radiologic assessment of lumbar intervertebral instability and degenerative spondylolisthesis

IMAGING OF LOW BACK PAIN I1 0033-8389/01 $15.00 + .OO

RADIOLOGIC ASSESSMENT OF LUMBAR INTERVERTEBRAL

INSTABILITY AND DEGENERATIVE SPONDYLOLISTHESIS

Remy S. Nizard, MD, PhD, Marc Wybier, MD, and Jean-Denis Laredo, MD

Despite many efforts, there is no clear and widely accepted definition of lumbar instability because there are no unquestionable and currently applicable clinical or radiologic criteria available for this entity. Despite this problem of definition, an increasing number of lumbar spine fusions are performed for this specific diagnosis.15, 82 Conversely, degenerative spondylolisthesis has an unequivocal radiologic definition. This article reviews the current concepts of lumbar instability and the different imaging modalities used to make the diagnosis as evident as possible. Degener- ative spondylolisthesis, which is a possible end stage of lumbar instability, is also discussed. Particular attention is paid to the radiologic analysis of vertebral displacement, which is of pathophysiologic and therapeutic concern.

INTERVERTEBRAL INSTABILITY

Definition

In 1985, Kirkaldy-Willi~,4~ as the president of the International Society for the Study of

the Lumbar Spine, in an introductive lecture during a symposium on lumbar spine instability raised three difficult questions: (1) what does the term lumbar instability mean, (2) how can it be detected, and (3) what are the best ways of treating varying degrees of this stage in the process of degeneration of the lumbar spine? Fourteen years later, and despite a large number of studies dealing with lumbar instability, including studies based on new imaging modalities, these questions remain unanswered. According to the American Academy of Orthopedic Surgeons: instability is defined as an abnormal response to applied loads, characterized by movement in the motion segment beyond normal constraints. Pope and P a ~ ~ j a b i ~ ~ explained this response by damage to the restraints (including ligaments and muscles) that hold the spine in a stable position. Stokes and F r y m ~ y e r ~ ~ defined seg- menta2 instability as loss of spinal motion segmental stiffness, in such a way that force ap- plication to that motion segment produces greater displacement than is seen in a normal structure. This state leads to a painful condi-

From the Department of Orthopaedic Surgery (RSN), Service de Radiologie Osteo-Articulaire (MW, J-DL), H6pital Lariboisikre, Assistance Publique des HBpitaux de Paris, Paris, France

RADIOLOGIC CLINICS OF NORTH AMERICA

VOLUME 39 NUMBER 1 JANUARY 2001 55

56 NIZARDetal

tion with the potential of progressive deformity and places neurologic structures at risk.

Biomechanical Background

During the movements of flexion-extension of the lumbar spine, rotation in the sagittal plane demonstrated by a change in the angle formed by the vertebral end plates at each intervertebral disk and translation in the sagittal plane (defined by a slip of one vertebra to the underlying one) may be observed. Ex- cessive displacements are restrained by normal intervertebral structures. According to Panjabi,56 the load-displacement curve of the typical spinal motion segment is nonlinear with high flexibility when motion is around the neutral position of the spine and increasing passive resistance to motion nearer the end ranges of spinal motion. The total range of motion of a spinal motion segment may be divided into the neutral zone and the elastic zone. The neutral zone is that part of the range of physiologic intervertebral motion, measured from the neutral position, in which the spinal motion is produced with a minimal internal resistance and minimal expenditure of muscular energy.

As theorized by P~mjabi,5~ spinal stability depends on three functionally interdependent subsystems: (1) a passive subsystem, (2) an active subsystem, and (3) a neural control subsystem. These three subsystems adjust spine stability both around the neutral position and in extreme positions. The passive subsystem includes vertebral bodies, facet joints and their capsules, spinal ligaments, and passive tension from the musculotendi- nous units. Serial cuttings of these structures and finite element analysis helped to determine their respective roles. In flexion, the main stabilizing structures are the posterior ligaments (interspinous and supraspinous ligaments); the facet joints and their capsule; and the intervertebral disk.’, In extension, the main stabilizing structures are the anterior longitudinal ligament, the anterior part of the annulus fibrosus, and the facet joints.” Rota- tional movements are mainly controlled by the intervertebral disk and the facet joints.2O For side-bending movements, the intertrans-

