What is the differential diagnosis for a patient with a possible cyst or swelling located to the proximal interphalangeal (PIP) joint of the 5th finger after a finger sprain?

Differential Diagnosis for PIP Joint Swelling After Finger Sprain

The most likely diagnoses for post-traumatic swelling at the PIP joint of the 5th finger include: volar plate injury with or without avulsion fracture, collateral ligament sprain, ganglion cyst formation, and less commonly, fracture-dislocation or septic arthritis if there was a penetrating injury. 1, 2

Primary Post-Traumatic Considerations

Volar Plate Injury with Avulsion Fracture

• This is the most critical diagnosis to exclude, as fractures involving ≥1/3 of the articular surface or interfragmentary gaps >3mm require surgical intervention 1, 2
• Obtain at least 3-view radiographs (posteroanterior, lateral, oblique) immediately to assess fracture pattern, displacement, and articular involvement 1, 2
• Look for volar base fracture fragments and degree of dorsal subluxation on lateral views 1
• MRI has 92-100% sensitivity for detecting flexor tendon avulsion and level of retraction if this is suspected 1

Collateral Ligament Sprain

• Complete ruptures at the PIP joint may require operative treatment, though this remains somewhat controversial 3
• Clinical examination should assess for joint instability and significant displacement 2
• Dynamic ultrasound can directly visualize PIP joint malalignment in the absence of fracture 1

Simple PIP Joint Sprain

• Prolonged swelling is the norm, not the exception - 63% of patients achieve resolution within 1 year, but 37% have persistent swelling beyond 12 months 4
• Even among those with resolution, 47% report persistent range of motion limitations and 41% have ongoing activity limitations 4
• Average pain scores remain elevated (VAS 2.6/10) at 1 year in patients without swelling resolution 4

Secondary/Delayed Presentations

Ganglion Cyst

• Ganglion cysts at the PIP joint are rare but well-documented, particularly in patients over age 65 5
• Trauma is postulated as an inciting factor for ganglion formation 5
• Ultrasound is highly accurate (94.1% sensitivity, 99.7% specificity) for superficial soft-tissue masses including ganglion cysts, showing characteristic features of minimal acoustic shadowing and minimal vascularity 6
• These can be confirmed by demonstrating fluid content on ultrasound in the appropriate clinical setting 6

Post-Traumatic Osteoarthritis (Early)

• While typically a chronic process, early degenerative changes can manifest as persistent swelling after trauma 6
• Clinical hallmarks include bony enlargement with or without deformity affecting the PIP joint (Bouchard nodes) 6
• Plain radiographs show joint space narrowing, osteophyte formation, subchondral sclerosis, and subchondral cysts 6

Critical Exclusions

Septic Arthritis

• Must be excluded urgently if there was any penetrating injury or skin breach 7
• Perform arthrocentesis with synovial fluid analysis for cell count with differential, Gram stain, aerobic/anaerobic culture, and crystal analysis 7
• Significantly elevated ESR and CRP indicate inflammatory arthritis or infection, while normal or mildly elevated markers suggest non-inflammatory causes 7
• The reference standard for diagnosis is positive culture from joint aspirate, though negative culture does not exclude the diagnosis if antibiotics were already started 6

Fracture-Dislocation

• Any joint incongruity after reduction requires operative treatment 3
• CT may be considered if initial radiographs are equivocal but clinical suspicion remains high 2

Imaging Algorithm

Start with plain radiographs in all cases - this is the gold standard initial study 6, 1, 2, 7

• If radiographs show avulsion fracture: measure fragment size and displacement to determine surgical candidacy 1, 2
• If radiographs are negative but swelling persists: consider ultrasound to evaluate for ganglion cyst, tenosynovitis, or soft-tissue fluid collections 6
• If concern for ligamentous injury: MRI can evaluate volar plate and collateral ligament integrity 2
• If concern for infection: urgent arthrocentesis takes precedence over advanced imaging 7

Common Pitfalls

• Underestimating the duration of symptoms: Patients should be counseled that prolonged swelling (>6-12 months) and functional limitations are common even with appropriate treatment 4
• Failing to obtain adequate radiographic views: At least 3 views are mandatory to avoid missing fracture-dislocations 1, 2
• Premature reassurance: Only 42% of patients with resolution of swelling report return to subjective normalcy 4
• Missing surgical indications: Avulsion fractures with specific criteria require immediate orthopedic referral 1, 2