Wear and tear of the spine
The locomotion of all vertebrates - including humans - is linked to the intact structure and function of the spine.
With advancing age, an unfavorable lifestyle such as lack of exercise or smoking can lead to signs of wear and tear of the spine (degenerative changes) and thus to pain and movement disorders. A genetic predisposition can also promote these symptoms.
These changes usually begin in the intervertebral discs, the elastic shock absorbers that lie between the bony vertebrae and allow the spine to move thanks to their deformability.
Structure and function of the intervertebral disc
A healthy intervertebral disc consists of a rough, firm ring of connective tissue fibers (collagen), the "annulus", which surrounds the disc nucleus. The nucleus has a high physical water-binding capacity and therefore consists of 80 percent water. Since water cannot be squeezed (compressed), the nucleus is not really soft, but it is so deformable that it can shift within the fibrous ring and stretch the fibrous ring (Fig. 1).
Figure 1: Displacement of the intervertebral disc nucleus when bending forward (Image: R. Schönmayr)
There is high load pressure inside the intervertebral disc, which is why it has no blood vessels or nerves.
Nevertheless, metabolic processes take place here that serve to supply the cartilage cells with nutrients and remove metabolic products. This happens with the tissue fluid that is pressed into the bones of the vertebral bodies via small sieve-like openings when the intervertebral disc is loaded. When the load is relieved - especially at night when lying down - tissue fluid is then sucked out of the vertebral bodies again.
The more frequent the alternation between loading and unloading, the better the metabolism of the intervertebral discs. Or vice versa: the less the intervertebral discs are loaded and unloaded, the worse the exchange of substances and the faster the ageing and wear of the intervertebral discs progresses.
Causes of a slipped disc
Similar to a car tire that loses air, the intervertebral disc loses its ability to bind water and thus its water content. It collapses and becomes brittle and cracked.
Because the intervertebral discs are no longer bulging, the vertebrae can now perform small slipping movements, which also place unnatural strain on the fibrous ring and ultimately damage it.
Tears can occur in the fibrous ring, through which parts of the nucleus can be pushed out - resulting in a herniated disc.
As the fibrous ring is thicker at the front and sides than at the back, herniated discs occur more frequently at the back of the vertebral bodies, where the spinal canal and nerve channels are located. This means that in addition to the back or neck pain caused by a herniated disc, nerve pain can also occur, which manifests itself as severe pain that sometimes feels like electricity.
Symptoms of a herniated disc
The pain is felt in the region of the body where the affected nerve runs. This is why herniated discs in the lumbar spine cause leg pain and herniated discs in the cervical spine cause arm pain.
This pain is very severe and usually depends on the position and movement of the spine, which is why people suffering from pain adopt a cautious relieving posture and avoid movement and strain on the spine. The back muscles tense up strongly as a reflex to prevent movement of the affected section of the spine.
If the prolapse presses very hard or for a long time on a nerve, its function can be disrupted. This can manifest itself in
- tingling sensations (formication),
- numbness
- and even numbness
but paralysis can also occur in the muscles supplied by this nerve.
Very large prolapses in the lumbar spine can also damage the nerves that control bladder emptying and the sphincter muscle in the rectum. This results in the inability to empty a full bladder and control bowel movements.
Prolapse of the cervical spine can also lead to pressure on the spinal cord. As a rule, this does not immediately mean paraplegia, but it can lead to
- considerable gait instability (ataxia),
- increased muscle stiffness in the legs (spasticity) and
- sensory disturbances in various parts of the body
can occur.
As long as herniated discs do not cause nerve damage, attempts can be made to treat them with a whole arsenal of non-surgical "conservative" measures. These include
- Medication,
- physiotherapy,
- manual therapy or
- physical applications (heat, water, stretching).
Surgery should only be considered if these therapy attempts fail or if there are signs of nerve damage.
Figure 2: Herniated disc of the lumbar spine in magnetic resonance imaging (MRI) (Image: R. Schönmayr)
Before an operation, imaging examinations are required following a physical and neurological examination. These are X-rays and either magnetic resonance imaging (MRI) or computer tomography (CT). Sometimes a contrast examination (myelography) with subsequent computer tomography is also required.
With these image examinations, the prolapse is visualized and its size and exact location can be seen (Fig. 2). This knowledge is essential for planning the subsequent surgical procedure.
