+E were similar. HPF also demonstrated significant increases of 29% to 4-fold in the yields of androgen metabolites in response to M, E, and M+E (n = 6; P<0.01). I and E similarly increased the yields of androgen metabolites, alone and in combination.

CONCLUSIONS

Adjunctive periodontal treatment with minocycline or indomethacin can contribute to hormone-modulated anabolic responses in males and females in gingival and periosteal fibroblasts derived from a chronically inflamed source.

Demineralized bone matrix and native bone morphogenetic protein in orthopaedic surgery

The recognition that demineralized bone matrix could induce bone formation when placed in mammalian skeletal muscle led to preclinical studies of crude native insoluble bone morphogenetic protein and noncollagenous protein, followed by the clinical application of demineralized bone matrix, chemosterilized autolyzed antigen-extracted allogenic bone, and autolyzed antigen-extracted allogenic bone matrix gelatin. Cultural norms and regulatory agencies influence the availability of different demineralized bone matrix preparations in different parts of the world, but there is continued interest in the biologic structure of native insoluble bone morphogenetic protein and noncollagenous protein aggregates and the applied science of osteoinduction and osteoconduction in reconstructive orthopaedic surgery. Demineralized bone matrix is not widely available in Asia, but tissue processing facilities in the United States distribute demineralized bone matrix materials with different carriers, handling properties, and possibly osteoinductive potential. The purpose of the current study was to review the development and use of various preparations of demineralized bone matrix materials.

Dexamethasone-mediated androgen metabolism in human gingival and oral periosteal fibroblasts

Dexamethasone modulates the effects of other hormones and mediates cell function; the periodontium is a target tissue for androgens. It was therefore relevant to investigate the modulation of androgen metabolism by dexamethasone in cultured human gingival (HGF) and oral periosteal fibroblasts (HPF). Each cell line was incubated in Eagle minimum essential medium with [(14)C]testosterone/[(14)C]4-androstenedione as substrates and serial concentrations of dexamethasone (0.5-50 microg/ml), for 24h; the medium was solvent-extracted, analyzed and quantified for steroid metabolites. In response to dexamethasone, both HGF (n=6) and HPF (n=4) showed up to two-fold increases in the formation of 5alpha-dihydrotestosterone and 4-androstenedione (P<0.01, one-way ANOVA), and 3.6- to 5-fold increases in the formation of testosterone (P<0.001), from [(14)C]4-androstenedione, with some inhibition at higher concentrations. Dexamethasone stimulated the formation of physiologically active androgen metabolites in a dose-dependent manner. These metabolites might therefore contribute to dichotomous effects in connective tissues of the periodontium, dependent on effective concentrations of dexamethasone.

Modulatory effects of indomethacin on androgen metabolism in human gingival and oral periosteal fibroblasts

The non-steroidal anti-inflammatory agent indomethacin (I) suppresses gingival inflammation and alveolar bone resorption. Androgens particularly 5 alpha-dihydrotestosterone (DHT) have anabolic effects on connective tissue and bone matrices. Human oral periosteal fibroblasts (HPF) and gingival fibroblasts (HGF) instigate healing in inflammatory periodontal lesions. The aim of this investigation was to compare the modulatory effects of I on the metabolism of two androgen substrates in human oral periosteal and gingival fibroblasts in culture. Monolayer cultures of both cell types (5(th)-9(th) passage) were established in Eagle's MEM and incubated with 14C-testosterone/14C-4-androstenedione and serial concentrations of I (0.5-50 microg/ml) for 24 h. The steroid metabolites were solvent extracted from the medium, separated by TLC and quantified using a radioisotope scanner. Both androgen substrates were metabolized mainly to DHT and 4-androstenedione/testosterone respectively, expressing 5 alpha-reductase and 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) activity in both HPF and HGF. There were 51% and 73% increases in the levels of DHT over controls, with HGF and HPF respectively (n = 6; n = 4, P < 0.01) in response to I at 1-5 microg/ml, often reaching control values at 50 microg/ml. The expression of 17 beta-HSD activity showed less stimulation than the levels of DHT. Both androgen substrates were effective in this metabolic conversion, which is applicable to healing responses in both males and females in vivo. There were 57% increases (n = 4; P < 0.01) over controls, in the formation of androstanediol from 14C-4-androstenedione at 10 microg of I, in HPF. This transformation may regulate androgen action in androgen-dependent tissue. In addition to its anti-inflammatory properties, indomethacin can contribute to anabolic reparatory responses, by increasing the expression of steroid metabolizing enzymes in gingival and periosteal fibroblasts, in the inflammatory periodontal lesion.

