Lumbar Anatomy

Original Editor - Lucinda hampton Top Contributors - Lucinda hampton and Kim Jackson

Introduction

The lower back (where most back pain occurs) includes the five vertebrae in the lumbar region and supports much of the weight of the upper body. The spaces between the vertebrae are maintained by intervertebral discs that act like shock absorbers throughout the spinal column to cushion the bones as the body moves. Ligaments hold the vertebrae in place, and tendons attach the muscles to the spinal column. Thirty-one pairs of nerves are rooted to the spinal cord and they control body movements and transmit signals from the body to the brain

  • The spine extends from the skull to the coccyx and includes the cervical, thoracic, lumbar, and sacral regions. The lumbar spine consists of 5 moveable vertebrae (numbered L1-L5). The lumbar vertebrae, as a group, produce a lordotic curve[1]
  • The intervertebral discs are responsible for the mobility without sacrificing the supportive strength of the vertebral column. The intervertebral discs, along with the laminae, pedicles and articular processes of adjacent vertebrae, create a space through which spinal nerves exit.
  • The complex anatomy of the lumbar region is a remarkable combination of these strong vertebrae (with their multiple bony elements) linked by joint capsules, and flexible ligaments/tendons, large muscles, and highly sensitive nerves. It also has a complicated innervation and vascular supply. 

Vertebrae

Typical lumbar vertebrae have several features distinct from those typical of cervical or thoracic vertebrae.[1]

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  • Presence of a large vertebral body.
  • Spinous process is short and thick, relative to the size of the vertebra, and projects perpendicularly from the body
  • Articular facets are markedly vertical, with the superior facets directed posteromedially and medially
  • Facets also have the unique feature of a curved articular surface. This is one feature that differentiates lumbar vertebrae to from thoracic
  • Addition of the mammillary process on the posterior aspect of the superior articular process.
  • One lumbar vertebra that may be considered atypical. L5 has the largest body and transverse processes of all vertebrae. The anterior aspect of the body has a greater height compared to the posterior. This creates the lumbosacral angle between the lumbar region of the vertebrae and the sacrum.

Structure a Reflection of Function[1]

  • The lumbar vertebrae have the largest bodies of the entire spine and an increase in size as the spine descends, a reflection of the responsibility of the lumbar spine of supporting the entire upper body. 
  • Due to the size of the intervertebral discs relative to the size of the vertebral body and the size and horizontal direction of the spinous processes, the lumbar spine has the greatest degree of extension of the vertebral column. 
  • The near-vertical orientation of the superior articular facets allows for flexion, extension, and lateral flexion, but prevents rotation. 
  • The mammillary processes provide a point of attachment for intertransversarii muscles and multifidus. 
  • The curvature of articular facets is thought to assist in the stabilization and weight-bearing capacity of lumbar vertebrae.

Each vertebral body is more or less a cylinder with a thin cortical shell, which surrounds cancellous bone. From L1 to L5, the posterior aspect changes from slightly concave to slightly convex, and the diameter of the cylinder increases gradually because of the increasing loads each body has to carry. At the upper and lower surfaces, two distinct areas can be seen: each is a peripheral ring of compact bone – surrounding and slightly above the level of the flat and rough central zone – which originates from the apophysis and fuses with the vertebral body at the age of about 16. The central zone – the bony endplate – shows many perforations, through which blood vessels can reach the disc. A layer of cartilage covers this central zone, which is limited by the peripheral ring. This is the cartilaginous endplate, forming the transition between the cortical bone and the rest of the intervertebral disc. A sagittal cut through the vertebral body shows the endplates to be slightly concave, which consequently gives the disc a convex form[2].

Meningeal branches of spinal nerves innervate all vertebrae[1]

Pedicles

The two pedicles originate posteriorly and attach to the cranial half of the body. Together with the broad and flat lamina, they form the vertebral arch. From L1 to L5, the pedicles become shorter and broader, and are more lateral. This narrows the anteroposterior diameter and widens the transverse diameter of the vertebral canal from above downwards. Together with the increasing convexity of the posterior aspect of the vertebral body, these changes in the position of the pedicles alter the shape of the normal bony spinal canal from an ellipse at L1 to a triangle at L3 and more or less a trefoil at L5 (Fig. 1).

Laminae

Each lamina is flat and broad, blending in centrally with the similarly configured spinal process, which projects directly backwards from the lamina. The two transverse processes project laterally and slightly dorsally from the pediculolaminar junction. The superior and inferior articular processes originate directly from the lamina.

