Is there an optimal force level for sutural expansion?

Authors

Sean Shih-Yao Liu, Lynne A. Opperman, Hee-Moon Kyung, Peter H. Buschang

Abstract

The purpose of this study was to establish the causal relationships between expansion force magnitudes, sutural separation, and sutural bone formation. Thirty-seven 6-week-old rabbits were randomly assigned to 4 force groups (0, 50, 100, or 200 g). Constant forces were delivered for 42 days by nickel-titanium open-coil springs to miniscrew implants (MSIs) placed in the frontal bone on both sides of the midsagittal suture. Inter-MSI and bone marker widths were measured biweekly to quantify sutural separation and MSI movements. Sutural bone formation was quantified based on the incorporation of fluorescent bone labels administered at days 18, 28, and 38. Nine of 74 MSIs failed between days 0 and 14, including 4 in the controls and 5 in the 50-g group. A decelerating curvilinear pattern of sutural separation was evident in the 50-g, 100-g, 200-g groups. Bone markers showed that sutural widths increased by 0.6, 3.2, 5.1, and 6.2 mm in the control, 50-g, 100-g, and 200-g groups, respectively. Except for the 200-g group, significantly greater amounts of bone formation were observed between days 18 and 28 than between days 28 and 38. Sutural bone formation also increased with increasing forces up to 100 g; there was no difference between the 100-g and the 200-g groups. Sutural separation explained 71% and 53% of the variations in bone formation between days 18 and 28 and days 28 and 38, respectively. Within the limits of this study, sutural bone formation is directly related to the amount of sutural separation, which is in turn related to the amount of force applied. The results suggest that there is a level of induced sutural separation that provides the greatest amount of bone formation.

Link to Article

http://dx.doi.org/10.1016/j.ajodo.2009.03.056

The presence of extracellular matrix alters the chondrocyte response to endoplasmic reticulum stress

Authors

Ashleigh E. Nugent, Denise L. McBurney, Walter E. Horton Jr.

Abstract

The objective of this study was to test the hypothesis that extracellular matrix (ECM) would alter the endoplasmic reticulum (ER) stress response of chondrocytes. Chondrocytes were isolated from calf knees and maintained in monolayer culture or suspended in collagen I to form spot cultures (SCs). Our laboratory has shown that bovine chondrocytes form cartilage with properties similar to native cartilage after 2–4 weeks in SCs. Monolayer cultures treated with ER stressors glucose withdrawal (–Glu), tunicamycin (TN), or thapsigargin (TG) up-regulated Grp78 and Gadd153, demonstrating a complete ER stress response. SCs were grown at specific times from 1 day to 6 weeks before treatment with ER stressors. Additionally, SCs grown for 1, 2, or 6 weeks were treated with increasing concentrations of TN or TG. Western blotting of SCs for Grp78 indicated that increased ECM accumulation results in delayed expression; however, Grp78 mRNA is up-regulated in response to ER stressors even after 6 weeks in culture. SCs treated with ER stressors did not up-regulate Gadd153, suggesting that the cells experienced ER stress but would not undergo apoptosis. In fact, SCs undergo apoptosis upon ER stress treatment after 0–1 day of growth; however, after 4 days and to 6 weeks, apoptosis in treated samples was not different than controls. Pro-survival molecules Bcl-2 and Bag-1 were up-regulated upon ER stress in SCs. These results suggest that presence of ECM confers protection from ER stressors. Future studies involving chondrocyte physiology should focus on responses in conditions more closely mimicking the in vivo cartilage environment.

Link to Article

http://dx.doi.org/10.1002/jcb.23025

Ovariectomy stimulates and bisphosphonates inhibit intracortical remodeling in the mouse mandible

Authors

DJ Kubek, DB Burr, MR Allen

Abstract

The pathophysiology of osteonecrosis of the jaw (ONJ) is thought to be linked to suppression of intracortical remodeling. The aim of this study was to determine whether mice, which normally do not undergo appreciable amounts of intracortical remodeling, could be stimulated by ovariectomy to remodel within the cortex of the mandible and if bisphosphonates (BPs) would suppress this intracortical remodeling. Skeletally mature female C3H mice were either ovariectomized (OVX) or SHAM operated and treated with two intravenous doses of zoledronic acid (ZOL, 0.06 mg/kg body weight) or vehicle (VEH). This ZOL dose corresponds to the dose given to patients with cancer on a mg/kg basis, adjusted for body weight. Calcein was administered prior to sacrifice to label active formation sites. Dynamic histomorphometry of the mandible and femur was performed. Vehicle-treated OVX animals had significantly higher (eightfold) intracortical remodeling of the alveolar portion of the mandible compared to sham – this was significantly suppressed by ZOL treatment. At all skeletal sites, overall bone formation rate was lower with ZOL treatment compared to the corresponding VEH group. Under normal conditions, the level of intracortical remodeling in the mouse mandible is minimal but in C3H mice it can be stimulated to appreciable levels with ovariectomy. Based on this, if the suppression of intracortical remodeling is found to be part of the pathophysiology of ONJ, the ovariectomized C3H mouse could serve as a useful tool for studying this condition.