Rapid quantitative bioassay of osteoinduction

We developed a reproducible, relatively rapid bioassay that quantitatively correlates with the osteoinductive capacity of demineralized bone matrix obtained from human long bones. We have found that Saos human osteosarcoma cells proliferate in response to incubation with demineralized bone matrix and that an index of this proliferative activity correlates with demineralized bone matrix-induced osteogenesis in vivo. The bioassay (Saos cell proliferation) had an interassay coefficient of variation of 23 +/- 2% and an intra-assay coefficient of 11 +/- 1%. Cell proliferation was normalized to a standard sample of demineralized bone matrix with a clinically high osteoinductive capacity, which was assigned a value of one. The Saos cell proliferation for each sample was related to the standard and assigned a value placing it into the low (0.00-0.39), intermediate (0.40-0.69), or high (0.70-1.49) osteoinductive index group. Osteoinduction of human demineralized bone matrix was quantitated by expressing new bone formation as a function of the total bone volume (new bone plus the demineralized bone powder). The demineralized bone matrix was placed in pouches formed in the rectus abdominis muscles of athymic rats, and endochondral bone formation was assessed at 35 days following implantation, when marrow spaces in the ossicles were formed by new bone bridging the spaces between demineralized bone matrix particles. The proliferative index correlated with the area of new bone formation in histological sections of the newly formed ossicles. When the proliferative index (the osteoinductive index) was divided into low, intermediate, and high groups, the correlation between it and new bone formation (osteoinduction) was 0.850 (p < 0.0005) in 25 samples of demineralized bone matrix. There was no overlap in the osteoinduction stimulated between the samples with low and high osteoinductive indices. We conclude that the proliferation assay is useful for the routine screening of bone allograft donors for osteoinductive potential. Furthermore, the two-dimensional area of new bone formation, as it relates to total new bone area, is a quantitative measure of osteoinduction.

The effect of minocycline on the metabolism of androgens by human oral periosteal fibroblasts and its inhibition by finasteride

The antimicrobial minocycline has matrix-stimulatory effects on connective tissue and bone. The aim here was to study the effect of minocycline on 5alpha reduction of androgen substrates to 5alpha-dihydrotestosterone (DHT) in periosteal fibroblasts and the influence of the antiandrogen finasteride on this conversion. Confluent cultures of periosteal fibroblasts established from oral periosteum isolated from the bone surface were incubated in duplicate in multiwell dishes with two androgen substrates, [(14)C]-testosterone/[(14)C]-4-androstenedione, in the presence or absence of serial concentrations of minocycline or the antiandrogen finasteride or the two in combination for 24 h. The metabolites formed were solvent-extracted with ethyl acetate, separated by thin-layer chromatography and quantified using a radioisotope scanner. Both androgen substrates were metabolized to DHT and 4-androstenedione or testosterone. Minocycline stimulated the synthesis of DHT from these substrates by 75-83% at 20-30 microg/ml (n=4; p<0.01). Finasteride inhibited the 5alpha-reductase activity of these substrates by 3-5-fold at 1 microg/ml and 40-80% at 0.01 and 0.1 microg/ml (n=4; p<0.01), with little change in 17beta-hydroxysteroid dehydrogenase activity. Minocycline and finasteride in combination showed an intermediate response with one substrate. As finasteride inhibits the type 2, 5alpha-reductase isoenzyme associated with anabolic functions, these findings demonstrate target-tissue androgen metabolic activity in periosteal fibroblasts at baseline and in response to minocycline. This has implications for the reparatory potential of the diseased periodontium during adjunctive treatment with minocycline.

