Relievant Medsystems is a medical startup that is planning large-scale clinical trials of its minimally-invasive device to treat lower back pain. Its device involves a spinal probe that uses radiofrequency ablation to deactivate nerves that cause back pain. The treatment is meant for early-stage back-pain sufferers who have not responded to noninvasive therapies. Relievant is located in the Redwood Junction office park off Middlefield.