The long-term success of an artificial disc in the cervical spine is governed by several factors. An important aspect is how well the disc is implanted or fused to the vertebra (bone). The bond formed by this implantation provides a foundation for the stability of the disc in the spinal motion segment. A well-implanted disc is less prone to future complications, such as instability and/or subsidence (caving-in of the disc) into the vertebra.
Artificial discs typically function to transfer loads to the next normal motion segment. However, no artificial disc developed so far can mimic the ability of a natural disc to absorb and cushion loads.
Implant-Enhancing Techniques for Artificial Cervical Discs
Several implantation techniques have been developed to increase the likelihood of a successful disc-bone fusion. Common examples include:
- Surface features. The surface of a disc that contacts the vertebra may be enhanced in order to encourage fixation with the bone to firmly hold the implant in place. A few ways of accomplishing this fusion is by creating:
- Wire mesh
- Specialized coatings. Certain coatings over the bone-contact ends of the disc may be sprayed or applied in order to enhance fusion with the bone. These materials stimulate bone cells to grow and attach on their surfaces. Commonly used coatings include:
- Plasma-sprayed titanium
- Aluminum oxide
- Calcium phosphate
- Screw fixations. Some artificial discs are fixed onto the adjacent vertebrae with screws.
All these techniques have been widely used to enhance the fixation of the disc to the vertebral bone. Some general risks include inadequate fixation, overgrowth of bone on the disc, and screw breakage.
Stability of Artificial Cervical Discs
The stability of an artificial disc refers to its ability to securely remain within a motion segment and effectively transfer loads to adjacent structures.
Factors that may affect the stability of an artificial cervical disc include:
- Design type: In general, the constrained (fused parts) and semi-constrained devices offer good stability.
- Fixation of disc to bone: If the disc-bone fixation is not complete or inadequate, the stability of the disc reduces.
- Subsidence. Subsidence occurs when the disc sags, caves, or sinks into the vertebral bone below it. Subsidence may cause abnormal transfer of loads in the spine and neck pain. This sagging can be avoided by adequately preparing the plates of the artificial disc to fit at the correct angle in the intervertebral space.
- Pseudo capsule. Some discs have a special polyurethane membrane that surrounds the ball in the center of the disc. This membrane, also called a pseudo-capsule, typically holds a small amount of saline that acts as a lubricant within the sheath. This design aids in the stability of the disc as it may act as a shock absorber for the motion segment.
- Vertebral uncinate process: Retaining all of most of the hook-shaped uncinate parts of a vertebra during surgery may lead to greater stability of an artificial disc.
Other causes affecting the stability of an artificial disc include size, surface design, and the technique used for implanting the disc in the motion segment.
Wear Debris of Artificial Cervical Discs
Over time, artificial discs may wear from the continued motion of the cervical spine. Wearing may result in the formation of wear debris—metallic or polyethylene particles that are generated over and dissipated into the body. These debris may lead to adverse effects including inflammatory hypersensitivity reactions, pseudotumor formation, destruction of bone, and loosening of the disc.
Potential causes for the production of wear debris
Production of wear debris depends on:
- Metal on metal design. Friction between the parts of the disc may produce wear debris upon neck movement. Typically, more debris are generated when the plates and ball are all made of metal.
- Materials used. In general, cobalt-chromium alloy produces less wear debris compared to other disc materials.
The formation of wear debris also slowly wears off parts of the artificial disc, leading to its gradual degradation over time. Research shows treating polyethylene with gamma radiation may help decrease the production of wear debris. This type of treatment may, however, lower the mechanical properties of the polymer.1 Some discs have a thin membrane that surrounds the entire disc in order to trap any wear debris, preventing them from being dispersed into the body tissues and fluids.
While the use of these discs in artificial cervical disc replacement surgeries has been largely successful, some cases have reported disc-related complications over time.2 Research suggests that in appropriately selected patients, the efficacy of artificial cervical discs is comparable or may even be better than artificial cervical discectomy and fusion (ACDF).3,4