Spine Joints. Facet joints are places on the spinal bones where two bones actually meet and articulate, therefore a facet joint injection is into that place where they meet. Many times arthritis or inflammation can arise here and cause pain. Sprain suggests the ligaments attaching next to the facet joint are stretched or irritated causing pain.
An injury. An injury to the soft tissues of this joint when the joint is stressed past it's nornal motion range. This can occur usually in the neck area and in the lower back. This can sometimes explain why some neck pains after an injury are chronic when a MRI will fail to reveal any disc problem but will not reveal a facet joint sprain which would require a facet block to diagnose.