====== Pars interarticularis ======

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The articular processes or zygapophyses (Greek ζυγον = "yoke" (because it links two vertebrae) + απο = "away" + φυσις = "process") of a vertebra, are projections of the vertebra that serve the purpose of fitting with an adjacent vertebra. The actual region of contact is called the articular facet.

Articular processes spring from the junctions of the [[pedicle]]s and laminæ, and there are two right and left, and two superior and inferior. These stick out of an end of a vertebra to lock with a zygapophysis on the next vertebra, to make the backbone more stable.

see [[superior articular process]]

The inferior processes or postzygapophysis project downward from a higher vertebra, and their articular surfaces are directed more or less forward and outward.

The articular surfaces are coated with hyaline cartilage.

In the cervical vertebral column, the articular processes collectively form the articular pillars. These are the bony surfaces palpated just lateral to the spinous processes.

The pars interarticularis, or pars for short, is the part of vertebra located between the inferior and superior articular processes of the [[facet joint]].

In the transverse plane, it lies between the lamina and pedicle. In other words, in the axial view, it is the bony mass between the facets that is anterior to the lamina and posterior to the pedicle. It is abnormal in spondylolysis, either due to fracture or congenitally.
((Ranu HS. Three dimensional surgical simulations of the spine.
J Biomed Eng. 1982 Oct;4(4):285-8.))
((Cyron BM, Hutton WC. The fatigue strength of the lumbar
neural arch in spondylolysis. J Bone Joint Surg Br. 1978
May;60-B(2):234-8.))

The pars interarticularis has been shown in myriad studies to play an integral role in maintaining the structural integrity of the spinal column: A finite element analysis conducted by Ranu

on intact and post-laminectomy vertebrae determined
that the region of the pars interarticularis is subject to high stresses, which increase when posterior elements are removed. The vulnerability of the pars was also highlighted in a study that described the susceptibility of the pars to fracture by fatigue due to repetitive stresses

An increased understanding of the pars interarticularis dimensions along the lumbar spine would help spine surgeons appreciate the window available for providing an adequate central canal decompression while
maintaining enough bone in the pars to ensure continuing structural integrity of the vertebral column.

On an anterior oblique radiograph of the lumbar spine, the pars is the neck of the imaginary Scottie dog; the Scottie dog's eye is the pedicle, its hindlegs the spinous process, its nose the transverse process, its ear the superior articular facet and its forelegs the inferior articular facet.

Stress fractures of the pars interarticularis are known to be associated with playing volleyball, although the mechanism is somewhat unclear.

see [[cervical pars fracture]]