Hanna Hebelka and Helena Brisby
Degenerative spondylolisthesis (DS) is an acquired sagittal translation of one vertebra relative to the subjacent one, without a pars interarticularis defect, caused by degenerative changes.1-3 DS is commonly referred to as an anterolisthesis even though it may also be a retrolisthesis, but to a lesser extent. The most common level for DS is L4-L5 followed by L5-S1, with anterolisthesis being most common at L4-L5 and retrolisthesis more common in L5-S1 and in individuals with reduced lordosis.4-11 This chapter aims to provide the best imaging strategy in obtaining the DS diagnosis. Clinical evaluation, with a thorough physical examination and detailed penetration of symptomatology, is an important step in the diagnosis of DS. The clinical presentation of DS has a wide spectrum ranging from no symptoms at all to back pain with or without radicular symptoms, leg pain, neurogenic claudication, muscle weakness, etc.4-12 Details regarding clinical evaluation of symptomatology are covered in Section 15 Chapter 1 of this publication. When the symptomatology and physical examination points to DS, radiological imaging is then used to confirm the diagnosis and possibly also advise regarding the clinical relevance of the DS in relation to other radiological findings.
RADIOGRAPHS IN THE DIAGNOSIS OF DS
Standard radiographs in anteroposterior (AP) and lateral views in a neutral position are considered the most appropriate imaging test to detect DS.1,13,14 Scoliosis and/or lateral translation are evaluated on AP images while vertebral body height and sagittal translation are best evaluated on the lateral radiographs.2 To achieve mechanical impact, studies recommend that these images be obtained in the standing position,1,12,15-19 since up to 1/3 of DS will be missed on imaging performed in a supine position.16,18,20,21 As an example, Segebarth et al. recently investigated 416 patients with degenerative spine conditions in whom supine MRI and standing radiographs (flexion/extension and lateral) had been performed.18 One hundred nine patients were found to have DS on standing radiographs, however 31 of those were undiagnosed on the supine MRI.18 This highlights the importance of performing weight-bearing imaging investigations when diagnosing and evaluating the grade of DS.
Radiographs are particularly able to demonstrate anterolisthesis of one vertebra relative to the subjacent, without a pars interarticularis defect (Fig. 2-1).14 Examples of concomitant changes that might be detected at radiographs are: disc space narrowing, endplate lesions, osteophytes and facet joint sclerosis.4,5,21,22 In addition, DS is frequently accompanied by degenerative scoliosis, sometimes even with elements of rotational translation, which might be a diagnostic challenge. Regarding alignment, degenerative spinal changes may, in addition to sagittal translation, lead to a kyphotic disc angle at the affected level.23 Sagittally-oriented facet joints are also associated with the presence of DS, though primarily with anterior translation and not posterior translation.11
Malalignment of the spinal processes on lateral radiographs can be seen in DS, due to anterior translation of the entire vertebra, since the neural arch is intact as opposed to in isthmic spondylolysis.5 Oblique AP views, or “scottie dog,” are traditionally used to detect isthmic spondylolisthesis and may reveal a collar on the “dog,” differentiating isthmic spondylolisthesis from DS.2,4,5 However a recent study did not show increased specificity and sensitivity in identifying spondylolysis by adding oblique views as compared with standard views alone.24
The magnitude of sagittal translation is often called “slip” and is measured in the lateral view (Fig. 2-2).23,25 Two methods are commonly used to grade spondylolisthesis, irrespective of its type (isthmic or degenerative): the Meyerding Classification26 and the Taillard Method,27 respectively. In the Meyerding Classification, the superior aspect of the inferior vertebra is divided into quartiles on the sagittal view and graded I-IV, representing a slip of the superior vertebra of I, II, III or IV quarters, respectively (Fig. 2-3A). Grade V refers to when one vertebra has slipped off the inferior vertebra entirely, i.e., spondyloptosis.3 According to Taillard, the sagittal translation is classified in percentage of the slip of the superior vertebra relative to the superior aspect of the inferior vertebra (Fig. 2-3B). Due to higher reproducibility, the Taillard Method is favored by most authors.4,5,28 Regarding progression, a slip ≥ 20% is considered relevant, since up to 15% error in intra/inter observer measurements has been reported.5,28,29
DS is often considered to be associated with pathological vertebral movement, i.e., instability.21,25 This instability theory is based on the occasional findings of increased translation on functional imaging. This theory has also contributed to the popularity of adding fusion surgery in patients with spinal stenosis in need of decompression surgery, aiming to stabilize a DS segment.23,30 A recent randomized controlled trial (RCT) challenges this instability theory by showing no clinical benefit to using fusion as an adjunct to decompression in DS patients.31 The importance of imaging and how to define potential “instability” in DS patients thus needs to be questioned. However, functional imaging is still widely used and the body of literature regarding DS “instability,” and how to best image it, is extensive, which is why this subject will be briefly covered here.
