Underlying mechanisms at the bone-biomaterial interface

Callus formation and fixation rigidity: a fracture model in rats

To produce different amounts of callus in rats, we devised a procedure to fix leg fractures using intramedullary nails that differ in bending rigidity. We inserted a silicone cannula into the intact diaphysis of the tibia of rats that had already been killed and then fractured the tibia by a three-point bending technique. The fracture was stabilised by insertion of either a stainless-steel or polypropylene nail into the silicone cannula. Biomechanical testing showed that the initial stiffness of the fractures differed between the two nail types by a factor of 16. In vivo, 16 Wistar rats were operated on by the same technique to study the formation of callus. Four weeks after fracture, the size of the callus differed significantly between the steel-nailed and polypropylene-nailed fractures. In conclusion, mechanically differing internal fixation devices led to different callus responses in rats when all other factors were kept equal.

Biomaterials and cardiovascular devices

In the field of cardiovascular surgery there is presently a lack of biomaterials possessing essential characteristics of the native tissue or organ which is to be replaced. This paper describes various biomaterials that have been introduced into the circulatory system and the complex reactions that subsequently occur. The risk of infection is also discussed as well as prevention and treatment regimes that can be used. Examples of future biomaterial development are outlined in an attempt to achieve biocompatibility.

Comparative study of tissue reactions to calcium phosphate ceramics among cancellous, cortical, and medullar bone sites in rabbits

In order to understand the influence of the implantation site on bone biomaterial evaluation, we implanted cylinders of HA and beta-TCP ceramics in the femoral diaphysis and condyle of rabbits. After 3, 8, 12, and 24 weeks of implantation, histological investigation and histomorphometry were performed on undecalcified samples. Our results show that spontaneous bone healing in the empty cavities is significantly different (p < 0.05) between cortical (SBH > 80%) and cancellous bone sites (SBH < 31%) and that no new bone is formed in marrow tissue. For both porous ceramics, the highest osteogenesis was obtained in the cortical site. Osteogenesis was intermediate in the cancellous site and weak in the medullar site. The material biodegradation was the strongest in the medullar site and higher in the cancellous site than in the cortical site. Both activities were better in the beta-TCP than in the HA (p > 0.05). The marrow tissue presents a foreign-body reaction more reliable, sensitive, and durable than other bone tissues. Therefore, the cancellous bone site is a good site for evaluation of the biofunctionality of biomaterials because of the equilibrium of the osteogenesis and the biodegradation activities, but marrow tissue seems to be better for testing material biocompatibility in vivo.

Biologic determinants of bone formation for osseointegration: clues for future clinical improvements

STATEMENT OF PROBLEM

Further improvement in and expansion of the application of dental implants requires control and improvement of bone mass for implant support.

PURPOSE

Although osseointegration involves both the formation and the maintenance of bone at implant surfaces, the aim of this article is to identify cellular and molecular determinants of bone formation that may be used in clinical attempts to enhance or expand the application of endosseous implants for dental and craniofacial prosthetics.

METHODS

A review of bone biology and dental and orthopedic implant literature was performed using Medline and published monographs.

RESULTS

This spectrum of information indicates that molecular and cellular approaches to creating and maintaining bone mass may be used to expand the application of dental implants and to improve dental implant success in bone-deficient sites.

Prostaglandins mediate the effects of titanium surface roughness on MG63 osteoblast-like cells and alter cell responsiveness to 1 alpha,25-(OH)2D3

