The facet joints are formed by the superior and inferior processes of each vertebra. The first cervical vertebra has an inferior articulating surface but, as it does not restrict lateral or posterior translation, is not always considered a proper zygoma (zygoma is Greek for "yoke," i.e. something that restrains movement). In the lumbar spine, facets provide about 20 percent of the twisting stability in the low back. Each facet joint is positioned at each level of the spine to provide the needed support especially with rotation. Facet joints also prevent each vertebra from slipping over the one below. A small capsule surrounds each facet joint providing a nourishing lubricant for the joint. Also, each joint has a rich supply of tiny nerve fibers that provide a painful stimulus when the joint is injured or irritated. Inflamed facets can cause a powerful muscle spasm.