Soft-Tissue Calcification versus Ossification

Q: What single radiographic feature most reliably distinguishes heterotopic ossification from soft-tissue calcification?

A: An organised internal trabecular pattern with a peripheral cortical rim. Calcification is amorphous and architecturally featureless regardless of how dense it appears. Say the word "ossification" only when you can point to both features — describing bone as calcification discards the entire differential.

Q: Describe the zoning phenomenon, its reversal, and how far you would trust it.

A: In myositis ossificans, mineralisation matures from the periphery inwards — a dense mature rim around an immature cellular centre. Extraskeletal osteosarcoma shows reverse zoning: dense disorganised mineral centrally with an immature or absent peripheral rim. CT is the investigation of choice. The honest qualification, which distinguishes a good answer: this is a widely reproduced pattern description, not a validated test — no study gives it a sensitivity or specificity, and in the early phase both imaging and histology may be non-characteristic. It shifts suspicion; it cannot exclude a sarcoma.

Q: What is the likelier mistake on this pattern — missing a sarcoma, or over-treating a benign lesion?

A: Over-treating the benign lesion. Across 60 published paediatric myositis ossificans cases, the pre-emptive diagnosis was neoplasia in 21.7 per cent and surgical excision was first-line in 46.7 per cent — for a condition that resolves spontaneously. The trigger the authors identify is specific and useful: absence of a remembered trauma is what pushes clinicians towards surgery, and idiopathic pseudomalignant myositis ossificans is precisely the variant with no injury. Caveat: these are published cases, which are selected for being difficult, so the true rate in unselected practice is lower.

Q: How often does heterotopic ossification follow total hip arthroplasty, and what predicts it?

A: 30 per cent — 1,939 of 6,468 hips across 14 studies. It is expected rather than exceptional, most of it low-grade and asymptomatic, so the clinical question is whether it restricts movement rather than whether it is present. The dominant risk factor is a pre-operatively ankylosed hip (odds ratio 9.85), then male sex (2.11), ankylosing spondylitis (1.90) and bilateral surgery (1.74). Rheumatoid arthritis is protective (0.51). Age, osteophytes and hypertrophic osteoarthritis were NOT risk factors, which is worth saying because all three are commonly assumed to be. The pooled figure does not separate Brooker grades — that is its main weakness.

Q: Which is more effective for heterotopic ossification prophylaxis around the hip, NSAIDs or radiation?

A: Neither. Nine randomised trials, 1,295 patients: pooled risk ratio for effectiveness 0.96 (95 per cent CI 0.88 to 1.06), and for complications 0.79 (0.45 to 1.41) — no significant difference either way, independent of arthroplasty versus fixation, sex, age and year. So choose on the patient: NSAIDs are out with renal impairment, peptic ulceration or anticoagulation; radiation is unattractive in the young, needs logistics, and carries a theoretical malignancy risk. Note what this evidence does NOT show — both arms are active treatments, so it says nothing about whether either beats no prophylaxis.

Q: Which tissue does calcium pyrophosphate preferentially calcify, and which does hydroxyapatite?

A: Calcium pyrophosphate deposits in fibrocartilage and hyaline cartilage — menisci, triangular fibrocartilage complex, symphysis pubis, acetabular labrum — producing linear calcification parallel to the joint surface. Hydroxyapatite deposits in tendon and periarticular soft tissue — supraspinatus, gluteus medius, longus colli — producing globular homogeneous calcification.

Q: A child presents with progressive stiffness, ribbons of ossification bridging the paraspinal soft tissues, and bilateral short great toes. What is the diagnosis and what is contraindicated?

A: Fibrodysplasia ossificans progressiva, caused by an activating ACVR1 mutation. Biopsy, intramuscular injection, dental blocks and attempted excision are all contraindicated because tissue trauma provokes catastrophic flares of new ossification. The short great toe is present from birth and is the diagnosis before any imaging.

Q: In which clinical settings is neurogenic heterotopic ossification most likely, and where does it form?

A: After traumatic brain injury, spinal cord injury and extensive burns. It forms in periarticular soft tissue below the neurological level — hip most commonly, then knee, elbow and shoulder — always outside the joint capsule with preservation of the joint space. Presentation is loss of range with local warmth and swelling, often mistaken for deep vein thrombosis or infection.

Q: What is calcific myonecrosis and why does it matter surgically?

A: A late sequela of compartment syndrome or peripheral nerve injury, typically decades later, in which an entire compartment — classically the anterior compartment of the leg — is replaced by a fusiform mass with peripheral plaque-like calcification and a liquefied centre. It matters because incision or biopsy commonly results in chronic infection, a non-healing sinus and amputation. Recognise it and leave it alone.