Reinventing Extremity Amputation in the Era of Functional Limb Restoration.

BACKGROUND: Recent progress in biomechatronics and vascularized composite allotransplantation have occurred in the absence of congruent advancements in the surgical approaches generally utilized for limb amputation. Consideration of these advances, as well as of both novel and time-honored reconstructive surgical techniques, argues for a fundamental reframing of the way in which amputation procedures should be performed. METHODS: We review sentinel developments in external prosthetic limb technology and limb transplantation, in addition to standard and emerging reconstructive surgical techniques relevant to limb modification, and then propose a new paradigm for limb amputation. RESULTS: An approach to limb amputation based on the availability of native tissues is proposed, with the intent of maximizing limb function, limiting neuropathic pain, restoring limb perception/proprioception and mitigating limb atrophy. CONCLUSIONS: We propose a reinvention of the manner in which limb amputations are performed, framed in the context of time-tested reconstructive techniques, as well as novel, state-of-the-art surgical procedures. Implementation of the proposed techniques in the acute setting has the potential to elevate advanced limb replacement strategies to a clinical solution that perhaps exceeds what is possible through traditional surgical approaches to limb salvage. We therefore argue that amputation, performed with the intent of optimizing the residuum for interaction with either a bionic or a transplanted limb, should be viewed not as a surgical failure, but as an alternative form of limb reconstruction.

Understanding patient preference in prosthetic ankle stiffness.

BACKGROUND: User preference has the potential to facilitate the design, control, and prescription of prostheses, but we do not yet understand which physiological factors drive preference, or if preference is associated with clinical benefits. METHODS: Subjects with unilateral below-knee amputation walked on a custom variable-stiffness prosthetic ankle and manipulated a dial to determine their preferred prosthetic ankle stiffness at three walking speeds. We evaluated anthropomorphic, metabolic, biomechanical, and performance-based descriptors at stiffness levels surrounding each subject's preferred stiffness. RESULTS: Subjects preferred lower stiffness values at their self-selected treadmill walking speed, and elected to walk faster overground with ankle stiffness at or above their preferred stiffness. Preferred stiffness maximized the kinematic symmetry between prosthetic and unaffected joints, but was not significantly correlated with body mass or metabolic rate. CONCLUSION: These results imply that some physiological factors are weighted more heavily when determining preferred stiffness, and that preference may be associated with clinically relevant improvements in gait.

Goats decrease hindlimb stiffness when walking over compliant surfaces.

Leg stiffness, commonly estimated as the 'compression' of a defined leg element in response to a load, has long been used to characterize terrestrial locomotion. This study investigated how goats adjust the stiffness of their hindlimbs to accommodate surfaces of different stiffness. Goats provide a compelling animal model for studying leg stiffness modulation, because they skillfully ambulate over a range of substrates that vary in compliance. To investigate the adjustments that goats make when walking over such substrates, ground reaction forces and three-dimensional trajectories of hindlimb markers were recorded as goats walked on rigid, rubber and foam surfaces. Net joint moments, power and work at the hip, knee, ankle and metatarsophalangeal joints were estimated throughout stance via inverse dynamics. Hindlimb stiffness was estimated from plots of total leg force versus total leg length, and individual joint stiffness was estimated from plots of joint moment versus joint angle. Our results support the hypothesis that goats modulate hindlimb stiffness in response to surface stiffness; specifically, hindlimb stiffness decreased on the more compliant surfaces (P<0.002). Estimates of joint stiffness identified hip and ankle muscles as the primary drivers of these adjustments. When humans run on compliant surfaces, they generally increase leg stiffness to preserve their center-of-mass mechanics. We did not estimate center-of-mass mechanics in this study; nevertheless, our estimates of hindlimb stiffness suggest that goats exhibit a different behavior. This study offers new insight into mechanisms that allow quadrupeds to modulate their gait mechanics when walking on surfaces of variable compliance.

Compliant Intramedullary Stems for Joint Reconstruction.

