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Lumbar Intervertebral Instability: A Review¹

¹ From the Department of Radiology, Catholic University, School of Medicine, Largo A. Gemelli 8, 00168 Rome, Italy (A.L., L.B.); Department of Radiology, Scientific Institute Hospital Casa Sollievo della Sofferenza, San Giovanni Rotondo and University of Foggia, Italy (G.G.); and Department of Diagnostic Imaging, the Robert Jones and Agnes Hunt Orthopaedic and District Hospital, Oswestry, Shropshire, England (V.N.C.). Received August 15, 2005; revision requested November 3; revision received February 5, 2006; accepted March 7; final version accepted June 1; final review and update by A.L. March 22, 2007. Address correspondence to A.L. (e-mail: a.leonemd{at}tiscali.it).

	ABSTRACT

TOP ABSTRACT INTRODUCTION DEFINITION OF INSTABILITY FUNCTIONAL ANATOMY AND... DEGENERATIVE LUMBAR... PAIN AND VERTEBRAL INSTABILITY... IMAGING EVALUATION OF LUMBAR... CLINICAL AND RADIOLOGIC... CONCLUSION ESSENTIALS References

 
Intervertebral instability of the lumbar spine is thought to be a possible pathomechanical mechanism underlying low back pain and sciatica and is often an important factor in determining surgical indication for spinal fusion and decompression. Instability of the lumbar spine, however, remains a controversial and poorly understood topic. At present, much controversy exists regarding the proper definition of the condition, the best diagnostic methods, and the most efficacious treatment approaches. Clinical presentation is not specific, and the relationship between radiologic evidence of instability and its symptoms is controversial. Because of its simplicity, low expense, and pervasive availability, functional flexion-extension radiography is the most thoroughly studied and the most widely used method in the imaging diagnosis of lumbar intervertebral instability. In this article, we provide an overview of the current concepts of vertebral instability, focusing on degenerative lumbar intervertebral instability, and review the different imaging modalities most indicated in diagnosing vertebral instability.

© RSNA, 2007

	INTRODUCTION

TOP ABSTRACT INTRODUCTION DEFINITION OF INSTABILITY FUNCTIONAL ANATOMY AND... DEGENERATIVE LUMBAR... PAIN AND VERTEBRAL INSTABILITY... IMAGING EVALUATION OF LUMBAR... CLINICAL AND RADIOLOGIC... CONCLUSION ESSENTIALS References

 
The spine is made up of segments, described as "motion segments," consisting of two vertebrae and the interconnecting soft tissue. In normal conditions of daily life, the spine is able to meet essential functional requirements: strength, mobility, and stability. This is the result of specific mechanical characteristics of each individual spinal component, as well as of the efficient integration of these components into the overall structure of the spine.

Spinal stability is defined as the ability for the vertebrae to maintain their relationship and limit their relative displacements during physiologic postures and loads (1). The requirement of stability is essential to the spinal column to prevent premature mechanical and biologic deterioration of its components. It is also fundamental to protect the spinal cord and nerve roots and to minimize energy expenditure.

One important mechanical function of the lumbar spine is to support the upper body by transmitting compressive and shearing forces to the lower body during the performance of everyday activities. To enable the successful transmission of these forces, mechanical stability of the spinal system must be ensured. Stability of the lumbar spine as a whole is maintained by the cooperation of disks, joints, ligaments, and muscles. Degenerative processes in the disk and facet joints affect the stability of the motion segment (2–8). Although segmental instability is often used synonymously with degenerative spondylolisthesis, it is clear there are numerous other conditions that are potentially unstable (spinal acute trauma, surgery, spondylolysis, tumors, or infections).

The primary objectives of this review are to summarize the current concepts of vertebral instability by focusing on degenerative lumbar intervertebral instability and to review the different imaging modalities used to make the diagnosis as evident as possible.

	DEFINITION OF INSTABILITY

TOP ABSTRACT INTRODUCTION DEFINITION OF INSTABILITY FUNCTIONAL ANATOMY AND... DEGENERATIVE LUMBAR... PAIN AND VERTEBRAL INSTABILITY... IMAGING EVALUATION OF LUMBAR... CLINICAL AND RADIOLOGIC... CONCLUSION ESSENTIALS References

 
Despite the effort of several authors to define lumbar spinal instability, no generally accepted definition is yet available. A major problem is that the concept of instability means different things to different specialists (clinicians, radiologists, bioengineers). However, a reasonable definition has been proposed by Pope and Panjabi (9) and Frymoyer and Selby (10). By advocating a biomechanical approach, they defined instability as a loss of motion segment stiffness, such that force application to that motion segment produces abnormally great motion compared to that of a normal spine. In other words, instability can be defined as an abnormal response to applied loads characterized kinematically by abnormal movement in the motion segment beyond normal constraints (10). This abnormal movement can be explained by damage to the restraining structures (ie, facet joints, disks, ligaments, and muscles) that, if damaged or lax, will lend to altered equilibrium and thus instability (9). In a biomechanical sense, stiffness is defined as the ratio of the load applied to a structure to the resulting motion.

The definition of instability as a loss of motion segment stiffness has also been supported by Panjabi et al (11). Their study was based on the concept that the loads applied to the motion segments of the human spine may be divided into those due to body posture and superimposed body weight (preload) and those due to various physical activities (physiologic loads). In that study, Panjabi et al (11) excised lumbar spine segments from cadavers within 16 hours of death and then applied a given axial preload to a motion segment followed by 12 physiologic loads (applied one at a time) for measuring the resulting three-dimensional motion. The results were calculated in the form of load displacement curves. For each combination of a preload and a physiologic load component, there were six load displacement curves: one curve representing main motion and five curves representing coupled motions. Two of their conclusions were (a) the main and the coupled motion curve are affected by the inclusion of preloads and (b) the application of any one of the 12 physiologic loads may produce six motion components, namely, three translations (one along each of the x-, y-, z-cartesian axes) and three rotations (one around each of the x-, y-, z-cartesian axes); therefore, a motion segment has six degrees of freedom (ie, six possible relative displacements of one vertebra relative to its neighbor) (Fig 1).

View larger version (10K): [in this window] [in a new window] [Download PPT slide]   Figure 1: Drawing shows three-dimensional coordinates system proposed by Panjabi and White (12) identifying translational (straight arrows) and rotational (curved arrows) movements along and/or around the x-, y-, and z-axes.

