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As we get older, the spongy discs between the vertebrae in our spines (see spine anatomy sidebar below) slowly dehydrate. They become less able to cushion against the shocks of daily living. This happens often, and without symptoms. Surprisingly, studies using magnetic resonance imaging (MRI) reveal that about a third of people in their thirties show signs of disc degeneration.
By the time we reach our seventies, degenerated discs are common. About 70 per cent of people in that age group are affected. As gravity makes vertebrae press down on the discs, we literally feel the weight of years in our spines. Indeed, up to several centimetres in height may be lost as the discs and spine settle with normal aging.
Disc thickness is not the only change to affect the spine with age. Sometimes, the tough outer lining (annulus fibrosus) that encircles the disc’s spongy centre weakens and tears.
The smooth gliding surfaces of the facet joints may also roughen and undergo bony remodelling, much as osteoarthritis thickens the knuckles. After years of tiny movements, even the dense bone of the vertebrae can wear at the edges. Bony spurs may also develop, irritating surrounding soft tissues.
Such structural changes can lead to spinal stenosis, a narrowing of the spinal canal’s interior. Like degenerating discs, spinal stenosis becomes common as we age. Many people have few symptoms.
Over time we all experience changes in the structure of our spines. As the spine ages, a disc bulge is likelier to become a more serious herniated disc. (This is also referred to as a ruptured, extruded, sequestrated or slipped disc.) Still, a disc bulge or even herniation is common, and often does not lead to significant clinical symptoms.
Disc herniation occurs when a weakness in the structure of the annulus fibrosus gives way. A small portion of the inner disc’s more gelatinous spongy centre is pushed out (extruded). It is a bit like squeezing toothpaste out of a tube once you have broken the aluminum seal. Many people with this problem do not remember a specific event - pain just appears one day. Disc tissue is most often forced out between the L4 and L5, or L5 and S1 vertebrae. If disc tissue pushes into the spinal canal and presses on nerve tissue, severe sciatica may result.
The sciatic nerve system branches out from the low back between the L4, L5 and sacral (S) vertebrae, and runs down through the buttocks and legs into the feet. Depending on which area of the sciatic nerve system the bulging disc is pressing against, the pain will follow a distinct pathway down the leg towards the foot. These pain patterns help diagnose where the problem disc is located.
Sciatica can vary from mild tingling to completely disabling pain. Sneezing, coughing or laughing can cause a stab of pain. Standing or sitting for long periods make pain worse. Nerve pathways that control the bladder and bowel can also be affected, as they branch off in the lower back and sacrum.
Before considering spine surgery to relieve sciatica symptoms, most doctors will prescribe treatment that might include:
If symptoms do not significantly improve after trying conservative treatment (see sidebar), surgery may be needed. Discs have a very limited capacity for self-repair and do not regenerate. Once the disc is squeezed out, it cannot be pushed back in — although the body often absorbs a portion that has extruded. With a herniated disc, the extended portion of the disc that is pressing on a nerve root may require removal. The bulk of the remaining disc is left intact to act as a shock absorber. Occasionally, sciatica eases even though the herniated part of the disc does not shrink. In such cases, the nerve has adjusted to its new environment.
The most common operation that spine surgeons perform is called a microdiscectomy. Done for severe disc herniation, this relieves pressure on the affected nerve root.
First, the ruptured disc is located using CT (also known as CAT) scans or MRIs. This helps to plan for surgery. During surgery, the space between the affected vertebrae is entered through the back of the spine. To get 
there, a bit of bone from the bottom and top of the adjacent lamina often must be cut away. The lamina forms the arching walls that make up the back of the spinal canal. Using this widened entry, the surgeon searches for the nerve root that is under pressure. Once it is found, the nerve fibre is protected and gently teased to one side to allow access to the ruptured portion of the disc. Finally, the extended portion of disc tissue is removed, leaving the remaining disc largely intact. Taking pressure off the nerve root helps stop the irritation and creates more room for the nerve.
Rates of recovery from surgery vary. Patients having routine disc surgery often go home the same day. Depending on age, general health and motivation, some return to their daily routines in a few weeks. Others can take three to six months to recover.
