BACKGROUNDMyositis ossificans is an uncommon form of metaplasia in which bone tissue forms in muscle after an injury to the muscle fibers, most commonly due traumatic injury-be it by repetitive minor trauma or one significant single event. This is a case report of a patient with myositis ossificans of femoral and hip muscles due to injury related to tonic-clonic seizures. CASE REPORTA 33-year-old female, with epilepsy associated with autoimmune limbic encephalitis since 2006, presented to the Rheumatology Clinic with chronic pain of both knee and hip joints bilaterally since the seizures began. The patient had begun previous treatment with a rheumatologist in another institution with loss of follow-up, presenting no response to treatment with alendronate. On physical examination, she presented with significantly limited range of motion of the hips, as well as swelling and pain during mobilization of both knees and hips; and also crackles during mobilization of the knees. X-rays showed heterotopic calcifications in both hips, as well as ossification of heterotopic calcification in the medial facet of both knees in the general topography of the medial collateral ligaments. Further investigation showed negative ANA, anti-DNA antibody and rheumatoid factor, as well as normal complement levels. The patient is currently undergoing treatment with risedronate, and awaiting computed tomography scans to be evaluated by an orthopedic surgeon; however, due to the severity of her neurologic symptoms and difficulty in controlling seizures, the risk of trauma to the already injured muscles persists. CONCLUSIONMyositis ossificans is a heterogeneous group of diseases that vary between post-traumatic and inflammatory injuries to genetic disorders; trauma being the most commonly associated etiology to the development of heterotopic ossification in muscles. It is believed that hematomas formed in muscle fibers related to injury are the main factor associated with the inflammatory cascade that leads to the formation of osteoblast in muscle and in turn ossification of said muscle. Treatment usually consists in controlling the risk factors (ongoing trauma) and avoiding formation or enlargement of muscle hematoma in acute cases; however, the use of bisphosphonates as an adjunct treatment, has been reported to be beneficial in some cases. In chronic cases, such as this one, surgical removal of these ossifications can be considered to improve range of motion of the joints and to better control pain.