A total knee arthroplasty does in no way preclude an MRI of the lumbar spine. Patients with metal in their back from prior spine surgeries can have an MRI.
Just as an understanding of the CT scan, a CT scan can be performed in the case of metal in the spine but will not define the nerve roots. A myelogram would be needed prior to the CT scan to visualize the nerve roots.
Dr. Corenman