What is the most likely diagnosis for a soft‑tissue swelling of the medial first metatarsophalangeal joint with an amorphous calcific density?

Most Likely Diagnosis: Acute Calcific Periarthritis (Hydroxyapatite Pseudopodagra)

The presentation of soft tissue swelling at the medial first metatarsophalangeal (MTP) joint with amorphous calcification is most consistent with acute calcific periarthritis (ACP), also known as hydroxyapatite pseudopodagra, caused by basic calcium phosphate deposition (BCPD).

Diagnostic Reasoning

The first MTP joint is a disease-specific target site for gout 1, making this location critical for distinguishing between crystal-induced arthropathies. However, the key distinguishing feature here is the amorphous calcific density, which is characteristic of hydroxyapatite (basic calcium phosphate) deposition rather than monosodium urate crystals.

Why This is ACP/Hydroxyapatite Pseudopodagra:

• Amorphous calcification on imaging is the hallmark finding of ACP at the first MTP joint 2, 3
• The periarticular location (medial aspect) is typical for this condition 2
• This presentation classically mimics gout (hence "pseudopodagra") but represents BCPD rather than monosodium urate deposition 3, 4
• Predominantly affects young to middle-aged women, though can occur in men 4

Key Differentiating Features from Gout:

While gout commonly affects the first MTP joint, the amorphous calcification pattern is not typical for gout. Gout produces:

• Monosodium urate crystal deposits (not amorphous calcium)
• Tophi (soft tissue masses) or double-contour sign on ultrasound 1
• Erosions with overhanging edges in chronic disease

Diagnostic Approach

For BCPD diagnosis, imaging is necessary; conventional radiography or ultrasound is the recommended modality 1. In your case, the plain radiograph has already revealed the diagnostic amorphous calcification.

Next Steps:

1. Clinical correlation: Look for acute onset pain, swelling, erythema, and tenderness that developed rapidly 2, 3

2. Laboratory evaluation: Check inflammatory markers and serum uric acid, but expect them to be normal or only mildly elevated 5

3. Joint aspiration (if diagnostic uncertainty remains):

   • Should be negative for monosodium urate and calcium pyrophosphate crystals 2
   • May show hydroxyapatite crystals (though these are difficult to identify on standard microscopy)
   • Ultrasound guidance recommended if aspiration is challenging 1

Important Caveat:

Do not mistake this for septic arthritis or cellulitis 3. The clinical presentation can be dramatic with erythema and swelling, but if inflammatory markers are normal/mildly elevated and imaging shows characteristic calcification, infection is unlikely. Unnecessary antibiotics or surgical drainage should be avoided 5, 3.

Management Implications

This diagnosis is critical because ACP is self-limiting and responds well to conservative management 5, 2:

• Corticosteroid injection provides rapid symptom relief with complete resolution typically within 2 weeks 5, 2
• Ultrasound-guided barbotage (aspiration and lavage) can be performed if calcific deposits are large 5
• No antibiotics or surgical intervention needed 5
• The calcifications may disappear spontaneously over time 3

Serial imaging is not recommended unless there is unexpected change in clinical characteristics 1, as this condition resolves with treatment.