An in vitro bioassay to assess biological activity in demineralized bone

Osteoinductive potential of small intestinal submucosa/ demineralized bone matrix as composite scaffolds for bone tissue engineering

Background: Demineralized bone matrix (DBM) is extensively used in orthopedic, periodontal, and maxillofacial application and investigated as a material to induce new bone formation. Small intestinal submucosa (SIS) derived from the submucosa layer of porcine intestine has widely utilized as biomaterial with minimum immune response. Objectives: Determine the osteoinductive potential of SIS, DBM, SIS/DBM composites in the in vitro cell culture and in vivo animal bioassays for bone tissue engineering. Materials and methods: Human periosteal (HPO) cells were treated in the absence or presence SIS, DBM, and SIS/DBM. Cell proliferation was examined by direct cell counting. Osteoblast differentiation of the HPO cells was analyzed with alkaline phosphatase activity assay. The Wistar rat muscle implant model was used to evaluate the osteoinductive potential of SIS, DBM, and SIS/DBM composites. Results: HPO cells could differentiate along osteogenic lineage when treated with either DBM or SIS/DBM. SIS/ DBM had a tendency to promote more cellular proliferation and osteoblast differentiation than the other treatments. In Wistar rat bioassay, SIS showed no new bone formation and the implants were surrounded by fibrous tissues. DBM demonstrated new bone formation along the edge of old DBM particles. SIS/DBM composite exhibited high osteoinductivity, and the residual SIS/DBM was surrounded by osteoid-like matrix and newly formed bone. Conclusion: DBM and SIS/DBM composites could retain their osteoinductive capability. SIS/DBM scaffolds may provide an alternative approach for bone tissue engineering.

Safety and efficacy of use of demineralised bone matrix in orthopaedic and trauma surgery

Demineralised bone matrix (DBM) acts as an osteoconductive, and possibly as an osteoinductive, material. It is widely used in orthopaedic, neurosurgical, plastic and dental areas. More than 500,000 bone grafting procedures with DBM are performed annually in the US. It does not offer structural support, but it is well suited for filling bone defects and cavities. The osteoinductive nature of DBM is presumably attributed to the presence of matrix-associated bone morphogenetic proteins (BMPs) and growth factors, which are made available to the host environment by the demineralisation process. Clinical results have not been uniformly favourable; however, a variable clinical response is attributed partly to nonuniform processing methods found among numerous bone banks and commercial suppliers. DBMs remain reasonably safe and effective products. The ultimate safe bone-graft substitute, one that is osteoconductive, osteoinductive, osteogenic and mechanically strong, remains elusive.

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.

Arthrodesis of lumbar spine transverse processes using nacre in rabbit

This study compares the osteogenic effects of nacre and autogenous bone grafts in a rabbit model of lumbar spine transverse process arthrodesis. A total of 15 rabbits were processed for arthrodesis between the fifth and sixth lumbar vertebrae using nacre powder mixed with autologous blood or autogenous iliac crest bone. Control rabbits were sham operated. Sample vertebrae were removed from the nacre-implanted rabbits at 2, 5, and 11 weeks postsurgery. The autogenous bone graft and sham-operated groups were processed for histological study 11 weeks postsurgery. The results for the three groups were compared at 11 weeks. The nacre-implanted samples taken at 2 weeks showed that the nacre was well tolerated by the host tissue. Endochondral bone formation was seen in the region of the dissolving nacre particles by 5 weeks. The newly formed bone formed a solid fusion between the transverse processes in one-third of the rabbits. There was still new bone formation at 11 weeks at the nacre implant site. Two-thirds of the rabbits had formed a solid fusion. Light microscopy also showed new bone formation 11 weeks after the autologous bone graft. All rabbits had a solid fusion. This initial study indicates that nacre can induce spinal fusion in an acceptable percentage of cases.

Effect of water-soluble matrix fraction extracted from the nacre of Pinctada maxima on the alkaline phosphatase activity of cultured fibroblasts

A new approach to the isolation of the water-soluble factors from nacre without any demineralization is described and examined their effect on fibroblast cells in culture. The soluble matrix in pure water from the nacre of Pinctada maxima was analysed by size-exclusion HPLC. Four fractions (SE1-SE4) of the water-soluble matrix (WSM) were further analysed by anion-exchange HPLC. The amino acid composition of the WSM showed that it is mainly composed of glycine and alanine. SE1 and SE4 had different amino acid compositions from the whole WSM. The WSM and SE4 tested on a culture of human foetus lung tissue fibroblasts increased the alkaline phosphatase (ALP) activity. SE1 caused a decrease in ALP activity. Our results support the hypothesis that WSM promotes the differentiation of cells in vitro.

