Uncovering the secrets of bone...
Bone has remarkable properties that allow it to support the demanding loads placed on the body while balancing strength with flexibility. Composed mainly of hard but brittle calcium salts and strong yet flexible collagen fibres, bone has the same strength as cast iron while remaining as light as wood. Some have called it the ultimate biomaterial – light, strong, adaptable and able to repair itself.
These properties come from a complex hierarchical structure but until recently it has been difficult to characterise the multiple levels of bone. However, synchrotrons now allow researchers to view the nano- and micro-structure of bone at high spatial resolution while simultaneously measuring its mechanical properties. Diamond has provided facilities for many UK and international groups studying bone, with more than 30 papers being published over the last two years, which should lead to new ways to tackle bone disease.
Studies using the wide range of techniques available at Diamond were conducted by a team from Imperial College, London. The research demonstrated the power of synchrotron imaging which allows clinicians, scientists and engineers to study the fracture and elastic properties of bone at the micro- and nanoscale. The group’s findings give a useful broad overview of synchrotron imaging methods in bone research which include microscale X-ray tomography (micro-CT), and nanoscale X-ray imaging.
Although these procedures cannot be applied to bone in vivo due to the high radiation dose associated, they are already improving the understanding of the aging process and the pathophysiology of bone fracture. In time, these methods should allow clinicians to design novel devices to assess bone strength, diagnostic tests for bone diseases and improve the design and monitoring of medical therapies and orthopaedic implants to treat bone conditions such as osteoporosis.
Bone structure and healing
A number of recent studies focus on understanding fracture risk and the mechanisms of bone healing.
Bisphosphonates are well studied and powerful drugs that help to maintain bone tissue particularly in people with osteoporosis and in some patients undergoing cancer treatment. However, a recent studyin Nature has shown the value of the micro-computed tomography (micro-CT) facilities at Diamond, which allow high-resolution imaging deep within bone samples. The images revealed that bones of people treated with bisphosphonates have larger numbers of micro-cracks than seen in normal bone. These tiny cracks between 30–100 μm (about the width of a human hair) gradually weaken the bone structure and make it more prone to breaking. This startling new discovery suggests that bisphosphonates could make bones more fragile in some patients and further studies are planned to assess these early findings.
Another X-ray technique available at Diamond has been able to shed new light on mineral composition in bone tissues. A study from a German/Swiss group used ptychographic X-ray nanotomography (PXCT) to improve understanding of tooth dentine structure. The PXCT technique improves on existing methods and allows variations in mineral content distribution to be mapped in three dimensions. It does this by combining highly sensitive phase contrast X-ray imaging comparable to quantitative backscattered electron microscopy imaging with 3D X-ray absorption micro-CT, thus providing new insights into bone tissue healing and aging.
Further studies publishes in Plos One and the journal Bone using the nanoscale X-ray facilities at Diamond are providing new insights on the impact of arthritis on the strength of bones during later life, and highlighting the importance of micro- and nanoscale bone strengthon fracture mechanisms.
Studying different bone diseases
Several studies using mouse models have shown the value of using synchrotron techniques in investigating specific conditions affecting bone. The studies provide valuable information on these diseases and potential treatments that have not been revealed with traditional approaches.
Secondary osteoporosis, leading to increased fracture risk, can sometimes develop following long-term glucocorticoid use. Bone strength has usually been assessed by bone mineral density tests using techniques such as central dual-energy X-ray absorptiometry (DEXA) which can be performed in hospital. The DEXA scan compares bone density with that of a young healthy adult or a healthy adult of the subject’s own age, gender and ethnicity and provides a comparative score. However, new UK studies using synchrotron micro-CT conducted at Diamond have shown that micro- and nanoscale changes of bone quality rather than just quantity are vital in increasing fracture risk in these patients. These studies clearly show how bone quality changes cause an increase fracture risk in secondary osteoporosis, independent of bone quantity. It is hoped that these procedures developed at Diamond may be transferable for the assessment of other clinically important metabolic bone diseases.
Synchrotron micro-CT has also proved valuable in detailed understanding of Crouzon syndrome which includes premature fusing of skull bones affecting the shape of head and face. Quantitative analysis of the microscopic structure of the bone which has previously been unobserved will help to provide information for planning future surgery in patients with Crouzon syndrome.
Medical intervention in bone is a fast growing area of medicine where synchrotron technology is proving invaluable. Artificial joint replacements and new methods to grow bone tissue following illness or injury demand a comprehensive understanding of bone development at the smallest scale.
A novel approach to the development of tissue engineered bone involves the use of aggregate or micromass cultures. Various techniques such as histochemical staining, protein assay kits and reverse transcription-polymerase chain reaction (RT-PCR) have been used previously, but they fail to provide simultaneous information on cell proliferation and mineralisation. However, a collaborative project between Keele University and Diamond found that synchrotron-based Fourier transform infrared microspectroscopy(micro-FTIR) can accurately track the mineralisation process involved in developing tissue-engineered bone at the molecular scale. These new findings will support the ongoing development of these procedures in assisting bone growth following surgery, disease or bone damage.
Tribo-corrosion, caused by mechanical wear and electrochemical corrosion, occurs at surfaces of bone and metal hip replacements. This results in raised cobalt and chromium concentrations in the synovial fluid and peripheral circulation which negatively affects bone mass and turnover. A team from the University of Sheffield used X-ray fluorescence and X-ray absorption spectroscopy at Diamond to better understand the transport, processing and effect of these metals on human osteoblasts and osteoclasts in order to mitigate their effects on patients undergoing hip replacement.
Synchrotrons are also proving useful in forensic medicine and archaeology. X-ray analysis of structural changes in burned skeletal remains can provide accurate estimates of the burning temperatures, which could provide vital information at crime scenes or archaeological sites. Using a combination of small- and wide-angle X-ray scattering (SAXS and WAXS) experiments at Diamond, the researchers from the University of Birmingham School of Dentistry were able to provide detailed information on cremation temperatures that had not been gained from the traditional X-ray scattering techniques that had been used previously in hard dental tissue.