Walking with Robotic Legs

Amputation dates back to prehistoric times. Although amputation surgery has considerably advanced over the years, with improved perioperative conditions, anesthesia, and hemostasis, technical enhancements are minimal.  The removal of a diseased limb is not the key challenge; the main challenge is to perform the surgery such that the patient can comfortably wear a prosthetic device post-surgery.

Rehabilitative efforts are significantly improved by knee joint salvage and the energy expenses needed for ambulation are considerably reduced. It is important that a patient is comfortable with using prosthesis, is able to put on and remove the prosthesis conveniently, walk on rough surfaces as well as use the commode at night. Hence treatment in this case is not confined to the surgeon alone; a primary care physician, prosthetist, physical therapist and a social worker must also be involved.

Lower-extremity amputations are mainly done due to trauma, tumors, peripheral vascular disease, infections or congenital limb deficiency. Lower limb or below knee also called transtibial amputations are the most common. One must understand that when the knee joint is preserved, using a prosthesis is much easier and the effort put in is much less.  The main goal of amputation is that the end organ is sensate, well-healed and functional that will combine effectively with a prosthesis. Based on laboratory and clinical tests as well as on etiological factors, length is selected.

The four post-surgical stages include:

• Post-operative prosthesis: This is provided within a day of amputation and mostly recommended for the younger persons going through amputation because of trauma, tumor or infection.
• Initial Prosthesis: The EPSF is normally fitted between 1 and 4 weeks of the surgery during the acute healing phase wherein the skin is capable of tolerating highly intimate fitting.
• Preparatory Prosthesis: These are normally used for three to six months basically preparing the amputee to use a definitive device.  
• Definitive Prosthesis: This is normally prescribed until stabilization of the residual limb making sure that the prosthesis is long-lasting. The lessons learnt during the use of the preparatory prosthesis help design the definitive prosthesis in terms of design, and any other possible problems.

Engineering a Prosthetic Leg

Convenience, function and cosmetics are all taken into consideration when designing a prosthetic leg. The health, weight, activity level of the patient need to be considered, also the shape, length, circulation, skin condition, maturation and range of motion must also be considered.

The prosthetic socket is the most important as it must support the body weight of the patient and hold in place the residual limb during all activities. A transtibial prosthesis includes a socket with an optional insert, adapter for fitting the socket to the shank and an artificial foot. The recent patellar-tendon bearing PTB socket design aims for load distribution over residual limb areas that are in line with the residual limb’s ability to tolerate load. The suspension sleeve used may be made of latex, neoprene, silicone or elastic.

The sleeve ensures excellent suspension. Suction is also alternatively used as a means of suspension. A shuttle locking pin enables its use in transtibial prosthesis. The residual limb is cushioned by foam or silicon inserts absorb and redistribute shear and compressive forces generated at the time of ambulation. Wool, synthetic or cotton socks, 1 to 15 ply are normally used by the transtibial amputee.

Normally transfemoral sockets are rigid. Recently, flexible sockets fabricated with a malleable thermoplastic are integrated into a semi-rigid or a rigid frame. The transfemoral socket and prosthesis is suspended mainly through suction, a Silesian belt, a total elastic suspension belt or a pelvic band/hip joint or waist belt. The hip disarticulation prosthesis includes a foot, shank, prosthetic knee, a control strap as well as a socket. Socket design is done using CAD and a CNC (milling) machine is used for carving the equivalent of the positive plaster socket mold. Socket fabrication is done by vacuum forming or lamination of the socket over this mold. The most common non-articulating prosthetic foot design is the SACH foot. This foot includes a rigid wooden keel with a flexible or compliant heel and forefoot.

The purpose of the prosthetic knee is to restore normal function and help the user to walk normally with the least amount of effort. Types of prosthetic knees include manual locking knees, alignment controlled knee units, polycentric linkages, friction brakes and fluid-resistive devices. Prosthetic fit is very important and includes evaluating using a test socket, checking the prosthetic suspension, whether the proximal trim lines are optimal in terms of comfort and mobility and prosthesis alignment.

