Wearable Powered Exoskeleton Use after Spinal Cord Injury

Proprietary names: Ekso™, Ekso GT™, Indego®, ProStep™, ReWalk™, ReWalk-I, ReWalk-P, ReWalk Personal 6.0, ReWalk Rehabilitation, REX P™, REX™, Robot Suit HAL® (Hybrid Assistive Limb®)

Generic names: active orthoses, ambulation exoskeleton, artificially-intelligent bionic exoskeleton, battery-operated bionic suit, biologically inspired robots, bionic ambulation, bionic gait exoskeleton, bionic legs, bionic suit, exoframe, exoskeleton walking suit, exosuit, legged-locomotion robotics, humane robots, humanoid robots, humanoids, lower extremity orthoses, lower-limb mobility device, motorized exoskeleton, neuroprosthetic training, neurorobotic training, overground bionic ambulation, paraplegic walker, powered armor, powered exoframe, powered exoskeleton, powered lower extremity exoskeleton, reciprocating gait orthosis, rehabilitative exoskeleton, robot-assisted gait therapy, robotic exoskeleton, robotic exoskeleton legs, robotic hip-knee-ankle-foot orthosis, upright mobility system, voluntary control system, walking assistance tool, wearable exoskeleton, wearable legs, wearable power-assist locomotor, wearable robot

Over the last decade, advancements in robotics, sensors, actuators, miniature computers, and control system software have spurred development of wearable powered exoskeletons with joints corresponding to those of the human body. These systems, intended for use by patients with neurologic conditions affecting the lower extremities, may be used in early rehabilitation for gait training (i.e., practice walking with an assistive device, brace, or support) and to help the brain compensate for injury; in late rehabilitation and community living as an exercise modality to promote physical, mental, and social wellness; and as a means to permit wheelchair-bound individuals to stand and walk independently in the home and community setting.1 Most of the wearable exoskeleton research to date has been performed in patients with impairment in motor or sensory function of the lower extremities after spinal cord injury (SCI), which is the focus of this report.1 (See Future Trends section below for discussion of research into use for other disorders.)

The spinal cord, which is very susceptible to injury, is the essential connection between the brain and peripheral nervous system.2,3 Trauma that fractures or dislocates vertebrae, which surround and protect the spinal cord, is the usual cause of SCI. Injury can also result from a penetrating wound such as a gunshot or stabbing.2,3 Furthermore, SCI can occur when occlusion or compression of the spinal arteries interrupts blood flow to the spinal cord.2,3

SCIs are defined as complete or incomplete.4 The American Spinal Injury Association Impairment Scale defines the extent of SCI using categories described in Table 1.4

SCIs to the cervical (neck) vertebrae (C1 through C7) can result in paralysis from the neck down (i.e., tetraplegia also known as quadriplegia), whereas SCIs below the neck, in the thoracic (T1 through T12), lumbar (L1 through L5), or sacral (S1 through S5) vertebral segments, can result in paralysis of part or all of the trunk and legs as well as pelvic organ dysfunction (i.e., paraplegia). After a paralyzing injury, loss of muscle activity and paradoxically increased activity (i.e., spasticity) ultimately leads to contracture and fibrous changes in the muscle.5 Degradation or loss of walking ability and mechanical unloading of the lower extremities result in profound muscle atrophy and bone loss.5 SCI also affects the autonomic nervous system, which involves control of the heart, glands, and smooth muscles,4 Over time, more than half of individuals with SCI develop chronic pain, which significantly affects quality of life (QOL).6

After acute treatment for SCI, rehabilitation specialists evaluate the person's functional abilities, determine the appropriate rehabilitation type, implement specific exercises and routines, and determine which assistive devices could help the person become more independent with daily living activities. Physical rehabilitation methods are intended to help strengthen and restore function in the muscle groups and the skeleton and improve coordination. Clinicians often prescribe exercise for patients with SCI for therapeutic purposes (i.e., to improve range of motion, for pain relief, and/or to improve bowel and bladder function).7 Initially, therapy consisted of body-weight-support systems that had a moving treadmill to initiate and sustain locomotion while therapists manually produced stepping motions for the legs.7 However, over the past two decades, robotic training devices have emerged in rehabilitation settings.

For functional purposes in the home/community setting, several assistive devices are used to enable standing or locomotion in this patient population (e.g., standing systems, knee-ankle foot orthoses KAFOs, reciprocal gait orthoses, functional electrical stimulation systems). However, many individuals with neurologic conditions affecting the lower extremities rely on conventional manual or powered-assisted wheelchairs for functional locomotion. Wheelchair users have limited access in many places and often experience secondary complications related to immobility (i.e., cardiovascular effects, depression, joint contracture, muscular atrophy, obesity, osteoporosis, skin breakdown, shoulder overuse syndrome, urinary tract infections, constipation) that together contribute to significant morbidity, cost, and mortality.8,9

The National Spinal Cord Injury Statistical Center (NSCISC) reports U.S. incidence of approximately 40 cases per million population, or about 12,500 people per year.10 In 2014, an estimated 276,000 people in the United States were living with an SCI.10 SCIs commonly occur among middle-aged adults (mean age 42 years) and are 4 times as likely to occur in males as in females.10 Since 2010, about 64% of SCIs have occurred in Caucasians, 23% in African Americans, 10% in Hispanics, 0.5% in Native Americans, and 1% in Asians.10 Since 2010, the percentage of SCIs classified in the NSCISC database by discharge diagnosis are as follows: complete tetraplegia: 14%; incomplete tetraplegia: 45%; complete paraplegia: 20%; and incomplete paraplegia: 21%.10

Worldwide, the reported incidence of traumatic SCIs ranges from 10.4 to 83.0 per million people per year.11 A few studies report on the incidence of nontraumatic SCI incidence. One study reports an estimated incidence of 68 per million in Canada.12 Australian estimates report an incidence of 26 per million.13-15 A hospital with a specialized SCI unit in Spain reports an incidence of 11.4 per million.16

Insufficient data exist to derive an exact global prevalence for traumatic SCI, which ranges between 236 and 1,009 per million people.17

Wearable powered exoskeletons are lower-extremity orthoses equipped with computer-controlled motors or actuators.1 The devices fall into two main categories: devices that are not self-supporting and require use of upper-extremity supports for balance (crutches, walker, or similar simple technologies), and fully self-supporting devices, which require no upper- extremity support.1 Users strap wearable exoskeletons over clothing to permit standing and walking for therapeutic or functional purposes. Exoskeletons have an external structural mechanism with joints corresponding to those of the human body and actuators that generate torques applied on the human joints.8

Powered exoskeletons are available for two care settings: rehabilitation models used for rehabilitation and therapeutic exercise or personal models used in the home/community setting for functional purposes, such as...