Orthopedic Approachs 2022, Nov, 19 | 12:00 am

Posterior Approach to the Lumbar Spine

Posterior Approach to the Lumbar Spine ‌

The posterior approach is the most common approach to the lumbar spine. Besides providing access to the cauda equina and the intervertebral discs, it can expose the posterior elements of the spine: The spinous processes, laminae, facet joints, and pedicles. The approach is through the midline, and it may be extended proximally and distally.

The uses of the posterior approach include the following:

Position of the Patient

 

The posterior approach can be undertaken with the patient in either of two positions:

  1. Logroll the patient into a prone position. Be sure that bolsters are placed longitudinally under the patient’s sides to allow the abdomen to be entirely free, reducing venous plexus filling around the spinal cord by permitting the venous plexus to drain directly into the inferior vena cava. The shoulders should be placed at no more than 90 degrees of abduction and should be slightly flexed forward to relax the brachial plexus. Careful padding of the ulnar nerve at the elbow and median nerve at the wrist must be assured. Position the head and neck in a relaxed, neutral position and be sure that no pressure is applied to the eyes. Avoid having the head lower than the rest of the body to reduce the risk of postoperative blindness (due to high hydrostatic pressure in the eyes leading to reduced blood perfusion).

     

    Pad the lower extremities carefully at the knees and feet. If the approach is to be used for decompression, flex the hips to create an increase in interlaminar or interspinous distance. Place the hips in neutral

    or slight extension for lumbar fusions to restore normal lordosis. Flex the knees and check that there is no pressure on the proximal fibula/common peroneal nerve region (Fig. 6-1A).

  2. Place the patient on his or her side, with the affected side upward. Flex the patient’s hips and knees to flex the lumbar spine and open up the interspinous spaces. Make sure that the patient is positioned with the involved spinal level over the table break. Jackknifing the table can open further the intervertebral space on the upper side of the patient by putting the lumbar spine into lateral flexion. One advantage of this position is that it allows the surgeon to sit. Also extravasated blood drains down, away from the operative field (see Fig. 6-1B).

For both positions, use a cold-light headlamp to illuminate the deepest layers of the dissection.

 

Landmarks and Incision

Landmarks

Palpate the spinous processes. Note that a line drawn between the highest points on the iliac crest is in the L4-5 interspace. The line is only a rough guide, however; the best means of determining the exact level is either to insert a small needle into the spinous process and obtain a radiograph or to carry the dissection distally and identify the sacrum.

Incision

Make a midline longitudinal incision over the spinous processes, extending from the spinous process above to the spinous process below the pathologic level. The length of the incision depends on the number of levels to be explored (Fig. 6-2).

 

 

Figure 6-1 A: The position of the patient for the posterior approach to the lumbar spine. B: Alternatively, place the patient in the lateral position with the affected side up.

 

Internervous Plane

 

The internervous plane lies between the two paraspinal muscles (erector spinae), each of which receives a segmental nerve supply from the posterior primary rami of the lumbar nerves.

 

Superficial Surgical Dissection

 

Deepen the incision through fat and fascia in line with the skin incision until the spinous process itself is reached. Detach the paraspinal muscles subperiosteally as one unit from the bone, using a dissector, such as a Cobb elevator, or with cautery (Fig. 6-3). Dissect down the spinous process and along the lamina to the facet joint. In a young patient, the tip

of the spinous process is a cartilaginous apophysis; it can be split in the midline, making subperiosteal muscle removal easier (Fig. 6-4).

If necessary, dissection can be continued laterally, stripping the facet joint capsule from the descending and ascending facets. To do this, strip the joint capsule in a medial to lateral direction across the posterior aspect of the descending facet; then, continue over the tip of the mamillary process of the more lateral ascending facet. If the transverse processes must be reached, continue dissecting down the lateral side of the ascending facet and onto the transverse process itself (see Fig. 6-4).

 

Deep Surgical Dissection

 

Remove the ligamentum flavum by cutting its attachments to the superior, or leading, edge of the inferior lamina using either a curette or sharp dissection. Immediately beneath are epidural fat and the blue-white dura. Using blunt dissection and staying lateral to the dura, carefully continue down to the floor of the spinal canal, retracting the dura and its nerve root medially (Figs. 6-5 to 6-8).

