= break in the interarticular portion of a vertebra

pars interarticularis = junction of vertebral pedicle, lamina, superior + inferior articular facets

Pars interarticularis abnormalities are a spectrum of nonunion, spondylolysis and stress without spondylolysis.

Prevalence: 3–7% of population; in 30–70% other family members afflicted

Age: early childhood; M÷F = 3÷1; Whites÷Blacks = 3÷1

Cause:

1. Chronic low-grade trauma: stress (fatigue) fracture of pars interarticularis from repetitive minor trauma (in most); during teenage growth spurt; common in gymnastics (30%), ballet, scrubbing floors, lifting heavy objects, diving, contact sports (college football player (20%), wrestler (28%), soccer, hockey, lacrosse)
2. Developmental deficiency:
   1. hereditary hypoplasia of pars → insufficiency fracture; eg, pars defect in 34% of Eskimos
   2. congenital malformation: frequently associated with spina bifida occulta of S1, dorsally wedge-shaped body of L5, hypoplasia of L5; HOWEVER: no pars defects have been identified in fetal cadavers
3. Secondary spondylolysis: neoplasm, osteomyelitis, Paget disease, osteomalacia, osteogenesis imperfecta

• activity-related low back pain + hamstring tightness in 50% (if associated with degenerative disk disease / spondylolisthesis)

Location: L5 (67–95%); L4 (15–30%); L3 (1–2%); in 75% bilateral

Plain film (57% PPV):

• radiolucent band ± sclerotic margin resembling the collar of a “Scottish dog” (on oblique view)
• may be associated with spondylolisthesis
• subluxation of involved vertebra (if pars defect bilateral)
• Wilkinson syndrome = reactive sclerosis + bony hypertrophy of contralateral pedicle + lamina ← stress changes related to weakening of neural arch in unilateral pars defect

CT:

• pars defect located 10–15 mm above disk space
• inner contour of spinal canal interrupted

NUC bone scintigraphy:

Sensitivity: SPECT/CT >SPECT >planar imaging

Spectrum: uni- / bilateral stress or break

• stress (50%) = uni- / bilateral focal radiotracer uptake of pars interarticularis WITHOUT break BUT osteosclerosis
• active spondylysis = uni- / bilateral focal radiotracer uptake + osteolysis (CLASSIC)
• nonunion (pseudarthrosis) = NO radiotracer uptake + break in pars interarticularis and sclerosis along margins of defect

Cx:

1. Spondylolisthesis (uncommon; most likely before 16 years of age)
2. Vertebral pedicle fracture (rare; typically unilateral fracture with contralateral spondylolysis; best seen on SAG reformatted CT)