The working unit of the spine is formed by two vertebrae (blocks of bone) and the disc in between. The blocks of bone articulate (connect) with each other via two facet joints (Roof Tiles) and the disc sandwiched between the bodies of the vertebrae. They work together in conjunction with other structures like ligaments and muscles (Core Muscle). 

X-ray of lumbar spine explaining The Working Unit – The doughnut cushion (Disc) outlined in red is sandwiched between two blocks of bone (Vertebra). The facet joints (outlined in yellow) are arranged like Roof Tiles one on top of each other.

The disc, which is similar to a squidgy doughnut, acts as a shock absorber and in the lower lumbar spine bears up to two times the body weight. Hence, weight reduction helps in the overall well-being of the disc. The disc is made of a stronger outer shell (Annulus) and a soft jelly-like inner core called the nucleus. I would say like a jam doughnut, where the outer shell of the doughnut is the annulus and jam inside is the nucleus. The water content of the nucleus maintains the cushion effect and helps us take the stress and strain of being upright and walking. The dehydrated disc is a term used when the water content starts decreasing, and it can affect the normal functioning of the unit itself.

MRI scan of Lumbar spine illustrating a worn out (Dehydrated) disc. The doughnut shaped disc (outlined in blue) is brighter in appearance when compared to one below (outlined in red); this is because it is hydrated or has more water content. The dehydrated disc in red appears to be darker (also called Black Disc), flatter and bulging.

The appearance of a normal disc on an MRI scan appears like an apple. Whereas, a dehydrated disc can be noted on an MRI scan as black disc due to reduced water content and loss of stalk divot. Dehydration of the disc leads to damage of the annulus (Annular tear) and escaping of the jam part (Nucleus) of the doughnut. The escaping or protruding disc is called disc herniation or slipped disc (Read Further). 

MRI scan of Lumbar spine comparing normal disc with dehydrated disc

The facet joints are roof tile like in structure. The two tiles have inner lining cartilage, as we have in any other joints like hips and knees. The tiles slide with every movement we make and are helped by slippery fluid (Synovial fluid). Like any other joint, they are prone to degeneration (arthritis) and pathological conditions (i.e. inflammation and infection). The joint is encased in a tough tissue called the capsule, which has nerve endings. These nerve endings send pain sensation to the brain when the joint is swollen, and is the area of target during the facet denervation procedure (Burning of nerve ending) for back pain management.

CT scan of Lower (Lumbar) Spine and Upper (Cervical) illustrating facet joints.Facet joint of the neck is  outlined in blue to demonstrate the Roof Tile pattern. The lumbar spine facet joint (outlined in red) is lined with cartilage (Blue) to facilitate movement at this joint.

The Core muscle structures help us maintain balance and keep us upright. We have evolved from apes to two-legged human beings. Various layers of abdominal or tummy muscles on the front of the spine, along with diaphragm (breathing muscle), works along with back muscles to maintain our upright posture. Age-related postural changes could cause increased strain on these muscle groups, especially the back.  This abnormal strain causes back pain and related manifestation (Read Further)

Spinal Surgeon

Mr. N K Rath

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