Radiological Evaluation of the Lumbar Instability

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Radiological Evaluation of LumbarInstability

Bidhya Bhusan Tamrakar, Huang , Orthopedics Department No. 2,Affiliated Hospital of Jiangsu University, Zhenjiang, Jiangsu Province,P. R. China

Abstract

Intervertebral instability of the lumbar spine is thought to be apossible pathomechanical mechanism underlying low back pain and sciatica and isoften an important factor in determining surgical indication for spinal fusion anddecompression. Instability of the lumbar spine, however, remains a controversialand poorly understood subject. At present, much controversy exists regarding theproper definition of the condition, the best diagnostic methods, and the mostefficacious treatment approaches. Clinical presentation is not specific, and therelationship 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 themost widely used method in the imaging diagnosis of lumbar intervertebralinstability. In this article, we provide an overview of the current concepts ofvertebral instability, focusing on degenerative lumbar intervertebral instability, andreview the different imaging modalities most indicated in diagnosing vertebralinstability.

The spine is made up of segments, described as motion segments, consisting oftwo vertebrae and the interconnecting soft tissue. In normal conditions of daily life,the spine is able to meet essential functional requirements: strength, mobility, andstability. This is the result of specific mechanical characteristics of each individual

spinal component, as well as of the efficient integration of these components intothe overall structure of the spine.

Spinal stability is defined as the ability for the vertebrae to maintain theirrelationship and limit their relative displacements during physiologic postures andloads. The requirement of stability is essential to the spinal column to preventpremature mechanical and biologic deterioration of its components. It is alsofundamental to protect the spinal cord and nerve roots and to minimize energyexpenditure.


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One important mechanical function of the lumbar spine is to support the upper bodyby transmitting compressive and shearing forces to the lower body during theperformance of everyday activities. To enable the successful transmission of theseforces, mechanical stability of the spinal system must be ensured. Stability of thelumbar 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. Although segmental instability is often used synonymouslywith degenerative spondylolisthesis, it is clear there are numerous other conditionsthat are potentially unstable (spinal acute trauma, surgery, spondylolysis, tumors,or infections).

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

DEFINITION OF INSTABILITYDespite the effort of several authors to define lumbar spinal instability, no generallyaccepted definition is yet available. A major problem is that the concept ofinstability means different things to different specialists (clinicians, radiologists,bioengineers). However, a reasonable definition has been proposed by Pope andPanjabi and Frymoyer and Selby. By advocating a biomechanical approach, theydefined instability as a loss of motion segment stiffness, such that force applicationto that motion segment produces abnormally great motion compared to that of anormal spine. In other words, instability can be defined as an abnormal response toapplied loads characterized kinematically by abnormal movement in the motionsegment beyond normal constraints. 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. In abiomechanical sense, stiffness is defined as the ratio of the load applied to astructure to the resulting motion.

The definition of instability as a loss of motion segment stiffness has also beensupported by Panjabi et al. Their study was based on the concept that the loadsapplied to the motion segments of the human spine may be divided into those dueto body posture and superimposed body weight (preload) and those due to variousphysical activities (physiologic loads). In that study, Panjabi et al excised lumbarspine segments from cadavers within 16 hours of death and then applied a givenaxial preload to a motion segment followed by 12 physiologic loads (applied one ata time) for measuring the resulting three-dimensional motion. The results were

calculated in the form of load displacement curves. For each combination of apreload and a physiologic load component, there were six load displacementcurves: one curve representing main motion and five curves representing coupledmotions. Two of their conclusions were (a) the main and the coupled motion curveare affected by the inclusion of preloads and(b) the application of any one of the 12physiologic loads may produce six motion components, namely, three translations(one along each of the x-, y-, z-cartesian axes) and three rotations (one around eachof the x-, y-, z-cartesian axes); therefore, a motion segment has six degrees of


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freedom (ie, six possible relative displacements of one vertebra relative to itsneighbor).

Figure 1:Drawing shows three-dimensional coordinates system proposed by Panjabi andWhite identifying translational (straight arrows) and rotational (curved arrows)movements along and/or around the x-, y-, and z-axes.

ANATOMY AND BIOMECHANICS OF THE LUMBAR

SPINE

The Vertebral Body

The vertebral body is the key element in the load-bearing system of the spine. Thevertebral body is made of a cortical bone shell and a core of cancellous bone, whichhas a honeycomb-like structure. Contrary to cortical bone, which is highly resistantbut scarcely adaptive to deformation, cancellous bone is able to manage load,accepting deformation without failure. Under increasing load, when the vertebralendplates deform, blood is forced out of the vertebra through multiple vascularforamina. It comes from the squeezing of bone marrow, which has been suggestedto be the main resistor of the dynamic peak loads. However, it should be pointed

out that since the strength of the vertebral body is directly related to its osseoustissue contents, vertebrae with reduced bone contents (such as in patients withosteoporosis) will be more likely to fail under load.

The Intervertebral Disk
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The intervertebral disk and the adjacent vertebral bodies form an integrated unit, inwhich collagen fibers of the disk are intimately related to the cartilaginousendplates. The normal intervertebral disk can be divided into nucleus pulposus andannulus fibrosus. The nucleus pulposus contains a much larger proportion ofhydrophilic proteoglycans than the annulus fibrosus, while the annulus fibrosuscontains a much higher amount of collagen. The annulus fibrosus consists of a

complex system of fiber bundles called lamellae, which become progressively morecompact centrifugally with differentiation into Sharpey fibers, whereby there is adirect bony anchorage at the peripheral attachment of the annulus with thevertebral body rim. These collagen fibers blend with the anterior and posteriorlongitudinal ligaments and act together to stabilize the vertebral motion segment.

