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Boning up: junior surgeons practice on model bones made of rigid polyurethane foam.

Practice makes perfect--a saying that naturally also applies to doctors. But how can an orthopedic surgeon practice a new technique, if not on a living specimen? The only alternative in the past was to use cadavers or the plentiful supply of sheep bones. The former brought up hygienic, logistical and ultimately ethical issues, while the latter was problematic because animal bone only resembles human bone to a limited degree. For some years now, the solution to this problem has been polyurethane--in the form of model bones manufactured by Synbone of Malans, Switzerland, using polyurethane raw materials from Bayer AG, Pittsburgh, PA. The choice of material is very important for use in the classroom setting: only the right balance of material properties in the hard outer layer and the foamed core of the synthetic bones can make student surgeons feel as if they are really operating.

"Solid materials, such as epoxy resins or urea systems, are just as unsuitable for realistic model bones as wood, for example," says Hans Pein, the Synbone engineer responsible for developing the model bones. "Human bone consists of a hard outer layer only a few millimeters thick known as the cortex, and a porous core referred to as cancellous bone. Realistic model bones have to imitate both perfectly." The reason is that cancellous bone offers a completely different type of resistance once a drill, screw or intramedullary pin has penetrated the hard cortex. This mechanical behavior is precisely what a surgeon must be familiar with when they want to precisely and firmly screw a metal plate onto a patient's broken upper arm bone, for example.

Apart from their somewhat lighter weight, the artificial bones imitate real ones almost perfectly in terms of surgical handling and appearance. Even the small pits and tendon insertions copied from the surface of real human bones are deceptively realistic. As a result, metal implants that have been prefabricated down to the last detail can be perfectly adapted to the model bones--something that is impossible with animal bones.

Precisely imitating the mechanical behavior of real bone involved a lot of development time. "Synbone basically uses the same raw materials as are used elsewhere to mold complex, impact-resistant housings for electrical devices, for example. After all, the Synbone teaching models are based on Bayer's Baydur 60 polyurethane," says Dieter Skoupi, polyurethanes expert at Bayer. The modeling of the cancellous bone admittedly called for a few tricks when it came to modifying the chosen polyurethane formulation, but Pein was able to rely on the technical support of Bayer AG's polyurethane applications specialists.

The bones are manufactured in two steps. The foamy cancellous bone is produced first in special multi-cavity molds using a low-pressure process. It is then provided with a hard outer layer by a high-pressure machine designed for small output volumes. The cycle time for each of these steps is about ten minutes. Polyurethane technology ensures more than just a realistic interior in the practice bones. "It enables economical production, because unlike with thermoplastic processing technology, the highly cost-effective molds used to manufacture polyurethane parts also allow small-batch production," explains Skoupi.

That's important, because although Synbone produces between 300 and 400 bones a day on four installations (or roughly 100,000 a year), they are divided among 200 different models--from simple thigh bones to complex spine or foot models comprising 28 or more different bones.

After demolding, the models need only be deflashed and fractured in a specific manner, as the surgeons have to use "standard fractures" in seminars to practice adapting implants to, for example, a multiple fracture of the shin bone. "The fact that the line of fracture forms just like in the original shows how realistic our polyurethane bones are," says Pein. "This is another example of what highly versatile polyurethanes can achieve today," remarks Skoupi. "The intelligent use of our polyurethane raw materials is not only helping an innovative company occupy a market niche, it could also indirectly benefit someone with a bone fracture who has to put his trust in the hands of a well-trained surgeon."

Circle 162--Synbone, or connect directly to their website via the Online Reader Service Program at

Circle 163--Bayer AG, or connect directly to their website at
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Geographic Code:4EUGE
Date:Nov 1, 2002
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