High-precision method for cyclic loading of small-animal vertebrae to assess bone quality

Radiation-induced changes in load-sharing and structure-function behavior in murine lumbar vertebrae

In this study, we used micro-CT-based finite element analysis to investigate the biomechanical effects of radiation on the microstructure and mechanical function of murine lumbar vertebrae. Specifically, we evaluated vertebral microstructure, whole-bone stiffness, and cortical-trabecular load sharing in the L5 vertebral body of mice exposed to ionizing radiation 11 days post exposure (5 Gy total dose; n = 13) and controls (n = 14). Our findings revealed the irradiated group exhibited reduced trabecular bone volume and microstructure (p < 0.001) compared to controls, while cortical bone volume remained unchanged (p = 0.91). Axially compressive loads in the irradiated group were diverted from the trabecular centrum and into the vertebral cortex, as evidenced by a higher cortical load-fraction (p = 0.02) and a higher proportion of cortical tissue at risk of initial failure (p < 0.01). Whole-bone stiffness was lower in the irradiated group compared to the controls, though the difference was small and non-significant (2045 ± 142 vs. 2185 ± 225 vs. N/mm, irradiated vs. control, p = 0.07). Additionally, the structure-function relationship between trabecular bone volume and trabecular load fraction differed between groups (p = 0.03), indicating a less biomechanically efficient trabecular network in the irradiated group. We conclude that radiation can decrease trabecular bone volume and result in a less biomechanically efficient trabecular structure, leading to increased reliance on the vertebral cortex to resist axially compressive loads. These findings offer biomechanical insight into the effects of radiation on structure-function behavior in murine lumbar vertebrae independent of possible tissue-level material effects.

Fatigue behaviour of load-bearing polymeric bone scaffolds: A review

Bone scaffolds play a crucial role in bone tissue engineering by providing mechanical support for the growth of new tissue while enduring static and fatigue loads. Although polymers possess favourable characteristics such as adjustable degradation rate, tissue-compatible stiffness, ease of fabrication, and low toxicity, their relatively low mechanical strength has limited their use in load-bearing applications. While numerous studies have focused on assessing the static strength of polymeric scaffolds, little research has been conducted on their fatigue properties. The current review presents a comprehensive study on the fatigue behaviour of polymeric bone scaffolds. The fatigue failure in polymeric scaffolds is discussed and the impact of material properties, topological features, loading conditions, and environmental factors are also examined. The present review also provides insight into the fatigue damage evolution within polymeric scaffolds, drawing comparisons to the behaviour observed in natural bone. Additionally, the effect of polymer microstructure, incorporating reinforcing materials, the introduction of topological features, and hydrodynamic/corrosive impact of body fluids in the fatigue life of scaffolds are discussed. Understanding these parameters is crucial for enhancing the fatigue resistance of polymeric scaffolds and holds promise for expanding their application in clinical settings as structural biomaterials. STATEMENT OF SIGNIFICANCE: Polymers have promising advantages for bone tissue engineering, including adjustable degradation rates, compatibility with native bone stiffness, ease of fabrication, and low toxicity. However, their limited mechanical strength has hindered their use in load-bearing scaffolds for clinical applications. While prior studies have addressed static behaviour of polymeric scaffolds, a comprehensive review of their fatigue performance is lacking. This review explores this gap, addressing fatigue characteristics, failure mechanisms, and the influence of parameters like material properties, topological features, loading conditions, and environmental factors. It also examines microstructure, reinforcement materials, pore architectures, body fluids, and tissue ingrowth effects on fatigue behaviour. A significant emphasis is placed on understanding fatigue damage progression in polymeric scaffolds, comparing it to natural bone behaviour.

