Microprocessor-Controlled Prostheses for the Lower Limb
DESCRIPTION
Microprocessor-controlled prostheses use feedback from sensors to adjust joint movement on a real-time as-needed basis. Active joint control is intended to improve safety and function, particularly for individuals who have the capability to maneuver on uneven terrain and with variable gait.
More than 100 different prosthetic ankle-foot and knee designs are currently available. The choice of the most appropriate design may depend on the individual’s underlying activity level. For example, the requirements of a prosthetic knee in an elderly, largely homebound individual will be quite different than a younger, active person. In general, key elements of a prosthetic knee design involve providing stability during both the stance and swing phase of the gait. Prosthetic knees also vary in their ability to alter the cadence of the gait, or the ability to walk on rough or uneven surfaces. In contrast to more simple prostheses, which are designed to function optimally at one walking cadence, fluid and hydraulic-controlled devices are designed to allow amputees to vary their walking speed by matching the movement of the shin portion of the prosthesis to the movement the upper leg. For example, the rate at which the knee flexes after “toe-off” and then extends before heel strike depends in part on the mechanical characteristics of the prosthetic knee joint. If the resistance to flexion and extension of the joint does not vary with gait speed, the prosthetic knee extends too quickly or too slowly relative to the heel strike if the cadence is altered. When properly controlled, hydraulic or pneumatic swing-phase controls allow the prosthetist to set a pace that is adjusted to the individual amputee from very slow to a race-walking pace. Hydraulic prostheses are heavier than other options and require gait training; for these reasons, these prostheses are generally prescribed for athletic or fit individuals. Other design features include multiple centers of rotation, referred to as “polycentric knees.” The mechanical complexity of these devices allows engineers to optimize selected stance and swing-phase features.
Microprocessor-controlled prosthetic knees have been developed. Examples of these include the Intelligent Prosthesis (Blatchford, U.K.), the Adaptive (Endolite, England), the Rheo (Ossur, Iceland), and the C-Leg (Otto Bock Orthopedic Industry, Minneapolis, MN). These devices are equipped with a sensor that detects when the knee is in full extension and adjusts the swing phase automatically, permitting a more natural walking pattern of varying speeds. For example, the prosthetist can specify several different optimal adjustments that the computer later selects and applies according to the pace of ambulation. In addition, these devices (with the exception of the Intelligent Prosthesis) use microprocessor control in both the swing and stance phases of gait. By improving stance control, they may provide increased safety, stability, and function; for example, the sensors are designed to recognize a stumble and stiffen the knee, thus avoiding a fall. Other potential benefits of microprocessor-controlled knee prostheses are improved ability to navigate stairs, slopes, and uneven terrain, and reduction in energy expenditure and concentration required for ambulation. The C-Leg was cleared for marketing in 1999 through the 510(k) process of the U.S. Food and Drug Administration (FDA, K991590).
Microprocessor-controlled ankle-foot prostheses are being developed for transtibial amputees. Examples of these include the Proprio Foot® (Ossur) and the iPED (developed by Martin Bionics LLC and licensed to College Park Industries). With sensors in the feet that determine the direction and speed of the foot’s movement, a microprocessor controls the flexion angle of the ankle, allowing the foot to lift during the swing phase and potentially adjust to changes in force, speed, and terrain during the step phase. The intent of the technology is to make ambulation more efficient and prevent falls in patients ranging from the young active amputee to the elderly diabetic patient. The Proprio Foot™ is the only microprocessor-controlled foot prosthesis that is commercially available at this time, and is a class I device that is exempt from 510(k) marketing clearance. The manufacturer must register the prosthesis with the restorative devices branch of the FDA and keep a record of any complaints, but does not have to undergo a full review. Information on the Ossur Web site indicates use of the Proprio Foot™ for low to moderate impact for transtibial amputees who are classified as level K3 (i.e., community ambulatory, with the ability or potential for ambulation with variable cadence).
