DANTE Study: The First Randomized, Double-Blind, Placebo and Active-Controlled, Parallel Arm Group Study Evaluating the Analgesic Efficacy and Safety of Dexketoprofen TrometAmol aNd Tramadol Hydrochloride Oral FixEd Dose Combination on Moderate to Severe Acute Pain in Patients with Acute Low Back Pain-Rationale and Design.