Technique

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.

FIGURE 20–2 Patient placed in right lateral decubitus position with table flexed to increase distance between the patient’s ribs and iliac crest.

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 20–3 Fluoroscopic image showing center positioning over disc space. A mark is made on skin corresponding to this mark.

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.

FIGURE 20–6 When retractor is deployed, soft tissue over disc space must be cleared away. Probe is used to detect any nerves that may cross the field.

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.

FIGURE 20–7 A and B, Cobb elevator is used to release contralateral anulus. This aids in coronal correction of deformity.

Complications

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.

Technique

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.)

To perform a decompression or a discectomy, it may be necessary to remove the ligamentum flavum. The superficial ligamentum flavum blends laterally into the facet joint capsule. A forward angled or small straight curet is used to detach the superficial and deep layers of the ligamentum flavum from the caudal edge of the cephalad lamina. The surgeon sweeps the curet medial to lateral and advances the curet with each successive sweep to detach the ligamentum flavum from the lamina. The ligamentum flavum typically inserts over the caudal 50% of the undersurface of the lamina. A small, angled elevator is placed under the ligamentum flavum to lift it off the dura and protect the latter. A Kerrison rongeur, pituitary rongeur, or knife can be used to remove the ligamentum flavum. The epidural fat, dura, nerve root, and epidural veins can be seen after the ligamentum flavum has been removed (Fig. 20–9).

FIGURE 20–9 Removal of ligamentum flavum.

(From Benzel E: Spine Surgery: Techniques, Complication Avoidance, and Management. Philadelphia, Churchill Livingstone, 2004.)

If a discectomy or exploration of the disc space is required, it can typically be performed through this opening. A portion of the lamina (laminotomy) may need to be removed to access the disc space adequately. A Penfield No. 4 dissector can be used to help mobilize the traversing nerve root, and a nerve root retractor can be used to retract the nerve roots gently medially. Care must be used not to retract too vigorously to avoid too much tension on the exiting nerve root. Bleeding from the epidural veins commonly occurs. Hemostasis can be obtained with bipolar cautery or the use of cottonoids, absorbable knitted fabric (Surgicel), or thrombin-soaked absorbable gelatin sponge (Gelfoam). Cottonoids can be placed in the cephalad and caudad extremes of the exposure to collapse the vessels and provide a working window.

The key to intracanal anatomy is the pedicle. The disc space is just cephalad to the pedicle, and the intervertebral foramen above the pedicle accommodates the exiting nerve root. The traversing nerve root lies just medial to the pedicle and exits the intervertebral foramen caudally. The disc space can be found by retracting the traversing nerve root medially and exploring the space above the pedicle (Fig. 20–10). The surgeon can use a Penfield No. 4 dissector to feel for the disc space. It should be a raised, white, soft structure.

FIGURE 20–10 Exposure of lumbar disc by retracting thecal sac medially.

(From Benzel E: Spine Surgery: Techniques, Complication Avoidance, and Management. Philadelphia, Churchill Livingstone, 2004.)

If a total laminectomy is needed to decompress or expose the dura and nerve roots, the fascia is removed entirely from the tip of the spinous process bilaterally. The muscles are dissected off of the spinous processes and lamina subperiosteally, taking care to protect the facet joints. The pars interarticularis must be exposed fully to avoid transecting it during the decompression. A rongeur can be used to remove the spinous processes.

The laminectomy may be performed many ways. A high-speed bur may be used to thin the lamina down to a thin cortical shell over the dura, then a Kerrison rongeur is used to remove the lamina. Alternatively, the tip of a rongeur may be inserted under the caudal edge of the cephalad lamina to remove the lamina. The rongeur is used to cut from the underside in an upward direction; this lessens the chance of catching dura. A Kerrison rongeur can be used to complete the laminectomy near the pars and the cephalad edge. To decompress the nerve roots adequately, the lateral recesses and intervertebral foramen must also be explored. A Woodson elevator or dural guide may be used to compress the dura gently and expose the lateral recesses; this exposes the ligamentum flavum in the lateral recess and intervertebral foramen. The ligamentum flavum should be removed to perform an adequate decompression. The medial aspect of the caudal pedicle marks the medial border of the intervertebral foramen.

Osteophytes from the facet joints often compress the exiting nerve root. Care must be taken when removing these osteophytes to avoid injury to the exiting nerve root and to avoid iatrogenic instability caused by too much removal of the facet joint. Typically, removal of less than 50% of the facet joint preserves its stability. This may necessitate the use of a 1- or 2-mm Kerrison rongeur. The use of a curved Kerrison rongeur can be helpful here. Bearing in mind that the facet joints are oriented sagittally in the lumbar spine, cutting the undersurface of the facet joint provides a greater means of decompressing the nerve roots, while preserving the overall stability of the joint.

