Robotic exoskeletons have emerged as a helpful rehabilitation tool for disabled and people suffering from several health-related consequences after a spinal cord injury (SCI).
Exoskeletons are wearable robotic units, controlled by computer boards to power a system of motors, pneumatics, levers, or hydraulics to restore locomotion and improve quality of life. Used by facilities for rehabilitation purposes in medical centers or home use, Exoskeletons have the potential to revolutionize rehabilitation following SCI.
Unlike previously existing solutions like wheelchairs, exoskeletons offer a great deal of freedom in the levels of physical activities. Different brands of powered exoskeletons are now commercially available for rehabilitation, but there is still limited accessibility to exoskeletons in clinical settings, partly because of their prohibitive cost and the high level of training. The technology is still premature to make clear recommendations about their clinical use after SCI.
This article is to highlight the main limitations and potential benefits of using exoskeletons in the rehabilitation. Clinical trials are currently underway to address some of these limitations and to maximize the benefits in rehabilitation settings.
We cover two main areas: designs of exoskeletons and significant health consequences. The design perspective refers to safety concerns, fitting time, and speed of exoskeletons, while the health perspective relates to factors like body weight, physical activity, pressure injuries, and bone health.
1. Safety and efficacy
A recent study indicates that exoskeletons are safe to use in all different settings. The study, which involved nine European rehabilitation centers, demonstrated the safety, feasibility, and training characteristics in patients with SCI following eight weeks of training. Out of 52 participants, three dropped out following ankle swelling, and four presented with grade II pressure injury but managed to continue the study.
Despite the possibility of fractures at distal tibia or calcaneus bone during walking, the study highlighted the potential health benefits in the use of exoskeletons in rehabilitation settings. The study also provided preliminary evidence on the efficacy of exoskeletons on cardiovascular health, energy expenditure, body composition, gait parameters, level of physical activity, and quality of life. Exoskeletons decrease seated time, increase standing and walking time as well as social engagements with family and friends. Reduced sitting time ameliorates several consequences that negatively impact health.
2. Fitting time across brands
Most brands today require special measurements to custom fit participants before donning/doffing, due to the factors like leg length discrepancy, pelvic obliquity, severe muscle wasting, or even highly sensitive skin.
Different brands have different donning/doffing process, and this usually takes up to 2-3 sessions to complete. After completing the initial measurements, the fitting requires at least 1 hour to complete safely before the individuals can stand up and walk. Some available brands have a shorter fitting time, while others need the patients to sit on a special chair to accomplish the fitting purpose.
One exception is the Indego exoskeleton that is easy to assemble when the patient sits in a wheelchair. It cuts the fitting time and provides higher safety. Therefore, all future brands need to consider shorter fitting time and allow fitting in wheelchairs without the need to transfer from one place to another.
3. Speed and community ambulation
Most common exoskeletons come with a modest speed that is slightly higher than 0.2 m/sec. The slow speed preserves balance and prevents frequent falling. However, the speed can increase up to 0.4 m/sec, when the patients gain confidence and stability following continuous training and walking. Most brands were tested indoors on tiled surfaces, but walking on uneven, muddy, pebbles, rainy and/or snowy terrains and weather conditions not suitable for exoskeletons creates additional challenges.
The market requires water-proof designs that can facilitate walking in different weather conditions or on uneven terrains. Future designs should also focus on choosing highly durable materials that provide less weight and allow faster speed without compromising balance.
4. Bodyweight and composition
The existing exoskeleton technology is only limited to those with a bodyweight of less than 100 kg (220 lbs.). It excludes two-thirds of individuals who are either overweight or obese. Notably, evidence suggests that several persons with SCI started a rigorous diet program to lose weight after initially being disqualified from enrolling because of exceeding the bodyweight cut-off limit recommended by the manufacture. They are motivated to engage in effective dietary plans and participate in exercise programs to maintain healthy body weight.
Furthermore, exoskeleton training independently helps people to lose weight, notably decreasing the percentage of the whole body and regional fat mass, improving cardio-metabolic health. A recent report demonstrated that improvement in cardiometabolic health is tightly associated with favorable body composition. According to the report, 15 weeks of exoskeleton training resulted in decreased body mass by 6 kg, including 2 kg loss in fat mass and 4 kg loss in fat-free mass in an individual. However, there is limited evidence to clearly suggest the positive effects of exoskeleton on body composition.
5. Physical activities
Disabilities lead people to a sedentary lifestyle and prolonged sitting, which is a risk factor for cardiovascular disease, cancer, etc. Recently released ISCOS guidelines recommended that persons with SCI engage in at least 20 min of moderate to vigorous-intensity aerobic exercise three times per week to improve cardio-respiratory fitness.
Exoskeletons decrease sitting time and improve physical activities by increasing the number of steps, duration, and distance of walking. Exoskeletons can also provide passive bodily movement of the lower extremity without muscle contraction. Exoskeleton training is undoubtedly an effective strategy to initiate muscle contraction and increasing energy expenditure.
6. Range of motion and natural neuro-recovery
To qualify for an exoskeleton training program, patients need to attain a certain degree of joint contractures at the hips, knees, and ankle joints. The hip extension requires a range of motion within 10-15 degrees, while knee extension needs less than 10 degrees’ flexion in standing position with ankle joints in a neutral position.
People who fail to attain this range of motion are encouraged to participate in an extensive stretching program to improve muscle flexibility around these joints. It might take up to 6 months to gain 6-10-degree improvement.
Anyways, unlike other forms of walking similar to knee-ankle foot orthosis (KAFO) or hip-knee-ankle-foot orthosis (HKAFO), exoskeletons facilitate a faster and natural neuro-recovery, primarily due to the functional range of motion during locomotion.
7. Bone health
Sixty percent of individuals with spinal cord injuries suffer from osteopenia or osteoporosis, a progressive disease that leads to bone loss in the distal femur and proximal tibia. Bone loss occurs subliminally at a rapid rate and approaching 1% of bone mineral density per week.
Most bone loss occurs within the first 12 to 24 months after injury and reaches a steady-state within 3-8 years. Furthermore, patients also have a higher risk of bone fractures. Therefore, it is essential to conduct an X-ray examination of knees, hips, and ankle joints to assess the risk of fracture, before exoskeleton training.
8. Pressure injuries
70%-75% of individuals with spinal cord injuries suffer pressure injuries with dramatic changes in their skin structures during their lifetime. It limits the number of candidates suitable for the exoskeleton. All exoskeletons have straps to maintain static and dynamic posture during standing and walking. They can cause excessive shear to the surrounding soft tissues and may lead to pressure injuries.
To circumvent this problem, researchers developed pressure sensors that can monitor pressure exerted by physical human-machine interfaces and provide feedback about levels of skin/body pressure in fastening straps. These sensors can protect against ischemia and necrosis by maintaining pressure in an adequate range.