CHAPTER 20 Lateral and Posterior Approaches to Lumbosacral Spine
When the decision to operate has been made, the surgeon must choose the best procedure and approach. When considering the options in the lumbar spine, many factors must be taken into account. First is the location of the pathology. Disease or deformity that primarily involves the vertebral bodies may be most easily approached through the abdomen or flank. The posterior elements are most easily approached through a posterior midline incision. Second, the morbidity of each approach must fit the risk tolerance of each individual patient. It would be advisable to avoid an anterior approach in a young man who has pathology at L5-S1 to avoid the small chance of retrograde ejaculation. With the advent of minimally invasive techniques, such as minimally invasive transforaminal lumbar interbody fusions and minimally invasive lateral interbody fusions, decreasing overall morbidity from blood loss and tissue dissection must be weighed against more complete visualization and disc removal provided by the anterior approach. These are some of the factors that should be considered when deciding on a surgical approach to maximize results and minimize patient morbidity.
The concept of minimally invasive spine surgery is attractive to patients and surgeons. Decreased postoperative pain, shorter hospital stay, and quicker return to activities support the use of minimally invasive techniques when achievable. One technique is the lateral access to the spine.1,2 This technique can be used for multilevel interbody fusions to correct kyphoscoliosis, for interbody support when treating adjacent segment degeneration or multilevel fusions, or to drain a psoas abscess (Fig. 20–1). With this approach, access to the spine from T7 down to L4-5 is possible. L4-5 is often difficult to reach, however, because of a high-riding iliac crest, and the means to access L5-S1 laterally has not been developed.
After the patient has been intubated and prophylactic antibiotics have been given, the patient is placed in the lateral decubitus position. When correcting a kyphoscoliosis, it is easier to perform the lateral approach on the side of the concavity. The table should be flexed to increase the distance between the iliac crest and the rib cage, and the patient should be secured with tape over the greater trochanter and chest wall (Fig. 20–2). The leg on top should also be flexed to relax the psoas. A cross-table anteroposterior radiograph should be taken, and the table should be rotated to place the patient in a true anteroposterior position. A corresponding lateral fluoroscopic image should also be taken to verify that access to the disc space is possible. Minor adjustments should be made to the table to obtain a true lateral image.
After the patient has been prepared and draped, the lateral image is obtained first. A radiopaque marker is placed over the center of the affected disc space (Fig. 20–3). When this point has been identified, a mark is made. Through this mark, a small incision is made for insertion of the dilators and an expandable retractor, which provides access to the lateral spine. A second mark is made posterior to this first mark at the border between the erector spinae muscles and the abdominal oblique muscles. At this second mark, a transverse incision about 2 cm long is made to accommodate the surgeon’s index finger (Fig. 20–4). Finger dissection is used down to the lumbodorsal fascia. A clamp, or scissors, can be used to spread the fascia and muscle fibers and provide entry into the retroperitoneal space.
FIGURE 20–4 Two-incision technique shown with lateral and posterolateral marks. Posterolateral incision is made about the length of the surgeon’s index finger away from lateral incision. From this mark, the surgeon should also measure the distance to spine to ensure that he or she can reach the psoas from this incision.
When an opening is created, the index finger is used to sweep the peritoneum anteriorly and to palpate the psoas muscle (Fig. 20–5A and B). The surgeon uses the index finger to sweep inferiorly to feel the inner table of the iliac crest (if in the lower lumbar spine) to verify that he or she is in the abdominal cavity. When the psoas is identified, the index finger is swept up to the previously made direct lateral mark. A 2-cm incision is made; the external and internal oblique muscles and the transverses abdominis muscles are split, and dilators are placed through this opening. The index finger, which is already in the retroperitoneal space, guides the initial dilator onto the psoas (Fig. 20–5C). The fibers of the psoas are split with the dilator using neurologic monitoring as a safety measure, if desired. A lateral radiograph should be obtained to verify the central position of the dilator at the desired disc space. After the position of the initial dilator is secured by placing a Kirschner wire through the dilator and into the disc space, larger dilators are used to spread the psoas under neurologic monitoring, and then an expandable retractor is placed over the dilators (Fig. 20–5D).
FIGURE 20–5 A and B, The surgeon uses digital palpation to sweep abdominal contents anteriorly and create a cavity in retroperitoneal space. C, Index finger guides initial dilator down to psoas. D, When initial dilator is secured in place with Kirschner wire, larger dilators are used to spread psoas under neurologic monitoring, and retractor is placed over dilators.
