Posterior and Posterolateral Approaches to the Lumbar Spine

Overview

The posterior approach is undoubtedly the most utilized approach in all of spine surgery. It remains the workhorse for exposure of the entire spine, from occiput to sacrum, during minimally invasive and deformity operations alike. In the lumbar and thoracic spine, the posterior approach and its variations provide exposure of the anterior vertebral bodies as has been described in tumor, trauma, and deformity surgeries. Thus it is with utmost importance that a spine surgeon develop acumen with the posterior approach.

In the lumbar spine, with the exception of the L5–S1 segment, the likeness of the dorsal elements lends itself to similarities and potential confusion during surgery. Precise marking of laminae and a thorough understanding of dorsal surface anatomy is critical to avoid surgery on unintended levels. This requires intraoperative interpretation of imaging studies and visible anatomic landmarks that have been indelibly marked with a radiolucent marker; adequate planning and study of preoperative imaging is critical at this step.

Good subperiosteal exposure of the spinous processes and laminae is the important next step, but irregularities in the shapes of the dorsal elements can create difficulties of their own. Spondylotic bone, body habitus, and excess bleeding can all obscure visualization. Exposure for a direct midline or paramedian approach requires differing entry points and muscle planes. Also, whether intertransverse process fusion technique will be used dictates the amount of lateral muscle stripping required.

Finally, wound closure is perhaps as important, if not more so, than exposure. Because of the dorsal skin incision, often close to the perineum, watertight wound closure remains a critical yet sometimes overlooked last step of posterior spinal surgery. Although few complications occur as a result of the posterior exposure per se, it is assumed that some infections and wound dehiscence occur because of poor exposure and closure.

Anatomy

The dorsal skin anatomy is straightforward, and skin incisions can be planned according to surface landmarks ( Fig. 38-1 ). Midline can easily be palpated, even in obese patients, using spinous processes (SPs). In the extremely obese, the thoracic SP or the sacrum/coccyx can be palpated. If using the paramedian approach, the recommendation is two fingerbreadths, or 2.5 to 3.0 cm, lateral to midline. The dorsoventral landmarks become more difficult, especially in obese patients. The iliac crests typically localize to L4–L5, although body habitus will often skew this landmark cephalad and will direct a surgeon toward L3–L4 ( Fig. 38-2 ). If any concern exists, I use the inner bore of a 20-gauge spinal needle and localize using a lateral radiograph ( Fig. 38-3 ).

Deep to the skin, the relevant anatomy includes subcutaneous fat, the Scarpa layer, and fascia. The Scarpa layer should be preserved with the intent to use it as an added layer during closure. Note that once the lumbodorsal fascia is reached, the perforating vessels will emerge. Midline should be easily palpable using SPs and the supraspinous ligament ( Fig. 38-4 ). The paraspinal muscles—the multifidus, longissimus, and iliocostalis—occupy the space flanking the SPs and laminae and extend to the transverse processes (TPs; Fig. 38-5 ).

Short intersegmental muscles, the interspinalis and intertransversarii medialis, originate at a caudal vertebra and insert on the adjacent vertebra. The short polysegmental muscles, the multifidus and lumbar erector spinae, span two to five vertebral levels. The multifidus is the most medial and is also the largest of the paraspinal muscles. Lateral to the multifidus lies the lumbar erector spinae, made up of the longissimus and the iliocostalis lumborum. Each muscle has thoracic and lumbar fascicles that originate on the mamillary and transverse processes and insert on the medial aspect of the posterior superior iliac spine.

The Wiltse paraspinal approach exploits the interval between the multifidus and longissimus. This plane can be palpated after the lumbodorsal fascia is split, and segmental vascular and neural structures are often encountered here. The segmental dorsal ramus must be found and followed into the foramen of interest ( Fig. 38-6 ), but note that it branches directly off the exiting nerve root, so it should be handled gently.

The relevant bony anatomy includes the SP, lamina, facet joint, and TP. Good subperiosteal dissection requires thorough understanding of the irregularities in lumbar anatomy. This includes understanding the depth and location of interspaces and the spatial relationships in between. From the perspective of the surgeon, the laminae will be found slightly cephalad in relation to the SP ( Fig. 38-7 ). More cephalad and lateral on the lamina, the pars interarticularis and then the facet joint are encountered. Especially when using cautery, keep in mind that direct ventral dissection caudal to the lamina may lead to violation of the interspace, particularly at the lumbosacral junction at L5–S1. The facet joint capsules must be preserved, unless a fusion at that level is planned. A clear plane of attachment of paraspinal muscles on the facet capsules can be effectively dissected ( Fig. 38-8 ). Coursing laterally, the accessory process—a typical pedicle screw entry point—is still seen, which then leads directly to the TP. Lateral dissection of the musculature out to the tips of the TPs then creates the posterior gutter for graft placement. The intertransverse membrane attaches from one TP to the next and should not be violated; it supports the fusion bed ( Fig. 38-9 ).

Indications/Contraindications

Indications

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Posterior surgery (diskectomy/laminectomy, posterior fusion, posterior interbody fusion)
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Symptomatic radiculopathy from disk herniation (paracentral and far lateral) or spinal stenosis
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Instability as a result of spondylolisthesis, trauma, or tumor