2.4.1 RP-generated anatomical models
With the advent of computed tomography scanning techniques in the 1970s, surgeons were given the first glimpse at spatially correct datasets produced non-invasively. This revolutionized the diagnosis and treatment of many different pathologies, and its scope has continued to expand since that time. Very early on, some surgeons stacked the bone structure image data, slice upon slice, to create crude 3D images. This allowed for unprecedented visualization in 3D and made possible the evaluation of pediatric deformities and traumatic injuries in an entirely new way. This modality was extremely useful, but still lacked the tactile element a physical model would provide.
Since the early 1980s there have been several methods for creating 3D physical models from imaging modalities such as CT scanning. Companies have produced these models for several years by milling materials such as foam and polyurethane. Milling is a subtractive process in which the model is formed by the controlled removal of material from a block of that material. While this technique can ultimately provide a fairly precise representation of a 3D structure, it lacks the ability to provide accurate internal and external structures for the assessment of the pathology presented.
Stereolithography (SLA), the first so-called 'rapid prototyping' process, was developed in the late 1980s to mitigate the weaknesses of the milling process. SLA allows designers to 'print' in 3D the parts they design. The process employs ultraviolet lasers which initiate a photopolymerization reaction that locally cures liquid resin into a solid plastic (Figure 2.3). Most of its immediate use was in the automotive and aerospace sectors, but gradually medical applications of this technology emerged. By the time 3D CT had been in use for about 10 years, surgeons in certain specialties were relying more heavily on 3D visualization tools. The merging of 3D medical imaging and stereolithography was fairly easy because both technologies relied on slices, or layers, as input. Software tools were developed specifically for this task, and surgeons have come to rely on this technology as a standard element in the planning of some complex treatments.
Stereolithographic anatomical modeling has been used in clinical practice in some form since the early 1990s. In basic terms, the process uses computed tomography (CT) images and computer-assisted machinery to produce physical models of the bone structure of a particular
patient. Surgeons have found uses for these tactile models in many specialties, but cranio-maxillofacial surgeons remain the largest group of their users worldwide.
This technique provides models that are extremely accurate and allow for visualization of internal structures, such as nerve canals and sinus cavities. Figure 2.4 shows an example of a stereolithography model produced from CT scan data. Rapid prototyping (RP) is characterized
by processes that create a physical object from computerized data using an additive, layer-based technique. The technology has evolved to encompass a family of manufacturing techniques including selective laser sintering (SLS), fused deposition modeling (FDM), 3D printing (3DP), laminated object manufacturing (LOM), multiJet modeling (MJM) and many others. Specialized software interfaces are required to take the medical imaging data typically used for anatomical modeling (i.e. CT or MRI) and convert it into the files needed to guide the RP apparatus.
The models produced are typically of the hard-tissue anatomy because of the need to reconstruct osseous defects. The appropriate imaging modality for these models is CT or CAT scanning, which, interestingly, can also be used to create soft-tissue models, most specifically of the external anatomy. Figure 2.5 shows a produced model of an infant with a severe cran-iofacial disorder. Occasionally, vasculature structures need to be modeled. A CT scan with a contrast agent is then used to locate the tumor, for its segmentation by the computer software. Figure 2.6 shows a model of a patient with a myxoma involving the right temporomandibular joint and cranial fossa. Using a two-color modeling process that has been developed, it is possible to segment separate structures apart from the bone. This is made possible by a specially formulated liquid resin that has two dose response levels to UV radiation. The first causes it to solidify and, if exposed to an additional dose from a UV laser, it will change color. In this case, the tumor shows up as a red material. This unique approach can also be used effectively for identification of the inferior alveolar nerve structures, tooth structures and soft- and hard-tissue tumors as well as existing implants.
Physical anatomical models (biomodels) produced from medical imaging data have been used more and more frequently over the last few years. These models are typically of the bony anatomy and are used for planning complex reconstruction cases which may or may not involve customization of a surgical device or implement to be used during the treatment. These models provide something that no other imaging study can: a physical object from which to make measurements; a tactile replica about which to bend devices; a simulacrum upon which to rehearse procedures, using actual surgical instruments (Christensen et al., 2004.)
Early uses for physical modeling included the design and fabrication of custom-made titanium mesh for cranial defects and mandibular trays. Models have since been used for the design and fabrication of implants ranging from total temporomandibular joint (TMJ) replacements to partial knee replacement devices. Apart from the creation of truly custom implants, models have been used increasingly for procedures such as jaw surgery or spinal fusion in planning the procedure.
Distraction osteogenesis of the facial skeleton can require more complex planning than traditional orthognathic surgery because of the gradual lengthening process and the devices to be manipulated. The learning curve for distraction is reportedly steep, resulting in a higher rate of complication when inexperienced surgeons perform the procedures than when more experienced surgeons perform them. Stereolithographic models have been used when planning distraction procedures in order to moderate the learning curve. A CT-based physical model allows for true 3D visualization, osteotomy planning and device adaptation to the anatomy prior to the procedure. This has been extremely helpful in saving time during Le Fort III or monobloc advancement cases utilizing distraction. Selectively colored stereolithography models highlighting the inferior alveolar nerve, teeth and teeth buds have also been used for precise planning of distractor attachment to avoid damage to vital structures.
