Why 3-D modelling is becoming the hip thing in hospitals

A new laser technique is helping orthopaedic surgeons to reconstruct patients' bones and assess their suitability for surgery, writes Simon Hadlington

Scientists and doctors are hoping that pioneering research into hi-tech model-making will give surgeons a better idea of what is inside a patient's body before they operate. Such information will help surgeons to decide whether or not an operation is feasible and, if it is, how best to go about it. In particular, the research could help to reduce the suffering endured by scores of elderly patients whose hip replacements have worn out.

By using a technique called selective laser sintering, it is becoming possible to take images from medical scanners and use them to create three- dimensional, hand-held models. The technique is best suited to creating facsimiles of patients' bones. Well-established medical imaging technologies such as computer tomography are used to generate data describing the three- dimensional characteristics of the patient's bone structure.

This information is then processed by a computer and fed into a machine in which a laser beam is directed on to a layer of powdered nylon one- tenth of a millimetre deep. Where the beam touches the powder, the nylon solidifies. After one "pass" of the laser beam, a second layer of powder is automatically placed over the first and the process is repeated, gradually building up the model slice by slice, in accordance with the information from the medical scan. Eventually, maybe 24 hours later, a solid facsimile of the patient's bone can be retrieved from the machine.

The University of Leeds bought such a machine, the first in the UK, 18 months ago. Since then, Dr Joanna Brown has been working with colleagues in the university's Departments of Mechanical Engineering and Applied Mathematical Studies and Centre of Medical Imaging Research to develop useful anatomical models.

At the General Infirmary in Leeds, Martin Stone, a consultant orthopaedic surgeon, is a specialist in "revision" hip surgery. Mr Stone says there are a group of patients whose artificial hip joints were removed some years ago because they had worked loose, sometimes damaging the "socket" part of the joint in the pelvis and rendering it unsuitable to accept a new artificial hip. In the absence of a solid joint, the body can grow a "fibrous" joint, which can function poorly and cause discomfort. These patients are being referred back to orthopaedic surgeons such as Mr Stone to see if it new techniques have made it possible to have a second artificial joint fitted.

"Because of advances in surgical techniques and the use of bone grafts, it is often possible to fit a second new hip to these patients," says Mr Stone. "But it is very difficult to get an accurate picture of the bone structure around the joint before surgery. We have tried everything: magnetic resonance imaging, plain X-rays, computed tomography reconstruction. But in all these cases you are still only getting a two-dimensional representation of what is a three-dimensional problem."

This means that patients can undergo the trauma of surgery only for the surgeon to find, when the bones are exposed, that the pelvis is not suitable to accept a new hip. Mr Stone says he is "tremendously hopeful" that the creation of 3-D models of the patient's bone structure will provide the solution: "Work that we have done so far suggests that this technique could be the way to avoid unnecessary surgery."

Furthermore, such technologies are likely to become increasingly important, Mr Stone believes. "The population is getting older, and there is going to be a huge wave of revision surgery which has not been accounted for by the Government. Demand is going to outstrip our ability to keep up with it."

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