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The determination of spine stability and instability is a challenge. It depends on the definition of the anatomic elements involved and the determination of the extent to which they are injured. The study of the biomechanics of the spine encompasses many controversial topics and continues to evolve. The goals of this chapter are to introduce the basic biomechanical principles of the intact and diseased cervical spine, to define the most accurate parameters regarding the definition of spine stability, and, through biomechanical scientific evidence, to assist treating physicians in crafting the optimal strategy for management of the unstable spine.
Spine Biomechanics
More important than biomechanical instability itself is the definition of clinical instability. In their classic biomechanical textbook, White and Panjabi introduced the most widely accepted definition of clinical spine instability: “Clinical instability is the loss of the ability of the spine, under physiologic loads to maintain relationships between vertebrae in such a way that there is neither initial or subsequent damage to the spinal cord or nerve roots, and in addition, there is neither development of incapacitating deformity nor severe pain.” Spine instability may be caused by trauma leading to bony or ligamentous injury, by infection, by tumor, or by iatrogenic resection of the spinal elements. Multiple in vitro and in vivo studies have been performed with the goal of defining the stability of the spine segment in question, and the results of these studies aid in treatment decisions.
Upper Cervical Spine Stability: Principles and Biomechanical Evidence
Unique bone and ligamentous anatomica features form the elements responsible for the stability of the upper cervical spine. Heller and colleagues tested the isolated biomechanical properties of the transverse ligament of C1 by simulating an anteroposterior shear injury mechanism. Eleven specimens failed in the midsubstance of the ligament, and 2 failed by bony avulsion. The mean load to failure was 692 N (range, 220 to 1590 N), and the mean displacement to failure was 6.7 mm (2 to 14 mm). These investigators concluded that anteroposterior translation of the C1 transverse ligament in relation to the C2 dens is essential for its fracture, and the rate of loading affects the type of injury (the greater the rate, the more probable it is a ligamentous injury, as opposed to a fracture). When the transverse ligament-dens complex fails, either by midsubstance tear or by dens fracture, the greatest increase in instability is in flexion and extension (42% or 22 degrees), followed by lateral bending (24% or 8 degrees), and least in axial rotation (5% or 5 degrees).
The alar ligaments have been extensively studied, and although the involved mechanics is more complex, the alar ligaments have been shown mainly to limit axial rotation. Their transection increases contralateral axial rotation by approximately 15%; as in the transverse ligament, alar ligament rupture is rate dependent. In one report, these ligaments failed at 13.6 Nm at 4 degrees per second and at 27.9 Nm at 100 degrees per second. Radiographically, occipitoatlantal instability should be considered when the patient has more than 2 mm of translation or 5 degrees of rotation between the occiput and C1. Significant variability exists from patient to patient, and patients with rheumatoid arthritis perhaps should be assessed by more lenient parameters. Radiographic instability is seen between C1 and C2 when the atlantodens interval (ADI) is greater than 3 mm in adults and 5 mm in children. When the ADI is greater than 5 mm, the transverse ligament is considered ruptured, and when the ADI is greater than 9 mm, both the transverse and alar ligaments are deemed incompetent. More than 50% of rotation between C1 and C2 is also considered a radiographic sign of instability.
Subaxial Cervical Spine Stability: Principles and Biomechanical Evidence
For many years, multiple clinical and biomechanical studies have been performed to understand the factors responsible for subaxial cervical spine stability. In 1978, Panjabi and associates axially loaded cadaveric cervical spines in increments of 5 kg until failure of the specimens. Ventral and dorsal soft tissue injuries were created. Ventral injuries with greater than 3.3 mm displacement at the disk level or greater than 3.8 degrees of rotation were considered unstable. Similarly, dorsal injuries resulting in 27 mm of interspinous space widening or an increase in angulation greater than 30 degrees with the axial loading were considered unstable.
The laminae and spinous processes serve as insertion points for such important dorsal stabilizers as the supraspinous and interspinous ligaments and the ligamentum flavum. Clinical studies showed that resection of these elements causes cervical spine instability in children and adults. In a cadaveric study, Goel and co-workers reported a 10% increase in the flexion-extension motion after multilevel cervical spine laminectomy. In another in vitro study, no instability was observed after multilevel laminoplasty, whereas a significant increase in motion in all planes was observed after multilevel laminectomy with 25% bilateral facetectomy.
In a clinical series, instrumentation was not performed after multilevel cervical laminectomy for resection of intramedullary tumors. The investigators wanted to avoid implant-related imaging interference on postoperative magnetic resonance imaging, to accurately monitor progression of disease more accurately. Cervical deformity developed in 52% of patients and cervical instability in 36%. Sixteen percent had moderate to severe disabling neck pain. Laminectomy of C2 was associated with cervical instability ( P = 0.02). Old age at the time of surgery correlated with cervical deformity ( P = 0.05), and multiple surgical procedures were associated with greater disability related to neck pain ( P = 0.01). The investigators ultimately concluded that only 12% of patients will develop clinical instability, and all will need postoperative magnetic resonance imaging. These investigators recommended that stabilization should be performed only in patients who develop instability.
