Combined exposure to big endothelin-1 and mechanical loading in bovine sternal cores promotes osteogenesis.

Increased bone formation resulting from mechanical loading is well documented; however, the interactions of the mechanotransduction pathways are less well understood. Endothelin-1, a ubiquitous autocrine/paracrine signaling molecule promotes osteogenesis in metastatic disease. In the present study, it was hypothesized that exposure to big endothelin-1 (big ET1) and/or mechanical loading would promote osteogenesis in ex vivo trabecular bone cores. In a 2×2 factorial trial of daily mechanical loading (-2000µÎµ, 120cycles daily, "jump" waveform) and big ET1 (25ng/mL), 48 bovine sternal trabecular bone cores were maintained in bioreactor chambers for 23days. The bone cores' response to the treatment stimuli was assessed with percent change in core apparent elastic modulus (ΔEapp), static and dynamic histomorphometry, and prostaglandin E2 (PGE2) secretion. Two-way ANOVA with a post hoc Fisher's LSD test found no significant treatment effects on ΔEapp (p=0.25 and 0.51 for load and big ET1, respectively). The ΔEapp in the "no load + big ET1" (CE, 13±12.2%, p=0.56), "load + no big ET1" (LC, 17±3.9%, p=0.14) and "load + big ET1" (LE, 19±4.2%, p=0.13) treatment groups were not statistically different than the control group (CC, 3.3%±8.6%). Mineralizing surface (MS/BS), mineral apposition (MAR) and bone formation rates (BFR/BS) were significantly greater in LE than CC (p=0.037, 0.0040 and 0.019, respectively). While the histological bone formation markers in LC trended to be greater than CC (p=0.055, 0.11 and 0.074, respectively) there was no difference between CE and CC (p=0.61, 0.50 and 0.72, respectively). Cores in LE and LC had more than 50% greater MS/BS (p=0.037, p=0.055 respectively) and MAR (p=0.0040, p=0.11 respectively) than CC. The BFR/BS was more than two times greater in LE (p=0.019) and LC (p=0.074) than CC. The PGE2 levels were elevated at 8days post-osteotomy in all groups and the treatment groups remained elevated compared to the CC group on days 15, 19 and 23. The data suggest that combined exposure to big ET1 and mechanical loading results in increased osteogenesis as measured in biomechanical, histomorphometric and biochemical responses.

Dependence of anisotropy of human lumbar vertebral trabecular bone on quantitative computed tomography-based apparent density.

Most studies investigating human lumbar vertebral trabecular bone (HVTB) mechanical property-density relationships have presented results for the superior-inferior (SI), or "on-axis" direction. Equivalent, directly measured data from mechanical testing in the transverse (TR) direction are sparse and quantitative computed tomography (QCT) density-dependent variations in the anisotropy ratio of HVTB have not been adequately studied. The current study aimed to investigate the dependence of HVTB mechanical anisotropy ratio on QCT density by quantifying the empirical relationships between QCT-based apparent density of HVTB and its apparent compressive mechanical properties--elastic modulus (E(app)), yield strength (σ(y)), and yield strain (ε(y))--in the SI and TR directions for future clinical QCT-based continuum finite element modeling of HVTB. A total of 51 cylindrical cores (33 axial and 18 transverse) were extracted from four L1 human lumbar cadaveric vertebrae. Intact vertebrae were scanned in a clinical resolution computed tomography (CT) scanner prior to specimen extraction to obtain QCT density, ρ(CT). Additionally, physically measured apparent density, computed as ash weight over wet, bulk volume, ρ(app), showed significant correlation with ρ(CT) [ρ(CT) = 1.0568 × ρ(app), r = 0.86]. Specimens were compression tested at room temperature using the Zetos bone loading and bioreactor system. Apparent elastic modulus (E(app)) and yield strength (σ(y)) were linearly related to the ρ(CT) in the axial direction [E(SI) = 1493.8 × (ρ(CT)), r = 0.77, p < 0.01; σ(Y,SI) = 6.9 × (ρ(CT)) − 0.13, r = 0.76, p < 0.01] while a power-law relation provided the best fit in the transverse direction [E(TR) = 3349.1 × (ρ(CT))(1.94), r = 0.89, p < 0.01; σ(Y,TR) = 18.81 × (ρ(CT))(1.83), r = 0.83, p < 0.01]. No significant correlation was found between ε(y) and ρ(CT) in either direction. E(app) and σ(y) in the axial direction were larger compared to the transverse direction by a factor of 3.2 and 2.3, respectively, on average. Furthermore, the degree of anisotropy decreased with increasing density. Comparatively, ε(y) exhibited only a mild, but statistically significant anisotropy: transverse strains were larger than those in the axial direction by 30%, on average. Ability to map apparent mechanical properties in the transverse direction, in addition to the axial direction, from CT-based densitometric measures allows incorporation of transverse properties in finite element models based on clinical CT data, partially offsetting the inability of continuum models to accurately represent trabecular architectural variations.

