Conjoined Twins' Separation a Model Surgery

Operation takes on-quarter the time, using rapid prototyping

—edited by Richard Mandel

In July of this year, surgeons and medical professions at the UCLA Mattel Children’s Hospital were faced with a unique and complex problem — the separation of craniopagus twins. Born July 24, 2001, Guatemalan twins Maria Teresa and Maria de Jesus Quiej-Alvarez were not only joined top to top, but rotated away from facing in the same direction, as well. Craniopagus twins — those who are fused at the tops of their heads — are one of the rarest types of conjoined twins, occurring an estimated 1 in 2.5 million births. Although the girls were cheerful and happy, such a condition precludes any chance for a normal life — indeed, their prospects of life beyond 10 years of age were rather poor since their brains would be pressing against each other’s as they continued to develop.

The good news was that the twins’ brains were separate and complete, with separated arteries and a dividing membrane. The veins draining the blood, however, were interwoven and fed into each others’ circulatory system. Separating conjoined twins is a highly complicated procedure, but surgeons determined that an operation was possible. The members in the surgical team all had different visualization needs in order to plan their role in the surgery. The plastic surgeons were most interested in the structure of the skull, while the neurosurgeons were particularly concerned with the arrangement of the blood vessels.

Compiling data

Because the blood vessels were crisscrossed, tracking them using standard, two-dimensional x-rays would be impossible. Dr. William J. Clearihue, a Biomedical Engineer and Anaplastologist at the West Los Angeles Veterans Administration, helped the surgeons to visualize the CT Scan Data through his own computer-based software program. He then suggested the use of 3D rapid prototyping to help the plastic surgeons practice how to separate the girls’ brain skulls, reroute the blood supply and plan skin grafting to cover the separated brains.

Boston-based Biomedical Modeling Inc. (BMI), an RP fabricator for medical uses, was asked to handle the conversion of imaging data into appropriate modeling files . The UCLA team had to supply BMI with three CT scans at different angles — it wasn’t possible to arrange the twins for one scan. The company registered and combined the scans of the twins’ brains and the intersection of the two skulls into a single, 3D model. BMI then used Materialise’s MIMICs software to merge the scans and process the data, including a biomodel of the skulls that included the maze of blood vessels.

Building an image

BMI asked InterPro Rapid Technologies Inc, a Connecticut-based RP service bureau, to build the three models. “We determined that our QuadraTempo system, manufactured by Objet Geometries, Rehovot, Israel, was the best choice for building these highly detailed models with the high accuracy needed,” says InterPRO co-owner Kevin Dyer. “The QuadraTempo’s ability to build the delicate features without support structures allowed us to clean the models much more easily after printing them. The fine details in the Tempo models proved especially critical to achieving success, since the surgeons needed to look inside the model and plan the rerouting of the blood vessels.” The QuadraTempo system builds parts by selectively jetting tiny droplets of acrylic photopolymer creating layers and then curing layer-by-layer, using UV light. A second, gel-like photopolymer material is used for support and is wiped off or removed by water jets.

BMI and InterPRO produced three biomodels — one of each of the twins' skulls, which could be studied separately or combined to provide the surgical team with a replica of the conjoined anatomy, and the third showing the region where the twins were joined, enabling the surgeons to easily see the complex architecture of arteries and veins. Dr. Clearihue adjusted each biomodel, orienting them to correct their position and connecting them so that the models could be manipulated in the operating room at the time of surgery.

By the first week of October, one of the twins had been moved out from intensive care, while the other’s condition was upgraded to “fair” though still being held in ICU. The surgeons reported that the renderings were invaluable in the planning process and rehearsal of this challenging surgical procedure. The biomodels enabled them to make important discoveries about the twins' anatomy which were not apparent from X-rays, or CT and MRI scans. “We’re so pleased to see that our technology was chosen for this humane undertaking, and this example is just one of many that we have envisioned for our company,” says Objet Geometries’ CEO Hanan Yosefi. “We believe that there are many benefits the Objet QuadraTempo can offer the medical community in applying prototyping for medical modeling and other essential applications.”

Besides defining internal structures, models from BMI have been applied in the creation of implants. The continuing progression, says BMI president Crispin Weinberg, in the quality of medical imaging “is providing us with better views of soft tissues. This means that, in future, not only will we be rendering models of bone, but more preparations for procedures involving blood vessels, nerves and internal organs.”

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.”