Companies that produce and supply RDM models are called service bureaux. A competent service bureau will employ a skilled radiological staff, trained to image the human body and empowered to make informed decisions about the production of these highly accurate models. Because the machinery is very expensive to purchase and maintain, most hospitals and medical centers are currently unable to bring this technology in-house. The first step in the process of acquiring a model involves the transfer of the data from the CT scan to the modeling facility. There is typically a lead time of 1-5 days for the production of a model from raw image data. Increasingly, these data can be transferred over the internet with file transfer protocol (FTP) directly from the radiology center. Reimbursement for models varies by country, with some health insurance providers covering the cost and some not covering the cost of models. Typical prices for anatomical models range from US$500 to US$3000.
Models are now available from medical modeling vendors in different materials. New processes such as 3D printing have allowed for less expensive models to become available for less complex cases. Models are also available that allow for intraoperative use. This expanded capability encourages surgeons to use models innovatively. One such use has reduced the morbidity of tissue grafts and their recipient sites by allowing for the manipulation of the grafted tissue upon a sterilized model.
The CT scan is the most important step of the entire modeling process. The ultimate accuracy of the model is utterly dependent upon the quality of the CT scan. The precise implementation of the modeling protocol is the second determinative factor, including the use of 1 mm x 1 mm continuous axial slices and a standard algorithm for the craniofacial skeleton. Other areas of anatomy, such as the pelvic bones, can be successfully imaged using 3 mm x 3 mm continuous axial slices which are subsequently reconstructed to less than 3 mm spacing. A competent service bureau providing modeling services should be able to provide specific protocols for many standard areas of anatomy prior to scanning.
Advanced manufacturing techniques can also help 'close the loop' between digital planning and treatment delivery. There are several examples of RP technology used for fabrication of devices to assist in the implementation of surgical interventions. Drill guides for dental implants (Figure 2.7), stents used in CMF surgery (Gateno et al., 2003b) and jigs to help align bone segments are a few examples. Such treatment aides incorporate patient-specific anatomical features, typical negatives of unique bone structure that are complementary to anatomy being targeted in the procedure. For example, drill guides used for dental implants fit snugly over bone of the jaw in only one possible position. Once in position the holes in the guide, which correspond to planned implant trajectories, constrain the physician's drill along the paths that were designed before the procedure in the computer. With the drill guides in place, the physician can quickly and confidently drill into bone without using traditional alignment techniques or
online X-ray imaging. This is an excellent example of how modern manufacturing can facilitate the implementation of a treatment plan that was designed virtually. Similar types of custom device have been suggested for spinal pedicle screw placement (Figure 2.8) and surgery for pelvic fractures. Along the same lines, templates for volumetric resections in neurosurgery have been proposed. These devices match the contours of surrounding bony anatomy and include a window cut out to indicate the borders of a lesion to be resected.
Custom stereotactic devices manufactured using RDM techniques have been suggested and are commercially available (D'Urso et al., 1999; Swain, 2004). These often rely on screw-in fiducial markers placed in the patient's head, surrounding the proposed surgical entry site, prior to imaging. These scans are then used as the basis for planning the trajectory of a biopsy needle or stylus which will introduce an electrical stimulator to a deep nucleus of the brain. Once this trajectory is planned with respect to the image data, and therefore the fiducial markers are also present in the scan, a physical guide can be manufactured (FHC Crop, Bowdoinham, ME, United States). This guide will mount to the fiducial markers, still present on the patient's head, and serve to aim instruments at the planned target point intraoperatively. The benefit of such an approach is that a bulky, invasive frame is avoided and the patient need not remain in the hospital between imaging and actual treatment.
The use of technologies generically grouped in the area of 'rapid digital manufacture' is growing in many sectors of industry, changing the way people buy products and services. The ability to custom-manufacture goods quickly and cost-effectively has aided several mainstream product lines and has impacted the production of consumer goods such as computers and running shoes. In the past, while the processes were too slow and expensive to allow their efficient, profitable use in consumer applications, they did make inroads into the clinical realm. One of the original uses of physical modeling from medical image data was the production of custom alloplastic implants and surgical implements. Notable early applications included custom total joint replacements like knee and hip prostheses as well as patient-specific TMJ replacements.
One of the most successful applications of RDM is the orthodontic product Invisalign (Align Technologies, Sunnyvale, CA, United States). In this application, 15-20 sets of tooth aligners serve the same purpose as traditional orthodontic braces. The process begins with a mold of the patient's teeth which is digitized to provide a virtual replica for planning the steps of straightening the teeth. From the computer plan a series of aligners are designed, which the patient wears in sequence in a process that lasts as long as 18 months. The aligners are not directly produced using stereolithography, but master models on which the designs are manufactured are created using stereolithography. The aligners are removable and translucent which provides many benefits in terms of comfort and cosmetic concerns.
Other kinds of computer-controlled manufacturing technique are used in medicine for customization on a large scale as well. Complex radiation treatment plans, specifically those employing a technique known as intensity-modulated radiation therapy (IMRT), are often implemented with custom-milled tissue compensating filters. With their design based on CT data and virtual treatment planning, these filters shape radiation beams in order to deliver treatment plans that seek to maximize dosage to tumor volumes while generally sparing the surrounding healthy tissue. The filters can be milled from metals that attenuate radiation beams (SPR, Sanford, FL, United States) and are thus able to vary the intensity of radiation across a beam, and to define the aperture of that beam (Zelinski, 2003). Computer treatment planning systems are increasingly automated, utilizing optimization algorithms to determine an arrangement of beams that will fulfill a physician's prescription. These systems provide clinicians with a virtual environment in which to design complex arrangements of beams. Often, equally complex devices are required to implement these virtual plans, and ADM techniques such as computer numerical control (CNC) machining is used with success. After a computer plan is approved by a physician, geometrical calculations convert the treatment plan to CNC instructions which can then be transferred electronically to a machining facility.
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