Lumbar Spine Flashcards
Lumbar Spine Overview
a. The lumbar spine consists of 5 moveable vertebrae numbered L1-L5.
b. The complex anatomy of the lumbar spine is a remarkable combination of these strong vertebrae, multiple bony elements linked by joint capsules, and flexible ligaments/tendons, large muscles, and highly sensitive nerves.
i. It also has a complicated innervation and vascular supply.
c. The lumbar spine is designed to be incredibly strong, protecting the highly sensitive spinal cord and spinal nerve roots.
d. At the same time, it is highly flexible, providing for mobility in many different planes including flexion, extension, side bending, and rotation.
Gross Anatomy
Bones of Lumbar Vertebrae
a. The lumbar vertebrae, numbered L1-L5, have a vertical height that is less than their horizontal diameter.
b. They are composed of the following 3 functional parts:
1. The vertebral body, designed to bear weight
- The vertebral (neural) arch, designed to protect the neural elements
- The bony processes (spinous and transverse), which function to increase the efficiency of muscle action
c. The lumbar vertebral bodies are distinguished from the thoracic bodies by the absence of rib facets.
i. The lumbar vertebral bodies (vertebrae) are the heaviest components, connected together by the intervertebral discs.
ii. The size of the vertebral body increases from L1 to L5, indicative of the increasing loads that each lower lumbar vertebra absorbs.
iii. Of note, the L5 vertebra has the heaviest body, smallest spinous process, and thickest transverse process.
Vertebral Bodies of the lumbar
a. The lumbar vertebral bodies are distinguished from the thoracic bodies by the absence of rib facets.
b. The lumbar vertebral bodies (vertebrae) are the heaviest components, connected together by the intervertebral discs.
c. The size of the vertebral body increases from L1 to L5, indicative of the increasing loads that each lower lumbar vertebra absorbs.
* i. Of note, the L5 vertebra has the heaviest body, smallest spinous process, and thickest transverse process.
d. The intervertebral discal surface of an adult vertebra contains a ring of cortical bone peripherally termed the epiphysial ring.
i. This ring acts as a growth zone in the young while anchoring the attachment of the annular fibers in adults.
e. A hyaline cartilage plate lies within the confines of this epiphysial ring.
Structures of the Lumbar Vertebrae
a. Each vertebral arch is composed of 2 pedicles, 2 laminae, and 7 different bony processes (1 spinous, 4 articular, 2 transverse) (see the following image), joined together by facet joints and ligaments.
b. The pedicle, strong and directed posteriorly, joins the arch to the posterolateral body.
i. It is anchored to the cephalad portion of the body and function as a protective cover for the cauda equina contents.
ii. The concavities in the cephalad and caudal surfaces of the pedicle are termed vertebral notches.
c. Beneath each lumbar vertebra, a pair of intervertebral (neural) foramina with the same number designations can be found, such that the L1 neural foramina are located just below the L1 vertebra.
d. Each foramen is bounded superiorly and inferiorly by the pedicle, anteriorly by the intervertebral disc and vertebral body, and posteriorly by facet joints.
i. The same numbered spinal nerve root, recurrent meningeal nerves, and radicular blood vessels pass through each foramen.
ii. Five lumbar spinal nerve roots are found on each side.
e. The broad and strong laminae are the plates that extend posteromedially from the pedicle.
i. The oblong shaped spinous processes are directed posteriorly from the union of the laminae.
Articular Process of Lumbar Vertebrae
a. The 2 superior (directed posteromedially) and inferior (directed anterolaterally) articular processes, labeled SAP and IAP, respectively, extend cranially and caudally from the point where the pedicles and laminae join.
i. The facet or zygapophyseal joints are in a parasagittal plane.
ii. When viewed in an oblique projection, the outline of the facets and the pars interarticularis appear like the neck of a Scottie dog
b. Between the superior and inferior articular processes, 2 transverse processes are projected laterally that are long, slender, and strong.
i. They have an upper tubercle at the junction with the superior articular process (mammillary process) and an inferior tubercle at the base of the process (accessory process).
ii. These bony protuberances are sites of attachments of deep back muscles.
c. The lumbar spine has an anterior, middle, and posterior column that is pertinent for lumbar spine fractures
Summary of Lumbar Anatomy
Lumbar vertebrae are characterized by massive bodies and robust spinous and transverse processes. Their articular facets are oriented somewhat parasagittally, which is thought to contribute the large range of anteroposterior bending possible between lumbar vertebrae. Lumbar vertebrae also contain small mammillary and accessory processes on their bodies. These bony protuberances are sites of attachment of deep lumbosacral muscles.
