The spinal column along with all its structures is built to withstand the forces of being a human being on this planet. This includes not only gravity but also the forces of sustaining life within that gravity force. This includes activities such as sitting, standing, walking, working at your job, playing sports, caring for your children, etc etc. The degree to which your spinal column can absorb, disperse and handle the forces which barrage it every millisecond of your existence factor greatly in how much stimulation pain receptive structures receive and how much pain your back generates. It also begins to give us an understanding of prescribing a long term treatment to back pain. It’s important to understand these forces, what causes them, how the body is built to respond to them and where structures fail through injury and overuse. This failure is what gives rise to back pain, which in many ways is a reaction to the bodies decreased capacity to handle external forces acting upon it.
Gravity and compression forces
Besides being a protective housing for the spinal cord the vertebral column also acts as protection against compressive forces, that is forces pushing down on us. The most obvious compressive force is gravity itself, but compressive forces are numerous, for example carrying a weight would apply a compressive force to the spine.
The spinal column has many ways to deal with compressive forces. When viewed from the side we can see the spinal column is not straight, but rather has several large curves in all of its sections.
These curves help to disperse the forces of gravity and compression In addition there are also several elastic components to the spine which help to disperse the force of compression. The vertebral body itself has a degree of compression, that is it actually compresses when a force is applied to it. Most people think of bone as being an immovable solid object, but the vertebral bone is made up of cancellous bone which is also known as “spongy bone”. The vertebral disc is made up of fibrocartilage which is also able to be compressed. Finally the articular facet joints are also made up of fibrous material which withstands compression. All of these structures provide their own elastic compression, which absorbs external compressive forces such as gravity or weight bearing activities. Vertebral compression will become an incredibly important concept later on when we talk about low back pain due to disc issues and also due to facet joint pain.
Shear Forces and lumbar flexion
In addition to compressive forces the spine has to cope with sheer forces. A sheer force is the force of one object in a horizontal direction against another object. In the spine with its “stacked” vertebrae a sheer force would be a vertebrae moving forward or backwards against the vertebrae below it.
Shear forces in the vertebral column are most commonly seen in lumbar flexion, that is when you bend forward at the low back your vertebrae tend to move forward over the vertebrae below it. What resists this shear force are your discs and facet joints. Other secondary but important resistance to this shear force is provided by ligaments such as the supraspinal ligament, interspinal ligament, and the posterior longitudinal ligament among others.
- Important ligaments which help to avoid shear forces
Shear forces, similar to compressive forces are extremely important in our discussion of low back pain.
Rotational forces are quite important, especially when examining the spinal disc. The disc is basically a structure which is formed of a gelatinous center with concentric fibro-cartilaginous rings which encircle the center.