verse ligaments probably play a significant role.% In the neutral zone of range of motion, muscles and tendons may act as transducers alerting changes in position and providing feedback to the neural control subsystem. In the end range of motion, these structures play a true mechanical role. The active subsystem consists of muscles and tendons. Differing roles have been assigned to unisegmental and plurisegmental muscle^.'^ Unisegmental muscles, such as intertransversarii and interspina- lis, located near the intervertebral center of rotation, may mainly act as transducers, send- ing information to the neural control subsystem for motion and position controls, whereas plurisegmental muscles (abdominal muscles and erector spinae) produce and control movements of the lumbar spine. The neural control subsystem includes the various transducers mentioned previously and the neural control centers. The various transducers pro- vide information to the control centers, which determine the requirements to achieve stability. Several studies have suggested a relationship between low back pain and persistent deficits in neuromuscular control,s, 47, 48 but more data are needed to confirm this relationship. Based on these biomechanical data, Pan- jabi57 proposed to define clinical instability as ”a significant decrease in the capacity of the stabilizing system of the spine to maintain the intervertebral neutral zones within the physiologic limits, so that there is no neurologic dysfunction, no major deformity and no incapacitating pain.” Precise in vivo definitions of these physiologic limits remain to be determined.

Postoperative Instability

Instability is one of the modes of failure of lumbar surgery. It may occur in an operated intervertebral level after disk excision, extensive decompression, or at a level adjacent to a spinal fusion. Decompressive surgery is responsible for muscular and ligamentous damage and bone removal that may impair stability at the operated levels. Preoperative risk factors of postoperative instability have been identified, and include the presence of an anterior spondylolisthesis,38 L4-5 level as the

RADIOLOGIC ASSESSMENT OF LUMBAR INTERVERTEBRAL INSTABILITY 57

site of surgery,22* 71 a sagittal orientation of the facet j0ints,2~, 68 and radiographic signs of mild disk degeneration.68 Surgical removal of the whole facet joint also increases the risk of postoperative in~tability.~~ If it is defined as occurrence or progression of spondylolisthesis, however, postoperative instability is not a frequent event.22* 26, 37* 38, 53

Spinal fusion has been shown to produce abnormal stresses on the adjacent cephalad or caudal nonfused segments. This is especially true with posterior fusion, which displaces the center of rotation of the lumbar spine in a cephalad and posterior direction.45, 46 Rigid fixation also is thought to produce additional stresses concentrated at the adjacent segments because of increased stiffness at the fused segments.45, 46 To evaluate the respective re- sponsibility of spinal fusion and the natural course of degenerative lesions in the occurrence of degenerative changes at the adjacent operated levels, Hambly et a131 compared, at an average follow-up of 22.6 years, the plain radiographs of 42 patients who underwent posterolateral spinal fusion with those of an age- and gender-matched control group of patients simply evaluated for low-back pain. No statistical difference was observed between the two groups in the frequency of radiographic degenerative changes at the transitional intervertebral levels (defined as the junction between fused and nonfused spine). The frequency of a dynamic instability (9% at the L2-3, 7% at the L3-4, and 13% at the L4-5 levels in the surgical group), defined as an anterior or posterior translation of more than 3 mm, was not statistically different between the two groups. Moreover, the rates of dynamic instability at each intervertebral level observed by Hambly et a131 were similar to those found by Hayes et a132 on functional radiographs in asymptomatic male patients on pre-employment physical examination. The rates of postoperative spondylolisthesis (11% at the L 3 4 level after an L P S l fusion and 14% at the L4-5 level after a L5-Sl fu- sion31 in the surgical group) were not statistically different between the two groups, but given the small number of patients in each group, the authors concluded that a degenerative spondylolisthesis was more likely to de-

velop in fusion patients than in the general p~pula t ion .~~

Radiologic Evaluation

Because no well-established clinical criteria are available, the diagnosis of intervertebral instability is based on the radiologic finding of an abnormal vertebral motion. Differentia- tion between normal and abnormal motion, however, remains uncertain and challenging.

Neutral Roentgenograms

Several indirect radiographic signs have been reported to be indicative of or associated with spinal instability. Moderate disk degeneration with mild space narrowing, osteo- sclerosis, and osteophytosis of vertebral end plates are thought to be associated with intervertebral instability; in contrast, a marked space narrowing is considered to be indicative of the late stabilization phase described by Kirkaldy-Willi~.~~? 85 According to Ma~rab,4~ the traction spur is a particular type of osteo- phyte that is located 2 or 3 mm from the end plate and has a horizontal orientation (Fig. 1). It is supposed to result from the tensile stresses exerted by the most external fibers of the annulus or those of the anterior longitudinal ligament on the vertebral body perios- teum in case of segmental instability.

The intervertebral vacuum phenomenon is defined by the presence of a gas collection

Figure 1. A T1 -weighted MR sagittal scan. Traction spur (arrowhead).

