The purpose of this study is to compare outcomes of subjects undergoing multilevel lumbar fusion (MLF) surgery with and without the iFuse 3-D implants in the "bedrock" trajectory.

This is a multicenter randomized controlled, postmarket clinical study to determine the differences between subjects undergoing standard MLF surgery with S2AI fixation in the pelvis vs the additional placement of iFuse 3-D in the bedrock configuration during MLF surgery. Subjects will be monitored for sacroiliac (SI) joint pain and at two years post surgery a CT scan will be taken to assess S2AI screw loosening or any other findings.

Inclusion Criteria:

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

Exclusion Criteria:

1. 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

2. Prior sacroiliac joint fusion/fixation on either side

3. Presence of spinal cord stimulator

4. Presence of severe hip pain that could impair functional and quality of life improvement from complex spine surgery

5. Surgeon plans to use iliac screw for pelvic fixation

6. Any known sacral or iliac pathology

7. Any condition or anatomy that makes treatment with the iFuse Implant System infeasible

8. Known metabolic bone disease

9. Severe osteoporosis

10. Known allergy to titanium or titanium alloys

11. Use of medications known to have detrimental effects on bone quality and soft-tissue healing

12. Neurologic condition that would interfere with postoperative physical therapy

13. Current local or systemic infection that raises the risk of surgery

14. 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.

15. Currently pregnant or planning pregnancy in the next 2 years

16. Prisoner or a ward of the state.

17. Known or suspected drug or alcohol abuse

18. Uncontrolled psychiatric disease that could interfere with study participation

19. Fibromyalgia

Plan to share data through Yale Open Data Access. Interested researcher may submit analysis plans to Yale.

After 2-year data are complete.

Interested researcher may submit analysis plans to Yale Open Data Access program