Summary of Key Points
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Facet fixation has several advantages over the more frequently used pedicle screw fixation, including lower cost, less dissection, more room for bone graft, lower complication rates, and a lower rate of infection.
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Facet fixation has been shown to decrease pseudarthrosis rates over noninstrumented fusion and to have a low incidence of complications.
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Facet fixation is a viable alternative to pedicle fixation in the field of lumbar stabilization, and although pedicle screw placement is more widely utilized for lumbar fixation than facet screws, facet fixation is a valuable technique that should be in the armamentarium of all spine surgeons.
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. This was modified by Boucher in 1959 by using longer screws and slightly altered placement. Translaminar placement was introduced in 1984 by Magerl in a paper describing its use as an adjunct to external spinal fixation.
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 did not require the presence of intact dorsal elements, as did the translaminar approach for facet screws. However, facet fixation is once again emerging as a viable alternative to pedicle fixation 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, and at a distance from the instantaneous axis of rotation, their orientation can dictate the direction of movement allowed in different areas of the spine. Facet joint 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 emanating from the neuroforamina of the same and often the upper level innervate the joint. 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 proprioceptive and tactile sense.
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% in direct axial loading. 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, the facet joints, 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 articular surface area of the facet 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. This sagittal orientation of the facet joint at L4-5 is hypothesized to lead to the higher incidence of degenerative spondylolisthesis at L4-5, with translation contained by the facet interface at the lower level. The L5-S1 joint is the exception in the lumbar spine, with a more coronal orientation of the facet joint and its facet-facet interface. Therefore, at L5-S1 anteroposterior translation is limited by the facet joint, and a large percentage of cases of subluxation at this level are secondary to pars defects. Lordosis at these levels also increases the shear forces exerted in the lumbar spine 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.
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. Internal fixation has been used extensively to assist with fusion procedures in modern spine surgery. Facet fixation specifically has been shown to decrease pseudarthrosis rates over noninstrumented fusion and to have a low incidence of complications.
Dorsal fusions were largely supplanted by dorsolateral fusions in the 1980s owing to a decrease in pseudarthrosis rates. Kornblatt and coworkers also showed that internal fixation, specifically with facet fixation or pelvic rods, improved the rate of fusion (87% versus 76% without fixation) and time to radiographic fusion (6.2 months versus 10.5 months) significantly. Jacobs and colleagues also arrived at this result using translaminar facet fixation and judging fusion by oblique and flexion-extension films. 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.
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 main use involves restriction of movement to facilitate fusion, either because a discectomy has been performed or because the patient experiences painful symptoms with motion.
One procedure that has led to a resurgence in facet fixation popularity is its application 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. 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 after ALIF by returning motion to preoperative levels. Kandziora and associates showed the equivalence of ipsilateral facet screws to translaminar screws with regard to range of motion, neutral zone, and elastic zone. 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. 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 ( Figs. 87-1 and 87-2 ).
Segmental Instability
Patients who have become unstable through degeneration or trauma may benefit from dorsal arthrodesis. Although pedicle screw fixation with rods is increasingly used, facet fixation remains a viable alternative. However, spinal 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 the treatment of low back pain secondary to degenerative disc disease is the elimination of motion at the affected segment. Outcomes have been shown to be equivalent to interbody procedures in multiple studies. 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 the earlier discussion). However, dorsal fusion alone has significant advantages, such as shorter operative time, lower for complication rates, lower cost, preservation of the anterior column, and ease of procedure.
Painful Facet Syndrome
In the healthy spine, the nociceptors in the facet joint capsule fire only under supranormal physiologic conditions. However, it has been shown that with inflammation, chemical mediators can sensitize the pain sensors to fire excessively or spontaneously. One study suggested that 15% of patients with low back pain had pain of facet origin. In patients with lumbar facet syndrome, Helbig and Lee demonstrated a positive response to facet block and facet rhizolysis in 50% to 60% of cases.
The existence of the so-called facet syndrome is still rather controversial. In 1996, Cavanaugh and colleagues showed in rabbits that algesic chemicals, such as carrageenan or substance P, can sensitize pain receptors or induce persistent firing of these neurons with small movements.
Results of studies examining the therapeutic potential of facet injections have been inconclusive, with some showing significant improvement and others showing equivalent results with the injection of normal saline. Once nonsurgical treatment has been exhausted, facet fixation can eliminate motion at the segment, thereby decreasing or eliminating pain.
Translaminar Facet Screws
Technique
The first description of the translaminar approach was by Magerl in 1984. However, the topic of the paper was actually an external fixator and the method of placing facet screws was only vaguely referenced. The trajectory of translaminar facet screws is well documented in the article with the screws entering at the base of the spinous process, traveling the length of the contralateral lamina, crossing the facet joint, and ending at the base of the transverse process ( Fig. 87-3 ). Multiple authors have described percutaneous translaminar screw placement using fluoroscopy ( Fig. 87-4A–E ).
Related posts:
Anatomy of Nerve Root Compression, Nerve Root Tethering, and Spinal Instability
Ventral and Ventrolateral Subaxial Decompression
Complex Lumbosacropelvic Fixation Techniques
Timing of Surgery Following Spinal Cord Injury
Spinal Wound Closure
Explant Analysis of Wear, Degradation, and Fatigue in Motion Preserving Spinal Implants
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