Diagnostic Fitting for Cylinder Power Effect Bitoric Lenses: Joel A. Silbert, OD
Diagnostic fitting is identical for all cases of high corneal astigmatism when using diagnostic gas permeable (GP) bitoric fitting sets, whether one is going to design a Spherical Power Effect (SPE) bitoric or a Cylinder Power Effect (CPE) bitoric lens design. The steps in examination and fitting are the same, with one exception: the over-refraction.
When there is not good agreement between the corneal and refractive astigmatism, then a SPE bitoric or spherical optical system will no longer be able to provide good vision. This is due to the presence of “residual astigmatism” (typically lenticular in origin, that is responsible for the difference between the corneal and refractive cylinders). For these cases, the use of a CPE bitoric lens is indicated.
Test Procedures, Fitting/Refitting, Design & Ordering
The clinician only has to modify the steps already described by this author in a previous SPE Case by substituting an SCOR for the spherical over-refraction. Adding the results of this SCOR to the labeled package powers of the test lens will provide the correct powers to order. The following refractive data was obtained:
OD: -2.00 -4.25 x 090
OS: -1.75 -4.50 x 090
OD: 47.00 @ 180 /44.00 @ 090
OS: 47.75 @ 180 /44.50 @ 090
Inspection of the above data shows that there is poor agreement between corneal and refractive cylinders. The corneal astigmatism is high, necessitating the use of a GP toric lens. However, an SPE bitoric or spherical optical system will be unable to provide good vision as each eye in this case manifests residual astigmatism in the amount of -1.25 x 090. The amount of this residual cylinder must be put back into the contact lens in order to achieve good vision.
Using the same SPE Diagnostic GP Fitting Sets described in an earlier case, the choice of the 2D fitting set is preferred in this case. Even though the cornea has 3D of toricity, we want to undercorrect this amount by about 1D. if we are to achieve a fit that provides adequate tear exchange. Thus, the 2D set is chosen.
The following diagnostic 2D SPE Bitoric lenses are found to provide the best fit:
Base Curve Radii (BCR)Overall/Optical Zone Diameter (OAD/OZD)Labeled SPE powers
|OD||7.67 / 7.34||9.2 (90) / 7.8 (180)||PL (90) / -2.00 (180)|
|OS||7.58 / 7.26||9.2 (90) / 7.8 (180)||PL (90) / -2.00 (180)|
(Note that the flat meridian is fitted “On K.” Also, note that because we are using an SPE diagnostic lens, the air powers of the test lenses have the same toricity as the back toricity in the lens.)
Performing a spherical over-refraction yields -2.00 D. OU, with predictable poor acuity of 20/40- each eye.
Thus, a SCOR or sphero-cylindrical over-refraction is performed in the phoropter, with good visual acuity. The results of the SCOR are as follows:
OD: -2.00 -1.25 x 090 (20/20-)
OS: -1.75 -1.25 x 090 (20/20-)
Add the results of the SCOR to the labeled powers of the SPE test lens. It is important to remember to be careful when adding these powers, to be sure to place the correct meridional power observing proper spectacle notation.
SPE Diagnostic Lens
PL (90) / -2.00 (180)
SCOR Add -2.00 to axis 90; Add -3.25 to axis 180
Final CPE Power -2.00 (90) / -5.25 (180)
SPE Diagnostic Lens
PL (90) / -2.00 (180)
SCOR Add -1.75 to axis 90; Add -3.00 to axis 180
Final CPE Power -1.75 (90) / -5.00 (180)
When placing the order with the laboratory, it is important to provide these powers along with the respective BCR values determined above for the best fit. The final lens will be a CPE bitoric, as it is no longer spherical. Unlike compensated SPE bitorics that have no visual effect should they rotate, CPE or conventional bitorics could potentially produce reduced acuity if they were to significantly rotate away from the desired axis (much like soft toric lenses). However, their inherent design makes them typically quite rotationally stable.
Discussion/Alternative Management Options
Although CPE bitorics (which are essentially synonymous with conventional bitorics) can be designed empirically (on paper), the use of diagnostic fitting sets using the SPE design provides the practitioner with a tool that makes fitting easy and accurate, for the same reasons articulated above in the SPE Bitoric Case. This case has shown that such fitting sets are quite utilitarian, because they not only can be used to specifically design a SPE bitoric, but can also be used when residual astigmatism is present and a CPE is thus indicated.
We have seen that the only step that differs is the treatment of the over-refraction, which in this case has to be spherocylindrical rather than spherical. Care must be taken when adding the powers of the SCOR to the labeled powers on the diagnostic SPE lens, but otherwise there are little differences in the fitting methodology.
In rare cases, the use of diagnostic lenses will not be successful. These atypical cases occur when there is poor agreement in the axis of the residual astigmatism, not matching well with the principal axes of the cornea. In those cases, a soft toric is usually necessitated.
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