Inclusion Criteria:

• Age 21-75 at time of screening.
• Patient scheduled for multilevel (> 3 levels) spinal fusion surgery with planned fixation to the pelvis using S2AI screws.
• Patient has signed study-specific informed consent form.
• Patient has the necessary mental capacity to participate and is physically able to comply with study protocol requirements. 

Exclusion Criteria:

• Indication for multilevel spine fusion surgery is any of the following: 1. Congenital neuromuscular disease 2. Prior pelvic fixation (i.e., patient already has S2AI or iliac bolts in place, current surgery indicated to revise this hardware) 3. Grade IV spondylolisthesis.
• Prior sacroiliac joint fusion/fixation on either side.
• Presence of spinal cord stimulator.
• Presence of severe hip pain that could impair functional and quality of life improvement from complex spine surgery.
• Surgeon plans to use iliac screw for pelvic fixation.
• Any known sacral or iliac pathology.
• Any condition or anatomy that makes treatment with the iFuse Implant System infeasible.
• Known metabolic bone disease.
• Severe osteoporosis.
• Known allergy to titanium or titanium alloys.
• Use of medications known to have detrimental effects on bone quality and soft-tissue healing.
• Neurologic condition that would interfere with postoperative physical therapy.
• Current local or systemic infection that raises the risk of surgery.
• Patient currently receiving or seeking short- or long-term worker's compensation and/or currently involved in injury litigation related to the SI joint or low back pain.
• Currently pregnant or planning pregnancy in the next 2 years.
• Prisoner or a ward of the state.
• Known or suspected drug or alcohol abuse.
• Uncontrolled psychiatric disease that could interfere with study participation.
• Fibromyalgia.