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September 2008 • Vol. 4, No. 4
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Advancing Orthotic and Prosthetic
Care Through Knowledge
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Jennifer Dowell, CPO
The first exploration of CAD/CAM in the O&P industry began in the 1970s with applications targeted at lower-limb prosthetics, with a transtibial focus. Until recently, development continued to be concentrated on lower-limb prosthetics. The technological requirements for capturing most residual limbs, which are typically cylindrical, are not quite as complicated as they are for most orthotic shapes; for example, an AFO, which is not a simple cylinder but, instead, a bent cylinder. However, recent advancements in technology, accompanied by a decrease in the cost of this type of technology, have opened the door for orthotic applications to be developed much more aggressively. In the last two years, more development has been focused on the expansion and advancement of orthotic applications, with the greatest emphasis on the challenge of shape capture/acquisition.
Shape Capture
In prosthetic CAD applications, it is common to take an electronic image of the limb without the need for any plaster at all. When a foot and ankle are brought into the equation, though, the process is not as straightforward. Different approaches are required according to factors such as anatomy, pathology, and the amount of detail required in the final device.
If the patient has little or no deformity or contracture, scanning the patient directly is an appropriate clinical application. This process actually involves two scans.
The foot is placed in a foam impression box to provide an impression of the plantar surface and to aid in the stabilization of the limb during scanning. Reflective markers on the patient identify landmarks to be transferred into the 3D image. Reference landmarks are also placed on the flat surface of the foam impression box and are included in both scans. For the first scan, the entire lower leg, including the ankle region and dorsum of the foot, is scanned. Once the first scan is complete, the limb is removed from the foam impression box and the plantar imprint is scanned. The software uses the reference landmarks to assemble a complete model from the two scans.
For patients with moderate to severe contractures or little muscle tone who require a significant amount of manual correction in order to return the leg to a more functional and anatomically normal position (preferably subtalar neutral if functionally possible), an intermediate step is required. A cast is taken, and then the cast—not the patient—is scanned. This is accomplished by scanning the outer surface in a single step, or by splitting the cast in half and scanning the inner surface.
If the patient displays a lack of definition in the foot and ankle, or if the device will not include any highly prominent anatomical features (as, for example, in a posterior-trimmed AFO), then it is clinically appropriate to scan the outer surface of a well-contoured, smooth cast that has uniform wall thickness. Landmarks are identified with reflective markers on the outer surface of the impression.
On the other hand, if the patient's anatomy is well defined, if significant muscle tone or contractures are present, or if the device will include highly prominent anatomical features (as, for example, in a tone-reducing or articulated AFO), a scan of the outside of a cast does not provide the required anatomical detail. In this case, the most clinically appropriate scanning option is to scan the inner surface of a cast that is split into medial and lateral halves. Landmarks are identified with reflective markers and each half of the cast is marked with three corresponding reference landmarks along the cut edge. After each half is scanned, the software uses the reference landmarks to assemble a complete model.
Modification
Once the model is captured and reassembled (if applicable), the model is modified to achieve the desired design by using a variety of "tools" in the software.
Alignment correction is achieved by using a selection of tools designed specifically for the correction of the foot and ankle. Plantarflexion/dorsiflexion, inversion/eversion, internal/external rotation, pronation/supination, and forefoot adduction can all be adjusted across a user-defined area of the model.
Surface modification is accomplished by using a series of tools designed for the specific modification needs of an AFO. The arch can be raised or lowered, a relief pad can be applied and modified, and freehand buildups and carvings can be accomplished. The toebox area can be flattened to accommodate a full foot plate and can also be lengthened to allow adequate room for fabrication and finish work.
Finally, a trim line is established to provide a guide during the finishing process. Multiple trim lines can be applied to create a floor-reaction-style AFO; a popliteal shelf can also be applied to create a PTB-style model.
If desired, the user may select a group of tools, arranged in a preferred order, and save it as a custom "modification wizard" for future use. Different wizards can be created for various modification needs. After selecting a modification wizard according to the needs of the patient and the type of device to be fabricated, the user is automatically prompted through each of the steps in that particular group.
Fabrication
The AFO model is then carved from either one block of foam or from two smaller blocks. The two-part process can be utilized by carvers that cannot reach beyond the rotational axis. An average adult-sized AFO will carve from a single block of foam in approximately 17–20 minutes (or 35–40 minutes for higher resolution), while an average two-part AFO will carve in approximately half that time.
Fitting
AFOs created from scanning the outside of a cast may require a radial or global volume reduction depending on the wall thickness of the cast. Additional reduction may be needed in the calf area, depending on the amount of musculature or soft tissue of the patient. If this is the case, a reduction can be made in an isolated area in addition to or instead of a global reduction.
AFOs created from a scan of the inner surface of a cast, as well as those created from a direct scan, typically require a 1mm radial increase to accommodate for the thickness of a sock to be worn under the device.
Documentation
The use of CAD/CAM provides the ability to document changes in a patient's volume and length over time, as well as the ability to view a model before and after modification. Several of OMEGA® Tracer®'s software tools, such as those that allow two images to be overlaid or compared to each other, provide documentation for changes in length, A/P, M/L, circumference, and volume. All modifications performed to the model are recorded in a non-editable format within the software. These capabilities also provide benefits for applications other than AFOs; for example, these tools allow documentation of the correction over time that is provided by a cranial remolding helmet.
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Model before modification shown in green. Model after modification shown in red. |
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Posterior dual model view of patient model pre- and post-correction. |
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Future of CAD/CAM
The future of orthotic applications for CAD/CAM continues to be the focus for both short and long-range research and development efforts. The immediate future offers electronic shape capture for spinal applications and the direct milling of insoles.
Summary
CAD/CAM technology, when applied and implemented correctly, affords consistency, repeatability, and standardization of patient care. The utilization of a non-contact method of shape capture establishes a repeatable and consistent method of capturing and tracking shape and volume changes, and generates an electronic record of the patient on demand whenever needed. The customizable modification wizards provide the flexibility and control of creating a specialized modification protocol that can be applied consistently. Advanced comparison capabilities allow us to compare and contrast models before and after modifications, as well as over time, to check for changes. The benefits for the creation of AFOs are significant, but are also applicable to a wide range of orthotic and prosthetic applications.
About the Author
Jennifer Dowell, CPO, earned a bachelor of science degree in prosthetics and orthotics from the University of Washington, Seattle, in 1990. She spent the first ten years of her practice in traditional clinical care before specializing in the development, testing, and instruction of CAD/CAM technology for the O&P profession. She is the OMEGA research clinician for Ohio Willow Wood, Mt. Sterling, Ohio.
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