Extending Mirror Therapy into Mixed Reality-Design and Implementation of the Application PhantomAR to Alleviate Phantom Limb Pain in Upper Limb Amputees

Phantom limb pain (PLP) is a restrictive condition in which patients perceive pain in a non-existent limb, incapacitating them from performing daily activities. Mirror therapy, during which patients look into a mirror reflecting their sound limb and imagining it as healthy on their amputated site, has proven to alleviate that pain. However, it is limited to unilateral movements which take place in a seated position We developed an assistive mixed reality (MR) tool on the Microsoft HoloLens 2 to extend conventional mirror therapy by enabling users to freely explore their environment and to perform bi-manual tasks. Thereby, the patient's residual limb was augmented by a superimposed virtual arm that is controlled by the residual limb. We evaluated the usability of this system with ten able-bodied individuals and two transradial amputees. Patients additionally rated the system for its motivational aspect using the IMI questionnaire and a user-centered survey. PhantomAR showed a high usability rate of 78.5%; immersion, positive affect and game flow were rated most highly in both patients, while PLP slightly decreased after using the application. We critically examined the use and implementation of a therapy environment on HoloLens 2 and proposed how to address potential pitfalls in development. Based on these findings, we expect the PhantomAR mixed reality therapy tool to positively impact the outcomes on PLP scores and motivation to carry out the therapy even in absence of a therapist.