58 NIZARD et a1

within the disk with two main radiologic ap- pearances. The typical central vacuum phenomenon is a gas collection that fills a large neocavity occupying both the nucleus and the annulus and is indicative of an advanced disk degeneration. Because instability may create excessive intervertebral distraction and nega- tive intradiskal pressure, allowing interstitial nitrogen to become gaseous, it is assumed that the intervertebral vacuum phenomenon is often associated with spinal instability. The other type of vacuum phenomenon is characterized by a gas collection located at the out- ermost part of the annulus fibrosus close to the vertebral corner. It is believed to be sec- ondary to a rupture of the insertion of Shar- pey's fibers and may also be the result of vertebral instability. Transitional vertebrae with a decreased motion at the L5-S1 level may induce overloading and instability at the L4-5 level. Unfortunately, the diagnostic value of these indirect signs of spinal instability remains unknown because their respective sensibility, specificity, and likelihood ratio cannot be determined in the absence of a well-defined gold standard.

Functional Radiography

Functional radiographs are the only imaging technique that can demonstrate intervertebral instability or at least what is considered to be an abnormal motion between two vertebrae (Fig. 2). This method is challenging and debatable, however, for three reasons.

First, reproducibility of functional radiographs is difficult and, according to Danielson et al,I4 a slight variation in patient positioning or in gantry tilting may result in a 10% to 15% variation in the range of vertebral displacement. These two factors (i.e., patient positioning and direction of the X-ray beam) have to be accurate and reproducible to allow optimal measurement. Second, the way to perform functional radiographs and the method to measure displacements are still not ~tandardized.~~ Radiographic landmarks used for the measurements vary between reported studies and radiographic magnification is not always taken into account. In addition, the measured displacement is usually of the same magnitude as the precision error. Third, because of the lack of gold standard to define intervertebral instability, the diagnostic value of functional radiographs cannot be determined. These methodologic difficulties probably account for the wide variation of the measurement results reported in studies conducted in asymptomatic subjects (Table

Vertebral instability is generally multidirec- tional,h3 whereas the displacement is evaluated in one plane at once. The sagittal and coronal planes are evaluated on plain radiographs and the axial plane by CT or MR imaging. In symptomatic patients, pain may act as a checkrein against bending of the trunk and so far results in underestimation of the true intervertebral motion. Functional radiographs performed with passive positioning theoretically increase the range of the in-

1).7, 16, 32, 41, 61, 75

Figure 2. Functional lateral radiographs during a myelogram showing an L S L 4 intervertebral instability with a 16" angulation between flexion (A) and extension (6) and encroachment on the thecal sac.


Table 1. VALUES OF OBSERVED DISPLACEMENTS IN ASYMPTOMATIC SUBJECTS ON FUNCTIONAL RADIOGRAPHS

~~

Author Number Translation Rotation (Level)

Pearcy"' 11 - 13-16 (L1-L5)

Hayes et a13* 59 - 7-13 (Ll-W)

Boden and Wkd7 40 1.3 f 0.8 mm 7.7-8.2 (LI-W) 9.4 f 6.1 (L5-S1)

Dvorak et all6 41 2.63.1 l~un (Ll-W) 11.9-18.2 (Ll-L5) 17 f 4.3 (L5-Sl)

Tallroth et aIm 72 1470 2 5 nUn (L3-U) 13.3-16.4 (Ll-L5)

14 2 5 (E-S1)

14 (WS1)

-0.9 l~un k 1.5 (L5-Sl)

29% 2 5 mm (LA-L5) 7.1% 2 5 l~un (L5-S1)

17.3 f 5.1 (Women), 16.4 3.7 (Men) (E-Sl)

tervertebral motion because response to pain and upright position is attenuated.

In the coronal plane, a slight disruption of the alignment of the spinous processes and of the lateral border of the vertebral bodies with or without a lateral slip (laterolisthesis) should be checked carefully.

Sagittal displacements have been studied extensively. Functional radiography in the sagittal plane can be achieved either in maximal flexion and extension or with passive traction and compression. Measurements achieved on these radiographs are the translation of a vertebra with respect to the underlying one and the variation of the angle between the vertebral end plates adjacent to the disk, which is commonly named rotation. Fribergu and Kalebo et al4I studied the sagittal displacements in the particular case of spondylolytic spondylolisthesis using the technique of traction-compression radiography. To produce traction the patient is positioned to hang by hands from a horizontal bar perpendicular to the film plane with the toes slightly touching the ground; axial compression of the spine is brought about by positioning the patient to stand erect with an additional load of a knap- sack filled with 20 kg of sand. Recently, Pitka- nen et a P compared traction-compression with flexion-extension radiographs in a group of 306 patients with clinically suspected instability based on (1) central low back pain on a prolonged static weight-bearing posture, (2) a visible and palpable step in the low lumbar spine, and (3) a so-called "instability catch." They found that flexion-extension demonstrated an abnormal motion in 23% of the patients, whereas traction-compression films

showed abnormal motion in only 2% of the patients. They concluded that traction-compression radiographs were of questionable value in the diagnosis of lumbar instability. Comparison of lateral views performed in the upright position with those obtained with horizontal X-ray beam with the patient supine may also demonstrate an abnormal motion with spontaneous reduction of spondylolisthesis when the patient is supine. This reduction is sometimes seen on the lateral scout view performed during a CT examination or on MR images (Fig. 3). Finally, it can be concluded from this profuse literature that the value of functional radiographs remains debatable. Many surgeons, however, use functional radiographs to disclose abnormal spinal motion before deciding on fusion. Unfortunately, even an abnormal motion is not necessarily related to the patient's clinical symptoms.

