We're still writing about our backs. This week I'm writing about 'slipped discs' in the spine. This is a topic that I get questions from most often among my friends and patients alike.
Patients have always been told that the intervertebral disc (IVD) causes referred pain down the back/ side of your legs. They often think that discs are very fragile and get easily injured.
Almost every single one of my patients are fearful of having a 'slipped intervertebral disc' whenever they have low back pain (LBP). The discs have a really bad reputation for causing significant pain and disability in many people
Are these common beliefs accurate? Let's go through the anatomy of the spine and the IVD.
The IVD consists of a very tough outer layer called the annulus fibrosis (AF). It is made of of several layers of fibrocartilage consisting of Type I and II collagen fibers. The AF protects the soft, gel-like substance in the middle known as the nucleus pulposus (NP). The NP helps distribute pressure evenly across the IVD and prevent excessive forces on the spine.
See how thick the AF is from the picture above? Here's something else you need to know. There is a cartilaginous endplate between the AF and the vertebra (the spine). The endplates hold the IVD in place. It allows load to be spread evenly and to provide attachment to the IVD. This creates a super strong connection to the AF making it impossible for the IVD to 'slip' out of position.
How strong are our IVD's? In a published study on thoracic discs in the young (28 years old plus minus 8 years) , it took about 740 pounds of force to compress the disc height 1 mm. For the older subjects (70 years young plus minus 7 years), it took almost 460 pounds of force. Note that these are on cadavers with the muscles and bones cut away (Stemper et al, 2010).
The endplates also allow for hydration of the disc to take place (see picture above).
Just like your ACL is often injured by shearing forces, it is also shearing forces that is most likely to hurt your discs. Twisting, rotating your back while lifting a heavy load is definitely not recommended.
What happens after your disc is injured? Find out more next week as I write more on that topic.
Reference
Stemper BD, Board D et al (2010). Biomechanical Properties Of Human Thoracic Spine Disc Segments. J Craniovert Junct Sp. 1(1): 18-22. DOI: 10.4103/09774-8237.65477
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