Percutaneous Transosseous Translaminar Approach for Thecal Sac Access in Advanced Ankylosing Spondylitis with Instrumented Posterior Spinal Fusion

A, Axial CT scan through the L2–3 level with the patient prone shows the cannula (arrow) traversing a 2.5-cm thickness of bone graft, lamina, and ossified ligamentum flavum. B, Axial CT scan demonstrates placement of the 22-gauge spinal needle (arrowhead) through the cannula (arrow) and into the SAS. The needle tip was ventral, and though we did not puncture the ventral dura, that is a theoretic risk. In future attempts, we plan to measure the distance to the middle of the thecal sac and use a depth gauge or collar to identify our intended depth. We will then secure that depth with a clamp on the spinal needle so that it cannot inadvertently advance with gravity. C, Sagittal reconstruction of the CT myelogram demonstrates subarachnoid compartmentalization, adhesions, and a compete myelographic block ventrally at T8 (arrow) and dorsally at T12 (arrowhead).