Spine Exposure

Historical Background

Anterior intervertebral body fusion was described in 1932 by Carpenter and promoted by Hodgson and Stock in 1956 as a definitive treatment of anteriorly oriented spine pathology while avoiding posterior element injury. Anterior discectomy, decompression, and intervertebral fusion are usually combined with supplemental posterior fixation to increase the rate of complete spinal fusion. Spine exposure for lumbosacral (LS) pathology was originally performed through an open retroperitoneal exposure using a large incision, with extensive tissue manipulation and significant patient morbidity. The anterior retroperitoneal approach is currently performed using a miniature open technique with minimal tissue manipulation, taking advantage of the space between the peritoneal contents and kidney anteriorly and the quadratus lumbar and psoas muscles posteriorly to afford excellent anterior spine exposure. Other recently described surgical approaches, such as an endoscopic retroperitoneal exposure and endoscopic lateral transpsoas exposure, capitalize on the retroperitoneal approach without compromising anterior spine exposure. Laparoscopic transperitoneal spine exposure also has it proponents. Regardless of the preferred technique, all surgical approaches necessitate dissection and mobilization of the thoracic aorta, abdominal aorta, or iliac arteries with the accompanying vena cava and iliac veins depending upon the disc levels that require fusion.

Notwithstanding the potential for major arterial or venous injury during anterior spine exposure, significant injury to other vulnerable structures may also occur during spine exposure. Recognition of these possible complications has led most centers of excellence to develop a two-team approach, consisting of vascular surgery and either orthopedic surgery or neurosurgery, to minimize morbidity and to maximize optimal operative exposure for reconstructive spine surgery. Advantages of the anterior spine exposure include preservation of posterior elements, decreased incidence of nerve injury, and avoidance of epidural scarring. From the perspective of the vascular surgeon, the key to success is providing the spine surgeon with adequate exposure so that discectomy, decompression, and partial corpectomy can be safely performed from vertebra side to side and implants accurately inserted based on the center of the disc space.

Preoperative Preparation

• Risk assessment. Patients who need spine exposure are often younger, have few to no risk factors for coronary or peripheral arterial disease, and are frequently overweight because of inactivity. Patients often have chronic back pain with radiculopathy and a long-standing pain syndrome that is managed by multiple specialists. Many have had a work-related injury. Older patients may need preoperative cardiac stress testing or pulmonary function studies, particularly if the procedure requires thoracic spine exposure.

• Peripheral vascular examination. LS spine exposure involves extensive mobilization of major arteries within the abdomen and pelvis, as well as their accompanying major veins. Therefore the presence of an abnormality in the peripheral pulse examination should prompt preoperative noninvasive testing and, if indicated, computed tomography (CT) angiography or formal arteriography for further evaluation. An accurate assessment of peripheral pulses before surgery is mandatory, because temporary vessel occlusion may occur during miniature open retroperitoneal anterior spine exposure.

• Role of preoperative imaging. Significant aortoiliac artery calcification increases the risk of arterial injury or dissection as a consequence of retractor placement. The extent of calcification can be evaluated with plain radiographs or CT imaging.

• Surgical history. In addition to peripheral pulse examination, assessment of the thorax, abdomen, and flank for signs of previous chest or abdominal procedures may influence the placement of the operative incision. For instance, the lower LS spine can be exposed via a right retroperitoneal approach if the procedure is a redo operative procedure in which adjacent proximal or distal disc levels are being exposed, or if there has been prior abdominal surgery in the left upper or lower quadrant.

• Incision position and length. The proposed incision should be discussed with the patient before surgery. Incision length may need to be extended to facilitate adequate spine exposure when there is significant calcific atherosclerotic disease or the patient has a large body habitus.

• Obtaining consent. Possible complications of surgery should be reviewed with the patient, including major artery or vein injury, bowel or ureter injury, deep vein thrombosis, wound infection or seroma, and nerve injury. In addition, the patient should be aware of the potential risk of erectile dysfunction, which usually consists of retrograde ejaculation, as opposed to impotence, due to disruption of the intermesenteric nerve plexus and superior hypogastric nerve plexus that course over the left common iliac artery origin. Significant anterior spinal scarring is present in the setting of infection or a history of previous anterior or posterior laminectomy with an attendant increased risk of complication.