	FUNCTIONAL ANATOMY AND BIOMECHANICS OF THE LUMBAR SPINE

TOP ABSTRACT INTRODUCTION DEFINITION OF INSTABILITY FUNCTIONAL ANATOMY AND... DEGENERATIVE LUMBAR... PAIN AND VERTEBRAL INSTABILITY... IMAGING EVALUATION OF LUMBAR... CLINICAL AND RADIOLOGIC... CONCLUSION ESSENTIALS References

The Vertebral Body
The vertebral body is the key element in the load-bearing system of the spine (13). The vertebral body is made of a cortical bone shell and a core of cancellous bone, which has a honeycomb-like structure. Contrary to cortical bone, which is highly resistant but scarcely adaptive to deformation, cancellous bone is able to manage load, accepting deformation without failure. Under increasing load, when the vertebral endplates deform, blood is forced out of the vertebra through multiple vascular foramina. It comes from the squeezing of bone marrow, which has been suggested to be the main resistor of the dynamic peak loads (16). However, it should be pointed out that since the strength of the vertebral body is directly related to its osseous tissue contents, vertebrae with reduced bone contents (such as in patients with osteoporosis) will be more likely to fail under load.

The Intervertebral Disk
The intervertebral disk and the adjacent vertebral bodies form an integrated unit, in which collagen fibers of the disk are intimately related to the cartilaginous endplates. The normal intervertebral disk can be divided into nucleus pulposus and annulus fibrosus. The nucleus pulposus contains a much larger proportion of hydrophilic proteoglycans than the annulus fibrosus, while the annulus fibrosus contains a much higher amount of collagen (17). The annulus fibrosus consists of a complex system of fiber bundles called lamellae, which become progressively more compact centrifugally with differentiation into Sharpey fibers, whereby there is a direct bony anchorage at the peripheral attachment of the annulus with the vertebral body rim. These collagen fibers blend with the anterior and posterior longitudinal ligaments and act together to stabilize the vertebral motion segment (18).

The intervertebral disk is the primary load-bearing structure in the spinal motion segment. The nucleus with its high water content has hydrostatic properties, acts as a fulcrum for spinal movement, and provides for the radial transmission of forces. Loading perpendicular to the surface of the disk is transmitted radially by the nucleus and distributed transversally within the annulus, which resists strongly at the periphery. By providing dispersion of loading forces, the nucleus decreases the risk of mechanical failure.

In normal conditions, a positive pressure is present within the nucleus pulposus at rest and it increases as loads are applied to the spine. In axial compression, the increased intradiskal pressure is counteracted by annular fiber tension and disk bulging. In flexion, extension, and lateral bending, the same process occurs. In axial rotation, the annular fibers in one direction are stretched, whereas those contralaterally are shortened or crimped (13). Since rotation and lateral bending may be coupled to each other, the stresses in the disk are thus a combination of tension, compression, and shear.

The Facet Joints
Facet joints are extensions of the laminae and are covered by hyaline cartilage on their articulating surface. The inclination of these articulations with respect to the midline varies among individual vertebral segments. The almost sagittal alignment of the facet joint plane in the proximal lumbar spine gradually becomes a more coronal orientation in the lower lumbar spine. This articular configuration permits a large range of motion in the sagittal plane (flexion-extension), a more limited range of lateral (right or left) bending, and greatly limited axial rotation (19). The biomechanical importance of the lumbar facet joints and their capsules is well established (20–25). The facet joints are subject to substantial forces and help shield the lower lumbar disks from shear loads (22,23); moreover, they are the primary elements acting against rotational or torsional forces (21,24). In active extension, the facets can function as a fulcrum, thereby reducing load on the anterior and middle spinal columns of Denis (25) and, therefore, on the disk. By reducing the load on the disk, the fulcrum effect reduces disk protrusion (26). In 1983, Denis (25) proposed the "three-columns" theory. The spine has three load-bearing columns on the sagittal plane: anterior, middle, and posterior columns. The anterior column consists of the anterior longitudinal ligament and the anterior half of the vertebral body and intervertebral disk. The middle column is formed by the posterior longitudinal ligament and the posterior half of the vertebral body and intervertebral disk. The posterior column includes all bone and ligamentous structures posterior to the posterior longitudinal ligament and includes the pedicles, laminae, facets, spinous processes, and all associated ligaments.

The capsular structures around the facet joints are richly supplied with pain-sensitive nerve endings, thus being functionally important in low back pain. The facet capsule is probably the major stabilizing structure among the posterior elements in flexion forces. Posner et al (20) and subsequently Adams and Hutton (21), using simulated physiologic loads and motion, ablated various components and showed that the facet capsule is the major stabilizing structure and is capable of resisting about one-half of the full flexion forces.

The Spinal Ligaments
The spinal ligaments must allow for adequate motion, while ensuring fixed postural positions between vertebrae. They resist tensile forces but buckle when subject to compression.

The anterior longitudinal ligament is attached to the intervertebral disks and the adjacent endplate margins of the vertebrae, but is not tightly adherent to the anterior surfaces of the vertebral bodies (Fig 2). The posterior longitudinal ligament is less developed than its anterior counterpart in the lumbar region. It is incorporated into the collagen fibers of the posterior annulus at the disk level, but is not attached to the central part of the vertebral body posteriorly (Fig 2). The flaval ligaments are thick, broad structures that connect the laminae of adjacent vertebrae. These ligaments, owing to their high elasticity, exert a contracting force on the vertebral arches, pressing the vertebrae together and keeping them aligned (Fig 2).

View larger version (56K): [in this window] [in a new window] [Download PPT slide]   Figure 2: Drawing shows the anatomic relationship between spinal ligaments, disk, and vertebrae in a motion segment. Sagittal view. ALL = anterior longitudinal ligament, FL = flaval ligament, ISL = interspinous ligament, ITL = intertransverse ligament, PLL = posterior longitudinal ligament, SSL = supraspinous ligament. Note posterior bulging (arrowhead) of redundant posterior disk surface and posterior longitudinal ligament, which is consequence of acquired collapse of the intervertebral disk.

Muscle
The motion of each segment is actively controlled by muscles (30). The lumbar region is well endowed with active muscles. The erector spinae, abdominal, and psoas muscles are all actively involved in maintaining the functional upright and sitting stability of the lumbar spine. They also contribute to the very high loads to which the lumbar spine is subjected.

Mobility of the Lumbar Spine
Considering the motion segment under static conditions, the intrinsic stability of the spine would appear to be satisfactory: facet configuration, normal intradiskal pressure, which maintains ligaments tension, as well as geometric characteristics of the vertebrae, all seem adequate to confer stability to the spine. The challenge to the stability of the spine is in its mobility, which at any moment may modify the conditions of equilibrium by subjecting the vertebral segments to forces of acceleration.

