How to image a coccyx (tailbone) after a fall?

Imaging the Coccyx After a Fall

Start with lateral radiographs of the coccyx obtained in both standing and sitting positions to evaluate for fracture, dislocation, and dynamic instability. 1

Initial Imaging Approach

Plain radiographs are the first-line imaging modality for evaluating coccyx trauma after a fall, following the same evidence-based principles established by the American College of Radiology for other skeletal trauma. 2 While ACR guidelines don't specifically address coccyx imaging, the fundamental approach to post-traumatic skeletal imaging applies universally—radiographs first, advanced imaging second if needed.

Dynamic Radiographic Technique

• Obtain lateral views in both standing AND sitting positions to assess for hypermobility and dynamic instability that may not be apparent on static films. 1
• Dynamic imaging reveals hypermobility defined as >25% posterior subluxation or >25° flexion while sitting. 1
• Significant hypermobility (>35° posterior subluxation) is a key finding in idiopathic coccydynia. 1

What to Look For on Radiographs

• Fracture patterns: Flexion type 1, compression type 2, or extension type 3 fractures. 1
• Type II coccyx morphology: Associated with increased risk of coccydynia. 1
• Intercoccygeal joint subluxation: Indicates instability. 1
• Bony spicules: Can contribute to chronic pain. 1

Advanced Imaging Indications

Consider MRI or CT only when radiographs are negative but clinical suspicion remains high, or when planning surgical intervention. 3, 1

MRI Indications

• Persistent pain despite negative radiographs to evaluate soft tissue structures, ligamentous injuries, and bone marrow edema. 3
• Suspected occult fracture not visible on plain films. 3
• Preoperative planning when coccygectomy is being considered. 3

CT Indications

• Complex fracture characterization when surgical planning requires detailed bony anatomy. 3, 1
• Evaluation of bony spicules and morphometric features. 1

Clinical Context Matters

Physical examination findings should guide imaging decisions:

• Focal tenderness on palpation of the coccyx is the hallmark finding that warrants imaging. 3
• Pain worse with sitting or sit-to-stand transitions is characteristic. 3
• Evidence of visible trauma after a fall increases likelihood of finding injury on imaging. 4

Common Pitfalls to Avoid

• Don't skip dynamic views: Static lateral radiographs alone miss hypermobility, which accounts for many cases of post-traumatic coccydynia. 1
• Don't order MRI first: This violates cost-effective imaging principles and the stepwise approach established for all skeletal trauma. 2, 3
• Don't overlook this region: Coccyx injuries are frequently dismissed, leading to delayed diagnosis and chronic pain. 1
• Don't assume all coccydynia is traumatic: One-third of cases are idiopathic, but post-fall evaluation should still begin with imaging to rule out fracture. 1

Nuclear Medicine

Bone scintigraphy is NOT routinely indicated for acute coccyx trauma evaluation, consistent with ACR recommendations for other skeletal trauma where it serves only as a complementary study in specific circumstances. 2, 3