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Rapid Prototyping in the Operating Room Ranges from the Planning of Bone Cuts to the Custom Fit of Implants.

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Mechanical Engineering 121(06), 52-53 (Jun 01, 1999) (2 pages) doi:10.1115/1.1999-JUN-2

Rapid prototyping in the operating room ranges from the planning of bone cuts to the custom fit of implants. Several rapid prototyping methods are used to produce anatomical models for a wide range of both soft and hard tissue surgeries. One method, used to create anatomical models for nearly any part of the body, is stereo-lithography, which rapidly produces a three-dimensional object by curing a liquid resin under a computer-guided laser. The technology itself has limitations barring its use in widespread applications. Today's rapid prototyping machines are bulky and complicated to run, and the modeling materials are sometimes hazardous in liquid form. Rapid prototyping offers surgeons an option not available by any other means. As a tactile tool, anatomical modeling provides substantially more information than 2D imaging modalities. The future may lead to surgeons' acceptance of less expensive models using 3D printing or multijet modeling over stereolithography.

When patients go under the knife for surgery, their obvious intent is to emerge in better condition than before. In the disciplines of oral, reconstructive, and craniofacial surgery, the patient is not only aided medically, but his appearance is restored or improved as well. Botched procedures in these cases can be accentuated by the patient's unusual appearance at best, or an unfortunate medical prognosis at worst.

Through medical modeling techniques, stories of elephant men have been rendered more fiction than fact.

Several rapid pro to typing methods are used to produce anatomical models for a wide range of both soft and hard tissue surgeries. One method, used to create anatomical models for nearly any part of the body, is stereo-lithography, which rapidly produces a three-dimensional object by curing a liquid resin under a computer-guided laser.

Medical modeling provides an imaging modality; that is, a visual means by which a surgeon can make a diagnosis and suggest a procedure. This use is similar to that of an X-ray, except the model is a three-dimensional, tangible representation based on spatially accurate data.

Data for the model is obtained through noninvasive imaging techniques, typically computed tomography for hard tissue or magnetic resonance imaging for soft. The data is formed into a model through the use of rapid prototyping equipment and specialized imaging software.

The stereolithography process takes serial section data and directs the ultraviolet energy of a low-power laser onto a vat of polymer epoxy or acrylic resin. After final curing in an ultraviolet oven, each model is finished to specifications, creating a general replica of the anatomy to be used for surgical planning.

Surgeons use anatomical models for three basic purposes, according to Andy Christensen, general manager at Medical Modeling Corp., an anatomical modeling service bureau in Golden, Colo. "Models are used for preoperative planning and surgical simulation, for communication with the patient and other surgeons, and for customization of off-the-shelf implants or the creation of custom made devices," he said. Additionally, because most models can be sterilized , they can be brought into the operating room to provide an intraoperative reference during surgery.

Since surgeons can rehearse incisions, measure grafts, and fit surgical resections before they operate, they save time during the actual procedure.

They can shorten exposure and anesthesia time for patients, and decrease blood loss.

This patient suffered from a congenital birth defect called Crouzon's Syndrome, in which the coronal and saggital plates in his skull fused, resulting in severe facial deformities and an open bite.

Grahic Jump LocationThis patient suffered from a congenital birth defect called Crouzon's Syndrome, in which the coronal and saggital plates in his skull fused, resulting in severe facial deformities and an open bite.

These benefits are further facilitated by the availability of semitransparent and two-color models. Semitransparent models can illustrate marrow spaces, craniofacial sinuses, blood vessels, and other body and bone cavities. Two color models can help a surgeon to visualize radiopaque density differences between nerves and bone, tumors and unaffected tissues, and other areas where a perceptible difference may be critical to the operation. This, said Christensen, makes treatment options available to patients who would otherwise be difficult or impossible to treat.

Several novel reconstructive surgery techniques have emerged from Britain. Surgeons at the Doncaster Royal Infirmary in Doncaster, England, have used modeling to construct custom-milled titanium implants for the treatment of trauma cases, facial deformities, craniofacial skull base surgery, and other reconstructive operations.

Ninian Peckitt, a consultant and maxillofacial surgeon at Doncaster, who has performed several of these operations himself and holds patents on many of the titanium implants he's used, said that the use of stereo-lithographic models has not only shortened surgery times and facilitated the design of custom implants, but can also lead to a more promising long-term prognosis for the patient.

