Extrusion failure is one of the most common types of O-ring failure. When the pressure becomes too great, the O-ring will actually extrude into the clearance gap. This will quickly nibble the O-ring away causing a loss of material, and once sufficient material is lost, seal failure will quickly follow.
There are three options to prevent this, the first of which is to reduce the clearances to lower the extrusion gap. This is obviously an expensive option, so a cheaper solution is to raise the durometer of the O-ring. Although, a higher durometer O-ring offers superior extrusion resistance, the fact that harder durometer materials have limited low pressure sealability and is often difficult to source due to material availability. The last and best option is the addition of a backup ring. A backup ring is a ring of hard, extrusion resistant material such as high durometer nitrile, nylon, Hytrel, or PTFE. A back-up ring is designed to fit between the O-ring and the extrusion gap and prevent the extrusion of the O-ring.