During surgery on the lumbar spine, the prolapse is located and removed endoscopically or microscopically via a very small access in the middle or slightly to the side of the back. Sometimes it is also necessary to remove friable material from inside the disc.
Goals and prognosis of an intervertebral disc operation
After the operation, leg pain usually disappears quickly, as the pressure on the nerves has been removed. Nerve damage, on the other hand, takes longer to recover; if the damage is very severe, it may not improve at all or only partially.
As far as back pain is concerned, the success of the operation is not always so good: the disc damage has not been repaired and the unnatural slipping movements have not been eliminated. If it is not possible to stabilize the diseased disc by consistently building up the back muscles, the result can be permanent back pain that increases with movement and strain.
This can lead to subsequent damage to the vertebral joints(osteoarthritis) and the bony end plates of the vertebral body(osteochondrosis), which in turn can cause pain. It is obvious that inadequately trained back muscles also cause pain, especially in the case of incorrect posture or curvature of the spine.
Figure 3: Spondylodesis = fusion of two vertebral bodies by screwing the vertebrae together and inserting a plastic and bone implant between the vertebral bodies via a surgical approach from behind. (Image: R. Schönmayr)
If such "chronic" back pain cannot be managed in any other way, there are various surgical procedures that can be used.
Spondylodesis for a herniated disc
Spondylodesis, in which two adjacent vertebrae are firmly joined together, is an operation that has been known for decades and has been further developed using modern technology (Fig. 3). This "stiffening" leads to bony fusion of the two vertebrae together, while the other vertebrae remain mobile, so that the mobility of the spine is only slightly impaired overall.
Nevertheless, this method has disadvantages: On the one hand, not all patients who undergo surgery are free of pain, and on the other hand, there is increased strain on the neighboring intervertebral discs. Particularly if these are already showing signs of wear and tear, this can increase over time and lead to corresponding symptoms.
In order to avoid an unfavorable redistribution of the load after a spinal fusion (spondylodesis), it has long been considered to replace a damaged intervertebral disc in such a way that the mobility is maintained - or even better - restored as with a healthy intervertebral disc after removal of the prolapse.
Figure 4: M6 disc prosthesis with elastic core, surrounded by a polyethylene mesh similar to a natural fiber ring. The wedges on the end plates serve to anchor the prosthesis to the vertebral bodies. (Image: SpinalKinetics Inc., manufacturer of the M6)
However, the first attempts at an artificial intervertebral disc (also known as a disc prosthesis) were unsuccessful. In the 1950s, doctors failed to insert a steel ball in place of the disc. Artificial intervertebral discs have been used in the lumbar spine on a larger scale since 1984. Dr. Büttner-Janz and Prof. Schellnack in what was then East Berlin developed a construction consisting of a plastic sliding core and two prosthesis plates made of a metal alloy, which was placed between two lumbar vertebral bodies to replace the defective disc.
Initial problems with material durability have been overcome over the years, so that the 3rd generation of the "Charité Disc" available today is a mature implant that is used worldwide. There are now also a large number of artificial discs from other manufacturers and based on other design principles.
In my opinion, the most interesting design at present is a prosthesis that has an elastic core between two titanium plates, which is surrounded by a fiber mesh that is very similar to a natural annulus (Fig. 4).
If the movement pattern of this artificial disc is tested in a test apparatus and compared with that of an intact disc, the similarity of the movement pattern is striking.
Should everyone with a damaged intervertebral disc receive such an artificial disc? The answer is no. The operation to insert an artificial disc requires a relatively complex approach from the abdomen, during which the large blood vessels in front of the spine(abdominal aorta and large abdominal vein) have to be mobilized and moved to the side.
A nerve plexus that runs in front of the vertebrae and is important for sexual function can be injured and lead to disorders.
In the case of a "normal" slipped disc, the usual, significantly less stressful disc surgery is still sufficient, as it is successful in around 90 percent of cases.
It has also been shown that people with advanced disc damage who have already undergone bony changes to the vertebral bodies and especially to the vertebral joints are less likely to benefit from an artificial disc.
The reason becomes clear when we look at the vertebral joints. These joints connect two vertebrae to each other, providing guidance and limiting movement. Like all joints, they are also subject to wear and tear and can cause very characteristic pain, particularly pain associated with movement (Fig. 5).