Surface roughness affects proliferation, differentiation (alkaline phosphatase and osteocalcin), local factor production (transforming growth factor (TGF beta) and prostaglandin E2 (PGE2)], and response to 1,25-(OH)2D3 (1,25) of MG63 osteoblast-like cells. In this study, we examined whether the effect of surface roughness on MG63 cells is mediated by prostaglandins produced by the cells. Unalloyed titanium (Ti) disks were pretreated with HF/HNO3 (PT) and then machined and acid-etched (MA). Disks were also coarse grit-sandblasted (SB), coarse grit-sandblasted and acid-etched (CA), or plasma-sprayed with Ti particles (PS). The surfaces, from smoothest to roughest, were PT, MA, CA, SB, and PS. MG63 cells were cultured to confluence on the Ti disks in the presence or absence of 10(-7) M indomethacin (Indo), a specific inhibitor of cyclooxygenase activity, resulting in decreased prostaglandin production. When the cells reached confluence, cell number, cell layer alkaline phosphatase specific activity (ALPase), and osteocalcin (OC) and latent TGF beta (LTGF beta) production were determined. In addition, confluent cultures which had been grown in the absence of Indo were exposed to 10(-7) M 1,25, 10(-7) M Indo, or a combination of the two for 24 h. On the rougher surfaces, cell number was decreased and ALPase, OC, and LTGF beta were increased. When indomethacin was present throughout the culture period, the effect of surface roughness on cell number, OC, and LTGF beta was abolished. ALPase was reduced, but surface roughness-dependent effects were still observed. Addition of indomethacin to confluent cultures for 24 h had no effect on any of the parameters examined, with one exception: Cells cultured on MA surfaces exhibited a more differentiated phenotype. 1,25 increased all parameters examined on SB, CA, and PS surfaces. When indomethacin was added with 1,25, the 1,25-dependent effects on cell number and OC and LTGF beta production were abolished; however, ALPase was unaffected. This indicates that bone cell response to systemic hormones may be modified by implant surface roughness. This effect may be mediated, at least in part, by prostaglandins produced by the same cells.

Interfacial phenomena: an in vitro study of the effect of calcium phosphate (Ca-P) ceramic on bone formation

In previous studies we developed a RF magnetron sputter technique for the production of thin Ca-P coatings. With this technique coatings can be produced that vary in Ca/P ratio as well as in structural appearance. The aim of this investigation was to obtain more understanding of the biological behavior of these coatings by way of in vitro experiments. The effect of noncoated titanium (Ti) and three different Ca-P-sputtered surfaces on the proliferation and differentiation (morphology and matrix production) of osteoblast-like cells was studied. Proliferation was determined using counting procedures; morphology was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fluorescent markers and energy-dispersive X-ray microanalysis (EDX) were used to obtain quantitative and compositional information about the resultant calcified extracellular matrix (ECM). Results demonstrated that proliferation of the osteoblast-like cells was significantly (p < 0.05) higher on noncoated than on Ca-P-coated samples. On the other hand, more mineralized ECM was formed on the coated surfaces. In addition, TEM confirmed that the cells on the coated substrates were surrounded by ECM with collagen fibers embedded in crystallized, needle-shaped structures. On the basis of these findings, we concluded that: (1) the investigated Ca-P sputter coatings possess the capacity to activate the differentiation and expression of osteogenic cells, and (2) bone formation proceeds faster on Ca-P surfaces than on Ti substrates. Further, this bone-inductive effect appeared to be dependent on the Ca-P ratio of the deposited coatings.

RGD peptides regulate the specific adhesion scheme of osteoblasts to hydroxyapatite but not to titanium

Hydroxyapatite (HA) is a bioactive dental implant material which accelerates bone formation on its surface. The mechanism of this acceleration is not clear. The elucidation of the cell adhesion might be the key to the understanding of the bioactive mechanism of HA. In this study, we analyzed the adhesion of HOS human osteoblasts onto HA and titanium to find the particular adhesion to HA. In short-term cultures in fetal bovine serum-pre-coated materials, a significantly higher number of cells adhered to HA than to titanium. In addition, serum-free conditions with phosphate-buffered saline pre-coating or bovine serum albumin pre-coating materials were tested. The results were nearly the same among all pre-coating conditions, suggesting that the quantity of cell adhesion was not affected by serum components. However, in the morphological observations by SEM, the form of adhesion was found to differ among pre-coating conditions. The osteoblasts tightly adhered and spread onto both HA and titanium with serum pre-coating, whereas the cells loosely adhered and did not spread without serum. To evaluate the Arg-Gly-Asp (RGD) sequence-specific adhesion, we used synthetic RGD peptides for a competitive inhibition test. The results showed that RGD peptides remarkably inhibited the tight adhesion and spreading of osteoblasts onto HA, whereas they did not strongly inhibit adhesion and spreading onto titanium. These results demonstrate that the regulation of cell adhesion to HA is different from that to titanium. Our study suggests that the RGD-containing serum proteins might have a major role in regulating the specific adhesion of osteoblasts to HA, and in inducing enhanced cell growth and differentiation.