The intervertebral disk is the primary load-bearing structure in the spinal motionsegment. The nucleus with its high water content has hydrostatic properties, acts asa fulcrum for spinal movement, and provides for the radial transmission of forces.Loading perpendicular to the surface of the disk is transmitted radially by thenucleus and distributed transversally within the annulus, which resists strongly atthe 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 atrest and it increases as loads are applied to the spine. In axial compression, theincreased intradiskal pressure is counteracted by annular fiber tension and diskbulging. In flexion, extension, and lateral bending, the same process occurs. In axialrotation, the annular fibers in one direction are stretched, whereas thosecontralaterally are shortened or crimped (13). Since rotation and lateral bendingmay be coupled to each other, the stresses in the disk are thus a combination oftension, 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 themidline varies among individual vertebral segments. The almost sagittal alignmentof the facet joint plane in the proximal lumbar spine gradually becomes a morecoronal orientation in the lower lumbar spine. This articular configuration permits alarge range of motion in the sagittal plane (flexion-extension), a more limited rangeof lateral (right or left) bending, and greatly limited axial rotation. Thebiomechanical importance of the lumbar facet joints and their capsules is wellestablished. The facet joints are subject to substantial forces and help shield thelower lumbar disks from shear loads; moreover, they are the primary elementsacting against rotational or torsional forces. In active extension, the facets canfunction as a fulcrum, thereby reducing load on the anterior and middle spinalcolumns of Denis and, therefore, on the disk. By reducing the load on the disk, the

fulcrum effect reduces disk protrusion. In 1983, Denis 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 anteriorlongitudinal ligament and the anterior half of the vertebral body and intervertebraldisk. The middle column is formed by the posterior longitudinal ligament and theposterior half of the vertebral body and intervertebral disk. The posterior columnincludes all bone and ligamentous structures posterior to the posterior longitudinalligament and includes the pedicles, laminae, facets, spinous processes, and allassociated ligaments.
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The capsular structures around the facet joints are richly supplied with pain-sensitive nerve endings, thus being functionally important in low back pain. Thefacet capsule is probably the major stabilizing structure among the posteriorelements in flexion forces. Posner et al and subsequently Adams and Hutton , usingsimulated physiologic loads and motion, ablated various components and showedthat 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 posturalpositions between vertebrae. They resist tensile forces but buckle when subject tocompression.

The anterior longitudinal ligament is attached to the intervertebral disks and theadjacent endplate margins of the vertebrae, but is not tightly adherent to theanterior surfaces of the vertebral bodies. The posterior longitudinal ligament is lessdeveloped than its anterior counterpart in the lumbar region. It is incorporated intothe collagen fibers of the posterior annulus at the disk level, but is not attached tothe central part of the vertebral body posteriorly. 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.

Figure 2:Drawing shows the anatomic relationship between spinal ligaments, disk, andvertebrae in a motion segment. Sagittal view.ALL = anterior longitudinalligament, FL= flaval ligament, ISL = interspinous ligament, ITL= intertransverse

ligament, PLL = posterior longitudinal ligament, SSL= supraspinous ligament. Noteposterior bulging (arrowhead) of redundant posterior disk surface and posteriorlongitudinal ligament, which is consequence of acquired collapse of theintervertebral disk.

The interspinous, supraspinous, and intertransverse ligaments help unite adjacentvertebrae. The interspinous ligaments are thin membranous structures that connectadjacent spinous processes. Rissanen performed a detailed macroscopic andmicroscopic study on 30 cadavers (age range, 3070 years). He found that in 80%


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of cases, there was a similar type and degree of degeneration in the intervertebraldisk and the corresponding interspinosus ligament at the L4-5 and L5-S1 motionsegments. These degenerative changes in the two structures seemed to beconcurrent and not consecutive in time and included partial and total tears (30%),accumulation of fatty and mucopolysaccaride substances, and the formation ofintraligamentous spaces resembling joint spaces, termed cavitations. These findings

suggest that the interspinous ligaments make very little contribution to the clinicalstability of the lumbar spine in the adult. In contrast, the supraspinous ligamentsappear to play a major role in the lumbar spine. Myklebust et al studied ligamentsindividually by sectioning all but the ligament to be tested and found that theinterspinous ligaments failed in the range of 85185 N, whereas the supraspinousligaments yielded in the range of 293750 N. The intertransverse ligaments join thetransverse processes of the vertebrae; they are generally weak structures except inthe lumbar region. The iliolumbar ligaments and their posterior band regulatelumbosacral motion, particularly flexion. It has been reported that the verticalthickness of the transverse processes of L5 could provide an indication of thefunctional strength of the iliolumbar ligaments.

Mobility of the Lumbar Spine

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

In normal conditions, during the movements of flexion-extension of the lumbarspine, rotation in the sagittal plane, demonstrated by a change in the angle formedby two opposite vertebral endplates and translation in the sagittal plane (defined bya slip of one vertebra relative to the vertebra below), may be observed. Abnormalmovements are restrained by normal intervertebral structures. The total range ofmotion of a spinal motion segment may be divided into the neutral zone and theelastic zone. Panjabi defined the neutral zone as the range of motion in which arelatively large intervertebral motion is produced by a minimum effort; it is theinitial portion of the total range of motion. In the elastic zone, located at theextreme ends of the total range of motion, movement is produced againstsubstantial internal resistance. The neutral zone concept is based on theobservation that the load-displacement curve of the typical spinal motion segment

is nonlinear, with high flexibility for motion occurring around the neutral position ofthe spine and with increased passive resistance to motion nearer the end range ofspinal motion. An increase in the neutral zone may lead to higher probability ofoverstretching of ligaments and be a source of instability.Spinal stability depends on three functionally interdependent subsystems that limitthe excursion of spinal motion segments and maintain the proper ratio of neutral-to-elastic zone motion: a passive subsystem, an active subsystem, and a neuralcontrol subsystem. Muscular contraction of the trunk and spine muscles, under thecontrol of postural reflexes (neural control subsystem), provide the active part of


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stability. The passive part of stability is provided by vertebral bodies, facet jointsand their capsules, spinal ligaments, and the passive tension from themusculotendinous units.

