A 71-year-old male presents with bilateral leg pain for the last two years. His pain is exacerbated when walking and is relieved when his sits or bends forward. He notes occasional periods where his legs feel weak, but motor examination reveals 5/5 motor strength throughout his bilateral lower extremities. He has diminished sensation on the medial aspect of his feet bilaterally. Management thus far has included NSAIDS with occasional narcotic usage, physical therapy, and two epidural steroid injections. Figure A shows a flexion radiograph, Figure B shows an extension radiograph, and Figures C and D show his current MRI scan. He feels his pain is substantially worse than it was one year ago. What is the most appropriate management at this time?