If these joints are already damaged, the increased movement and strain that occur after the insertion of an artificial disc often lead to increased pain. In these cases, it is actually better to immobilize the mobility by means of a stiffening operation.
Figure 5: Advanced damage (osteoarthritis) to the vertebral joints (Image: R. Schönmayr)
Who is the right candidate for an artificial disc? Patients with chronic back pain that cannot be adequately treated conservatively and is mainly caused by the damaged intervertebral disc.
The bony structures of the neighboring vertebrae should not yet show any advanced damage - the vertebral joints in particular should still be functional and able to bear weight without pain. Whether this is the case can be checked before a planned operation.
Under these conditions, patients who have already undergone surgery on the affected intervertebral disc may also be considered. Patients who have disc damage at several levels are also eligible, although there are indications from various publications that the results are not quite as good afterwards.
Sometimes it can also make sense to combine surgical procedures: an intervertebral disc with severe concomitant damage to the bony structures is treated by means of vertebral body fusion (spondylodesis), while the neighboring, less severely damaged level (segment) is treated with an artificial intervertebral disc.
This has the advantage that the next overlying disc is not exposed to increased mechanical stress, i.e. the domino effect of "subsequent instability" is avoided.
At the neck, the static load on the spine is lower and the mobility is more pronounced; this is why an artificial disc is used more frequently here. Nevertheless, the bony wear and tear should not be too advanced here either.
In younger and middle-aged people in particular, herniated discs of the cervical spine also occur without severe joint damage (Fig. 6), so it may make sense to use an artificial disc in this group.
Figure 6: Herniated disc of the cervical spine in magnetic resonance imaging (MRI) (Image: R. Schönmayr)
The installation of an artificial disc in the cervical spine is also significantly less complex or stressful than in the lumbar spine. The diseased disc and the prolapse are removed - usually under a microscope - via a very simple approach from the front of the neck, in which the easily displaceable soft tissues of the neck are carefully pushed apart.
The disc space is then straightened slightly until it is back in the correct position and the prosthesis is inserted under fluoroscopic control and its mobility is checked (Fig. 7).
Postoperative stabilization of the cervical spine with a cervical collar (orthosis) is not necessary and the patient can leave the hospital after a few days.
After about 3 months, the artificial disc is so firmly anchored that all everyday activities are possible again. When certain sports can be practiced again must be decided on a case-by-case basis by the surgeon.
Figure 7: Implanted disc prosthesis (M6) between the 5th and 6th cervical vertebrae in the postoperative X-ray image from the front and from the side (Image: R. Schönmayr)
Possible complications of an operation with implantation of an artificial intervertebral disc should not be neglected. A distinction must be made here between complications that can occur during the surgical approach and generally during any such operation and those that can arise from the implanted prosthesis. Fortunately, such complications are very rare and, based on previous experience, the material durability of the implants does not appear to be a problem.
It is not yet sufficiently known what happens to a prosthesis when the bone quality of the vertebrae deteriorates with age (keyword: osteoporosis) and the relatively hard metal components could sink into the bone.
However, it can be concluded from similar cases with other implants that, in the worst case, ossification occurs around the implant, i.e. a spontaneous, slowly developing stiffening - exactly the result that a stiffening operation would have brought about.
Such spontaneous bony bridging has been observed with varying frequency in the various models available - admittedly without any detrimental effect on the surgical outcome in terms of improvement in pain and neurological symptoms. However, mobility in the operated segment is lost.
What you should know is that the success rate in terms of pain and neurological symptoms with a fusion operation is just as good as with the implantation of a disc prosthesis.
Whether maintaining mobility in the operated segment can prevent wear damage to the neighboring intervertebral discs in the long term has yet to be proven by long-term studies.
In summary, it can be said today that the artificial intervertebral disc has found a firm place in the surgical treatment of damaged intervertebral discs and wear and tear of the spine, despite its very limited field of application.
As with all such interventions, it is crucial for success that only patients who are suitable for the procedure are selected. This requires very precise preliminary diagnostic examinations and a great deal of experience on the part of the operating doctor.
However, when they are used correctly and implanted precisely, they represent progress and a real improvement in the treatment of disc diseases.