In normal conditions, during the movements of flexion-extension of the lumbar spine, rotation in the sagittal plane, demonstrated by a change in the angle formed by two opposite vertebral endplates and translation in the sagittal plane (defined by a slip of one vertebra relative to the vertebra below), may be observed. Abnormal movements are restrained by normal intervertebral structures. The total range of motion of a spinal motion segment may be divided into the neutral zone and the elastic zone. Panjabi (30) defined the neutral zone as the range of motion in which a relatively large intervertebral motion is produced by a minimum effort; it is the initial portion of the total range of motion. In the elastic zone, located at the extreme ends of the total range of motion, movement is produced against substantial internal resistance. The neutral zone concept is based on the observation that the load-displacement curve of the typical spinal motion segment is nonlinear, with high flexibility for motion occurring around the neutral position of the spine and with increased passive resistance to motion nearer the end range of spinal motion. An increase in the neutral zone may lead to higher probability of overstretching of ligaments and be a source of instability (31).

Spinal stability depends on three functionally interdependent subsystems (30) that limit the excursion of spinal motion segments and maintain the proper ratio of neutral-to-elastic zone motion: a passive subsystem, an active subsystem, and a neural control subsystem (30). Muscular contraction of the trunk and spine muscles, under the control of postural reflexes (neural control subsystem), provide the active part of stability. The passive part of stability is provided by vertebral bodies, facet joints and their capsules, spinal ligaments, and the passive tension from the musculotendinous units (30).

The most pronounced movements of the lumbar spine are flexion and extension in the sagittal plane; other motions are axial rotation and lateral bending. More complex motions involve combinations of forward flexion, side bending, and twisting. Measurements of horizontal displacements and angulations between the vertebrae loaded in vitro after sequential sectioning of the motion segment structures helped to determine their respective roles.

Forward flexion results in anterior compression of the disk and in sliding separation of the facet joints. This movement is controlled by the posterior ligaments (interspinous and supraspinous ligaments), the facet joints and their capsules, the intervertebral disk, and the paraspinal muscles (31). In extension, the main stabilizing structures are the anterior longitudinal ligament, the anterior part of the annulus fibrosus, the facet joints, and the rectus abdominis muscle (32). Rotational movements are mainly controlled by the intervertebral disk and the facet joints (33). For side-bending movements, which are accompanied by some rotation with sliding separation of the facet joints, the intertransverse ligaments probably play an important role (34).

	DEGENERATIVE LUMBAR INTERVERTEBRAL INSTABILITY

TOP ABSTRACT INTRODUCTION DEFINITION OF INSTABILITY FUNCTIONAL ANATOMY AND... DEGENERATIVE LUMBAR... PAIN AND VERTEBRAL INSTABILITY... IMAGING EVALUATION OF LUMBAR... CLINICAL AND RADIOLOGIC... CONCLUSION ESSENTIALS References

 
Upright posture and upright weight bearing in humans cause excess stresses that are maximal at and suprajacent to the lumbosacral junction. This results in more severe age-related changes in these spinal segments. The degenerative processes of the lumbar spine generally initiate from the intervertebral disk, at the level at which progressive biochemical and structural changes take place, leading to a modification in its physical properties of elasticity and mechanical resistance. Disk degeneration, which affects the whole population, is commonly seen from age 30 years onward. The degenerative process in the disk results in a gradual disruption of the collagen fibers and reduction in the proteoglycan contents, with a gradual loss of water contents and elasticity of the disk (35). More than 50% of autopsy specimens obtained from individuals in their 3rd and 4th decade of life show peripheral tears of the annulus fibrosus (36). After age 40 years, the disk becomes progressively more fibrous and disorganized due to aging and degeneration; the final stage is represented by regions of amorphous fibrocartilage (37). This will at some point entail a superoinferior narrowing and eventual collapse of the intervertebral disk.

Three clinically relevant consequences of acquired collapse of the intervertebral disk are (a) pathologic changes in the vertebral bodies, with osteophyte development; (b) anterior bulging of the flaval ligaments and posterior bulging of the posterior longitudinal ligament, with consequential narrowing of the central spinal canal (Fig 2); and (c) posterior bulging of redundant posterior disk surface, with narrowing of the central spinal canal and of the inferior recesses of the neural foramina (Fig 2). Moreover, intervertebral disk degeneration and acquired collapse permit the adjacent vertebrae to slide back and forth over each other. This results in laxity of the ligamentous network responsible for binding the vertebrae together and leads to craniocaudal partial subluxation of the facet joints, which may be asymmetric from side to side. Subsequent stresses on the facet joints then result in osteoarthritis with ostheophytosis, which in turn causes narrowing of the lateral recesses of the central spinal canal and of the neural foramina. Furthermore, partial subluxation of the facet joints leads to the collision of the apex of the superior articular facet process with the overlying pars interarticularis and pedicle. Continued collision of these structures results in ostheophytosis and consequently to further narrowing of the central spinal canal and of the neural foramina.

Osteoarthritis of the facet joints, which may occur independently of the disk, is characterized by the thinning of the cartilage, sclerotic changes in the subchondral bone, osteophyte formation, synovial inflammation, and capsular ligament laxity (38). In more severe forms of the process, osteoarthritis of the facet joints may allow hypermobility of the facet joint and then may lead to a spondylolisthesis (Fig 3) (6). This term refers to the forward slippage (by any cause) of a vertebra on the subjacent one in the sagittal plane. In 1930, Junghanns (39) defined lumbar vertebral slippage in the absence of a bone defect in the pars interarticularis as pseudospondylolisthesis. This was later categorized as degenerative lumbar spondylolisthesis by Newman and Stone (40). Backward vertebral slippage, a type of spondylolisthesis, has been called retrolisthesis (41).

View larger version (127K): [in this window] [in a new window] [Download PPT slide]   Figure 3: Transverse CT scan through L4 vertebra at bone window in 62-year-old woman with degenerative spondylolisthesis. Subluxated inferior apophyseal processes of the superior vertebra (arrows) cause central spinal canal and lateral recesses stenosis. Note sagittal orientation of right facet joint.

The relationship between lumbar instability and degenerative spondylolisthesis was suggested by Kirkaldy-Willis and Farfan (2) who, in a functional sense, proposed three clinical and biomechanical stages of lumbar spine degenerative changes: temporary dysfunction, unstable phase, and stabilization. Spinal degenerative changes included disk degeneration, facet joints osteoarthritis, ligamentous degeneration, and muscle alterations. The duration of each stage varies greatly, and there are no clear-cut clinical signs or symptoms to distinguish one stage from the next. The first phase, defined as the temporary dysfunction phase, is associated with slight reversible anatomic changes. The second, or unstable, phase is characterized by disk height reduction, ligament and joint capsule laxity, and facet joint degeneration. In the third, or stabilization, phase, osteophytes and marked disk space narrowing lead to stabilization of the motion segment with a reduction (partial or complete) in its range of motion, sometimes after spondylolisthesis has already occurred (Fig 4). On the basis of this model, the radiologic observation of degenerative spondylolisthesis does not necessarily indicate that intervertebral instability is still present at the time of imaging because a new stabilization may have already occurred.

