These are used to retract, expose or push tissue, muscles, organs or bones during surgery. Hand-held retractors must be held during surgery. Self-retaining models stay in place by themselves. The depth of the incision determines the size and model of the retractor. The smaller models are used on the surface to hold back skin and tissue, while the larger ones are designed to retract muscles, organs and bones in deeper cavities. To minimize trauma, the position of the retractors must be changed frequently.