This method uses the transverse processes, psoas muscle, and lumbar plexus to calculate a safe zone for entering the psoas muscle through a dorsal-spinal approach. The main advantage of this technique is that it improves patient safety by providing the measurements for calculating a safe zone for the irrigation and debridement of a psoas abcess. Before beginning the procedure, place the previously thawed cadaveric specimen on the surgical table in the supine position and illuminate the field with surgical lights, then cover the specimen with a blanket, sparing the upper and lower abdomen.
For the isolation and identification of key anatomical structures using a ventral approach, use a blade to make a midline cranial to caudal incision down to the hypogastric region and directly perpendicular to the anterior superior iliac spines. Make a transverse cut between the anterior superior iliac spines and use a surgical blade and scissors to reflect and remove the abdominal musculature and fascia to improve visualization and to aid in measurement accuracy. Identify the transverse processes, laminae, pars interarticularis of the lumbar vertebrae, psoas muscles, and the nerve roots of the lumbar plexus exiting the neuroforamina.
Once all of the key anatomic structures have been identified as undamaged and intact, carefully turn over the specimen to begin the dorsal approach. With the specimen in the prone position, use a surgical blade, scissors, Cobb Elevator, and other surgical tools to resect the latissimus dorsi and paraspinal muscles, starting sequentially from the posterior spinous process to the transverse processes. Use the Cobb Elevator and other surgical tools as appropriate to isolate the transverse processes from any remaining soft tissue attachments to aid in visualization and accurate measurements.
With the paraspinal muscles now removed, place a surgical ruler parallel to the surgical field to obtain digital photographs of the intact psoas muscle to demonstrate its relationship to the transverse processes at each vertebral level. When a sufficient number of good quality images have been acquired, turn the specimen back to the supine position, use a surgical blade to strip the psoas muscle from its medial attachment at each lumbar body and use blunt-edged scissors to carefully expose the entire lumbar plexus and to reflect the psoas muscle laterally. With a surgical ruler placed parallel to the surgical field, take additional photographs to capture the relationship of the lumbar plexus to the transverse process at each level.
Finally, turn the specimen on its side and obtain additional photographs to demonstrate the relationship of the transverse processes to the lumbar plexus in the sagittal plane. For calibration of the tissue measurements, open the image to be analyzed in ImageJ and use the magnifying glass tool to zoom into the surgical ruler in the photograph. Use the line-drawing tool to carefully draw a straight line between the margins of a one-millimeter segment on the ruler and, under Analyze, select Set Scale to set the known distance to one and the unit of length to one millimeters, then place a check in the box next to Global and click OK.The measurements will be calibrated according to the number of pixels per millimeter on the ruler.
To calculate the maximal irrigation and debridement and safe zone, use the line-drawing tool to draw a straight line between the lateral superior tip of each transverse process to the lateral edge of the psoas at the same level. When all of the lines have been drawn, select Measure from the Analyze menu. A new Results window will open and the measurements will appear in millimeters in the new window in the Length column in the order that they were obtained, then repeat the calculation for each of the measurements indicated in the table.
The area for maximum medial to lateral irrigation and debridement in the coronal plane is determined by measuring the distance from the superior and inferior tips of the transverse processes to the lateral edge of the psoas muscle at the corresponding vertebral level. On average, the maximal extent of lateral debridement can be performed 5.5 and 5.7 millimeters medial to the inferior and superior tips of the transverse processes at L1, 4.7 and 5.1 millimeters medial at L2, 1.8 and 2.5 millimeters medial at L3, 0 and 0.4 millimeters medial at L4, and 3.8 and 3.7 millimeters lateral at L5.In the sagittal plane, the lumbar plexus is typically approximately 15 to 20 millimeters anterior to the transverse processes. Remember to be careful when working around the transverse processes as they can be fragile and any damage would disqualify any measurements obtained in relation to these landmarks.
