Thanks to advances in microelectronics, hydraulics, and motors, medical technicians can help nearly amputee achieve goals and accomplish feats that weren’t possible at the turn of the century.
Shane Wurdeman, PhD, CP
Hanger Clinic Certified Prosthetist and Research Scientist, Hanger Clinic
“We can rebuild him. We have the technology. We can make him better than he was. Better, stronger, faster.” These words launched the popular 1970s television series The Six Million Dollar Man, giving birth to a generation of innovators and inventors.
In the 70s the words were nothing more than science fiction. Today, advances in prosthetic rehabilitation technologies are transforming this quote into non-fiction. Prosthetists – the healthcare professionals who design and build prostheses – can arguably make some individuals better than they were before their amputation. And thanks to advances in technology, prosthetists can help nearly every amputee achieve goals and accomplish feats that were not possible at the turn of the century. Here are a few examples.
The age of microprocessors is upon us. It’s been a decade since the Proprio foot was introduced, using a microprocessor and sensors to detect when to provide powered dorsiflexion of the foot. That motion is an upward rotation of the ankle that raises the toe – the action that effectively increases the ground clearance of the foot as it swings through the air. However, after Proprio, the use of microprocessors in foot and ankle systems seemed to stall. Meanwhile, an increasing number of studies showed the benefits of using hydraulics to increase the ability of the foot and ankle to adapt to uneven surfaces in real-world environments. Subsequently, microprocessors were added to provide reactive adjustments to the hydraulic resistance to motion of the foot and ankle. This increased the foot-ankle unit’s responsiveness to changes in speed and on uneven terrain beyond that provided only through resistance to flow. Microprocessors are also being used to send output signals to motors. For example, a foot prosthetic from BiomX Medical will detect user’s instance within the walking cycle. Then, at the appropriate time, when the user is in the phase of the walking cycle referred to as propulsion, a motor is cycled for a power burst replicating the burst produced by the anatomical ankle-foot.
Prosthetic feet are also experiencing advances in areas such as materials and geometry. Rush feet, a new line of prosthetic feet, use a fiberglass construction originally designed for Apache helicopters. The fiberglass material is an alternative to the near-standard-of-care carbon fiber material. The fiberglass material seems to decrease hysteresis as the foot cycles through a sequence of steps.
With regard to geometry, the new Pro-Flex effectively engages a series of levers as the foot progresses forward from the moment it contacts the ground to later in stance while shifting the ankle’s rotation point. This action increases the power burst from the prosthetic foot far beyond what’s been measured in previous prosthetic feet using only passive elastic-energy return.
Although it’s been more than 15 years since a microprocessor was used to assist with control of a prosthetic knee joint, recent advances in sensor technologies and algorithm improvements have greatly enhanced this technology. The Genium, for example, uses a combination of accelerometers, gyroscopes, and strain gauges to calculate knee angle, the lower-limb position relative to the environment, and the moments at the knee and ankle. The data from these sensors is read at 100 Hz and analyzed in real time by algorithms that translate the readings into output signals.
These control servo-motors that open and close valve ports on hydraulic cylinders to reduce or increase the knee’s resistance to flexion and extension. Most impressively, the control algorithms now learn and optimize the activity based on the user’s walking patterns – artificial intelligence at work. By being able to appropriately sense the knee’s position, and being programmed to learn the proper response to maximize users’ function and safety, microprocessor-controlled knees can effectively increase users’ range of walking speeds, reduce the incidence of falls, and improve the user’s ability to participate in a range of activities. Stairs and ramps no longer create stressful situations for an individual with an amputation above the knee. In addition, microprocessor-controlled knees are now being manufactured with increased resistance to the elements, including several waterproof models and a few others with weatherproof ratings.
Perhaps the most critical component of any prosthesis is the socket – the connection between the living biological system and the artificial prosthesis. An amputee will place their residual limb into the socket, which then distributes and transmits forces through the prosthesis to accomplish a task such as walking, running, or moving a prosthetic hand to grasp an object. Modern socket designs, such as the Comfort Flex socket for lower limb prosthesis, isolate muscle groups and are a more anatomically correct design than previous sockets for the individual with an amputation above the knee. But despite being anatomically correct, the socket was a static design, unable to change shape to accommodate the routine daily volume changes in an amputee’s residual limb.
These sockets can now be equipped with the Boa closure system, which lets users tighten and loosen the socket according to the volume of their limb. The Boa closure system has also been added to sockets for individuals with amputations below the knee, marketed as the Revo Limb. The ability of the user to dynamically adjust the fit of the socket at any given instance lessens the need to carry around prosthetic limb socks used to restore fit. This method, however, does not let users tighten the socket in the regions where the residual limb loses volume. As a result, socks can also create sites of increased pressure and shear force.
In addition to advances to the rigid supportive element of the socket, the protective interface applied between the user’s residual limb and the rigid socket has also evolved. The interface helps improve the lives of prosthesis users who may have previously been unable to obtain a comfortable fit. The interface or liner is constructed from various silicones, thermoplastic elastomers, or urethanes. But these liners also insulate the residual limb, creating a hot and uncomfortable environment. New silicone liners are now being embedded with Outlast technology, a material that uses phase changes to capture and retain heat, which ultimately aids in cooling the user’s residual limb. Increased comfort for individuals using prostheses translates to increased activity.
Finally, the tools and instruments used by prosthetists have also evolved, improving their ability to assist the prosthesis user. One such development, the Europa+, attaches to a prosthesis to provide moment data at the connection point just below the socket. This information can be read and interpreted by the prosthetist to make changes to the positioning of the parts of the prosthesis attached to the socket, termed the alignment. This lets the prosthetist minimize stresses to the user’s residual limb, as well as mechanical stress to the prosthetic’s parts to decrease mechanical wear. In addition, the sensor can record temporal symmetry in the user’s walking pattern, a valuable tool that helps monitor the user’s progress through rehabilitation.