Imagine for a moment that you were charge with designing a human. Where would you begin? Would you start with a brain? Well . . . the brain can't live without blood supply, so maybe scratch that, lets go with the heart? But wait, the heart needs to pump oxygenated blood, and it can't do that without the lungs. But wait one second . . . you get the idea.
The spine intact is one of the most important aspects of the human skeleton. In fact, the presence of a spine helps differentiate entire class of animals, the vertebrates and the invertebrates (lacking a spine). The spine is a series of bones that provides axial support to the thorax and abdomen, helps to hold an animal in an upright position, and acts as protection to the spinal cord and .nerves
Therefore, in designing a spine, a simple hollow cylinder would seem appropriate, very similar in fact to the femur. This design would connect the thorax and abdomen with the arms and legs, all while providing protection to the spinal cord. But ask Mr. Stick-man how he feels about is spine and he would tell you that he has a terrible time catching his food or making a living as he has no ability to bend, extend or rotate. So how do we modify Mr. Stick-man to make him more functional?
I trust the image will lead you to the answer. Herein lies the design we are after. Instead of one long bone, we have connected several smaller bones by a series of joints, thus allowing for freedom of movement and protection of the spinal cord/nerves. The smaller bones are known as vertebrae. The vertebrae are connected by a series of joints known as facet joints and the intervertebral disc. The disc is not a joint in the true sense of the term. Rather, it is a pad of cartilage the exist between the vertebral bodies. Together the disc, facets, and ligaments help bind the spine together while providing with flexion, extension, and rotation. A video of more detailed anatomy can be seen here: https://www.spine-health.com/video/spine-anatomy-overview-video?jwsource=cl.
The spine then is not one long bone. It is multiple bones connected by multiple joints. This design allows for movement about the axial spine while supporting the abdomen and thorax and protecting the spinal cord. The downside to all these moving parts, however, is that the spine can become very complex to diagnose and treat. This is quite different than the femur. The femur rarely develops pathology because there are no moving parts. It is one long bone that malfunctions only with a severe trauma resulting in fracture. The spine has multiple moving parts. Simple mechanics says that more moving parts can lead to more problems. In fact, that is exactly what we see. The most common cause of chronic low back pain are the moving parts, specifically the intervertebral discs and facet joints.
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