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

Forward flexion results in anterior compression of the disk and in sliding separationof the facet joints. This movement is controlled by the posterior ligaments(interspinous and supraspinous ligaments), the facet joints and their capsules, theintervertebral disk, and the paraspinal muscles. In extension, the main stabilizingstructures are the anterior longitudinal ligament, the anterior part of the annulusfibrosus, the facet joints, and the rectus abdominis muscle. Rotational movementsare mainly controlled by the intervertebral disk and the facet joints. For side-bending movements, which are accompanied by some rotation with sliding

separation of the facet joints, the intertransverse ligaments probably play animportant role.

LUMBAR INSTABILITY

Upright posture and upright weight bearing in humans cause excess stresses thatare maximal at and suprajacent to the lumbosacral junction. This results in moresevere age-related changes in these spinal segments. The degenerative processesof the lumbar spine generally initiate from the intervertebral disk, at the level atwhich progressive biochemical and structural changes take place, leading to a

modification in its physical properties of elasticity and mechanical resistance. Diskdegeneration, which affects the whole population, is commonly seen from age 30years onward. The degenerative process in the disk results in a gradual disruptionof the collagen fibers and reduction in the proteoglycan contents, with a gradualloss of water contents and elasticity of the disk. More than 50% of autopsyspecimens obtained from individuals in their 3rd and 4th decade of life showperipheral tears of the annulus fibrosus . After age 40 years, the disk becomesprogressively more fibrous and disorganized due to aging and degeneration; thefinal stage is represented by regions of amorphous fibrocartilage. This will at somepoint entail a superoinferior narrowing and eventual collapse of the intervertebraldisk.

Three clinically relevant consequences of acquired collapse of the intervertebral

disk are (a) pathologic changes in the vertebral bodies, with osteophytedevelopment; (b) anterior bulging of the flaval ligaments and posterior bulging ofthe posterior longitudinal ligament, with consequential narrowing of the centralspinal canal ; and (c) posterior bulging of redundant posterior disk surface, withnarrowing of the central spinal canal and of the inferior recesses of the neuralforamina. Moreover, intervertebral disk degeneration and acquired collapse permitthe adjacent vertebrae to slide back and forth over each other. This results in laxityof the ligamentous network responsible for binding the vertebrae together and


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leads to craniocaudal partial subluxation of the facet joints, which may beasymmetric from side to side. Subsequent stresses on the facet joints then result inosteoarthritis with ostheophytosis, which in turn causes narrowing of the lateralrecesses of the central spinal canal and of the neural foramina. Furthermore, partialsubluxation of the facet joints leads to the collision of the apex of the superiorarticular facet process with the overlying pars interarticularis and pedicle.

Continued collision of these structures results in ostheophytosis and consequentlyto further narrowing of the central spinal canal and of the neural foramina.Osteoarthritis of the facet joints, which may occur independently of the disk, ischaracterized by the thinning of the cartilage, sclerotic changes in the subchondralbone, osteophyte formation, synovial inflammation, and capsular ligament laxity. Inmore severe forms of the process, osteoarthritis of the facet joints may allowhypermobility of the facet joint and then may lead to a spondylolisthesis. This termrefers to the forward slippage (by any cause) of a vertebra on the subjacent one inthe sagittal plane. In 1930, Junghanns defined lumbar vertebral slippage in theabsence of a bone defect in the pars interarticularis as pseudospondylolisthesis.

This was later categorized as degenerative lumbar spondylolisthesis by Newmanand Stone . Backward vertebral slippage, a type of spondylolisthesis, has been

called retrolisthesis.

Figure 3:Transverse CT scan through L4 vertebra at bone window in 58-year-old woman withdegenerative spondylolisthesis. Subluxated inferior apophyseal processes of thesuperior vertebra (arrows) cause central spinal canal and lateral recesses stenosis.Note sagittal orientation of right facet joint.

Osteoarthritis of the facet joints, with consequential loss of their normal structuralsupport, plays an important role in the development of degenerativespondylolisthesis. In a study by Grobler et al, the facet joint orientation of the lowerlumbar spine in a normal population and in a population of patients withdegenerative spondylolisthesis at L4-5 level was also characterized. At the L4-5level, a more sagittal orientation of facet joints was found in the degenerative

spondylolisthesis group, when compared with the normal group. This sagittalorientation facilitates vertebral slippage when the other predisposing factors arepresent. Because of these abnormalities and the preponderance of coronalorientation of the L5-S1 facet joints, the majority of degenerative spondylolisthesisoccurs at the L4-5 level.

The relationship between lumbar instability and degenerative spondylolisthesis wassuggested by Kirkaldy-Willis and Farfan who, in a functional sense, proposed threeclinical and biomechanical stages of lumbar spine degenerative changes: temporarydysfunction, unstable phase, and stabilization. Spinal degenerative changes


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included disk degeneration, facet joints osteoarthritis, ligamentous degeneration,and muscle alterations. The duration of each stage varies greatly, and there are noclear-cut clinical signs or symptoms to distinguish one stage from the next. The firstphase, defined as the temporary dysfunction phase, is associated with slightreversible anatomic changes. The second, or unstable, phase is characterized bydisk height reduction, ligament and joint capsule laxity, and facet joint

degeneration. In the third, or stabilization, phase, osteophytes and marked diskspace narrowing lead to stabilization of the motion segment with a reduction(partial or complete) in its range of motion, sometimes after spondylolisthesis hasalready occurred. On the basis of this model, the radiologic observation ofdegenerative spondylolisthesis does not necessarily indicate that intervertebralinstability is still present at the time of imaging because a new stabilization mayhave already occurred.