Finally, prosthetic alignment is ensured during the design of the leg ensuring that the user has good gait, a comfortable leg, physical security, minimal effort while walking, optimal posture making sure that even if used for long time periods the residual limb is not impacted.

Recent Controversy – Competitive Runner vs. Amputee Sprinter

Research by Weyand PG et al (2009) involved three tests of functional similarity between a competitive male runner with intact limbs and an amputee sprinter also the metabolic cost of sprinting endurance, running and running mechanics.

The first hypothesis and second hypotheses were mainly physiological comparisons of the metabolic cost of running and sprinting endurance, respectively.  The results show that physiological function was quite similar and almost identical between the amputee and intact limb subjects. The third hypothesis showed that there was considerable dissimilarity while sprinting. Hence it was concluded that running for an amputee is physiologically similar though mechanically dissimilar to normal running. The similarity is due to the reliance of both the amputee and the normal runner on the extensor muscles across the knee and hip joints. The mechanical dissimilarities is mainly due to the fact that it is not possible to provide these mechanical functions with a simple prosthetic design and anyway offering something like nature is not possible.

Into the Future

Recent innovation in prosthesis such as the Vanderbilt Exoskeleton as demonstrated in the video below helps improve the quality of life of people with spinal cord injuries. This specially designed exoskeleton helps individuals with complete paraplegia to walk, stand, move from a sitting position to a standing position and vice versa as well as walk up and down the stairs. It weighs 27 lbs, has embedded sensors and microprocessors to monitor user movements and can enable joint torque at the knees and hips to generate motion.

For transfemoral amputees, presently available prostheses are not very effective. A prosthetic device with actively powered knee and ankle joints has been suggested to improve the mobility of transfemoral amputees by both reducing biomechanical disparity between healthy individuals and transfemoral amputees.

The most recent prototype is a comprehensive, self-contained, powered knee and ankle prosthesis. The control system of this prosthesis helps the user to do several daily activities, which include standing on a rough terrain, walking on different cadences as well as climbing up and sown slopes and stairs.

Future Work and Breaking News

In a major breakthrough, the Rehabilitation Institute of Chicago has recently designed the world’s first neural control bionic leg (see video below) that will allow amputees to have more control over the prosthesis. This unique bionic leg interacts with 31-year-old Vawter, who lost his leg in an accident. When Vawter indicates to the device that he wants to stand by pushing a button, the leg responds by moving one step.

The Department of Defense Army’s Telemedicine and Advanced Technology Research Center has funded this research, which aims at adding neural information to the control system so that they improve the control of the prosthetic leg. The amazing part of this innovation is that one need not drag the prosthetic but can walk very much like a normal individual.

References

• Weyand P.G, Bundle M.W, McGowan C.P, Grabowski A, Brown M.B, Kram R and Herr H. The fastest runner on artificial legs: different limbs, similar function? Journal of Applied Physiology. 2009;107:903–911.
• M. Barbara Silver-Thorn. 2004. Chapter 33: Design of Artificial Limbs for Lower Extremity Amputees. Published in Standard Handbook of Biomedical Engineering and Design. The McGraw-Hill Companies.
• Gailey R, McFarland LV, Cooper R.A, Czerniecki J, Gambel J.M, Hubbard S, Maynard C, Smith D.G, Raya M, Reiber G.E. Unilateral lower-limb loss: Prosthetic device use and functional outcomes in service members from Vietnam war and OIF/OEF conflicts. Journal of Rehabilitation Research & Development. 2010;44:317–332.
• Pitkin M. On the way to total integration of prosthetic pylon with residuum, J Rehabil Res Dev. 2009;46(3): 345–360.
• Kahle J.T, Highsmith M.J, Hubbard S.L. Comparison of non-microprocessor knee mechanism versus C-Leg on Prosthesis Evaluation Questionnaire, stumbles, falls, walking tests, stair descent, and knee preference. Journal of Rehabilitation Research and Development. 2008;45(1):1–14.
• Powered Exoskeleton
• Powered Lowered Limb Devices