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CHAPTER 1: DEFINING TERMS

The first step in understanding disc problems is to define terms commonly used by doctors to describe the patient's condition. Doctors are often not very good at explaining things to their patients, and some do not even make the attempt. Let's begin with the explanation of commonly used terms that you might hear your doctor say, or you might read on a medical report.

Disc (or Disk - either spelling can be used):
The disc is a soft-tissue structure that lies between the bones of the spine (the vertebrae). It has two components. There is an outer ring of cartilage fibers, similar to the tissue that makes up the ear. This outer ring is firm but flexible and it is called the annulus. You might hear or read terms like "annular bulge" or "annular tear", and these terms refer to this outer "retaining wall" of the disc.

The second part of the disc is an inner core that is a gel-like material that is normally the consistency of toothpaste. This inner core is called the nucleus pulposis, or simply the nucleus. The nucleus is what gives the disc its shock-absorbing quality.

The following illustration shows the location and components of the spinal discs in the low back.

Figure 12. Spinal anatomy showing side view of spine and vertebra and top view of disc

Disc Herniation or Bulge:

When the disc is damaged and the retaining wall (the annulus) is weakened, the pressure from the gel in the center (the nucleus) will cause the retaining wall to bulge outward. Technically, the terms "herniation" and "bulge" can be used interchangeably, but many doctors will call larger bulges "herniations" and smaller bulges "bulges". For particularly mild bulges, you may see the term “diffuse bulge”. Another term that is sometimes used to describe a bulge is a disc "protrusion". Many doctors will use the term "protrusion" when the disc bulge is very focal - where the distance the bulge sticks out from the disc is about the same or is more than the width of the bulge.

"Slipped" Disc:

Although a commonly-used term, a disc does not actually "slip" - the disc is very firmly attached to the bones above and below it and it would take a complete dislocation of the spine to displace it (in which case, you have far greater problems than the disc itself). A "slipped" disc is actually a herniated or bulging disc as just discussed.

Figure 13. Disc herniation / near rupture. Top view of vertebra and disc showing disc bulging and pressing against spinal cord and nerve.

Ruptured Disc:
When a disc is damaged to the point where the retaining wall (the annulus) is breached, the disc is said to be ruptured. This is sometimes called a disc
"extrusion". This is a more severe injury than a herniated or bulging disc, although some doctors will tell patients they have a ruptured disc even when it is only bulging/herniated. In the case of a true ruptured disc, the inner gel (the nucleus) actually leaks out of the disc. In some cases, part of the gel will come all the way out of the disc and separate from the main part of the disc. This is what is called a "sequestrum" (also called a sequestered fragment or a sequestered disc).

“Pinched” Nerve:
Like the term "slipped disc", "pinched nerve" is also a commonly-used term that is not really an accurate description of what actually happens. Most people imagine a nerve being trapped between bone surfaces and being "pinched". It is extremely rare for a nerve to become pinched between bones.

In the case of a bulging or ruptured disc, the disc material may push up against the sensitive part of the nerve where it branches off of the spinal cord (see Figure 2) and cause sufficient compression to cause pain, tingling, weakness, and possibly other symptoms. It has been shown that the part of the nerve right where it branches off of the spinal cord (the "nerve root") is so sensitive that just the weight of a dime can cause changes in the nerve's function. In some cases, the nerve may also have pressure placed on it by swelling caused by the body's inflammatory response to the disc injury. If you have ever hit your finger with a hammer or stubbed your toe really badly, you know all too well how much pressure (and pain) swelling can create.

Disc Degeneration:
Besides bulges and ruptures, the other main problem that can occur with discs is degeneration. Degeneration is a gradual loss of fluid from the disc over a long period of time that results in the disc becoming thinner and stiffer. This is the long-term effect of a disc injury and can be seen with or without a disc bulge or herniation.

