Spondylolysis

Spondylolysis is a defect in the pars interarticularis of the neural arch, the portion of the neural arch that connects the superior and inferior articular facets. It is commonly known as pars interarticularis defect or more simply as pars defect. 

Epidemiology

Spondylolysis is present in ~5% of the population ² and higher in the adolescent athletic population.  It is more common in men than in women ¹.

Clinical presentation

Spondylolysis is commonly asymptomatic. Symptomatic patients often have pain with extension and/or rotation of the lumbar spine. Approximately 25% of individuals with spondylolysis have symptoms at some time.

It is a common cause of low back pain in adolescents and in particular athletes. 

Pathology

Spondylolysis is believed to be caused by repeated microtrauma, resulting in a stress fracture of the pars interarticularis. A dysplastic pars is usually present. Genetics are also believed to be a factor.

Traumatic pars defects result from high-energy trauma where there is hyperextension of the lumbar spine and are rare in a congenitally normal vertebra.

Location

• ~90% of cases of spondylolysis occur at the L5 level and ~10% occur at L4 level ^1,2
• the process may be unilateral or bilateral

Associations

• ~65% of patients with spondylolysis will progress to spondylolisthesis ², which is seen radiographically in ~25% ⁴ In most patients this occurs before the age of 16.
• spina bifida occulta

Radiographic features

Imaging features depend on the age of the lesion.

Plain radiograph

• limited sensitivity compared to SPECT and CT ⁴
• Scotty dog sign: on oblique radiographs, a break in the pars interarticularis can have the appearance of a collar around the dog's neck
• inverted Napoleon hat sign

CT/MRI

• wide-canal sign may be present on sagittal images when there is spondylolisthesis ³
• deviation of the spinous process
• sclerosis of the contralateral pedicle

Classification

Hollenberg classification ⁶

This is a classification system mostly based on MRI features

• grade 0: normal pars interarticularis; MRI: no signal abnormality, pars interarticularis intact
• grade I: stress reaction; MRI: marrow edema; intact cortical margins.
• grade II: incomplete stress fracture; MRI: marrow edema; incomplete cortical fracture or fissure. 
• grade III: acute complete stress fracture; MRI: marrow edema; complete cortical fracture extending through pars interarticularis. 
• grade IV: chronic stress fracture); MRI: no marrow edema. Fractures completely extending through pars interarticularis. 

Treatment and prognosis

Surgery is only considered in rare circumstances as most cases respond to conservative management ². 

Differential diagnosis

• osteoid osteoma
• osteomyelitis