Lower Limb Prostheses
A lower limb prosthesis is an artificial replacement for any or all parts of the lower extremity (leg).
A prosthesis is used to provide an individual who has an amputated limb with the opportunity to perform functional tasks, particularly ambulation (walking), which may not be possible without the limb. In 2000, there were more than 1.5 million people in the United States with amputations. Amputation surgery most often is performed due to complications of peripheral vascular disease or neuropathy; trauma is the second leading cause of amputation. Amputations performed because of tumor or congenital limb deficiency are less common.
There are several levels of lower limb amputation, including partial foot, ankle disarticulation, transtibial (below the knee), knee disarticulation, transfemoral (above the knee), and hip disarticulation. The most common are transtibial (mid-calf) and transfemoral (mid-thigh). The basic components of these lower limb prostheses are the foot-ankle assembly, shank, socket, and suspension system.
The foot-ankle assembly is designed to provide a base of support during standing and walking, in addition to providing shock absorption and push-off during walking on even and uneven terrain. Four general categories of foot-ankle assemblies are non-articulated, articulated, elastic keel, and dynamic-response. One of the most widely prescribed foot is the solid-ankle-cushion-heel (SACH) foot, due to its simplicity, low cost, and durability. It may be inappropriate, however, for active community ambulators and sports participants. Articulated assemblies allow motion at the level of the human ankle; this motion may occur in one or more planes, depending on whether it is a single-axis or multi-axis foot. These assemblies offer more mobility at the cost of less stability and increased weight. The elastic keel foot is designed to mimic the human foot without the use of mechanical joints; the dynamic-response foot is designed to meet the demands of running and jumping in athletic users.
The shank corresponds to the anatomical lower leg, and is used to connect the socket to the ankle-foot assembly. In an endoskeletal shank, a central pylon, which is a narrow vertical support, rests inside a foam cosmetic cover. Endoskeletal systems allow for adjustment and realignment of prosthetic components. In an exoskeletal shank, the strength of the shank is provided by a hard outer shell that is either hollow or filled with lightweight material. Exoskeletal systems are more durable than endoskeletal systems; however, they may be heavier and have a fixed alignment, making adjustments difficult.
The socket contacts the residual limb and disperses pressure around it. A hard socket offers direct contact between the limb and the socket, resulting in decreased friction, no liner bulk, easy cleaning, and increased durability. It is, however, difficult to fit and adjust in response to residual limb changes. A soft socket includes a liner as a cushion between the socket and residual limb. This provides additional protection for the limb but may increase friction and bulk. Transtibial socket types include: patellar tendon-bearing (PTB), silicone suction, energy-storing, or bent-knee designs. Transfemoral socket types include: quadrilateral, ischial containment, and contoured adducted trochanteric-controlled alignment method (CAT-CAM) designs. A prosthetic sock is usually worn to help cushion the limb from forces and accommodate for volume changes. Prosthetic socks are available in a variety of materials and thickness, and may be worn in layers to achieve the most comfortable fit.
Suspension devices should keep the prosthesis firmly in place during use and allow comfortable sitting. Several types of suspension exist, both for the transtibial and transfemoral amputation. Common transtibial suspensions include sleeve, supracondylar, cuff, belt and strap, thigh-lacer, and suction styles. Sleeves are made of neoprene, urethane, or latex and are used over the shank, socket and thigh. Supracondylar and cuff suspensions are used to capture the femoral condyles and hold the prosthesis on the residual limb. The belt and strap method uses a waist belt with an anterior elastic strap to suspend the prosthesis, while the thigh-lacer method uses a snug-fitting corset around the thigh. The suction method consists of a silicone sleeve with a short pin at the end. The sleeve fits over the residual limb and the pin locks into the socket. With a transfemoral prosthesis, suction and several types of belt suspension also are available.
Transfemoral amputations also provide the additional challenge of incorporating a prosthetic knee unit. The knee unit must be able to bend and straighten smoothly during ambulation, in addition to providing stability during weightbearing on that limb. Knees are available as single-axis, polycentric, weight-activated, manual-locking, hydraulic, and pneumatic units. Technology using microprocessors in knee units is becoming a reality, although costs can be prohibitive.