Titanium surface roughness alters responsiveness of MG63 osteoblast-like cells to 1 alpha,25-(OH)2D3

Surface roughness has been shown to affect differentiation and local factor production of MG63 osteoblast-like cells. This study examined whether surface roughness alters cellular response to circulating hormones such as 1 alpha,25-(OH)2D3. Unalloyed titanium (Ti) disks were pretreated with HF/HNO3 (PT) and then were machined and acid-etched (MA). Ti disks also were sandblasted (SB), sandblasted and acid etched (CA), or plasma sprayed with Ti particles (PS). The surfaces, from smoothest to roughest, were: PT, MA, CA, SB, and PS. MG63 cells were cultured to confluence on standard tissue culture polystyrene (plastic) or the Ti surfaces and then treated for 24 h with either 10(-8) M or 10(-7) M 1 alpha,25-(OH)2D3 or vehicle (control). Cellular response was measured by assaying cell number, cell layer alkaline phosphatase specific-activity, and the production of osteocalcin, latent (L) TGF beta, and PGE2. Alkaline phosphatase activity was affected by surface roughness; as the surface became rougher, the cells showed a significant increase in alkaline phosphatase activity. Addition of 1 alpha,25-(OH)2D3 to the cultures caused a dose-dependent stimulation of alkaline phosphatase activity that was synergistic with the effect caused by surface roughness alone. 1 alpha,25-(OH)2D3 also caused a synergistic increase in osteocalcin production as well as local factor (LTGF beta and PGE2) production on the rougher CA, SB, and PS surfaces, but it had no effect on the production on smooth surfaces. The inhibitory effect of surface roughness on cell number was not affected by 1 alpha,25-(OH)2D3 except on the SB surface. 1 alpha,25-(OH)2D3 decreased cell number, increased alkaline phosphatase activity and osteocalcin production, and had no effect on LTGF beta or PGE2 production by MG63 cells grown on tissue culture polystyrene. These data suggest that bone cell response to systemic hormones is modified by surface roughness and that surface roughness increases the responsiveness of MG63 cells to 1 alpha,25-(OH)2D3. They also suggest that the endocrine system is actively involved in normal bone healing around implants.

Phenotype expression of gingival fibroblasts cultured on membranes used in guided tissue regeneration

Human gingival fibroblasts were cultured in vitro using as substrates an extracellular matrix (matrix) and polytetrafluoride (PTFE) membranes, which are used in guided tissue regeneration. To test the degree of biocompatibility of these membranes, the cellular proliferation and the accumulation of extracellular matrix (ECM) macromolecules were considered as parameters. The fibroblasts were cultured in vitro for 24 and 48 hours without serum on plastic, matrix, and PTFE membranes in the presence of 3H-thymidine, 3H-glucosamine, and 3H-proline to study the neo-synthesis of DNA, glycosaminoglycans (GAG), and collagen proteins, respectively. Studies on cell proliferation showed that fibroblasts grown on matrix membrane significantly increased 3H-thymidine incorporation, while fibroblasts grown on PTFE membrane decreased 3H-thymidine incorporation, compared to plastic used as a control. Moreover, the PTFE membrane induced a marked decrease of collagen and GAG accumulation both in the cellular and extracellular pool, while the matrix membrane provoked a decrease of the two macromolecules in the cellular pool and an increase in the extracellular one, compared to the control. The data we obtained demonstrate that matrix membranes are the most suitable to stimulate both cellular proliferation and ECM macromolecule accumulation.