The part of the lamina between the superior and inferior articular processes is called the ‘pars interlaminaris’. It runs obliquely from the lateral border of the lamina to its upper medial border. This portion of the lamina is subjected to considerable bending forces, as it lies at the junction between the vertically oriented lamina and the horizontally oriented pedicle. This ‘interlaminar part’ will therefore be susceptible to fatigue fractures or stress fractures (spondylolysis).

Facet Joints

The joints between the lower and upper articular processes are called zygapophyseal joints, apophyseal joints or ‘facet’ joints. They are true synovial joints, comprised of cartilaginous articular surfaces, synovial fluid, synovial tissue and a joint capsule [2]

The superior articular surface is slightly concave and faces medially and posteriorly. The convex inferior articular surface points laterally and slightly anteriorly. In general terms, there is a change from a relatively sagittal orientation at L1–L3, to a more coronal orientation at L5 and S1

Unlike the disc, the facet joints normally do not bear weight and during normal loads they are not subjected to compression strain. In degenerative fragmentation of the disc, however, intervertebral height diminishes and the articular surfaces are subjected to abnormal loading, setting up spondylarthrosis. The main function of the facet joints is to guide lumbar movements and keep the vertebrae in line during flexion–extension and lateral flexion. Because of the more sagittal slope of the articular surfaces, very little rotation takes place at the four upper lumbar levels. More distally, at the lumbosacral level, the joint line has a more coronal plane, which makes rotational movements potentially possible, but these are limited by the iliolumbar ligaments. The total range of rotation in the lumbar spine is therefore very limited, although not completely zero. Fibers of the medial branch of the dorsal root innervate the facet joints. The same nerve supplies the inferior aspect of the capsule and the superior aspect of the joint below.
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Intervertebral Discs

Two adjacent vertebral bodies are linked by an intervertebral disc. Together with the corresponding facet joints, they form the ‘functional unit of Junghans’n The disc consists of an annulus fibrosus, a nucleus pulposus and two cartilaginous endplates. The distinction between annulus and nucleus can only be made in youth, because the consistency of the disc becomes more uniform in the elderly. For this reason, nuclear disc protrusions are rare after the age of 70. From a clinical point of view, it is important to consider the disc as one integrated unit, the normal function of which depends largely on the integrity of all the elements. That means that damage to one component will create adverse reactions in the others.

Endplate

An upper and a lower cartilaginous endplate (each about 0.6– 1 mm thick) cover the superior and inferior aspects of the disc. The endplate permits diffusion and provides the main source of nutrition for the disc. The hyaline endplate is also the last part of the disc to wear through during severe disc degeneration.

  • Plates of cartilage that bind the disc to their respective vertebral bodies.
  • Each endplate covers almost the entire surface of the adjacent vertebral body; only a narrow rim of bone, called the ring apophysis, around the perimeter of the vertebral body is left uncovered by cartilage.
  • The portion of the vertebral body to which the cartilaginous endplate is applied is referred to as the vertebral endplate.
  • The endplate covers the nucleus pulposus in its entirety; peripherally it fails to cover the entire extent of the annulus fibrosus.
  • The collagen fibrils of the inner lamellae of the annulus enter the endplate and merge with it, resulting in all aspects of the nucleus being enclosed by a fibrous capsule.

Annulus Fibrosus

  • The annulus fibrosis is made up of 15–25 concentric fibrocartilaginous sheets or ‘lamellae’ each formed by parallel fibres, running obliquely at a 30° angle between the vertebral bodies. Because the fibers of two consecutive layers are oriented in opposite directions, they cross each other at an angle of approximately 120°. This arrangement of the annular fibers gives the normal disc great strength against shearing and rotational stresses, while angular movements remain perfectly possible.
  • The outermost fibers are attached directly to bone, around the ring apophysis, and for that reason they are referred to as the ligamentous portion of the annulus fibrosis.
  • The inner third merges with the cartilaginous endplate and is referred to as the capsular portion of the annulus fibrosis

Nucleus Pulposus

This consists of a gelatinous substance, made of a meshwork of collagen fibrils suspended in a mucoprotein base, which contains mucopolysaccharides and water.

  • As the anterior part of the vertebral body grows faster than the posterior part, the nucleus comes to lie more posteriorly. Consequently, the anterior part of the annulus will have thicker and stronger fibres, which means that the annulus gives better protection against anterior than posterior displacements of the nucleus; this is disadvantageous with respect to the contiguous nerve roots and dura.

Functions of the Disc

The primary function of the disc is to join the vertebrae and allow movement between them. The other functions are typical of the erect spine: a shock absorber; a load distributor; and a separator of the posterior facets to maintain the size of the intervertebral foramen.

