Fessler is a board-certified neurosurgeon who specializes in minimally invasive spine surgery. He is dedicated to finding innovative methods to repair spinal cord injuries, and he has been a pioneer in minimally invasive spine surgery. Fessler has made significant contributions to endoscopic and microendoscopic surgical developments, including microendoscopic discectomy and microendoscopic compression of lumbar stenosis. He has a special interest in spinal cord transplantation and was the first physician in the United States to perform human embryonic spinal cord transplantation.