Anterior Exposure to Lumbosacral Spine: Anatomy and Techniques

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CHAPTER 19 Anterior Exposure to Lumbosacral Spine

Anatomy and Techniques

There has been an increasing need for access surgery to the lumbosacral spine. In the past, the need for access was due to the prevalence of Pott disease from tuberculosis with associated spinal osteomyelitis, severe fractures of the spine, or malignant involvement and destruction of the spine. With evolution of the understanding of the pain mechanism in spine disease, there has been a significant increase in the number of spinal fusion procedures. With progress continuing in the development of artificial disc constructs, the need for complete anterior access to the lumbar spine continues to increase. The surgeon who performs anterior spine access procedures must be intimately familiar with the anatomy of the lumbar spine region and adept at recognizing potential hazards to minimize the risk of complications during approach surgery.

Anatomy

Detailed knowledge of the anatomy surrounding the lumbar spine is mandatory to approach the spine with a minimum of risk. By taking advantage of the avascular tissue planes of the abdominal wall and with a thorough knowledge of the anatomic structures to be encountered, the surgeon can gain access to the retroperitoneal space with only a small amount of tissue division.

Abdominal Wall

The superficial layers of the abdominal wall include the skin, subcutaneous fascia, and fatty layers separating them. The muscular wall of the abdomen is composed of differing layers depending on the medial or lateral location of examination. The abdominal wall is composed of three muscle layers lateral to the abdominal rectus muscle—the external and internal oblique muscles and the transversus abdominis muscle. Each layer has associated fascial extensions that come together to make up the anterior and posterior rectus sheath surrounding the rectus muscle medially. Deep to these layers is the transversalis fascia and the peritoneum with a variable amount of preperitoneal fat interspersed between these two layers. As this layer progresses laterally, the peritoneum thickens and becomes less densely adherent to its adjacent tissue layer.

The posterior musculature includes the psoas major muscle immediately lateral to the lumbar spine. Lateral to the psoas muscle is the quadratus lumborum muscle superiorly and the iliacus muscle inferiorly transitioning at approximately the L4 level. At the more superior disc levels (approximately L2-3 and cephalad), the tendinous and muscular slips of the diaphragm start to form anteriorly on the spine itself. The right crus of the diaphragm inserts into L3, and the left crus inserts into L2. They are tendinous at this point and need to be taken down to expose L2-3. For L3-4, the right crus needs to be taken down for exposure all the way to the right side. These do not need to be repaired.

Vascular Anatomy: Variability, Bifurcations, Iliolumbar Vein, Segmental Vessels, and Middle Sacral Vessels

The distal aorta and the distal vena cava with their respective bifurcations provide the primary structures of consequence to the immediate anterior surface of the lumbosacral region. The aorta sits to the left of the vena cava, in approximation with the midline of the spine, with its major branches typically crossing anterior to the venous branches of the cava. The vena cava sits to the right side of the spine, which is why right-sided approaches are more hazardous.

The vascular anatomy is reviewed in a caudad-to-cephalad order because this is the order of most common to least common levels of approach. The vasculature at the L5-S1 level generally consists of the middle sacral vessels, which can be divided between ligatures or cauterized, depending on their size. The vena cava bifurcation is generally superior to the L5-S1 disc and overlies the L5 vertebral body on the right side, although in a small percentage of individuals this is seated as low as the inferior edge of the L5-S1 disc itself. It bifurcates to the left and right common iliac veins.

Rarely, anomalous venous drainage may be present across the sacral spine. This may include a single vessel as a bridging iliac vein to a confluent venous plexus across the region. The iliolumbar vein typically branches off posterior to the common iliac vein at the L5 vertebral level and is an important consideration for exposure for the L4-5 disc space above. Most of the time, it may be identified within 2 cm of the L4-5 disc space. In a small percentage of patients, it may not be present. The common iliac artery can be easily visualized lateral to the vein and is usually bifurcating into the internal and external iliac arteries at the L5-S1 level.