Computed Tomography Imaging

Computed tomography scan has a higher sensitivity than plain radiographs for the de- piction of a vacuum phenomenon within the intervertebral disk or the facet joints. As previously suggested, however, the relationship between this finding and intervertebral instability is questionable. Following Farfan et a1,I9, 2o Graf 24 described a technique of functional CT named twist-test that aimed to demonstrate a gap of the facet joint space during rotation of the trunk. The CT scan is taken through the facet joint while the patient twists the torso with the pelvis tightly strapped to

60 NIZARD et a1

Figure 3. Intervertebral instability. A, Degenerative spondylolisthesis is visible on the lateral roentgenogram with the patient in the upright position. B, There is spontaneous reduction of the L 4 L 5 slippage on the sagittal T1-weighted MR image, with the patient supine.

Figure 4. Functional CT scan. Gap between the facet joints at the L4-L5 level during rotation. A, Neutral position with bilateral widening of the facet joint spaces. 8, Right rotation; no significant change. Left rotation, subluxation and joint space narrowing of the right facet joint.


the CT table. The gap appears as a vacuum phenomenon into the facet joint space during rotation. It is not known, however, whether this technique allows the differentiation between normal and unstable spine and at what extent this finding is related to clinical symptoms (Fig. 4).

Magnetic Resonance Imaging

Bram et a19 compared the MR imaging findings and those of flexion-extension radiographs in 60 patients (300 mobile lumbar units). They found no relationship between the occurrence of a sagittal translation of at least 3 mm on functional radiographs and the presence of abnormalities in the bone marrow adjacent to the end plates at MR imaging. Conversely, they found a significant association between radiographic instability and an- nular tears defined, according to April1 and Bogduk? as a high signal intensity dot on T2- weighted or postcontrast T1-weighted images and between instability and traction spurs. Unfortunately, lack of a good description of the population studied precludes a clear conclusion on the relevance of these findings.

Lumbar Myelography

This is an invasive procedure that remains the only imaging modality allowing current evaluation of the compression of nerve roots and thecal sac in the upright position, and functional positions including end points of flexion, extension, and lateral bending of the lumbar spine. Dynamic myelography is the best method to determine which interverte- bra1 levels are responsible for the neurologic compression symptoms and the respective participation of the intervertebral disk and posterior elements in the production of the spinal stenosis. Myelography is still performed in some institutions as a preoperative procedure in case of neurologic impairment.

Other Diagnostic Methods

Several devices aiming at stabilizing the lumbar spine preoperatively have been tested

to predict the results of a lumbar fusion. The preoperative value of a lumbar cast is not established?, 66 A percutaneously settled external fixation device theoretically allows a selective intervertebral segment immobiliza- tion. Several studies have tried to demonstrate the value of this procedure. The methodologic flaws of these studies prevent one from drawing a definitive conclusion on the relevance of this method?, 17, 36, n, ;rg Recently, Bednar and Raducan? in a prospective ran- domized study, showed that external fixation accurately predicted the result of a lumbar fusion. The aggressiveness of this procedure, however, and the high rate of complications including a rate of 9% of infection around the pins and a 9.8% rate of nerve root damage are dissuasive for an extensive and systematic use.

Clinical and Radiologic Correlation

Clinical criteria of lumbar spine instability are still lacking." Low back pain has a very low specificity. The radiologic demonstration of disk degeneration is the only criterion significantly associated with low back pain as recently suggested by a meta-analysis."' Sev- eral clinical signs of lumbar instability have been identified by Paris.59 Complaints of "giv- ing away," a sudden catch, shake, or hitch during forward bending have not been rigor- ously evaluated.

As mentioned previously, Pitkanen et aP2 found that clinical signs of lumbar instability were poorly correlated with abnormalities found on functional radiographs. This dis- crepancy makes either the radiologic evaluation or the clinical definition of lumbar instability questionable. Sat0 and KikuchP9 tried to evaluate the natural history of radiologic lumbar instability, excluding isthmic or degenerative spondylolisthesis. Fifty patients who had a radiographic diagnosis of lumbar instability were reexamined after 10 years. All patients suffered from incapacitating low back pain. Radiologic instability was defined as a 5-degree disk space widening or more by the Boxall et ale method or as a forward translation in flexion or posterior slippage in extension of at least 5% by the Taillard

62 NIZARD et a1

meth0d.7~ Significant clinical symptoms were still present at the 10-year follow-up evaluation in 48% of the patients, whereas radiologic criteria of instability persisted in only 20% of patients. The association of a posterior disk space widening with an anterior slippage of the upper vertebra at the end point of flexion on initial radiographs was the only finding that was correlated with persisting and incapacitating low back pain. Unfortunately, the lack of clear definition of the inclusion criteria in this study limits the value of its conclu- sions.

