Gaining Medical Certainty with Solid Imaging

Solid imaging is a technology that is well known within the engineering community and is relied upon by product manufacturers worldwide. But other disciplines—including medicine, architecture, art, and archaeology— are also realizing the benefits of this technology as well.

In particular, Zürich University Hospital’s Maxillofacial Surgery Clinic has made a name for itself through its use of stereolithography. A recent project involved stereolithography models for craniofacial surgery, which provide a graphic representation of specific body structures.

The source data for the model is obtained from special imaging techniques, such as computerized tomography (CT scanning), magnetic resonance tomography (MRT) or ultrasound tomography, which produce an image of the human body. The source data consists of a stack of cross-sections of grey scale dots, with the grey scale value acting as a measure of tissue density. When placed one above another, these cross-sectional images produce a three-dimensional data set that reveals spatial structures.

Doctors and radiologists examine these cross-sections individually and mentally compile them into a three-dimensional physical object. But human imagination can quickly reach its limits. Thanks to the help of modern computer technology and 3-D reconstruction techniques, it is now possible to convert digital images into physical models.

A three-year study was carried out jointly between Zürich University Hospital and the Institute of Information Technology in Zürich. In order to evaluate the benefits of stereolithography models in craniofacial surgery, Professor H. F. Sailer of Zürich University Hospital selected 30 patients due to undergo craniofacial operations and prepared two stereolithographic models for each patient. The objective was to simplify the diagnosis and therapeutic planning process. The 30 patients presented a variety of conditions.

In some diseases, unusual anatomical conditions arise. Our example (see illustration) shows an operation for which Zürich University Hospital became famous. Dr. F. Carls, Senior Physician in Maxillofacial Surgery, commented: "The models are of great importance in explaining the procedure to the patient. With the help of the stereolithography model, patients and their families can be shown how the operation will be carried out. This sort of model not only gives the doctor the confidence he needs, but also results in a better informed patient."

For complicated procedures, pre-operative planning becomes critically important, as the
smallest error can have devastating effects. For example, in a hypertelorism operation, the eye sockets have to be repositioned. This is done by detaching the eye sockets completely, moving them toward the center of the face and adjusting their height. If the procedure is not handled properly, the patient could lose his or her sight.

"The stereolithography model not only gives the doctor the confidence he needs, but also results in a better informed patient."
                                                Dr. F. Carls, Zurich University Hospital

A hypertelorism operation normally takes up to 23 hours. Now, with the help of pre-operative planning using 3-D models, only eight to twelve hours are needed. Shorter operating time ultimately means reduced blood loss, less need for anesthesia and a lower risk of infection. Such benefits easily justify the cost of producing the models.

Once the cranium data is obtained from computerized tomography, a three-dimensional model can be prepared. In this study, the models were produced using 3D Systems’ SLA 250 machine. Professor Sailer and his team then planned the operations using these models as a guide. Although detailed information can also be obtained with a 3-D CT scanned image, the surgeon gains the certainty he needs for the forthcoming operation when he can hold a three-dimensional model in his hand, view it from different angles, and rehearse the procedure.

Since the models cannot be sterilized at present, they are typically used in the advance planning for the operation. But, as Dr. Warnke confirms, the benefits of 3-D models are significant: "We have been able to improve our pre-operative planning significantly, and in some cases, an operation has been possible strictly as a result of having a model."