Musculoskeletal ImagingMyositis ossificans
a disorder in which the muscles (and frequently also other soft tissue such as tendons, ligaments, fasciae, aponeuroses and joint capsules) are involved by heterotopic ossification or
cartilage formation.
Two main categories are recognized.
Myositis ossificans traumatica
(also termed myositis ossificans circumscripta) frequently occurs after some type of
trauma, although in other cases the patient may have one of the systemic problems in which soft tissue ossification occurs, such as neurologic conditions, burns and tetanus (
Fig.1). Some lesions develop spontaneously (
pseudomalignant osseous tumour of soft tissue). Some patients may not have the
inflammation or the muscle involvement suggested by the term "myositis."
Areas commonly affected are the elbow, thigh, buttocks, shoulder and calf. Highly diagnostic on radiographs is the presence of a peripheral rim of calcification and ossification about a more lucent centre. Another important finding is a radiolucent band between the lesion and the subjacent cortex.
In
myositis ossificans progressiva
, also called fibrodysplasia ossificans progressiva, a hereditary disorder of connective tissue, the pattern of inheritance remains unknown, but most reported cases result from spontaneous mutations. Most patients have bilateral microdactyly of the first toes, often associated with synostosis of the phalanges. The pathologic abnormalities of connective tissue are similar to those of myositis ossificans traumatica. However, in myositis ossificans progressiva, the first spicules of bone can appear in the centre of the nodules, which is the opposite of the peripheral pattern of ossification (the zonal phenomenon) occurring in myositis ossificans traumatica. Eventually the entire muscle or muscle group is replaced by columns of lamellar bone. Smooth muscles are spared. Typically, myositis ossificans progressiva is characterized by erratic remissions and exacerbations, with new episodes often being precipitated by minor
trauma, injections or surgery.
In myositis ossificans progressiva, joint ankylosis, including fusion of the hips, may occur as a consequence of ossification of the surrounding soft tissues. Radiographically, digital abnormalities may be seen at birth and precede the soft tissue ossification. Hallux valgus is the most common abnormality and is associated with microdactyly of the first toes and hypoplasia or synostosis of the phalanges. Broad femoral necks, bilateral cortical thickening, an abnormal carrying angle at the elbow, and an increased frequency of spina bifida are also seen. In the soft tissues, a mass develops that gradually shrinks in size and ossifies. Ultimately, lesions may appear as a column of bone replacing the entire muscle of the neck or extremities. Pseudarthrosis and pseudoexostoses (not true osteochondromas) may also occur. Also, see ankylosis (III:1), Fig. 1.
DR/RB