What are the differential diagnoses for a 20-year-old male with severe mid to low back pain after a direct fall onto his coccyx, severe enough to prevent him from running?

Differential Diagnoses for Mid/Low Back Pain After Direct Coccyx Fall in a 20-Year-Old Male

The primary differential diagnoses following direct coccyx trauma include coccygeal fracture/dislocation, coccygeal hypermobility, sacrococcygeal joint injury, lumbar spine fracture (particularly if pain radiates above the coccyx), and less commonly spinal epidural hematoma or ligamentous injury requiring urgent imaging if neurological symptoms develop.

Traumatic Coccygeal Pathology (Most Likely)

Coccyx Fracture or Dislocation

• Coccyx fractures are classified into three types: flexion type 1, compression type 2, and extension type 3 1
• Direct fall onto the coccyx is the most common mechanism for coccygeal injury, with trauma being the predominant etiologic factor in coccydynia 2, 3
• Fractures, subluxation, and luxation of the coccyx can all result from direct trauma and cause severe pain that limits sitting and functional activities like running 2

Coccygeal Hypermobility/Subluxation

• Abnormal mobility of the coccyx is the most common pathological finding in patients with coccydynia, occurring in 70% of cases 2
• Hypermobility is defined as more than 25% posterior subluxation while sitting or more than 25° flexion while sitting, with more than 35° posterior subluxation considered significant 1
• This can result from acute injury and manifests as severe pain with sitting and transitions from sitting to standing 3

Sacrococcygeal or Intercoccygeal Disc Injury

• Disc degeneration or injury at sacrococcygeal (SC) and intercoccygeal (IC) segments can occur from direct trauma 2
• These injuries may not be visible on plain radiographs but can cause significant functional impairment 4

Lumbar Spine Pathology (Critical to Exclude)

Lumbar Vertebral Fracture

• Mechanical loading and impact stress on the lower spine from falls can cause Schmorl's nodes and degenerative changes, particularly in lumbar vertebrae 5
• CT is the gold standard for identification of spine fractures, outperforming radiographs 5
• If pain extends into the mid-back (above the sacrum), lumbar fracture must be excluded as 20% of spine trauma patients have noncontiguous injuries at multiple levels 6

Ligamentous Injury Without Fracture

• Up to 25% of cervical spine injuries involve NO fracture but represent unstable ligamentous injuries that appear normal on X-rays and CT 6
• While this statistic applies to cervical spine, the principle extends to thoracolumbar injuries where ligamentous disruption can occur without bony injury 5

Neurological Complications (Red Flags Requiring Urgent Imaging)

Spinal Epidural Hematoma

• Over 13% of post-traumatic epidural hematomas have normal CT scans, making MRI essential when neurological symptoms are present 6
• Spinal epidural hematoma may occur after minor trauma and can cause local or radicular pain, though less commonly ataxia 5
• Any tingling, numbness, or neurological deficit requires urgent MRI regardless of normal plain films or CT 6

Spinal Cord Compression or Contusion

• Cord compression can occur from fractures, malalignment, or epidural hematoma in the trauma setting 5
• Normal CT does NOT exclude significant injury in patients with neurological symptoms, as purely ligamentous injuries and cord contusions are invisible on CT 6

Less Common but Important Differentials

Sacroiliac Joint Injury

• Fall-related trauma can cause sacroiliac joint dysfunction or ligamentous injury 5
• This typically presents with pain in the lower back/buttock region that worsens with sitting and standing transitions

Pelvic Fracture

• Direct coccyx trauma with sufficient force can extend to involve the sacrum or pelvic ring 5
• Requires evaluation if pain extends laterally or anteriorly beyond the midline coccygeal region

Soft Tissue Injury

• Levator ani muscle strain or injury to muscle attachments around the coccyx can occur from direct trauma 2
• Pilonidal cyst or perianal abscess should be considered in the differential, though less likely with acute traumatic presentation 2

Diagnostic Approach

Initial Imaging

• The first step is dynamic X-ray of the coccyx in standing and sitting position to evaluate morphologic parameters and hypermobility 1
• Lateral radiographs in both positions allow assessment of coccygeal mobility and alignment 2, 3

Advanced Imaging Indications

• If initial X-ray is inconclusive but clinical suspicion remains high, MRI or CT can reveal radiographic findings of coccydynia not visible on plain films 4
• MRI is superior for detecting soft tissue abnormalities, disc injury, ligamentous disruption, and epidural hematoma 6, 4
• CT with multiplanar reformatted images is necessary for exclusion of fracture if neurologic symptoms develop or pain is severe 5

Red Flags Requiring Urgent MRI

• Any neurological symptoms (numbness, tingling, weakness, bowel/bladder dysfunction) 6
• Pain that extends significantly above the coccyx into the lumbar region 5
• Inability to bear weight or severe functional limitation beyond expected for simple coccygeal contusion 6

Critical Pitfalls to Avoid

• Do not assume normal plain radiographs exclude significant injury, as coccygeal hypermobility and ligamentous injuries are not visible on static films 1, 4
• Do not mobilize the patient without excluding unstable injury if neurological symptoms are present, as this can convert incomplete to complete cord injury 6
• Do not dismiss severe functional impairment (inability to run) as simple contusion without dynamic imaging to assess for hypermobility or occult fracture 2, 1
• Plain X-rays miss up to 77% of spine abnormalities in symptomatic patients, making them inadequate for definitive clearance 6