Despite numerous efforts, determination of the relationship between clinical symptoms and radiographic abnormalities remains challenging or even impossible because of the absence of a gold standard for the definition of lumbar instability.

Clinical Relevance

Many different surgical procedures have been proposed to achieve lumbar spine stabilization. The soft tissue stabilization system proposed by Graf," which is designed to pro- vide stabilization without fusion, has not been studied extensively and no convincing results have been reported.10, 25, 29, 51 Lumbar spine fusion remains the most widely used procedure to achieve stabilization. The suc- cess rates, as reported in a meta-analysis car- ried out in 1992, range from 16% to 95y0.~ Most of the studies reviewed in this meta- analysis, however, and those performed later, have major methodologic bias. Although lumbar fusion has been performed for more than 80 years, its clinical efficiency in lumbar spine degenerative diseases, especially in cases with no neurologic signs, remains to be established.

Yone et aim, 84 found in two studies that the results of decompression and fusion were better than those of decompression alone in patients who met preoperatively the radiologic criteria of Posner et a P (Fig. 5). Results of these studies suggest that fusion is indicated when instability is radiologically demonstrated. Unfortunately, inclusion criteria and outcome measures were not defined clearly in these studies, and additional dem-

onstration is needed before accepting this conclusion as a rule.

DEGENERATIVE SPONDYLOLISTHESIS

Definition

Spondylolisthesis is easier to define than spinal instability. Etymologically, this term means "vertebral slipping." The so-called olisthetic vertebra slips along the underlying vertebra. first differentiated the degenerative spondylolisthesis from the spondylolytic type; afterward he noticed on cadaveric specimens that the vertebral slip was not always associated with a neural arch discontinuity. Because this type of vertebral slipping mainly occurs in the elderly," New- man and Stone55 proposed the term degeneru- tive spondylolisthesis. It is caused by facet joint erosion and loosening of the muscular, capsu- lar, and ligamentous structures, which unite two adjacent vertebrae as the result of intervertebral degeneration.63 The incidence and prevalence of degenerative spondylolisthesis remain incompletely known.

Recently, Kauppila et al" studied the Fra- mingham cohort regarding degenerative spondylolisthesis. Among the 2824 subjects who had a first radiograph in 1967 to 1968, 1558 were dead by the follow-up examination and 617 accepted a follow-up radiograph in 1992 to 1993. The initial radiograph was done at a mean age of 54 years and the follow-up radiograph at a mean age of 79 years. At the follow-up examination, 12% of the men and 25% of the women had developed a degenerative slippage of more than 3 mm. Mean incidence when both sexes were considered to- gether was 19.7%. The forward displacement was the more frequent type (68.3% of the slippage). The average anterior slippage was 18% k 5.5 and the average posterior slippage was 15% 4.0.

Vogt et alsl evaluated the prevalence of spondylolisthesis on the radiographs of 788 white women with a mean age of 71.5 years who were followed-up in a study on osteopo- rosis. The vertebral slip was measured at the L34, L4-5, and L5-Sl levels using a digitized


Anterior translation

>8%

L5 - s1 > 6%

L1- L2 to L4- L5

I I

= (RON)

Posterior translation Angdation

> 9 Yo > 9"

> 9 Yo > 1"

x X

100 ibd

0-

a

film measurement. The prevalence of an anterior slip of more than 3 mm was 28.4%, whereas the prevalence of a posterior slip of more than 3 mm was 14.2%. If a 5-mm cutoff point was used, the prevalence of anterior and posterior displacements was 14.2% and 3.2%, respectively. Seventy-three percent of the spondylolistheses were located at the L4-5 level, 28% at the L5-Sl level, and 12% at the L3-4 level. As expected, the prevalence of spondylolisthesis increased with age.

Facet joint degeneration probably plays a significant role in the genesis of degenerative spondylolisthesis,28 because in their normal arrangement facet joints prevent anterior slippage.3O. 40, 70 A facet joint degeneration with

marked subchondral bone erosion is almost always present in degenerative spondylolisthesis. Moreover, at the level of the spondylolisthesis, facet joints usually have a relative sagittal orientation, which facilitates vertebral slipping and may be either a cause or a conse- quence of subchondral bone erosion.

