Thomas Stanley, Michael J. Lee, Mark Dumonski, and Kern Singh
ANATOMY
Superficial landmarks allow for gross determination of anatomic level. Proximally, C7 and T1 are the largest spinous processes and act as landmarks for determining anatomic level. Distally, the intercrestal line approximates the L4–5 interspace.
There are three layers to the posterior musculature of the spine (FIG 1, Table 1):
Superficial layer: trapezius, latissimus dorsi, rhomboid major and minor, and levator scapulae
Intermediate layer: serratus posterior superior and inferior, and levatores costarum
Deep layer: erector spinae, transversospinalis, interspinalis, and intertransversarii
The superficial and intermediate layers receive their nervous supply from peripheral nerves, which are not encountered through the posterior approach (FIG 2). The deep layer receives its nervous supply segmentally from the posterior dorsal rami. There is a large amount of redundancy in the innervation of the deep layer.
The midline approach is a true internervous plane, and nerve injury occurs only with excessive lateral discsection.
The vascular supply to the deep layer is from segmental branches of the aorta. These vessels enter the operative field at the level of the intertransverse ligament and can be a source of significant bleeding.
The facet joint capsules have a shiny white appearance and the individual fibers can be seen inserting onto the lateral edge of the laminar trough. Care should be taken to avoid violating the capsular fibers unless that segment is being fused.
The ligamentum flavum has a yellow appearance, with the fibers running in a cephalad–caudad direction. The cephalad end of the ligament has a broad insertion from the base of the spinous process to between 50 and 70 percent of the anterior surface of the lamina. The caudad end of the ligament inserts from the superior edge of the lamina to between 2 and 6 mm of the anterior surface of the lamina.4
Particularly at the L5–S1 level, the interspace may be widened or the posterior bony anatomy only partly formed. Care should be exercised when exposing this level as inadvertent plunging into the canal may occur.
Laterally, the intertransverse membrane overlies the iliopsoas and protects the neural structures that lie beneath.
In children, the spinous process apophysis has not fused. During dissection, the apophysis is split down to the bone and then elevated with the paraspinal musculature.
FIG 1 • The superficial, intermediate, and deep musculature of the back.
SURGICAL MANAGEMENT
Positioning
Patients should be placed in the prone position on a radiolucent table (FIG 3A).
Care is taken to ensure that the neck is in a neutral position with no hyperextension.
The arms are positioned at 90 degrees or less of abduction to minimize the likelihood of rotator cuff impingement. The arms are allowed to slightly hang down in a forward-flexed position about 10 degrees. The axilla should be clear from any padding to prevent brachial plexus palsy.
Elbow pads are placed along the medial epicondyle to protect the ulnar nerve.
FIG 2 • Cross-sectional anatomy of the thoracic and lumbar spine.
Pads are placed at the chest and iliac crests.
The chest pad is placed just proximal to the level of the xiphoid process and distal to the axilla. In women, care is taken to tuck the breasts and ensure that the nipples are pressure-free.
The iliac pads are placed two fingerbreadths distal to the anterior superior iliac spine, allowing the abdomen to hang free and reducing any unnecessary epidural bleeding.
Proper placement of the chest and iliac pads allows for restoration of normal sagittal alignment via gravity.
Alternatively, for lumbar decompressive procedures alone, the knees are positioned in a sling, allowing the hips to flex and eliminating lumbar lordosis and widening the laminar interspaces (FIG 3B). This position improves access to the lum-
bar spinal canal but should be avoided when instrumenting as lumbar lordosis is decreased.
Approach
Two approaches are used: midline and paraspinal.
The midline approach is used for most spinal procedures as it allows direct access to the spinal canal.
The paraspinal approach, also known as the Wiltse approach, was initially described for spondylolisthesis but is now being used during far-lateral discectomies and minimally invasive muscle-sparing techniques.
There is increased interest in the paraspinal approach, particularly in conjunction with transforaminal lumbar interbody fusion procedures.
FIG 3 • A. Prone position on a radiolucent table. The abdomen is not compressed. B. The knee–chest position is obtained using a Wilson frame.
TECHNIQUES
MIDLINE POSTERIOR APPROACH
Incision and Dissection
Anatomic landmarks are identified to center the skin incision appropriately (TECH FIG 1A).
A midline incision is made over the spinous processes down to the level of the fascia.
A Cobb elevator is used to create 2-mm full-thickness skin flaps with subcutaneous fat. This allows for better visualization of the fascia during closure (TECH FIG 1B,C).
The location of the spinous processes is again verified, and electrocautery is used to reflect the fascia from the tips of the spinous processes.
Electrocautery is used to subperiosteally elevate the paraspinal musculature laterally to the trough of the lamina. The surgeon should avoid going beyond this point to protect the insertion of the facet joint capsule.
A sponge and Cobb are then used to gently dissect the paraspinal musculature off the facet joint capsule.
Cautery
Two venous bleeders are encountered that require electrocautery (TECH FIG 2A).
The first is located adjacent to the pars interarticularis (TECH FIG 2B,C).
The second is located just lateral to the facet joint.
Electrocautery is used to elevate the paraspinal musculature off the transverse processes. Care should be taken to stay on the transverse process and not to violate the intertransverse membrane.
Bipolar cautery should be used at the intertransverse ligament to avoid damage to the spinal nerves.
TECH FIG 1 • A. Anatomic landmarks. B,C. The fascia is exposed with full-thickness skin flaps.
TECH FIG 2 • A. Venous bleeding sites are adjacent to the pars interarticularis and at the junction of the facet and the transverse process. B,C. Probes (arrows) indicate the location of venous bleeders adjacent to the pars interarticularis (B) and the facet joint (C).
Paraspinal Resection
In large and muscular patients, often it is necessary to excise a portion of the paraspinal muscles overlying the transverse processes to be fused.
The muscle is resected beginning underneath the fascia and extending toward the lateral edge of the transverse processes. This creates a pocket over the transverse processes that serves as a bone graft cavity (TECH FIG 3).
TECH FIG 3 • A,B. Electrocautery is used to excavate a muscular pocket for the fusion mass. C,D. Complete posterior exposure.
PARASPINAL APPROACH
The approach is typically performed two fingerbreadths lateral to the spinous process.
After the fascia has been exposed, the paraspinal muscles are palpated and the interval between the multifidus medially and longissimus laterally is identified.
A sharp incision through the fascia is made at this interval (TECH FIG 4).
The interval is defined with blunt dissection down to the lateral edge of the facet joint and transverse process junction.
TECH FIG 4 • Cross-section of spine showing Wiltse interval.
COMPLICATIONS
Major and minor complication rates of up to 80% have been reported in some series (Table 2).2
Risk factors for complications include patient age, length of surgery, levels exposed, blood loss, and postoperative urinary incontinence. Diabetes and other medical comorbidities have not been shown to be independent risk factors for the development of postoperative complications.1–3
REFERENCES
1. Benz RJ, et al. Predicting complications in elderly patients undergoing lumbar decompression. Clin Orthop Relat Res 2001;384:116–121.
2. Carreon LY, et al. Perioperative complications of posterior lumbar decompression and fusion in adults. J Bone Joint Surg Am 2003; 85A:2089–2092.
3. Olson MA, et al. Risk factors for surgical site infection in spinal surgery. J Neurosurg 2003;98:149–155.
4. Olszewski AD, et al. The anatomy of the human lumbar ligamentum flavum: new observations and their surgical importance. Spine 1996;21:2307–2312.