The range of segmental vertebral mobility in DS is wide, without any universally accepted definition for either the term “instability” or which imaging techniques should be adopted to verify it – uniform reference standards are lacking.1 Various studies have also reported conflicting results regarding the association between symptoms and DS slip detected with radiology.8,9,21,32,33
To quantify mobility in DS, many doctors employ the use of functional imaging techniques, such as lateral flexion/extension radiographs, since they have the potential to reveal an increased translation.15,34-39 A difference in vertebral slip of ≤ 3 mm between flexion and extension (functional radiographs) has been seen in a majority of asymptomatic subjects.3,40,41 Further, a static translation of > 3 mm, on either extension or flexion radiographs, has been reported in up to 42% in asymptomatic individuals.34 A cut off value of > 3 mm has therefore been accepted by many studies as a sign of abnormal sagittal translation,21,31,42-45 whereas others adopt a change in slip of ≥ 8% or ≥ 4 mm.7,34,46-48
Because of its simplicity, low expense and wide availability, flexion/extension radiographs are the most widely used functional imaging method regarding DS instability.21,38,49-51 Knutsson et al. were one of the first to state that flexion/extension radiographs revealed instability, with many followers also claiming that such functional imaging is important for assessing grade of pathological translation in DS.52 Functional radiographs have in several studies revealed findings not shown on conventional radiographs.6,16,25,44,46,47 For example, Cabraja et al. found what they defined as a pathological slip in 11% of their 100 studied spondylolisthesis patients (83% DS) with flexion/extension radiographs, which had not been apparent in standing/recumbent position radiographs.46 Despite this study’s finding, in most patients, standing radiographs compared with recumbent position radiographs reveal greater sagittal translation compared to flexion/extension radiographs.46 Several others support the use of standing/supine radiographs instead of flexion/extension radiographs.15,16,18,46 Regarding posterior translation, increased incidence has been reported adjacent to discs that are ankylosed or fused. For example, retrolisthesis has been shown postoperatively in 15% of segments cranial to fused disc levels on extension radiographs.53
Various other functional imaging methods have been proposed, such as functional radiographs in seated position and traction/compression radiographs, whereas others claim that flexion/extension in the supine position is preferable to reveal movement in a segment with DS.25,47,48,54,55 The argument for the latter is that pain-induced spasms in the paraspinal muscles are inhibited in supine position and therefore might reveal movement in the segment better than in the standing position.46-48,54,56
It is important to highlight that although functional imaging methods may reveal/increase translation and movement in a spinal segment, this is not synonymous with required changes to clinical decision making. For example, Försth et al. showed in their RCT that despite a sagittal slip over 7 mm on lateral radiographs, there was no difference in clinical outcome comparing decompression combined with fusion with decompression alone.31 A vast majority of doctors today seem to believe that the additional value of functional imaging techniques is limited in the routine clinical situation.15,18,20,21,40,57 Another important reason to question routine use of functional imaging is the non-standardized technique, i.e., lack of routine reference standards and reproducibility.20 Patient positioning relative the X-ray beam, differences in magnifying factor, vertebral tilt and rotation can affect reproducibility with measuring variations up to 10–15%.28,38,50,58 Further arguments for the limited value of functional radiographs is the large variation in the general population and the decreased range of motion of the spine in patients suffering from pain.40,57,59,60
To summarize, considering the limited evidence of its value, cost and radiation, the use of routine functional imaging is not recommended.1,35 Functional imaging should rather be reserved for clinically challenging patients in whom the results of the investigations are likely to affect therapy.15,20,35,36
MAGNETIC RESONANCE IMAGING (MRI) AND DS