 

 

 

Figure 6-2 Make a longitudinal incision over the spinous processes, extending

from the spinous process above to the spinous process below the level of pathology. A line drawn across the highest point of the iliac crest is in the L4-5 interspace.

 

 

 

Figure 6-3 Deepen the incision through the fat and fascia in line with the skin incision until the spinous process itself is reached. Detach the paraspinal muscles subperiosteally.

 

 

Figure 6-4 A: Dissect the paraspinal muscles from the spinous process and lamina to the facet joint. Remove the paraspinal muscles subperiosteally as one unit from the bone. B: Continue dissecting laterally, stripping the joint capsule from the descending and ascending facets. Note the branches of the lumbar vessels that bleed during stripping of the muscles.

 

 

Figure 6-5 A: Remove the ligamentum flavum by cutting its attachment to the superior or leading edge of the inferior lamina. B: Immediately beneath the ligamentum flavum and epidural fat is the blue-white dura. Identify the spinal nerve. Note the overlying epidural veins.

 

 

Dang

 

 

Vessels

The vessels supplying the paraspinal muscles on a segmental basis are close to the facet joints, in the area between the transverse processes. These branches of the lumbar vessels frequently bleed as the dissection is carried out laterally. Vigorous cauterization of these vessels may be necessary to stop the bleeding. Note that the posterior primary rami of the lumbar nerves, which also supply the paraspinal muscles segmentally, run with these vessels and will be damaged by cautery. Fortunately, loss of

some of these nerves does not totally denervate the paraspinal muscles, because they are innervated segmentally (see Fig. 6-4).

The venous plexus surrounding the nerves and the floor of the vertebra may bleed during the blunt dissection needed to reach the disc (see Fig. 6-7). The bleeding can be stopped with Gelfoam or cotton patties soaked in thrombin. Bipolar Malis cautery also may be used, although it must be done with great care because of the proximity of the nerve roots.

The iliac vessels lying on the anterior aspect of the vertebral bodies may be injured if instruments pass through the anterior portion of the annulus fibrosus (see Fig. 6-21).6

Nerves

Each spinal nerve must be identified individually and protected. The more lateral the surgical field, the easier it is to identify the nerve and retract it so the disc space can be seen. If a larger exposure is needed, incise part of the lamina on the distal portion of the involved vertebra.

 

 

 

Figure 6-6 A: Insert a blunt dissector under the cut edge of the ligamentum flavum. B: Use a Kerrison rongeur to remove the distal end of the lamina. Note

that the ligamentum flavum attaches halfway up the undersurface of the lamina. C: Remove additional lamina and the remaining portion of the ligamentum flavum at its attachment to the undersurface of the lamina.

 

How to Enlarge the Approach

Local Measures

  1. To gain better exposure of the dura, nerve root, and disc, remove additional portions of the lamina, both from the leading edge of the lamina below and from the caudal edge of the lamina above. A portion of the facet joint itself even can be removed. Remember that it is safer to remove bone than to retract nerve roots or dura excessively. If the wound is tight, dissect the paraspinal muscles off the posterior spinal elements above and below the exposed level to make the muscles easier to retract.

  2. To gain access to other parts of the posterior aspect of the spine, carry the dissection as far laterally as possible, onto the transverse processes. Complete lateral dissection exposes the facet joints and transverse processes, permitting facet joint fusion and transverse process fusion, if necessary (see Fig. 6-4).

Extensile Measures

To extend the approach, merely extend the skin incision proximally or distally and detach the posterior spinal musculature from the posterior spinal elements. The approach can be extended from C1 down to the sacrum.

 

 

Figure 6-7 A: Using blunt dissection, carefully continue down the lateral side of the dura to the floor of the spinal canal; retract the dura and its nerve root medially. Reveal the posterior aspect of the disc. B: Cross section revealing the retraction of the dural tube and a herniated nucleus pulposus impinging on a nerve root.