3.1 Traditional Concepts

White and Panjabi define clinical stability of the spine as the ability of the spine under physiologic loads to limit patterns of displacement so that the spinal cord and nerve roots are not damaged or irritated and, in addition, to prevent incapacitating deformity or pain caused by structural changes. ¹ Spinal stability is a phenomenon of increments (shades of gray, so to speak); it is not absolutely absent or present. Depending on circumstance, the spine is expected to provide varying degrees of support (stability). ² Therefore, spinal stability should be defined according to circumstances.

The converse of stability, obviously, is instability. Whereas stability is difficult to define, instability is somewhat more easily quantified and assessed. Because instability is possibly more appropriate to consider clinically, it is discussed here. Instability should be defined generally, with specific consideration given to the type of instability. Instability is the inability to limit excessive or abnormal spinal displacement. The use of the word excessive reflects the difficulty of clinical quantitation. This chapter focuses on the understanding of, and how to deal with, the uncertainty associated with the quantitation of instability.

There are two fundamental categories of instability: acute and chronic. Acute instability may be broken down into two subcategories: overt and limited. Chronic instability can likewise be broken down into two subcategories: glacial instability and the instability associated with dysfunctional segmental motion. These subcategories are not distinct from each other.

3.2 The Quantitation of Acute Instability

One has only to read the voluminous literature on acute spinal instability to appreciate the difficulties associated with the definition process. ^{3 – 25} Many authors have attempted to quantitate the degree or extent of acute instability by a point system approach. White and Panjabi describe a region-specific point system in which an accumulation of five or more points indicates the presence of an unstable spine. Their regional point system emphasizes differences between the cervical, thoracic and thoracolumbar, and lumbar regions. ¹ It is emphasized that these are essentially assessments of overt and limited instability, as defined in the following pages.

Stability determination algorithms are ultimately intended to delineate the most appropriate management scheme in any given clinical situation. It perhaps should not rely significantly on regional differences within the spine. In this vein, the multiple schemes of White and Panjabi ¹ for determining the extent of acute instability are combined here into a scheme for the subaxial spine that is not region-specific (Table 3.1). This “point” system attempts to objectively define the extent of stability, or the lack thereof. It must be recognized that a determination of the extent of acute instability is often difficult and depends on the philosophy or orientation of the surgeon and on the limitations of the available diagnostic armamentarium.

From a biomechanical perspective, clinical instability is more closely associated with the neutral zone than with the overall range of motion. Thus, the greater the neutral zone (increased laxity), the less the stability (see Chapter 1).

In light of the foregoing, White and Panjabi recommend a stretch test for the assessment of acute cervical spine stability. ¹ This involves the progressive addition of cervical traction weight (to 33% of the patient’s weight) with serial radiographic and clinical assessments. A positive test (indicating the presence of instability) shows a disc interspace separation of more than 1.7 mm or a change in angle between vertebrae of more than 7.5 degrees between the prestretch and poststretch conditions. The merits of this test are uncertain. First, it is clearly not without risks, whether those risks be immediately obvious or occult. The risk for tethering the spinal cord over a ventral mass goes without saying. Perhaps the most significant and least immediately recognized risk of such a procedure is the risk of a false-negative test—that is, the seeming presence of stability in an unstable situation. Unfortunately (in the author’s opinion), this test has been used as a determinant of eligibility for participation in contact sports. The surgeon must remember that, particularly in athletes, the resistance to stretching by muscle action (voluntary or involuntary) may easily conceal ligamentous deficiencies. Furthermore, physical contact during many sports results in a far greater transmission of force to the spine than that achieved during a stretch test. Finally, the loads are of a different nature (stretch vs axial loading).

