Triethyleneglycol dimethacrylate addition improves the 3D-printability and construct properties of a GelMA-nHA composite system towards tissue engineering applications.

In tissue engineering, there is a growing interest in the development of 3D printable bone tissue-inspired nanocomposites. However, most such nanocomposites have poor mechanical properties, owing to poor dispersion of the mineral phase (e.g. nano-hydroxyapatite, nHA) within the organic phase (e.g. methacrylated gelatin, GelMA) and low volume fractions of each phase. Triethyleneglycol dimethacrylate (TEGDMA) is commonly added to dental resin-based composites to improve the properties of the dental resin. Here, the effects of substituting a portion of the water phase in a GelMA-nHA composite with TEGDMA were evaluated. TEGDMA improved the dispersion of nHA within the highly-concentrated GelMA-based composite ink, as well as increased the ink's shear yield strength and reduced the critical energy for ink cure. As a result, the printability of the composite ink was greatly improved upon TEGDMA inclusion. Lastly, while the swelling of the cast composite in 37 °C water increased slightly, the mechanical properties (tensile strength, toughness, and stiffness) of the cast composite increased by at least an order of magnitude upon TEGDMA addition, and all composites demonstrated MSC cytocompatibility after 24 h. Overall, TEGDMA shows promise as an additive to tune properties of the GelMA-nHA system towards use in tissue engineering applications.

Enhanced levels of non-enzymatic glycation and pentosidine crosslinking in spontaneous osteoarthritis progression.

OBJECTIVE: To test the hypothesis that heightened advanced glycation endproducts (AGEs) content in cartilage accelerates the progression of spontaneous osteoarthritis (OA) in the Hartley guinea pig (HGP) model. METHODS: Twenty-eight male, 3-month-old HGPs were used. Eight were left untreated as a baseline control group and sacrificed at 3 months of age (n = 4) and 9 months of age (n = 4; age-matched controls). The other 20 HGPs received intra-articular knee injections in the right knee whereas the left knees acted as contra-lateral non-injected controls. Injections consisted of 100 µl phosphate buffered saline (PBS; n = 10) or PBS+2.0 M D-(-)-Ribose (n = 10). Injections were given once weekly for 24 weeks. At the end of the treatment period, the tibiae were fixed with formalin, scanned with microCT for sub-chondral bone mineral density, and then histological slides were prepared, stained with Safranin-O with Fast Green counter stain and scored using the OARSI-HISTOgp scheme. Cartilage pentosidine (established biomarker for AGEs) content, collagen content (% dry mass), glucosaminoglycan GAG-to-collagen ratio (µg/µg), GAG-to-DNA ratio and DNA-to-collagen ratio were measured. RESULTS: Pentosidine content increased greatly due to PBS + Ribose injection (P < 0.0001) and reached levels found in cartilage from 80-year-old humans. Surprisingly, mean OARSI-HISTOgp scores for both the injected and contra-lateral controls in the PBS + Ribose group were not detectably different, nor were they different from the mean score for the age-matched control group. CONCLUSION: AGEs accumulation due to intra-articular ribose-containing injections in the HGP model of spontaneous knee OA did not enhance disease progression.

Mechanism of bone collagen degradation due to KOH treatment.

BACKGROUND: The mechanisms underlying the effect of alterations in type I collagen on bone mechanical properties are not well defined. In a previous study, male and female emu tibiae were endocortically treated with 1M potassium hydroxide (KOH) solution for 1-14days. This treatment resulted in negligible mass loss (0.5%), collagen loss (0.05%), no differences in geometrical parameters but significant changes in mechanical properties. The objective of this study was to determine the mechanism of collagen degradation due to KOH treatment in order to explain the previously observed mechanical property changes. METHODS: Bone mineral was assessed using x-ray diffraction (XRD), microhardness and backscattered electron imaging (BSE). Bone collagen was assessed using α-chymotrypsin digestion, differential scanning calorimetry (DSC), gel electrophoresis (SDS-PAGE) and polarized light microscopy (PLM). RESULTS: BSE, microhardness and XRD revealed no changes in bone mineral due to KOH treatment. DSC showed an altered curve shape (lower and broader), indicating a change in collagen organization due to KOH treatment. Decreased α-chain band intensity in 14-day KOH treated groups detected using SDS-PAGE indicated α-chain fragmentation due to KOH treatment. PLM images revealed differences in collagen structure in terms of pattern distribution of preferentially oriented collagen between the periosteal and endocortical regions. CONCLUSION: These results suggest that endocortical KOH treatment causes in situ collagen degradation, which explains the previously reported altered mechanical properties. GENERAL SIGNIFICANCE: Compromising the organic component of bone contributes to an increase in bone fragility.

The fatigue resistance of rabbit tibiae varies with age from youth to middle age.

