When spondylolysis is suspected, anteroposterior, lateral, oblique left and right, and collimated lateral views of the spine are performed. Using these five views, most spondylolytic defects are detected. In the acute setting, the defect is narrow with irregular borders, whereas more chronic lesions have smoother and more rounded edges. The width of the gap depends on the grade of spondylolisthesis. Indirect signs of spondylolysis on the AP view are lateral deviation of the spinous process and sclerosis of the contralateral pedicle. The oblique views may show the “scotty dog” sign (a break in the neck of the dog representing the isthmic defect). Although 45° oblique views have long been thought to be the best projections for detecting spondylolysis, the study is only 32 % sensitive (Haus 2012). They demonstrate isthmic defects only well if the X-ray beam is perpendicular to the pars, while the orientation of lysis is in the majority of cases close to the coronal plane. This is why the collimated lateral view actually shows the majority of defects (Leone et al. 2011). Also a hypoplastic L5 vertebral body is evidence of an L5 spondylolytic defect. The forward displacement in spondylolisthesis is determined on the lateral view. Listhesis can be graded according to the percentage of slipping by dividing the subjacent vertebral body into four equal parts. A slip in the first quarter is grade I and a slip in the last quarter is grade IV. In symptomatic spondylolisthesis, dynamic flexion and extension views should be obtained to determine lumbar intervertebral instability. Most spondylolisthesis occur in flexion. Axial or extension spondylolisthesis more often give nerve root compression. Sometimes this is the only anomaly found in young people with low back pain complaints. The distinction between normal and abnormal movement is however difficult with a substantial overlap between symptomatic and asymptomatic motion patterns. General values of 10 % for sagittal rotation (i.e., variation of the angle between vertebral endplates adjacent to the disc) and 4 mm for sagittal translation (i.e., slip of one vertebra relative to the vertebra below) are used as cutoff.

When radiographs are negative for spondylolysis, but clinical suspicion is high, the more sensitive test is single-photon emission computed tomography (SPECT). SPECT is useful in early stages as it can identify patients with symptomatic (pre-)spondylolysis before radiography does. Positive SPECT suggests symptomatic spondylolysis, stress reaction, stress fracture, or healing (Fig. 11.6). It can differentiate between acute spondylolysis and a nonunion of the pars. Radiographic pars defects without uptake on SPECT are often asymptomatic. There has, however, also been documented positive SPECT imaging in asymptomatic athletes. Furthermore, activity on SPECT is nonspecific and has to be differentiated from other entities, for example, facet arthritis, infection, or osteoid osteoma. Therefore, if SPECT is positive, a CT scan should be performed for further evaluation. Therefore, SPECT is not recommended as screening tool in young patients. SPECT can be used to guide therapy. SPECT-positive spondylolytic lesions may better respond to conservative therapy (Leone et al. 2011; Papanicolaou et al. 1985; Maxfield 2010).