What is the best initial imaging study to rule out a pars fracture in a young athlete?

Best Imaging Study to Rule Out Pars Fracture in a Young Athlete

Start with plain radiographs (AP, lateral, and oblique views) of the lumbar spine as your initial imaging study, but if clinical suspicion remains high despite negative radiographs, proceed directly to MRI without contrast as it is the most sensitive modality for detecting early pars stress injuries and can guide treatment decisions that may prevent progression to complete fracture. 1, 2

Initial Imaging Approach

Plain Radiographs First

• Begin with anteroposterior, lateral, and oblique radiographs of the lumbar spine as the first-line screening tool for suspected pars fractures 1, 2
• Oblique views are particularly valuable as they show the classic "Scotty dog" appearance, with a collar or break in the dog's neck representing the pars defect 2
• However, recognize that standard radiography has low sensitivity (approximately 77.6%) for detecting spondylolysis, especially in the absence of spondylolisthesis or when fractures are non-displaced 1

Critical Pitfall to Avoid

• Do not stop at negative AP and lateral radiographs alone—they miss a significant proportion of pars defects, and oblique views substantially improve detection rates 2
• The combination of negative radiographs and negative clinical examination has only 0.81 negative predictive value, meaning you will miss cases if you stop here 1

Advanced Imaging When Radiographs Are Negative

MRI Without Contrast: The Preferred Next Step

When radiographs are negative but clinical suspicion remains high, MRI without contrast is your best next imaging study for several compelling reasons:

• MRI has 92% sensitivity for detecting pars injuries, comparable to bone scan but without radiation exposure 3
• MRI uniquely detects stress reactions (pre-fracture stage) before complete lysis occurs, showing bone marrow edema in the pars interarticularis or adjacent pedicle when CT and radiographs are still negative 1, 4, 3
• This early detection is clinically critical because identifying and treating stress reactions can prevent progression to complete fracture 1, 4
• MRI demonstrates vertebral marrow edema that correlates with active, symptomatic spondylolysis 1, 2, 4
• Resolution of MRI signal abnormalities indicates response to therapy, making it useful for monitoring treatment effectiveness 1
• MRI can evaluate additional pathology including disc disease, apophyseal injuries, and radiculopathy that may coexist or mimic spondylolysis 1

When to Consider SPECT Bone Scan

• Area-of-interest SPECT is very sensitive for identifying spondylolysis and may be useful when radiographs are negative with high clinical suspicion 1
• SPECT detects increased bone turnover in areas of stress reaction, stress fracture, or established spondylolysis 1
• Some evidence suggests bone scan with SPECT may be more sensitive than MRI for active spondylolysis 1
• However, SPECT has lower specificity than MRI and exposes the patient to radiation 1
• SPECT may be negative in chronic spondylolysis with wide separation and smooth margins 1

Role of CT

• CT has increased sensitivity for detecting non-displaced fractures and established spondylolysis compared to radiographs 1, 2
• CT is complementary to SPECT and MRI for higher specificity and sensitivity 1
• However, CT is less sensitive than MRI in detecting early stress injuries involving the pars interarticularis without complete lysis, which is frequently seen in pediatric patients 1
• Reserve CT primarily for follow-up imaging to assess bony healing of known pars defects after treatment, not as the initial advanced imaging study 1, 2
• CT best demonstrates fracture size, extent, and cortical integrity during healing assessment 4

Clinical Context That Matters

High-Risk Athletes

• Maintain particularly high suspicion in adolescents involved in gymnastics, football, weightlifting, wrestling, soccer, track and field, baseball, tennis—sports involving repetitive hyperextension and rotational forces 1, 2
• Young soccer athletes are prone to multilevel spondylolysis from compressive stress injuries, while baseball players sustain injuries from high rotational forces 1

Presentation Pattern

• Chronic mechanical low back pain worsening with activity and improving with rest is the hallmark symptom 2
• Do not assume all back pain in young athletes is muscular—pars defects must be ruled out 2

Algorithmic Summary

1. Start with AP, lateral, and oblique lumbar spine radiographs 1, 2
2. If radiographs are positive: Consider CT for detailed fracture characterization if it will change management 1
3. If radiographs are negative but clinical suspicion remains high: Proceed to MRI without contrast as the preferred next study 1, 2, 3
4. Alternative to MRI: SPECT bone scan if MRI is unavailable or contraindicated 1
5. Reserve CT for follow-up imaging to assess healing of known fractures 1, 2