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Myossitis ossificans (MO) is an aberrant reparative process that causes benign heterotopic ossification in soft tissue. We report a case of MO presenting as a large mass located at the dorsal aspect of the distal thigh, with no history of trauma, with radiological and clinical features mimicking parosteal sarcoma. An incisional biopsy was performed and the mass was excised. The histological features identified the lesion as MO. In half of the cases, these ossifications may adhere to the periosteum. In these cases, the lesion is known as parosteal MO, which may be confused with a parosteal osteosarcoma. This parosteal MO seldom becomes malignant. We emphasize the importance of a differential diagnosis of MO, since these lesions may simulate tumours and lead to misdiagnosis.

Fibrodysplasia (or Myositis) Ossificans Progressiva (FOP) is a rare genetic disease with variable expression, characterized by the association of congenital anomalies of the toes and fingers and progressive appearance of ectopic bone within the skeletal muscles, often following a trauma or an infection. FOP initially affects the nape and thoracic paravertebral muscles. With age, there is a progression of ossifications to other muscular groups following a proximodistal and cranio-caudal extension. Patients develop a restrictive respiratory insufficiency with atelectasis. The diagnosis of FOP is clinical and does not require biopsy. Circumscribed post-traumatic ossifying myositis is the most important differential diagnosis. It is characterized by the appearance of painful ossifications, in young adults, following a trauma and is limited to one localisation. The conservative treatment of FOP remains unsatisfactory. Surgical removal of osteomas to restore joint mobility leads to the development of additional heterotopic ossifications. Each surgical attempt brings about a quasi-inevitable recurrence. Anaesthesia of patients with FOP is difficult because of spinal rigidity and ankylosis of the jaw. Surgery is indicated only with a focused indication to correct an invalidating deformity.

Myositis ossificans (MO) is a condition characterised by focal, benign and self-limited idiopathic heterotopic bone formation. It is extremely rare in the hand and wrist and may lead to concomitant nerve compression. Because of the rare incidence of pseudomalignant MO at the wrist and hand, we found it of interest to report a case of this condition localised to the wrist. A 31-year-old female patient presented with swelling and pain of her left wrist. The physical examination findings, magnetic resonance imaging and Tc-99m bone scan suggested acute osteomyelitis or a tumoral condition. Incisional biopsy and pathological examination was done. The microscopic findings confirmed that the lesion was pseudomalignant MO. The lesion was removed totally and decompression of the ulnar nerve and artery was achieved. The patient regained full asymptomatic range of motion of all digits and wrist and the numbness of the fourth and fifth digits had subsided at follow-up five months later.

The authors report the case of a 22-year-old female suffering from thigh pain for 5 months. The first diagnosis considered was circumscribed myositis ossificans but the duration of symptoms and the radiological aspect mimicking some malignant features led to consider parosteal sarcoma as a possible diagnosis. Surgical resection was therefore requested and histological analysis confirmed the diagnosis of circumscribed myositis ossificans. This case emphasizes the difficulty in differentiating circumscribed myositis ossificans from a slowly growing tumor as both can be associated with increasing pain and a malignant radiological aspect. A surgical approach is therefore frequently necessary for definitive diagnosis and treatment.

We report a 15-month-old girl with myositis ossificans at the shoulder. Because of the unusual presentation at this age the differential diagnosis with osteomyelitis or a tumoral process had to be considered. It had a benign evolution without specific treatment.