Introduction
In March of 2020, I will be interning at the Barcelona Spine Centre, a world-leading centre for non-invasive spine surgery and a pioneer in artificial disc replacement (ADR) surgery. In preparation for my internship, I've written a blog entry about spinal anatomy and non-invasive spine surgery.
The Vertebral Column
The spine is an interconnected complex of bones, muscles, nerves, tendons and ligaments. A healthy spine constitutes strong muscles and bones, flexible tendons and ligaments, and sensitive nerves. The vertebral column is made up of 33 individual interlocking bones called vertebrae. These provide support to the body while protecting your spinal cord from injury. The top 24 vertebrae are movable and make up the cervical, thoracic and lumbar spine, while the sacral and coccyx vertebrae are fused and hence fixed.
C1 is a ring-shaped atlas that connects directly to the skull and pivots around C2, providing the greatest range of motion to the neck. The thoracic spine has a limited range of motion. It holds the ribcage and protects the heart and lungs. The main role of the lumbar spine is to bear the weight of the body. L1-L5 are very large vertebrae, facilitating the absorption of stress upon lifting heavy objects. The sacrum connects the spine to hip bones, while the coccyx provides attachment for ligaments and muscles of the pelvic floor.
The spine has natural curvatures that act like a coiled spring to absorb shock and allow a range of motion throughout the spinal column. The curvature is maintained by muscles at the back of the spine (extensors) and by abdominal muscles (flexors). Excess body weight, weak muscles and other forces can pull at the spines alignment, causing:
- Lordosis, an abnormal curve of the lumbar spine.
- Kyphosis, an abnormal curve of the thoracic spine (hunch back).
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Scoliosis, an abnormal side-to-side curvature.
The structure of a Vertebra
The vertebral body is a thick oval segment of bone consisting of cancellous bone tissue, encircled by a protective layer of compact bone. Bony structures called pedicles protrude from either side of the vertebral body and join with the laminae to form the vertebral arch. The upper and lower surfaces of the vertebral body are flat to provide attachment for the vertebral discs that lie between each vertebra. Since vertebral bodies bear major loads, they are prone to developing compression fractures, particularly in patients with osteoporosis.
The vertebral foramen is an opening formed between the anterior body and the posterior vertebral arch. This foramen creates a tunnel that begins at C1 and runs down to L5, known as the spinal canal. The spinal canal houses the spinal cord and its meninges (membranous coverings of the spinal cord).
A process is a bony projection which serves for the attachment of muscles and ligaments. Seven processes project from each vertebra:
- The spinous process protrudes where the laminae of the vertebral arch join and provides the point of attachment for the muscles and ligaments of the spine.
- Two transverse processes, one on each side of the vertebral body serve for the attachment of muscles and the inter-transverse ligaments.
- Four articular processes serve the purpose of attaching to the adjacent vertebra and increase the stability of the vertebral column. The superior articular processes project upwards from the lower vertebra, while the inferior processes project downwards from the higher vertebra. The points at which the vertebrae attach are called articular facet joints.
The thoracic vertebrae have additional facet joints for attachment to the ribs; on each of the transverse processes, there is a facet joint that articulate to the tubercle of the rib, and one that articulates with the head of the rib.
Discs
Every cervical, thoracic and lumbar vertebra is separated from other vertebrae by an intervertebral disc. Discs act as shock absorbers, keep bones from rubbing together and allow for increased mobility in the spine. An individual disc is made up of:
- A gel-filled nucleus, which provides elastic resistance;
- An outer ring called the annulus, which has fibres that attach between the bodies of each vertebra
Discs reabsorb fluid when the body is at rest. With age, discs lose this ability and become more brittle and flat. This explains why we lose height as we age.
Ligaments
A ligament is the fibrous connective tissue that connects bones. In the vertebral column, ligaments hold the vertebrae together, stabilise the spine, and protect the discs. They also allow for flexion and extension of the spine while keeping the bones aligned. The three major ligaments of the spine are:
- Ligamentum Flavum
- Anterior Longitudinal Ligament (ALL)
- Posterior Longitudinal Ligament (PLL)
ALL and PLL are continuous bands that run from the top to the bottom of the spinal column. The Ligamentum Flavum attaches to the laminae of each vertebra.
The Spinal Nerves
The hollow spinal canal contains the spinal cord, fat, ligaments and blood vessels. The spinal cord runs from the brainstem to L1. An adult human's spinal cord is around 45cm long and as thick as a thumb. In the lumbar spine, the spinal cord fibres separate into the cauda equina and continue down through the spinal canal to the tailbone before branching off to the legs and feet.
Under each pedicle, a pair of spinal nerves exits the spinal cord and passes through the intervertebral foramen to branch out to the body. Spinal nerves are numbered according to the vertebrae above which they exit the spinal canal (i.e. C1-C8, T1-T12, L1-L5, S1-S5, and one coccygeal nerve).
Each spinal nerve has two roots:
1) Ventral (front) roots carry motor impulses from the brain to the body;
2) Dorsal (back) roots carry sensory signals from the body to the brain.
Once a nerve passes through the intervertebral foramen it branches. Each branch has sensory and motor fibres.
The spinal cord both carries motor signals from the brain to the body, and sensory signals from the body to the brain. It can also react without sending messaged through the brain in a spinal reflex. Any damage to the spinal cord can cause loss of sensory and motor function below the level of injury, e.g. a thoracic or lumbar injury can cause paraplegia. Surgeons often remove the lamina of the vertebral arch (laminectomy) to access the spinal cord and nerves to treat stenosis, tumours or herniated discs.
Dermatome Pattern
Spinal nerves innervate specific areas of the body and form a striped pattern across the body called a dermatome. Doctors use this pattern to diagnose the location of a spinal problem based on the location of pain or muscle weakness. For example, leg pain is usually associated with damage to the L4-S3 nerves.
Artificial Disc Replacement Surgery
ADR surgery can relieve pain caused by pinched nerve roots or as a result of diseased/damaged discs. The artificial implants are made out of metal and polyethylene. Typical motives for receiving ADR surgery are:
- Disc herniation, when a fragment of the disc nucleus is pushed out of the annulus.
- Collapsed discs, when an intervertebral disc suffers a loss in overall height from degeneration through age or injury.
- Lumbar Facet Syndrome, an irritation of the lumbar spine caused by dysfunction at the level of facet joints.
- Disc dehydration, a common feature of degenerative disc disease.
- Cervical spinal stenosis, the narrowing of the spinal canal at the level of the cervix, often due to chronic degeneration.
Procedure
References:
1) Articular processes. Retrieved 24 February 2020, from https://en.wikipedia.org/wiki/Articular_processes
2) Keith Bridwell, M., & Gerard Malanga, M. Facet Joints of the Spine's Anatomy. Retrieved 24 February 2020, from https://www.spineuniverse.com/anatomy/facet-joints-spines-anatomy
3) Ligament. Retrieved 24 February 2020, from https://en.wikipedia.org/wiki/Ligament
4) Ligament. Retrieved 24 February 2020, from https://en.wikipedia.org/wiki/Ligament
5) Spine Anatomy, Anatomy of the Human Spine. Retrieved 24 February 2020, from https://mayfieldclinic.com/pe-anatspine.htm
6) Thomas Scioscia, M. Spinal Discs. Retrieved 24 February 2020, from https://www.spine-health.com/conditions/spine-anatomy/spinal-discs
7) Vertebra. Retrieved 24 February 2020, from https://en.wikipedia.org/wiki/Vertebra
8) Vertebral Body. Retrieved 24 February 2020, from https://www.spine-health.com/glossary/vertebral-body