Spondylolysis

MRI Lumbar spine sagittal T2 right far lateral image, left far lateral image, sagittal T1 right far lateral and left far lateral image, shows non displaced break in L5 pars intercularis on either side. No listhesis.

Non displaced breaks in L5 pars inter articularis on either side confirmed on CT study. 

SPONDYLOLYSIS

Spondylolysis refers to an osseous defect within the pars interarticularis, an isthmus of bone located between the superior and inferior articular processes. 

Spondylolysis most commonly affects the L5 level (in 85  to  95% of cases) with the majority of the remaining cases occurring at L4 (5  to 15% of cases). 

While the exact etiology of spondylolysis is unknown, it is generally believed to represent a stress fracture caused by repetitive loading, although there are hereditary and genetic contributing factors.

Spondyolysis on MRI needs demonstration of an osseous break in pars on sagittal far lateral images which may not be possible every time due to limitations of sagittal plane of imaging. 

Spondyolysis with an associated Spondylolisthesis is readily identified on the midline sagital images. However in most cases of spondyolysis have normal vertebral alignment, several ancillary findings have been described that may aid in the diagnosis of spondylolysis. 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.

1. Widened anteroposterior 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. 

A midline sagittal T2 weighted image:  The AP diameter of the canal at L1 measures 1.7cm and the AP diameter of the canal at L5 measures 2.5cm. The ratio of L5:L1 is 1.47, which is above the normal value of 1.25. Even without this calculation, it is clear the canal is enlarged at the L5 level with the posterior elements displaced slightly posterior when compared with the posterior elements of the more cranial vertebral bodies.

2. Reactive marrow changes : Altered marrow signals similar to those observed in patients with degenerative disc disease are identified within the posterior elements adjacent to pars defects in a significant number of patients in ~ 40% of patients with spondylolysis 

3. Abnormal wedging: An abnormal wedging of the posterior aspect of the vertebral body at the level of the pars defect. 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.Therefore, such wedging may suggest the presence of pars defects.

A midline sagittal T2 image lines have been drawn to show the height of the anterior and posterior vertebral body. Posterior wedging of the vertebral body is seen, which is an ancillary finding suggesting the presence of spondylolysis at this level.

Hypoplasia is common in spondylolysis and might mimic spondylolisthesis so careful not to misdiagnose spondylolysis as spondylolisthesis, when hypoplasia is present.