Quadhelix expander
A Quadhelix is a fixed expander.  It is used to widen the upper arch by moving the upper back and side teeth through the bone into a wider arch-form. 
It can be used both in adults and children and is often used at the same time as the fixed appliances although in some cases it is placed before the fixed appliances.

Quadhelix in adult male worn with his fixed appliances

Quadhelix in 8 year old child