View larger version (134K): [in this window] [in a new window] [Download PPT slide]   Figure 4a: Functional lateral radiographs of lumbar spine in 64-year-old woman with L5 spondylolysis and grade I axial spondylolisthesis. (a) Flexion and (b) extension views obtained in 1992 show L5-S1 instability with 10° sagittal rotation between flexion and extension and an anterior translation exceeding 4 mm (unstable phase). (c) Flexion and (d) extension views obtained in 2005 show disappearance of the range of movement (late stabilization phase).

View larger version (123K): [in this window] [in a new window] [Download PPT slide]   Figure 4b: Functional lateral radiographs of lumbar spine in 64-year-old woman with L5 spondylolysis and grade I axial spondylolisthesis. (a) Flexion and (b) extension views obtained in 1992 show L5-S1 instability with 10° sagittal rotation between flexion and extension and an anterior translation exceeding 4 mm (unstable phase). (c) Flexion and (d) extension views obtained in 2005 show disappearance of the range of movement (late stabilization phase).

View larger version (126K): [in this window] [in a new window] [Download PPT slide]   Figure 4c: Functional lateral radiographs of lumbar spine in 64-year-old woman with L5 spondylolysis and grade I axial spondylolisthesis. (a) Flexion and (b) extension views obtained in 1992 show L5-S1 instability with 10° sagittal rotation between flexion and extension and an anterior translation exceeding 4 mm (unstable phase). (c) Flexion and (d) extension views obtained in 2005 show disappearance of the range of movement (late stabilization phase).

View larger version (130K): [in this window] [in a new window] [Download PPT slide]   Figure 4d: Functional lateral radiographs of lumbar spine in 64-year-old woman with L5 spondylolysis and grade I axial spondylolisthesis. (a) Flexion and (b) extension views obtained in 1992 show L5-S1 instability with 10° sagittal rotation between flexion and extension and an anterior translation exceeding 4 mm (unstable phase). (c) Flexion and (d) extension views obtained in 2005 show disappearance of the range of movement (late stabilization phase).

To test the validity of this three-stage hypothesis, Axelsson and Karlsson (47) assessed the intervertebral mobility for the two most distal lumbar disk levels in 18 adult patients with low back pain, disk degeneration, and no prior spinal surgery. Each spinal segment was placed in one of five categories according to the grade of disk degeneration. They observed that intervertebral mobility undergoes changes throughout the degenerative process and that a stage of relative stabilization is reached after the degenerative process has reduced the disk height by at least 50% (category III). Even so, they concluded that absolute stability could not be assumed even for spine segments with greater than 50% disk height reduction, as some mobility may still persist in such segments.

	PAIN AND VERTEBRAL INSTABILITY

TOP ABSTRACT INTRODUCTION DEFINITION OF INSTABILITY FUNCTIONAL ANATOMY AND... DEGENERATIVE LUMBAR... PAIN AND VERTEBRAL INSTABILITY... IMAGING EVALUATION OF LUMBAR... CLINICAL AND RADIOLOGIC... CONCLUSION ESSENTIALS References

 
Vertebral instability is thought to be a major cause of low back pain and is often an important factor in determining the surgical indication for spinal fusion with decompression. Weiler et al (44) reported that in approximately 20%–30% of patients with low back pain a vertebral instability was diagnosed, but low back pain has a very low specificity (several quite distinct lesions commonly yield much the same symptoms complex) and the radiologic demonstration of degenerative disk disease remains the only criterion substantially associated with low back pain (48).

The etiology of intervertebral disk degeneration is largely unknown, but it is thought that disk nutrition is involved (49,50). The normal intervertebral disk is avascular and receives its nutrition by passive diffusion from vessels in the endplate and around the annulus. In man, the relative contributions are unknown, but the importance of the vascular channels in the endplate has been stressed (51).

Kauppila (52) performed angiographic examinations in 22 cadaveric lumbar spines to determine the presence of any new blood vessels between adjacent lumbar vertebrae and the corresponding disks. It was found that the normal anastomosing arteries in the posterior longitudinal ligament were substantially obliterated with advancing degeneration of the disk, whereas several tiny tortuous arteries were seen in the anterolateral aspects of the intervertebral spaces connecting the adjacent vertebrae. Histologic examination of the anterolateral part of the annulus demonstrated that the vascularity of the annulus increased substantially with degeneration of the disk. Regression analysis showed that vascular changes occurred before degeneration of the disk at every lumbar level, which suggests that disturbances in the nutritional supply may precede degeneration.

Brown et al (50) used immunohistochemical techniques to demonstrate vascularity and innervation in the vertebral endplate and body and attempted to relate this to disk disease. They suggested that disorders of nutrition, possibly caused by neural disturbances, may lead to disk degeneration and demonstrated that in the most degenerate disks at least, a process of neovascularization may occur. Moreover, they found endplate cartilage defects and a marked increase in the density of nerve fibers containing sensory calcitonin gene–related peptide in highly localized areas with greater degeneration. These findings would suggest that the endplates and vertebral bodies are sources of pain.

Fagan et al (53), using a sheep model that allowed evaluation of the whole lumbar motion segment, demonstrated that the endplate innervation is denser centrally, adjoining the nucleus, but the densest innervation is in the periannular connective tissue. On the whole, the normal disk has a meager nerve supply that is limited to the very outermost structures. Hence, for any noxious stimulus to result in diskogenic pain, there must be stimulation of the nerves in the endplate or the periannular region. However, further studies are required to elucidate the functional implication of these works.

