The sterno-occipital-mandibular immobilizer (SOMI) cervical orthosis is a rigid cervico thoracic orthosis used for supporting the ervical spine . SOMI does not provide complete immobilization though. It is somewhere between a Philadelphia collar and a halo brace.
Structurally, it has a chest plate that goes up to the notch where the collarbones meet in the front and metal, aluminum, or plastic bars that curve over the shoulder.
Straps from the bars go over the shoulder and cross to the opposite side of the anterior plate to hold it in place.
A removable chin piece attaches to the chest plate with an optional headpiece that can be used when the chin piece is removed for eating.
The SOMI is used to immobilize an unstable neck due to fracture or ligamentous injury or disruption.
Image Credit: http://www.ketteringsurgical.co.uk/orthopaedics/orthotic-cervical-co/somi-brace-sternal-occipital-mandibular-immobilizer.71437.htm