With the advent of pedicle screw fixation for the lumbar vertebrae, there are now several additional anatomic relationships that are important at the level of the posterior bony elements.⁴ The location of the pedicles is identified by anatomic landmarks and by radiography or image intensification fluoroscopy in the operating room.

In the lumbar region, the center of the pedicles is usually at the inferolateral edge of the facet joint, on an imaginary transverse line bisecting the transverse processes (Fig. 20–11). If there is severe facet arthrosis, the lateral edge of the facet joint may be lateral to the true pedicle entry site, however. In these cases, the surgeon should also refer to the pars interarticularis. The lateral border of the pars typically corresponds to the medial border of the pedicle. In the lumbar region from this point, one may use a pedicle finder, with a 20-degree medial inclination at L5, 10-degree inclination at L4, 5-degree inclination at L3 and L2, and no inclination at L1 (Fig. 20–12). One may follow the progress of the pedicle finder by feeling inside the pedicle with a pedicle feeler and by checking with the image intensifier or by radiographs. In the lateral view, the probe or marker should be parallel to the disc space.

FIGURE 20–11 Illustration of lumbar pedicle entry sites. In lumbar region, the center of pedicles is usually at inferolateral edge of facet joint.

(From Herkowitz H, Eismont FJ, Garfin SR, et al [eds]: Rothman-Simeone The Spine, 4th ed. Philadelphia, WB Saunders, 1999.)

FIGURE 20–12 Transaxial position of pedicle screws.

(From Herkowitz H, Eismont FJ, Garfin SR, et al [eds]: Rothman-Simeone The Spine, 4th ed. Philadelphia, WB Saunders, 1999.)

Technique

General endotracheal anesthesia is recommended for this procedure. The patient is placed on the operating table in the prone position with chest rolls on either side of the thorax to protect ventilation or on a radiolucent table with chest and hip pads.

A longitudinal paramedian incision is made at the lateral border of the erector spinae muscles (approximately two fingerbreadths from midline) centered over the level of interest. The incision is extended to the lumbar fascia, and the erector spinae muscles are identified. The interval between the erector spinae muscles and the multifidus is found after opening the fascia, and dissection proceeds between these muscles down to the facet joints and the transverse processes of the vertebrae (Fig. 20–13). The paraspinal muscles are retracted medially, the transverse process at the desired level is tagged with a radiopaque marker, and radiographs are obtained to confirm the vertebral level. For a minimally invasive transforaminal interbody fusion, this exposure is adequate to perform a decompression and fusion.

FIGURE 20–13 Cross section of lumbar spine and paraspinal structures at level of L3.

(From Herkowitz H, Eismont FJ, Garfin SR, et al [eds]: Rothman-Simeone The Spine, 4th ed. Philadelphia, WB Saunders, 1999.)

If access to the vertebral body is desired, the dissection can be carried further anteriorly. The transverse process is divided with an osteotome and is retracted laterally with its musculotendinous attachments. The vertebral pedicle is palpated, and the lumbar nerves are identified and protected as they leave the foramina above and below the pedicle (Fig. 20–14). The psoas muscle is carefully separated from the vertebra using a periosteal elevator. The lumbar vessels lie on the waist or mid-portion of the vertebral body posterior to the psoas muscle and should be separated from the body during this portion of the dissection. The vessels may be clamped and cauterized if necessary. An opening may be made in the lateral aspect of the vertebral body anterior to the pedicle, using a curet or drill (Fig. 20–15).

FIGURE 20–14 Lumbar vertebrae as viewed from posterolateral approach. Dissection proceeds directly anterior to stump of transverse process, along pedicle to vertebral body in front. Note lumbar segmental vessels draped over waist or mid-portion of vertebral bodies. By dissecting directly anterior to pedicles, one can avoid these vessels and lumbar nerves leaving neural foramina below pedicles.

(From Herkowitz H, Eismont FJ, Garfin SR, et al [eds]: Rothman-Simeone The Spine, 4th ed. Philadelphia, WB Saunders, 1999.)

FIGURE 20–15 Posterolateral approach to lumbar vertebrae, lateral to erector spinae muscle mass and behind psoas. Transverse process is divided and retracted laterally with musculotendinous insertions to gain access to lateral aspect of vertebral body.

(From Herkowitz H, Eismont FJ, Garfin SR, et al [eds]: Rothman-Simeone The Spine, 4th ed. Philadelphia, WB Saunders, 1999.)

The lesion may be identified grossly at this time but should be verified radiographically with a curet placed within the lesion. Through this approach, specimens may be obtained from the lateral, central, or anterior aspect of the vertebral body or pedicle. The lesion may be curetted, and small chips of cancellous bone graft may be installed to stimulate osteogenesis within a sterile defect. The wound is copiously irrigated with saline and inspected for hemorrhage. The margins are allowed to fall together, and the lumbar fascia is closed with interrupted sutures. The skin is repaired, and the patient is nursed with some form of external spinal support, depending on the postoperative stability of the spine.