Degeneration is characterized by a loss of fluid from the disc gel and is sometimes referred to as disc "dessication" (meaning a drying out of the disc).
When discs degenerate, other problems often occur as a consequence. As the disc gets thinner, the joints at the back of the spine (called the "facet joints") start to get compressed and the joint surfaces rub together more than normal. This can cause damage to the joints and can in turn result in extra bone growth around them. This extra bone growth can cause bone spurs or ridges around the spine. When this occurs around the spinal joints, it may be called "facet arthrosis" ("arthrosis" simply means "abnormal condition of a joint"). When this occurs, it will often be called degenerative arthritis.

Figure 15. The pictures above compare a normal section of spine (on the left) with a section of spine with disc degeneration (on the right). Notice that the disc in the normal spine is quite thick, while the degenerated disc is almost completely gone.
More importantly, notice how thick and healthy the nerve is in the normal spine, while the nerve in the degenerated spine is shriveled up because it is literally being strangled by the excess bone growth around it. Also see how jagged the edges of the bones are in the degenerated spine. As the disc continues to decay and die, the bones will actually grow together as one solid piece, further compressing the nerves.

Stenosis:
Stenosis simply refers to a narrowing of an opening. This term is also used to describe changes in blood vessels as well as changes in the spine. In the spine, there are two main types of stenosis that describe which openings are being narrowed.

"Spinal stenosis", which is also called "central canal stenosis" refers to a narrowing of the opening where the spinal cord passes through a section of the spine.

"Foraminal stenosis", also called "neural stenosis" or "neuroforaminal stenosis", refers to a narrowing of the openings where the nerves exit the spine.

Stenosis can be caused by a few different things. One cause is disc bulges and ruptures, where the disc material takes up part of the opening in question (as shown in Figure 2). Stenosis can also be caused by additional bone and/or soft tissue formation seen with degenerative arthritis. Finally, although rare, stenosis may occur from a tumor, or anything else that takes up space in the openings of the spine. One other term that relates to stenosis is "patent", and this refers to the openings of the spine being normal.

Miscellaneous Radiology Terms:
There are various terms you may see on X-ray or MRI reports that have little to do with the condition of the spine. MRI reports in particular often make reference to the techniques used to produce the images. For instance, you may see reference to "T1" or "T2" "weighted images", or "spin echo". These have nothing to do with the condition of the spine. Be careful not to confuse references to "T1" or "T2" "weighted images" or "views" with the spinal bones T1 and T2 (see next section).

Spinal Numbering:
The adult spine is usually composed of 24 movable bones (vertebrae). There are 7 cervical (neck) bones, 12 thoracic (upper and mid back - the bones that have ribs attached to them) bones, and 5 lumbar (low back) bones. The bones are numbered starting at one at the highest portion of each section. In other words, the top bone in the neck is C1 ("C" for cervical) and then they are numbered down to C7 at the base of the neck. The first bone in the upper back is T1 ("T" for thoracic, or sometimes "D" for dorsal) and then down to T12 (the last bone in the spine with ribs). The first bone in the low back is L1 ("L" for lumbar) and then down to the last movable bone at the bottom of the spine, which is L5. In adults, there is a single triangular bone that lies right below the last lumbar bone. This bone is called the sacrum. In children, the sacrum is actually 5 separate bones, with the top of the sacrum being S1. This numbering system for the sacrum remains in effect in adults even though the 5 bones are fused together as one piece. There are a few exceptions where a given individual will have more or less of one or more types of spinal bones, but they usually are numbered as just described.
Discs are numbered by the spinal bones that they lie between. The lowest disc in the spine lies between L5 and S1, and so it is called the L5/S1 disc. The next disc up would be L4/L5 (alternately named L4/5) and so on. In the neck, although there are 7 bones, there is no disc between the first two spinal bones, so the top disc in the neck would be C2/C3 (or C2/3), the next would be C3/C4 and so on down to where the section of the spine changes name and the disc becomes C7/T1.