Link to Article

http://dx.doi.org/10.1111/j.1601-6343.2010.01497.x

Osteoclast Response to Low Extracellular Sodium and the Mechanism of Hyponatremia-induced Bone Loss

Authors

Julia Barsony, Yoshihisa Sugimura, and Joseph G. Verbalis

Abstract

Our recent animal and human studies revealed that chronic hyponatremia is a previously unrecognized cause of osteoporosis that is associated with increased osteoclast numbers in a rat model of the human disease of the syndrome of inappropriate antidiuretic hormone secretion (SIADH). We used cellular and molecular approaches to demonstrate that sustained low extracellular sodium ion concentrations ([Na+]) directly stimulate osteoclastogenesis and resorptive activity and to explore the mechanisms underlying this effect. Assays on murine preosteoclastic RAW 264.7 cells and on primary bone marrow monocytes both indicated that lowering the medium [Na+] dose-dependently increased osteoclast formation and resorptive activity. Low [Na+], rather than low osmolality, triggered these effects. Chronic reduction of [Na+] dose-dependently decreased intracellular calcium without depleting endoplasmic reticulum calcium stores. Moreover, we found that reduction of [Na+] dose-dependently decreased cellular uptake of radiolabeled ascorbic acid, and reduction of ascorbic acid in the culture medium mimicked the osteoclastogenic effect of low [Na+]. We also detected downstream effects of reduced ascorbic acid uptake, namely evidence of hyponatremia-induced oxidative stress. This was manifested by increased intracellular free oxygen radical accumulation and proportional changes in protein expression and phosphorylation, as indicated by Western blot analysis from cellular extracts and by increased serum 8-hydroxy-2′-deoxyguanosine levels in vivo in rats. Our results therefore reveal novel sodium signaling mechanisms in osteoclasts that may serve to mobilize sodium from bone stores during prolonged hyponatremia, thereby leading to a resorptive osteoporosis in patients with SIADH.

Link to Article

http://dx.doi.org/10.1074/jbc.M110.155002

Contrast-enhanced micro-computed tomography of fatigue microdamage accumulation in human cortical bone

Authors

Matthew D. Landrigan, Jiliang Li, Travis L. Turnbull, David B. Burr, Glen L. Niebur, Ryan K. Roeder

Abstract

Conventional methods used to image and quantify microdamage accumulation in bone are limited to histological sections, which are inherently invasive, destructive, two-dimensional, and tedious. These limitations inhibit investigation of microdamage accumulation with respect to volumetric spatial variation in mechanical loading, bone mineral density, and microarchitecture. Therefore, the objective of this study was to investigate non-destructive, three-dimensional (3-D) detection of microdamage accumulation in human cortical bone using contrast-enhanced micro-computed tomography (micro-CT), and to validate micro-CT measurements against conventional histological methods. Unloaded controls and specimens loaded in cyclic uniaxial tension to a 5% and 10% reduction in secant modulus were labeled with a precipitated BaSO4 stain for micro-CT and basic fuchsin for histomorphometry. Linear microcracks were similarly labeled by BaSO4 and basic fuchsin as shown by backscattered electron microscopy and light microscopy, respectively. The higher X-ray attenuation of BaSO4 relative to the bone extracellular matrix provided enhanced contrast for the detection of damage that was otherwise not able to be detected by micro-CT prior to staining. Therefore, contrast-enhanced micro-CT was able to nondestructively detect the presence, 3-D spatial location, and accumulation of fatigue microdamage in human cortical bone specimens in vitro. Microdamage accumulation was quantified on segmented micro-CT reconstructions as the ratio of BaSO4 stain volume (SV) to total bone volume (BV). The amount of microdamage measured by both micro-CT (SV/BV) and histomorphometry (Cr.N, Cr.Dn, Cr.S.Dn) progressively increased from unloaded controls to specimens loaded to a 5% and 10% reduction in secant modulus (p<0.001). Group means for micro-CT measurements of damage accumulation were strongly correlated to those using histomorphometry (p<0.05), validating the new methods. Limitations of the new methods in the present study included that the precipitated BaSO4 stain was non-specific and non-biocompatible, and that micro-CT measurements exhibited greater variability compared to conventional histology. Nonetheless, contrast-enhanced micro-CT enabled non-destructive imaging and 3-D spatial information, which are not possible using conventional histological methods.

Link to Article

http://dx.doi.org/10.1016/j.bone.2010.10.160

Temporal and spatial expression of osteoactivin during fracture repair

Authors

Samir M. Abdelmagid, Mary F. Barbe, Michael Hadjiargyrou, Thomas A. Owen, Roshanak Razmpour, Saqib Rehman, Steven N. Popoff, Fayez F. Safadi

Abstract

We previously identified osteoactivin (OA) as a novel secreted osteogenic factor with high expression in developing long bones and calvaria, and that stimulates osteoblast differentiation and matrix mineralization in vitro. In this study, we report on OA mRNA and protein expression in intact long bone and growth plate, and in fracture calluses collected at several time points up to 21 days post-fracture (PF). OA mRNA and protein were highly expressed in osteoblasts localized in the metaphysis of intact tibia, and in hypertrophic chondrocytes localized in growth plate, findings assessed by in situ hybridization and immunohistochemistry, respectively. Using a rat fracture model, Northern blot analysis showed that expression of OA mRNA was significantly higher in day-3 and day-10 PF calluses than in intact rat femurs. Using in situ hybridization, we examined OA mRNA expression during fracture healing and found that OA was temporally regulated, with positive signals seen as early as day-3 PF, reaching a maximal intensity at day-10 PF, and finally declining at day-21 PF. At day-5 PF, which correlates with chondrogenesis, OA mRNA levels were significantly higher in the soft callus than in intact femurs. Similarly, we detected high OA protein immunoexpression throughout the reparative phase of the hard callus compared to intact femurs. Interestingly, the secreted OA protein was also detected within the newly made cartilage matrix and osteoid tissue. Taken together, these results suggest the possibility that OA plays an important role in bone formation and serves as a positive regulator of fracture healing.

Link to Article

http://dx.doi.org/10.1002/jcb.22702