Figure 1. Replacement hip joint made from Titanium - Zimmer's Allofit Acetabular Shell with Metasul insert.

Horn describes the process of plaster moldmaking, using tracings, pictures, and measurements from the surviving limb. The residual limb (stump) is tested for its ability to bear pressure; this affects the design of the socket. In a central fabrication facility, a foam leg is created with a carbon fiber and aluminum core (the “pylon”)—this usually requires further refinement using highspeed routers and sanding machines. “Women are more cosmetically onscious,” Horn says. “We can make prosthetics that can change with the press of a button from a foot that can wear a sneaker to a foot that can wear a 2½-inch heel.”

Robert Lipschiutz, director of Prosthetics and Orthotics at the Rehabilitation Institute of Chicago, also learned modeling on the job, though he did study drafting in school as part of his education as a mechanical engineer. “In some cases, especially with upper extremities, there can be microprocessor involvement within a prosthesis, externally powered to make a hand or a knee work better.” He describes finishing techniques that have become increasingly sophisticated. “LivingSkin is a company that specializes in custom silicone restorations; for example, hands—they will take the finished limb and spray on a rubber coating which feels very realistic to the touch. It simulates the three dermal layers of natural human skin. Like human skin, LivingSkin’s epidermal layer also contains pores and skin detail. This is customized with a matching skin tone, freckles, even painted-on hair. In some situations, actual hair can be grafted into the rubber coating.”

Philip Castorie from LivingSkin hires practicing artists from many fields—metalsmiths, ceramicists, woodworkers, and painters—to create super-real effects. “Our artists are happy to use their talent to directly improve the lives of others,” he says.

The design and finishing of artificial joints requires many skill sets, not the least of which is artistry. While many enter the field from the realms of medicine and engineering, it is also increasingly the case that some whose careers and areas of study have been in art are joining the ranks.

Michael Smerka received his BFA at SUNY Purchase, majoring in sculpture with a concentration in furniture making. “At school I did a lot of plaster and bronze casting, welding, and joining.” After graduation he created custom furniture for the affluent residents of Westchester County, then worked at the Guggenheim Museum in New York City in the fabrication department.

Through a chance contact, he met a prosthetist who encouraged him to pursue related studies (chemistry, physics, and math, for starters), and Smerka is now doing his residency to complete his certification. “Our work is always governed by design,” says Smerka. “There are two aspects of prosthetic design: how the prosthesis is physically constructed, and the cosmetic aspect, which is more organic. The field has changed dramatically with the advent of plastics—we now have a large selection of possible choices in terms of construction and finishing.”

Smerka describes the process: “First, as clinicians, we work with patients hands-on to cast the limb, deciding which components to include and assessing how the patient will function. Once we are satisfied with how it will work—taking into account biomechanical principles involved in gait, for example—we serve as technicians in fabricating the cover, using materials like silicone and rubber, custom pigmented and painted. I’ve done quite a bit of cosmetic covers work—we call this the cosmesis, the outer appearance, to hide what doesn’t look natural.

“Many of the materials we use come from the art world: modified epoxy resins, ‘skins’ made of carbon fiber, nyglass [a combination of nylon and fiberglass].” Smerka contends that there is a direct correlation between art and the making of prosthetics as a hands-on activity. “What’s different is the psychology of dealing with limb loss,” he says.

Dan Shamp, from Shamp Bionics in Akron, Ohio, holds two patents in the field of prosthetics, and is also a certified prosthetist-orthotist. “Many of the technologies used for prostheses have been borrowed from other fields. What’s exciting is that now we are beginning to see technologies developed specifically for prostheses that are being borrowed by other industries.”

Ossur, a prostheses company, developed “Rheo Knee,” which uses magnetic fields to vary resistance. This advanced sensing and processing technology provides multiple safeguards against stance release, and more responsive motion. Another new technology will soon be seen in sports cars—it involves a fixed orifice and electromagnetic changes in fluid viscosity as it passes through the orifice, allowing greater control over the speed of the reaction.

As technology, materials, and bionics evolve, replacement options will continue to increase. Short of cloning ourselves to replace our entire bodies, defective parts can be replaced as needed using a variety of high-tech materials and techniques. And as engineers and designers work to mimic nature’s functions, the objects that they create will grace our imperfect bodies with permanence, indestructibility, and beauty.