Heterotopic ossification (HO) refers to the presence of bone in soft tissue where bone normally does not exist. The acquired form of HO most frequently is seen with either musculoskeletal trauma, spinal cord injury, burns or traumatic brain injury injury.
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.
A simpler modified classification for HO (Della Valle) has 3 grades 7:
- grade A: absence of HO (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 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 HO is therefore the presence of a space of more or less than 1 cm between opposing surfaces of bone.
The most common presentation with with pain around the site of HO. Associated features can include fever, soft tissue swelling, and poor mobility of the affected joint.
The pathophysiology is unknown but most theories describe a cellular response to local tissue injury which lead to the release of chemical mediators that stimulate exaggerated bone proliferation. Muscle trauma seems to be a significant triggering event.
This may be the initial investigation done. Imaging features evolve as the ossification process progresses.
- typical finding is a soft tissue mass without calcific change
- these can often be missed since radiographs are typically done for vague symptoms of pain
- 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 to the periphery
- lesions can also be poorly organised without a recognisable mineralisation pattern
- mature cortical bone is formed if no treatment is used for the evolving heterotopic ossification
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 HO from other causes and serial imaging may be required to confirm the evolution of the lesion along the typical course for HO.
- low-attenuation soft tissue mass with indistinct surrounding soft tissue planes
- it may show contrast enhancement
- zonal mineralisation pattern as described previously
- a central fatty marrow component can occasionally be seen
- mature cortical bone at the periphery
There is no specific role for MRI once the diagnosis of HO 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.
- soft tissue mass with heterogenous high T2 signal
- lesion may manifest simply as enlargment of an involved muscle
- surrounding ill defined high T2 signal representing oedema
- enhancement of the soft tissue lesion and surrounding oedematous tissue
- this is seen as peripheral low T1 signal in the zonal pattern described
- high T1 signal centrally representing fatty marrow change
- low T1 signal peripherally in keeping with cortical bone
- persisting T2 signal components within the lesion
- low signal on STIR with little residual oedema
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