The relationship between lumbar instability and degenerative spondylolisthesis was suggested by Kirkaldy-Willi~~~ who proposed a subdivision of lumbar spine degeneration into three phases. The first phase of joint dysfunction is associated with slight reversible anatomic changes. In the second phase, instability takes place. The intervertebral disk has a reduced height and loses its mechanical

64 NIZARD et a1

properties. Ligaments and joint capsules become loose and degenerative changes occur in the facet joints. The result of these changes is an increased and abnormal range of movements. In the third and last stage, osteophytes and advanced disk space narrowing lead to restabilization of the intervertebral level with decrease or disappearance of the range of movement, sometimes after a vertebral slipping has occurred. Finally, if degenerative spondylolisthesis is the result of an intervertebral instability that occurred at a given period in the natural course of the intervertebral degenerative process, the presence of a spondylolisthesis does not mean that vertebral instability is still present at the time of imaging because restabilization, as previously defined, may have already occurred.

Radiologic Evaluation

Direction of Slipping

The direction of slipping is defined according to the displacement of the upper (olisthetic) vertebra. Once degenerative changes have unlocked the intervertebral joint, the vertebral slipping occurs in a plane and along a direction that roughly depend on two factors: (1) the symmetry of facet joint lesions and (2) the distribution of weight- bearing forces at each intervertebral level. When facet joint degeneration and subluxation are symmetric, vertebral slipping is mainly sagittal without significant rotatory

displacement. In the case of asymmetric subluxation, a rotatory displacement is associated with the anteroposterior displacement, the less dislocated joint being the axis of the rotation. In the latter situation, the orientation of the joint spaces is asymmetric with the most sagittally oriented joint space being the site of the most marked dislocation.

Anterior inclination of the disk space, which is usually present at the L 3 4 and the L4-5 levels, especially in cases of increased lumbar lordosis, induces anterior slipping, whereas a posterior inclination, which is usually present at the L2-3 and L1-2 levels, may lead to posterior slipping (Fig. 6). In addition, a lateral slipping (laterolisthesis) may occur at one or two lumbar levels in elderly patients. This lateral slipping is almost always associated with vertebral rotation and scoliosis. This vertebral rotation may be induced by a pre-existing idiopathic scoliosis that in turn is worsened by the rotatory laterolisth- e s i~ . '~ If these patterns are the most frequent, however, anterior slipping may be observed at the upper vertebral levels, posterior slipping at the lower lumbar levels, and lateral slipping may occur without a pre-existing scoliosis and further result into de novo progressive scoliosis." The anterior, posterior, and lateral degenerative spondylolistheses have a moderate magnitude of displacement. According to Kauppila et a1,4* average anterior slippage is 18% * 5.5% and average posterior slippage 15% & 4%. Only 10.6% of the 123 patients with vertebral slippage in this

Figure 6. An L4-L5 posterior slipping during extension. Flexion (A); extension (6).


study had a displacement exceeding 25% according to Meyerding scale.54

Consequences of Vertebral Slipping on the Lumbar Spinal Canal

Degenerative spondylolisthesis is frequently associated with a significant narrowing of the lumbar spinal canal and compression of the neural s t ruc ture~ .~~ Studies of the natural history of degenerative lumbar spondylolisthesis have suggested that progression of the slippage occurs in 30% of pa- t i e n t ~ . ~ ~ Compression of the neural structures results from different mechanisms. In the case of symmetric anterior facet joint subluxation with anterior spondylolisthesis, the two lower facets of the upper (olisthetic) vertebra ventrally impinge on the spinal canal and lateral recesses (Fig. 7). Osteophytes frequently increase the impinging effect of the subluxated facets. The narrowing of the spinal canal and the lateral recesses lead to a compression of the thecal sac in the central spinal canal or the nerve root in the lateral recess on both sides. On a lateral view, the central spinal canal narrowing is located between the neural arch of the upper vertebra and the posterior

Figure 7. Degenerative anterior spondylolisthesis with symmetric facet joint subluxation. Axial CT scan. The subluxated inferior apophyseal processes of the superior vertebra (arrows) almost completely narrow the central spinal canal.

and superior corner of the vertebral body of the underlying vertebra. In the case of symmetric posterior facet joint subluxation, which results in a retrospondylolisthesis, a central spinal canal narrowing is less frequent because the range of slipping of the upper vertebra is lower. The backward displacement of the inferior facets of the upper vertebra results in a widening of the facet joint spaces. On a lateral view, the central canal narrowing, when present, is located between the posterior and inferior corner of the upper vertebral body and the lamina of the underlying vertebra (Fig. 8). In the case of asymmetric anterior subluxation of the facet joints, the inferior facet located on the side of the most im- portant dislocation impinges ventrally on the ipsilateral nerve root into the lateral recess.44 Narrowing of the central spinal canal is usually milder than in the case of symmetric anterior subluxation. In addition, a foraminal disk herniation is frequently present at the same level and on the side of maximal subluxation and is attributable to the shear effect of the vertebral rotation on the intervertebral disk.44 This association of an asymmetric facet joint subluxation with a unilateral recess stenosis and an ipsilateral foraminal disk herniation on the side of maximal facet joint subluxation is a characteristic triad of rotatory spondylolisthesis (Fig. 9).44

