Role of Nuclear Medicine in Heterotopic Ossification

Musculoskeletal · 2 min read

Heterotopic ossification is the formation of true bone in soft tissue. It is bone, with trabeculae and marrow, not dystrophic calcification. It is typically periarticular. It follows hip arthroplasty, fracture and surgery around a joint. It also follows spinal cord injury, traumatic brain injury and burns. It matters because it restricts movement and causes pain.

Why the scan is requested

Two questions are asked. Only the second is a nuclear medicine question.

• Is this heterotopic ossification? Usually answered by CT, which shows the characteristic appearance and location.
• Is it still active? Excision performed while the bone is immature carries a higher risk of recurrence. This is what the bone scan is for.

The scan is the functional counterpart to the CT. The CT gives the anatomy and the plane of resection. The scan gives the activity.

Three-phase bone scan

Early HO has increased blood flow. Developing HO also forms new bone. A three-phase ^99mTc-MDP bone scan measures these changes. The flow and blood-pool phases turn positive earliest — activity may appear before ossification is visible on radiographs.

Assessing maturity

• Serial scans. Compare the lesion against a fixed reference region on the delayed images. Declining, then plateauing, activity supports maturation.
• Localisation. SPECT/CT confirms that the uptake lies in soft tissue rather than in bone or at an implant interface.
• Comparability. Quantify rather than eyeball. An impression that a lesion looks less hot than last time is unreliable if acquisition or display has changed.
• Key caution. A completely “cold” scan is not required before every excision.
• The decision to operate. Pain, function, complications and CT anatomy. A scan alone cannot predict recurrence.

Pitfalls

• Uptake is nonspecific. Injury, surgery, healing and infection all produce a positive scan.
• Mistaken for infection. Maturing heterotopic bone is intensely active for months around a prosthesis. Correlate with the CT; its appearance and location are usually characteristic.
• Adjacent structures. Uptake at a fracture, a healing osteotomy or a loosened prosthesis may lie close to the heterotopic bone. SPECT/CT separates them.
• Timing. A single scan gives a value, not a trend. Maturity is a direction of travel, and one study cannot show it.

Fuller version, with the rest of benign bone disease: Benign & sports injury.