The ‘Why’ & ‘What’ of Centroid SIA involving astigmatic correction.
As the art of correcting corneal astigmatism became popular with Toric IOLs, a predominant concept has been to provide incision on the steep axis so as to steepen the flat axis. The concept of Surgically Induced Astigmatism (SIA) became popular, and quickly adapted as an important factor in the overall planning of astigmatism correction with Toric IOLs.
On the other hand, to correct low amount of astigmatism, a LRI was also looked forward to, or an OCCI (opposite clear corneal incision) practiced.
How far are incisions on the cornea predictable in terms of the resultant vector forces acting at a right angle away. Does incision at the same location for two patients have the same resultant effect on the postoperative cornea.
When you press a bottle filled with some water in your hand, the resultant folds on the plastic would depend on the fluid filled pressure level of the bottle. In our can, the eye with it's IOP postoperatively, will be amongst various factors determining the post operative corneal curvature that results from the incision. Not to mention that each cornea would have it's own corneal biomechanism which would significantly influence the post operative cornea. The elasticity, corneal hysteresis and corneal stiffness are different for different patients. And hence, providing an incision on the steep meridian would steepen the flat axis predictably and repeatedly remains a wishful assumption in practice.
This brings us to the concept of Centroid SIA. The Centroid SIA plotted on a Double Angled Plot has revealed that the impact of corneal incision is spreaded across the meridians, the mean of which is much lower than the previously averaged and wide spread believed SIA of. 50 diopters roughly. It also is more scientific as the vector analysis involved in this calculation is based on a Double Angled Plot, rather than the single angle plot previously applied. The thought behind this is that to do a vector analysis, one must match the 0/90/180 degrees of the eyeto mathematical plots. For example, in the eye, 0 and 180 degree is considered the same axis. In maths, you have no such assumption ( zero and 360 degree is same in maths ). Hence each meridian of the eye is doubled to make it at par with maths for vector analysis through Cartesian coordinates. In Doubled Angled Plot, the entire WTR SIA is plotted on one side, while the ATR SIA is plotted on the other end (see picture )
In summary, Centroid SIA is a new concept that frees us from some oversimplified assumption of cornea both in terms of it's biomechanism and mathematical interpretation. The result, the impact of SIA may be negligible compared to the importance it was given a few years back. Warren Hill, Douglas Koch, Graham Barrett and many early adopters have advocated a Centroid SIA as low as to have no effect on overall outcomes at all. This together with the overall consideration of posterior corneal astigmatism may go a long way in providing predictable outcomes to patients.