Figure 4a:Functional lateral radiographs of lumbar spine in 59-year-old woman with L5spondylolysis and grade I axial spondylolisthesis.(a) Flexion and (b)extension viewsobtained in 1999 show L5-S1 instability with 10 sagittal rotation between flexionand extension and an anterior translation exceeding 4 mm (unstablephase). (c) Flexion and (d) extension views obtained in 2011 show disappearance

of the range of movement (late stabilization phase).

Figure 4b:Functional lateral radiographs of lumbar spine in 59-year-old woman with L5spondylolysis and grade I axial spondylolisthesis.(a) Flexion and (b)extension viewsobtained in 1999 show L5-S1 instability with 10 sagittal rotation between flexionand extension and an anterior translation exceeding 4 mm (unstablephase). (c) Flexion and (d) extension views obtained in 2011 show disappearanceof the range of movement (late stabilization phase).
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Figure 4c:Functional lateral radiographs of lumbar spine in 59-year-old woman with L5spondylolysis and grade I axial spondylolisthesis.(a) Flexion and (b)extension viewsobtained in 1999 show L5-S1 instability with 10 sagittal rotation between flexionand extension and an anterior translation exceeding 4 mm (unstablephase). (c) Flexion and (d) extension views obtained in 2011 show disappearanceof the range of movement (late stabilization phase).

Figure 4d:Functional lateral radiographs of lumbar spine in 59-year-old woman with L5spondylolysis and grade I axial spondylolisthesis.(a) Flexion and (b)extension viewsobtained in 1999 show L5-S1 instability with 10 sagittal rotation between flexionand extension and an anterior translation exceeding 4 mm (unstablephase). (c) Flexion and (d) extension views obtained in 2011 show disappearanceof the range of movement (late stabilization phase).

Authors of several biomechanical and clinical studies have reported the associationof disk degeneration with segmental instability, confirming Kirkaldy-Willis andFarfan's concept . However, this association was not confirmed in other studies.

To test the validity of this three-stage hypothesis, Axelsson and Karlsson assessed

the intervertebral mobility for the two most distal lumbar disk levels in 18 adultpatients with low back pain, disk degeneration, and no prior spinal surgery. Eachspinal segment was placed in one of five categories according to the grade of diskdegeneration. They observed that intervertebral mobility undergoes changesthroughout the degenerative process and that a stage of relative stabilization isreached 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 assumedeven for spine segments with greater than 50% disk height reduction, as somemobility may still persist in such segments.


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Segmental lumbar spinal instability is a temporary phase in the degenerative

process of the lumbar spine. This process of degeneration has been sub-divided into

three phases.

(a) Dysfunction - is the earliest phase in which the affected level of the lumbar spine

does not function normally but pathological changes are minimal.

(b) Instability- intermediate phase in which the disc height is diminished and the

annulus fibrosus bulges all around the circumference of the disc, the ligaments and

capsule of the posterior facet joint are lax and the articular cartilage is degenerated.

This leads to increased and abnormal movement.

(c) Restabilization - fibrotic and osteophytic stabilization of the segment occurs.

This phase is associated with fibrosis within the intervertebral joint, enlargement

and locking of the facets and periarticular fibrosis. It is also associated with loss of

nuclear material within the disc and peripheral osteophyte formation. These

changes result in increasing stiffness of the joint .

Causes of the spinal instability:A, degenerative diseases,B, postoperative status,C, trauma to the spine or its surrounding structures,D, Development disorders like scoliosis and other congenital spine lesions &

E, infection.

RADIOLOGICAL EVALUATION OF LUMBAR

INSTABILITY

The diagnosis of vertebral instability is commonly based on the imaging finding ofabnormal vertebral motion. There may be abnormal translation and/or rotationaround the x-, y-, and z-axes of the three-dimensional coordinates system proposedby Panjabi and White. In this system, 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 horizontalin the sagittal plane, from front to back. Vertebral instability is generally

multidirectional, whereas the resulting displacement is evaluated in one plane at atime. 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 planeare evaluated on computed tomographic (CT) or magnetic resonance (MR) images.

Structural Changes on Neutral Radiographs


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Several radiographic findings have been proposed as indicators of vertebralinstability. To our knowledge, Knuttson was the first to report the vacuumphenomenon in the intervertebral disk and to present its association with lumbarspine instability. Because instability may create excessive intervertebral distractionand subsequent negative intradiskal pressure, allowing interstitial nitrogen in thesurrounding tissues to become gaseous and to accumulate within clefts of the

degenerated disk, it is assumed that the vacuum phenomenon is often associatedwith vertebral instability. Moderate disk degeneration with mild disk spacenarrowing and osteosclerosis also have been associated with vertebral instability. Incontrast, a marked disk space narrowing has been considered to be indicative of thelate stabilization phase described by Kirkaldy-Willis and Farfan.Another classic indirect radiographic sign associated with instability is the tractionspur, which is located 2 or 3 mm from the endplate and has a horizontal orientation.

The proposed mechanism is that the traction spur is caused by increased tensilestresses exerted by the Sharpey fibers or by those of the anterior longitudinalligament 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 theintervertebral 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 aresult of compression and a sign of stability restoration. Traction spurs and clawosteophytes are thought to represent different stages of the same pathologicprocess and frequently coexist on the same vertebral rim.

Figure 5:

Lateral radiograph of lumbar spine in 51-year-old man shows traction spur (arrow).

MacGibbon and Farfan suggested that elongated L5 transverse processes (those atleast as long as the L3 transverse processes) and a deep-seated L5 vertebra (thatsituated below the intercrestal line) confer stability on the lumbosacral joint andexpose the L4-5 joint to rotational stresses. In contrast, when the intercrestal line

passes through the L5 vertebra or through the L5-S1 disk and the transverseprocesses are short, the lumbosacral joint is at risk to strain. However, Frymoyerand Selby reported no relation between disk degeneration and either intercrestalline position or transverse processes length.