The Weak Zone of the Disc

Several anatomical, biochemical and biomechanical properties make the posterior aspect of the disc the most critical and vulnerable part of the whole intervertebral joint.

  • The posterior annular fibers are sparser and thinner than the anterior.
  • Because the area available for diffusion is smaller posteriorly than anteriorly, the posterior part of the nuclear–annular boundary receives less nutrition and again the posterior part of the disc is the most strained part.
  • The posterior longitudinal ligament affords only weak reinforcement, whereas the anterior fibers are strengthened by the powerful anterior longitudinal ligament.
  • Because of the special mechanical arrangements of the annular fibers, the tangential tensile strain on the posterior annular fibers is 4–5 times the applied external load.
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All these elements explain the predominance of the posterior part of the disc in the development of weakening, radiating ruptures and posterior nuclear displacements. This is unfortunate, because most nociceptive tissues responsible for backache and sciatica (nerve roots and dura mater) emerge just beyond the posterior aspect of the disc.


Ligaments

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The broad, thick anterior longitudinal ligament  originates from the anterior and basilar aspect of the occiput and ends at the upper and anterior part of the sacrum. It consists of fibers of different lengths: some extend over 4–5 vertebral bodies; the short fibers attach firmly to the fibers of the outermost annular layers and the periosteum of two adjacent vertebrae.

The posterior longitudinal ligament is smaller and thinner than its anterior counterpart: 1.4 cm wide (versus 2 cm in the anterior ligament) and 1.3 mm thick (versus 2 mm). The posterior longitudinal ligament is narrow at the level of the vertebral bodies, and gives lateral expansions to the annulus fibrosis at the level of the disc, which bestow on it a denticulated appearance. Although the posterior ligament is rather narrow, it is important in preventing disc protrusion. Its resistance is the main factor in restricting posterior prolapse and accounts for the regular occurrence of spontaneous reduction in lumbago. This characteristic is also exploited in manipulative reduction, when a small central disc displacement is moved anteriorly when the ligament is tightened.

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The ligamentum flavum connects two consecutive laminae and has a very elastic structure with an elastin content of more than 80%. The lateral extensions form the anterior capsule of the facet joints and run further laterally to connect the posterior and inferior borders of the pedicle above with the posterior and superior borders of the pedicle below. These lateral fibers form a portion of the foraminal ring and the lateral recess.

The interspinous ligament lies deeply between two consecutive spinal processes. Unlike the longitudinal ligaments, it is not a continuous fibrous band but consists of loose tissue, with the fibers running obliquely from posterosuperior to anteroinferior. This particular direction may give the ligament a function over a larger range of intervertebral motion than if the fibers were vertical. The ligament is also bifid, which allows the fibers to buckle laterally to both sides when the spinous processes approach each other during extension.

The supraspinous ligament is broad, thick and cord-like. It joins the tips of two adjacent spinous processes, and merges with the insertions of the lumbodorsal muscles. Some authors consider the supraspinous ligament as not being a true ligament, as it seems to consist largely of tendinous fibers, derived from the back muscles. The effect of the supraspinous ligaments on the stability of the lumbar spine should not be underestimated. Because the ligament is positioned further away from the axis of rotation and due to its attachments to the thoracolumbar fascia, it will have more effect in resisting flexion than all the other dorsal ligaments.

The intertransverse ligaments are thin membraneous structures joining two adjacent transverse processes. They are intimately connected to the deep musculature of the back.

The iliolumbar ligaments are thought to be related to the upright posture. They do not exist at birth but develop gradually from the epimysium of the quadratus lumborum muscle in the first decade of life to attain full differentiation only in the second decade. The ligament consists of an anterior and a posterior part. The anterior band of the iliolumbar ligament is a well-developed, broad band.

The iliolumbar ligaments play an important role in the stability of the lumbosacral junction by restricting both side flexion and rotational movement at the L5–S1 joint and forward sliding of L5 on the sacrum[2].

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Muscles and Fasciae 

The spine is unstable without the support of the muscles that power the trunk and position the spinal segments. Back muscles can be divided into four functional groups: flexors, extensors, lateral flexors and rotators[2]

Extensors, arranged in three layers

  1. Most superficial is the strong erector spinae or sacrospinalis muscle. Its origin is in the erector spinae aponeurosis, a broad sheet of tendinous fibers attached to the iliac crest, the median and lateral sacral crests and the spinous processes of the sacrum and lumbar spine.
  2. Middle layer is the multifidus. The fibers of the multifidus are centered on each of the lumbar spinous processes. From each spinal process, fibers radiate inferiorly to insert on the lamina, one, two or three levels below. The arrangement of the fibers is such that it pulls downwards on each spinal process, thereby causing the vertebra of origin to extend.
  3. Third layer is made up of small muscles arranged from level to level, which not only have an extension function but are also rotators and lateral flexors.