Glass peek composite promotes proliferation and osteocalcin production of human osteoblastic cells

An isoelastic intramedullary implant has been developed using a composite of polyetheretherketone and 10% random, chopped E-glass fibers (GPEEK). The effect of this novel material on human bone cells has not been defined. The objective of this study was to test whether GPEEK supported the proliferation of the human bone cell line MG63, which exhibits osteoblastlike characteristics. Cells (1 x 10(5)/mL) were propagated on GPEEK discs with three different surface roughnesses (3, 6, and 9 microns) and on polystyrene plates, for comparison. The reaction of MG63 osteoblastlike cells to the GPEEK polymer composite was analyzed by determination of cell yield, osteocalcin production, and levels of alkaline phosphatase. The viable cells that were retrieved from the GPEEK discs of all three surface roughness had an approximate sixfold increase in number. Osteoblastic function of the cells, indicated by osteocalcin production, was unimpaired after a 5-day culture on the three surfaces of GPEEK. The highest level of osteocalcin was produced by osteoblastic cells propagated on GPEEK with a 9 microns surface roughness. The levels of alkaline phosphatase of these cells were similarly greater for the different degrees of surface roughness. Overall, this study demonstrates that GPEEK supported proliferation of osteoblastlike cells and provided a favorable environment for the continued production of osteocalcin in vitro.

Underlying mechanisms at the bone-surface interface during regeneration

The goal of regenerative therapy around teeth and implants is to create a suitable environment in which the natural biological potential for functional regeneration of periodontal ligament and/or bone can be maximized. In order for the regenerative process to be successful, the following factors must be addressed: prevention of acute inflammation from bacteria, mechanical stability of the wound, creation and maintenance of blood clot-filled space, isolation of the regenerative space from undesirable competing tissue types, and the creation of a desirable surface chemistry, energy, roughness and microtopography that can directly influence cellular response, ultimately affecting the rate and quality of new tissue formation and, therefore, the regeneration process. This paper will review how surface characteristics (chemistry and roughness) can affect cell response and local factor production. To evaluate the effect of surface chemistry on cell proliferation and differentiation costochondral chondrocytes were grown on standard tissue culture plastic dishes sputter-coated with different materials. The results indicate that surface materials can elicit differential responses in cell metabolism and phenotypic expression in vitro. In a second study, the effect of varying titanium surface roughnesses on osteoblast-like cell behavior was examined. Surface roughness was found to alter osteoblast proliferation, differentiation and matrix production in vitro. In addition, production of PGE2 and TGF beta by these cells was also shown to increase with increasing surface roughness, indicating that substrate surface roughness also affects cytokine and growth factor production. The role of surface roughness in determining cellular response was further explored by comparing the response of osteoblasts grown on new and previously used surfaces. The results of these latter studies showed that cell proliferation, expression of differentiation markers and overall matrix production are not altered when cells are grown on used vs. virgin surfaces. This suggests the possibility that implants may be re-used, especially in the same patient, if they are appropriately treated. In this context, it should also be noted that rougher titanium surfaces may require more extensive cleaning procedures. From a global perspective, these studies provide some insight into how bone regeneration can be optimized in the presence of an implant or tooth root residing at the site of a bony defect. Since the new bone being produced, during regeneration, grows from a distal area toward the implant or tooth root surface, it is hypothesized that the osteoblasts growing on the surface of the implant may produce local factors that can affect the bone healing process distally. In short, it appears that the surface characteristics of an implant, particularly roughness, may direct tissue healing and, therefore, subsequent implant success in sites of regeneration by modulating osteoblast phenotypic expression.