	IMAGING EVALUATION OF LUMBAR INSTABILITY

TOP ABSTRACT INTRODUCTION DEFINITION OF INSTABILITY FUNCTIONAL ANATOMY AND... DEGENERATIVE LUMBAR... PAIN AND VERTEBRAL INSTABILITY... IMAGING EVALUATION OF LUMBAR... CLINICAL AND RADIOLOGIC... CONCLUSION ESSENTIALS References

 
The diagnosis of vertebral instability is commonly based on the imaging finding of abnormal vertebral motion. There may be abnormal translation and/or rotation around the x-, y-, and z-axes of the three-dimensional coordinates system proposed by Panjabi and White (12). In this system (Fig 1), the x-axis is horizontal in the coronal plane, from left to right, the y-axis is vertical, or craniocaudal, and the z-axis is horizontal in the sagittal plane, from front to back. Vertebral instability is generally multidirectional, whereas the resulting displacement is evaluated in one plane at a time. Sagittal (front to back, or z-axis) and coronal (side to side, or x-axis) displacements are evaluated on radiographs, and displacements on the axial plane are evaluated on computed tomographic (CT) or magnetic resonance (MR) images.

Structural Changes on Neutral Radiographs
Several radiographic findings have been proposed as indicators of vertebral instability. To our knowledge, Knuttson (5) was the first to report the vacuum phenomenon in the intervertebral disk and to present its association with lumbar spine instability. Because instability may create excessive intervertebral distraction and subsequent negative intradiskal pressure, allowing interstitial nitrogen in the surrounding tissues to become gaseous and to accumulate within clefts of the degenerated disk, it is assumed that the vacuum phenomenon is often associated with vertebral instability. Moderate disk degeneration with mild disk space narrowing and osteosclerosis also have been associated with vertebral instability (Fig 4a, 4b). In contrast, a marked disk space narrowing has been considered to be indicative of the late stabilization phase described by Kirkaldy-Willis and Farfan (2) (Fig 4c, 4d).

Another classic indirect radiographic sign associated with instability is the traction spur (10,54–56), which is located 2 or 3 mm from the endplate and has a horizontal orientation (Fig 5). The proposed mechanism is that the traction spur is caused by increased tensile stresses exerted by the Sharpey fibers or by those of the anterior longitudinal ligament on the periosteum of the vertebral body, in the case of spinal instability. The claw osteophyte is a bony outgrowth arising very close to the margin of the intervertebral disk, from the vertebral body apophysis, directed with a sweeping configuration toward the corresponding part of the vertebral body opposite the disk. The claw osteophyte is not strictly associated with instability; it is regarded as a result of compression and a sign of stability restoration. Traction spurs and claw osteophytes are thought to represent different stages of the same pathologic process and frequently coexist on the same vertebral rim (57).

View larger version (137K): [in this window] [in a new window] [Download PPT slide]   Figure 5: Lateral radiograph of lumbar spine in 48-year-old man shows traction spur (arrow).

Functional Radiography
Functional radiography in the sagittal plane can be achieved either in flexion and extension (59–63) or with passive axial traction and compression (64,65). In axial traction and compression radiography, lateral radiographs are obtained with the patient in standing position. Axial traction is accomplished by letting the patient hang by his or her hands from a horizontal bar, whereas compression radiography is performed when the patient has sandbags of approximately 30% of his or her weight on the shoulders (66). However, Pitkanen et al (66), comparing traction-compression with flexion-extension lateral views in a group of 306 patients with clinically suspected instability, concluded that traction-compression radiographs were of questionable value in the diagnosis of lumbar instability.

Because of its simplicity, low expense, and wide availability, functional flexion-extension radiography is the most thoroughly studied and the most widely used method in the imaging diagnosis of lumbar intervertebral instability (2,3,66,67). Many surgeons use flexion-extension lateral views to disclose abnormal vertebral motion before deciding on surgical fusion. However, as reported by Nizard et al (68), this method is challenging and debatable for the following three reasons: (a) Its diagnostic value cannot be determined because of the lack of a nontraumatic and routinely applicable reference standard to define intervertebral instability; (b) its reproducibility is difficult, a slight variation in patient positioning or in the direction of the x-ray beam may result in a 10%–15% variation in the range of vertebral displacement (66); and (c) the appropriate way to obtain flexion-extension radiographs and the method to measure displacements are still not standardized.

The choice of patient position, lateral decubitus versus standing, which best optimizes the flexion-extension radiographs, has been subjective (3,10,56,64,67,69–73). Several authors (64,70–72) have evaluated patients with low back pain and/or spondylolisthesis and found intervertebral motion to be lower when flexion-extension radiographs were obtained with the patient in the recumbent position compared with standing. However, in patients with unstable spondylolisthesis, to maximize the chances of detecting maximum abnormal translational movement in the sagittal plane, Wood et al (73) recommended that flexion-extension radiographs should be obtained in the lateral decubitus position. In their study, more abnormal translation was observed with the patient in this position than while standing. One possible explanation for their results could be that splinting of the spine from the paraspinal postural or abdominal musculature may reduce the spine's range of motion when the patient is standing (2,3). Moreover, in symptomatic patients, pain can inhibit muscle function, resulting in an underestimation of the true intervertebral motion (64).

Flexion-extension lateral views allow measurement of the sagittal translation of a vertebra with respect to the underlying one and the amount of vertebral rotation in the sagittal plane (defined by the variation of the angle between two opposite vertebral endplates observed between the extremes of movement) (Fig 6). Historically, much interest has been focused on the excessive translation in the sagittal plane. The implications of translation overestimation in the sagittal plane (ie, in the diagnosis of instability) are inappropriate clinical decisions possibly resulting in unnecessary fusion surgery. Three sources of error in measuring translation in the sagittal plane are (a) the technique used to measure translation, (b) the quality of radiographs, and (c) the concomitant rotation in the sagittal plane (sagittal rotation) and/or rotation about the vertical axis of the spine (axial rotation) (74).

View larger version (109K): [in this window] [in a new window] [Download PPT slide]   Figure 6a: Functional lateral radiographs of lumbar spine in 56-year-old man show L4-5 intervertebral instability with 17° sagittal rotation between (a) flexion and (b) extension. Sagittal rotation corresponds to the variation of the angle (lines) between vertebral endplates adjacent to the disk.

View larger version (103K): [in this window] [in a new window] [Download PPT slide]   Figure 6b: Functional lateral radiographs of lumbar spine in 56-year-old man show L4-5 intervertebral instability with 17° sagittal rotation between (a) flexion and (b) extension. Sagittal rotation corresponds to the variation of the angle (lines) between vertebral endplates adjacent to the disk.

In the study of Shaffer et al (74), the measurement technique described by Morgan and King (75) demonstrated the overall best performance and the least interference due to concomitant motions. Other generally used techniques have been described by Posner et al (20) and Dupuis et al (3) (Fig 7); these techniques supposedly avoid inaccuracies that result from magnification by measuring translation as a percentage of vertebral body width.

