The Center Of Rotation (COR) is an important concept in artificial disc replacement surgery and it is often misunderstood. In the three dimensional planar world of movement of the artificial disc replacement, the Center Of Rotation (COR) of concern is parallel to the arms as they are extended horizontally to the patient’s sides. In geometric terms the x-axis of the body, as one person faces another. It involves the bend the body forward and extending the spine backwards. In the natural healthy patient the Center Of Rotation (COR) is posterior to the spinal cord; however, artificial discs are unable to reconstruct the exact Center Of Rotation (Of course, no device can be placed in the spinal canal.) COR of the natural discs for several reasons, despite the construction of the artificial disc. This also includes the elastomer types of ADR.

The key issue of Center Of Rotation (COR) of an ADR implant are the how the load is distributed and whether or not it causes pressure in the facet joints. Improperly placed artificial discs replacements are most often the reason ADR patients experience facet pain. Deviation of more than 1.5 mm from the centerline of the spine (as seen in the AP view) and deviation from the optimal depth makes the patient a candidate for facet pain and possible revision. (Unfortunately many inexperienced surgeons place the disc too anteriorly for fear of disturbing the spinal canal.) Radiographic control with a fluoroscope of the placement of the artificial disc should be considered as an essential part of any artificial disc replacement surgery.

Semi-constrained devices control the center of rotation of the ADR and prevent undue stress on the facets in extension motion, even more so than the natural disc. They also offer direct resistance to horizontal lateral movement (translation). Uncontrolled horizontal lateral movement (translation) can lead to facet damage and a popping sensation of the affected level. Horizontal rotation (the y-axis, twisting of the spine) is dampened by muscles and eventually limited by the facet joints, which is the natural function of the facet joints. The is no need for the ADR to dampen y-axis rotation (twisting), as this is done naturally by the facets, every day of a persons life.

Some speculate that the arc of rotation of the semi-constrained artificial disc causes facet problems, but this not born out by research or simple mechanical analysis. The arc is so large, that extension pressure of the facets remains close to that of the natural or healthy disc. Indeed the bigger the arc the more natural the motion of the extension is.

Elastomer discs offer no control over the center of rotation. Ideally, they would shift the Center Of Rotation (COR) posterior of the cord like the natural disc. Indeed this is what is hoped by the designers. However, this would only be true if the vertebrae anchoring the elastomer disc were perfectly remodeled and all the ligaments, tendons and muscles surrounding the affected level were balanced as they are in a complete healthy segment. But this balancing is highly unlikely, because they have most likely adapted to the degenerative condition of the spine. Therefore, they would pull the elastomer disc out of alignment and could cause facet problems. If structures are shortened and calcified, the surgeon must even release or resect them, to allow for the regaining of disc height and mobility. Thereafter, they are no longer patent to support and guide a disc. Unconstrained discs are fully dependent on the guidance of these structures. They are hence adequate only in very selected case. In some patients with severally damaged facets, TSMS can be a solution.