Bone morphogenetic protein excipients: comparative observations on poloxamer

Clinicians await the availability of synthetic bioimplants that will replace the need for autogeneic bone grafts in bone reconstructive surgery. For more than a decade, researchers have evaluated delivery vehicles for the tissue morphogen bone morphogenetic protein. The object of this investigation was to measure induced bone development when bone morphogenetic protein was delivered by human tendon collagen, human demineralized bone matrix, hydroxyapatite, a composite of human tendon collagen and human demineralized bone matrix (tendon collagen + demineralized bone matrix), Poloxamer 407, and a composite of human demineralized bone matrix and Poloxamer 407. Sixty-three adult male Swiss Webster mice (Harlan Sprague-Dawley, Indianapolis, Ind.) received 126 implants. The animals were divided into seven groups of nine animals, depending on carrier (six carriers plus the positive control group) used. Each animal received a bone morphogenetic protein-enhanced carrier in one hindquarter muscle mass, with the contralateral leg being implanted with the carrier alone. Implants were evaluated by quantitative radiomorphometry validated by histologic methods. Radiographically, no significant differences were identified among any of the implants evaluated (p > 0.05). Histomorphometric analysis demonstrated that Poloxamer 407 was significantly (p < 0.05) better at delivering bone morphogenetic protein than the other carriers involved in this investigation. The new bone developed in a tubular or spherical shape. Interaction of endogenous and exogenous delivery systems seems to be essential for optimal transmission of bone morphogenetic protein. The importance of the excipient to deliver bone morphogenetic protein and develop a bone morphogenetic protein concentration gradient has been emphasized by other investigators and confirmed by our research on poloxamer. With further research on the physicochemical mechanisms of localization and transmission of bone morphogenetic protein, it may be possible to avoid hazardous operations with autogeneic bone.

Evaluation of 2 novel approaches for assessing the ability of demineralized freeze-dried bone allograft to induce new bone formation

BACKGROUND

Because of the wide variation in the ability of human demineralized freeze-dried bone allograft (DFDBA) to reproducibly induce new bone formation, there is a need for a reliable measure of bone induction activity. In this study we examined an immature osteoprogenitor cell line for its potential utility in measuring the activity of DFDBA in vitro.

METHODS

We characterized the response of 2T9 cells, an immature osteoprogenitor cell line derived from the calvariae of transgenic mice containing the SV40 T-antigen driven by the mouse bone morphogenetic protein (BMP)-2 promoter, to recombinant human BMP-2 by measuring alkaline phosphatase specific activity, osteocalcin production, and matrix mineralization. Responses were compared to those obtained with 1,25-(OH)2D3. In addition, 2T9 cells were cultured with active or inactive human DFDBA in the presence or absence of BMP-2. We also tested the hypothesis that radio-opacity of tissue following implantation of DFDBA in vivo correlates with the ability of human DFDBA to induce new bone. DFDBA from 9 different donors, stratified by age, were implanted subcutaneously in the thorax of 18 nude (nu/nu) mice. Tissue was harvested at 36 days postoperatively and examined histologically and biochemically for calcium and phosphorus uptake.

RESULTS

2T9 cells exhibited a dose- and time-dependent response to soluble BMP-2. Proliferation was decreased and alkaline phosphatase activity, osteocalcin production, and mineralized nodule formation were increased. The effects were dose- and time-dependent. Peak effects on alkaline phosphatase and osteocalcin were noted on day 8, whereas mineral deposition did not begin to occur until day 12. 1,25-(OH)2D3 did not regulate these effects unless used with BMP-2. When the cells were exposed to active or inactive DFDBA in the presence or absence of BMP-2, no effect on 2T9 cell differentiation was observed. This indicated that DFDBA released no soluble factors with bone inductive ability and that if any active factors were adsorbed to the DFDBA, they were inactivated. When DFDBA was implanted subcutaneously in the thorax of nude mice, there was no histologic evidence of new bone formation. However, there was a donor age-dependent decrease in Ca and P uptake of the implanted tissue, reflecting a donor age-dependent decrease in remineralization of DFDBA.

CONCLUSIONS

These data indicate that cell culture assays like the one used in this study may not be appropriate indicators of bone induction ability by DFDBA since soluble factors may not be responsible for bone induction in vivo. Nonetheless, in vitro assays are still needed. While Ca and P uptake by DFDBA-implanted tissue in the present study correlated with the age-dependent decrease in bone induction at intramuscular sites in a previously reported study, these data show that early x-rays may actually detect remineralization and not new bone formation. Thus, assessment of bone induction ability may still depend on histologic analysis of animal models.