Our study aims to investigate the safety and efficacy of high intensity focused ultrasound in the management of low back pain due to sacroiliitis. Sacroiliitis is the inflammation of a joint in the lowest part of the spin and it is one of the cause of low back; the diagnosis is essentially based on clinical examination and the confirmation of this diagnosis relies on a significant pain relief following an infiltration of local anesthetic in the joint. One of the potential treatment is to ablate the nerves which carry the painful stimuli from the joint to the central nervous system through; these nerves enter the spinal canal through the sacral foramina which are two series of four pairs of openings located on the sacrum, the triangular bone at the base of the spine. Traditionally, these nerves are ablated by the heat produced by radiofrequency or the cold produced by cryoablation; both techniques require percutaneous needle insertion in close proximity to the targeted nerves, posing potential discomfort and risk of infection for the patient. High intensity focused ultrasound delivered by the device Neurolyser XR uses ultrasounds which are converged to the targeted point thanks to a gel pad positioned on the back of the patient; this mechanism of action does not require sterile conditions as the skin is not pierced and the discomfort of patient is dramatically reduced during the procedure for there is non need to insert and move any needle to properly position it. The correct orientation of the ultrasound beam is guaranteed by the alignment of the targeting system of the device and the X-ray image. The lesion is produced only if there is a bone right under the focus of the ultrasound beam and this is a safety system which assures that no direct injuries of other structures (e.g., vessels, spinal root) can be caused. To date, Neurolyser XR has been extensively used to treat facet joint syndrome through the ablation of the medial branches. However, its application in sacroiliitis has not yet been as deeply investigated.

Inclusion Criteria:

• Adult males and females, legally able and willing to participate in the study and come for follow-up visits
• Able and willing to fill the research questionnaires and to communicate with investigator and research team
• Patient with bilateral or unilateral sacroiliac joint pain of > 6 months duration
• Patients presenting with a) a positive (>70% pain relief) to a previous nerve ablation procedure of the sacroiliac joint and / or b) with a positive (>70% pain relief) to a previous nerve block procedure of the sacroiliac joint within the last six months)
• Average pain score of 4 or higher in the last month, (on 0-10 scale).

Exclusion Criteria:

• Pregnant or breastfeeding patient
• Patients presenting with neurological deficits (including lumbosacral radiculopathy but not solitary radicular pain).
• Patients with history of lumbar and / or sacral spine surgery
• Patients with the presence of metal hardware at the lumbosacral spine
• Patients with history of pelvic pathology that may increase procedural risk and / or influence symptoms and / or generate unrelated adverse event (per the discretion of the study principal investigator)
• Patients unable to understand and complete the research questionnaires in Italian.
• Patients presenting with any severe medical condition preventing the patient from safely and effectively being treated in the study or reporting study outcome.
• Patient with extensive scarring in the skin and tissue overlying the treatment area.
• Patients enrolled in or planned to be enrolled in another clinical trial during the duration of this research project.