Lumbar vertebral joints
a. The mobility of the vertebral column is provided by the symphyseal joints between the vertebral bodies, formed by a layer of hyaline cartilage on each vertebral body and an intervertebral disc between the layers.
b. The synovial joints between the superior and inferior articular processes on adjacent vertebrae are termed the facet joints (also known as zygapophysial joints or Z-joints ).
i. They permit simple gliding movements.
c. The movement of the lumbar spine is largely confined to flexion and extension with a minor degree of rotation (see the image below).
d. The region between the superior articular process and the lamina is the pars interarticularis.
i. A spondylolysis occurs if ossification of the pars interarticularis fails to occur.
Lumbar intervertebral discs
a. Discs form the main connection between vertebrae.
i. They bear loading during axial compression and allow movement between the vertebrae.
ii. Their size varies depending on the adjacent vertebrae size and comprises approximately one quarter the length of the vertebral column.
b. Each disc consists of the nucleus pulposus, a central but slightly posterior mucoid substance embedded with reticular and collagenous fibers, surrounded by the annulus fibrosus, a fibrocartilaginous lamina.
c. The annulus fibrosus can be divided into the outermost, middle, and innermost fibers.
d. The anterior fibers are strengthened by the powerful anterior longitudinal ligament (ALL). The posterior longitudinal ligament (PLL) affords only weak midline reinforcement, especially at L4-5 and L5-S1, as it is a narrow structure attached to the annulus.
e. The anterior and middle fibers of the annulus are most numerous anteriorly and laterally but deficient posteriorly, where most of the fibers are attached to the cartilage plate.
f. The annular fibers are firmly attached to the vertebral bodies and are arranged in lamellae.
i. This annular arrangement permits limiting vertebral movements, reinforced by investing ligaments.
Summary of the Lumbar Disc
a. Each disc consists of the nucleus pulposus, a central but slightly posterior mucoid substance embedded with reticular and collagenous fibers, surrounded by the annulus fibrosus, a fibrocartilaginous lamina.
b. The annulus fibrosus can be divided into the outermost, middle, and innermost fibers. The anterior fibers are strengthened by the powerful anterior longitudinal ligament (ALL).
c. The posterior longitudinal ligament (PLL) affords only weak midline reinforcement, especially at L4-5 and L5-S1, as it is a narrow structure attached to the annulus.
d. The anterior and middle fibers of the annulus are most numerous anteriorly and laterally but deficient posteriorly, where most of the fibers are attached to the cartilage plate.
Lumbar vertebral ligaments
a. The ALL covers the ventral surfaces of lumbar vertebral bodies and discs.
i. It is intimately attached to the anterior annular disc fibers and widens as it descends the vertebral column.
ii. The ALL maintains the stability of the joints and limits extension.
b. The PLL is located within the vertebral canal over the posterior surface of the vertebral bodies and discs.
i. It functions to limit flexion of the vertebral column, except at the lower L-spine, where it is narrow and weak.
ii. The supraspinous ligament joins the tips of the spinous processes of adjacent vertebrae from L1-L3.
iii. The interspinous ligament interconnects the spinous processes, from root to apex of adjacent processes.
c. Sometimes described together as the interspinous/supraspinous ligament complex, they weakly resist spinal separation and flexion.
d. The ligamentum flavum (LF) bridges the interlaminar interval, attaching to the interspinous ligament medially and the facet capsule laterally, forming the posterior wall of the vertebral canal.
i. It has a broad attachment to the undersurface of the superior lamina and inserts onto the leading edge of the inferior lamina.
ii. Normally, the ligament is taut, stretching for flexion and contracting its elastin fibers in neutral or extension. It maintains constant disc tension.
e. The intertransverse ligament joins the transverse processes of adjacent vertebrae and resists lateral bending of the trunk.
f. The iliolumbar ligament arises from the tip of the L5 transverse process and connects to the posterior part of the inner lip of the iliac crest.
i. It helps the lateral lumbosacral ligament and the ligaments mentioned above stabilize the lumbosacral joint
Lumbar spine musculature
Four functional groups of muscles govern the lumbar spine and can be divided into extensors, flexors, lateral flexors, and rotators. Synergistic muscle action from both the left and right side muscle groups exist during flexion and extension of the L-spine
Lumbar spine musculature- Extensors
The extensor muscles are arranged in 3 layers.
