The Ewing Amputation: The First Human Implementation of the Agonist-Antagonist Myoneural Interface

Abstract

Background: The agonist-antagonist myoneural interface (AMI) comprises a surgical construct and neural control architecture designed to serve as a bidirectional interface, capable of reflecting proprioceptive sensation of prosthetic joint position, speed, and torque from and advanced limb prosthesis onto the central nervous system. The AMI surgical procedure has previously been vetted in animal models; we here present the surgical results of its translation to human subjects.

Methods: Modified unilateral below knee amputations were performed in the elective setting in 3 human subjects between July 2016 and April 2017. AMIs were constructed in each subject to control and interpret proprioception from the bionic ankle and subtalar joints. Intraoperative, perioperative, and postoperative residual-limb outcome measures were recorded and analyzed, including electromyographic and radiographic imaging of AMI musculature.

Results: Mean subject age was 38 ± 13 years, and mean body mass index was 29.5 ± 5.5 kg/m2. Mean operative time was 346 ± 87 minutes, including 120 minutes of tourniquet time per subject. Complications were minor and included transient cellulitis and one instance of delayed wound healing. All subjects demonstrated mild limb hypertrophy postoperatively, and intact construct excursion with volitional muscle activation. All patients reported a high degree of phantom limb position perception with no reports of phantom pain.

Conclusions: The AMI offers the possibility of improved prosthetic control and restoration of muscle-tendon proprioception. Initial results in this first cohort of human patients are promising and provide evidence as to the potential role of AMIs in the care of patients requiring below knee amputation.

Figures

Fig. 1.

Conceptual illustration of the AMI. Mechanical linkage of innervated muscles restores natural contracture-stretch relationships, leading to activation of native mechanoreceptors and bi-directional communication with the central nervous system. AMI function is described as follows: at rest, both AMI muscles are at resting length and tension. When the agonist muscle contracts under volitional or reflexive activation, the antagonist is passively stretched, causing increased spindle discharge in the antagonist. This spindle activity is interpreted by the central nervous system (CNS) as a change in phantom joint position. Force feedback is provided to the CNS as artificial stimulation of the antagonist muscle causes it to contract in opposition to the agonist, creating tension at the agonist musculotendinous junction and increasing agonist Golgi tendon organ (GTO) discharge. This is interpreted by the CNS as torque about the phantom joint. A representation of muscle spindle and GTO state is shown for each condition.

Fig. 2.

Operative technique. A, Standard below knee amputation incision pattern. B, Tension marking sutures on component muscles. C, Disinsertion of component muscles. D, Isolation of tibia and fibula in preparation for osteotomies. E, Harvesting of tarsal tunnels from amputated distal limb. F, Tarsal tunnel with native tendon component. G, Placement of suture anchors for tarsal tunnel fixation to anterior tibia. H, Position of tarsal tunnels on anterior tibia following fixation.

Fig. 3.

Displays operative site closure.

Fig. 4.

Schematic illustration of the Ewing amputation. Two AMIs are constructed in the residuum at the time of primary transtibial amputation. Tarsal tunnels harvested from the amputated ankle joint are affixed to the medial flat of the tibia and serve as pulleys for the AMIs. When the patient is connected to a robotic prosthesis, the proximal and distal AMIs are myoelectrically linked to the prosthetic ankle and subtalar joints, respectively. In the diagram, suture points are denoted by blue crosses and tantalum beads are denoted by yellow circles. Positioning of these elements are representative, and not to scale.

Fig. 5.

Postoperative sequelae. A, Mild cellulitis was noted overlying the AMI constructs in 2 subjects 2 weeks after amputation; both resolved with oral antibiotic therapy. B, Development of a nonpainful pseudobursa overlying the AMI constructs was noted in 2 patients. C, Preservation of muscle bulk in the residual limb and slight prominence of AMI constructs required minor modifications in prosthesis socket design for all three study subjects.

Fig. 6.

Plain radiograph of residual limb. Radiopaque tantalum beads illustrate intact position of AMI constructs in the residuum, 6 weeks after the operative procedure.

Fig. 7.

Ultrasound evidence of dynamic AMI muscle relationships. Patients were asked to cyclically move the phantom ankle from approximately 80% dorsiflexion through approximately 80% plantar flexion, while ultrasound from the proximal tarsal tunnel and EMG from AMI muscles were simultaneously recorded. A representative sample is shown from patient 2. Tendon travel measurements are normalized to the length of the patient’s medial tibial flat. EMG values are normalized to the maximum EMG recorded from a given trial.

Fig. 8.

EMG amplitude during volitional movement of the phantom limb. AMI patients showed a repeatable generation of activation in muscles typically associated with each desired movement (A), an improved ability to limit muscle activation in muscles not associated with the desired movement (B), and a decrease in unintended co-contraction (C). Error bars depict standard error. Asterisk indicates significant difference at α = 0.02.