Computer-Aided Design and Manufacturing in Prosthetics and Orthotics

Where Did We Come From? Where Are We Now? Where Are We Going?

David M. Gerecke, CPO, FAAOP

Since the days of carving wooden sockets with pulling tools and draw knives, prosthetists and orthotists have searched for faster, easier, and more accurate ways to design and manufacture prosthetic sockets and orthotic devices. With the introduction of computer-aided design and manufacturing (CAD/CAM) in the 1980s, clinicians had a tool with the promise of highly accurate shapes and repeatable procedures that would still allow the artistry of device fabrication to prevail. CAD/CAM continues to develop into a mainstream tool for the orthotic and prosthetic profession. Current systems allow shape capture, manipulation, storage, and modeling of most orthotic and prosthetic devices in today's practices.

Photograph courtesy of Paul Prusakowski, CPO, FAAOP.

Capturing the Shape

Shape-capture options continue to increase and improve. Multiple shape-capture options are available, including laser scanning, digitization of positive or negative models, and the creation of template-based shapes. Digital models can be rectified in a wide variety of ways, including overlaying photographs or radiographs and by creating shapes from other imaging sources such as computed tomography (CT) or magnetic resonance (MR) scans.¹ Laser scanners are the current state of the art. The two most common variations are positioned in three axes by either magnetic tracking or optical indexing.

Both magnetic-tracking and optical-indexing scanners produce finely detailed images. Both require practice to master their respective techniques. Potentially their greatest asset lies in the non-contact nature of the image capture. Since manipulation does not distort the image, volume and shape remain consistent from image to image, which allows predictable volume changes in the rectification process. However, non-contact image capture is also the main drawback; underlying tissue density is not apparent. Ultrasound may provide one way of registering varying tissue density. References to using ultrasound to aid image capture have been published since at least 1987,^2,3 and shapes derived from CT or MRI data continue to hold promise. However, these techniques are not yet feasible unless data is available from diagnostic procedures used in medical cases. Using MR or CT only for the sake of image capture has not proved worth the risk or the expense.⁴

Photograph courtesy of Ohio Willow Wood.

Shape Rectification

CAD owes its efficiency in large part to the powerful shape-modification tools available to the clinician. Digital models can be quickly and accurately rectified. Very precise corrections for alignment, symmetry, or practitioner error can easily be made, undone, or enhanced. Multiple versions of the model can be created for various trial applications. The systems offer numerous modification, alignment, and symmetry tools that give the practitioner the ability to perform alterations that would be difficult and time-consuming to do by hand. These tools can be grouped in a variety of ways such as sequences, wizards, and similar means to add consistency and improve efficiency. For instance, model-specific tools exist to adduct the forefoot of an AFO shape⁵ or copy TLSO modifications from left to right. Tools for modifying cranial-remolding orthoses make it possible to correct asymmetry to a percentage to prevent exceeding target circumference or diameter measurements. Liner locking mechanism tooling bosses, monolithic pylons, and multiple trimlines are easily added, adjusted, edited, and saved for future use.

Image overlay has proven to be useful in developing shapes. Digital photos are the most common type of image overlay. They are very useful in showing surface features and profiles as long as the photo is taken in an anatomical position (anterior, lateral, etc.) and is referenced so the digital model can be scaled to the image.

The image overlay technique is immediately useful in developing spinal shapes. Radiographs are especially useful as overlays. Two excellent examples are rectifying scoliosis shapes and identifying underlying heterotopic ossification in residual limbs. Given their increasing availability, digital radiologic images are often readily available to the prosthetist/orthotist. As in the use of photo overlays, shapes are easily rectified to the profiles of these images.


Photograph courtesy of Polhemus.	Photograph courtesy of David Gerecke, CPO, FAAOP.

Consistency is one of the greatest strengths of digital shape rectification. Graduated volume reductions are accomplished proportionately and accurately. Prosthetic models can be shortened or lengthened by the same proportion every time. Ply values can be adjusted to reflect the particular brand of stump sock the practitioner or patient favors so that ply adjustments to socket shapes are predictable.

Photograph courtesy of David Gerecke, CPO, FAAOP.

Computer-Aided Manufacturing (CAM)

Computer-aided manufacturing of O&P devices is where image design comes to fruition. Computer-controlled carvers turn three-dimensional digital models into three-dimensional foam models. Modern foam carvers are faster, cleaner, and more accurate than their forebears of the 1980s. The expense of owning a carver is comparatively less than 25 years ago as well. Multiple manufacturers and suppliers of foam blanks keep prices in check.

