Sacroiliac Joint Fusion With iFuse Implant System (SIFI)

Inclusion Criteria: 1\. Age 21-70 at time of screening 2\. Patient has lower back pain for \>6 months inadequately responsive to conservative care 3\. Diagnosis of sacroiliac joint disruption or degenerative sacroiliitis based on ALL of the following: 1. Patient has pain at or close to the posterior superior iliac spine (PSIS) with possible radiation into buttocks, posterior thigh or groin and can point with a single finger to the location of pain (Fortin Finger Test), and 2. Patient has at least 3 of 5 physical examination maneuvers specific for the SI joint (see Table 3), and 3. Patient has improvement in lower back pain numeric rating scale (NRS) of at least 50% after injection of local anesthetic into affected SI joint(s) (see Section 3.6.4), and 4. One or more of the following: i. SI joint disruption: 1. Asymmetric SI joint widening on X-ray or CT scan 2. Leakage of contrast on diagnostic arthrography ii. Degenerative sacroiliitis: 1. Radiographic evidence of SI joint degeneration, including sclerosis,osteophytes, subchondral cysts, or vacuum phenomenon on CT or plain film, or 2. Due to prior lumbosacral spine fusion 4\. Baseline Oswestry Disability Index (ODI) score of at least 30% 5. Baseline SI joint pain score of at least 50 on 0-100 mm visual analog scale 6. Patient has signed study-specific informed consent form 7. Patient has the necessary mental capacity to participate and is physically able to comply with study protocol requirements Exclusion Criteria: 1. Severe back pain due to other causes, such as lumbar disc degeneration, lumbar disc herniation, lumbar spondylolisthesis, lumbar spinal stenosis, lumbar facet degeneration, and lumbar vertebral body fracture 2. Other known sacroiliac pathology such as: 1. Sacral dysplasia 2. Inflammatory sacroiliitis (e.g., ankylosing spondylitis or other HLA-associated spondyloarthropathy) 3. Tumor 4. Infection 5. Acute fracture 6. Crystal arthropathy 3. History of recent (\< 1 year) major trauma to pelvis 4. Previously diagnosed osteoporosis (defined as prior T-score \<-2.5 or history of osteoporotic fracture). 5. Osteomalacia or other metabolic bone disease 6. Chronic rheumatologic condition (e.g., rheumatoid arthritis) 7. Any condition or anatomy that makes treatment with the iFuse Implant System infeasible 8. Chondropathy 9. Known allergy to titanium or titanium alloys 10. Use of medications known to have detrimental effects on bone quality and soft-tissue healing 11. Prominent neurologic condition that would interfere with physical therapy 12. Current local or systemic infection that raises the risk of surgery 13. Patient currently receiving or seeking worker's compensation, disability remuneration, and/or involved in injury litigation. 14. Currently pregnant or planning pregnancy in the next 2 years 15. Patient is a prisoner or a ward of the state. 16. Known or suspected drug or alcohol abuse 17. Diagnosed psychiatric disease (e.g., schizophrenia, major depression, personality disorders) that could interfere with study participation 18. Patient is participating in an investigational study or has been involved in an investigational study within 3 months prior to evaluation for participation