Hip, Hip, Hooray!

By Emma Pewsey
University of Cambridge, UK

This summary was awarded 1st place for Access to Understanding 2013

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X-rays can now be used not only to show where bones have fractured, but also to investigate why these bones break in the first place. Results suggest the possibility of preventing the trauma of thousands of broken hips using drugs already commonly used for treating osteoporosis.

Normal healthy bones can be thought of as nature’s scaffold poles. The tightly packed minerals which make up the cortical bone form a sheath around an inner core of spongy bone and provide the strength which supports our bodies. Throughout our lives, our skeletons are kept strong by the continuous creation of new, fresh bone and the destruction of old, worn out bone. Unfortunately, as we age destruction becomes faster than creation, and so the cortical layer thins, causing the bone to weaken and break more easily. In severe cases, this is known as osteoporosis. As a result, simple trips or falls which only bruise a younger patient can cause serious fractures in the elderly. However, half of elderly patients admitted to hospital with a broken hip do not suffer from osteoporosis.

So why do those hips break? This is a question of great importance, as hip fractures are debilitating. Repairing one requires traumatic surgery that, even if successful, may not enable a patient to regain the full mobility they had beforehand. The National Osteoporosis Society estimates that 13,800 people in the UK die every year as a direct result of hip fractures. This is over 10% of patients injured. (http://www.nhs.uk/conditions/hip-fracture/Pages/introduction.aspx) Therefore, understanding how these fractures occur and acting to prevent them is vital for improving the quality and length of life of our aging population.

To better understand why hips fracture, researchers in Cambridge and Prague analysed CT scans performed on the opposite, unbroken hips of a group of elderly women admitted to Bulovka University Hospital, Prague with hip fractures. Previous studies have shown that these tend to be in a similar condition to the broken hip pre-fracture.

CT scanners are now standard pieces of equipment in most hospitals, and are used to examine organs and tissues inside the body. Essentially a rotating X-ray machine, a CT scanner takes many X-ray snapshots at different angles around a body part to produce a 3D image of its internal structure. X-rays are energetic waves of energy which are partially absorbed by the materials they pass through. The amount of absorption depends on the density of the structure encountered – denser structures, like bone, absorb more of the X-ray energy, leaving less energy to be measured by the detector on the other side. However, the resolution of the images collected by a standard hospital CT scanner is not sensitive enough to accurately determine the thickness of the cortical bone.

A new image processing technique has changed that. Using it, the researchers in Prague and Cambridge were able to extract information from clinical CT scans that was sensitive enough to produce coloured maps on the surface of a model hip showing the variation of cortical bone thickness in more detail than ever. Variations in thickness of only 30 microns – the size of a grain of dust – could be detected.

The results were striking. Not only did the women with fractured hips have generally thinner cortical bone than normal, but some patients also had local patches of even thinner bone. This was the case even in women who did not suffer from osteoporosis. Most importantly, the extra-thin regions were found on the femoral neck – the part of the hip bone where fractures most commonly occur. In some patients, these patches were 30% thinner than the surrounding bone, and as big as a thumbnail. These weaker points provide the ideal conditions for a crack to form and subsequently grow into a fracture. Further studies are needed to confirm whether these localised regions do act as the starting point for a fracture, but at the very least they affect the type, and hence severity, of fracture which occurs. They could also explain the mystery of spontaneous hip fracture, which accounts for 6% of hip fractures – 4000 broken hips a year in the UK, which break for no known reason.

The research team have named these local patches of thinner bone ‘focal osteoporosis’. However, despite the name it is not yet known if these areas can be strengthened using standard osteoporosis drugs, which slow down the natural destruction of bone cells. An extensive clinical trial will be needed to investigate further, but if the focal patches do respond to treatment it raises the tantalising possibility of a future where many fractures could be treated before they even form. The improvement this would have on our quality of life in our old age would be invaluable.

This article describes the research published in:

Cortical thickness mapping to identify focal osteoporosis in patients with hip fracture (2012) K. E. S. Poole, G. M. Treece, P. M. Mayhew, J. Vaculík, P.Dungl, M. Horák, J. J. Štěpán, A. H. Gee PLoS One 7(6), e38466
http://EuropePMC.org/articles/PMC3372523

This article was selected for inclusion in the competition by Arthritis Research UK.