Anatomy and Physiology of the Facet Joints Amin Jahanbakhshi M.D. Guive Sharifi M.D. Departement of Neurosurgery Loghman Hakim Hospital

The motion segment The functional unit of the vertebral column two adjacent vertebral bodies an intervertebral disc two zygoapophyseal joints. It constitutes a three joint complex which forms a universal joint with many possible motions

There are different kinds of spinal movements that are ususally coupled in a single spinal motion: Flexion-extension (bending): rotation about the x-axis. Axial rotation (torsion): rotation about the y-axis. Lateral bending (side bending): rotation about the z-axis. Translational movements : movements of the whole vertebra The movements of each spinal segment are limited by anatomic structures such as ligaments, intervertebral disks, and facets.

Center of rotation (COR) is the axis about which the rotation occurs. The COR for flexion-extension: posterior third of the inferior vertebral body The COR for axial rotation of lumbar spine: in the midline, posterior to the disk but anterior to the facet joints The COR for axial rotation of cervical spine: aligned with the facet joints When coupled: variable COR decreases loads because the working distance of the spinal muscles and ligaments

loss of disk height Pathologic loading: loss of disk height Pathologic loading: Degeneration of the intervertebral disk alterations in sagittal alignment

Orientation of the facet joints are different in different spinal levels cervical: coronally oriented (horizontal plane of the screen): resist translation facilitate flexion, extension, and rotation (the greatest degree of flexibility of any part of the spine).

Orientation of the facet joints are different in different spinal levels Lumbar: sagittally oriented, perpendicular to the screen resist rotation Allowing flexion and extension as well as side bending change of orientation from the L1 to L5: L1 is perpendicular to the screen, L5 angled outwards by 20 degrees prevents the L5 from slipping

Rotation is prevented In the upright position but not when the lumbar spine is fully flexed forwards (the position of greatest vulnerability to lifting sprains)

Orientation of the facet joints are different in different spinal levels Thoracic: intermediately oriented Allow bending forwards and backwards and rotate to left and right Side bending is limited (because of Ribs 1 to 10) allowing more side bending at these levels 11-12. "intermediate”restriction of translation and rotation.

Nerve Supply main spinal nerve at foramen posterior primary ramus Anterior primary ramus Medial branch Intermediate and lateral branches one half of a facet joint, the central spinal muscles (multifidus), the spinal ligaments and a small area of skin in the midline of the back Supplies back muscles further away from the midline. Each facet joint is supplied by two medial branch nerves, denervation requires lesions at two levels

The facet joints (zygoapophyseal joints) are diarthrodial, gliding joints with a synovial lining. Because of the laxity of their joint capsules, there is a considerable range of movement in different directions. The width of the articular cartilage is 2.5-4 mm The cartilage is thickest toward the center of the joint erosion, fissuring and fringing of the joint cartilage The joint surfaces are slightly curved: the superior facets concave the inferior facets convex

The posterior joint recess is more generous in size than the anterior one

radiograph of a 3 mm thick specimen slice: a layer of hyaline cartilage is seen to fit the space between the opposing but incongruent cortical margins.

MR slice (SE 2000/20) Hyaline cartilage within the black cortical lining exhibits intermediate signal intensity.

MR slice (SE600/20) of a different patient the delicate black lines in the centers of the facet joints represent the joint spaces.

MR slice (SE800/25) a fat tab in the superior portion of the joint the posterior joint recess.

parasagittal plane: A 3-mm thick oblique cut

radiograph of the same specimen: Note the superior joint recess.

MR image (SE2000/20) of the same specimen signal voids: the subchondral cortices of the facet joints The triangular high signal intensity: asynovial fat pad in the superior joint recess The thin layer of intermediate signal intensity: hyaline cartilage, seen below the fat pad

oblique MR cut (SE 600/20) through the facets Note: The ‘Scotty dog” appearance An exiting nerve root which is contrasted against the high intensity signal of fat. Obliterated joint spaces because of facet arthrosis.

In recesses there are fat pads (meniscoid structures), about the size of a grain of rice coated with synovial tissue having numerous synovial fringes continuous with the adipose tissue surrounding the exiting spinal nerves Entrapment of a synovial tab may cause “sudden catch” type of low back pain

MR cuts show the triangular synovial fat pad in the superior joint recess there is a slight decrease in the signal intensity of the fat pad on the more T2 weighted image. SE 2000/20 SE 2000/70

superior joint recess with a synovial tab extended into the joint space. Note the erosion of the joint surface

The angle of a facet joint: The angle between a line parallel to the surfaces of a joint and a line between the centers of the two facet joints the mean angulotion is 39.1 on the left and 42.9 on the right Asymmetry of the facet joints occurs in 25-32% of cases It is reported a higher incidence of sciatica and disc protrusion on the side of the more oblique facet. curvature of the facet joint surface becomes more pronounced when the facet angle increases

normal variations of the facet angles and facet asymmetry

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