Apparent elastic modulus of ex vivo trabecular bovine bone increases with dynamic loading.

Although it is widely known that bone tissue responds to mechanical stimuli, the underlying biological control is still not completely understood. The purpose of this study was to validate required methods necessary to maintain active osteocytes and minimize bone tissue injury in an ex vivo three-dimensional model that could mimic in vivo cellular function. The response of 22 bovine trabecular bone cores to uniaxial compressive load was investigated by using the ZETOS bone loading and bioreactor system while perfused with culture medium for 21 days. Two groups were formed, the "treatment" group (n = 12) was stimulated with a physiological compressive strain (4000 µÎµ) in the form of a "jump" wave, while the "control" group (n = 10) was loaded only during three measurements for apparent elastic modulus on days 3, 10, and 21. At the end of the experiment, apoptosis and active osteocytes were quantified with histological analysis, and bone formation was identified by means of the calcium-binding dye, calcein. It was demonstrated that the treatment group increased the elastic modulus by 61%, whereas the control group increased by 28% (p<0.05). Of the total osteocytes observed at the end of 21 days, 1.7% (±0.3%) stained positive for apoptosis in the loaded group, whereas 2.7% (±0.4%) stained positive in the control group. Apoptosis in the center of the bone cores of both groups at the end of 21 days was similar to that observed in vivo. Therefore, the three-dimensional model used in this research permitted the investigation of physiological responses to mechanical loads on morphology adaptation of trabecular bone in a controlled defined load and chemical environment.

High affinity binding of an engineered, modular peptide to bone tissue.

Bone grafting procedures have become common due in part to a global trend of population aging. Native bone graft is a popular choice when compared to various synthetic bone graft substitutes, owing to superior biological activity. Nonetheless, the insufficient ability of bone allograft to induce new bone formation and the insufficient remodeling of native bone grafts call for osteoinductive factors during bone repair, exemplified by recombinant human bone morphogenetic protein 2 (rhBMP2). We previously developed a modular bone morphogenetic peptide (mBMP) to address complications associated with the clinical use of rhBMP2 as a bone graft substitute. The mBMP is designed to strongly bind to hydroxyapatite, the main inorganic component of bone and teeth, and to provide pro-osteogenic properties analogous to rhBMP2. Our previous in vivo animal studies showed that mBMP bound to hydroxyapatite-coated orthopedic implants with high affinity and stimulated new bone formation. In this study, we demonstrate specific binding of mBMP to native bone grafts. The results show that mBMP binds with high affinity to both cortical and trabecular bones, and that the binding is dependent on the mBMP concentration and incubation time. Importantly, efficient mBMP binding is also achieved in an ex vivo bone bioreactor where bone tissue is maintained viable for several weeks. In addition, mBMP binding can be localized with spatial control on native bone tissue via simple methods, such as dip-coating, spotting, and direct writing. Taken together with the pro-osteogenic activity of mBMP established in previous bone repair models, these results suggest that mBMP may promote bone healing when coated on native bone grafts in a clinically compatible manner.

Exercise for Prevention of Osteoporosis: A Review.

In brief This paper reports some of the current research about the effects of physical activity on bone mineral and its implications in preventing osteoporosis. For men, bone mineral loss does not usually present a problem until about the eighth decade, but some women may lose 30% of their bone mineral mass by age 70. Although physical activity is only one of many factors in osteoporosis, it is clear that bone atrophy occurs when activity is lacking and that bone hypertrophy occurs when sufficient physical activity is present.