Musculoskeletal
Heterotopic ossification prophylaxis
A single post-operative radiation treatment that prevents painful bone formation around a hip or elbow after surgery or trauma.
Definition
Heterotopic ossification (HO) is the formation of mature lamellar bone in soft tissues where bone does not normally exist, most commonly in the musculature around the hip after total hip arthroplasty, acetabular fracture fixation, or traumatic brain/spinal cord injury. Clinically significant HO (Brooker grade III–IV) causes pain, stiffness, and loss of joint motion, and can ultimately block the range of motion needed for activities of daily living or wheelchair sitting.
How radiation treats it
Radiation blocks the transformation of pluripotent mesenchymal stem cells in the surgical bed into osteoblastic cells. Inhibited cells lose their ability to produce osteoid, so the mineralization cascade that would normally begin in the first days after surgery never proceeds. Radiation must be delivered during the narrow window before those cells differentiate, which is why timing matters more than dose.
Side effects
- Acute effects are minimal because the dose is low and delivered in a single fraction: mild transient skin erythema or dry desquamation within the field is possible.
- The radiation is delivered within the surgical field, so any late effects occur in tissue that was already operated on; clinically significant late toxicity is rare.
- Special care is taken to avoid irradiating any residual reproductive cells or underlying bowel when the pelvis is treated.
Simulation process
Most patients are simulated the same day as the CT planning scan. The patient is positioned supine on a flat tabletop (no hip prosthesis leg holder is required), often with the feet internally rotated and secured. CT or orthogonal kilovoltage images define the volume at risk based on the surgical approach (lateral, posterolateral, or anterior) — typically the periacetabular gluteal muscles, hip abductors, and the soft tissue of the surgical bed. Field borders are kept off weight-bearing bone stock and hardware-dense regions are outlined so the planner can account for attenuation.
Treatment process
A single fraction of 700 cGy (7 Gy) prescribed to mid-separation using parallel-opposed AP/PA fields, ideally delivered within 24–72 hours after surgery (some centers deliver 500 cGy pre-operatively). Treatment takes only minutes. Studies show pre-operative and post-operative prophylaxis are equally effective, but the dose must be delivered before osteogenic precursor cells begin their first division. Patients are usually treated as outpatients and sent home the same day.
Key takeaways for students
- 7 Gy in a single fraction within 24–72 hours post-op is the classic regimen.
- Target the surgical approach, not the whole hip.
- Timing is everything — once osteoid forms, radiation cannot reverse it.