However, manufacturing over CAM modelstypically foampresents a unique set of advantages and challenges. Carvers, just like any precision instrument, need regular maintenance and calibration. Inaccurate fabrication can result from poorly maintained or calibrated carvers coupled with inconsistent fabrication techniques. Properly maintained and calibrated carvers and expert fabrication skills produce prosthetic sockets with volume variations of only 1.1 percent.⁶

Carvers were originally designed to carve prosthetic shapes, which are generally straight-axis models. Three-axis carvers are still common and can carve most bent-axis models such as AFOs and upper-limb models by tilting the model on the carving mandrel. Four-axis carvers improve the ability to carve complex shapes such as advanced upper-limb designs. Multiaxial carvers are common in the engineering world and will likely find their way to O&P, most likely in larger fabrication facilities.

Photograph courtesy of Jose Miguel Gomez, MD, LO; Gomez Orthotic Systems LLC.

Vacuum-forming thermoplastics over foam models presents particular challenges. Since foam is a thermal insulator, thermoplastics stay at forming temperature longerwhich can cause problems with adhesion to the parting barrier, usually nylon stockinette. In the same way that reliable, predictable vacuum-forming over plaster models did not develop overnight, thermoplastic-forming over foam models is a developing process. Care must be taken in plastic temperature, amount of vacuum, amount of time the vacuum is applied, and, of course, how the model is prepared.

Automated thermoformers simplify this task when fabricating plastic prosthetic sockets. Preformed plastic cones are heated for a specified period of time and then blister-formed over a positive model.

Future fabricators could bypass carvers and vacuum-fabricating fixtures entirely and utilize Solid Freeform Fabrication (SFF) technology. Once rare and very expensive, these machines are becoming more common as the engineering community continues to develop rapid prototyping techniques. SFF has yet to see common use in everyday clinical practice, but it is used in a variety of research capacities.⁷

Practice Models

Digital shape capture and modification open many practice-model possibilities. One important point is that CAD/CAM can allow the clinician to concentrate on the clinical aspects of our profession by outsourcing fabrication. This frees the clinician from lab supervision, training, inventory, Occupational Safety and Health Administration (OSHA) regulatory activities, and many other tasks that do not directly relate to clinical patient care. Outsourced fabrication allows the prosthetic and orthotic clinic to be located in medical office buildings or hospital campuses without the need for extensive and expensive hazardous material-handling systems. Clinicians sending digital shape files are not limited by the logistic challenges and expense of shipping heavy, bulky, and sometimes fragile physical molds or models. In fact, computer-aided manufacturing encourages the use of outsourced fabrication, especially helpful in certain situations, such as remote locations where full fabrication facilities are impractical. CAD/CAM grants easy access to specialized manufacturers, such as those who fabricate autoclaved carbon laminates or cranial-remolding orthoses regulated by the Food and Drug Administration (FDA). Conversely, very large companies have the ability to e-mail files from far and wide to central manufacturing facilities, where expertise and efficiency can be concentrated and maximized. At the other end of the spectrum, very small practices can effectively care for patients and their referral sources while minimizing overhead expenses.⁸


Photograph courtesy of Ohio Willow Wood.	Photograph courtesy of Rodin 4D.

Photograph courtesy of Rodin 4D.

Photograph courtesy of Provel.

At University of Texas Health Science Center at San Antonio, we have a variable call for fracture and burn masks. Digital modeling allows us to make the most effective use of our lab by outsourcing this often labor- and equipment-intensive fabrication to manufacturing facilities that specialize in these processes.

Since no shipping time or expense is involved, faster delivery times can be achieved. CAD/CAM will probably never replace plaster modeling entirely; however, CAD/CAM adds significantly to the arsenal of tools available to manage our widely varied patient population.

David M. Gerecke, CPO, FAAOP, is the owner of Active P&O, San Antonio, Texas, 210.639.2061.

References

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Malinauskas M, Krouskop TA, Barry PA. Noninvasive measurement of the stiffness of tissue in the above-knee amputation limb. Journal of Rehabilitation Research and Development. 1989;26(3):4552.
Faulkner VW, Walsh NE. Computer designed prosthetic socket from analysis of computed tomography data. Journal of Prosthetics and Orthotics. 1989;1(3):154164.
Dowell, J. CAD/CAM applications for the AFO. The Academy TODAY. 2008; 4(4):A1213.
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Rogers W, Bosker G, Faustini M, Walden G, Neptune RR, Crawford, R. Case report: Variably compliant transtibial prosthetic socket fabricated using solid freeform fabrication. Journal of Prosthetics and Orthotics. 2008;20(1):17.
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