Dynamic radiography may provide utility. ^{8 – 10 ,​ 26} However, the surgeon must keep in mind that flexion and extension radiographs may not be helpful. In fact, following trauma, they may be misleading. If pathology is observed and iatrogenic injury via the act of flexion and extension is not incurred, they are useful. Regardless, they are not without risk if spinal instability is present. Perhaps the most significant “risk” is associated with a scenario in which a “normal” flexion–extension radiograph is interpreted as indicating a safe clinical situation, when indeed such is not so. In this case, the test result is falsely negative. Incomplete patient cooperation and “guarding” against excessive spinal movement because of underlying acute pathology can disguise a pathologic process that may lead to catastrophe if treated improperly. If flexion–extension radiographs are used, the surgeon must have a good grasp of the normal flexibility characteristics of the spine. ^{27 ,​ 28} Degenerative disease is associated with similar concerns. ²²

Acknowledgement of the extent of instability is critical for surgical decision making. It, however, is also critical for other reasons, such as turning, ambulation, and intubation considerations. The notion that nasal intubation is safer than endotracheal intubation may not be valid, at least regarding the upper cervical spine. ²⁹

The determination of the presence of more chronic forms of instability should be considered separately. These clinical situations are obviously different from acute trauma, as are the surgeon’s expectations and the patient’s risks.

3.3 “Column” Concepts of Spinal Integrity

Many instability definition schemes use point systems to quantitate the extent of spinal integrity (or loss thereof) and to ultimately determine the presence or absence of spinal stability (see Table 3.1). These schemes are usually based on a “column” concept of spinal structural integrity, such as those described by Holdsworth, by Bailey, by Denis, by Kelly and Whitesides, and by Louis (Fig. 3.1). ^{14 ,​ 15 ,​ 30 – 33} The consideration of “columns” in defining the extent of instability is of some value because it helps the physician to conceptualize and categorize case-specific phenomena. ^{16 ,​ 17 ,​ 34} The three-column (one ventral column and two lateral columns) theory of Louis ³³ is based on the fact that the spine bears axial loads principally by accepting these loads along the three vertical bony and soft tissue columns (the vertebral body and intervertebral discs and the two facet joint complexes) at each segmental level (see Fig. 3.1). Although this is indeed true, the concept of Louis assists in the instability assessment process only when predominantly axial loads are considered. It assesses the bony component of failure much more effectively than the soft tissue component of failure because of its obvious association with the bony columns of the spine (vertebral body and facet joints). This aspect of stability is easily assessed by radiography and computed tomography (CT). It can be quantitated by assessing the extent of collapse or fracture. However, except for the case of significant vertebral body failure, a correlation between the extent of bony injury and the presence of overt spinal instability may be tenuous. Furthermore, Louis’s three-column theory does not facilitate assessment of the distraction, flexion, and extension components of an injury.

The two- and three-column concepts of Holdsworth, Bailey, and Kelly and Whitesides (two columns) and of Denis (three columns) ^{14 ,​ 30 – 32} are more applicable to this situation (see Fig. 3.1). They not only assist in assessing the bony collapse associated with axial load bearing but also offer insight into the assessment of the distraction, flexion, and extension components of the injury (i.e., injury to the dorsal elements) of the spinal elements of the spinal column. Denis’s three-column theory, which adds the concept of a middle column to the two-column theories, allows specific assessment of that component of the spinal column in the region of the neutral axis. The neutral axis is that longitudinal region of the spinal column that bears a significant portion of the axial load and about which spinal element distraction or compression does not excessively occur with flexion or extension (Fig. 3.2). Usually, the neutral axis is located in the region of the middorsal aspect of the vertebral body—that is, the middle column of Denis. Usually, the instantaneous axes of rotation (IARs) in the sagittal plane are located close to or within the neutral axis (see Chapter 2). ^{35 ,​ 36}

The three columns of Denis ³¹ are conceptually useful for determining the presence or absence of acute instability. Point systems designed to quantitate stability often use the three-column concept of Denis.

3.4 Categorization of Instability

To facilitate understanding of and, therefore, clinical application of the terms stability and instability (overt and limited; see below), a more simplistic approach is taken here for the subaxial cervical, thoracic, and lumbar spine. ³⁷ This is done because strict criteria for the universal definition of stability and instability are impossible to derive. Therefore, the surgeon must realize up front that the clinical decision-making process, as it pertains to the definition of instability, is somewhat tenuous, and that it relies heavily on clinical judgment and the surgeon’s intuition and “savvy.”

Instability is divided here into two categories: acute and chronic. Each is unique. However, neither of these categories is clearly defined. Each uses, at least in part, the concepts of each of the column theories discussed.