View larger version (9K): [in this window] [in a new window] [Download PPT slide]   Figure 7: Measurement technique of Dupuis et al (3). Sagittal translation is measured by drawing lines U and L along the posterior cortices of upper and lower vertebral bodies. A third line I along inferior endplate of the superior vertebral body is drawn and a fourth line R is drawn parallel to L through the intersection point of lines I and U. Translation is defined as the perpendicular distance between parallel lines L and R. To obviate inaccuracies due to x-ray magnification factor, translation is measured as percentage of the width of the upper vertebral body (W). Sagittal rotation is measured by drawing perpendicular lines to posterior body lines (U and L). If apex of the angle is posterior to vertebral body, the angle is positive; if it is anterior, the angle is negative.

Side bending in the lumbar spine is a composite motion consisting of rotation about the z-axis (lateral bending) coupled with y-axis rotation (axial rotation) that occurs toward the convexity of the curve created by the z-axis rotation. Normal axial rotation may cause the spinous processes to move toward the concavity of the curve (ie, toward the direction of bending), where the disk spaces normally close, while the vertebral bodies rotate away from the side of bending (2,67). Pathologic axial rotation can be detected on side-bending radiographs if the spinous processes move to the convex side, producing an asynchronous spinous process line (2,67). Pathologic rotation can also manifest as a lateral translation (laterolisthesis) of one vertebra on another during lateral bending (3). Additional signs that have been proposed as radiographic indicators of abnormal movement include a loss of vertebral body movement and paradoxical opening of the disk space on the bending side (2).

Pitkanen and Manninen (67), analyzing retrospectively flexion-extension and side-bending radiographs of 300 patients with clinically suspected lumbar spinal instability, reported that side-bending radiographs are complementary to flexion-extension radiographs. Side-bending radiographs should be obtained if side-bending instability is clinically suspected, especially when flexion-extension radiographs are normal, but are unlikely to be helpful on a routine basis.

It can be concluded that the value of functional radiography remains debatable. Some may consider flexion-extension lateral views to be a rough and imprecise method to detect lumbar intervertebral instability; however, the majority of surgeons still seem to believe in their usefulness and use them as indicators for instability.

CT Imaging
CT provides a detailed view of spinal degenerative changes and facet joint orientation (Fig 2). CT can demonstrate underlying predisposing anatomic factors, such as facet joint asymmetry, that may lead to an abnormal axial rotation of a vertebra on the subjacent one (rotatory spondylolisthesis). This results in accelerated stresses and asymmetric disk and facet joint degenerative changes, particularly asymmetric anterior subluxation of the facet joints, unilateral recess stenosis, and a foraminal disk herniation on the side of maximal facet joint subluxation (78) (Fig 8).

View larger version (121K): [in this window] [in a new window] [Download PPT slide]   Figure 8a: Transverse CT scans at L4-5 level in 54-year-old woman with characteristic triad of rotatory spondylolisthesis. (a) Asymmetric subluxation of L4-5 facet joints maximal on the right side (arrow). (b) Narrowing of the right lateral recess (arrow). (c) Ipsilateral foraminal L4-5 disk herniation (arrow) impinging on the right L4 nerve root.

View larger version (120K): [in this window] [in a new window] [Download PPT slide]   Figure 8b: Transverse CT scans at L4-5 level in 54-year-old woman with characteristic triad of rotatory spondylolisthesis. (a) Asymmetric subluxation of L4-5 facet joints maximal on the right side (arrow). (b) Narrowing of the right lateral recess (arrow). (c) Ipsilateral foraminal L4-5 disk herniation (arrow) impinging on the right L4 nerve root.

View larger version (140K): [in this window] [in a new window] [Download PPT slide]   Figure 8c: Transverse CT scans at L4-5 level in 54-year-old woman with characteristic triad of rotatory spondylolisthesis. (a) Asymmetric subluxation of L4-5 facet joints maximal on the right side (arrow). (b) Narrowing of the right lateral recess (arrow). (c) Ipsilateral foraminal L4-5 disk herniation (arrow) impinging on the right L4 nerve root.

View larger version (158K): [in this window] [in a new window] [Download PPT slide]   Figure 9a: Functional CT: twist test in 65-year-old woman. (a) Left rotation, subluxation (arrow) of the left facet joint. (b) Right rotation, narrowing of the left facet joint; subluxation (arrow) of the right facet joint.

View larger version (119K): [in this window] [in a new window] [Download PPT slide]   Figure 9b: Functional CT: twist test in 65-year-old woman. (a) Left rotation, subluxation (arrow) of the left facet joint. (b) Right rotation, narrowing of the left facet joint; subluxation (arrow) of the right facet joint.

View larger version (139K): [in this window] [in a new window] [Download PPT slide]   Figure 10a: Functional CT in 61-year-old man with vertebral instability. (a) Extension and (b) flexion sagittally reformatted images show intradiskal vacuum phenomenon during extension (arrow in a) and degenerative spondylolisthesis during flexion (arrow in b) with anterior sagittal translation exceeding 4 mm.

View larger version (131K): [in this window] [in a new window] [Download PPT slide]   Figure 10b: Functional CT in 61-year-old man with vertebral instability. (a) Extension and (b) flexion sagittally reformatted images show intradiskal vacuum phenomenon during extension (arrow in a) and degenerative spondylolisthesis during flexion (arrow in b) with anterior sagittal translation exceeding 4 mm.

Degenerative diskogenic vertebral changes can be noted on endplates bordering the intervertebral disks (Modic types 1–3) (37). The association of vertebral instability with changes in the bone marrow adjacent to the endplates has been discussed, but without consistent results (37,81–84). Modic et al (37) stated that the clinical importance of these changes in the bone marrow is unknown. Lang et al (81) observed bone marrow changes adjacent to the endplates in postoperative instability, but no statistically significant correlation exists between segmental instability and abnormalities of the bone marrow adjacent to the endplates in patients without spinal fusion, as resulted from a study of Bram et al (P = .26) (82). Conversely, Bram et al (82) found a significant association between radiographic instability and traction spurs and between radiographic instability and annular tears.

In their study of patients with chronic low back pain, Aprill and Bogduk (83) first described annular tears as a high-signal-intensity dot on sagittal T2-weighted images (Fig 11). Therefore, flexion-extension radiographs should be considered in patients with annular tears or traction spurs. Unfortunately, additional studies supporting this conclusion are necessary before it can be generally accepted. A high-signal-intensity zone in the posterior annulus fibrosus on sagittal T2-weighted images has been found much too frequently in asymptomatic subjects to be considered a reliable independent diagnostic indicator (84).

View larger version (129K): [in this window] [in a new window] [Download PPT slide]   Figure 11: Intervertebral disk annular tear in 51-year-old woman. Sagittal T2-weighted (3700/103 [repetition time msec/echo time msec]) image of lumbar spine shows dehydration of nucleus pulposus and annular tear (arrow) of L4-5 disk.

