Acetaminophen combined with tramadol is more effective than acetaminophen or tramadol to reduce neuropathic root pain: an experimental study with application of nucleus pulposus in a rat model. - Sato R; Sekiguchi M; Konno SI (European spine journal : official publication of the European Spine Society, the European Spinal Deformity Society, and the European Section of the Cervical Spine Research Society, 2020)