Some saddle pads are built up at the withers, and made of quilted cotton with panels to customize correction.
Contoured (or "built-up", "wither-relief", "swayback" or "orthosport") saddle pads are raised in the front to lift the saddle off of the withers. Built-up saddle pads are designed to fit horses which have very high (shark fin) withers. Swayback saddle pads are designed for horses which have swaybacks or very prominent withers. Swayback pads have an elevated area over the withers, and also provide extra padding in the middle of the pad. Orthosport pads have a breathable core, and extra padding behind the shoulders and along the spine, to relieve pressure from the withers and kidney areas.
Memory foam is an open cell, polyurethane material that molds to the body and prevents pressure sores. The drawback to memory foam is that it is quite heavy, and expensive.