Flexors

  1. intrinsic group (psoas major, psoas minor and iliacus)
  2. extrinsic group (abdominal wall muscles).

Lateral flexors and rotators

  • internal and external oblique, the intertransverse and quadratus lumborum muscles.
  • remember that pure lateral flexion is brought about only by the quadratus lumborum.

Spinal Canal

The spinal canal is made up of the canals of individual vertebrae so that bony segments alternate with intervertebral and articular segments. The shape of the transverse section changes from round at L1 to triangular at L3 and slightly trefoil at L5 (Fig. 1). An anterior wall and a posterior wall, connected through pedicles and intervertebral foramina, form the margins of the canal.

The anterior wall consists of the alternating posterior aspects of the vertebral bodies and the annulus of the intervertebral discs. In the midline these structures are covered by the posterior longitudinal ligament, which widens over each intervertebral disc.

The posterior wall is formed by the uppermost portions of the laminae and the ligamenta flava. Because the superoinferior dimensions of the laminae tend to decrease at the L4 and L5 levels, the ligamenta flava consequently occupy a greater percentage of the posterior wall at these levels. The spinal canal contains the dural tube, the spinal nerves and the epidural tissue.

Dura Mater 

The dura mater is a thick membranous sac, attached cranially around the greater foramen of the occiput, where its fibers blend with the inner periosteum of the skull, and anchored distally to the dorsal surface of the distal sacrum by the filum terminale.

At the lumbar level, the dura contains the distal end of the spinal cord (conus medullaris, ending at L1), the cauda equina and the spinal nerves, all floating and buffered in the cerebrospinal fluid. The lumbar roots have an intra- and extrathecal course. Emerging in pairs from the spinal cord, they pass freely through the subarachnoid space before leaving the dura mater. In their extrathecal course and down to the intervertebral foramen, they remain covered by a dural investment. At the L1 and L2 levels, the nerves exit from the dural sac almost at a right angle and pass across the lower border of the vertebra to reach the intervertebral foramen above the disc. From L2 downwards, the nerves leave the dura slightly more proximally than the foramen through which they will pass, thus having a more and more oblique direction and an increasing length within the spinal canal.

The dura mater has two characteristics that are of cardinal clinical importance: mobility and sensitivity.[2]

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Nerve Roots

The radicular canal contains the intraspinal extrathecal nerve root. The nerve root consists of a sheath (dural sleeve) and fibres. Each structure has a specific behaviour and function, responsible for typical symptoms and clinical signs. This has some clinical consequences: slight pressure and inflammation only involve the sleeve and provoke pain and impaired mobility. More substantial compression of the root will also affect the nerve fibres, which leads to paraesthesia and loss of function.[2]

Significant Facts

Spinal nerves increase in size as the spinal cord descends, however, the intervertebral foramen decrease in size. This combination, in addition to pathology such as intervertebral disc degeneration that brings two adjacent vertebra closer together, commonly leads to spinal stenosis, a condition in which the vertebral foramen compresses the spinal nerves. This may be treated with a laminectomy, a process in which the spinous process, laminae, and pedicles are removed to create more room for the spinal cord and spinal nerves[1]

The lumbar region has a lesser incidence of neurological injury due to fractures as compared to those in the thoracic region. This is due to the large size of the vertebral canal, the inferior end of the spinal cord at the level of L2, as well as the relative resilience cauda equina nerve roots. This is why spinal taps are performed inferior to L2; the roots forming the cauda equina, suspended in the cerebrospinal fluid (CSF), move out of the way of the spinal needle[1].

The valve-less vertebral venous plexuses allow for the metastasis of cancer from the pelvis, such as that of the prostatic, to the vertebral column[1]

Resources

Lumbar Spine Anatomy

Spinal Cord Anatomy

References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Waxenbaum JA, Futterman B. Anatomy, back, lumbar vertebrae. InStatPearls [Internet] 2018 Dec 13. StatPearls Publishing. Available from: https://www.ncbi.nlm.nih.gov/books/NBK459278/ (last accessed 24.1.2020)
  2. 2.0 2.1 2.2 2.3 2.4 2.5 Musculoskeletal key Applied anatomy of the lumbar spine Available from:https://musculoskeletalkey.com/applied-anatomy-of-the-lumbar-spine/ (last accessed 24.1.2020)