Overview
The spine is made up of the neck (cervical spine), midback (thoracic spine), and low back (lumbosacral spine – lumbar spine and sacrum). Here we are discussing the low back. The spine is a very complex structure and is difficult to understand. I have tried to simplify it to help you understand it, but have also described all the different structures so that if they are mentioned by the doctor or in an x-ray report, you can get an idea of where that structure is.
Lumbosacral Spine
In the lumbar spine, there are five lumbar vertebrae, numbered L1, L2, L3, L4 and L5. The sacrum is made up five fused vertebrae (S1 to S5). The discs between the vertebra are numbered L1-2, L2-3, L3-4, L4-5 and L5-S1.
Coccyx
This is a small piece of bone that projects from the tip of the sacrum. It is said to be a left-over tail, and thus it is called the tail-bone. It can vary in size, shape and the angle it makes with the sacrum.
Transitional vertebrae
Up to 10% of people do not have the typical arrangement of five lumbar vertebra. It is usually a spectrum which goes from L5 being fused to the sacrum completely (sacralisation of L5) where there are only four lumbar vertebrae, to S1 being separated from the sacrum (lumbarisation of L5) where there are six lumbar vertebra, and many variations in between.
Parts
Vertebral Body
The vertebral body is a strong cylinder that is in the front of the spinal column. Stacked on top of each other the bodies form the supporting column.
Intervertebral Disc
The disc separates adjacent vertebral bodies. It is made of two parts – the Annulus Fibrosis and Nucleus Pulposis.
Annulus Fibrosis
This makes up the tough outer layer. It can be thought of like a tyre. It is strongly attached to the vertebral bodies, holding them together and resisting excessive movement.
Nucleus Pulposis
This is the inner gel. It can be thought of as like silicone inside the tyre. It allows some movement, and acts like a shock absorber.
Vertebral Arch
This is the horseshoe shaped structure attached to the back of the vertebral body. It contains the spinal cord and nerves. It is made up of a number of bony parts.
Pedicle
This is the tubular piece of bone on each side that directly attaches to the vertebral body. They are the prongs on the horseshoe.
Lamina
This is the flat piece of bone on each side that attaches to the pedicle. The lamina from each side curves toward the middle and joins the other at the spinous process. They form the arch of the horseshoe.
Spinous Process
This is the flat piece of bone that projects out at the back in the middle. It merges with the lamina on each side. It sticks out backwards from the middle of the arch of the horseshoe.
Transverse Process
This is the small piece of bone on each side that projects out sideways from the pedicle near the vertebral body. It does not form part of the arch.
Articular Processes
There are two superior and two inferior articular processes. From the top of the vertebral arch near where the lamina joins the pedicle, a superior articular process projects upwards on each side. From the bottom of the vertebral arch from the lower edge of the lamina, an inferior articular process projects downwards on each side. They are discussed further in the section on facet joints below.
Pars Interarticularis
This is a small area of bone that is difficult to clearly identify but is structurally important. It can be considered as the part of the lamina between the superior and inferior articular processes.
Facet Joint
This is also called the zygopophyseal or apophyseal joint. Between each pair of vertebrae, there is a pair of facet joints. It is a complex structure. The inferior articular processes that project from the upper vertebrae form a joint with the superior articular processes that project from the lower vertebrae. The forming the joint overlap like roof tiles. They are important in controlling movement between adjacent vertebra, and preventing one vertebra slipping forward on the one below.
Nerves
Spinal Cord and Cauda Equina
The spinal cord is a tube the thickness of a finger running down from the brain through the spinal column. It is enclosed by the vertebral body in front and the vertebral arch behind. It gives off pairs of nerves at every vertebra. The spinal cord actually finishes at the top of the lumbar spine (L1) and below this, there are nerves only. This is because the body grows more quickly than the spinal cord, and the spinal cord ends up shorter than the spinal column. The string of nerves that is in the lumbar spine is called the cauda equina. (This is because it is said to resemble a horse’s tail which in Latin is cauda equina).
Dura and CSF
The spinal cord and cauda equina are enclosed by a tough sleeve of tissue called the dura. The dura is filled with fluid which the cord and nerves float in. This fluid is called cerebrospinal fluid and is called so because it surrounds both the brain and spinal cord. A pair of nerves exit at each vertebral level. On each side, a nerve with its sleeve of dura passes under the pedicle and sideways out of the spinal canal (beneath the pars interarticularis). The nerve is given the name of the vertebra whose pedicle it runs under. In other words, the right L4 nerve root runs out between L4 and L5, under the pedicle of L4.
Epidural space
There is a small space between the bony canal formed by the vertebral body and arch, and the dura containing the spinal cord and nerves. It is called the epidural (or extradural) space. It is filled with blood vessels and fat.
Ligaments
Ligamentum Flavum
This is the thick yellow (flavum is Latin for yellow) ligament that runs between the adjacent laminae. It fills the gap at the back on each side between the lamina above and below.
Interspinous and Supraspinous Ligaments
These ligaments run between the spinous processes. They help prevent the spine from bending forward excessively.
Other Ligaments
Capsular ligaments bind together the facet joints. The intertransverse ligaments run between adjacent transverse processes. The anterior and posterior longitudinal ligaments run from top to bottom along the front and back of the vertebral bodies respectively.
Muscles
Extensor Muscles
They run at the back along the spinal column on each side of the spinous processes, under the skin. There are three layers.
Erector Spinae
This is the most superficial layer. It is a strong fleshy layer of muscle running lengthways along each side of the spine.
Multifidus
This is the middle layer. It is made up of numerous small slips of muscle. The slips run from one vertebra, upwards and inwards to another vertebra one two or three levels above. It starts from a small bony projection near the base of the transverse process (called the mamillary process) on the lower vertebra and attaches to the lamina and spinous process of the upper vertebra.
Deep
This layer is made up of multiple small muscles running between adjacent levels.
Flexor Muscles
They run in front along the spinal column in the groove between the vertebral bodies and the transverse processes. They are the psoas and iliacus muscles.
Motion Segment
A motion segment consists of:
- two adjacent vertebrae
- the disc and two facet joints that join them
- the ligaments that join them
Thus any single vertebra is part of two motion segments, one with the vertebra above it, and the other with the vertebra below it. It is a complex structure, as it must allow movement in all directions (flexion, i.e., forward bend, extension, i.e., backward bend and lateral flexion, i.e., lateral bend). It must also provide stability and avoid excessive movement in any direction.