Vertebral body alignment and height are best assessed on well-positioned lateral radiographs (Fig. 6.1). Optimal positioning yields a single line denoting the posterior cortex of each vertebral body; a line along these posterior cortices will form a smooth, uninterrupted curve when the vertebral alignment is normal. However, lateral views are often compromised by patient rotation. With rotation, two posterior vertebral body cortices may be evident at the rotated levels; a line connecting the midpoints of the spaces between these cortices can be visualized and should again form a smooth curve when the alignment is normal. Alternatively the midpoints of the anterior aspects of the vertebral bodies should be smoothly aligned (Fig. 6.3). Vertebral anatomic landmarks demonstrated on lateral radiographs include the pedicles, superior and inferior articular facets, facet joints, neural foramina, intervertebral disc spaces, and spinous processes. The portion of the neural arch between the superior and inferior articular facets, the pars interarticularis (plural, pars interarticulari; Latin plural partes interarticulares) can be seen, of particular interest in spondylolisthesis (Fig. 6.4). Most of the radiographic measurements related to spondylolisthesis are performed on lateral views.

Oblique views are performed in the AP projection with the patient rotated 45° to his or her left (left posterior oblique) and right (right posterior oblique). In these views, the shape of the vertebral posterior elements is reminiscent of a Scottish terrier; hence, the term “Scottie dog” is used. The parts of the Scottie dog that can be identified include the eye (pedicle), snout or nose (transverse process), ear (superior articular facet), foot (inferior articular facet), and neck (pars interarticularis) (Fig. 6.5).

The diagnosis of spondylolisthesis is most commonly made on lateral lumbar spine radiographs where anterior slippage (anterolisthesis) or posterior slippage (retrolisthesis) of a vertebral body is noted in relation to the vertebral body directly below (Fig. 6.6). Spondylolisthesis, either anterior or posterior, is usually found at a single level but may be seen at multiple levels (Fig. 6.7) [2]. Again, the radiographic finding of spondylolisthesis, especially when mild, may not correlate with symptomatology.

In the radiographic grading of spondylolisthesis, the most commonly used tools are the classification systems of Meyerding [3] and Taillard [4] both of which have been shown to yield results with high intra- and inter-observer agreement [5].

Meyerding’s system is based on division of the superior endplate of the vertebra below the slipped vertebra into four equal parts. Alignment of the listhesed or slipped vertebra in relation to the divisions in the endplate below determines the grade. Slippages between 0 and 25 % of the endplate below are grade I, between 25 and 50 % are grade II, between 50 and 75 % are grade III, and between 75 and 100 % are grade IV. Anteriorly slipped vertebrae that descend anterior and inferior to the endplate below are classified as grade V, also called spondyloptosis (Fig. 6.8).

Taillard’s assessment method is also referred to as the “percentage slip” measurement. The distance between the posterior margins of the slipped vertebra and the vertebra below is divided by the anteroposterior dimension of the inferior vertebral endplate and expressed as a percentage (Fig. 6.9).

In the cervical spine and thoracic spine, spondylolisthesis is commonly measured in millimeters rather than grades or percentage slip.

Progression of spondylolisthesis can be assessed radiographically, provided that follow-up examinations are similar to prior studies in technique and positioning (Fig. 6.10).

Lateral views in flexion and extension (Fig. 6.20) are used to assess stability at the site of spondylolisthesis or to elicit spondylolisthesis. These can be performed on the tabletop with the patient in a lateral decubitus position, or with the patient standing. Proponents of standing views note that weight-bearing views may better approximate normal daily activities. However, patients may achieve a higher degree of flexion and extension in the decubitus tabletop position. In other attempts to elicit the greatest movement at sites of spondylolisthesis, axial compression–traction techniques have been used [17]. Putto proposed that the flexion view be done with the patient seated and their hips flexed, while the extension view be done with the patient standing with hips against the radiography table [18].

Two types of instability are assessed. Parallel instability refers to movement of the upper vertebra in relation to the lower vertebra anteriorly with flexion or posteriorly with extension. The angle between the two vertebral endplates does not change significantly (Fig. 6.21). Angular instability is defined as an abnormal change in the angle between the endplates of the listhesed vertebra and the vertebra below (Fig. 6.22). Wide variations in vertebral body motion on flexion and extension have been reported. In an extensive review, Leone et al. concluded that between flexion and extension, the upper limit of normal total parallel excursion is 4 mm and the upper limit of normal angular change is 10° [19]. It should be noted that patients with normal alignment in the neutral position may demonstrate spondylolisthesis on flexion or extension (Fig. 6.23).

While flexion and extension radiographs are the most common method of assessing stability, comparison between modalities may provide similar information. For instance, if the severity of spondylolisthesis changes between a neutral radiograph and a supine MRI study, instability has been effectively demonstrated (Fig. 6.24).

Progression of instability in spondylolisthesis can be assessed on serial studies. However, reproducibility of flexion and extension views is difficult and slight variations in patient positioning or angulation of the radiographic beam can result in discrepancies in the range of vertebral displacement.

MRI is a powerful cross-sectional imaging modality that detects the behavior of the nuclei of hydrogen atoms, the most abundant atoms in the human body, in the context of a strong magnetic field. The physics concepts related to MRI are complex and are beyond the scope of this text. Very simply put, patients undergoing MRI are placed within the strong magnetic field of the machine; this magnetic field is always “on.” Current is applied to a coil over the body part to be imaged and the coil produces energy in the form of a rapidly changing magnetic field. This energy falls within the energy frequency range commonly used in radio broadcasts, and is therefore called radiofrequency energy. While radiofrequency energy is on the same EM spectrum as X-rays, it is of a much higher wavelength (lower frequency and therefore lower energy) and cannot ionize tissues. Thus the risks associated with the ionizing radiation of radiography, computed tomography, and nuclear medicine studies do not apply to MRI.

The radiofrequency energy from the coil causes alterations in the spin of the hydrogen nuclei. When the radiofrequency waves are turned off, the hydrogen nuclei “relax” to assume their original orientation. As they relax, they give off energy which is detected by a receiver coil; the information is then processed by computer and an image is generated. An array of coils and MRI scanning parameters are available for use depending on the body part and the type of information sought.

MRI is the cross-sectional imaging modality of choice in the workup of spondylolisthesis as excellent soft tissue differentiation is achieved without exposing the patient to ionizing radiation.

The studies should include sequences using a variety of parameters. T1-weighted (T1W) images are useful for visualizing fracture lines and excluding abnormal marrow infiltration. Proton-density (PD) or T2-weighted (T2W) images provide good spatial resolution. Short-tau inversion recovery (STIR) sequences are excellent for detection of bone marrow edema as are fat-suppressed PD or T2W sequences. Various gradient-echo (GE) sequences are available and are useful in evaluating the intervertebral disc contours particularly in the cervical spine.

Sagittal images demonstrate vertebral body heights and alignment, and are also used to evaluate intervertebral disc heights and disc hydration. The central spinal canal and neural foramina can be assessed on both sagittal and axial images. The axial sequences should include a “stacked” set of slices that are contiguous and parallel to each other, rather than angled for disc evaluation, in order to gain optimal visualization of the pars interarticulari. Coronal images are useful for visualization of scoliosis and lateral spondylolisthesis. Figure 6.25 shows normal lumbosacral spine anatomy on MRI.