Structure of the Spine
When you think of “the spine,” you are probably thinking about one of two things: the bony column that provides physical support, or the nervous system circuits that the vertebral column protects. The spine is really both of these things, and more.
In spine surgery, the surgeon begins from the outside and works toward the location of the surgical problem. The following description of the spine is organized the same way. The figures are from the 1918 edition of Gray’s Anatomy, which is the public domain and viewable online at www.bartleby.com. Fortunately, changes in surgical technique occur more rapidly than changes in human anatomy.
The vertebral column is made up of stacked bony segments called vertebrae.
The vertebrae are held together by ligaments. The resulting structure is strong and stable enough to let us stand up and sit up, but also flexible enough to allow movement.
Although all vertebrae have features in common, there are clear differences in size and shape along the length of the column.
For example, the thoracic vertebrae are where the ribs attach to the spine, while the cervical vertebrae at the top of the spine have a special joint to allow rotation of the head.
The vertebral column is the outermost covering of the spinal cord, and helps to protect it. The menginges are between the vertebrae and the spinal cord and are made up of three layers: the thick, tough dura mater, the arachnoid, and the pia mater.
The arachnoid is partially separated from the other two layers by the subdural and subarachnoid cavities, both of which are filled with cerebrospinal fluid. These meninges sheath the spinal nerves as they exit between the vertebrae. The pia mater directly contacts the spinal cord.
The cord contains both neurons, the cells which carry information to and from the body, and glial cells, which act as support systems for the neurons.
The cervical and lumbar swellings are where nerves leave for the arms and legs. An injury to the surface of the cord at the cervical swelling can damage the nerves going to the arms but leave the nerves going to the legs relatively intact; this can produce central cord syndrome patients, so-called “walking quads.”
Tumors are classified by where they appear in the spine.
Types of Tumors in the Spine
The anatomical location of tumors can be determined by modern imaging technology, and will determine the surgical approach used. Further information on the tumor’s type and origin can come from tissue pathology, but this requires a tissue specimen to be taken from the patient. What this tissue specimen can tell us is the natural history of the tumor. Even though some tumors can be very aggressive, the 5-year survival rate for spinal tumor patients is greater than 90%.
95% of spinal tumors are not in the spinal cord itself. 55% are in the vertebrae or surrounding tissues, outside the dura mater. 40% are in the dura. The remaining 5% arise in the spinal cord itself.
INTRADURAL EXTRAMEDULLARY TUMORS – inside the dura, outside the spinal cord
65% of intradural extramedullary tumors (themselves 40% of spinal tumor) are due to tumors in the nerve sheath (neurofibromas and schwannomas). Neurofibromas can arise from neurofibromatosis, which is one of the most common genetic disorders in the United States, affecting one in every 3,000 children. Neurofibromas are benign tumors, but can progress to malignancy in no more than 12% of cases, and may take 10-20 years to do so. Schwannomas arise from the “Schwann cells” that produce myelin around nerve axons; nerve roots giving rise to schwannomas are usually useless.
25% of intradural extramedullary tumors occur in the meninges (meningiomas). Of these, at least 75% occur in women. Usually, meiningiomas occur on the side or the back of the spinal cord, which is convenient for surgical access. However, meningiomas near the neck more commonly occur towards the front of the spine. Like neurofibromas, meningiomas are benign.
INTRAMEDULLARY TUMORS – inside the spinal cord
Intramedullary spinal cord tumors are 2%- 4% of all adult nervous system tumors, corresponding to the ratio of neurons in the spinal cord to neurons in the brain. However, the percentage is higher (10%) in the pediatric population, for currently unknown reasons.