Long-term performance and stability of implanted neural interfaces in individuals with lower limb loss

<i>Objective.</i>High-density nerve cuffs have been successfully utilized to restore somatosensation in individuals with lower-limb loss by interfacing directly with the peripheral nervous system. Elicited sensations via these devices have improved various functional outcomes, including standing balance, walking symmetry, and navigating complex terrains. Deploying neural interfaces in the lower limbs of individuals with limb loss presents unique challenges, particularly due to repetitive muscle contractions and the natural range of motion in the knee and hip joints for transtibial and transfemoral amputees, respectively. This study characterizes the long-term performance of these peripheral nerve interfaces, which is crucial for informing design modifications to optimize functionality.<i>Approach.</i>We evaluated the longitudinal performance of 16-contact nerve cuffs and their associated components implanted in four participants with unilateral transtibial limb loss over five years. Key outcome measures included charge density at sensory thresholds and electrical impedance.<i>Main results.</i>Out of 158 channels (i.e. individual contacts within the nerve cuffs and their corresponding leads), 63% were consistently responsive, 33% were partially responsive, and 4% were non-responsive. Smaller connector assemblies and increased lead length near the cuffs significantly enhanced performance, with the final two participants demonstrating notably improved responses where 77% and 96% of channels were consistently responsive, respectively, compared to 50% and 6% in the first two participants.<i>Significance.</i>Overall, the implanted nerve cuffs showed robust stability in the residual limbs of highly active individuals with limb loss. Furthermore, employing strategies to reduce stress on transition points in the components significantly improved overall system performance.