Pre- and postoperative 3 ft standing scoliosis X-rays demonstrating an adult deformity case with global sagittal imbalance and the application of ACR technique at L3/L4 level
Pre- and post-op lateral X-rays showing with detail the amount of segmentary lordosis at L3/L4 after ACR procedure
27.3 Regional Anatomy
The MIS-ACR exposes spine surgeons to unfamiliar regional anatomy associated with the anterior spinal column such as visceral organs, the autonomic plexus, and the great vessels. The ALL itself is a very strong band of fibers that extend along the anterior aspect of the vertebrae consisting of three layers: superficial, intermediate, and deep. It is typically thicker and more narrow at the level of the vertebral body (VB) and thinner and wider at the level of the disc space and more adherent to the disc than the VB .
Directly anterior to the ALL at its lateral border lies the sympathetic plexus. It typically resides where the psoas major meets the ALL and is in direct communication with the lumbar plexus through white and gray rami communicantes (via the paravertebral ganglia). These communicating fibers reside along the lateral vertebral body and are rarely encountered at the level of the disc space where an ALL release is performed (Fig. 27.3).
Cadaveric anatomical dissection showing the anatomical relationship of the anterior longitudinal ligament and nearby structures
The great vessels (aorta, inferior vena cava, common iliac arteries/veins) reside along anterior lumbar vertebral bodies, immediately anterior to the ALL and the sympathetic plexus. The aorta typically bifurcates 18 mm rostral to the L4–5 disc space, and the inferior vena cava bifurcates within 2 mm of the L4–5 disc space . Between the ALL and the great vessels resides an adipose-lined anatomic plane that allows blunt dissection immediately dorsal to the vessels to safely isolate the ALL for sectioning.
27.4 Surgical Technique
The MIS-ACR entails a more extensive dissection of critical anatomic structures (listed above) from the lateral transpsoas retroperitoneal approach. This procedure is highly dependent on anterior-posterior (AP) and lateral fluoroscopy and detailed patient positioning. The patient is positioned in the lateral decubitus position in the same manner used for the traditional lateral transpsoas approach. Laterality is typically dictated by the concavity of the coronal deformity. We choose to approach from the concave side so that more levels can be accessed from a single incision. If there is not a significant coronal deformity, then we choose to approach from the right to place the more durable aorta or iliac artery to the blind side and keep the veins on the side of our operative field. The most common levels for ACR at our institution are L2/L3 and L3/L4. The L4/L5 level is typically avoided due to the anterior migration of the lumbar plexus.
The anterior lumbar column is accessed through the typical lateral retroperitoneal transpsoas approach with the use of directional triggered electromyogram (tEMG) to help prevent a lumbar plexus injury. Once the retractor is docked in a safe location (typically posterior third of the disc space or working zone 3) , a discectomy is performed, and the endplates are prepared in the same manner as a basic lateral interbody fusion. The retractor is then opened more anteriorly, and the ALL is identified (Fig. 27.4). It is a thick (white) fibrous structure bridging the two vertebral bodies anteriorly.
Intraoperative view of the L3/L4 lumbar interspace with the retractor in place after discectomy and anterior longitudinal ligament dissection
A natural anatomic plane is then developed (with a blunt instrument) directly ventral to the ALL and dorsal to the autonomic plexus and great vessels. Under AP fluoroscopic guidance, the blunt dissector is advanced while applying slight posterior pressure against the ALL. Very little resistance should be encountered during this maneuver. Resistance means that the wrong plane has been accessed and risk for vessel injury is substantial. We only advance to the medial border of the contralateral pedicle (under AP fluoroscopy) for safety reasons.
While the blunt dissector is protecting the ventral vascular structures, bipolar cautery is used to coagulate the ALL, and an annulotomy blade is slid down the dissector making cuts in and anterior to posterior direction. The surgeon should never cut in a downward manner. Only the first two-thirds of the ligament need to be sectioned (once again, for safety reasons), and the remaining component is broken with a specifically designed disc space distractor. The dissector is slowly removed and the disc space distractor is placed under AP fluoroscopic guidance. The distractor is opened in a gradual manner (multiple minutes) until the final third of the ligament of broken. A hyperlordotic cage is then placed with unicortical lateral screws to prevent anterior migration (Figs. 27.5, 27.6, and 27.7).
Intraoperative fluoroscopic anteroposterior images showing (from a to d). The progression of dissection and sectioning of the anterior longitudinal ligament and final widening of the disc space
Intraoperative lateral fluoroscopic images showing before and after sectioning of the anterior longitudinal ligament and lordosis gain
Pre- and postoperative CT sagittal images showing the amount of lordosis achieved after the ACR procedure
27.5 Potential Pitfalls
A strict contraindication for ACR is the presence of a large anterior osteophyte such as those seen in diffuse idiopathic skeletal hyperostosis (DISH). The natural anatomic plane is typically obliterated and cannot be safely dissected without significant risk of major vascular injury.