A backup assembly is provided for backing up a seal assembly that seals a gap between two components of a joint assembly. The backup assembly may include an elastomer, a spring element embedded within the elastomer, and first and second rings located adjacent to the elastomer on opposite sides of the spring element. The spring element may be a garter spring, and the elastomer may be over-molded around the spring. The elastomer may have opposing wing extensions that extend respectively along the first and second rings, and the rings may be steel rings. The elastomer, spring element, and rings form an annular backup ring. The backup assembly may be employed in a seal assembly for sealing a gap between two components of a joint assembly. The seal assembly may include a diaphragm seal component, and the backup assembly operates to provide backup and extrusion resistance for the seal component.