The longevity of current joint replacements is limited by aseptic loosening, which is the primary cause of non-infectious failure for hip, knee, and ankle arthroplasty. Aseptic loosening is typically caused either by osteolysis from particulate wear, or by high shear stresses at the bone-implant interface from over-constraint. Our objective was to demonstrate feasibility of a compliant intramedullary stem that eliminates over-constraint without generating particulate wear. The compliant stem is built around a compliant mechanism that permits rotation about a single axis. We first established several models to understand the relationship between mechanism geometry and implant performance under a given angular displacement and compressive load. We then used a neural network to identify a design space of geometries that would support an expected 100-year fatigue life inside the body. We additively manufactured one representative mechanism for each of three anatomic locations, and evaluated these prototypes on a KR-210 robot. The neural network predicts maximum stress and torsional stiffness with 2.69% and 4.08% error respectively, relative to finite element analysis data. We identified feasible design spaces for all three of the anatomic locations. Simulated peak stresses for the three stem prototypes were below the fatigue limit. Benchtop performance of all three prototypes was within design specifications. Our results demonstrate the feasibility of designing patient- and joint-specific compliant stems that address the root causes of aseptic loosening. Guided by these results, we expect the use of compliant intramedullary stems in joint reconstruction technology to increase implant lifetime.

Prosthetic Limb Attachment via Electromagnetic Attraction through a Closed Skin Envelope.

OBJECTIVE: Current socket-based methods of prosthetic limb attachment are responsible for many of the dominant problems reported by persons with amputation. In this work, we introduce a new paradigm for attachment via electromagnetic attraction between a bone-anchored ferromagnetic implant and an external electromagnet. Our objective was to develop a design framework for electromagnetic attachment, and to evaluate this framework in the context of transfemoral amputation. METHODS: We first used inverse dynamics to calculate the forces required to suspend a knee-ankle-foot prosthesis during gait. We then conducted cadaveric dissections to inform implant geometry and design a surgical methodology for covering the implant. We also developed an in silico framework to investigate how electromagnet design affects system performance. Simulations were validated against benchtop testing of a custom-built electromagnet. RESULTS: The physical electromagnet matched simulations, with a root-mean-square percentage error of 4.2% between measured and predicted forces. Using this electromagnet, we estimate that suspension of a prosthesis during gait would require 33 W of average power. After 200 and 1000 steps of simulated walking, the temperature at the skin would increase 2.3℃ and 15.4℃ relative to ambient, respectively. CONCLUSION: Our design framework produced an implant and electromagnet that could feasibly suspend a knee-ankle-foot prosthesis during short walking bouts. Future work will focus on optimization of this system to reduce heating during longer bouts. SIGNIFICANCE: This work demonstrates the initial feasibility of an electromagnetic prosthetic attachment paradigm that has the potential to increase comfort and improve residual limb health for persons with amputation.

An Ankle-Foot Prosthesis for Rock Climbing Augmentation.

This research presents the design and preliminary evaluation of an electromyographically (EMG) controlled 2-degree-of-freedom (DOF) ankle-foot prosthesis designed to enhance rock climbing ability in persons with transtibial amputation. The prosthesis comprises motorized ankle and subtalar joints, and is capable of emulating some key biomechanical behaviors exhibited by the ankle-foot complex during rock climbing maneuvers. The free space motion of the device is volitionally controlled via input from EMG surface electrodes embedded in a custom silicone liner worn on the residual limb. The device range of motion is 0.29 radians of each dorsiflexion and plantar flexion, and 0.39 radians each of inversion and eversion. Preliminary evaluation of the device was conducted, validating the system mass of 1292 grams, build height of 250 mm, joint velocity of 2.18 radians/second, settling time of 120 milliseconds, and steady state error of 0.008 radians. Clinical evaluation of the device was performed during a preliminary study with one subject with transtibial amputation. Joint angles of the ankle-foot, knee, and hip were measured during rock climbing with the robotic prosthesis and with a traditional passive prosthesis. We found that the robotic prosthesis increases the range of achieved ankle and subtalar positions compared to a standard passive prosthesis. In addition, maximum knee flexion and hip flexion angles are decreased while climbing with the robotic prosthesis. These results suggest that a lightweight, actuated, 2-DOF EMG-controlled robotic ankle-foot prosthesis can improve ankle and subtalar range of motion and climbing biomechanical function.

Caprine Models of the Agonist-Antagonist Myoneural Interface Implemented at the Above- and Below-Knee Amputation Levels.

