Back Anatomy

The back in general is such a large area, incorporating everything from the pelvis to the very top of the neck and scapula (shoulder blades). For that reason this section focuses solely on the spine.

Bones of the spine

In total there are 32 bones which form the spinal column which are split into 5 sections:

  • Cervical: This section forms the neck and consists of 7 vertebrae. These are the smallest of the vertebrae as they do not have to carry as much weight. The top two cervical vertebrae are called the Axis and Atlas and allow the head to rotate on the neck. The cervical vertebrae allow the movements of flexion, extension, rotation and lateral flexion (side-bending).
  • Thoracic: The thoracic spine runs from shoulder level down to the level of the lowest ribs and includes 12 vertebrae which increase in size the lower down the spine they are positioned. Each vertebrae also forms a joint with the adjacent rib (known as a costovertebral joint). The thoracic spine does not move as freely as the cervical or lumbar sections as its main purpose is to provide stability for the rib cage and protection for the organs within the thoracic cavity.
  • Lumbar: Contains 5 vertebrae and forms the lower back. These are the largest vertebrae due to the additional weight they must carry. The lumbar region also allows a lot of movement, into flexion, extension, rotation and lateral flexion which means it is the most frequently injured section of the back.
  • Sacral: The sacral spine (or sacrum) consists of 4 fused vertebrae which cannot move independently of each other. This part of the spine is shaped like a triangle and bridges the gap between the two sides of the pelvis, connecting the spine to the lower body. The joints with the ilium (pelvis), either side of the sacrum are known as Sacroiliac (SI) joints. 
  • Coccyx: Contains 4 small fused bones known as the tail bone. These have no real function, although can occasionally be the source of pain known as coccydynia.

The bones of each section are named with a letter (C for cervical, T for thoracic, L for lumbar) followed by a number which represents its position within that section e.g. C1-C7, T1-T12 and L1-L5.

With the exception of those forming the sacrum and coccyx, vertebrae take the same general form, with some small differences from section to section. The basic shape of a vertebra includes a ‘body’ which is the large flat circular section. This part carries the weight of the vertebrae above it. Inbetween each of the vertebral bodies is a cartilaginous disc which provides cushioning and shock absorption. The very first vertebrae, the Atlas (or C1), does not have a vertebral body. Instead it is a ring of bone which attaches to the second vertebral body (Axis or C2) by the Odontoid process (a small upward protruding piece of bone) about which the atlas rotates.

Protruding from the back of the vertebra are three bony processes. The two either side are called transverse processes with the central one being the spinous process. It is the spinous process’ which are visible running down the backs of most people. These processes form the main site for muscles to attach to the spine.

 In the middle of these three processes is a hole known as the foramen in which the spinal cord passes, from the brain, through the foramen of each vertebra as shown opposite. Between each of the vertebra the spinal cord branches nerves sideways which then travel around the body to supply all muscles and organs.

Joints

The main joints which have been associated with lower back pain are Facet joints (sometimes known as zygapophysial joints) and the Sacroiliac joint.

  • Facet joints occur in pairs at the back of each vertebrae. They connect neighbouring vertebrae and allow movement between them. The facet joints direct the plane of motion at each vertebral segment, which is dependant on their angle and orientation. Throughout the spine the angles and orientations differ which alters the possible movement allowed in that area. Facet joint pain may arise directly from the facet joint either from inflammation or nerve impingement
  • Costovertebral joints are formed between the heads of the ribs and the bodies of the thoracic vertebrae. There is very little movement available here. The joint can sublux causing mid back pain which radiates into the chest.
  • Costotransverse joints are between the tubercle of the rib and the transverse processes of the thoracic vertebrae.
  • The Sacroiliac Joint is formed where the Sacrum meets each side of the pelvis. The joint only allows very small gliding movements when the legs are moved. These joints can often get stuck or in some cases one half of the pelvis can glide forwards or backwards, which is often referred to as a twisted pelvis.

Ligaments

The ligaments surrounding the spine connect individual vertebrae and provide support to the whole structure. There are two types of ligament:

  • Intrasegmental ligaments: These connect one vertebrae to another and include the Ligamentum Flavum (a strong ligament which connects the posterior surfaces of each vertebrae), Interspinous ligaments (inbetween each spinous process) and intertransverse ligaments (connecting transverse processes to the one above and below on the same side).
  • Intersegmental ligaments: These ligaments are long stabilising ligaments which provide support to the spine as a whole. They include the Anterior and Posterior Longitudinal ligaments which run down the front and back of the vertebral bodies respectively and the Supraspinous ligament which is a cord-like ligament which attaches to the tip of every spinous process.

Deep muscles of the back

The number of muscles having one or both attachments within the back as a whole is huge. For the purpose of this section we will look at the muscles which are most often involved in lower back pain.

The deepest and most important muscles when it comes to stability of the spine are collectively known as the Transversospinalis muscles. This group consists of three ‘layers’, the deepest being Rotatores, then Multifidus with Semispinalis being the most superficial. It is thought that these muscles provide precise movements of each vertebra and aid stability in the back.

A second group of muscles which attach only to the spine are known as Erector Spinae. These muscles are more superficial and larger then the Transversospinalis muscles and arise from the thick, broad band of fascia known as the Lumbar Aponeurosis. Erector Spinae consists of three groups of muscles, the Iliocostalis which is the most lateral (outer) and attaches to the ribs, close to their attachment to the spine. The Longissimus arises from the Lumbar Aponeurosis and inserts to the transverse processes of the vertebrae. Spinalis is the most medial (central) muscle which attaches to the lumbar and thoracic spinous processes. All of these muscles are thought to be responsible for extending the spine (leaning backwards).

Superficial muscles of the lower back

There are a large number of muscles in the lower back. Those which are most commonly involved in back pain are:

  • Latissimus Dorsi: This is the largest muscle of the lower back which is responsible for pulling the arm downwards and backwards. It originates from the spinous processes of T6-T12 and the Iliac crest (top of the pelvis), travels upwards across the entire lower back and inserts onto both humerus’ (upper arm bone).
  • Quadratus Lumborum:  This muscle is responsible for side bending and also aids extension of the lumbar spine. It originates at the Iliac crest and passes upwards to attach to the lowest rib and to the transverse processes of L1-L4
  • Gluteus Medius: Although strictly a gluteal muscle, it is often associated with low back pain. Its action is to internally rotate (turn the knee inwards) and abduct the hip (take the leg away from the centre of the body). It also has a very important function in maintaining and correcting hip level, which can be a cause of overuse and the development of trigger points. It arises from the outer surface of the Ilium (pelvis) and inserts onto the Greater Trochanter at the top of the Femur (thigh bone).