We’ve probably all had heard someone say before that they “blew out a disc.” In most cases there is some prior acute injury that led to the symptoms or the determination of disc involvement.
This phrase leads to an interesting, but somewhat inaccurate, perception about disc protrusions and the role they play in pain and injury complaints. A brief review of intervertebral disc mechanics shows how the “blown-out disc” concept is not very accurate.
The intervertebral disc is designed to manage compressive load between adjacent vertebrae. As a result, it is designed to handle very high compressive loads. The idea many people have when they hear about a “blown-out disc” is that some acute force caused a previously normal disc to rapidly protrude to one side as a result of the sudden force. In the lumbar region the blown-out disc is frequently suggested to result from lifting heavy loads and having sudden back pain.
When a disc protrudes outside of its normal boundaries it is called a herniation. Consequently, the blown-out disc would be a rapid and severe herniation. One reason this concept is not accurate is that most people don’t realize that acute herniation of a healthy disc is very unlikely.
In addition, the type of force loads required to cause the disc herniation generally don’t match those of the injury condition. It is certainly possible to have an acute disc herniation, but it is much less common than many realize.
The biomechanical structure of the disc is designed to resist compressive loads. The disc is also designed such that rapid compressive loads are resisted more significantly than loads applied over a prolonged period of time. It is the load applied over a prolonged period that most commonly leads to disc herniation.
For example, it is more detrimental to the lumbar discs to sit in a slumped posture every day than it is to lift something heavy all of a sudden. If there is an acute disc herniation, it’s generally the last straw, occurring to a disc that was already protruding and seriously weakened.
Researchers investigated how much pressure applied to a lumbar intervertebral disc would cause it to burst or protrude. They placed cadaver specimens of lumbar spine in a pressure vise and measured the increased pressure load on the spine. What they discovered from these experiments was quite surprising.
Pressure was continually increased, but before the disc ruptured, the vertebral endplates of the bone actually fractured. This suggested that the disc was actually more resistant to the pressure increase than the bone.
Commonly used expressions like the blown-out disc (or slipped disc; also erroneous because the disc doesn’t slip), often lead to inaccurate models of pathophysiology. As a result, treatment decisions may be based on these incorrect models and lead to poor outcomes.
When you hear these types of phrases, make sure you understand the real underlying pathology and make treatment decisions based on accurate physiology.