The Neurolyser XR is a device used to deliver high-intensity focused ultrasound for the ablation of the lateral sacral branch nerve in patients with Sacroiliitis.

The sacroiliac joint connects the sacrum (a triangle shaped bone) with the iliac bone on both sides of the lower spine. These joints transmit all the forces from the upper body to the pelvis and their respective legs. Repeated trauma and aging affect this joint resulting in inflammation and arthritic changes. These changes result in misalignment of the joint or limitation to normal motion which can create pain typically very low in the back or in the buttocks,

High-Intensity Focused Ultrasound (HIFU) technology uses sound waves. These sound waves are concentrated into a specific spot in order to raise the local tissue temperature causing a burn, this burn is expected to intentionally damage the nerve causing pain.

The HIFU device used in this study is called, the Neurolyser XR. It focuses sound wave energy directly on the nerve of the painful sacroiliac joint, causing an increase in temperature, which is expected to directly damage the nerve fibers which carry pain signals to the brain.

Inclusion Criteria:

1. Men and women aged 18 and older
2. BPI < 40
3. Patients who are able and willing to give consent and able to attend all study visits.
4. Patients who are able to communicate with the treating physician.
5. Patients must have chronic lower back pain (LBP) attributed to degenerative sacroiliitis diagnosed by a pain physician using the following guidelines:
6. At least 1 positive sacroiliac joint provocation test (Gaeslen's test, FABER (Patrick's test), and pelvic rock)
7. Back pain predominantly below L5
8. 50% relief following either sacroiliac joint (SI) joint injection and/or lateral branch blocks corresponding to the expected duration of the medication utilized. (i.e SI joint injection-expected pain relief duration of steroid, lateral branch blocks-expected pain relief duration of local anesthetic utilized).
9. All other possible sources of low back pain have been ruled out as the primary pain generator, including but not limited to: the intervertebral discs, bone fracture, the zygapophyseal joints, the hip joint, symptomatic spondylolisthesis, tumor, and other regional soft tissue structures (this is done by physical exam, medical history, and MRI/CT/X-ray as required)
10. Patients with NRS (0-10 scale) LBP average score ≥ 4
11. Patients with chronic LBP for at least 12 months.
12. Patients with Oswestry Disability Questionnaire (ODQ) for LBP score > 40%

Exclusion Criteria:

1. Patients on dialysis
2. Patients with evidence of lumbosacral radiculopathy on MRI, CT or physical exam findings, including radicular leg pain (symptomatic moderate or severe foraminal or central canal stenosis).
3. Spinal pathology that may impede recovery such as spina bifida occulta, grade II or higher.
4. Spondylolisthesis at L5/S1, or scoliosis
5. Patients with radiofrequency ablation (RF/RFA) treatment for sacroiliitis LBP
6. Patients with bilateral sacroiliitis LBP
7. Patients with previous low back surgery
8. Patients who are pregnant
9. Patients with existing malignancy
10. Patients with allergies to relevant anesthetics
11. Patients with motor deficit or any other indication for surgical intervention
12. Patients with contraindications for MRI
13. Patients with an acute medical condition (e.g., pneumonia, sepsis, systemic infection, uncontrolled immunosuppression-AIDS, cancer, DM) that is expected to hinder them from completing this study.
14. Patients with unstable cardiac status including:
15. Unstable angina pectoris on medication
16. Patients with documented myocardial infarction less than 40 days prior to protocol enrolment
17. Patients with Severe Congestive Heart Failure, New York Heart Association (NYHA) class 4.
18. Patients on anti-arrhythmic drugs or with uncontrolled and/or untreated arrhythmia status
19. Patients with severe cerebrovascular disease (CVA within last 6 months)
20. Patients with severe hypertension (diastolic BP > 100 on medication)
21. Patient Body Mass Index > 45 or <20
22. Patients with known intolerance or allergies to the MRI contrast agent (e.g. Gadolinium or Magnevist) including advanced kidney disease
23. Patients with an active infection or severe hematological, neurological, or other uncontrolled disease.
24. The patient is unable to communicate with the investigator and staff.
25. Patients who are not able or willing to tolerate the required prolonged stationary position during treatment (approximately 2 hours)
26. Patient with acute pelvic inflammatory conditions
27. Ongoing/ unresolved Worker's compensation, injury litigation, military medical board, or disability remuneration claims
28. Morphine Milligram Equivalents (MME) > 60 per 24 hrs.
29. Patients with platelets < 100.