A 5-year-old boy underwent arthroscopic surgery because of a lateral d
iscoid meniscus, which was followed by an overly strenuous rehabilitat
ion program. At 6-week follow-up, a painful mass at the distal part of
the thigh and a flexion restriction of the operated knee were detecte
d, caused by myositis ossificans inside the vastus medialis muscle. Th
e symptoms disappeared with an appropriate rehabilitation program and
the patient recovered.