In development are lower-limb prostheses that also replace muscle activity in order to bend and straighten the prosthetic joint. For example, the Power Foot (developed at the Massachusetts Institute of Technology and licensed to iWalk) is a myoelectric prosthesis for transtibial amputees that use muscle activity from the remaining limb for the control of ankle movement. This prosthesis is designed to propel the foot forward as it pushes off the ground during the gait cycle, which in addition to improving efficiency, has the potential to reduce hip and back problems arising from an unnatural gait with use of a passive prosthesis. This technology is limited by the size and the weight required for a motor and batteries in the prosthesis. The Power Knee (Ossur), which is designed to replace muscle activity of the quadriceps, uses artificial proprioception with sensors similar to the Proprio Foot in order to anticipate and respond with the appropriate movement required for the next step. The Power Knee is currently in the initial launch phase in the United States.
POLICY
A microprocessor-controlled knee in amputees is considered medically necessary if the medical appropriateness criteria are met. (See Medical Appropriateness below.)
A microprocessor-controlled knee in individuals who do not meet medical appropriateness criteria is considered not medically necessary.
A powered knee is considered investigational.
A microprocessor-controlled or powered foot is considered investigational.
MEDICAL APPROPRIATENESS
A microprocessor-controlled knee in an amputee is considered medically appropriate when ALL the following criteria are met:
Physical ability, including adequate cardiovascular and pulmonary reserve, for ambulation at faster than normal walking speed
Demonstrated need for long distance ambulation at variable rates or for ambulation on uneven terrain (use of the limb in the home or for basic community ambulation, including negotiation of stairs, is not sufficient to justify provision of the computerized limb over standard limb applications)
Adequate cognitive ability to master use and care requirements for the technology
Has successfully completed a trial period of one month to evaluate the tolerability and efficacy of the prosthesis in a real life setting.
IMPORTANT REMINDER
We develop Medical Policies to provide guidance to Members and Providers. This Medical Policy relates only to the services or supplies described in it. The existence of a Medical Policy is not an authorization, certification, explanation of benefits or a contract for the service (or supply) that is referenced in the Medical Policy. For a determination of the benefits that a Member is entitled to receive under his or her health plan, the Member’s health plan must be reviewed. If there is a conflict between the Medical Policy and a health plan, the express terms of the health plan will govern.
ADDITIONAL INFORMATION
Amputees should be evaluated by an independent qualified professional to determine the most appropriate prosthetic components and control mechanism. Typically, this assessment is by a certified orthotist and/or prosthetist who is a healthcare professional specifically educated and trained to manage comprehensive orthotic and/or prosthetic patient care. This includes patient assessment, formulation of a treatment plan, implementation of the treatment plan, follow-up and practice management.
For individuals in whom the potential benefits of the microprocessor knees are uncertain, individuals may first be fitted with a standard prosthesis to determine their level of function with the standard device.
SOURCES
Berry, D. (2006). Microprocessor prosthetic knees. Physical Medicine and Rehabilitation Clinics of North America, 17, 91-113.
BlueCross BlueShield Association. Medical Policy Reference Manual. (2:2010). Microprocessor-controlled prostheses for the lower limb (1.01.25). Retrieved May 17, 2010 from BlueWeb. (23 articles and/or guidelines reviewed)
Hafner, B. J., & Smith, D. G. (2009). Differences in function and safety between Medicare functional classification level-2 and -3 transfemoral amputees and influence of prosthetic knee joint control. Journal of Rehabilitation Research & Development, 46 (3), 417-433.
ORIGINAL EFFECTIVE DATE: 11/13/2010
MOST RECENT REVIEW DATE: 11/13/2010
ID_BA
Policies included in the Medical Policy Manual are not intended to certify coverage availability. They are medical determinations about a particular technology, service, drug, etc. While a policy or technology may be medically necessary, it could be excluded in a member's benefit plan. Please check with the appropriate claims department to determine if the service in question is a covered service under a particular benefit plan. Use of the Medical Policy Manual is not intended to replace independent medical judgment for treatment of individuals. The content on this Web site is not intended to be a substitute for professional medical advice in any way. Always seek the advice of your physician or other qualified health care provider if you have questions regarding a medical condition or treatment.
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