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Principles of Fabrication


Key Points

  • Fabrication can be classified into three subsets: (1) constructed from raw materials; (2) combined from raw materials and specially designed components; or (3) mass-produced, generic, completed devices that can be modified to fit the patient.

  • Although meticulous fabrication remains essential for creating an optimal orthosis, the methods and materials currently used create much stronger, thinner, lighter, and more biomechanically sophisticated orthoses than previously possible.

  • Computer-aided design and computer-aided manufacturing methods are available to even the smallest orthotic facility and can be used to expedite fitting as well as to facilitate off-site manufacturing at a specialized fabrication center.

  • Sound clinical judgment, combined with conscientious fitting and adjustment of well-designed orthoses, remains the hallmark of optimal orthotic treatment.

Orthoses are either custom made by the orthotist or are generic devices fabricated in a range of sizes. The techniques used to produce traditional metal and leather orthoses and thermoplastic orthoses have not changed. They are extensively described in earlier editions of this Atlas and the orthotic manuals of the 1970s. The process that has changed is where these devices are fabricated and whether they are custom made for the individual patient or are mass-produced generic devices, which are either modified to fit the patient or simply placed on the patient.

Custom-fit or “off-the-shelf” devices are usually used when there is an acute injury or when the size of the individual falls within the normal small, medium, or large size ranges. When greater technical judgment is required because of either the complexity of the diagnosis or the anatomical shape of the patient and a prefabricated device cannot address those issues, then the orthotist will need to fabricate a custom-made orthosis.

Metal and leather devices still require a two-dimensional tracing of the limb ( Fig. 3.1 ), and thermoplastic orthoses still require a three-dimensional model for the device to be formed around ( Fig. 3.2 ). Both require accurate measurements to ensure that the orthosis will fit and function properly.

Figure 3.1
Two-dimensional tracing process used to produce a metal ankle–foot orthosis.

Figure 3.2
Right lower limb being prepared for impression to produce the mold for a plastic ankle–foot orthosis. Heavy-duty shears can be used to open the semihardened mold.

Computer-aided design (CAD) has been used for several decades for orthoses, but because of advancements in both data collection and software development, its use has increased. Newer scanning techniques allow for more accuracy. Advances in computer-aided manufacturing (CAM) now make possible the carving of more complex anatomic shapes and the direct fabrication of some orthoses so that the carved model is bypassed altogether.

Data Collection: Measurements, Impressions, and Scans

The process of measuring a patient for an orthosis has evolved; device-specific instruments have been developed to improve the fit of the orthosis. This is more critical when the individual orthotist does not produce the device but instead uses a central fabrication facility or the manufacturer of a proprietary orthosis. To reduce data-collection errors, several knee orthosis manufacturers have developed measurement tools ( Figs. 3.3 and 3.4 ) that uniformly record the same data (abduction and joint angles, length and circumference measurements) to help reduce production errors. This enhances the fit and function of the orthoses and reduces the number of devices that have to be remade. Some manufacturers also require very accurate photographs to accompany the measurement form ( Fig. 3.5 ).

Figure 3.3, The Custom Contour Measuring Instrument (CCMI) is used to measure knees and elbows for custom DonJoy Defiance and A22 braces. For the knee, the device takes into account 14 separate anatomical reference points from the patient's leg, indicating thigh and calf circumference, thigh and calf contours, knee width, and varus and valgus dimensions.

Figure 3.4, The Custom Configuration System makes it possible to use a cell phone camera or digital camera and proprietary Image Alignment Guide and Tibia Contour Gauge to take photographs and measurements that can be transmitted by email to the manufacturer.

Figure 3.5, Accurate digital images must be taken at the correct distance and perpendicular to the knee. These are used to create an exact 1 : 1 ratio image.

The use of plaster of Paris or synthetic casting tape to make a three-dimensional mold of the patient is still the most commonly used technique. An indelible pencil is used to mark bony prominences and joint axes before the plaster is applied. The water-soluble pencil allows the marks to transfer to the interior of the plaster impression and later transfer a second time to the liquid plaster. Although the cast itself is inherently accurate, if the patient moves or the modifications are not done correctly, the fit or function of the orthosis will be affected. This is when the skill of the orthotist becomes apparent.

With the evolution of three-dimensional scanning equipment, its use has increased in the fabrication of orthotic devices. Some scanners are made specifically for the field, and some can be used or modified from other industries. Both white-light and blue-light optical scanners have been developed to take ultra-accurate measurements ( Fig. 3.6 ). These are primarily used for spinal orthoses and knee orthoses. As the orthotic software has advanced, commercial infrared scanners can be used with tablet devices for orthotic data collection ( Figs. 3.7 and 3.8 ). Cranial scanning is now less invasive, which is especially beneficial for very young children, with the use of a video camera capable of converting the video into a three-dimensional image ( Fig. 3.9 ).

Figure 3.6, Vorum three-dimensional scanner capturing millions of measurements in a few minutes for the fabrication of a thoracolumbosacral orthosis.

Figure 3.7, A commercially available Structure Sensor infrared camera attached to an iPad tablet with proprietary computer-aided design software can be used to create scans for multiple types of orthoses.

Figure 3.8, Tracing a limb with an infrared camera attached to iPad tablet. As the image is captured, the image on the tablet turns white, as shown on the screen on the lower right.

Figure 3.9, The patient's head is covered by the SmartSoc, and landmark stickers are applied in a specific way. The practitioner takes a quick video of the entire head to capture the information needed to re-create the three-dimensional image.

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