LAUSR

I really appreciate the questions raised in the letter about our paper [1]. As the designer of the technique, I am more than happy to answer. The rationale behind all this comes from my personal experience in performing this kind of lateral mass posterior fixation. It is a concept-based technique: the deformation that is usually found in lateral masses of real patients, osteophytes and underlying pathology makes it really difficult to define precise midpoints and measure 1 or 2 mm distances as described in classic techniques, let alone measuring any angle. Also, leaning on spine processes, as suggested by others, may be also hindering and misleading. So, I found really not reproducible any kind of measurement in an anatomically distorted scenario. The conceptual idea is to build a 3D image of the actual lateral mass you need to work on in your brain, with the root and vertebral artery paths added, and work on it, letting aside any other consideration. The key point being dissection of the whole posterior lateral mass, and then, identifying the valley between lamina and mass and the ridge of the lateral mass, later on going lateral to define the lateral border. With this in the visual field, you build a slanted cube in your mind and define the longest trajectory, which is a diagonal in all three dimensions. That defines your trajectory from bottom medial to top lateral [2]. Now, let me outline my personal pearls, as I have been using this in my practice for years. As for the entry point, care should be taken regarding the joint, so a little safety margin had to be kept. Then you put a Penfield at the upper lateral corner of the mass and you aim at its ventral zone; in this step, in order to get proper drill bit alignment, sometimes you have to go through the posterior interspinous ligament, sometimes the posterior spinous processes need to be trimmed or a trough burred in the upper border, and in order to ensure that the drill bit goes where you want, most of the times you have to skip drill guides because then the diameter of the composite drill + guide does not allow for proper trajectory alignment due to impingement against anatomical structures and the drill bit is forced out of the desired direction. After the trajectory is selected, the entry point cortex is burred with high speed power drill and the drill bit is advanced with a powered device, kept at low speed with your finger pressure control of power, with small pushes by the leading hand and a security hand holding in front of the other to stop when any loss of resistance is sensed; the aim is always a bicortical purchase. A long drill bit is preferred (10–15 cm) in order to allow for some flexible bending to ensure the trajectory in case the shaft of the drill bit abuts against any bony prominence that could deviate the tip. Having said all that, now I come to answer the specific questions. The entry point: Yes, as you go caudally, you need a more angulated trajectory, sometimes I open the skin and make a stab wound in the fascia (I avoid any implant or instrument touching the epidermis) to get the desired trajectory, and sometimes a trough is burred in the spinous process, as described above. The facet joint: Visual control of entry point allows for a safety margin, and also aiming to the upper lateral corner takes you away from the joint, although sometimes aiming up is tough and limited; a homogeneous resistance is felt when you are in spongious bone, if resistance is felt too * Máximo-Alberto Díez-Ulloa [email protected]