Relative Effects of Radiation-Induced Changes in Bone Mass, Structure, and Tissue Material on Vertebral Strength in a Rat Model

The observed increased risk of fracture after cancer radiation therapy is presumably due to a radiation-induced reduction in whole-bone strength. However, the mechanisms for impaired strength remain unclear, as the increased fracture risk is not fully explained by changes in bone mass. To provide insight, a small animal model was used to determine how much of this whole-bone weakening effect for the spine is attributable to changes in bone mass, structure, and material properties of the bone tissue and their relative effects. Further, because women have a greater risk of fracture after radiation therapy than men, we investigated if sex had a significant influence on bone's response to irradiation. Fractionated in vivo irradiation (10 × 3 Gy) or sham irradiation (0 Gy) was administered daily to the lumbar spine in twenty-seven 17-week-old Sprague-Dawley rats (n = 6-7/sex/group). Twelve weeks after final treatment, animals were euthanized, and lumbar vertebrae (L4 and L5 ) were isolated. Using a combination of biomechanical testing, micro-CT-based finite element analysis, and statistical regression analysis, we separated out the effect of mass, structural, and tissue material changes on vertebral strength. Compared with the sham group (mean ± SD strength = 420 ± 88 N), the mean strength of the irradiated group was lower by 28% (117 N/420 N, p < 0.0001). Overall, the response of treatment did not differ with sex. By combining results from both general linear regression and finite element analyses, we calculated that mean changes in bone mass, structure, and material properties of the bone tissue accounted for 56% (66 N/117 N), 20% (23 N/117 N), and 24% (28 N/117 N), respectively, of the overall change in strength. As such, these results provide insight into why an elevated clinical fracture risk for patients undergoing radiation therapy is not well explained by changes in bone mass alone. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Short Cyclic Regimen With Parathyroid Hormone (PTH) Results in Prolonged Anabolic Effect Relative to Continuous Treatment Followed by Discontinuation in Ovariectomized Rats

Despite the potent effect of intermittent parathyroid hormone (PTH) treatment on promoting new bone formation, bone mineral density (BMD) rapidly decreases upon discontinuation of PTH administration. To uncover the mechanisms behind this adverse phenomenon, we investigated the immediate responses in bone microstructure and bone cell activities to PTH treatment withdrawal and the associated long-term consequences. Unexpectedly, intact female and estrogen-deficient female rats had distinct responses to the discontinuation of PTH treatment. Significant tibial bone loss and bone microarchitecture deterioration occurred in estrogen-deficient rats, with the treatment benefits of PTH completely lost 9 weeks after discontinuation. In contrast, no adverse effect was observed in intact rats, with sustained treatment benefit 9 weeks after discontinuation. Intriguingly, there is an extended anabolic period during the first week of treatment withdrawal in estrogen-deficient rats, during which no significant change occurred in the number of osteoclasts, whereas the number of osteoblasts remained elevated compared with vehicle-treated rats. However, increases in number of osteoclasts and decreases in number of osteoblasts occurred 2 weeks after discontinuation of PTH treatment, leading to significant reduction in bone mass and bone microarchitecture. To leverage the extended anabolic period upon early withdrawal from PTH, a cyclic administration regimen with repeated cycles of on and off PTH treatment was explored. We demonstrated that the cyclic treatment regimen efficiently alleviated the PTH withdrawal-induced bone loss, improved bone mass, bone microarchitecture, and whole-bone mechanical properties, and extended the treatment duration. © 2021 American Society for Bone and Mineral Research (ASBMR).

Relations Between Bone Quantity, Microarchitecture, and Collagen Cross-links on Mechanics Following In Vivo Irradiation in Mice

