Why an artificial disc?
In the cervical spine, the main reason for designing and implanting cervical artificial discs is to avoid adverse biomechanical effects on adjacent segments (cervical discs) that can result from cervical vertebral fusion surgeries.
Biomechanical in vitro studies have shown that constructs including fusion between vertebral bodies can increase the motion of the adjacent segment and the stress on the adjacent disc.
Clinically, it has been shown that over time the radiographic and clinical deterioration of adjacent cervical discs after cervical fusion is 2% to 3% per year, and it is estimated that between 7% and 15% of patients with previous anterior cervical fusions will require another operation at an adjacent level*.
Therefore, the artificial disc potentially reduces or even eliminates the harmful effects of vertebral fusion on adjacent levels by preserving motion.
Cervical spine biomechanics
The biomechanical factors to consider include the shape of the articular surfaces, their centers of rotation, and the subsequent effect these factors can have on other parts of the cervical spine, including the facet joints.
Regarding the materials from which the disc prosthesis is made, consideration must be given to the composition of the part of the prosthesis that will be in contact with the vertebral endplates, the method of implant fixation to the bone, and the materials used in the moving parts (joint).
Perhaps one of the most important aspects of artificial cervical disc design is understanding the bony anatomy of the cervical vertebra, which is where the prosthesis will be implanted.
After various studies, it is known that the densest bone in the cervical vertebrae is found in the lateral parts of the vertebral endplate. This higher bone density is partly caused by higher flexion loads and high lateral mobility observed in the cervical spine. In fact, bone density was found to be higher in the cervical spine than in the lumbar spine.
Thus, based on these aspects, certain design properties are recommended: (1) the implant must have a sufficient contact surface with the endplate to maximize the contact area with the vertebral body, especially in zones of higher bone density; (2) the implant must have a design that preserves the uncovertebral joints.
The normal movement of a natural cervical disc consists of movement, but limited (restrictive) in six axes: flexion, extension, rotation, compression, translation, and distraction. Although a disc with a high degree of mobility might seem desirable, biomechanical studies have shown that a certain restriction of motion is beneficial in terms of preventing the transmission of excessive forces to adjacent structures (vertebral discs), especially to the posterior facet joints. We then speak of semi‑constrained motion.
Goals of cervical disc replacement
The goals of cervical artificial disc placement are:
1- To maintain or restore disc height and the physiological curvature of the cervical spine.
2- To maintain or restore disc mobility.
3- To avoid long‑term degeneration of adjacent levels.
Which cervical artificial disc should I choose?
First, you should be guided by your surgeon’s experience. Currently, there are many vertebral artificial discs on the market. The latest‑generation discs are those that allow motion in the six aforementioned axes.
It should be noted that in the USA, the FDA has approved the use of artificial vertebral discs at a single level, and at two levels only the Mobi‑C prosthesis is approved. In Europe, however, the CE approves the use of different artificial discs at multiple levels. Up to four levels have been implanted in Europe.
We generally use prostheses with six degrees of freedom such as the M‑6, Baguera, and Mobi‑C.
Source:
*Hilibrand AS, Carlson GD, Palumbo MA, et al. Radiculopathy and myelopathy at segments adjacent to the site of a previous anterior cervical arthrodesis. J Bone Joint Surg Am 1999;81:519–28.
