Myositis ossificans is described as extra-osseous, localised, non-neoplastic formation of bone and cartilage [2
]. It can be classified in to three types [3
(1) Myositis ossýficans progressiva which is a metabolic disorder occurring in children with widespread metamorphosis of muscle into bone, all of the skeletal muscles becoming involved progressively.
(2) Traumatic myositis ossificans circumscripta which follows local trauma which may be either acute or chronic repeated injuries.
(3) Myositis ossýficans circumscripta without history of trauma. This is usually found in paraplegia, chronic infections, burns and poliomyelitis, but may occur independently of these conditions.
The aetiology of traumatic myositis ossificans is unknown. The most common sites to be affected are the hip, anterior thigh (quadriceps), and anterior arm (brachialis) [4
]. It appears to be a self limiting disease with spontaneous resolution after maturation in most cases, though some can take years to resolve [6
Histologically, the "zone phenomena" has been described by Ackerman [2
] where the lesion is separated in to inner, middle and outer zones.
• Central zone: extreme variation of cells and atypical mitotic figures
• Middle zone: orientated osteoid
• Outer zone: well formed bone
Myositis ossificans of the sternocleidomastoid muscle was first described in 1950 [7
] though this was due to myositis ossificans progressiva, rather than following trauma. Post-traumatic cases of myositis ossificans of the sternocleidomastoid have been described, but no case has previously been reported as a consequence of a fracture of the clavicle.
This case highlights that traumatic myositis ossificans circumscipta can arise in the sternocleidomastoid muscle following a fracture of the medial third of the clavicle.