More than 294,000 people in the United States are living with spinal cord injuries (SCI), and over 17,000 new injuries occur every year. Individuals with SCI experience a sudden decrease in mechanical loading in their lower limbs, leading to severe bone loss known as SCI-induced osteoporosis. One proposed method to treat SCI-induced bone loss is to use exoskeleton walking therapy to load the lower limbs, but it is currently unknown whether the loading experienced is sufficient for any therapeutic benefits. An ongoing clinical trial (NCT02533713) associated with this thesis allows 30 individuals with chronic SCI to complete exoskeleton walking therapy for 6 months. We hypothesized that exoskeleton walking therapy would be sufficient to provide therapeutic benefit, resulting in a significant increase in the bone mineral content (BMC; g) in the distal femur and proximal tibia. We also proposed to assess the kinetics and kinematics associated with walking therapy for each participant. Finally, we hypothesized that there would be a positive relationship between the change in BMC and a participant-specific measure of “bone loading dose”.
The present analysis includes data from 9 participants in the clinical trial. Computed tomography (CT) scans including at least 15 cm of the distal femur and proximal tibia were taken before and after six months of exoskeleton walking therapy. The scans were analyzed quantitatively to calculate BMC and participant-specific measures of bone strength. The bone strength measures were used to create candidate loading dose calculations for each participant. The clinical trial intervention included 6 months of exoskeleton walking therapy performed 3 hours a week, using either the EKSO or Indego exoskeleton. Video data and pressure sensing insole data was captured during the final walking therapy sessions. The data were used to calculate candidate measures of loading dose. We used paired t-tests to compare pre- versus post-intervention values of BMC. We also used independent t-tests to compare the kinetics and kinematics of walking between the two exoskeletons. Finally, Pearson’s correlations were calculated to assess the relationship between the changes in BMC and the various loading dose calculations.
There was a significant 2.3±3.6% increase in femur BMC following exoskeleton walking therapy (p=0.026). There was a non-significant 4.6±8.6% increase in BMC of the epiphyseal region of the tibia (p = 0.062). The kinematics and temporal parameters varied between the two exoskeletons, with the Indego having a higher range of motion at the knee and ankle, and a faster stride time and walking speed than the EKSO. Loading dose was more closely associated with change in tibia BMC versus femur BMC (tibia r = 0.73 to 0.79, p<0.005 versus femur r = 0.41 to 0.60).
BMC increased in regions of the distal femur following exoskeleton walking therapy. High positive correlations between participant specific loading dose calculations suggest that bone changes in a dose-dependent manner as a result of exoskeleton walking therapy. Assessing changes in bone mass compared to the amount of loading during walking therapy can provide important information to identify therapeutic targets and predict response due to exoskeleton walking therapy.
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