Partial removal of the disc can affect the spine’s ability to carry heavy loads. The remainder of the disc must adapt to make up for the missing part. The risk of another portion of the disc herniating is approximately 15 to 20 per cent. Patients can expect significant improvement following surgery, but they may not have a 100 per cent recovery.
After decades of wear and tear, the spine’s vertebrae may no longer align properly. They are like a wobbly column of building blocks. One vertebra may slip forward on top of another, grinding back and forth, often putting pressure on nerve roots. Thanks to this, spinal fusion is the second most common procedure that spine surgeons perform. In this surgery, a section of the spine is immobilized to provide stability and pain relief. Usually, it is needed in the lower back.
The surgeon places a bone graft along the spine segment to be fused. The graft may come from the patient, an organ donor, or be made of a synthetic material. As it can take a year or two for the graft to fully integrate into nearby bone, metal alloy (often titanium) rods and screws form an internal splint to keep the vertebrae in place. Corrective wedges made of metal alloy or synthetic materials may be placed between two vertebrae to restore balance and supply more area for additional bone graft. Once the spine has fused, the rods and screws stay in place unless they cause irritation.
Most people are comfortable after a month or two. However, it often takes six months to a year to really get back to normal.
For stenosis, the goal is to enlarge the spinal canal to provide more room for the spinal nerve elements. As with disc herniations, stenosis can cause severe nerve pain that radiates into the legs. Pain is often made worse by walking and relieved by rest. In people for whom conventional non-surgical therapies do not work, pain due to a compressed nerve can be improved with surgery. However, back pain from the wear-and-tear process that caused the stenosis will remain.
During surgery, ‘surgical decompression’ widens the canal where it has narrowed by removing part of the lamina. If necessary, some disc material and bony spurs will be removed too. This takes pressure off the nerve. If the spine is also unstable, spinal fusion (see above) may be done along with surgical decompression surgery. Recovery can take three to six months.
Patients often ask spine surgeons about success rates. This question must be answered carefully since it is vital to have realistic expectations.
Spine surgery can be very successful. It usually helps pain from nerve compression (the radiating nerve symptoms in the legs). Surgery also relieves painful nerve pressure and stabilizes the spine. However, it is hard to relieve the low back pain that comes from mechanical degeneration. Nor can surgery reverse or cure arthritis – one reason why about one in four people may need a number of spinal procedures throughout life.
You may wonder why we can replace hips and knees but not discs. Artificial discs have been around for some 30 years now. The first ones did not work well at all. Today’s discs are better. However, while early results are encouraging, mid- to long-term results are less so. Much controversy still exists about artificial discs. Part of the difficulty is that often people have more than one problem with their spines.
At present, we do not have a good way to replace discs and joints over multiple levels or do a comprehensive ‘total back replacement.’ Still, with better understanding of hip and knee replacements, we may yet translate that knowledge to successfully treat arthritic backs. Emerging research into biologic strategies holds tremendous promise. For instance, tissue engineering and regenerative medicine may one day slow or ward off degenerative disc disease.
The spine serves as a mechanical connection between the arms and legs. It is also the central pathway for the spinal cord and the nervous system. At each level of the spine, individual nerves branch off from the spinal cord and spread through the body. They allow the brain to control body movement and function.
The spine is a column of 24 bones, which doctors think of in four sections.
Between each vertebra are shock-absorbing discs. They have a gel-like core surrounded by a tough fibrous ring called the annulus fibrosus. This makes the discs both elastic and durable. The lumbar spine and pelvis support much of the body’s weight. This pressure is why most bulging or ruptured discs happen between the fourth and fifth lumbar vertebrae and the sacrum (labelled as L4, L5, and S1).
Seen from above, a single vertebra looks a bit like a bird’s foot. The spinous process, flanked on each side by the transverse processes, could be the toes, while the curved body of the vertebra could be the heel. The lamina is the smooth back covering of the spine between the transverse and spinous processes. Just above the lamina, and level with the top of the body of the vertebra, are two structures like pedestals, called the facet joints. Together, these various parts form an arched central canal through which the spinal cord passes. When the spinal cord reaches the lumbar region, it splits into bundled strands of nerve root.
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