Ultrasound Prescan vs Conventional Landmark-guided Technique in Spinal Anesthesia in Orthopedic Obese Patients

This is a open-label randomised controlled study. 90 patients will be recruited and divided into two groups of 45 patients per group. Patient recruitment will be performed by patient visit and checking inclusions and exclusion criteria. After written informed consent, every single patient will be allocated in one of the two groups following a computer generated randomization list. Patients in both groups will undergo: * standard monitoring with Non Invasive Blood Pressure (NIBP) cuff, three-lead electrocardiography (ECG) and Pulse-oximetry (SpO2). * Peripheral intravenous access * Sedation (as prescribed by the OR anesthesiologist) * Spinal anesthesia performed by anesthesiologists skilled in both techniques (conventional landmark technique or Accuro guided) at the L3-L4, L4-L5 or L5-S1 level, with a 25 or 27 Gauge needle, with the surgical side declive or proclive according to the baricity of the local anesthetic used (sitting position, when needed, will be reported in the CRF) In both groups an observer will monitor and register spinal procedure duration (starting point: anesthesiologist wearing sterile gloves, ending point: end of local anesthetic injection). Control group (landmark technique): The anesthesiologist will identify lumbar spinous processes with traditional landmark palpation. Once the correct interspinous level and the mid-line will be identified, the anesthesiologist will proceed with needle insertion and spinal injection. Treatment group (ultrasound pre-procedural scan with Accuro): By using Accuro US probe the anesthesiologist will perform a pre-procedural lumbar spine scan to detect the needle entry site. After image optimization, the Accuro probe will be aligned with the spine mid-line, as indicated by a dashed red-line on the screen. After that the interlaminar space at the desired intervertebral level will be detected, as indicated by orange overlay in the screen. The depth and the device angle used to detect the interlaminar space will be annotated. The anesthesiologist will then disengage the Accuro Locator needle guide and press gently against the skin. Then the probe will be removed and the spinal injection will be performed.