Although the lumbar spine undergoes large compression loads in normal activities, MR imaging is routinely performed with the patients supine and, therefore, with the spine unloaded. Recent advances in the design of magnets and gradient coils have made possible the development of open MR imaging systems, which provide new opportunities to investigate spinal kinematics, particularly vertebral instability. Early studies were limited to assessing spinal kinematics by imaging the patient in the supine position in combination with several different axial loading MR imaging–compatible devices (85). More recent studies have used open MR imaging systems, which provide gradient capabilities and field homogeneity sufficient for the evaluation of the lumbar spine under upright weight-bearing conditions in either seated (flexion-extension in sagittal plane) or standing body positions (86–90). However, the reported results are not really convincing and, despite continuous development of MR imaging equipment, essential problems still arise during attempts to perform examinations in upright posture for patients with spinal disorders (89,90).

Weishaupt et al (89) evaluated whether positional (seated) MR imaging can demonstrate nerve root compromise not visible at conventional (supine) MR imaging in 30 patients with chronic low back pain unresponsive to nonsurgical treatment but without compression of neural structures. Positional pain differences were related to position-dependent changes in foraminal size. Positional MR imaging more frequently demonstrated minor neural compromise than did conventional MR imaging, but no convincing signs of canal or foraminal encroachments were found.

Wildermuth et al (90) investigated the influence of various body positions on the dural sac and the intervertebral foramina in 30 consecutive patients with combined low back pain and sciatica, who were examined in the supine, upright flexion, and upright extension positions with an open MR imager. The authors found only small position-dependent differences in the sagittal diameter of the dural sac and foraminal size, and the information gained in addition to that from standard MR imaging was limited. Moreover, the overall examination time created severe pain problems. Motion artifacts and difficulties in reproducing the positioning between the sequences occurred regularly. This impaired the possibilities for analyzing the content of the spinal canal.

In conclusion, although such techniques may increase the sensitivity of MR imaging for identification of lumbar nerve root compression, further studies are required before the true management value of the techniques can be determined.

Radiostereometric Analysis and Distortion-compensated Roentgen Analysis
Radiostereometric analysis (RSA) was developed in 1974 by Selvik (91) as a method for performing accurate three-dimensional measurements in vivo over time from sequential radiographs. Since then, the method has been subjected to several updates (92,93). RSA has proved to be a precise quantifying method for evaluating micromotions between different structures and has been used in many orthopaedic fields such as prosthetic fixation (94), joint stability and kinematics (95), fracture stability (96), and spinal fusion stability (97,98), as well as to assess three-dimensional translatory and rotatory motion in the lumbar spine (99,100). This method requires the insertion of at least three radio-opaque tantalum markers in each vertebra to determine the geometric characteristics of the vertebral anatomy. Two x-ray tubes angled 40° to each other are necessary for simultaneous exposures of the patient's tantalum-marked vertebrae together with a calibration cage supplied with 0.8–1.0 mm tantalum markers in well-defined positions and placed between the patient and the film plane (Fig 12). The relative movements of marked vertebrae, induced by positional changes from flexion to the neutral position in the supine spine, can be calculated with repeated radiographic examinations and computer analysis. The RSA accuracy determined from the results of repeated radiographic examinations has amounted to a standard deviation of 0.7°, 0.2°, and 0.3° for rotatory motion around the transverse, vertical, and sagittal axes, respectively, and to a standard deviation of 0.2 mm for translatory motion along these axes (101,102). RSA has proved to be the best method to detect very small movements between vertebrae (91,101,102); unfortunately, this method is technically difficult, time consuming, and requires specific apparatus. Moreover, because of its invasive nature, it is unsuitable for studies of large patient series.

View larger version (11K): [in this window] [in a new window] [Download PPT slide]   Figure 12: Schematic presentation of the RSA apparatus.

Leivseth et al (100), measuring sagittal-plane translatory and rotatory motion with DCRA and RSA in 15 lumbar segments of eight patients, found that measurement precision of DCRA is inferior to that of RSA but higher than that of conventional protocols assessing lumbar segmental motion. Compared with the reference standard RSA, DCRA measured sagittal-plane rotation with an error in the order of 1.4° and measured sagittal translation with an error in the order of 1.25 mm. They concluded that if measurement errors of this order can be tolerated, DCRA might be the method of choice for sagittal rotatory and translatory measurements.

	CLINICAL AND RADIOLOGIC CONSIDERATIONS

TOP ABSTRACT INTRODUCTION DEFINITION OF INSTABILITY FUNCTIONAL ANATOMY AND... DEGENERATIVE LUMBAR... PAIN AND VERTEBRAL INSTABILITY... IMAGING EVALUATION OF LUMBAR... CLINICAL AND RADIOLOGIC... CONCLUSION ESSENTIALS References

 
Clinical criteria for lumbar spine instability have not yet been clearly defined. Recurrent, acute episodes of low back pain produced by mechanical stresses have been considered to be indicative of instability (2). If a full return from the bent position fails because of a sudden attack of low back pain (ie, instability catch), if a patient is unable to get a raised, straightened leg to move down and suddenly drops the leg due to a sharp pain in the low back (ie, painful catch), and if a patient feels anxiety resulting from a sensation of collapse of the low back because of a sudden attack of back pain during movement (ie, apprehension), the patient fulfills the three criteria for instability described by Kotilainen and Valtonen (104). A loss of tone in the legs or in the low back and pelvic region (ie, giving away phenomenon) has also been observed in some patients with lumbar instability (105). However, these clinical criteria have not been rigorously evaluated.

Overall, the relationship between imaging instability and its symptoms is controversial. Pitkanen et al (56) found poor correlation between clinical signs of lumbar instability and abnormalities found on functional radiographs. Dvorak et al (106) found that the analysis of the segmental motion of the lumbar spine using functional radiographs does not aid in differentiating the underlying pathologic condition of a patient with low back pain. Conversely, Iguchi et al (107) measured sagittal translation and rotation at the L4-5 segment in flexion-extension radiographs of 1090 outpatients with low back and/or leg pain by using a three-landmark measuring method. The symptoms of four groups with and without 3-mm translation and with and without 10° sagittal rotation were compared for all patients and for 280 age-matched patients by using the scoring system proposed by the Japanese Orthopaedic Association (107) for assessment of surgical treatment of low back pain. This scoring system is based on subjective symptoms and clinical signs; the total score ranges from 0 to 15, with only a score of 15 representing an asymptomatic patient with no objective signs (107). Results showed that patients with 3-mm or greater translation had been suffering from low back and/or leg pain the longest and had significantly lower scores than patients with less than 3-mm translation; however, no difference was observed between the groups in terms of sagittal rotation.