Functional Radiography


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Functional radiography in the sagittal plane can be achieved either in flexion andextension or with passive axial traction and compression. In axial traction andcompression radiography, lateral radiographs are obtained with the patient instanding position. Axial traction is accomplished by letting the patient hang by hisor her hands from a horizontal bar, whereas compression radiography is performedwhen the patient has sandbags of approximately 30% of his or her weight on the

shoulders. However, Pitkanen et al, comparing traction-compression with flexion-extension lateral views in a group of 306 patients with clinically suspectedinstability, concluded that traction-compression radiographs were of questionablevalue 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 usedmethod in the imaging diagnosis of lumbar intervertebral instability. Many surgeonsuse flexion-extension lateral views to disclose abnormal vertebral motion beforedeciding on surgical fusion. However, as reported by Nizard et al, this method ischallenging and debatable for the following three reasons: (a) Its diagnostic valuecannot be determined because of the lack of a nontraumatic and routinelyapplicable reference standard to define intervertebral instability; (b) its

reproducibility is difficult, a slight variation in patient positioning or in the directionof the x-ray beam may result in a 10%15% variation in the range of vertebraldisplacement ; and (c) the appropriate way to obtain flexion-extension radiographsand the method to measure displacements are still not standardized.

The choice of patient position, lateral decubitus versus standing, which bestoptimizes the flexion-extension radiographs, has been subjective. Several authorshave evaluated patients with low back pain and/or spondylolisthesis and foundintervertebral motion to be lower when flexion-extension radiographs were obtainedwith the patient in the recumbent position compared with standing. However, inpatients with unstable spondylolisthesis, to maximize the chances of detectingmaximum abnormal translational movement in the sagittal plane, Wood et alrecommended that flexion-extension radiographs should be obtained in the lateral

decubitus position. In their study, more abnormal translation was observed with thepatient in this position than while standing. One possible explanation for theirresults could be that splinting of the spine from the paraspinal postural orabdominal musculature may reduce the spine's range of motion when the patient isstanding. Moreover, in symptomatic patients, pain can inhibit muscle function,resulting in an underestimation of the true intervertebral motion.Flexion-extension lateral views allow measurement of the sagittal translation of avertebra with respect to the underlying one and the amount of vertebral rotation inthe sagittal plane (defined by the variation of the angle between two oppositevertebral endplates observed between the extremes of movement). 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 thediagnosis of instability) are inappropriate clinical decisions possibly resulting inunnecessary fusion surgery. Three sources of error in measuring translation in thesagittal plane are (a) the technique used to measure translation, (b)the quality ofradiographs, and (c) the concomitant rotation in the sagittal plane (sagittal rotation)and/or rotation about the vertical axis of the spine (axial rotation).


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Figure 6a:Functional lateral radiographs of lumbar spine in 58-year-old man show L4-5intervertebral instability with 17 sagittal rotation between(a) flexionand (b)extension. Sagittal rotation corresponds to the variation of the angle (lines)between vertebral endplates adjacent to the disk.

Figure 6b:Functional lateral radiographs of lumbar spine in 58-year-old man show L4-5intervertebral instability with 17 sagittal rotation between(a) flexionand (b)extension. Sagittal rotation corresponds to the variation of the angle (lines)between vertebral endplates adjacent to the disk.

Shaffer et al developed an experimental model of the L4-5 motion segment, wherethe actual amount of sagittal translation was known, and designed a set of studies

to assess the consistency and accuracy of measuring translations on radiographs ofvarying quality by using seven measurement techniques on models displayingvarying degrees of concomitant motion. These studies suggested that highconsistency and accuracy indices do not ensure acceptable false-positive and false-negative rates. When radiograph quality is low and concomitant motion is involved,even relatively large measured translations may occur when the actual translationsare substantially lower (large false-positive rates). Even with high-qualityradiographs, minimal (5 mm) are less often overestimated.In the study of Shaffer et al, the measurement technique described by Morgan andKing demonstrated the overall best performance and the least interference due toconcomitant motions. Other generally used techniques have been described by

Posner et al and Dupuis et al ; these techniques supposedly avoid inaccuracies thatresult from magnification by measuring translation as a percentage of vertebralbody width.


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Figure 7:Measurement technique of Dupuis et al (3). Sagittal translation is measured bydrawing lines U and L along the posterior cortices of upper and lower vertebralbodies. A third line Ialong inferior endplate of the superior vertebral body is drawnand a fourth line R is drawn parallel to L through the intersection point oflines I and U. Translation is defined as the perpendicular distance between parallellines L and R. To obviate inaccuracies due to x-ray magnification factor, translationis measured as percentage of the width of the upper vertebral body (W). Sagittalrotation 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; ifit is anterior, the angle is negative.

