Updated: Dec 13, 2020
We often hear the term 'slipped disc'. But what is this actually describing and how do spinal discs actually get injured...
Intervertebral Disc Anatomy
Discs are made on rings of fibrous tissue called the Annulus Fibrosis and a middle material (which resembles crab meat which has absorbed water!) called the Nucleus Pulposus. The Nucleus Pulposus is 'viscoelastic' which means under certain conditions it becomes liquified and can deform.
The outer Annulus senses pressure and position so contains nerves which relay signals to the brain, including pain signals. Nerve receptors are also found in the vertebral end plates.
It is normal for the disc to move and change shape based on pressure, for example, when we bend forward pressure is dissipated to the back of the disc and vice versa. The Nucleus Pulposus helps with this pressure dissipation. It is normal for the Annulus to move in any direction. The abnormality comes when we almost 'sprain' the annulus with certain movements e.g. movements too quick, too strong or too often for too long.
Types of disc issues...
The most common areas we see disc issues in are the lower back (lumbar spine) and neck (cervical spine), although they can more rarely happen in the mid back (thoracic spine). Instead of a disc necessarily 'slipping', disc problems can be described using the following terms...
'Bulge'- general widening of the disc in all directions. If excessive, this can cause pain by overstretching of the outer annulus, as the outer ligamentous type region stretches and becomes less resistant to force.
'Herniation' - is defined as a bulge that is confined to less than 25% of the circumference of the disc and can be separated into...
'Protrusion'- when the neck of the disc material is wider than the dome. This is a more substantial bulge. The inner disc can crack a little and the nucleus pulposus can move to the outer portion of the disc.
'Extrusion'- when the neck of the disc material is narrower than the dome. This is when the inner material extrudes out and the outer annulus breaks, like a hernia in the groin.
'Sequestration'- when this material then breaks off and is free floating. This can contain bits of bone from the vertebra, cartilage from the end plate and nuclear and annular material.
It's important to know that discs are living tissues so they can adapt over time, albeit very slowly usually. Herniations can resorb as the immune system produces inflammation as a healing process. This inflammatory response is also why pain is felt. Some research has even shown that the larger the disc herniation, the more likely it is to reduce in size in one year if left alone. This is potentially due to a larger immune system response that is triggered in large herniations, meaning the body is more likely to clear up the issue quicker!
Overall the causes of pain due to discs may be due to a mixture of: Biomechanical factors and changes in load, microfractures, inflammatory response, and some are even looking into a low level of bacterial infection being another cause.
MRI scans- needed or not?
It is important to realize that over 50% of those over 50 years of age, asymptomatic or symptomatic, have disc degeneration shown on their MRI scans (I believe the statistics are even as high as 80% of those at 50 years of age, 93% of those at 70 years of age and around 30% of those under 50 years of age!). So disc degeneration may be seen as 'normal' in a lot of us and doesn't always cause symptoms.
However, research has also shown that disc issues are two times more common in people with lower back pain and MRI scan degenerative changes can correlate with increased likelihood of recurrence of lower back pain. So, although disc issues don't always cause pain, there is still clinical relevance to seeing degenerative changes on MRI scans and correlating the imaging with the patient with symptoms.
'Modic Changes' are a way of describing disc changes seen in MRI scans. There are three levels including:
Type 1- Oedema (excess fluid) in the end plate and bone marrow of the vertebra, with blood vessels moving into the area.
Type 2- Fatty change in the bone marrow.
Type 3- Sclerosis (hardening) of the bone with thickened trabeculae and end plate. Normally this is seen in patients over 50/60 years old.
The tricky bit with imaging is that it is still not easy to always know exactly which specific degenerative changes seen on the scan are actually causing the patients pain, especially when the main complaint is lower back pain alone (without sciatica symptoms e.g. leg pain). For example, it may be the disc itself or could be the small facet joints of the spine very near by instead. Often these structures go hand in hand, so when the disc loses height when degenerating over time, the facet joints take more load which may also cause facet joint degeneration. So with generalized degenerative changes and lower back pain alone it is more tricky.
However, with sciatica (nerve root irritation/'radiculopathy') research has shown that luckily as clinicians we are often able to identify the level and side of disc issues/nerve irritation in 90% of cases, without even needing an MRI scan.
So all in all, MRI scans may only be needed to rule out other pathology and may not be necessary in those with lower back pain alone.
Sciatica and 'radiculopathy'...
As mentioned above, as clinicians we are often able to work out which disc is likely causing patient's pain without needing an MRI scan. This is based on a number of factors including: the distribution of the pain or sensation change (e.g. pins and needles/numbness) in the arm/leg/hand/foot (if it is travelling that far), changes in Deep Tendon Reflexes (when we tap on your knee/ankle/elbow etc with a reflex hammer), muscle strength testing and spinal palpation. Different areas of skin, reflexes and muscles correspond with different levels of the spine e.g. the achilles reflex links to the S1 nerve root, which often correlates with an L5/S1 disc issue.
This is because the nerves that are in charge of our skin sensation, reflexes and muscle control in our arms/legs come from our cervical spine (neck) and lumbar spine (lower back) respectively, via the spinal cord and nerve roots (the thoracic spine mainly supplies the trunk). The nerve roots link together to form larger nerves in our arms or legs, such as the median nerve or sciatic nerve. This is why a disc issue in the lower back can cause pain or pins and needles all the way down in the toes and a disc issue in the neck can cause weakness of grip in the hand!
Disc issues can often cause either irritation/inflammation around the nerve roots or can in some cases cause a true impingement of the nerve root. Pain arising from the nerve root is called 'radicular pain syndrome' and, if accompanied by sensation/motor/reflex changes it can be called a 'radiculopathy'.
Radiculopathies may also be caused by other issues in the spine such as spinal stenosis due to osteoarthritis and subsequent bone spurs, spondylolisthesis (where one vertebra has moved forward on the one below) and other less common conditions. Again, certain examination procedures alongside a detailed history and, in some cases, imaging can help look into these other differential diagnoses.
How can disc issues/radiculopathies be treated?
Chiropractic care (/osteopathy/physiotherapy)- hands on treatment aimed at improving function at the level of and/or around the vertebra with the disc injury. This may include: spinal mobilisation (including flexion-distraction/traction) or manipulation, massage, dry needling/acupuncture and pelvic blocking. This will often be alongside a gradual home exercise program, aimed at improving flexibility and strength around the spine in the long run.
If conservative care like this fails to improve symptoms, further referral for epidural injections (which often consist of steroid and anesthetic) usually follows. This can be done in different ways, some of which are 'transforaminal', 'interlaminar' and 'caudal' depending on if one or both sides of nerve roots are affected.
Further to this, surgery such as a microdiscectomy may be needed for a smaller number of patients (in particular if weakness in the arm/leg is progressing and not responding to treatment).
With all of these options, pain medication may be beneficial for some patients, to manage symptoms alongside care.
And, as always, looking into lifestyle factors such as nutrition, smoking, sleep and stress/depression/anxiety can help in individual management.
'Chiropractic Science' Podcast #050 with Dr Tue Secher Jensen
'The Back Pain Podcast'- 'Do I need an MRI for my back pain?' and 'Discs don't slip!' episodes.
NICE guidelines for management of sciatica: Scenario: Management | Management | Sciatica (lumbar radiculopathy) | CKS | NICE