Maigne et al (108) studied 42 patients with low back pain that occurred immediately on sitting down and was relieved on standing up by using functional radiographs and found an important association between this symptom and imaging signs of instability (100% specificity, 31% sensitivity) or severe anterior loss of disk space in flexion (87% specificity, 55% sensitivity).

	CONCLUSION

TOP ABSTRACT INTRODUCTION DEFINITION OF INSTABILITY FUNCTIONAL ANATOMY AND... DEGENERATIVE LUMBAR... PAIN AND VERTEBRAL INSTABILITY... IMAGING EVALUATION OF LUMBAR... CLINICAL AND RADIOLOGIC... CONCLUSION ESSENTIALS References

 
Determination of the relationship between imaging instability and its symptoms remains challenging if not impossible. In the case of degenerative spondylolisthesis and concomitant spinal stenosis at the slip level, the clinical pattern includes buttock and leg pain usually associated with low back pain. These symptoms are brought on with walking and are relieved with resting. Spinal stenosis can cause compression of the cauda equina or individual nerve roots. The classic description of neurogenic claudication from spinal stenosis is bilateral radicular pain, disorders of sensory function, and motor deficits (109).

Surgical treatment is indicated in degenerative spondylolisthesis associated with neurologic symptoms and lumbar intervertebral instability. Surgery for pain in spinal instability has a wide spectrum of options, and the surgeon's preference is probably the most important factor governing the choice that is offered to the patient. Fusion is the most commonly offered procedure that can be performed anteriorly, posteriorly, posterolaterally, or in combination. There are a large variety of surgical internal fixation devices for stabilization. The objective is to fix the unstable segment, which will, if successful and solid, become painless (76). The second alternative is to restore stability but retain mobility of the segment by flexible stabilization such as the Graf ligament by using tension banding and pedicle screws (110). Third, a number of disk replacement prostheses have been promoted to restore the necessary structural and biomechanical properties of the spine that need to exist to achieve lumbar lordosis and stability of the segmental motion segments (111). These surgical procedures, however, have various drawbacks, including high surgical morbidity, risk of neural injury, and risk of instrumentation failure. Therefore, it is important that the indication is based on a thorough clinical and imaging assessment of the patient. Psychometric tests should be included in the preoperative evaluation (112).

Indications for surgical treatment are (a) a persistent or recurrent leg pain despite a reasonable trial of nonsurgical treatment (minimum of 3 months); (b) progressive neurologic deficit related to the spinal stenosis; (c) important reduction in the quality of life; and (d) confirmatory imaging study: anterior translation greater than 3 mm and sagittal rotation greater than 10°. In a study by Yone et al (77), the radiographic criteria of Posner et al (20) proved useful for selecting patients with instability for fusion treatment.

Patients treated with spinal fusion combined with instrumentation following posterior decompression seem to have the best outcome (76,113,114). However, no sensitivity, specificity, predictive value, or accuracy of the tests used in establishing instability was reported in the articles that reported the outcome after spinal surgery for instability (76,113,114).

Imaging has been harnessed to play an increasing role in the diagnosis and treatment of patients suspected of having instability. The pathologic changes in the spine in the three phases postulated by Kirkaldy-Willis and Farfan (2) in 1982 can be accurately identified, but how they relate to the functional concepts of dysfunction, instability, and stabilization is not always clear. These pathologic changes along with the complications of disk herniation and spinal stenosis have also all been demonstrated in asymptomatic individuals (115). To complicate matters further, it is generally agreed that the physical disorder of segmental instability is associated with an emotional and/or psychological reaction, which also needs to be recognized and treated appropriately to avoid an inferior therapeutic outcome (110). It is salutary to note that in several retrospective studies, the psychological examination was proved to be the most predictive before fusion surgery in patients with chronic low back pain and unproved diagnostic labels. The validated pain score forms and psychosocial abnormalities identified by the Oswestry Low Back Pain Disability Questionnaire (116), Short Form 36 health survey questionnaire (117), and Distress and Risk Assessment Method (118) were far better than any radiographic, CT, MR imaging, ordinary clinical examination, or lumbar injection studies, including discography, for predicting the outcome of fusion operations (119,120).

All of this uncertainty and controversy creates an ethical burden for all doctors from all disciplines involved in the diagnosis and treatment of patients with low back pain who are thought to be suffering from segmental instability. Currently, imaging helps to select those patients who have supportive evidence of a cause-and-effect relationship in their spine that shows the degeneration process associated with segmental instability (121). It is, however, still far from satisfactory, with significant gaps in the knowledge that will formulate a unified concept of this condition. The quantification of normal and abnormal spinal motion is likely to be still dependent on imaging. It is unlikely that any future agreement of definition, clinical syndromes, and therapeutic regimes can be reached if clinically useful measurements are not a fundamental component of the whole concept of instability.

	ESSENTIALS

TOP ABSTRACT INTRODUCTION DEFINITION OF INSTABILITY FUNCTIONAL ANATOMY AND... DEGENERATIVE LUMBAR... PAIN AND VERTEBRAL INSTABILITY... IMAGING EVALUATION OF LUMBAR... CLINICAL AND RADIOLOGIC... CONCLUSION ESSENTIALS References

 

• The diagnosis of vertebral instability is commonly based on the imaging finding of abnormal vertebral motion.
  
• Functional flexion-extension radiography is the most thoroughly studied and the most widely used method in the imaging diagnosis of lumbar intervertebral instability.
  
• Determination of the relationship between imaging instability and its symptoms remains challenging if not impossible.
  
• Surgical treatment is indicated in degenerative spondylolisthesis associated with neurologic symptoms and lumbar intervertebral instability.
  

	ACKNOWLEDGMENTS

 
The authors thank Thomas J. Re, MD, MSEE, Catholic University, School of Medicine, Rome, Italy, for his editorial assistance in the preparation of the manuscript.

	FOOTNOTES

 

Abbreviations: DCRA = distortion-compensated roentgen analysis • RSA = radiostereometric analysis

Authors stated no financial relationship to disclose.

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TOP ABSTRACT INTRODUCTION DEFINITION OF INSTABILITY FUNCTIONAL ANATOMY AND... DEGENERATIVE LUMBAR... PAIN AND VERTEBRAL INSTABILITY... IMAGING EVALUATION OF LUMBAR... CLINICAL AND RADIOLOGIC... CONCLUSION ESSENTIALS References

 

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