A 52-year-old man with chronic low back pain due to lumbar spondylosis presents to the pain clinic. He has been on oral paracetamol 1 g thrice daily and ibuprofen 400 mg twice daily for 6 months without adequate relief. Physical examination reveals tenderness over L4–L5 with restricted lumbar flexion. He denies radiculopathy or neurological deficit. What is the most appropriate next step in management?