A "joystick" technique to aid in odontoid resection for basilar invagination
The traditional method for transoral dens resection is to bur it down or transect it at the base and pull it out with a clamp. With burring, the tip can break off and retract into the foramen magnum. Pulling the fragment with a clamp can be cumbersome; the dens can slip out and "bounce" back into the cord. Because these are rare cases, we sought to develop and test a reliable technique that novice surgeons could easily perform.
MATERIALS AND METHODS
Four spine fellows and 1 experienced surgeon performed transoral approaches to the odontoid in 5 cadaveric specimens. None had experience with dens resections. One specimen couldn't be used due to an anomalous C1-C2 articulation. In the rest, the C1-2 area was exposed. An artificial basilar invagination was created by removing 5 mm of the C1-2 joint bilaterally to allow superior migration of the odontoid. This basilar invagination was visualized under fluoroscopy while pushing the C2 body superiorly to bring the odontoid tip beyond the clivus(figure). The anterior ring of C1 was removed with a high-speed drill. A 4.0 mm diameter, 18 mm length screw was placed into the odontoid a few mm above the base, in the midline (figure). The screw had tight purchase into the cortical bone, despite the osteoporotic cadavers. The odontoid was then transected at its base using a high-speed drill. Our screwdriver had a tip that expands to lock it into the screw that allowed us to keep it as one unit. The odontoid was freed from its soft tissue and ligamentous attachments using curettes and removed using the screw/screwdriver as a "joystick".
Even in the hands of those inexperienced in dens resection techniques, in 3 of the 4 specimens, the odontoid was easily and expeditiously removed. The screw, used as a "joystick," provided an excellent anchor to grab, as well as manipulate the loose fragment. Compared to grabbing the wet, oily, rounded and slippery odontoid itself, we felt that this technique provided a more secure and reliable technique. This became more evident with our 4th specimen when the screw cut out of the bone, making the odontoid removal much more difficult. The error with this specimen was that the fellow inserted the screw too caudally, close to where the dens was transected, resulting in screw cut-out while manipulating the fragment during extrication. We noted that in the other 3 specimens, the screw had been placed at least 3 mm above the transection.
Even in the hands of those inexperienced in odontoid resection, the dens was easily extricated in ¾ of the cases. The screw should be placed >3mm from the transection to prevent screw cutout. Because transoral dens resection is rarely indicated, it is quite conceivable that most surgeons performing the procedure will be inexperienced in the technique, mimicking our situation. While we used a screw that rigidly attaches to the screwdriver, a tap can suffice. Clinical trials are necessary to determine its true potential.