The cutoff between normal and abnormal movement is also difficult to determine. Alarge range of normal motion has been reported with a substantial overlap ofsymptomatic and asymptomatic motion patterns ; sagittal rotation may be as highas 25 in healthy young volunteers. This hypermobility may or may not bepathologic, depending on the ability of the vertebral and soft-tissue structures toaccommodate the movement. However, values of 10 for sagittal rotation and 4mm for sagittal translation are typically used to infer instability. In two studies, Yoneand Sakou confirmed the usefulness of Posner et al's definition of spinal instabilityfor selecting patients with instability for fusion treatment. The radiographic criteriaof Posner et al are an anterior translation greater than 8% (L1-2 to L4-5) or greaterthan 6% (L5-S1) of the vertebral body width, posterior translation greater than 9%

(L1-S1), and angular displacement (sagittal rotation) in flexion greater than 9 (L1-5) or greater than 1 (L5-S1). These values are relatively similar to those given byNachemson et al.Side bending in the lumbar spine is a composite motion consisting of rotation aboutthe z-axis (lateral bending) coupled with y-axis rotation (axial rotation) that occurstoward the convexity of the curve created by the z-axis rotation. Normal axialrotation may cause the spinous processes to move toward the concavity of thecurve (ie, toward the direction of bending), where the disk spaces normally close,while the vertebral bodies rotate away from the side of bending. Pathologic axialrotation can be detected on side-bending radiographs if the spinous processesmove to the convex side, producing an asynchronous spinous process line.Pathologic rotation can also manifest as a lateral translation (laterolisthesis) of one

vertebra on another during lateral bending. Additional signs that have beenproposed as radiographic indicators of abnormal movement include a loss ofvertebral body movement and paradoxical opening of the disk space on the bendingside.Pitkanen and Manninen, analyzing retrospectively flexion-extension and side-bending radiographs of 300 patients with clinically suspected lumbar spinalinstability, reported that side-bending radiographs are complementary to flexion-extension radiographs. Side-bending radiographs should be obtained if side-bending
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instability is clinically suspected, especially when flexion-extension radiographs arenormal, 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 imprecisemethod to detect lumbar intervertebral instability; however, the majority ofsurgeons still seem to believe in their usefulness and use them as indicators forinstability.

CT Imaging

CT provides a detailed view of spinal degenerative changes and facet jointorientation. CT can demonstrate underlying predisposing anatomic factors, such asfacet joint asymmetry, that may lead to an abnormal axial rotation of a vertebra onthe subjacent one (rotatory spondylolisthesis). This results in accelerated stressesand asymmetric disk and facet joint degenerative changes, particularly asymmetricanterior subluxation of the facet joints, unilateral recess stenosis, and a foraminaldisk herniation on the side of maximal facet joint subluxation.

Figure 8a:Transverse CT scans at L4-5 level in 57-year-old woman with characteristic triad ofrotatory spondylolisthesis. (a)Asymmetric subluxation of L4-5 facet joints maximalon the right side (arrow). (b) Narrowing of the right lateral recess(arrow). (c) Ipsilateral foraminal L4-5 disk herniation (arrow) impinging on the rightL4 nerve root.

Figure 8b:


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Transverse CT scans at L4-5 level in 57-year-old woman with characteristic triad ofrotatory spondylolisthesis. (a)Asymmetric subluxation of L4-5 facet joints maximalon the right side (arrow). (b) Narrowing of the right lateral recess(arrow). (c) Ipsilateral foraminal L4-5 disk herniation (arrow) impinging on the rightL4 nerve root.

Figure 8c:Transverse CT scans at L4-5 level in 57-year-old woman with characteristic triad ofrotatory spondylolisthesis. (a)Asymmetric subluxation of L4-5 facet joints maximalon the right side (arrow). (b) Narrowing of the right lateral recess(arrow). (c) Ipsilateral foraminal L4-5 disk herniation (arrow) impinging on the rightL4 nerve root.

Kirkaldy-Willis and Farfan described a technique of functional CT (twist test), inwhich the CT scan is obtained through the facet joint while the patient twists thetorso and the pelvis is tightly strapped to the CT table. The aim of the twist test wasto demonstrate increased abnormal motion, such as a gap of the facet joint spaceor an abnormal motion during rotation of the trunk, not clearly evident at functional

radiography. A facet joint shows increased motion when the cartilage spaceincreases on rotation and when the superior articular process on that side isdisplaced forward to narrow the root canal. The gap may appear as a vacuumphenomenon into the facet joint space during rotation. However, according toNizard et al, it is not known whether this technique allows the differentiationbetween normal and unstable spine.

Figure 9a:


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Functional CT: twist test in 59-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.

Figure 9b:Functional CT: twist test in 59-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.

CT is the procedure of choice to detect a vacuum phenomenon within thedegenerating disks or facet joints, although this finding has no known clinicalsignificance (Fig 9). Functional CT may demonstrate what is considered to be anabnormal motion between two vertebrae, but it is unsuitable for large patient series

in view of the exposure to ionizing radiation).Figure 10a:Functional CT in 63-year-old man with vertebral instability. (a) Extensionand (b) flexion sagittally reformatted images show intradiskal vacuum phenomenonduring extension (arrow in a) and degenerative spondylolisthesis during flexion(arrow in b) with anterior sagittal translation exceeding 4 mm.
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Figure 10b:Functional CT in 63-year-old man with vertebral instability. (a) Extensionand (b) flexion sagittally reformatted images show intradiskal vacuum phenomenonduring extension (arrow in a) and degenerative spondylolisthesis during flexion(arrow in b) with anterior sagittal translation exceeding 4 mm.

MR ImagingMR imaging is generally considered to be the most accurate imaging method fordiagnosing degenerative abnormalities of the spine, except for the vacuumphenomenon, and is often used as the diagnostic modality of choice for patientswith chronic low back pain. Identification of patients with an increased chance ofinstability on MR images can be clinically relevant and can influence indications forflexion-extension radiography.

Degenerative diskogenic vertebral changes can be noted on endplates borderingthe intervertebral disks (Modic types 13). The association of vertebral instabilitywith changes in the bone marrow adjacent to the endplates has been discussed, but

without consistent results. Modic et al stated that the clinical importance of thesechanges in the bone marrow is unknown. Lang et al observed bone marrow changesadjacent to the endplates in postoperative instability, but no statistically significantcorrelation exists between segmental instability and abnormalities of the bonemarrow adjacent to the endplates in patients without spinal fusion, as resulted froma study of Bram et al (P = .26). Conversely, Bram et al found a significantassociation between radiographic instability and traction spurs and between


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radiographic instability and annular tears.

