Working Diagnosis:
Acute dislodging of chronic post-traumatic myositis ossificans of the anterior compartment of the lower leg, with tear through the anterior compartment fascia.

Treatment:
The patient was offered referral for orthopedic consultation to discuss surgical resection and fascia repair but he declined given his limited symptomology. No other therapeutic management was offered.

Outcome:
The patient continued to do well without intervention despite persistence of the mobile mass in his lower leg.

Author's Comments:
Myositis ossificans is characterized as a benign, self-limiting ossification of soft tissue. A common cause is blunt soft-tissue trauma. This patient likely developed myositis ossificans as a result of being kicked in the shin over 30 years prior. The resultant chronic dorsiflexion weakness, which he reported although was not evident on exam, may have been secondary to direct ossification of muscle fibers or from the ossifying mass causing mechanical blockage of the surrounding musculature as evidenced by the patient's limited range of motion with active dorsiflexion. The "pop" and new mobile mass was likely the result of an ossification tearing the anterior compartment fascia.

Editor's Comments:
Myositis ossificans is most commonly found in the large skeletal muscles like the quadricep (1). This case illustrates a rather unusual location for the myositis ossificans in the anterior shin. MRI is usually the preferred method of imaging for a soft tissue mass, but this case also shows the importance of musculoskeletal ultrasound as an imaging modality (2). It has been reported that ultrasound is even able to detect the formation of hetertopic bone even before CT (2).

References:
1. Mooney, Caitlyn, and George Pujalte. "MYOSITIS OSSIFICANS: Sports Medicine Today." MYOSITIS OSSIFICANS | Sports Medicine Today, 2020, www.sportsmedtoday.com/myositis-ossificans-va-122.htm.

2. Walczak, Brian E., Christopher N. Johnson, and B. Matthew Howe. "Myositis ossificans." JAAOS-Journal of the American Academy of Orthopaedic Surgeons 23.10 (2015): 612-622.

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