Osteogenic differentiation of cultured marrow stromal stem cells on the surface of bioactive glass ceramics

To investigate the significance of apatite-wollastonite-containing glass ceramic (AW ceramic) surfaces and the biological apatite layer formed on these surfaces, rat marrow cell culture, which shows osteogenic differentiation, was carried out on four different culture substrata (control culture dish, two AW ceramics, each having a different surface roughness, and a ceramic on which an apatite layer was formed. A culture period of 2 weeks in the presence of beta-glycerophosphate, ascorbic acid, and dexamethasone resulted in abundant mineralized nodule formations that were positive for alkaline phosphatase (ALP) stain on all substrata. The stain on the apatite-formed AW ceramic was the most intense, the enzyme activity being about twice that of the control culture dish, which had the lowest stain and activity of the four substrata. Northern blot analysis of bone Gla protein (BGP) showed the same tendency, that is, the amount of BGP mRNA from cultured cells on the apatite-formed AW ceramics was the highest and the mRNA on the control dish was the lowest. These data indicate that the glass ceramic surface promotes osteoblastic differentiation and that the promotion can be further enhanced by the formation of a biological apatite layer on the ceramic surface.

The role of implant surface characteristics in the healing of bone

The surface of an implant determines its ultimate ability to integrate into the surrounding tissue. The composite effect of surface energy, composition, roughness, and topography plays a major role during the initial phases of the biological response to the implant, such as protein adsorption and cellular adherence, as well as during the later and more chronic phases of the response. For bone, the successful incorporation (and hence rigid fixation) of an alloplastic material within the surrounding bony bed is called osteointegration. The exact surface characteristics necessary for optimal osteointegration, however, remain to be elucidated. This review will focus on how surface characteristics, such as composition and roughness, affect cellular response to an implant material. Data from two different culture systems suggest that these characteristics play a significant role in the recruitment and maturation of cells along relevant differentiation pathways. In the case of osteointegration, if the implant surface is inappropriate or less than optimal, cells will be unable to produce the appropriate complement of autocrine and paracrine factors required for adequate stimulation of osteogenesis at the implant site. In contrast, if the surface is appropriate, cells at the implant surface will stimulate interactions between cells at the surface and those in distal tissues. This, in turn, will initiate a timely sequence of events which include cell proliferation, differentiation, matrix synthesis, and local factor production, thereby resulting in the successful incorporation of the implant into the surrounding bony tissue.

Surface roughness modulates the local production of growth factors and cytokines by osteoblast-like MG-63 cells

Titanium (Ti) surface roughness affects proliferation, differentiation, and matrix production of MG-63 osteoblast-like cells. Cytokines and growth factors produced in the milieu surrounding an implant may also be influenced by its surface, thereby modulating the healing process. This study examined the effect of surface roughness on the production of two factors known to have potent effects on bone, prostaglandin E2 (PGE2) and transforming growth factor beta 1 (TGF-beta 1). MG-63 cells were cultured on Ti disks of varying roughness. The surfaces were ranked from smoothest to roughest: electropolished (EP), pretreated with hydrofluoric acid-nitric acid (PT), fine sand-blasted, etched with HCl and H2SO4, and washed (EA), coarse sand-blasted, etched with HCl and H2SO4, and washed (CA), and Ti plasma-sprayed (TPS). Cells were cultured in 24-well polystyrene (plastic) dishes as controls and to determine when confluence was achieved. Media were collected and cell number determined 24 h postconfluence. PGE2 and TGF-beta 1 levels in the conditioned media were determined using commercial radioimmunoassay and enzyme-linked immunosorbent assay kits, respectively. There was an inverse relationship between cell number and Ti surface roughness. Total PGE2 content in the media of cultures grown on the three roughest surfaces (FA, CA, and TPS) was significantly increased 1.5-4.0 times over that found in media of cultures grown on plastic or smooth surfaces. When PGE2 production was expressed per cell number, CA and TPS cultures exhibited six- to eightfold increases compared to cultures on plastic and smooth surfaces. There was a direct relationship between TGF-beta 1 production and surface roughness, both in terms of total TGF-beta 1 per culture and when normalized for cell number. TGF-beta 1 production on rough surfaces (CA and TPS) was three to five times higher than on plastic. These studies indicate that substrate surface roughness affects cytokine and growth factor production by MG-63 cells, suggesting that surface roughness may modulate the activity of cells interacting with an implant, and thereby affect tissue healing and implant success.