Additional Features

Nonspecific diffuse degenerative changes are usually present on plain radiographs at several lumbar intervertebral levels. The range of sagittal and coronal slipping at the level of spondylolisthesis can be evaluated on the lateral and anteroposterior radiographs. A significant rotatory displacement associated with the spondylolisthesis may result on the anteroposterior view in a subtle bayonet-like disruption in the alignment of the spinous processes at the level of the olisthetic vertebra.

The participation of each anatomic degenerative structure in the narrowing of the spinal canal can be evaluated on MR imaging and CT examinations. A facet joint synovial cyst in the lateral foramen or in the central

66 NIZARD et a1

Figure 8. Myelogram with lateral functional views in extension (A) and flexion (6): L4-L5 degenerative retrolisthesis with compression of the thecal sac between the postero-inferior corner of L4 and the lamina of L5.

Figure 9. Characteristic triad of rotatory spondylolisthesis. Axial CT scans at the L4-L5 level. A, Asymmetric subluxation of the L4-L5 facet joints maximal on the right side (arrow) narrowing the right lateral recess. 6, lpsilateral foramina1 L4-L5 disk herniation (open arrow) impinging on the right L4 nerve root.

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50

Dec

ompr

essi

on

Dec

ompr

essi

on a

nd

noni

nstr

umen

ted

post

erol

ater

al fu

sion

44

Dec

ompr

essi

on

Dec

ompr

essi

on a

nd

Dec

ompr

essi

on a

nd

noni

nstn

unen

ted

inst

rum

ente

d po

ster

olat

eral

fusi

on

post

erol

ater

al fu

sion

inst

rum

ente

d po

ster

olat

eral

ex

tens

ive

inst

rum

ente

d fu

sion

po

ster

olat

eral

fusi

on

inst

rum

ente

d po

ster

olat

eral

fu

sion

(10

)

post

erol

ater

al fu

sion

inst

rum

ente

d po

ster

olat

eral

fu

sion

(19

)

45

Dec

ompr

essi

on

Dec

ompr

essi

on a

nd li

mite

d D

ecom

pres

sion

and

17

Dec

ompr

essi

on (7

) D

ecom

pres

sion

and

19

Dec

ompr

essi

on (1

1)

Dec

ompr

essi

on a

nd

33

Dec

ompr

essi

on (14)

Dec

ompr

essi

on a

nd

Alte

rnat

ely

assi

gned

to

trea

tmen

t

Ran

dom

izat

ion

met

hod

not

stat

ed

Ass

igne

d w

ith th

e da

te of

ad

mis

sion

to

hosp

ital

Unc

lear

Unc

lear

Patie

nts’

cho

ice

Tabl

e 3.

RE

SU

LTS

OF

CO

MP

AR

ATI

VE

STU

DIE

S O

N L

UM

BA

R F

US

ION

FO

R D

EG

EN

ER

ATI

VE

SP

ON

DY

LOLI

STH

ES

IS A

ND

LU

MB

AR

INS

TAB

ILIT

Y

FOIIO

W-U

P

Ref

eren

ce

(yea

rs)

Mai

n O

utco

me

Her

kow

itz a

nd K

~r

z~

3

(2.4

-4)

Eval

uatio

n ba

sed

on p

ain,

Brid

wel

l et all2

3.2

(2-7

) Sl

ip p

rogr

essi

on

activ

ity, a

nd p

aink

iller

use

Gro

b et

aIz

6 2.

5 (2

-3)

Eval

uatio

n ba

sed

on p

ain,

ac

tivity

, and

pai

nkill

er u

se

Yon

e et

alw

3

(2-5

) Ja

pane

se O

rtho

paed

ic

Feff

er e

t alz

l 4 (1-6)

Not

wel

l def

ined

A

ssoc

iatio

n sc

ore

Yon

e an

d Sa

kou"

2

min

imum

Ja

pane

se O

rtho

paed

ic

Ass

ocia

tion

scor

e

Sec

onda

ry

Res

ult

Out

com

es

T2 >

T1

(P=O

.OO

Ol)

Slip

pro

gres

sion

N

onfu

sion

T1

: 4/9

N

onfu

sion

T2

: 7/1

0 T

3 1

/24

(P =

0.00

1)

Wal

king

abi

lity

No

diff

eren

ce

T2>T

18O

Yo

good

and

ex

celle

nt v

s 28

% (P

c0.0

5)

