Scientific Fact 1 – The 2/3 rule If you were to look at a lumbar spine disc from above, it would look like an onion cut in half. You would see a softer jelly like centre and a system of concentric surrounding rings surrounding this jelly centre. The jelly is called the nucleus pulposis, and the rings are called the annular fibres. The interesting thing about the disc is that only the outer 1/3 of the disc has a nerve supply, the inner 2/3 has no nerves. This means that the inner 2/3 can be damaged, you simply just don’t feel it. However when discs fail, the rings progressively tear from the centre outwards. It is only when the fibres from the outer 1/3 start tearing that you will feel sharp back pain.
This explains many patients situation. Some of you have likely been progressively tearing fibres in the disc. Whatever incident that brought you into the office was the proverbial straw that broke the camel’s back. When the disc starts improving, the opposite happens. The outer fibres repair first. Even when you feel 100%, still give yourself 10 days of being careful as it is quite easy to tear back out to the outer 1/3.
Scientific Fact 2 – Pressure changes. In the 1960’s a clever Northern European by the name of Nachemson performed a neat experiment whereby he measured pressure changes in lumbar spine discs as body position changed. What he found out was that when we stand upright, the pressure on the lowest lumbar disc is about 100kg. When we sit straight, it jumps to 150kg. When we sit slumped (as most of us do), the pressure jumps to 200kg. When we stand and bend slightly over at the spine (imagine picking something up of a chair), the pressure is 225kg. Moral of the story is that activities or occupations that are flexion based will create the most disc pressure and have a greater chance of causing disc damage. The flexion based movements can be sudden or sustained. The 2 cohorts of the population most likely to suffer back pain are office workers (too much sitting in flexion) and manual labour workers (too much load in bending and lifting).
Scientific Fact 3 – Morning Pressure – When we lie horizontal at night, the pressure in our disc drops to about 25kg (on our backs) and 75kg (lying on our side). The decrease in pressure compared with standing results in the disc uptaking fluid. They effectively swell in your sleep. When you awake in the morning you are then actually a little taller for this reason. However, you are also more susceptible to either feeling a damaged disc (this explains why those with fresh disc injuries find it hard to get out of bed) or actually damaging a disc if you lift or bend over. Most acute back pain episodes admitted into Casualty wards occur before 9am in the morning. If you are going to hurt a disc, you have more chance of doing it first thing in the morning.