A Novel Approach to Upper Extremity Amputation to Augment Volitional Control and Restore Proprioception

Upper extremity amputation is among the oldest known surgical procedures in medical history, with many of its technical principles having first been elucidated by Hippocrates. Despite the passage of more than two millennia, relatively little has changed in the operative approach to upper limb sacrifice. An estimated 58,000 patients in the United States currently suffer from upper extremity limb loss at either the above elbow (AEA) or below elbow (BEA) level, and the prevalence of upper limb amputation is expected to rise to approximately 95,000 patients by 2050.

Normal function of the upper limb is enabled through the dynamic interplay of multiple muscle groups acting in concert. Manual dexterity 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 upper extremity exist in a balanced agonist/antagonist situation in which volitional activation of one muscle leads not only to its contracture, but also to passive stretch of its opposite. Changes in muscle tension manifest through this interaction of agonist and antagonist units lead to stimulation of specialized receptors within the muscle fibers (e.g., muscle spindle fibers and Golgi tendon organs) 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.

Unfortunately, the standard operative approach to upper limb amputation at either the AEA or BEA level obliterates many of the dynamic relationships characteristic of the uninjured upper extremity. Initial surgical exposure is typically accomplished through a fishmouth-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 construct in order to achieve definitive closure. The rudimentary approximation of discordant tissues in the distal limb in this approach 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 leads to impaired limb function with prostheses, but also manifests as pathological sensory perception of the extremity in the form of phantom limb and phantom pain symptoms.

To date, the limitations of these approaches have been tolerated due to the fairly simplistic goal of upper limb amputation: to provide a stable, padded surface for mounting a prosthesis. Historically, upper limb prostheses have afforded amputees the opportunity to recover at least some measure of upper limb function. However, such devices have generally not been able to recapitulate the complex biomechanics of the human upper limb due to limited ranges of motion and lack of feedback control. These limitations have resulted in reported upper limb prosthesis rejection rates ranging from 23% to 45%, including both body-powered and myoelectric devices.

However, the capabilities of modern prostheses are now expanding remarkably. Technological advances including increasingly miniaturized electronics, wireless communications and ever-refined positional sensors have enabled prosthetic developers to create next-generation bionic limbs with greatly enhanced degrees of freedom over prior models. Even more advanced prostheses are currently being developed that have the potential to offer sensory feedback - both tactile and positional - in a manner never before seen. Such prosthetic devices, while not yet available commercially, are presently being studied in experimental settings. For example, the Defense Advanced Research Projects Agency (DARPA) recently issued a request for proposals under the Hand, Proprioception and Touch Interfaces (HAPTIX) Program incorporating an upper limb prosthesis including six degrees of freedom at the wrist, thumb and all digits, 10 pressure sensors capable of providing sensory feedback, and joint angle and velocity sensors capable of providing joint position data.