Indication

Central stenosis due to ligamentum flavum thickening without considerable listhesis, disc herniation, or deformity. 

Measure the pelvic incidence-lumbar lordosis mismatch on standing films. Patients with a PI-LL mismatch of <5 are good candidates for the procedure. If there is a considerable mismatch they need to be given lumbar lordosis and require instrumentation. 

This patient had no PI-LL mismatch and a good sagittal balance. MRI demonstrated severe canal stenosis with redundancy of the nerve roots. 

Positioning

Prone, bumpy Jackson table. Chest pad is high and hip pads are low

Localization

Preop: Anatomic landmarks only, no x-ray. Check the location of the top of the iliac crest on axial MRI, see which disk space this corresponds to and mark appropriately. 

Intraop: flat plate. Place an Allis clamp at the expected location of the pedicle in the same orientation as for a screw. In the picture below, the Allis clamp was not correctly oriented. 

Bear in mind that the lamina of that level is caudal to the pedicle. So, in this case, the L4 lamina is caudal to the location of the Allis. 

Monitoring

SSEPs only, no motors. 

Procedure

Expose in standard fashion, taking special care not to violate the posterior tension band. Exposed the medial facets but do not violate the joint capsules. 

Use the M8 to drill an upside down U shaped trough in the appropriate hemilamina to expose the ligamentum flavum. 

Take care not to violate the pars interarticularis. You will take some medial facet, this is unavoidable. 

The apex of the "U" should extend rostrally until you see epidural fat. 

Place a blunt nerve hook under the ligamentum at the apex of the "U", in the epidural fat, pointed rostrally. Then, rotate the nerve hook medially and use it to scrape the attachment of the ligamentum off the deep surface of the superior lamina. Sweep the hook medially then inferiorly to disconnect the top of the ligament. 

Remove the ligament with pituitary and Kerrison rongeurs. 

Do the same thing on the contralateral hemilamina. Once adequately decompressed, the thecal sac should be loose and you should be able to place a Woodson in the epidural space, reach under the intact spinous process, and see its tip from the other side. You may only see the dura moving on the other side and that's okay too. 

Verify appropriate levels of decompression by placing a Woodson in the epidural space at the topmost decompressed level and another instrument at the bottom-most, then take a flat plate x-ray. 

Once all levels are decompressed place two drains, one on each side of the spinous processes, and close.