The overall goal of this Lateral Retroperitoneal approach, Drill Bit Intervertebral Disc Injury method is to create a safe, consistent, and reproducible Ovine model of degenerative intervertebral discs. This Ovine Lumbar Intervertebral Disc Degeneration model can help answer key questions in the field of translational spinal research by providing another method for investigating the path or physiology of disc degeneration and the potential investigation of regenerative therapies. The main advantage of this approach is that it allows for consistent surgical injury through a safe, surgical approach in an appropriate animal model.
After anesthetizing and intubating a sheep and preparing the operating suite according to the text protocol, place the animal on the operating table in the right lateral position. Using electronic clippers, shave the region defined superiorly by the lower ribs, inferiorly by the Iliac bone, medially by the Contralateral Lumbar Transverse Processes and approximately 10 centimeters lateral to the Ipsilateral Lumbar Transverse Processes. Palpate the Iliac crest, lumbar transverse processes, and cross a vertebral angle for the landmarks for this surgical incision site.
Then, use a sterile pen to mark these landmarks. Next, with chlorhexidine and alcohol iodine antiseptic wash, disinfect the lateral abdomen. Then, place a sterile fenestrated square drape over the surgical site and a large sterile square drape on the overhead table followed by the sterile instruments.
Under surgical loupe magnification and a headlight, inject local anesthetic, and then use a 22 scalpel blade attached to a four scalpel handle to make a longitudinal incision parallel to and one centimeter anterior to one to two lumbar transverse processes above and below the intervertebral disc levels of interest. Using monopolar diathermy, divide the underlying subcutaneous tissue and the lateral aspect of the abdominal wall musculature. Directing the dissection towards the tips of the lumbar transverse processes, above and below the intervertebral discs of interest.
Next, divide the thoracolumbar fascia longitudinally at it's attachment to the transverse processes. Visualize and preserve the underlying quadratus lumborum, soleus muscles and the traversing neurovascular bundles. Using diathermy, maintain hemostasis through the procedure.
Sweep the fingers between the plane of the peritoneum and posterior abdominal wall musculature at the exposed intervertebral disc levels to perform digital blunt dissection. Now, using diva retractor, retract the quadratus lumborum and soleus muscles posterior laterally to further expose the intervertebral discs. Then, palpate for the concave intervertebral bodies and the convex intervertebral discs.
Position the retractors immediately over the discs and take care to ensure lumbar vessels are not damaged. Then, using surgical loupe magnification with the headlight, identify the lumbar vessels which are located approximately one centimeter caudal to the inferior end plate. After performing an intraoperative lateral x-ray to confirm the disc level, depending on the disc levels desired, expose the intervertebral discs by separating the surrounding structures.
For levels L three, four, and above, use digital blunt dissection to sweep aside the muscular attachments over the discs. For levels L four, five, and below, use bipolar diathermian scissors to sharply divide the thicker tendonus muscular attachments over the disc. Define the drill bit injury point by observing the left lateral and interior extremities at the intervertebral disc.
The disc level should be confirmed with lateral x-ray prior to drill bit injury. Fit a brad point drill bit into the power drill. Ensure the diameter of the drill bit is slightly less than the intervertebral disc height.
For example, 3.5 millimeters for lumbar intervertebral discs in 60 to 70 kilogram sheep. Apply a drill bit stop to provide an unprotected drill bit length of approximately half the diameter of the lumbar intervertebral disc. For example, 12 millimeters for lumbar intervertebral discs in 60 to 70 kilogram sheep.
Next, apply the drill bit to the entry point and direct it in the trajectory slightly cranial to the center of the intervertebral disc, which will minimize the risk of end plate injury. With the drill on low power, slowly advance the drill bit into the intervertebral disc for one second. If excessive resistance is encountered, adjust the trajectory in a slight cranial or caudal fashion.
Once hemostasis is achieved, use Ringer's solution to irrigate the wound. Then, perform layered closure, preferably using 2-0 absorbable synthetic braided sutures to the lateral abdominal wall tissues and continuous 3-0 undyed absorbable synthetic subcuticular suture to the skin. Shown here is a pre-operative sagittal plane slice from a 3T MRI of ovine lumbar one half to the lumbosacral intervertebral disc with no evidence of significant pre-operative intervertebral disc degeneration.
This intraoperative lateral radiograph confirms the intervertebral disc level and enables calculation of the disc height index. The gross morphological appearance presented here demonstrates the drill bit injury tract penetrating the annulus fibrosus and extending into the nucleus pulposus. The injury is also evident in this 9.4T MRI.
While attempting this approach, it's important to direct the drill bit cranially and advance the drill bit slowly to minimize the risk of end plate injury. After watching this video, you should have a good understanding of how to produce drill bit injury in the ovine model through a lateral retroperitoneal approach. Don't forget that working with scalpels and cautery can be hazardous.
Personal protective equipment and appropriate safety precautions must be followed at all times. It is important to maintain surgical sterility.
