1) Description of Measurement

Assessment of the pars interarticularis evaluates for spondylolysis, stress reactions, or chronic non-union, which are common in young athletes, degenerative spondylolisthesis, and in association with lumbosacral transitional vertebrae (Bertolotti’s syndrome). Key features include:

• Defect width

• Cortical continuity

• Marginal sclerosis

These features distinguish acute stress fractures from chronic pars defects.

2) Instructions to Measure

• Scroll to the true parasagittal slice through the pars at the affected level.

• Identify the pars region between the superior and inferior articular processes.

• Assess Continuity:

• Intact cortex → normal

• Lucent cleft → pars defect
  
  

• Measure Defect Width (mm):

• At the widest part of the lucent gap, measure from one cortical margin to the other.
  
  

• Evaluate Sclerosis:

• Note increased cortical density along the defect margins (chronic stress reaction).
  
  

• Repeat bilaterally and at adjacent levels.

3) Normal vs. Pathologic Ranges

• Cortical continuity

• Normal: Intact

• Acute Stress Reaction: Partial disruption

• Chronic Pars Defect: Complete cleft

• Defect Width

• Normal: 0 mm

• Acute Stress Reaction: < 2 mm

• Chronic Pars Defect: ≥ 2-3 mm

• Sclerosis

• Normal: None

• Acute Stress Reaction: Minimal

• Chronic Pars Defect: Prominent marginal sclerosis

• Alignment

• Normal: No slip

• Acute Stress Reaction: No or minimal slip

• Chronic Pars Defect: Associated with spondylolisthesis

4) Important References

Hammerberg KW. New concepts on the pathogenesis and classification of spondylolisthesis. Spine (Phila Pa 1976). 2005 Mar 15;30(6 Suppl):S4-11. doi: 10.1097/01.brs.0000155576.62159.1c.

Campbell RS, Grainger AJ, Hide IG, et al. Juvenile spondylolysis: a comparative analysis of CT, SPECT and MRI. Skeletal Radiol. 2005 Feb;34(2):63-73. doi: 10.1007/s00256-004-0878-3. Epub 2004 Nov 25.

Pereira Duarte M, Camino Willhuber GO. Pars Interarticularis Injury. 2023 Feb 5. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan–.

Rakauskas TR, Gallup S, Mohamed AA, et al. An update on the prevalence and management of Bertolotti's syndrome. Front Surg. 2024 Dec 12;11:1486811. doi: 10.3389/fsurg.2024.1486811.

5) Other info....

Defect width > 3 mm with sclerosis strongly indicates chronic non-union.

Early stress reactions may be CT-negative; MRI STIR is more sensitive for marrow edema.

Bilateral pars defects commonly precede isthmic spondylolisthesis.