Heterotopic ossification

Heterotopic ossification (HO)refers to the presence of bone in soft tissue where bone normally does not exist. The acquired form is usually secondary to musculoskeletal trauma, spinal cord injury, burns, or traumatic brain injury.

Clinical presentation

The most common presentation is with pain around the ossification site. Associated features can include fever, soft tissue swelling, and poor mobility of the affected joint.

Pathology

The pathophysiology is unknown but most theories describe a cellular response to local tissue injury which leads to the release of chemical mediators that stimulate exaggerated bone proliferation. Muscle trauma seems to be a significant triggering event.

Approximately 90% of hip joint replacement patients may have heterotopic ossification ¹².

Classification

The severity of heterotopic bone formation has been classified according to several systems. One traditionally used method is the Brooker classification system which divides severity into 4 types ⁷.

The Della Valle classification is a simpler modified classification system with three grades ⁷:

• grade A: absence of HOheterotopic ossification (may be ≥1 island of bone of <1 cm in length)
• grade B: presence of ≥1 islands of bone of at least 1 cm in length and bone spurs from the pelvis or femur.; 1 cm distance between opposing bone surfaces of bone
• grade C: bone spurs arising from the pelvis or femur with <1 cm between opposing surfaces or apparent bone ankylosis

The important distinction in reporting the presence of HOheterotopic ossification is therefore whether the presence of a space of morebetween opposing bone surfaces is greater or lesssmaller than 1 cm between opposing surfaces of bone.

Radiographic features

Plain radiograph

Plais film is often the initial imaging investigation. Imaging features evolve as the ossification process progresses.

• early stage
  • a typical finding is a soft tissue mass without calcific changechanges
  • these can often be missed since radiographs are typically done for vague symptoms of pain
• mineralisation
  • can occur within 10 days after the causative insult
  • calcification usually starts peripherally, though cases associated with fibrodysplasia ossificans progressiva can calcify from the central zone out totowards the periphery
  • lesions can also be poorly organised without a recognisable mineralisation pattern
• maturation
  • mature cortical bone is formed if no treatment is used for the evolving heterotopic ossification is left untreated

CT

Findings on CT mirror those of plain radiographs but CT is able to identify lesion mineralisation earlier and has good overall specificity. It can sometimes be difficult to distinguish the soft tissue lesion seen early on in the evolution of HOheterotopic ossification from other causes and serial imaging may be required to confirm the evolution of the lesion along the typical course for HOheterotopic ossification.

• early stage
  • low-attenuation soft tissue mass with indistinct surrounding soft tissue planes
  • it may show contrast enhancement
• mineralisation
  • zonal mineralisation pattern as described previously
  • a central fatty marrow component can occasionally be seen
• maturation
  • mature cortical bone at the periphery

MRI

There is no specific role for MRI once the diagnosis of HOheterotopic ossification has already been made. Instead, MRI is usually used in the assessment of a soft tissue mass. It has the added advantage of evaluating for other possible causes such as neoplasms (i.e. sarcoma) or underlying osteomyelitis.

• early stage
  • soft tissue mass with heterogeneous high T2 signal
  • lesion may manifest simply as enlargement of an involved muscle
  • surrounding ill-defined high T2 signal representing oedema
  • enhancement of the soft tissue lesion and surrounding oedematous tissue
• mineralisation
  • peripheral low T1 signal in the zonal pattern described
  • high T1 signal centrally representing fatty marrow change
• maturation
  • low T1 signal peripherally in keeping with cortical bone
  • persisting T2 signal components within the lesion
• delayed
  • low signal on STIR with little residual oedema

See also

• myositis ossificans
• soft tissue calcification
• dystrophic calcification
• periarticular soft tissue calcification