Fusi

on: g

ood

(5),

fair

(3)

Dec

ompr

essi

on: g

ood

(5),

fair

(3),

poor

(3)

T

2>T

1 80

% go

od a

nd

exce

llent

vs

43%

Blo

od l

oss

Ope

rativ

e tim

e N

onfu

sion

D

evic

e-re

late

d co

mpl

icat

ions

Su

bjec

tive r

esul

t N

onfu

sion

In

stab

ility

Low

bac

k pa

in

Wal

king

abi

lity

Non

fusi

on

Res

ults

T1: 9

6%, T

2 28

%

36%

T2: 3

/10,

T3:

21/

24

28/3

1 be

tter i

f no

slip

T1 <

TZ

<T3

Tl<

T2<

T3

1 in

T3

4 de

vice

rem

oval

s

(P =

0.00

2)

prog

ress

ion

T2>

T1

(P<0

.05)

10%

Dec

ompr

essi

on (4

)

Sign

ifica

ntly

bet

ter i

n T2

Si

gnifi

cant

ly b

ette

r in T2

1/19

Com

plic

atio

ns

T2

nonf

usio

n T3

: inf

ectio

n, 1

scre

w o

ut

the

pedi

cle,

1 in

stab

ility

ab

ove

the

fusi

on

T2

1 fr

actu

re a

bove

, 1 ho

ok d

islo

catio

n -

T =

tre

atm

ent.


spinal canal is sometimes associated with the degenerative spondylolisthesis.@', 67 The range of rotation between two adjacent vertebrae may be measured with CT, but is mostly of theoretical interest. When spine fusion is considered, T2-weighted M R images help in the evaluation of disk degeneration.

In our practice, lumbar myelography including functional views is still performed as a preoperative evaluation of the spinal stenosis with neurologic symptoms. This technique may disclose a vertebral instability associated with the degenerative spondylolisthesis, which may explain to some extent why some cases with an apparently moderate spinal canal narrowing on CT or MR imaging are symptomatic.

Clinical and Radiologic Correlation

The relationship between low back pain and degenerative spondylolisthesis is unclear. According to Vogt et a1,8l 37.5% of patients presenting with anterior spondylolisthesis had low back pain within the past 12-month period, whereas 32% of women without spondylolisthesis reported symptoms of low back pain. In addition, these authors found no significant correlation between anterior spondylolisthesis and the Occurrence of severe or incapacitating pain. Conversely, Kauppila et a14* in the Framingham cohort study showed that at the time of the second evaluation (at an average of 25 years after the first evalua- tisn) the rate of patients who experienced pain, aching, or stiffness in their back on most days was significantly greater in the spondylolisthesis group than in controls. In the case of degenerative spondylolisthesis with nerve root compression, the anatomic type of vertebral slippage has, to some extent, an influence on the pattern of the neurologic symptoms, especially their unilateral or bilateral distribution. In the case of symmetric (anterior or posterior) spondylolisthesis, with no or only mild rotatory component, nerve root involvement that is typically bilateral and pluriradi- cular is related to the compression of the thecal sac in the central spinal canal and to a bilateral lateral recess stenosis. Conversely, in the case of asymmetric spondylolisthesis,

with a marked rotatory displacement, nerve root involvement is frequently unilateral in- volving one or two nerve roots on the side of maximal facet joint subluxation because of the compression of the nerve roots in the ipsilateral lateral recess and foramen. In a meta-analysis dealing with surgically treated degenerative spondylolisthesis, radiculopa- thy and neurogenic claudication were found preoperatively in 32% and 3% of the patients, re~pectively.~~

Clinical Relevance

Surgical treatment is indicated in degenerative spondylolisthesis associated with neurologic symptoms and when medical treatment has failed. Decompression with or without fusion may be discussed. In a meta-analysis of 25 studies dealing with the treatment of degenerative spondylolisthesis, Mardjetko et a150 suggested that patients who had spinal fusion associated with decompression had a better outcome than those treated with decompression alone. Moreover, the fusion rate increased when fixation devices with pedicular screws were used. It must be noted, however, that only three of these studies were prospective and quasirandomized with im- perfect methods of treatment allocation (Table 2). In addition, these studies have several other serious weaknesses including lack of blinding and independent assessment of the outcome, which gave additional potential for bias. Despite these limitations, there is weak evidence that adding spinal fusion to decompression in degenerative spondylolisthesis produces less progressive slipping and better clinical outcome than decompression alone (Table 3).

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Address reprint requests to

R6my S. Nizard, MD, PhD Department of Orthopaedic Surgery

H6pital LariboisiGre 2 rue Ambroise Par6

75010 Paris France

e-mail: [email protected]

radiologic assessment of lumbar intervertebral instability and degenerative spondylolisthesis

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