MRI is the most appropriate modality to image spinal canal stenosis or foraminal stenosis accompanying DS.1,2,4,14,22,61 Other MRI findings frequently accompanying DS are facet joint hypertrophy and effusion, synovial cysts, high grade osteoarthritis and thickened ligamentum flavum.1,4,62,63 Spinal MRI should at minimum include conventional sagittal and axial T1 and T2-weighted sequences.2,22 There are no universally accepted quantitative criteria for which MRI parameters that are best for evaluation of spinal stenosis. Examples of various measurements used are cross-sectional dural sac area, AP diameter of the osseous spinal canal, foraminal diameter, lateral recess height and angle and ligamentous interfacet distance.61,64 T1-weighted imaging provides detailed information regarding anatomical structures of foremost soft tissues, and pathology within these, while T2-weighted images are best to visualize the spinal canal and foraminal recesses.3 The cross-sectional dural sac area and AP diameter of the dura are best evaluated at axial T2-weighted images while parasagittal images can display nerve root entrapment by osteophytes, or protruding disc, at the level of the foramina.3 Diminished fat surrounding the root in the foramina is a qualitative parameter indicating foraminal stenosis affecting the nerve root (Fig. 2-4). In 2010, Barz et al. introduced the “positive sedimentation sign,” in which the nerve roots are “floating” within the dural sac as opposed to, due to gravity, being packed together within the posterior aspect of the dura on supine axial MRI images (Fig. 2-5). This sign, the absence of nerve root sedimentation, has been reported to be associated with symptoms of lumbar spinal stenosis.65,66
Facet joint effusion is highly indicative of DS16,62,63,67 and appears to have a linear correlation with the degree of slip.39,42,63,68,69 Facet joint effusion size, best seen on axial T2-weighted images, is measured at the greatest distance between the articular surfaces (Fig. 2-6). According to North American Spine Society’s (NASS) Guidelines 2016, facet joint effusion > 1.5 mm on supine MRI suggests DS and if detected without previous radiographs, standing lateral radiographs are recommended, since DS can be occult/reduced in supine position and facet joint effusion indicate possible movements of the affected segment.1,39,42,62,68
FUNCTIONAL MRI IMAGING
The development of open MRI systems provides new opportunities to image the spine in various positions (standing, sitting, flexion/extension). Both axial loading during MRI (alMRI) and upright MRI have, similar to functional radiographs, been reported to reveal/increase DS slip as compared to in the supine position.12,19,44,70,71 For example, alMRI has been shown to reduce the cross-sectional dural area compared with recumbent MRI without load71,72 and MRI performed in a sitting position can reveal anterolisthesis to a greater extent and in more patients as compared to supine MRI.70 Recently, Splendiani et al. investigated how the change of MRI position, from supine to upright, influenced spinal changes like disc herniations, spinal stenosis, sagittal translation and lumbar lordosis angle in 4305 low back pain patients.44 1178 of these patients had a sagittal translation (> 3 mm) on either supine or upright MRI. Upright MRI revealed significant alterations, defined as the appearance of, or an increase of changes between the positions, in 715 of these 1178 patients. These MRI techniques provide an excellent opportunity to investigate the effect of kinematics on spinal structures, thus having the potential to add clinical useful information for individual DS patients. However, as with functional radiographs, the evidence is insufficient to recommend for or against routine functional MRI.1,21 Rather, alMRI or upright MRI should only be used in selected cases in which supine MRI/ radiographs are negative but symptoms indicate neurogenic claudication.12 It is crucial to recall that it is not a question of whether increased slip can be proven with either method, but rather if this information adds to the clinical decision-making regarding treatment. Prospective, appropriately powered studies are warranted to better assess the utility of functional MRI in the detection and evaluation of stenosis in the setting of DS.1
COMPUTED TOMOGRAPHY (CT) AND DS
With development of the MRI technique, providing more detailed anatomic information without radiation, the value of computed tomography (CT) with regard to DS has diminished. CT may however still be of value, primarily to assess details regarding bony structures such as degenerative changes of the facet joints and bony spurs protruding into the spinal canal causing central and forminal narrowing (Fig. 2-7).22 In patients with contraindications for MRI, in whom clinicians suspect DS with accompanying spinal and/or foraminal stenosis, CT myelography is recommended.1,2,14,22 CT myelography may also be valuable postoperatively, when artifacts from surgical hardware often restrict the MRI assessment.22 High radiation exposure, the former drawback of CT, has decreased since both hardware and software CT equipment are continuously developing. Therefore “low dose CT protocols,” which decrease the radiation dose without sacrificing image quality, might become potential tools to evaluate spinal biomechanics, since they provide the possibility to obtain spinal images in three-dimensions with high resolution.73-75 For example, Försth et al. have reported that three-dimensional (3D) CT provides a tool for measuring spinal 3D movements safely, reliably and with high accuracy.76
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