What is a pars defect at L5 (Lumbar 5)?

What is a Pars Defect at L5?

A pars defect at L5 is a stress fracture or break in the pars interarticularis—the narrow bridge of bone connecting the superior and inferior articular processes of the fifth lumbar vertebra—most commonly caused by repetitive hyperextension and rotational forces, particularly in young athletes. 1

Anatomical Location and Pathophysiology

• The pars interarticularis is the specific bony segment between the superior and inferior articular facets of the vertebra, and when fractured, this condition is called spondylolysis. 2, 3

• L5 is the most common site for pars defects, accounting for the majority of cases, with L4 being the second most frequent location. 2, 4

• These defects result from repetitive stress injuries rather than acute trauma, developing over time from activities involving repeated spinal hyperextension and rotation. 1

• If left untreated or if bilateral defects occur, the vertebra can slip forward (spondylolisthesis), potentially causing dynamic canal stenosis, chronic low back pain, and radiculopathy. 5

Clinical Presentation

• Chronic low back pain is the hallmark symptom, particularly in young athletes and weightlifters who engage in repetitive hyperextension activities. 1, 2

• The pain is typically mechanical in nature, worsening with activity and improving with rest. 1

• Most pars defects are bilateral, though unilateral defects can occur. 3, 4

• Acute severe back pain is uncommon, occurring in only rare cases. 2

Diagnostic Imaging Approach

Initial imaging should be anteroposterior and lateral radiographs of the lumbar spine, with oblique views added specifically to better visualize pars interarticularis defects. 1

• Standard radiography has low sensitivity for detecting spondylolysis in the absence of spondylolisthesis, particularly for non-displaced fractures. 1

• Oblique radiographic views are particularly useful for visualizing pars defects, showing the classic "Scotty dog" appearance with a collar or break in the dog's neck representing the defect. 1

Advanced Imaging When Radiographs Are Negative or Equivocal

• MRI without contrast is the preferred next step if radiographs are negative but clinical suspicion remains high, as it detects stress injuries involving the pars interarticularis without complete lysis and identifies vertebral marrow edema. 1

• CT has increased sensitivity for detecting non-displaced fractures and established spondylolysis compared to conventional radiography, and is complementary to MRI for higher specificity. 1

• CT is particularly useful for follow-up imaging to assess healing of known pars defects after treatment. 1

• SPECT bone scanning can detect early stress injuries before fracture completion, though it is less commonly used as first-line advanced imaging. 1

Common Clinical Pitfalls

• Do not assume all back pain in young athletes is muscular—maintain high suspicion for pars defects in adolescents with mechanical low back pain, especially those involved in gymnastics, football, weightlifting, or other hyperextension sports. 1, 2

• Standard AP and lateral radiographs alone may miss pars defects—oblique views significantly improve detection rates. 1

• Hypoplasia of the neural arch can be associated with pars defects and may predict poorer healing outcomes. 2

• Multiple-level pars fractures involving consecutive or non-consecutive segments (such as L3 and L5) are rare but do occur, requiring careful evaluation of the entire lumbar spine. 4, 6

Treatment Considerations and Outcomes

• Most patients initially undergo conservative management with activity modification and bracing. 2, 3, 6

• Surgical repair may be indicated for patients who fail conservative treatment, with various techniques including direct pars repair with bone grafting and screw fixation. 2, 3, 5

• Union rates after surgical repair are high (82% at 12 months, 94.3% at 24 months), with significant pain relief and return to athletic activity. 3

• Even skeletally mature intercollegiate athletes with acute L3 or L4 spondylolysis can successfully return to full athletic competition following either conservative treatment or surgical repair. 6