"There needs to be a functional result in the reconstruction that minimizes additional reconstructive surgery and lowers the mortality rate during the recovery period," he said.

"It's important that the reconstruction be maintained or easily salvaged during this period."

Peckitt said the use of modeling in the design of sometimes complex "anatomical facsimiles" lets designers take into consideration several factors, ranging from the patient's age to the force of gravity. Peckitt said that some of his reconstructive surgery makes use of the titanium-coated hollow-screw reconstruction plate, or THORP, system developed jointly by two Swiss entities, the Department of Maxillofacial Surgery in Bern and the Institute Straumann in Waldenburg. He uses stereolithography to fit the THORP implant to the patient before anyone enters the operating room. These implants, he said, work well as bone surfaces grow, reform, or decay.

He said reforming bone has been shown to grow into the hollow sections of the screw, further strengthening the anchorage of a THORP system implant.

More work is being done in the medical modeling arena in the United States than overseas, but it is still not common practice. Christensen said that cost and speed generally keep rapid prototyping out of most operating rooms.

Surgeons used an anatomical model incorporating stereolithography to perform reconstructive measures before surgery, which corrected deformities in the patient's jaw and midface.

Grahic Jump LocationSurgeons used an anatomical model incorporating stereolithography to perform reconstructive measures before surgery, which corrected deformities in the patient's jaw and midface.

Justifying Cost and Speed

Each year, thousands of surgeries are performed that could benefit from the use of models, yet "models of the craniofacial skeleton, for instance, are sold for an average cost of$1,500 to $3,000," Christensen said. "Despite the benefits of its use, it can be a hard sell."

Between 70 and 80 percent of anatomical models in the States are produced using the stereolithography process.

The future may lead to surgeons' acceptance of less expensive models using 3-D printing or multijet modeling over stereolithography. "Not only is stereolithography equipment expensive," Christensen explained, "but the high cost of the exotic materials used with it combine to make acquisition of an anatomical model for only a few hundred dollars much more attractive, even if it doesn't produce a superior prototype."

Modeling's use today is essentially limited to more complex and time-consuming procedures. "These are currently the only types that can justify the $1,000-plus modeling bill," Christensen said. Lower costs, he predicted, will bring the technology into use in more "routine" surgical procedures.

The technology itself has limitations barring its use in widespread applications. Today's rapid prototyping machines are bulky and complicated to run, and the modeling materials are sometimes hazardous in liquid form, Christensen said. "These machines require too much special expertise for existing hospital personnel to run them," he explained. As far as wider use is concerned, he said, "To get there, the equipment must become more cost-effective, become easier to use, and occupy a smaller footprint."

Other advances in imaging software and CT scanner technology are also necessary to bring rapid prototyping into more general surgical use, according to Christensen. "As projects become more complex, current rapid prototyping tools will be pushed to the limit," he said. The software needs to reach a more "pushbutton" stage, to allow more users into the field. Additionally, he said there is also growing interest in the development of software packages that will allow for interaction by the surgeon in the office setting.

CT scanners, like X-rays, expose the patient to radiation, which Christensen would like to see minimized over time. "Faster scanners producing finer data will be key to getting models produced for more purposes, and with less radiation exposure to the patient," he said.

Insurance companies and HMOs have also been slow to add modeling to coverage plans. Code setups have been lagging because of the price tag attached to the process, said Christensen, but there is still room for optimism.

Christensen credits the companies and doctors who thus far have played a role in the development of the anatomical modeling market, and is confident that its role in surgical applications will grow. He said the benefit of medical modeling, be it stereolithography or another method yet to be developed, cannot be argued.

"Rapid prototyping offers surgeons an option not available by any other means," he said. "As a tactile tool, anatomical modeling provides substantially more information than 2-D imaging modalities." He said that further development by rapid prototyping equipment vendors, and by the doctors using it in practice, can be expected to drive up the quality and usage, and to lower the costs.

"The future almost guarantees that growth will be seen in this area with better, faster, and cheaper machines and materials," he said. "One day we may see every patient who could benefit from this service get it."

Copyright © 1999 by ASME
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