The facet joint capsule is an area of connective tissue that covers and closes the facet joint (which is also known as the zygapophysis or zygapophyseal joint) from top to bottom. This connective tissue creates a sort of bulky seam that holds the two bones forming the facet joint together.
According to an anatomical description study that was published in the July 2011 issue of the Journal of Biomechanical Engineering, the facet joint capsule resists forces of tension that are developed across the joint when it rotates and/or translates. Translational movements are those that go in a straight line. Walking is an example, but when you talk about joint movement, the meaning is more subtle. In this case, it refers to the linear (i.e. no rotation or twisting involved) motion of one spinal bone relative to the one above or below.
Together with the rest of the structures that make up the facet joint, the capsule helps to transfer different types of loads during a variety of spinal movements.
Each vertebra has four facet joints—two on the top (called superior facets) and two on the bottom (called inferior facets), so there are four facet joint capsules for each spinal bone. The fibrous connective tissue of the capsule encases the whole joint. On the inside of the joint is a lining (called the synovial lining) that makes synovial fluid. Synovial fluid is like WD-40 for the joint—it lubricates the bones that make up the facet joint, as well as the space between them, which helps make for smooth, frictionless motion in the joint.
The capsule of the facet joint - as well as other aspects of the joint—is richly supplied with nerves—which play a role in pain perception. A study published in the issue of American Journal of Neuroradiology says that one possible way back pain is created is by a capsular stretch. (The authors also name nerve impingement and inflammation as other potential pain generators.) They say that facet joints may be responsible for up to 45% of low back pain cases and up to 55% of cases of chronic neck pain without disc herniation.