Degenerative spondylolisthesis describes the slip of one spinal segment relative to the adjacent segment. It is usually associated with spinal stenosis (which limits one's ability to walk and stand) and is the most common indication for surgery in adults over age 65 years. A variety of surgical techniques are available; the most common are the Posterolateral Instrumented Fusion (PLF), and Interbody Fusion (IF) including Posterolateral Interbody Fusion (PLIF), and Transforaminal Interbody fusion (TLIF). IF uses a cage that is placed within the cleaned out disc space between the vertebral bodies being fused. Although this approach achieves a good fusion rate and deformity correction it is associated with a higher surgical cost and potential intra-operative complication rate. What is more, no consensus exists as to whether IF provides better patient rated functional outcome and quality of life. If the advantages of IF do not translate into superior patient rated outcomes, then the risk-benefit ratio would be tipped in favor of PLF. The purpose of this study is to determine if IF is equivalent to PLF for the treatment of degenerative spondylolisthesis. The investigators will conduct a prospective randomized control trial comparing these two procedures. Our primary outcome measure will be the Oswestry Disability Index, which evaluates spinal pain and function.

Inclusion Criteria:

1. Patients aged 50 years or older
2. Grade I or II (less than 50% slip of the cephalad vertebra compared to the caudal vertebra) degenerative spondylolisthesis at one or two contiguous levels between L1 and L5.
3. Lumbar spinal stenosis at the same levels of the degenerative spondylolisthesis producing radiculopathy or neurogenic claudication unresponsive to a minimum of 3 months of non surgical treatment
4. Patients who are medically suitable for surgical management
5. Patients who have consented for surgical treatment
6. Patients able to provide informed consent for the study and complete the questionnaires

Exclusion Criteria:

1. Lytic spondylolisthesis
2. Non degenerative stenosis (example: tumor, trauma, epidural lipomatosis)
3. Severe vertical foraminal stenosis necessitating interbody insertion to re-establish foraminal height
4. Segmental kyphosis at the level of the spondylolisthesis
5. Segmental scoliosis >10 degrees at the level of the spondylolisthesis
6. Rheumatoid arthritis
7. Active infection
8. On long term disability or workers compensation claim
9. Drug or alcohol misuse
10. Lack of permanent home residence
11. Previous surgery at the proposed surgical level
12. Previous fusion in the lumbar spine
13. Contraindication to surgery: medical co morbidities
14. Unable to complete questionnaire (e.g. Dementia)
15. Unable to give voluntary consent