After the retractor is secured to the table, the dilators are removed to provide lateral access to the disc (Fig. 20–6). A neurologic monitoring probe can be used to check for any nerves that may be crossing the working window of the retractor. If a nerve is detected, the Kirschner wire should be repositioned away from the nerve, and the psoas should be redilated. If this fails, conversion to another means of interbody fusion should be considered because repeated positioning of the retractor, or pressure on the nerve, could result in postoperative paresthesias or palsies.
Anteroposterior and lateral radiographs should be taken at this point to verify that the retractor is docked on the disc space and that the retractor is positioned over the center of the disc. When appropriate positioning has been confirmed, the retractor should be secured in place. A lateral discectomy is performed in standard fashion with shavers, curets, and rasps. A Cobb elevator should be used to release the contralateral anulus (Fig. 20–7). Releasing the contralateral anulus loosens the spine in the coronal plane and aids in the correction of coronal plane deformities. Sizers and trials are used to determine the optimal implant size. The implant is filled with the surgeon’s graft or fusion enhancer of choice and impacted across. The wound is closed in layers. No drains are typically necessary.
Because minimally invasive lateral access to the spine is a newer procedure, publications regarding the efficacy and complication rates are sparse. Numbness in the lateral thigh and psoas weakness have been noted by some physicians; however, the rate still remains unknown. To minimize this risk, the retractor is opened just enough to perform the lateral discectomy. Exuberant deployment of the retractor may place undue pressure on the nerve roots or the psoas itself, or both. Also, neurologic monitoring is advised to decrease the possibility of nerve injury.
Injuries to the bowel and vessels have not been reported, but these may occur. It is recommended that this procedure be done at a facility where a general or vascular surgeon is available. One method to decrease the rate of bowel or vascular injury is to place the initial dilator under direct visualization. After the lateral incision has been made, the retractor can be passed down to the psoas. The initial dilator is then placed through the psoas under direct visualization.
The posterior approach through a midline longitudinal incision is the most common approach to the lumbar spine.3 It provides direct access to the spinous processes, laminae, facets, and even pedicles and lateral aspects of the vertebral bodies at all levels of the lumbar spine. The pedicle starting holes and transverse processes can be reached by dissecting and retracting the paraspinal muscles laterally. Through this approach, it is possible to perform most of the spine procedures currently practiced today, including microdiscectomies, laminectomies, and most fusion procedures. The posterior aspect of the vertebral body and disc space over the lower lumbar levels can be reached after laminectomy by retracting the dura, but the exposure is limited.
Anatomic variations exist among individuals, and this must be taken into account when planning surgery. The intercrestal line typically crosses at L3-4, but this is not a rigid anatomic finding. A lateral radiograph shows where the intercrestal line is. Also, lumbarization or sacralization of the last vertebral segment can confuse the surgeon when localizing the level of pathology. Additionally, a spina bifida occulta or an unusually wide interlaminar space may exist. To avoid inadvertent injury to the dura or nerve roots with a bovie or periosteal elevator during the exposure, the surgeon should study radiographs before surgery to look for these abnormalities.
The patient is positioned prone to allow the abdomen to hang free of pressure. This position reduces venous plexus filling around the cauda equina by permitting the venous plexus to drain directly into the inferior vena cava. The anesthesiologist should check the eyes, and the surgeon and nurses should assess the bony prominences to ensure that they are well padded. If a microdiscectomy or decompression is to be performed, flexing the lumbar spine on a Wilson frame, or similar table, is recommended to open up the interspinous spaces. If a fusion also is to be performed, placing the patient on a Jackson table is recommended to maintain the lumbar lordosis. A solution containing epinephrine in a 1 : 500,000 concentration may be injected into the subcuticular tissues and muscles to decrease blood loss.
A midline incision is made between the spinous processes of the levels to be exposed, and the erector spinae and multifidus muscles are dissected from the bony elements (spinous processes, interspinous ligaments, laminae, facet joints, and transverse processes) as needed for the levels that must be visualized, using electrocautery or sharp dissection (Fig. 20–8). The paraspinal muscles should be elevated subperiosteally to minimize blood loss. Care should be taken not to injure the facet joint capsules and interspinous ligaments in areas where motion would be expected after the operation. If the transverse processes must be reached, dissection is continued down the lateral side of the facet joints and onto the transverse process itself. The vessels supplying the paraspinal muscles segmentally are close to the facet joints and the pars interarticularis. If these vessels are cut, they can bleed vigorously. Cauterization is necessary to stop these bleeders. The posterior primary rami of the lumbar nerves run with these vessels.
FIGURE 20–8 Dorsal subperiosteal exposure of lumbar spine.
(From Benzel E: Spine Surgery: Techniques, Complication Avoidance, and Management. Philadelphia, Churchill Livingstone, 2004.)