UNLABELLED: Young adults are at risk of stress fractures. Risk is higher in younger and female individuals. Stress fractures occur due to repeated loading of the bone (fatigue). We modeled this with rabbit tibiae. Age increased fatigue resistance which correlated with bone mineral density. A sex difference was not detected. INTRODUCTION: Younger adults who engage in intense physical activity with a sudden increase in intensity level (military recruits/college athletes) are at risk of bone stress fractures. Risk is greater in females and diminishes with aging. Stress fractures may be the result of fatigue damage, which is not repaired rapidly enough to avoid fracture. It was hypothesized that the fatigue resistance of whole rabbit tibiae would be less in female specimens but greater as animal age increased. METHODS: Rabbit tibiae were harvested from three age groups (4, 7, and ≥ 12 months (females only)). The tibiae were scanned with dual energy X-ray absorptiometry to determine bone mineral density (BMD), computed tomography to quantify geometry, and then fatigue tested in three-point bending. RESULTS: In the ≥ 12-month group, BMD was approximately 20% higher, while the fatigue resistance was found to be approximately ten times higher than the other age groups. Sex was not a factor in the 4- and 7-month groups. Multiple linear regression revealed that fatigue life was negatively correlated with applied stress range and positively correlated with BMD (adjusted r (2) = 0.69). CONCLUSIONS: A difference in fatigue behavior due to sex was not detected, but there was a large increase in fatigue resistance with age. This correlated with increased BMD and parallels a reduced risk of stress fracture due to age in military recruits. Skeletal "maturation" may play an important role in determining stress fracture risk. Increased risk in females may be due to mechanisms other than those that determine material behavior.

Deleting Rac1 improves vertebral bone quality and resistance to fracture in a murine ovariectomy model.

SUMMARY: The roles of Rac1 and Rac2 in regulating osteoclast-mediated bone quality in postmenopausal osteoporosis were evaluated using an ovariectomized murine model. Animals' bone composition and architecture were evaluated. Our results demonstrate that the deletion of Rac1 increases vertebral bone quality compared to wild-type bones in an ovariectomized model. INTRODUCTION: To determine the roles of the Rho family small GTPases Rac1 and Rac2 in regulating osteoclast-mediated bone quality in a model of postmenopausal osteoporosis. METHODS: Twelve-month-old female mice from three genotypes-wild type (WT), Rac1 null (LysM.Rac1 KO), and Rac2 null (Rac2KO)--were studied in control and ovariectomized groups (mice previously ovariectomized at 4 months of age). Animals were sacrificed at 12 months of age, and the femora and vertebrae were harvested for mechanical testing, bone densitometry, micro-computed tomography, and histomorphometric analyses to evaluate bone mineralization and architecture. The results were compared between groups using ANOVA and LSD post-hoc tests. RESULTS: We observed that LysM.Rac1 KO mice showed higher vertebral bone mineral density compared to WT in both control and ovariectomized groups. Consistent with this finding, LysM.Rac1 KO vertebrae showed increased resistance to fracture and increased trabecular connectivity compared to WT in both groups. Micro-CT analysis revealed that Rac2KO ovariectomized vertebrae have more trabecular bone compared to WT and LysM.Rac1 KO, but this did not translate into increased fracture resistance. CONCLUSION: Our results demonstrate that the deletion of Rac1 increases vertebral bone quality compared to WT bones in a postmenopausal osteoporosis model.

The long-term effects of water fluoridation on the human skeleton.

Municipal water fluoridation has notably reduced the incidence of dental caries and is widely considered a public health success. However, ingested fluoride is sequestered into bone, as well as teeth, and data on the long-term effect of exposure to these very low doses of fluoride remain inconclusive. Epidemiological studies suggest that effects of fluoride on bone are minimal. We hypothesized that the direct measurement of bone tissue from individuals residing in municipalities with and without fluoridated water would reveal a relationship between fluoride content and structural or mechanical properties of bone. However, consonant with the epidemiological data, only a weak relationship among fluoride exposure, accumulated fluoride, and the physical characteristics of bone was observed. Analysis of our data suggests that the variability in heterogenous urban populations may be too high for the effects, if any, of low-level fluoride administration on skeletal tissue to be discerned.

Artefacts in the mechanical characterization of porcine articular cartilage due to freezing.

Many experimental protocols for investigating articular cartilage mechanics have involved the use of a freeze-thaw cycle for storage or tissue manipulation. It was hypothesized that mechanical properties are altered due to freeze-thaw cycling. The aim of this study, therefore, was to examine the possibility of protocol-induced artefacts in the mechanical properties of porcine articular cartilage specimens related specifically to freeze-thaw events. Twenty-eight osteochondral specimens [14 from the femoral condyles (FCs) and 14 from the patella-femoral (PF) groove] were tested in confined compression before and after being frozen at -20 degrees C for 7 days. The fluid-independent and fluid-dependent mechanical properties (aggregate modulus of the solid phase and the half-life of stress relaxation respectively) were determined and compared. The aggregate modulus decreased by 13.5 per cent and 20.1 per cent for the PF and FC regions respectively (p = 0.002) and the half-life of the stress relaxation at 10 per cent strain decreased by 6.4 per cent and 12.6 per cent for the PF and FC specimens respectively (p = 0.0341). In conclusion, it has been shown that the protocol used, which involved freezing to -20 degrees C and thawing after 7 days, caused artefacts in the mechanical properties of porcine osteochondral specimens. It is suggested that protocols requiring freezing must be critically reviewed to eliminate such artefacts.