Rapid Prototyping Custom fabrication of bone replacements

by David Orenstein free-lance writer

The obvious limit for computing in medicine—the body is physical, but information technology is abstract—is becoming less obvious. If a Navy-sponsored research project is successful, orthopaedic surgeons soon could print out implantable segments of bone the way one prints out a digital photo today.

Advanced Ceramics Research of Tucson is testing a system called Plasti-Bone, which uses 3-D image data captured by a CT scan or MRI to fabricate a new bone segment from strong materials into which the body can re-grow natural bone. The fabrication is accomplished using “rapid prototyping,” a process that uses what amounts to a 3-D printer. The machine literally carves a plastic or ceramic material in the shape specified by the 3-D image data sent by a
computer.

In other words, the following scenario may cease to be imaginary in the next few years: A patient presents at a hospital with a crushed left tibia. The orthopaedic surgeon orders a CT scan and, in consultation with the radiologist, determines the proper shape and length of bone needed to replace the crushed section of tibia. After generating a 3-D model, the physicians hit the “print” command and the rapid prototyping machine begins precisely extruding the materials to form the custom replacement bone segment. In about 8 hours, the new segment is ready to be inserted into the patient.

Rapid prototyping has advanced far enough in medicine and manufacturing that the computing involved is relatively standardized and straightforward. Software called Mimics from the Belgian company Materialise handles the conversion of CT scan image data into the STL file format used by rapid prototyping machines. ACR uses a specially retrofitted rapid prototyping machine made by Minnesota-based Stratasys Inc, according to Ranji Vaidyanathan, the engineer at ACR who developed Plasti-Bone.

Some physicians already use rapid prototyping to create models of body parts that help them visualize and simulate upcoming procedures they will perform on patients. Materialise’s Web site (www.materialise.com/Medical/ main_ENG.html) offers numerous case studies of European physicians using rapid prototyping.

Plasti-Bone takes the idea a few steps further by proposing to implant prototyped parts into patients. Mr Vaidyanathan says the company’s key advance is the materials. Plasti-Bones are made of a blend of polybutylene terephthalate, or PBT, a plastic similar to that in milk jugs. The bones are made to be porous, with holes of .1 mm to .2 mm, he says. After fabrication, the bones are dipped in a calcium phosphate solution that can be doped with bone growth factors. Fabrication and coating take about half a day; then the implantation can be done.

The result is a bone implant that surrounding natural bone slowly will grow into over the next 4 to 6 months, Mr Vaidyanathan says. After 1 or 2 years, the PBT will have dissolved, having given way to a new, healthy, natural bone. Although the area still is mostly PBT, the material is strong enough to act as bone for the patient. In tests, rats with Plasti-Bones have been up and running around a few weeks after implantation, he says.

The company’s testing has followed implantations only up to 6 months, however, so it has not confirmed that the PBT actually disappears or that the area heals exactly as predicted. While testing continues, Mr Vaidyanathan says, ACR likely will not bring any product to market for 3 to 4 years.

When it does, it will mark another beneficial blurring of the line between computing and care. It is becoming less and less true that computers merely are for billing and keeping calendars.


David Orenstein is a technology and business writer in Silicon Valley. To learn more about a technology topic in Computing Care, e-mail him at davealli@comcast.net.