Use of an actual prosthesis usually follows a period of postoperative management that includes addressing issues of pain, swelling, and proper positioning. In addition, physical therapy for range of motion, strength, bed mobility, transfers, and single limb ambulation often takes place during the initial rehabilitation period. In some cases, an individual may be fitted with an immediate post-operative prosthesis to allow for early double-limb ambulation. Many individuals will be fitted with a temporary prosthesis when the wound has healed. A temporary prosthesis allows for ambulation and continued shrinkage of the residual limb until a definitive prosthesis is fit.
When evaluating a prosthesis before use, the prosthetist and physical therapist should ensure that the inside of the socket is smooth and that all joints move freely. The socket should fit securely on the residual limb, and the overall prosthesis length should match the length of the intact leg. The patient must learn how to properly put on the residual limb sock and the prosthesis itself. A variety of techniques are used, depending on the type of socket and suspension system.
The user should be aware of how to properly care for and maintain the prosthesis, liner, and socks. Most plastic sockets and liners can be wiped with a damp cloth and dried. Socks should be washed and changed daily. Due to the wide variety of componentry and materials used in the fabrication of prostheses, the prosthetist should be the source for instructions regarding proper care and maintenance for each individual. In general, the patient should return to the prosthetist for any repairs, adjustments or realignments.
Health care team roles
The patient's primary care physician, surgeon, neurologist, prosthetist, physical and occupational therapists, nurses, and social worker are all important players in the multidisciplinary health care team. Surveys of patients with amputations have shown that the physical therapist, along with the physician and prosthetist, plays one of the most valued roles in providing information and help both at the time of amputation and following amputation. The entire team's input, along with the patient's input, is vital in determining whether a prosthesis should be fit and the specific prescription for the prosthesis. Input should be provided regarding the patient's medical history, premorbid level of function, present level of function, body build, range of motion, strength, motivation, and availability of familial and social support.
The physical therapist usually plays a major role in training an individual to walk with a prosthesis, and also is the health care professional who can evaluate prosthetic function immediately and over time. The physical therapist is trained in gait assessment and should watch for compensations and gait deviations that may indicate a problem with the prosthesis.
The main goal of prosthetic training usually is smooth, energy-efficient gait. This includes the ability of the individual to accept weight on either leg, balance on one foot, advance each leg forward and adjust to different types of terrain or environmental conditions. Principles of motor learning often are used in training, progressing from simple to complex tasks. Individuals begin with learning to keep their bodies stable in a closed environment with no manipulation or variability. An example may be practicing standing balance on one or both legs. Mobility, environmental changes, and task variability are added slowly to further challenge the individual as tasks are mastered. In the end, an example of a more complex task practiced may be the ability walk in a crowded hallway while carrying an object in one hand. In addition to ambulation training, the patient also should be taught how to transfer to and from surfaces, assume a variety of positions such as kneeling or squatting, and manage falls. Depending upon the individual's previous and present level of function, use of a traditional cane, quad cane, or crutches may be indicated. Patient motivation, comorbidity, level of amputation and level of function are all factors in determining the outcome of rehabilitation.
Gailey, Robert S. One Step Ahead: An Integrated Approach to Lower Extremity Prosthetics and Amputee Rehabilitation. Miami: Advanced Rehabilitation Therapy, Inc.,1994.
Lusardi, Michelle M., and Caroline C. Nielsen. Orthotics and Prosthetics in Rehabilitation. Boston: Butterworth-Heinemann, 2000.
May, Bella J. Amputations and Prosthetics: A Case Study Approach. Philadelphia: F.A. Davis Company, 1996.
Hsu, Miao-Ju, et al. "Physiological Measurements of Walking and Running in People with Transtibial Amputations with 3 Different Prostheses." Journal of Orthopedic and Sports Physical Therapy 29 (Sept. 1999): 526-33.
Peggy Campbell Torpey, MPT