Spondylolysis is a bone defect in the pars interarticularis of the vertebra, most commonly occurring at L5 (85%). It develops as a stress fracture in young athletes (gymnastics, football, diving, wrestling) due to repetitive lumbar extension and rotational stress. Its prevalence in the general population is approximately 3-6%, with a 2:1 male predominance. When bilateral pars defects are present, loss of posterior element stabilization may lead to anterior vertebral translation (spondylolisthesis). The Meyerding classification grades the degree of spondylolisthesis. In early stress reaction, MRI shows bone marrow edema, while in chronic cases CT axial images demonstrate a clear defect in the pars with sclerotic margins. Most cases are managed conservatively; surgery is considered only in the presence of advanced spondylolisthesis or neurological deficit.
Age Range
5-35
Peak Age
15
Gender
Male predominant
Prevalence
Common
Spondylolysis is a stress fracture that develops from repetitive mechanical stress on the pars interarticularis. The pars interarticularis is the weakest point of the vertebra — the thin bony bridge between the superior and inferior articular processes is subjected to both compression and tensile forces during axial loading. During lumbar hyperextension, the load on the L5 pars increases dramatically because the inferior articular process contacts the lamina of the vertebra below, creating a shearing effect. Repetitive microtrauma exceeds bone remodeling capacity and a stress reaction begins — at this stage, MRI shows bone marrow edema (STIR hyperintensity) because edema contains water that produces high signal on T2/STIR sequences. Continued stress without healing progresses to a complete cortical fracture — visible on CT as a hypodense defect traversing the pars with reactive sclerosis. L5 is most commonly affected because it is the level where lordosis is most pronounced and the greatest axial loading occurs. Bilateral defects eliminate the stabilizing function of the posterior elements and allow anterior translation of the vertebral body (spondylolisthesis).
On oblique sagittal CT reformat or oblique radiograph, the posterior elements of the vertebra resemble a Scottie dog (Scottish terrier) silhouette. In spondylolysis, the pars interarticularis defect is seen as a fracture/lucency in the neck region of the dog. This pathognomonic finding is the most classic and recognizable imaging sign of the pars defect.
Full-thickness bone defect in pars interarticularis on axial thin-section CT. The defect is usually obliquely oriented and seen between the lamina and pedicle on axial images. In chronic cases, reactive sclerosis and rounding of defect margins are seen. In acute cases, defect lines are sharp with minimal sclerosis.
Report Sentence
A bilateral/unilateral bone defect in the pars interarticularis at L___ is identified, consistent with spondylolysis; sclerosis along defect margins suggests a chronic process.
Scottie dog (Scottish terrier) appearance on oblique sagittal reformat or oblique radiograph: transverse process = nose, pedicle = eye, superior articular process = ear, inferior articular process = front leg, lamina = body. In spondylolysis, the dog's neck is broken — representing the pars defect.
Report Sentence
A defect in the Scottie dog neck region is identified on oblique sagittal reformatted images, consistent with pars interarticularis spondylolysis.
Hyperintense signal in pars interarticularis and surrounding bone marrow on MRI STIR/T2 fat-sat — indicating acute or subacute stress reaction. Hypointense signal in the same region on T1. Bone marrow edema reflects the stress reaction stage before cortical fracture develops and is critical for early diagnosis. MRI may be positive while CT does not yet show a defect.
Report Sentence
Hyperintense bone marrow edema in the pars interarticularis at L___ on STIR sequences is identified, consistent with acute/subacute stress reaction; thin-section CT is recommended with a preliminary diagnosis of spondylolysis.
Anterior translation of the vertebral body in the presence of bilateral pars defects (spondylolisthesis). Meyerding classification: Grade I (1-25% slip), Grade II (26-50%), Grade III (51-75%), Grade IV (76-100%), Grade V (spondyloptosis — complete slip). Sagittal CT reformat evaluates slip amount and neural foramen narrowing.
Report Sentence
Grade ___/V (Meyerding) anterior spondylolisthesis secondary to bilateral pars interarticularis defect at L___ is identified with a slip of ___ mm.
Neural foramen narrowing and nerve root compression due to spondylolisthesis. Evaluated by decreased foraminal fat and cerebrospinal fluid (CSF) signal on T2 sagittal images. Foraminal stenosis is the main cause of radiculopathy. The L5-S1 foramen is most commonly affected.
Report Sentence
Neural foraminal narrowing and ___ nerve root compression secondary to spondylolisthesis at L___-S___ is identified.
Focal increased uptake in the pars interarticularis region on bone scintigraphy (Tc-99m MDP) — indicating active bone turnover. Sensitivity increases with SPECT (Single Photon Emission Computed Tomography). Positive in cases with stress reaction and active healing potential. Uptake may normalize in chronic stable defects.
Report Sentence
Focal increased uptake in the pars interarticularis at L___ on bone scintigraphy/SPECT is identified, consistent with active stress reaction/spondylolysis.
Criteria
Bone marrow edema in pars interarticularis on MRI present but no full-thickness cortical defect on CT. Clinically, pain present in active athletes.
Distinct Features
MRI positive, CT negative or stress line. High healing potential — complete healing possible with bracing and activity modification. Focal increased uptake on scintigraphy.
Criteria
Full-thickness defect in pars on CT + sclerotic margins. No bone marrow edema on MRI. May be asymptomatic or minimally symptomatic.
Distinct Features
Prominent sclerosis and rounding of defect margins. Low healing potential — fibrous nonunion. Scintigraphy may be normal. Spondylolisthesis may or may not accompany.
Criteria
Bilateral pars defect + anterior translation (slip) of the vertebral body. Graded by Meyerding Grade I-V.
Distinct Features
Risk of neural foramen narrowing and nerve root compression. Grade I-II usually conservative, Grade III-V may require surgery. Disc degeneration may accompany. L5-S1 is most commonly affected level.
Distinguishing Feature
AS has sacroiliitis + syndesmophytes, no pars defect; spondylolysis has no sacroiliitis, isolated pars defect
Distinguishing Feature
Sacral insufficiency fracture has edema in sacral ala (H-shape), spondylolysis has edema in pars interarticularis; different age groups (elderly vs young)
Distinguishing Feature
DISH has anterior ligament ossification, no pars defect; spondylolysis has no ossification, pars defect present
Distinguishing Feature
Osteoid osteoma has nidus (<1.5 cm) + surrounding sclerosis, no pars defect; night pain relieved by NSAIDs; spondylolysis has no nidus, defect present
Urgency
routineManagement
conservativeBiopsy
Not NeededFollow-up
6-monthSpondylolysis is generally managed conservatively. In acute stress reaction, healing potential is high with activity modification and bracing (6-12 weeks). Symptom-based approach is applied for chronic stable defects. Surgery (pars repair or posterolateral fusion) is considered in advanced spondylolisthesis (Grade III+) or progressive neurological deficit.
Spondylolysis is generally managed with conservative treatment (activity modification, bracing, physiotherapy). Surgery (pars repair or fusion) may be needed in symptomatic bilateral defect with spondylolisthesis. Asymptomatic cases do not require treatment.