Clinical History
A 19-year-old male soccer player complaining of left thigh inner mass mid section with pain and stiffness. History of thigh trauma 2 months ago during a football match.
Imaging Findings
Left thigh lateral radiograph demonstrates well-circumscribed peripherally calcified appearance at 2 months.
Ultrasound revealed a heterogeneous hypoechoic mass (51.7 x 21.9 mm) of the left thigh embedded in the vastus Intermedius muscle with disruption of the muscle fibers displaced by complex echoes (calcified fragments) with a central fluid collection and no vascularisation on colour doppler.
On CT, soft tissue ossifications arranged along the muscle fibers with central noncalcified area, subjacent to the distal femur.
On magnetic resonance imaging (MRI), Left midthigh vastus intermedius lesion with heterogeneous signal (Fig. 1), heterotopic curvilinear calcifications with blooming artifact, internal areas of hemorrhage (Fig. 2) and surrounded by hyper intense signal of edema (Fig. 2) and postcontrast enhancement of the inflamed soft tissue (Fig. 3).
Discussion
Myositis Ossificans (MO) is heterotopic ossification in skeletal muscle with no definite aetiology. Some cases may be associated with trauma, however, more than 80% of cases have been reported to have no causative factors [1]. It is usually detected within the large muscles of the extremities [2, 3].
Patients usually present with a lump, pain, and tenderness where the lesion is located. Erythema of the overlying skin may be present. Due to inflammatory response in early stages of MO, acute phase proteins may elevate. C reactive protein is reported to better correlate with the inflammatory activity of heterotopic ossification compared to erythrocyte sedimentation rate [4].
In the initial stages (1-2 weeks), MO is revealed as a well-defined solid mass with MRI. It appears to be isointense on T1- and hyperintense on T2-weighted image compared to muscle. Following intravenous gadolinium-based contrast medium injection, it enhances vividly. In initial stages, adjacent muscle may have increased signal on T2 compatible with oedema. Peripheral calcification detected > 2 weeks is pathognomonic of MO and CT has been reported to be superior to demonstrate compared to other imaging modalities [5].
During the maturation stage (up to 5-6 months), peripherally located calcification proceeds through central part of MO.
In the presence of a peripherally calcified mass within the skeletal muscles with/without a history of trauma, the diagnosis of MO is established.
If biopsy is done it must not be performed during the early phases of MO, as there are high false positive rates for the misdiagnosis of sarcomatous tumour. (6)
Differential diagnosis consists of pyomyositis, extra skeletal osteosarcoma, and soft tissue sarcomas. The T2 hyperintensity of the muscle (muscle oedema) adjacent to MO on MRI may raise the suspicion of pyomyositis radiologically. However, lack of systemic (e.g. fever) and laboratory (e.g. leukocytosis) findings of infection lead to the diagnosis of MO [7].
In the initial stages of MO, MRI may be confusing and mimic soft tissue sarcomas. When peripheral calcification appears, other soft tissue sarcomas including osteosarcoma (central calcification is expected) may be ruled out.
As MO is self-limiting and may resolve completely in time, the management is conservative, including application of ice and resting in initial stages and physiotherapy with passive stretching and strengthening.
References:
[1] Nuovo MA, Norman A, Chumas J, Ackerman LV (1992) Myositis ossificans with atypical clinical, radiographic, or pathologic findings: A review of 23 cases. Skeletal Radiol 21:87–101.
[2] Parikh J, Hyare H, Saifuddin A (2002) The imaging features of post-traumatic myositis ossificans, with emphasis on MRI. Clin Radiol 57:1058-66.
[3] Beiner JM, Jokl P (2002) Muscle contusion injury and myositis ossificans traumatica. Clin Orthop 403:110-9.
[4] Estrores IM, Harrington A, Banovac K (2004) C-reactive protein and erythrocyte sedimentation rate in patients with heterotopic ossification after spinal cord injury. J Spinal Cord Med 27(5):434-7.
[5] Amendola A, Glazer M, Agha P, et al. (1983) Myositis Ossificans Circumscripta: Computed Tomographic Diagnosis. Radiology 49: 775-9.
[6] Alexis Lacout, et al (2012) Myositis ossificans imaging: keys to successful diagnosis. Indian J Radiol Imaging v.22(1).
[7] Mitsionis GI, Manoudis GN, Lykissas MG, Sionti I, Motsis E, Georgoulis AD, Berisa AE (2009) Pyomyositis in children: early diagnosis and treatment. J Pediatr Surg 44(11):2173-8.