The cervical facet joint and its capsule provide significant contribution to the stability of the cervical spine. In cadaveric studies, Zdeblick and associates showed that resection of more than 50% of bilateral cervical facets, or more than 50% of the bilateral cervical facet joint capsule, results in instability. In a finite element modeling (FEM) study, Voo and colleagues confirmed that instability, indeed, begins to develop after resection of more than 50% of both joints at the same level. An evaluation of unilateral facetectomy showed that even resection of 75% or 100% of one facet joint was more stable than resection of 50% of both joints, in flexion and extension. Unilateral complete cervical facetectomy and 75% facet resection were more unstable than bilateral 50% facet resection in lateral bending and axial rotation. Unilateral 50% facetectomy was more stable than bilateral 50% facetectomy in all planes of range of motion. A cadaveric study assessed bilateral and unilateral complete cervical facetectomy. Furthermore, Cuisick and co-workers found that bilateral facetectomy reduced the stability of the joint in 53% of specimens and that unilateral facetectomy resulted in stability reduction of 32%.
Even though ventral diskectomy without fusion was performed for many years, ultimately it was determined that the anterior elements also play a role in spine stability. Therefore, fusing the segment after complete anterior diskectomy became the gold standard of treatment. After C5-C6 diskectomy, Shulte and associates noticed an increase in the range of motion between segments (66% in flexion, 69% in extension, 41% for lateral bending, and 40% for axial rotation).
Biomechanics of Cervical Spine Instrumentation
Dens Screw Fixation
Odontoid fixation of a dens fracture was first described in the early 1980s. Initially, the use of two screws was advocated, with the theoretical advantage of increased stability and rotation resistance properties. However, because of the difficulty of placing two screws in such a small space and the findings of subsequent studies that showed no difference in biomechanical stability or fusion rate when using one or two screws, the current standard of practice is to use one screw only for the fixation of dens fractures.
Dorsal C1 and C2 Instrumentation
Initially, dorsal fixation of C1 and C2 was accomplished with dorsal wiring techniques. More rigid modern constructs improved biomechanical stability and have achieved fusion rates that approach 100%. Biomechanically, transarticular C1-C2 screws using the Magerl technique have shown a 10-fold increased rotational stiffness over dorsal wiring techniques, with similar lateral bending stiffness. C1 lateral mass and C2 pars constructs have been shown to have superior biomechanical stability characteristics in lateral bending and axial rotation when compared with dorsal wiring techniques. This same study also found no difference in stability between C1 lateral mass and C2 pars fixation when using the technique popularized by Harms and Magerl transarticular screws. Adding a dorsal tension band (wires) to the transarticular C1-C2 construct has been shown to increase flexion-extension stability biomechanically, thus making this combination construct a popular choice among clinicians when transarticular screws are used.
Dorsal Instrumentation of the Subaxial Cervical Spine
Wire constructs for stabilizing the subaxial cervical spine have been used for many years. In a biomechanical study, Cuisick and colleagues showed that either facet-lamina wiring or interfacet wiring can partially restore (20% of the intact joint strength) the stability of unilateral or bilateral complete facet joint resection. Currently, one of the most popular techniques among spine surgeons for dorsal cervical instrumentation is the use of lateral mass screws. This relatively simple technique has a low complication rate and is associated with excellent results overall. Biomechanically, the fixation strength of lateral mass screws is strongest at C4, and it becomes progressively weaker toward either end of the cervical spine (C2 and C7). In addition, bone quality has been consistently shown to be worse at the lateral mass of C7 when compared with the other cervical levels. The lateral mass fixation at C2 and C7 is often hindered by lack of high-quality or sizable bone mass. At these two levels, other fixation techniques are usually used. Biomechanically, pedicle screws are superior to lateral mass fixation in any level of the cervical spine. Pedicle screws have demonstrated a significantly lower rate of loosening at the bone-screw interface, greater strength after fatigue testing, and greater pull-out strength when compared with lateral mass screws. Their placement is, however, technically demanding and not free of complications. Cervical pedicle screw insertion has been considered too risky and maybe unnecessary, except at the C2 and C7 levels.
Intralaminar Screw Fixation for the Upper and Lower Cervical Spine
The use of intralaminar (also known as translaminar) screws for the fixation of C2, C7, T1, and T2 has become increasingly popular. Minimal complications and high fusion rates have been reported when using intralaminar screws for constructs at C2 and C7. Intralaminar screws from C3 to C6 are not recommended because the laminar thickness of these segments is usually less than 3.0 mm, too small to accept a 3.5- or 4.0-mm screw safely. C2 intralaminar screw fixation has been shown to be biomechanically equivalent to more traditional C2 screw fixation techniques while decreasing the risk of the vertebral artery injury ( Fig. 7-1 ). Moreover, this type of intralaminar screw fixation has also been used as an alternative technique for patients with intact dorsal elements who require fixation in the upper thoracic spine and C7 vertebra, where pedicle screw insertion is possible but not risk free, and the lateral mass bone quality and size are not optimal.
