Intra-articular synovial protrusions in the lower lumbar apophyseal joints.

Two types of joint inclusions are demonstrated in the lumbosacral facetal joints--a large, fat-filled, vascular intra-articular synovial protrusion projecting into the medial part of the joint from below, and a dense, fibrous, intra-articular synovial protrusion projecting from the joint capsule into the upper medial part of the joint. Although nerve fibers have not yet been demonstrated in the intra-articular synovial protrusions, it is reasonable to assume their presence in such a rich vascular network. These intra-articular synovial protrusions are vulnerable to being nipped between moving parts of the joints, in this way possibly transmitting traction forces to the related parts of the fibrous joint capsule. This could constitute a likely source of low back pain with resultant reflex muscle spasm.