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.

Effect(s) of the demineralization process on the osteoinductivity of demineralized bone matrix

The relationships between residual calcium levels and particle size of ground demineralized bone matrix and its osteoinductive potential were investigated using in vitro and in vivo assays. The effects of variable residual calcium levels, variable particle sizes, and donor age and gender were studied using a tissue culture-based bioassay (in vitro) as well as an athymic mouse (in vivo) bioassay. The osteoinductive potential of the bone-derived biomaterial was assessed by measuring the degree of new bone formation (change in percent calcium content after 4 weeks of implantation) in the in vivo assay and levels of alkaline phosphatase activity associated with cultures of human periosteal cells (HPO cells) in the in vitro assay, respectively. Slightly demineralized bone matrix and overly demineralized bone matrix possessed a degree of osteoinductive potential whereas bone demineralized to levels of approximately 2% residual calcium provided for maximum osteoinductive potential in both assay systems. The osteoinductive potential of ground demineralized bone varied relative to the particle size such that DBM particles ranging from 500 to 710 microns provided for the highest level of calcium deposition (increase of 8.1 weight percent calcium) after 4 weeks of implantation in muscle pouches of an athymic mouse, whereas explanted particles less than 250 microns showed the lowest level of calcium deposition (increase of only 2.8 weight percent calcium). In the donor age and gender study, DBM from different donors were divided into 5 age groups for both female and male donor derived bone: less than 20, 21 to 30, 31 to 40, 41 to 50, and 51 to 60 year old age groups. This study indicated that DBM from female donors in the 31 to 40 years old age group and male donors in the 41 to 50 year age group possess the highest osteoinductive potential, whereas DBM derived from donor bone from both female and male donors in the 51 to 60 year age group presented the lowest osteoinductive potential. DBM derived from male and female donors did not in general show significant differences in osteoinductive potential.

A quantitative assessment of osteoinductivity of human demineralized bone matrix

Demineralized bone matrix (DBM) is widely used in the repair of pathologies associated with skeletal defects and periodontal diseases. The present study was directed at establishing in vivo and in vitro models for a quantitative assessment of the osteoinductivity of DBM before clinical use. Athymic mice were used in an in vivo assay to overcome the species limitations (for human DBM) found in xenogeneic animal models. Calcium contents of explants, as an indicator of new bone formation, were assayed and expressed as a change in the weight percent calcium in the explant as compared to the weight percent of calcium in the implanted material. A total of 82 mice (2 implants per mouse) were used in this study. Significant amounts of new bone were induced in this animal model in response to implantation of DBM. Muscular implantation was found to be more osteoinductive (increases of 10.0 +/- 0.4 calcium weight percent of explant) than subcutaneous implantation (increases of 1.62 +/- 0.27 calcium weight percent of explant) and new bone formation in muscular implantation sites of athymic mice mimics endochondral bone formation. Between weeks 1 to 4, the weight of explanted materials did not significantly differ from the weight of the implanted material; however, by week 5 the explant weight began to increase. Calcium deposition over the 5 weeks of implantation increased in a nearly linear fashion. Consequently week 4 was chosen as the optimum time for explantation in the in vivo assay in that sufficient calcium levels had been achieved without a significant increase in explant dry weight. Aliquots of 10, 20, 30, and 40 mg per implantation site were used in dose response studies in the in vivo bioassay. Dose response curves with DBM exhibited maximal activity at the 20 mg DBM implant dose in the in vivo bioassay. An in vitro bioassay was also developed where human periosteal (HPO) cells were chosen because osteoprogenitor cells found in bone repair typically come from periosteal tissue. Alkaline phosphatase (ALP) activity in confluent cell cultures of HPO cells exposed to DBM, as an indicator of osteoblast induction, reached its highest level on day 5 of DBM treatment. Aliquots of 2, 5, 10, 20, 30, and 40 mg DBM per flask were chosen in dose response studies using the in vitro bioassay. These dose response studies with DBM revealed that quantities approximating 5 to 10 mg DBM in the in vitro model provided for maximal levels of ALP in cell extracts. A linear correlation (R2 = 0.7397) was demonstrated between the in vivo calcium remineralization assay and the in vitro ALP assay of osteoinductivity of DBM, suggesting that the in vitro assay can be used to quantitatively assess the osteoinductive potential of DBM where production and distribution of clinically usable DBM dictates rapid analysis.