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

Figure 11:

Intervertebral disk annular tear in 54-year-old woman. Sagittal T2-weighted

(3700/103 [repetition time msec/echo time msec]) image of lumbar spine showsdehydration of nucleus pulposus and annular tear (arrow) of L4-5 disk.

Degenerative disk disease and facet joint osteoarthritis affect the stability of themotion segment. However, the exact relationship between degenerative diskdisease, facet joint osteoarthritis, and vertebral instability at MR imaging has notbeen defined. Murata et al compared disk degeneration at MR imaging with that atflexion-extension radiography and found no statistically significant relation betweensegmental instability and disk degeneration. Fujiwara et al also compared MR


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imaging and functional radiography of the lumbar spine to examine the relationsamong segmental instability, disk degeneration, and facet joint osteoarthritis inpatients with low back pain; they reported that an anterior translation of 3 mm orgreater was positively associated with disk degeneration and facet jointosteoarthritis.Although the lumbar spine undergoes large compression loads in normal activities,

MR imaging is routinely performed with the patients supine and, therefore, with thespine unloaded. Recent advances in the design of magnets and gradient coils havemade possible the development of open MR imaging systems, which provide newopportunities to investigate spinal kinematics, particularly vertebral instability. Earlystudies were limited to assessing spinal kinematics by imaging the patient in thesupine position in combination with several different axial loading MR imagingcompatible devices. More recent studies have used open MR imaging systems,which provide gradient capabilities and field homogeneity sufficient for theevaluation of the lumbar spine under upright weight-bearing conditions in eitherseated (flexion-extension in sagittal plane) or standing body positions. However, thereported results are not really convincing and, despite continuous development ofMR imaging equipment, essential problems still arise during attempts to perform

examinations in upright posture for patients with spinal disorders.Weishaupt et al evaluated whether positional (seated) MR imaging can demonstratenerve root compromise not visible at conventional (supine) MR imaging in 30patients with chronic low back pain unresponsive to nonsurgical treatment butwithout compression of neural structures. Positional pain differences were related toposition-dependent changes in foraminal size. Positional MR imaging morefrequently demonstrated minor neural compromise than did conventional MRimaging, but no convincing signs of canal or foraminal encroachments were found.Wildermuth et al investigated the influence of various body positions on the duralsac and the intervertebral foramina in 30 consecutive patients with combined lowback pain and sciatica, who were examined in the supine, upright flexion, andupright extension positions with an open MR imager. The authors found only small

position-dependent differences in the sagittal diameter of the dural sac andforaminal size, and the information gained in addition to that from standard MRimaging was limited. Moreover, the overall examination time created severe painproblems. Motion artifacts and difficulties in reproducing the positioning betweenthe sequences occurred regularly. This impaired the possibilities for analyzing thecontent of the spinal canal.

In conclusion, although such techniques may increase the sensitivity of MR imagingfor identification of lumbar nerve root compression, further studies are requiredbefore 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 as a method forperforming accurate three-dimensional measurements in vivo over time fromsequential radiographs. Since then, the method has been subjected to severalupdates. RSA has proved to be a precise quantifying method for evaluatingmicromotions between different structures and has been used in many orthopaedic


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fields such as prosthetic fixation , joint stability and kinematics , fracture stability,and spinal fusion stability, as well as to assess three-dimensional translatory androtatory motion in the lumbar spine. This method requires the insertion of at leastthree radio-opaque tantalum markers in each vertebra to determine the geometriccharacteristics of the vertebral anatomy. Two x-ray tubes angled 40 to each otherare necessary for simultaneous exposures of the patient's tantalum-marked

vertebrae together with a calibration cage supplied with 0.81.0 mm tantalummarkers in well-defined positions and placed between the patient and the film plane. The relative movements of marked vertebrae, induced by positional changes fromflexion to the neutral position in the supine spine, can be calculated with repeatedradiographic examinations and computer analysis. The RSA accuracy determinedfrom the results of repeated radiographic examinations has amounted to a standarddeviation 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 fortranslatory motion along these axes. RSA has proved to be the best method todetect very small movements between vertebrae; unfortunately, this method istechnically difficult, time consuming, and requires specific apparatus. Moreover,because of its invasive nature, it is unsuitable for studies of large patient series.

Figure 12:Schematic presentation of the RSA apparatus.

For this reason, there has been an interest in alternative, noninvasive methods suchas distortion-compensated roentgen analysis (DCRA) protocol presented by Frobinet al. By using advanced methods of image analysis, DCRA measures rotation andtranslation in the sagittal plane from lateral flexion-extension radiographs of thelumbar spine. The DCRA method is based on the (a) analysis of vertebral contours inthe lateral view;(b) identification of geometric measurements that are virtuallyindependent of distortion, axial rotation, or lateral tilt; and (c) determination of thepattern of translational and rotational motion, applying a new protocol based onthose geometric measures.

Leivseth et al, measuring sagittal-plane translatory and rotatory motion with DCRAand RSA in 15 lumbar segments of eight patients, found that measurementprecision of DCRA is inferior to that of RSA but higher than that of conventionalprotocols assessing lumbar segmental motion. Compared with the referencestandard RSA, DCRA measured sagittal-plane rotation with an error in the order of1.4 and measured sagittal translation with an error in the order of 1.25 mm. Theyconcluded that if measurement errors of this order can be tolerated, DCRA might bethe method of choice for sagittal rotatory and translatory measurements.


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CLINICAL AND RADIOLOGIC CONSIDERATIONS

Clinical criteria for lumbar spine instability have not yet been clearly defined.