BACKGROUND: Traditional approaches to amputation are not capable of reproducing the dynamic muscle relationships that are essential for proprioceptive sensation and joint control. In this study, the authors present two caprine models of the agonist-antagonist myoneural interface (AMI), a surgical approach designed to improve bidirectional neural control of a bionic limb. The key advancement of the AMI is the surgical coaptation of natively innervated agonist-antagonist muscle pairs within the residual limb. METHODS: One AMI was surgically created in the hindlimb of each of two African Pygmy goats at the time of primary transtibial amputation. Each animal was also implanted with muscle electrodes and sonomicrometer crystals to enable measurement of muscle activation and muscle state, respectively. Coupled agonist-antagonist excursion in the agonist-antagonist myoneural interface muscles was measured longitudinally for each animal. Fibrosis in the residual limb was evaluated grossly in each animal as part of a planned terminal procedure. RESULTS: Electromyographic and muscle state measurements showed coupled agonist-antagonist motion within the AMI in the presence of both neural activation and artificial muscle stimulation. Gross observation of the residual limb during a planned terminal procedure revealed a thin fibrotic encapsulation of the AMI constructs, which was not sufficient to preclude coupled muscle excursion. CONCLUSIONS: These findings highlight the AMI's potential to provide coupled motion of distal agonist-antagonist muscle pairs preserved during below- or above-knee amputation at nearly human scale. Guided by these findings, it is the authors' expectation that further development of the AMI architecture will improve neural control of advanced limb prostheses through incorporation of physiologically relevant muscle-tendon proprioception.

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

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.

Proprioception from a neurally controlled lower-extremity prosthesis.

Humans can precisely sense the position, speed, and torque of their body parts. This sense is known as proprioception and is essential to human motor control. Although there have been many attempts to create human-mechatronic interactions, there is still no robust, repeatable methodology to reflect proprioceptive information from a synthetic device onto the nervous system. To address this shortcoming, we present an agonist-antagonist myoneural interface (AMI). The AMI is composed of (i) a surgical construct made up of two muscle-tendons-an agonist and an antagonist-surgically connected in series so that contraction of one muscle stretches the other and (ii) a bidirectional efferent-afferent neural control architecture. The AMI preserves the dynamic muscle relationships that exist within native anatomy, thereby allowing proprioceptive signals from mechanoreceptors within both muscles to be communicated to the central nervous system. We surgically constructed two AMIs within the residual limb of a subject with a transtibial amputation. Each AMI sends control signals to one joint of a two-degree-of-freedom ankle-foot prosthesis and provides proprioceptive information pertaining to the movement of that joint. The AMI subject displayed improved control over the prosthesis compared to a group of four subjects having traditional amputation. We also show natural reflexive behaviors during stair ambulation in the AMI subject that do not appear in the cohort of subjects with traditional amputation. In addition, we demonstrate a system for closed-loop joint torque control in AMI subjects. These results provide a framework for integrating bionic systems with human physiology.

A murine model of a novel surgical architecture for proprioceptive muscle feedback and its potential application to control of advanced limb prostheses.

OBJECTIVE: Proprioceptive mechanisms play a critical role in both reflexive and volitional lower extremity control. Significant strides have been made in the development of bionic limbs that are capable of bi-directional communication with the peripheral nervous system, but none of these systems have been capable of providing physiologically-relevant muscle-based proprioceptive feedback through natural neural pathways. In this study, we present the agonist-antagonist myoneural interface (AMI), a surgical approach with the capacity to provide graded kinesthetic feedback from a prosthesis through mechanical activation of native mechanoreceptors within residual agonist-antagonist muscle pairs. APPROACH: (1) Sonomicrometery and electroneurography measurement systems were validated using a servo-based muscle tensioning system. (2) A heuristic controller was implemented to modulate functional electrical stimulation of an agonist muscle, using sonomicrometric measurements of stretch from a mechanically-coupled antagonist muscle as feedback. (3) One AMI was surgically constructed in the hindlimb of each rat. (4) The gastrocnemius-soleus complex of the rat was cycled through a series of ramp-and-hold stretches in two different muscle architectures: native (physiologically-intact) and AMI (modified). Integrated electroneurography from the tibial nerve was compared across the two architectures. MAIN RESULTS: Correlation between stretch and afferent signal demonstrated that the AMI is capable of provoking graded afferent signals in response to ramp-and-hold stretches, in a manner similar to the native muscle architecture. The response magnitude in the AMI was reduced when compared to the native architecture, likely due to lower stretch amplitudes. The closed-loop control system showed robustness at high stretch magnitudes, with some oscillation at low stretch magnitudes. SIGNIFICANCE: These results indicate that the AMI has the potential to communicate meaningful kinesthetic feedback from a prosthetic limb by replicating the agonist-antagonist relationships that are fundamental to physiological proprioception.