Practice of a testing bench to study the effects of cyclic stretching on osteoblast-orthopaedic ceramic interactions

Effects of physiological mechanical strains on the release of growth factors and the expression of differentiation marker genes in human osteoblasts growing on Ti-6Al-4V

Mechanical loading factors at the bone-implant interface are critical for the osseointegration and clinical success of the implant. The aim of the present investigation was to study the effects of mechanical strain on the orthopedic biomaterial Ti-6Al-4V/osteoblast interface, using an in vitro model. Homogeneous strain was applied to human bone marrow derived osteoblasts (HBMDOs) cultured on Ti-6Al-4V, at physiological levels (strain magnitudes 500 microstrain (microepsilon) and 1000 microepsilon, at frequencies of load application 0.5 Hz and 1 Hz), by a mechanostimulatory system, based on the principle of four-point bending. Semi-quantitative reverse transcription-polymerase chain reaction (sqRT-PCR) was used to determine the mRNA expression of Cbfa1 and osteocalcin at different loading conditions. The release of growth factors as a response to stretch was also investigated by transferring stretch-conditioned media to nonstretched cells and by measuring their effect on the regulation of DNA synthesis. Mechanical loading was found to contribute to the regulation of osteoblast differentiation by influencing the level of the osteoblast-specific transcription factor Cbfa1, both at the mRNA and protein level, and also the level of osteocalcin, which is regarded as the most osteoblast-specific gene. Both genes were differentially expressed shortly after the application of different mechanical stimuli, in terms of strain frequency, magnitude, and time interval. Media conditioned from mechanically stressed HBMDOs stimulate DNA synthesis more intensely compared to media conditioned from unstressed control cultures, indicating that mechanical strain induces the release of a mitogenic potential that regulates cell proliferation.

Estimation of hydrodynamic shear stresses developed on human osteoblasts cultured on Ti-6Al-4V and strained by four point bending. Effects of mechanical loading to specific gene expression

The aim of the present investigation was to study the effects of mechanical strain on the orthopedic biomaterial Ti-6Al-4V-osteoblast interface, using an in vitro model. Homogeneous strain was applied to Human Bone Marrow derived Osteoblasts (HBMDOs) cultured on Ti-6Al-4V, at levels which are considered physiological, by a four-point bending mechanostimulatory system. A simple model for the estimation of maximum hydrodynamic shear stresses developed on cell culture layer and induced by nutrient medium flow during mechanical loading, as a function of the geometry of the culture plate and the load characteristics, is proposed. Shear stresses were lower than those which can elicit cell response. Mechanical loading was found that contributes to the regulation of osteoblast differentiation by influencing the expression of the osteoblast-specific transcription factor Cbfa1, both at the mRNA and protein level, and also the osteocalcin expression, whereas osteopontin gene expression was unaffected by mechanical loading at all experimental conditions.

Modulation of the responses of human osteoblast-like cells to physiologic mechanical strains by biomaterial surfaces

In a previous study we demonstrated that MG-63 cells cultured on Ti-6Al-4V discs covered by alumina ceramic and submitted to intermittent mechanical strain (IMS) presented morphological alteration associated with enhanced differentiation. Here we examine how the mechanical response of osteoblasts can be modulated by the nature of the substrate. MG-63 cells were cultured on four materials: polystyrene and Ti-6Al-4V (average roughness = 0.48 microm) as smooth substrates; Ti-6Al-4V (average roughness = 5.76 microm) and Ti-6Al-4V covered with alumina (average roughness = 5.21 microm) as rough substrates. Mechanical strains were applied for 15 min, three times a day for 1-5 days with a 600 microstrains magnitude and a 0.25 Hz frequency. IMS stimulated alkaline phosphatase activity by 25-35% on all substrates and had no effect on cell growth on either substrate. Fibronectin (FN) was chosen as representative of cell-matrix interaction. FN production was increased by 60% after 1 day of stretching only on alumina-coated discs. FN organization examined on smooth substrates was affected by 5 days of IMS, showing a thickening of the fibres. The same modifications induced by IMS were previously observed on alumina-covered discs. Vinculin expression was not affected by IMS whatever the substrate. Cell-cell interactions were determined by N-cadherin immunoblotting. N-cadherin expression was increased by IMS specifically on rough substrates. Our results suggest that the nature of the surface did not influence the up-regulation of alkaline phosphatase activity induced by IMS, but modulates specifically cell-substrate as well as cell-cell interactions in response to IMS.

Effects of substrate characteristics on bone cell response to the mechanical environment

The effect of substrate characteristics on primary human bone cell response to mechanical loading was investigated in this study. The substrates comprised organic and inorganic materials with a range of hydrophilic and hydrophobic features. Substrate surface topography varied from smooth to particulate to porous. It was found that hydrophilic substrates such as borosilicate glass facilitated bone cell adhesion, in contrast to hydrophobic substrates such as poly(L-lactic acid), in which clumps of cells grew unevenly across the substrate surface. All primary bone cells cultured in the various collagen-coated substrates were responsive to mechanical stimulation. The study showed that, at a low strain level of 1000 microstrain, mechanical stimulation enhanced bone cell differentiation rather than proliferation. Coating the substrates with collagen type I enhanced cell adhesion and promoted an elongated cell morphology, indicating that the presence of specific binding sites on a substrate may be more important than its hydrophilic properties, regardless of the substrate topography.

