by David Wadsworth

This series of 2 blogs on low back pain caused by disc injury is designed as a resource for patients of Pain Solutions Queensland to refer to in their recovery.

A disc injury causes significant back pain that results in a prolonged recovery time.  It requires careful management to become pain free and just as importantly, remain pain free throughout life.  This is no easy task and requires the patient to be well educated about disc injuries and actively involved in their treatment.

The things you need to know as a patient to successfully overcome disc injuries include understanding the anatomy, how you might inadvertently damage the disc whilst it’s healing and what you can do to assist the disc to repair as fast as possible.  Remember that pathology means permanent structural damage to the disc (see our earlier persistent pain post part 6 for the difference between pathology and dysfunction).  This means you need to learn how to manage the damaged disc for life as discs are not capable of regenerating to their original strength and condition. 

Disc Anatomy:

In the spinal column each vertebrae is linked to the next via 3 joints: a disc at the front and two smaller “facet” joints at the back.  The disc has two main parts:

• A central gelatinous “nucleus” and
• An outer “annulus” that consists of a series of 10-20 strong fibrous rings (arranged in layers much like an onion).

The disc permits small multi-axial movement between the vertebrae.  The high fluid content of the nucleus causes it to exert pressure against the annular rings whose tension constrain the nucleus.   The tension in the annulus also helps to spread the compressive loads from the body and legs evenly onto the vertebral bones.  Excess pressure against the fibrous rings can cause them to tear, often gradually over a period of years, with tears typically happening from the inside-out.

As we age the nucleus tends to dry out.  By middle age it becomes more fibrous than gelatinous, which can see a change in the type of disc injury occurring.

Discs are unique in that they are the largest structures in the body lacking a blood supply.  The blood supply to the disc only reaches the outer layers of the annulus….. so how does a disc get nutrition if it has a minimal blood supply?  Mainly by diffusion of fluid and nutrients, which is aided by cyclical loading and unloading of the spine (as occurs when moving or walking under normal, non-extreme conditions).  This has huge implications for healing and recovery for anyone who has a disc injury.

In general terms there are four types of disc injury:

1. Annular tear: a radial tear or fissure through the fibrous annular rings. An acute tear can be very painful but may not necessarily involve a disc bulge.  The tear can allow the nucleus to push through the annular rings and even exit the normal boundary of the disc (see sequestration below).
2. Degeneration: wear and tear damage to the disc. This may include a number of changes such as dessication (drying out of the disc), annular tearing, generalised bulging, disc losing height (like a car tyre deflating), and formation of bone spurs.  As the degeneration progresses a range of symptoms are possible, depending on several factors.  For example, if a disc bulge or bone spur compresses a nerve on one side, nerve pain may occur down one leg (“sciatica”), whereas if the compressive force is central onto the spinal cord then stenosis (narrowing of the spinal canal) occurs, causing pain down both legs (“neurogenic claudication”).  The inability of the disc to take its normal loads in the spine can lead to additional force on the facet joints at the back of the vertebrae, causing facet joint pain and eventually arthritis.
3. Herniation / Protrusion: is a disc bulge which may be painful due to disc damage, or may cause inflammation or compression of nearby nerves resulting in nerve pain as well as disc pain.
4. Sequestration / Extrusion: occurs when a fragment of disc breaks off. This may cause pain from the disc or from adjacent nerve irritation.

Discogenic Low Back Pain

Now we know a little about the anatomy and types of disc pathology, let’s look at how discs cause pain.

Discs only have nerves supplying the outer rings of the annulus.  Because discs usually tear from the inside-out, the early stages of damage are often painless as the inner annular rings have no nerve supply.  It is only when the damage eventually extends to the outer 1/3 of the disc that you feel disc pain.  As a result sometimes a trivial movement (like bending over to tie your shoe lace) results in the final tear to the outer rings and causes severe disc pain, although clearly most of the damage was caused long before such a minor incident. 

Disc Healing: What you Need to Know as a Patient

Discs struggle to heal because they contain very few cells and almost no blood supply, which are the two things which ARE required if a structure in the body is to repair itself quickly and completely.

Discs heal an annular tear initially by growing capillaries (called granulation tissue) into the outer edge of the tear.  These capillaries take 2-4 weeks to fully form and are fragile, being easily damaged again if the disc is overloaded.  The new blood vessels could be considered the body’s way of creating a supply chain to provide nutrients and the raw materials to repair the tear.

The next stage involves the formation of scar tissue in the outer few millimetres of the annulus (in other words a tear heals with a thin 1-2mm layer of scar which is weaker than the original disc and is no longer arranged in rings with strong parallel collagen fibres).   Scar tissue is type 3 collagen, which is weaker than the type 1 collagen it is replacing.  The scar gradually forms more collagen fibres and the fibres mature (become stronger) over 3-6 months.  Even more strength is noted after 12 months in healing scar tissue.  Think of this healing as the body creating a weak “putty plug” into the tear in the disc – putty is not a very good way to structurally repair a huge crack in a wall, nor does it work terribly well for repairing a disc!

What does this mean for you if you have a disc injury?

1. You must unload the disc FOR A PROLONGED PERIOD OF TIME in order for it to heal. Every time you exceed the fragile strength of the capillaries or scar tissue you may re-tear the healing “wound” and the process starts all over again.
2. How do you unload a disc? See our next post about disc loading and unloading.