Endoscopic Lumbar Canal Decompression
Pathophysiology of lumbar stenosis is complex and multifactorial. Compression of neural elements generally occurs from a combination of degenerative changes, including ligamentum flavum hypertrophy, bulging of intervertebral disc and facet thickening with arthropathy. It has become evident that the major neurological compression is typically seen at the level of interlaminar window. Considering this fact multilevel focal laminotomy is an alternative to wide laminectomy. Such laminotomies also preserve the midline bony and ligamentous complex, thereby allowing for improved postoperative muscle reattachment and function.
Ideal operation for the lumbar canal stenosis would be one that could simultaneously achieve adequate decompression of the neural elements and minimize damage to posterior muscular, ligamentous and bony complex.
The facet of superior articular process forms the roof of lateral canal / recess, while it is bounded anteriorly by posterior surface of vertebral body and disc. The nerve root leaves the sac approximately at the level of disc and then enters the lateral recess which turns into inter-vertebral canal at the level of superior margin of pedicle.
Under pathological conditions, the inter- vertebral disc, with increased loading of joints, the medial border of superior articular process develops a spondylotic scalloping and as a result of this with added disc protrusion the nerve root is compressed. ( Microsurgical Anatomy of Lumbar Stenosis- Hans-Jorgen Rulen MD, Adolf Muller MD, -Neurosurgery- 39: 345-351-1996)
Patient is placed in Knee-Chest position, with abdomen lax and slight interspinous distraction. Exact localization of each of the disc space is done with help of localizing pin under image guidance. For L4/5 and L5/S1 levels same incision can be used. If the angle of L5 to sacrum is more obtuse then two separate incisions can be used. Length of the paramedian incision and the distance away from midline depends upon the patients corpulence. After putting the outer tube, we should see, in the cranial half of the tube the exposed lamina, in the caudal half the ligamentum flavum with the interlaminar window, laterally the medial aspect of the facet joint and medially the base of the spinous process.
Laminectomy with routine 3mm Kerrison bone punch or 3mm. micro drill is started from the medial most part of the lamina where it is attached to the base of the spinous process and then carried out laterally to detach the ligamentum flavum. After detaching the ligamentum flavum, cotton wool is inserted to push the dura anteriorly which gives access to the epidural space. As the canal is narrow, at each step dura is protected with cotton wool. Ligamentum flavum is excised with 90 degree Kerrison's punch as much laterally as possible. Hypertrophied medial facet is undercut to appreciate the lateral margin of the dura . Dura should not be retracted medially unless and until you see and appreciate the lateral margin of the dura to prevent dural injuries.
Undercutting of the base of the spinous process and opposite lamina is done with 45 degree punch or 3mm. micro drill. Lateral recess decompression is achieved by excision of the lig. flavum. After adequate lat. recess decompression root is mobilized with penfield nerve root retractor. For L4/L5 and L5/S1 after excision of the L4 lamina, lig. flavum is excised caudally to reach cranial end of L5 lamina. L5 hemilaminectomy is done to decompress the traversing L5 root and S1 root. Endospine is inclined to opp. side to undercut the L5 lamina and L4 lamina to decompress the opp. traversing root.
ADVANTAGES OF THE ENDOSCOPIC CANAL DECOMPRESSION
Easier access to opposite side from same side, by tilting 30-450
We can address the damaged areas that we cannot access by direct vision. It is possible to resect the hypertrophied lig. flavum and superior facet of contralateral side. More over we can confirm the compressed nerve root directly under the hypertrophied superior facet, which is not possible by direct vision. We can keep track of anatomical position and perform the decompression procedure while observing the compressed nerve root on video monitor during endoscopic surgery.
The method has the added advantage that a two segment approach can be carried out with one skin incision for neighboring segments.