Age / Sex : 69 / M

Myositis ossificans

Discussion

Findings:

 Radiography : no abnormal density in soft
tissue and bony structure.

 MRI : There is a ill-defined, T2 high SI, T1
iso to slightly high SI soft tissue lesion with enhancement in the vastus
medialis muscle at the mid level of the thigh. Muscle fibers are relatively preserved
and it does not show adjacent soft tissue and bone marrow signal change.

 USG : About 6cm relatively well defined, oval
hyperechoic lesion with inner hypoechoic portion is noted in the vastus medialis
muscle. Probably it contains multiple microcalcifications and they are shown by
very echogenic portion of the lesion. On power doppler image, It shows little
vascular flow.

Differential Diagnosis:

 Proliferative myositis

 Lymphoma

 Sarcoidosis

Diagnosis:
 

 Myositis ossificans.

Discussion:

•       Heterotrophic
ossification, formation of non-neoplastic bonelike tissue in any soft tissue
location including skeletal muscle, tendon, fascia, ligament, other connective
tissue.

–      3 phases : early (< 1 week),
subacute (>10 day), late (maturation, 2-5 wks)

–      Bony production : peripheral,
centripetal

•       Etiology : unknown, may secondary to
trauma

•       Clinical manifestation : very
inflammatory, rapidly growing, painful muscular mass, dramatic onset

•       Imaging finding

[Radiolography]

–      The earliest radiological signs appear within
7 to 14 days, describing a well-circumscribed soft-tissue mass associated with
a faint periosteal reaction

[USG]

–      homogeneous or heterogeneous hypoechoic
mass

•        

[CT]

–      Gold standard in characterizing the
typical features, extensive muscle edema and perilesional edema without bone
marrow or cortical abnormalities.

[MRI]

–      Lack of invasion of adjacent tissues
within lesion

–      T1 and T2 : iso to slightly high SI

•      
T2-weighted
images of early stage lesions, i.e., before peripheral ossification has
occurred, show an inhomogenous localized mass with high central signal
intensity. With time, the lesion matures and the peripheral ossification
becomes denser (visible on conventional radiograph). On T2-weighted MRI, the
classical finding of a hyperintense center surrounded by a hypointense rim can
be recognized.

•      
On
T1-weighted images, the lesion appears isointense to muscle and can only be
identified by its mass effect. Specific STIR images on T1 make further
distinction possible by yielding the lesion slightly hyperintense to muscle and
by distinguishing it from the surrounding edema, which is a typical feature.

–      Gd T1-CE : rim enhancement (zone
phenomenon : active hypervascularized osteoid matrix)

–      Subacute : low SI on all sequences
mineralization

•       Differential diagnosis

–      At the early stage, it can be
distinguished from soft-tissue neoplasms by its extensive muscle edema without
bone marrow or cortical abnormalities. Other lesions presenting surrounding
edema are infection, rhabdomyolysis and hematoma. The differential diagnosis
with other slowly calcified lesions includes synovial sarcoma, rhabdomyosarcoma
and malignant fibrous histiocytoma. At the advanced stage, it must always be
distinguished from osteosarcoma or chondrosarcoma. Those entities present a
pattern of mineralization with peripheral mature ossification. However,
osteosarcomas usually have a dense, calcified center, while in myositis
ossificans the most calcified zone is at the periphery.

References:

Resnick
D. diseases of soft tissue and muscle. In: Resnick D, Kransdorf MJ, eds. Bone
and joint imaging. 3rd ed. Philadelphia, Pa: Saunders, 2005; 1386-1387

Saussez
S1, Blaivie C, Lemort M, Chantrain G. Non-traumatic myositis ossificans in the
paraspinal muscles. Eur Arch Otorhinolaryngol. 2006 Apr;263(4):331-5. Epub 2005
Aug 24.

Lacout
A, Jarraya M, Marcy PY, et al. Myositis ossificans imaging: keys to successful
diagnosis. Indian J Radiol Imaging 2012;22(1):35-39

Vanden
Bossche L, Vanderstraeten G. Heterotopic ossification: a review. J Rehabil Med
2005;37(3):129-136