Lumbar Facet Fixation Techniques

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Chapter 154 Lumbar Facet Fixation Techniques

Marc Eichler, Jennifer Orning, Eldad Hadar

The concept of lumbar spine facet fixation has existed since 1948, with King’s description of a novel method of internal fixation in the lumbosacral spine as an alternative to immobilization in plaster.¹ This was modified by Boucher in 1959 by using longer screws and slightly altered placement.² Translaminar placement was introduced in 1984 by Magerl in a paper describing its use as an adjunct to external spinal fixation.³

Despite the longevity of facet fixation as a method for spine immobilization, its use was largely usurped by pedicle screw fixation. Pedicle screws were believed to increase the stability and stiffness of the construct and do not require the presence of intact dorsal elements, as does the translaminar approach for facet screws. However, facet fixation is once again emerging as a viable alternative and useful maneuver in the field of lumbar stabilization.

Anatomy

Facet joints, or zygapophyseal joints, are the only truly mobile interfaces in the lumbar spine. They are described as either apophyseal or diarthrotic joints, consisting of sliding cartilaginous surface and capsules containing synovial fluid. Although situated dorsally, far from the instantaneous axis of rotation, their orientation can dictate the movement allowed in different areas of the spine. This orientation changes from the cervical to lumbar spine, and even within the lumbar spine itself.

The capsules of the facet joints are highly innervated. Medial branches from the dorsal rami of the same and often the upper level innervate them, emanating from the neuroforamina. Pain fibers, which are the smallest somatosensory neurons, innervate the capsule. The pain fibers are small myelinated A-delta fibers or unmyelinated C fibers with unencapsulated endings. Studies have demonstrated the presence of pacinian and Ruffini endings, which serve as mechanoreceptors for proprioception and movement sense.⁴

Wear and tear on the facet joints can cause remodeling consistent with arthritis in other mobile joints of the body. Perhaps meant to stabilize the joint, such remodeling also leads to enlargement of the overall facet interface and possible compression of the nerve roots.

Biomechanics

In direct axial loading, facet joints bear a relatively small amount of the overall load. However, with extension and hyperextension, they bear a larger portion of the load—approximately 30%, compared with 10% to 20% direct axial loading.⁵ When flexed, they are reported to handle nearly 50% of the ventral shear load. Because of their motion and distance from the instantaneous axis of rotation, they, along with dorsal ligaments, facilitate the majority of movement in the flexed posture. This opens the joint and stretches the capsule. The capsule is viscoelastic. As such, the elastic zone may diminish over time. Without the ability to return to its neutral state, mobility may increase as the joint capsule is stretched.

The facet surface area of the articular surfaces increases as one descends the lumbar spine. The relatively sagittal orientation of the facet joints in most of the lumbar spine restricts rotational movements. Flexion and anteroposterior translation are not restricted by this portion of the vertebral column. The L5-S1 joint is the exception, with a more coronal orientation of the facet joint and its facet-facet interface. This is one of the main causes hypothesized to lead to the higher incidence of degenerative spondylolisthesis at L4-5, with translation contained by the facet interface at the lower level. Instead, pars defects occur at this level, causing a large percentage of the cases of subluxation. The lordosis at these levels also increases the shear forces at the lower levels as the orientation of the spine itself becomes more horizontal with respect to gravity in the upright posture. This places increased strain on the facet joints.

Tropism must also be considered. Tropism is manifested by asymmetry in the bilateral facet joints with respect to their angles, with one having a more coronal orientation than the other. The incidence of tropism is increased in degenerative disc disease—perhaps suggesting a contributory factor. The vertebral body rotates toward the more oblique facet with axial loading, possibly leading to increased stress on the anulus fibrosus and accelerated disc degeneration.⁵

Indications

Adjunct to Noninstrumented Fusions

Pseudarthrosis rates are reported at 10% with bone graft alone during fusion procedures for one level, and possibly greater than 30% with more than two or three levels.⁶ Internal fixation has been used extensively to assist with fusion procedures in modern spine surgery.^7,⁸ Facet fixation specifically has been shown to decrease pseudarthrosis rates over noninstrumented fusion and to have a low incidence of complications.⁹

Dorsal fusions were largely supplanted by dorsolateral fusions in the 1980s owing to a decrease in pseudarthrosis rates. Kornblatt et al. also showed that internal fixation, specifically with facet fixation or pelvic rods, improved the rate of fusion (87% vs. 76% without fixation) and time to radiographic fusion (6.2 months vs. 10.5 months) significantly.⁶ This was also shown by Jacobs et al., using translaminar facet fixation and judging fusion by oblique and flexion-extension films.¹⁰ Both studies used patients who had a pseudarthrosis from a prior procedure. They demonstrated that facet fixation can promote fusion after failed procedures—with the caveat that outcomes deteriorate with each successive surgery in most spinal procedures.¹¹

Adjunct to Anterior or Posterior Lumbar Interbody Fusion

Posterior lumbar fusion may be used for a variety of indications. Because facet fixation does not allow distraction or manipulation of alignment, the majority of uses involve restriction of movement to facilitate fusion, either because a discectomy has been performed or because the patient experiences painful symptoms with motion.

One of the uses that has caused a resurgence in facet screw popularity is as an adjunct to anterior lumbar interbody fusion (ALIF), when there is no need for posterior nerve root decompression. Failure of fusion with ALIF alone has been reported in up to 24% of cases.^12,¹³ Cadaveric studies have shown that ALIF alone allows more movement during extension with little preload than does the preoperative spine—a risk for graft displacement and poor fusion. Facet fixation with translaminar screws enhances stability, returning motion to the level allowed preoperatively.¹⁴ Kandziora et al. showed the equivalence of ipsilateral facet screws to translaminar screws with regard to range of motion, neutral zone, and elastic zone.¹⁵ However, they also demonstrated improved parameters in all test modes with pedicle screws.

For similar reasons, facet screws are also used as an adjunct to posterior lumbar interbody fusion; this was described as early as 1988, with only 1 complication in 35 patients and fusion apparent in all with the use of postoperative thoracolumbosacral orthosis immobilization.¹⁶ This procedure is useful to enhance fusion acquisition and prevent motion that may lead to graft displacement. With posterior lumbar interbody fusion, no further surgical exposure is necessary to place the screws (Fig. 154-1).

FIGURE 154-1 Model demonstrating orientation of cortical lag screws across inferior facet into superior facet and underlying pedicle.

Segmental Instability

Patients who have become unstable though degeneration or trauma may benefit from a dorsal arthrodesis. Although pedicle screw fixation with rods is increasingly used, facet fixation remains a viable alternative. However, the alignment must already be appropriate because facet fixation does not permit the application of enough leverage to mobilize spinal segments and alter the alignment.

Painful Disc Syndromes

The theory behind posterior lumbar fusion for degenerative disc disease is elimination of motion at the affected segment. Outcomes have been shown to be equivalent to interbody procedures in multiple studies.^17,¹⁸ It is thought that the painful portion of the segment is the disc as a whole or the anulus fibrosus and, therefore, dorsal fusion alone may yield poor outcomes unless the disc material is removed, such as with an interbody procedure (see earlier discussion). However, dorsal fusion alone has significant advantages, such as shorter operative time, less risk for complication, lower cost, preservation of the anterior column, and ease of procedure.