Pitfalls and Danger Points

• Morbid obesity. In a morbidly obese patient, the risks of surgery often exceed the benefits until significant weight loss has been achieved.

• Redo surgery. Redo operative procedures are always associated with significant scarring of the anterior spine, which results in adherent vessels and ureteral fixation. This is particularly true for explantation of previously placed artificial discs, which occasionally become displaced or have component failure. Alternate spine access strategies are often used for these potentially complicated cases. Arteriorrhaphy or venorrhaphy with pledgeted polypropylene suture is recommended when an injury is encountered, because these vessels tend to be friable when redissected off the anterior spine surface. Large venous rents are best controlled with a side-biting vascular clamp, because it may not be possible to occlude the vein proximal and distal to the injury site without risking further injury.

• Calcified arteries. Calcified arteries present a unique problem because of the retraction required to expose the disc space. Even pliable vessels are frequently retracted at rather acute angles to limit dissection yet afford complete disc space exposure. Therefore the risk of arterial injury or dissection increases significantly when the vessel is heavily calcified. The best way to avoid calcified vessel injury is to mobilize more length of the artery proximal and distal to the disc levels of interest, even if this requires extension of the incision or more mobilization of adjacent structures.

• Vascular injury. Significant arterial injury is rarely encountered during anterior spine exposure, but if it is noted, appropriate arterial reconstruction should be performed as needed, as opposed to risking acute arterial occlusion by applying multiple blindly directed sutures and pledgets to an injured vessel. To appropriately expose and securely repair a seemingly uncontrollable major venous injury, it is best to partially excise and subsequently repair the common iliac artery rather than risk exsanguinating hemorrhage by multiple failed attempts at vascular control.

• L4-5 discectomy. Although tempting in some cases, it is unwise to perform discectomy and spinal instrumentation at more than one disc space at a time.Attempting to expose multiple levels at once either occludes the aorta or common iliac artery or risks significant damage to the confluence of the common iliac veins and vena cava. This is especially true when the L4-5 disc space is involved, because there can be significant variability in the arterial and venous anatomy. In the usual scenario, the disc space is exposed after left-to-right mobilization of the aorta and left common iliac artery across the anterior spine. However, the left common iliac artery and vein occasionally need to be separated and retracted away from each other to fully exposure the disc space without prolonged arterial occlusion or arterial injury. Less common dissection schemes occasionally need to be used at the L4-5 disc space, in cases with a high aortic bifurcation or duplicated vena cava. These alternate schemes are particularly helpful for redo cases, in which the iliac veins may be densely adherent to part of the anterior surface of the spine, or after previous posterior LS spine surgery, when significant scarring of the anterior spine surface may be encountered.

• Right-sided discectomy. Spine exposure performed from a right-sided miniature open retroperitoneal approach necessitates mobilization of the vena cava and to some extent the right common iliac vein for lower LS spine procedures. Retraction of the vena cava medially all the way across the anterior surface of the spine often occludes the vessel and results in significant decrease in preload, which is not well tolerated in elderly patients. Depending on arterial or venous anatomy, the disc spaces in these patients may be better exposed by dissecting between the vena cava and the abdominal aorta. There is less compression of each structure with lateral retraction away from the midline.

• Self-retaining retractors. Self-retaining renal vein retractors (Omni-Tract Surgical, Minneapolis) are used to maintain disc space exposure and to protect the adjacent vessels and peritoneal contents. These retractors are slightly angled forward at their tip, and this angled portion can usually be positioned to hug the disc space laterally, supplying the spine surgeon with just enough room to operate even in very large patients. These retractors should be placed with care to avoid excessive traction on the mobilized structures and positioned just medial to the sympathetic chains that course along the lateral aspects of the vertebra to protect this structure from inadvertent injury.