The vascular anatomy at the L4-5 level is the most variable of all the levels and can provide either a boon or a bane to the access surgeon depending on its layout. Much has been described in scoring systems for the degree of difficulty this variability presents; however, it does not alter the general approach strategy. The aortic bifurcation and the cava bifurcation make up the variable vascular distribution for L4-5. This consideration is only for the amount of relative length that would be obtained when mobilizing the left common iliac artery, not for dissecting out the aortic bifurcation itself.

Generally, the caval bifurcation is at the L5 vertebral body. The aortic bifurcation sits between L4 and L5, although it is usually located superior to the L4-5 disc space. The left common iliac artery courses laterally from this and has a variable degree of length and tortuosity to it. Cephalad to the L4-5 level are the segmental vessels that cross over the vertebral bodies to enter the aorta. The dissection is to elevate the left common iliac artery to decrease the amount of tension on the vessel. In some instances of a high-riding aortic bifurcation, the common iliac artery is able to be maintained in a lateral position. Otherwise, complete mobilization is generally needed to maintain this vessel in a right-sided location relative to the face of the L4-5 disc during a left-sided approach.

Surgical Approach

Preoperative Considerations

Communication is the mainstay to a successful partnership between the spine surgeon and access surgeon and the supporting surgical team. Before the day of surgery, relevant radiologic imaging will have been obtained and reviewed to determine the surgical plan. Any additional imaging should be pursued at that time to take into account disease processes that the patient may have. Examining plain films for signs of arterial calcifications, which may provide indirect evidence of vascular disease (Fig. 19–1), is particularly useful for the access surgeon. Additionally, the plain film should be evaluated for osteophytes and spondylolisthesis, which can exaggerate misleading anatomic bony features or be an indicator of potential inflammatory changes. A thorough vascular examination should be repeated at the time of surgery. Pulse oximetry on the patient’s left lower extremity is a useful adjunct for monitoring arterial flow distal to the area of dissection. Positioning of the patient is important to facilitate intraoperative placement of personnel, fixed retraction, and fluoroscopy machines.

Open Approaches

Retroperitoneal

The retroperitoneal approach can proceed from various incisions, including vertical midline, paramedian, oblique, and transverse. Extensive lateral incisions should be avoided because this may denervate medially situated rectus muscle. The incision needs to take into account the spinal level and the number of lumbar levels to be exposed. An infraumbilical transverse incision can accommodate most approaches to the L4-5 and L5-S1 disc levels, whereas a more obliquely oriented incision is favored for access to disc levels above L4 (Figs. 19-2 and 19-3). This incision allows for access to the L2-3 disc level and possibly L1-2 disc level in patients with a favorable body habitus.

The operative procedure should begin after an appropriate review of the films and after surgical goals have been established between the access surgeon and the spine surgeon. Communication with all members of the team is important to establish the surgical plan of the case and to decrease the risk of complications. Review of relevant radiographic material by the access surgeon can show potential deviations from normal anatomy and is the first step toward risk reduction. At that time, the relative position of the L4-5 disc level to the superior iliac crest can be identified.

Lateral fluoroscopy can be used to assess the level of the incision (Fig. 19–4). This imaging is done in conjunction with a radiopaque probe or rod to determine the angle of the approach and level, which can be marked on the patient’s abdomen. Fluoroscopy can be especially important in obese patients because there may be no other anatomic landmarks palpable to guide the placement of the abdominal incision (Fig. 19–5). In most nonobese patients, palpation by an experienced access surgeon can be used to locate the sacral prominence to identify the L5-S1 disc space. A transverse approach to the L4-5 level should be directed at approximately the level of the superior anterior iliac spine, although fluoroscopic confirmation should be used if there is any question. All of the landmarks can be confirmed by fluoroscopic guidance, and fluoroscopy is especially important in patients who have distorted spine anatomy, had prior spine surgery, or are obese.