a. The largest group of intrinsic back muscles and primary extensor is the erector spinae (or sacrospinalis).
i. In the lower L-spine, the erector spinae appears as a single muscle.
ii. At the upper lumbar area, it divides into 3 vertical columns of muscles (iliocostalis, longissimus, spinalis).
b. Located posterolateral to the vertebral column, they have a common origin from a thick tendon that is attached to the sacrum, the lumbar spinous processes, and the iliac crest.
i. The iliocostalis is the most lateral, and the spinalis (smallest muscle) is the most medial.
ii. The longissimus (largest muscle) inserts on to the skull base, whereas the iliocostalis inserts onto the angles of the ribs and transverse processes of the lower cervical vertebrae.
iii. As these muscles ascend up the vertebral column, they divide regionally depending on where the muscle attaches superiorly.
c. A 3-layered fasciculated muscle, the transversospinal muscle group, lies deep to the erector spinae and originates on the mamillary processes in the lumbar spine.
i. In the sacrum, it originates from the laminar area just medial to the posterior sacral foramina, from the tendinous origins on the erector spinae, and the medial surface of the posterior superior iliac spine (PSIS).
ii. Each fascicle is directed superomedially toward the inferior and medial margin of the lamina and adjacent spinous process.
iii. The superficial layer attaches from 3-4 levels above, the intermediate layer attaches 2 levels above, and the deep layer attaches 1 level above. The transversospinal muscle group acts both as an L-spine extensor and a rotator.
d. A multitude of small, segmental muscles are the deepest layer of the lumbar extensors.
i. They can be divided into 2 groups, both innervated by the dorsal rami of spinal nerves.
ii. The levatores costarum are not typically present in the lumbar spine. The second group contains the interspinales and intertransversarii.
iii. The interspinales consists of short fasciculi attached between the spinous processes of contiguous vertebrae.
iv. The intertransversarii consist of 2-3 slips of muscles, which pass between adjacent transverse processes.
v. They are postural stabilizers and increase the efficiency of larger muscle group action.
Lumbar spine musculature- Forward Flexors
a. Flexors of the L-spine are divided into an iliothoracic (extrinsic) group and a femorospinal (intrinsic) group.
b. The iliothoracic group is made up of the abdominal wall muscles: rectus abdominis, external abdominal oblique, internal abdominal obliquus, and the transversus abdominis.
c. The femorospinal group is made up of the psoas major and iliacus muscles. The psoas major originates from multiple areas: the anterior surface and lower border of transverse processes of L1-L5, from the bodies and discs of T12-L5.
d. It inserts on to the lesser trochanter of the femur and is innervated by direct fibers of the lumbar plexus (L1-L3). Its primary action is hip and trunk flexion.
Lumbar spine musculature- Lateral Flexors
a. True lateral flexion is normally a combination of side bending and rotation.
b. Normally, side bending is brought about by ipsilateral contraction of the oblique and transversus abdominal muscles and quadratus lumborum.
i. Of these, only unilateral contraction of the quadratus lumborum can bring about pure lateral flexion and elevation of the ilium, whereas bilateral contraction produces some lumbar extension.
c. The quadratus lumborum is attached below to the iliolumbar ligament and to the adjacent part of the iliac crest above the lower anterior surface of the 12th rib and to the apexes of the L1-4 transverse processes.
Lumbar spine musculature- Rotators
a. Rotation of the lumbar spine is brought about by the unilateral contraction of muscles that follow an oblique direction of pull; the more oblique the course, the more important the rotational effect.
b. Most of the extensors and lateral flexors follow an oblique course and produce rotation when their primary component has been neutralized by antagonist muscle groups.
c. The transversospinal muscle group, innervated by the dorsal rami of spinal nerves, is deep to the erector spinae muscle and runs obliquely (superomedially) from the transverse processes to the spinous processes.
i. As a group, they act to extend the vertebral column.
ii. But, when contracted unilaterally, they cause the trunk to rotate in the contralateral direction.
d. The transversospinal muscle group are divided into 3 groups: the semispinalis, multifidus, and rotatores lumborum muscles.
i. The rotatores lumborum are small, irregular, and variable muscles connecting the superoposterior part of the transverse process of the vertebra below to the inferolateral border of the lamina of the vertebra above.
