Lumbar vertebrae

Vertebral body

The lumbar vertebrae are perhaps most easily defined by their stout and think vertebral bodies; they are relatively large, wider laterally compared to anteroposteriorly and thicker in its anterior half compared to posterior (which contributes to the lumbar lordosis). Their intervertebral surfaces are somewhat flattened or slightly concave superiorly and inferiorly, concave behind and deeply restricted at the front and laterally.

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Vertebral arch

The vertebral arches of lumbar vertebrae consist of paired of pedicles and laminae which encircle the vertebral foramen and ultimately support seven main processes. They are small in appearance relative to the size of the vertebral bodies. The pedicles are stout and strong, projecting posteriorly from the upper part of the vertebral body. As a result, the inferior vertebral notches have considerable depth compared to their superior counterparts. The pedicles also change in morphology from vertebrae L1 to L5, increasing in width from 9 mm up to 18 mm. The angle of orientation in the axial plane also increases from 10 to 20 degrees from vertebrae L1 to L5.
The laminae are strong, broad and short in morphology and form the posterior portion of the vertebral arch. The upper lumbar laminae are taller instead of wider, whereas the lower lumbar laminae are wider instead of tall. The laminae merge together at the midline to complete the posterior part of the vertebral arch, providing a base for a spinous process.
The vertebral foramen enclosed by the vertebral arch is triangular in appearance, smaller than that typically found among the cervical vertebrae but larger than those typical of the thoracic spine.

Transverse process

Transverse processes of lumbar vertebrae are long and slender compared to their thoracic counterparts, with changing morphology from vertebrae L1 to L5. These processes are horizontal in L1-L3 and incline slightly upward in L4-L5. In L1-L3, the transverse processes arise from the junctions of the pedicles and laminae, but in L4-L5, they arise from the pedicles and posterior portions of the vertebral bodies since they are set farther forward. The transverse processes are positioned anterior to the articular processes instead of behind them as in the thoracic vertebrae.

In the lumbar vertebrae, the costal element of each transverse process (pleurapophysis or costal process) occupies most of the anterior and lateral aspect of the transverse process, with only the root and parts of the posterior aspect (accessory process, see below) originating as ‘true’ transverse process elements (diapophysis).

Accessory process

Each lumbar transverse process usually bears a small tubercle along its posterior proximal aspect, known as an accessory process. They serve as attachment sites for the intertransversarii and longissimus muscles (see below).

Articular processes/facets

The superior and inferior articular processes are well-defined and project upward and downward from the junctions of pedicles and laminae, respectively.

The superior processes are concave and face medially (like when the palms of the hands are facing each other when about to clap), whereas the inferior processes are convex and face laterally towards the superior articular facets of the vertebra below, forming zygapophyseal (facet) joints. This anatomical conformation allows for a good degree of flexion/extension, but limited twisting/rotation of the lumbar spine.

When viewed posteriorly, the superior and inferior articular processes of vertebrae L1 and L2 collectively form a inverted vertically elongated trapezoid shape; those of vertebrae L3 and L4 form a square, while those of vertebra L5 form a horizontally elongated trapezoid.

Mammillary process

Mammillary processes are small tubercles located on the posterior aspect of the superior articular processes of lumbar vertebrae (and sometimes vertebra T12). They serve as attachment sites for the intertransversarii and multifidus muscles.

Spinous process

The spinous process is short, 'hatchet shaped' and blunt in appearance; it  projects almost horizontally from the posterior aspect of the vertebral arch. They are quadrilateral, hatchet shaped in appearance.

Specific lumbar vertebrae

• First lumbar vertebrae (L1): smallest of the series and lies roughly inline with the anterior end of the 9th rib at a level called the transpyloric plane (since the pylorus of the stomach is found at this level).
• Fifth lumbar vertebrae (L5): is significantly different in morphology, with its body being much deeper anteriorly than posteriorly, contributing to the lumbosacral angle. The spinous process is smaller than higher lumbar vertebrae and there is a wider interval between the inferior articular processes, which face almost anteriorly rather than laterally. The transverse processes are substantially thicker and arise from the body as well as the pedicles.

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