At times, direct visualization of a pars defect is difficult on sagittal MR images, and thus it may be difficult to determine whether a patient with spondylolisthesis has a degenerative origin or if the malalignment is due to spondylolysis. In such cases, characteristic ancillary findings can be utilized to differentiate degenerative spondylolisthesis from isthmic spondylolisthesis. An appearance that we have found to be highly characteristic of isthmic spondylolisthesis is the horizontal neural foramina sign. In patients with spondylolysis, the neural foramina often assume a horizontal configuration on far lateral sagittal images (9a,9b). This feature is not present in degenerative spondylolisthesis, and the configuration also accounts for radiculopathy due to foraminal stenosis in patients with more severe isthmic spondylolisthesis.
The final ancillary observation that may aid in the detection of spondylolysis is an abnormal wedging of the posterior aspect of the vertebral body at the level of the pars defect. This finding is a well-known radiographic finding that occurs at the level of spondylolisthesis. It is unclear if this finding is an effect of the spondylolisthesis, a predisposing condition, or a combination of both. On sagittal MR images, wedging of the posterior vertebral body is seen both in patients with spondylolisthesis and in those with spondylolysis and no significant subluxation.8 Therefore, such wedging may suggest the presence of pars defects (Figure 8).
6 Ulmer JL, Mathews VP, Elster AD, King JC. Lumbar spondylolysis without spondylolisthesis: recognition of isolated posterior element subluxation on sagital MR. Am J Neuroradiology. 7:1393, 1995.
3 Fredrickson BE, Baker D, McHolick WJ, Yuan HA, Lubicky JP. The natural history of spondylolysis and spondylolisthesis. J Bone Joint Surg Am. 66:699, 1984.
Spondylolysis is an osseous defect found in both symptomatic and asymptomatic individuals. It predisposes to pathologic intervertebral subluxation or spondylolisthesis, most commonly occurring at the L5-S1 level. The pars defects are thought to represent chronic stress related injuries. Although these often occur during the first decade of life, accompanying vertebral problems develop somewhat later in life. It is important for the interpreter of MR to recognize both the primary and ancillary findings of spondylolysis, and in patients with spondylolisthesis, characteristic MR findings allow differentiation of degenerative versus isthmic causes. MR’s ability to grade disease severity and directly visualize nerve root involvement assists in treatment decisions.
Unfortunately the patient may present with degeneration at more than one disc level. Until the advent of aware state surgical examination, the sources of Vertebral Slippage were hard to define and surgeons engaged in pre-operative ‘guesstimation’ based on the results of clinical examination, X-rays, MRI scans and CAT scans. The complexity of the spinal region means that a wide range of possible conditions exist to confound diagnosis. In the presence of back and leg pain and spondylolisthesis or retrolisthesis the surgeon will tend to focus upon this evident pathology and treat this. In our published studies the pain was arising at an adjacent level in almost 20% of cases. The patients body had adapted to the slippage and surgery at this level would not have modified the pain.
As in our patient, spondylolysis may lead to spondylolisthesis, a forward (ventral) subluxation of an upper vertebra on a lower vertebra. Wiltse and coworkers have classified spondylolisthesis into five types based upon etiology:12
Many patients with spondylolysis are asymptomatic. Of those with symptoms, approximately one-quarter are associated with spondylolisthesis.3 Pain is usually limited to the low back. If the pain radiates, it is usually to the buttocks or the back of the thigh and is often from hamstring tightness rather than lumbar radiculopathy. With associated higher grade spondylolisthesis, however, radiculopathy becomes more common due to associated foraminal stenosis.
Imaging evaluation of a patient with low back pain typically begins with a series of lumbar spine radiographs. Spondylolysis is usually evident on lateral radiographs, although oblique projections may be useful. On frontal projections, fragmentation of the lamina may be identified.4 If spondylolisthesis is present, it should be graded according to the Myerding system,5 with grade I indicating anterior subluxation of less than 25%; grade II, 25% to 50%; grade III, 50% to 75%; and grade IV, 75% to 100%.
Vertebral Slippage is associated with various combinations of back, buttock and leg pain, numbness and muscle weakness. The back pain may arise from irritation within the disc wall but more commonly arises from the pinching of the trapped nerve in the exit doorway (Foramen) from the spinal column. The foramen is distorted and the nerve is tethered by years of scarring reaction to repetitive bruising, can not evade the pinching by the bulging distorted disc wall or overriding facet joints (or fracture margins in the case of Spondylolytic Spondylolisthesis). When advanced the compression causes numbness and weakness to develop.
Spondylolisthesis is readily identified on the midline sagital images. However, as most cases of spondyolysis have normal lumbar vertebral alignment, other findings must be utilized to detect the pars defects. In addition to demonstrating cortical disruption of the pars (A,B), several ancillary findings have been described that may aid in the diagnosis of lumbar spondylolysis.6,7,8 These include a widened anteroposterior diameter of the spinal canal on sagittal images, reactive marrow changes in the posterior elements, and abnormal wedging of the posterior aspect of the vertebral body.
To evaluate the sagittal diameter of the spinal canal, a ratio of the AP diameter at the L5 level to the AP diameter at the L1 level is used. The canal is measured from the posterior cortex of the vertebral body to the anterior aspect of the lamina on a mid-sagittal image. A ratio of 1.25 is normal. This ratio is increased in patients with spondylolysis due to posterior subluxation of the posterior elements, even in cases where no spondylolisthesis is present. Usually, the posterior subluxation of the posterior elements is evident on the mid-sagittal image with a resultant increase in the canal size at the level of the pars defect and actual calculation of a ratio is unnecessary (6a).
(7a) T1- and (7b) T2- weighted sagittal images of the lumbar spine in a 35 year-old male who presented with 4-5 months of bilateral lower extremity pain are provided. A defect of the pars interarticularis is seen (arrows) with cortical interruption and a resultant grade I spondylolisthesis. Reactive marrow changes are also present adjacent to the pars defect with increased signal within the marrow (arrowheads) on both the T1 and T2 weighted images.