Zygapophysial

What it is

The paired joints at the back of the spine, between the articular processes of adjacent vertebrae. Universally called facet joints in clinical practice; zygapophysial is the formal term.

Each pair works alongside the intervertebral disc at the front, so every spinal segment is a three-joint complex: one disc and two facets.

They are ordinary synovial joints, with cartilage, a capsule and synovial fluid — unlike the disc, which is not.

What it does

They determine which movements each part of the spine can perform, and this is their most important property. The orientation of the joint surfaces differs by region:

• In the neck, the facets lie at a shallow angle, permitting generous rotation
• In the thoracic spine they are oriented more vertically and in a coronal plane, permitting rotation but limited by the ribs
• In the lumbar spine they face largely inward and outward, which permits flexion and extension and strongly limits rotation

That last point explains a lot. The lower back rotates very little by design; the rotation people believe they are producing there is mostly happening in the thoracic spine and hips.

They also share load with the disc, taking more of it as the spine extends.

What loads it

Extension and rotation of the spine load them most. Standing and walking load them continuously, and more so with an increased lumbar curve.

What is commonly got wrong about it

"Twisting from the lower back." The lumbar facets are built to prevent it. Rotation should come from the thoracic spine and hips, and this is one of the better-founded pieces of movement advice because it follows directly from joint orientation.

Facet joints "locking" or "going out". A popular explanation for sudden back pain. The mechanism proposed varies, evidence is limited, and the confident manual-therapy account of detecting and correcting it is not well supported.

That a joint being a pain source can be determined by feel. Diagnostic blocks exist precisely because clinical examination cannot reliably identify facet joints as a pain source. This is a clinical matter.