Somatotopic Configuration of Distal Residual Limb Tissues in Lower Extremity Amputations

Historical Background

Lower extremity amputation is among the oldest known surgical procedures in medical history. Despite the passage of over two millennia, however, relatively little has changed in the operative approach. Currently, lower limb amputation is indicated most frequently for lower extremity compromise due to severe peripheral vascular disease, followed in short order by trauma, tumors, infections and congenital limb deficiencies. Estimates of frequency of lower limb amputations range from 30,000-40,000 cases per year in the United States alone.

Normal function of the lower limb is enabled through the interplay of multiple muscle groups acting in concert. Ambulation is a remarkably orchestrated biomechanical process that is dependent upon a complex feedback loop involving the central and peripheral nervous systems and the musculoskeletal system. In their native state, the muscles of the lower extremity exist in a balanced agonist/antagonist milieu in which volitional activation of one muscle leads not only to its contracture, but also passive stretch of its opposite. Changes in muscle tension manifest through these changes lead to stimulation of specialized receptors within the muscle fibers that transmit joint position information to the cerebral cortex. Such feedback, in conjunction with cutaneous sensory information from skin mechanoreceptors, provides us with a sense of limb proprioception that ultimately enables high fidelity limb control, even in the absence of visual feedback.

However, the standard operative approach to lower limb amputation at either the below knee (BKA) or above knee (AKA) level obliterates many of the dynamic relationships characteristic of the uninjured lower extremity. Initial exposure is accomplished through either a stair-step (BKA) or fishmouth (AKA) pattern incision, followed by progressive transection of muscles, vessels, nerves and bone at the level of the incision. Tissues distal to the site of structural transection are discarded, regardless of whether or not there may be viable segments, and the proximal residual muscles are layered over the distal transected bone in order to provide insulation to this exposed osseous surface. The surrounding skin is then advanced over the bone/muscle infrastructure in order to achieve definitive closure. The rudimentary approximation of tissues in the distal limb in these approaches results in a disorganized scar mass in which normal dynamic muscle relationships are destroyed. The uncoupling of native agonist/antagonist muscle pairings results in isometric contraction of residual muscle groups upon volitional activation, producing incomplete, unbalanced neural feedback to the brain that results in aberrant perception of residual limb position. Such disturbed feedback not only results in impaired ambulatory function with prosthetics, but also manifests as pathological sensory perception of the extremity in the form of phantom limb and phantom pain symptomatology.

To date, providers and patients have tolerated the limitations of these approaches due to the fairly simplistic goal of lower limb amputation: to provide a stable, padded surface for prosthesis mounting. Historically, lower limb prostheses have afforded amputees the opportunity to recover at least some measure of ambulatory function. Standard lower limb prostheses currently afford the wearer the walk in a rudimentary fashion, as well as occasionally run. However, such devices have generally not been able to recapitulate the complex biomechanics of the human lower limb due to limited ranges of motion and lack of feedback control. These limitations have resulted in substantially altered kinematics in lower limb amputees that are associated with derangements in energy expenditure profiles that worsen with laterality and ascending level.