Stretch-mediated responses of osteoblast-like cells cultured on titanium-coated substratesin vitro

Cyclic stretching experiments on osteoblast-like cells have proven to be a useful tool in understanding the underlying mechanisms of load transduction at the bone-implant surface. However, most experimental setups use silicone rubber substrates, which are atypical for orthopedic and dental implant materials. Therefore, we investigated the responses of osteoblast-like cells to loading on titanium (Ti)-coated versus plain silicone substrates. Ti-coated substrates were made by a radio-frequency magnetron sputtering process, and characterized using Rutherford backscattering spectrometry, X-ray photoelectron spectroscopy, and contact-angle measurements. Osteoblast-like cells cultured from rat bone marrow were seeded on both types of substrates and stretched for 1 h continuously. Subsequently, cell proliferation, alkaline phosphatase activity, and calcium content were measured for up to 24 days after seeding. In addition light-, scanning electron-, and confocal laser scanning micrographs were made. The results showed that our Ti coating had a thickness of 50 nm and contained Ti/oxygen as 1:1. However, further characterization proved that the silicone material had a tendency to resurface through the coating. Osteoblast-like cells proliferated faster on the Ti-coated substrates, but differentiation was slower compared with the silicone substrates. It was concluded that that there was a definitive influence of the substrate material in mechanical stress models. Therefore, extrapolation of results obtained using silicone substrates cannot be translated directly toward the situation of metallic implant materials.

Intermittent versus continuous stretching effects on osteoblast‐like cellsin vitro

The objective of this study was to quantify and compare stretch-mediated responses of primary rat osteoblast-like cells to uniform cyclic strain applied intermittently or continuously. Primary rat osteoblast-like cells were seeded and cultured in silicone rubber dishes for 2 days. They were then subjected to 1000 microstrains at 1 Hz for periods of 60 consecutive minutes or to a series of 15-min stretch followed by 15-min rest, until a total stretch of 60 min. After stretching, cells were incubated and assayed on days 4, 8, 16, and 24 for DNA content, alkaline phosphatase (ALP) activity, and calcium (Ca) content. Additionally, qualitative information was obtained via scanning electron and confocal laser scanning micrographs. Significant increases in DNA were observed for cells stretched intermittently versus cells stretched continuously and versus controls. Results showed significant decreases (p < 0.05) in ALP for cells between stretched groups and between both stretched groups versus controls. Additionally, Ca content was greater in cells stretched intermittently versus controls on days 4 and 8 and versus cells stretched continuously on day 24. In conclusion, intermittently strained cells demonstrated significant decreases in ALP and increases in DNA and Ca versus cells strained continuously. This supports the theory that cells respond to mechanical loading in a "trigger-like" response.

Physiological strains induce differentiation in human osteoblasts cultured on orthopaedic biomaterial

We have developed an in vitro mechanical stretching model of osteoblastic cells cultured on metallic biomaterials in order to study the effects of mechanical strain on osteointegration of orthopaedic implants. Titanium alloy discs coated with alumina or hydroxyapatite were used as substrates. Three Dynacell devices were especially designed to apply cyclic strains on rigid biomaterials. The regimen (600 mu epsilon strains, 0.25Hz) was defined on the basis of physiological data and estimated deformation on hip stem prostheses. The performances of these apparatus were reproducible and provided controlled deformations. Human osteosarcoma cell line MG-63, human osteoblasts obtained from primary cultures and ROS 17/2.8 rat osteosarcoma cells were used as cell models. Cell behaviour was assessed in terms of growth and alkaline phosphatase (ALP) activity by in situ assays for two regimens: 15-min deformations repeated three times a day to mimic rehabilitation exercises and 24-h continuous deformations. We demonstrated that continuous deformation did not affect the growth and ALP activity of MG-63 cells, in contrast with sequential deformations which had no effect on cell number, but which stimulated ALP activity after 5 days of stretching. This sequential regimen can also modify the behaviour of human bone-derived cells resulting in increased proliferation after 5 days and stimulation of ALP activity after 15 days. ROS 17/2.8 rat osteosarcoma cells submitted to sequential deformations responded faster than other cell lines by increasing their ALP activity only after 1 day of stretching. Like MG-63 cells, proliferation of the ROS 17/2.8 rat osteosarcoma cell line was not affected by sequential deformations. This study suggests that short, repeated deformations defined to mimic rehabilitation exercises recommended after prostheses implantation are more likely to exert beneficial effects on implanted bone than continuous strains.