Functional Properties of Ball and Socket vs. Viscoelastic TDRs.
| Characteristic Compared to Natural Disc | Metal-on-Metal | Metal-on-Poly | Viscoelastic one-Piece |
|---|---|---|---|
| 1. Restoration of normal/adjacent Disc Height | (+) | (+) | (+) |
| 2. Restoration of Disc Angle | (+) | (+) | (+) |
| 3. Mimics Quantity of Motion (ROM) | - | - | (-) |
| 4. Mimics Quality of Motion (stiffness, COR, NZ) | - | - | (-) |
| 5. Stability (Passive Restraint) | - | - | (+) |
| 6. Shock Damping | - | (-) | + |
1. & 2. Restoration of normal disc height and disc angle depends on the assortment of available implants in relation to patient's disc height and disc angle variations. The disc height is most stable in the long run in metal-on-metal discs, followed by metal-on-poly implants. Most viscoelastic one-piece discs can better sustain the disc angle than functional 2- or 3-component discs.
3. No disc has physiological ROM to the different directions (sagittal, frontal, transversal plane). Spherical ball and socket discs imply always hypermobility.
4. There is no disc with complete qualitative physiological features.
5. Stability is not to separate from quantity and quality of motion. The intervertebral motion has much more resistance in viscoelastic discs.
6. Damping function is the pre-condition for any motion in viscoelastic one-piece discs. Material Poly has a low degree of elasticity.