Lumbar Artificial Disc Replacement
(ADR)

Rationale

Disc replacement is undertaken to help with symptoms of back pain (and often with leg pain symptoms too) arising from a disc which has become worn and damaged. Pain may be experienced in the damaged disc itself, as well as in other structures such as the small joints at the back of the spine (the facet joints) which can become overloaded as the disc collapses and narrows. This narrowing of the disc, which occurs as the wear and tear gets worse, causes the vertebra above to sink down towards the one below putting extra pressure on the facet joints. The loss of disc height also reduces the size of the openings ('neural foramen') at either side where the nerves leave the spine; this can result in a pinching of the nerve roots leading to increased leg pain and other less common symptoms.

Disc replacement surgery corrects these various problems. The pain-generating disc is completely removed and the artificial disc inserted in its place mimics a normal healthy disc, restoring normal movement at this part of the spine and avoiding the transfer of stress to other parts of the lower back. The height of the disc is returned to normal too, offloading the facet joints and opening out the neural foramen on either side.

The Artificial Disc

Artificial discs have evolved progressively over the years. The very earliest disc replacement, used in the 1960s, was a simple steel ball-bearing and it is rumoured that John F Kennedy had one of these implanted. Experience, together with advances in engineering and materials science, has led to ever more sophisticated designs and there are currently several different forms of disc replacement available. Over the past ten years, Mr Boeree has used both the ProDisc-L and the Charité (pronounced share-ih-tay) artificial discs. Both of these discs have the advantage of having been tried and tested for many years in a large number of centres across the world. The Charité has undergone some modifications recently, principally to make it easier to implant, and the latest form has been renamed the 'In-Motion artificial disc'. An addition to ProDisc, the ProDisc-Oblique, has also been introduced recently, a design which facilitates easier and safer insertion at the L4/5 level and above.

Both the ProDisc and Charité comprise of a very tough central polyethylene core held between two stainless steel endplates. These endplates are fixed to the adjacent vertebrae, by a central keel in the case of ProDisc, and by two rows of 'teeth' in the case of Charité (now called In-Motion). The central polyethylene core is fixed to the lower endplate in the ProDisc, the upper endplate essentially rocking over the domed upper surface of the core. In the Charité, the central core is not fixed to either endplate but instead has domed surfaces on both the top and the bottom which are held between two reciprocal polished concavities in the endplates.

The very latest generation of disc replacement is represented by the M6 artificial disc, available for both the lumbar spine (the M6-L) and the cervical spine (the M6-C). This disc replacement far more closely replicates the anatomical structure, the movement and the mechanical properties of a natural healthy disc.

The design of the M6 is extremely innovative and yet it really draws its inspiration from the natural disc. It has an artificial nucleus to allow for axial compression, which is important for the absorption of shock loads through the spine. Previous disc designs have not allowed compression. The M6 also has an outer annulus.

Just like the annulus of the natural disc, this creates properly controlled movement and its job is to provide progressive restraint at the extremes of movement. The annulus and nucleus together allow for the full combination of bending and translational movements at the motion segment being treated. Disc replacements such as ProDisc or Charité are unconstrained and must rely on other structures such as ligaments and the facet joints to limit movement. These structures may therefore be subject to stress and potentially to gradual wear and tear as a result. Similarly, their design does not allow for translation, permitting movement to only three degrees of freedom (flexion/extension, side bending and rotation). However, the M6's combination of an artificial nucleus and annulus allows for more natural, better quality movement in all six degrees of freedom (hence the name M6). These design features of the M6 disc replacement are shown in the following short video clip.

M6-L Design Features

Although the M6 disc replacement offers a number of potentially significant biomechanical advantages, it must be noted that our clinical experience with this particular artificial disc is still relatively limited when compared to ProDisc and Charité. The M6 has certainly undergone very robust and exhaustive testing in the laboratory but we have not yet amassed the many years of clinical evaluation available for both ProDisc and Charité.

How is the Disc Replacement adapted to an individual?

It is important to select the right implant for every individual. The dimensions from side to side and from front to back will vary according to the height and stature of an individual, and will also depend on the level of the disc being replaced.

The same factors will also affect the height of the implant. Similarly, it is equally important to restore the normal amount of angulation ('lordosis') between the back and the front of the disc since this can affect overall balance and other parts of the spine. There is a spectrum of sizes available in all disc replacements to allow for this wide variation. During the operation, trials are used and checked visually and with x-ray control to ensure optimum size, height and lordosis and that the implant is correctly placed.

 

 

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