Recurrent, acute episodes of low back pain produced by mechanical stresses havebeen considered to be indicative of instability. If a full return from the bent positionfails because of a sudden attack of low back pain (ie, instability catch), if a patient isunable to get a raised, straightened leg to move down and suddenly drops the legdue to a sharp pain in the low back (ie, painful catch), and if a patient feels anxietyresulting from a sensation of collapse of the low back because of a sudden attack ofback pain during movement (ie, apprehension), the patient fulfills the three criteriafor instability described by Kotilainen and Valtonen. A loss of tone in the legs or inthe low back and pelvic region (ie, giving away phenomenon) has also beenobserved in some patients with lumbar instability. However, these clinical criteriahave not been rigorously evaluated.Overall, the relationship between imaging instability and its symptoms is

controversial. Pitkanen et al found poor correlation between clinical signs of lumbarinstability and abnormalities found on functional radiographs. Dvorak et al foundthat the analysis of the segmental motion of the lumbar spine using functionalradiographs does not aid in differentiating the underlying pathologic condition of apatient with low back pain. Conversely, Iguchi et al measured sagittal translationand rotation at the L4-5 segment in flexion-extension radiographs of 1090outpatients with low back and/or leg pain by using a three-landmark measuringmethod. The symptoms of four groups with and without 3-mm translation and withand without 10 sagittal rotation were compared for all patients and for 280 age-matched patients by using the scoring system proposed by the JapaneseOrthopaedic Association for assessment of surgical treatment of low back pain. Thisscoring 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 patientwith no objective signs. Results showed that patients with 3-mm or greatertranslation had been suffering from low back and/or leg pain the longest and hadsignificantly 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 studied 42 patients with low back pain that occurred immediately onsitting down and was relieved on standing up by using functional radiographs andfound an important association between this symptom and imaging signs ofinstability (100% specificity, 31% sensitivity) or severe anterior loss of disk space inflexion (87% specificity, 55% sensitivity).

CONCLUSION

Determination of the relationship between imaging instability and its symptomsremains challenging if not impossible. In the case of degenerative spondylolisthesisand concomitant spinal stenosis at the slip level, the clinical pattern includesbuttock and leg pain usually associated with low back pain. These symptoms arebrought on with walking and are relieved with resting. Spinal stenosis can cause


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compression of the cauda equina or individual nerve roots. The classic descriptionof neurogenic claudication from spinal stenosis is bilateral radicular pain, disordersof sensory function, and motor deficits.Surgical treatment is indicated in degenerative spondylolisthesis associated withneurologic symptoms and lumbar intervertebral instability. Surgery for pain in spinalinstability 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. Fusionis the most commonly offered procedure that can be performed anteriorly,posteriorly, posterolaterally, or in combination. There are a large variety of surgicalinternal fixation devices for stabilization. The objective is to fix the unstablesegment, which will, if successful and solid, become painless. The secondalternative is to restore stability but retain mobility of the segment by flexiblestabilization such as the Graf ligament by using tension banding and pediclescrews . Third, a number of disk replacement prostheses have been promoted torestore the necessary structural and biomechanical properties of the spine thatneed to exist to achieve lumbar lordosis and stability of the segmental motionsegments. These surgical procedures, however, have various drawbacks, includinghigh surgical morbidity, risk of neural injury, and risk of instrumentation failure.

Therefore, it is important that the indication is based on a thorough clinical andimaging assessment of the patient. Psychometric tests should be included in thepreoperative evaluation.Indications for surgical treatment are (a) a persistent or recurrent leg pain despite areasonable trial of nonsurgical treatment (minimum of 3 months);(b) progressiveneurologic deficit related to the spinal stenosis; (c)important reduction in the qualityof life; and (d) confirmatory imaging study: anterior translation greater than 3 mmand sagittal rotation greater than 10. In a study by Yone et al , the radiographiccriteria of Posner et al proved useful for selecting patients with instability for fusiontreatment.Patients treated with spinal fusion combined with instrumentation followingposterior decompression seem to have the best outcome. However, no sensitivity,

specificity, predictive value, or accuracy of the tests used in establishing instabilitywas reported in the articles that reported the outcome after spinal surgery forinstability.Imaging has been harnessed to play an increasing role in the diagnosis andtreatment of patients suspected of having instability. The pathologic changes in thespine in the three phases postulated by Kirkaldy-Willis and Farfan in 1982 can beaccurately identified, but how they relate to the functional concepts of dysfunction,instability, and stabilization is not always clear. These pathologic changes alongwith the complications of disk herniation and spinal stenosis have also all beendemonstrated in asymptomatic individuals . To complicate matters further, it isgenerally agreed that the physical disorder of segmental instability is associatedwith an emotional and/or psychological reaction, which also needs to be recognizedand treated appropriately to avoid an inferior therapeutic outcome . It is salutary tonote that in several retrospective studies, the psychological examination wasproved to be the most predictive before fusion surgery in patients with chronic lowback pain and unproved diagnostic labels. The validated pain score forms andpsychosocial abnormalities identified by the Oswestry Low Back Pain DisabilityQuestionnaire , Short Form 36 health survey questionnaire , and Distress and RiskAssessment Method were far better than any radiographic, CT, MR imaging,


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ordinary clinical examination, or lumbar injection studies, including discography, forpredicting the outcome of fusion operations .All of this uncertainty and controversy creates an ethical burden for all doctors fromall disciplines involved in the diagnosis and treatment of patients with low back painwho are thought to be suffering from segmental instability. Currently, imaging helpsto select those patients who have supportive evidence of a cause-and-effect

relationship in their spine that shows the degeneration process associated withsegmental instability. It is, however, still far from satisfactory, with significant gapsin the knowledge that will formulate a unified concept of this condition. Thequantification of normal and abnormal spinal motion is likely to be still dependenton imaging. It is unlikely that any future agreement of definition, clinical syndromes,and therapeutic regimes can be reached if clinically useful measurements are not afundamental component of the whole concept of instability.

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Radiological Evaluation of the Lumbar Instability

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