Humans are exposed to ionizing radiation via spaceflight or cancer radiotherapy, and exposure from radiotherapy is known to increase risk of skeletal fractures. Although irradiation can reduce trabecular bone mass, alter trabecular microarchitecture, and increase collagen cross‐linking, the relative contributions of these effects to any loss of mechanical integrity remain unclear. To provide insight, while addressing both the monotonic strength and cyclic‐loading fatigue life, we conducted total‐body, acute, gamma‐irradiation experiments on skeletally mature (17‐week‐old) C57BL/6J male mice (n = 84). Mice were administered doses of either 0 Gy (sham), 1 Gy (motivated by cumulative exposures from a Mars mission), or 5 Gy (motivated by clinical therapy regimens) with retrieval of the lumbar vertebrae at either a short‐term (11‐day) or long‐term (12‐week) time point after exposure. Micro‐computed tomography was used to assess trabecular and cortical quantity and architecture, biochemical composition assays were used to assess collagen quality, and mechanical testing was performed to evaluate vertebral compressive strength and fatigue life. At 11 days post‐exposure, 5 Gy irradiation significantly reduced trabecular mass (p < 0.001), altered microarchitecture (eg, connectivity density p < 0.001), and increased collagen cross‐links (p < 0.001). Despite these changes, vertebral strength (p = 0.745) and fatigue life (p = 0.332) remained unaltered. At 12 weeks after 5 Gy exposure, the trends in trabecular bone persisted; in addition, regardless of irradiation, cortical thickness (p < 0.01) and fatigue life (p < 0.01) decreased. These results demonstrate that the highly significant effects of 5 Gy total‐body irradiation on the trabecular bone morphology and collagen cross‐links did not translate into detectable effects on vertebral mechanics. The only mechanical deficits observed were associated with aging. Together, these vertebral results suggest that for spaceflight, irradiation alone will likely not alter failure properties, and for radiotherapy, more investigations that include post‐exposure time as a positive control and testing of both failure modalities are needed to determine the cause of increased fracture risk. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research. This article has been contributed to by US Government employees and their work is in the public domain in the USA.

Dietary countermeasure mitigates simulated spaceflight-induced osteopenia in mice

Spaceflight is a unique environment that includes at least two factors which can negatively impact skeletal health: microgravity and ionizing radiation. We have previously shown that a diet supplemented with dried plum powder (DP) prevented radiation-induced bone loss in mice. In this study, we investigated the capacity of the DP diet to prevent bone loss in mice following exposure to simulated spaceflight, combining microgravity (by hindlimb unloading) and radiation exposure. The DP diet was effective at preventing most decrements in bone micro-architectural and mechanical properties due to hindlimb unloading alone and simulated spaceflight. Furthermore, we show that the DP diet can protect osteoprogenitors from impairments resulting from simulated microgravity. Based on our findings, a dietary supplementation with DP could be an effective countermeasure against the skeletal deficits observed in astronauts during spaceflight.

Effects of ex vivo ionizing radiation on collagen structure and whole-bone mechanical properties of mouse vertebrae

Bone can become brittle when exposed to ionizing radiation across a wide range of clinically relevant doses that span from radiotherapy (accumulative 50 Gy) to sterilization (~35,000 Gy). While irradiation-induced embrittlement has been attributed to changes in the collagen molecular structure, the relative role of collagen fragmentation versus non-enzymatic collagen crosslinking remains unclear. To better understand the effects of radiation on the bone material without cellular activity, we conducted an ex vivo x-ray radiation experiment on excised mouse lumbar vertebrae. Spinal tissue from twenty-week old, female, C57BL/6J mice were randomly assigned to a single x-ray radiation dose of either 0 (control), 50, 1000, 17,000, or 35,000 Gy. Measurements were made for collagen fragmentation, non-enzymatic collagen crosslinking, and both monotonic and cyclic-loading compressive mechanical properties. We found that the group differences for mechanical properties were more consistent with those for collagen fragmentation than for non-enzymatic collagen crosslinking. Monotonic strength at 17,000 and 35,000 Gy was lower than that of the control by 50% and 73% respectively, (p < 0.001) but at 50 and 1000 Gy was not different than the control. Consistent with those trends, collagen fragmentation only occurred at 17,000 and 35,000 Gy. By contrast, non-enzymatic collagen crosslinking was greater than control for all radiation doses (p < 0.001). All results were consistent both for monotonic and cyclic loading conditions. We conclude that the reductions in bone compressive monotonic strength and fatigue life due to ex vivo ionizing radiation are more likely caused by fragmentation of the collagen backbone than any increases in non-enzymatic collagen crosslinks.