Instability, being a phenomenon that is unique to a specific clinical circumstance, is most appropriately defined separately for each category, rather than in regard to the schemes of White and Panjabi, which define instability in a global sense but quantitate it on a region-specific basis. ¹ The scheme used here for acute instability categorizes instability (overt and limited) with regard to the potential for catastrophe. The scheme used here also differs from that of White and Panjabi by its de-emphasis of region specificity.

Four subcategories of instability are also defined here. These are referred to throughout the text. These categories are (1) overt instability, (2) limited instability, (3) glacial instability, and (4) the instability associated with dysfunctional segmental motion (Table 3.2). The first two are acute and the second two chronic. As already mentioned, these categories are not distinct from each other. None are applicable to all clinical situations. Overt instability and limited instability are applicable to acute posttraumatic situations or cases of spinal involvement by tumor or infection. Therefore, the relatively acute disruption of spinal integrity is implied. Both of these categories of instability may have a chronic component, as well. For example, if an overtly unstable spine is not surgically stabilized and does not acquire stability nonoperatively, the acute overt instability blends or merges into a chronic phase. Similarly, if a spine with acute limited instability does not heal properly, excessive ligamentous laxity may persist and become chronic. The latter may be difficult to differentiate, at times, from glacial instability or dysfunctional segmental motion. Glacial instability and dysfunctional segmental motion are usually manifestations of a process that is more chronic than overt or limited instability. They are usually associated with degenerative disease or the long-term sequelae of trauma, tumor, or infection.

The point system presented here for the quantification of overt and limited (acute) instability (see Table 3.1) is relatively independent of spinal level (excepting the occiput and upper cervical spine). It depends, instead, on the category of spinal instability considered. Therefore, Table 3.1 is appropriate for consideration only with regard to delineation of the presence or absence of acute spinal stability.

In any given clinical situation, the surgeon may ask if overt instability, limited instability, glacial instability, or dysfunctional segmental motion exists. If none of these is present, the spine is stable. If instability exists in one or more of these subcategories, the decision-making process is dictated by the clinical situation, including the anticipated loads to be applied to the spine.

Ultimately, the need to define the subcategory of instability present in any given clinical situation is based on the need to attend to the patient’s pathology. Obviously, there are numerous potential options for treatment. These include surgery for decompression and/or stabilization, bed rest, external splinting, and medications for pain and/or inflammation. Each subcategory of instability, therefore, is associated with a number of indications for treatment, as well as types of treatment strategies. These schemes may be complex. They should, however, be clearly established in the mind of the clinician. As long as the surgeon’s scheme is individually “thought out” and based on sound principles, it should serve the surgeon (and the patient) well.

To optimize patient management, the spine surgeon should first determine the extent of instability present (and the category of instability). Then, the patient’s symptoms (complaints) must be considered, as well as the extent of neurologic compromise, the risks for further neurologic injury, and the desires and concerns of the patient. For example, a patient with a spine injury that is moderately unstable (e.g., a fractured facet joint with interspinous ligament disruption identified clinically and by magnetic resonance (MR) imaging; limited instability) is at moderate risk for deformity progression and delayed neurologic injury. Therefore, the surgeon might recommend stabilization and fusion surgery. If the patient agrees, the surgery should be performed if it is not medically contraindicated. On the other hand, a patient who has significant laxity at the L4–L5 motion segment (dysfunctional segmental motion), but whose symptoms have responded to nonoperative management (see Chapter 37), should not have surgery, regardless of his or her desires. In the former case, surgery is used as a management option to prevent further harm. In the latter case, despite the presence of an unstable spine (ligamentous laxity; dysfunctional segmental motion), the patient is without symptoms and should not have surgery. Spinal instability has widely disparate implications in different clinical circumstances. In the former case, the ability of the spine to resist “excessive” displacement was thought to be deficient, requiring surgical stabilization. In the latter case, it was not thought to be deficient because the laxity was not progressive and did not cause refractory pain or neurologic deficit (chronic instability).

The terms overt instability and limited instability are applicable to situations in which there is a risk for acute loss of stability. The term glacial instability is applicable in more chronic situations. Glacial instability is confirmed by serial assessments or by incriminating evidence (e.g., a translational deformation of the spine in the presence of a pain syndrome consistent with the deformation). The term dysfunctional segmental motion is much less objectively defined. It applies to situations in which overt or limited clinical instability is not present, but in which pain, combined with abnormal significant spinal motion, is present. Each is defined in the following pages.