Date of Award
Master of Science in Vision Science
Before the fairly recent invention of soft toric contact lenses, astigmats had two lens options out side of spectacles: soft spherical lenses or hard contact lenses. Hard lenses provide great optics but the discomfort of these lenses eliminates them as an option to many. Soft spherical contact lenses provide desirable comfort but lack the cylinder correction needed by astigmats. A hybrid lens having the comfort of a soft lens and the optics of a hard lens would solve this problem. Thus, the toric soft lens was born. It had the comfort of the soft lens and the cylinder correction resulting in hard lens wear. The "flimsy" nature of soft lenses material allows for this comfortable fit, but also allows the lens to rotate freely orienting itself differently from blink to blink. This is of no concern for spherical soft lenses as the power is the same in all meridians, but with soft toric lenses orientation is key as there is both a spherical and a cylindrical correction. In an effort to stabilize the rotation of soft toric lenses, a prism ballast design was developed. This prism ballast results in a lens that is thicker over one end. This thicker end and it's interaction with the wearer's upper lid stabilizes the orientation of the lenses by what has been labeled "the watermelon seed effect." This "watermelon seed" phenomenon forces the thicker part of the lens inferior. The labeled power on the soft toric lens assume the lens has oriented it self with the thicker portion perfectly perpendicular. In actuality, the lens rarely perfectly aligns this way. To correct for this shift, many clinicians estimate the direction of rotation, from their perspective, and magnitude. Then applying, the LARS (Left Add, Right Subtract) rule they make their correction to the axis. This estimation of rotation is not easy. Even an experience clinician can expect + 5° error in their estimation. 5° of rotation is not that much, just less than 3% (5°/180° ~ 3%) of a full rotation, but can definitely affect and blur the wearer's vision. A method relying less on the clinician's subjective estimation would more accurately measure the amount of lens rotation. This strategy will help more accurately estimate the amount and direction of rotation. Very simply: The amount of rotation of the spherocylindricallens can be more accurately measured with an over-refraction and a few other measurements. First of all, the vertex corrected refractive error from phorometry is needed. From this value, a soft toric lens will be selected with an appropriate base curve. This lens will be as close to the vertex corrected refraction as possible. The fit of the lens is also of utmost importance as a lens that is too flat will be uncomfortable for the patient and will rotate differently and unpredictably with each blink. Once a suitable lens based on comfort, stability, and the patient's refractive error has been determined, an over-refraction will be performed "over" this lens. This over-refraction value along with the vertex corrected refractive error of the patient can be used to calculate the amount of rotation of the lens. This magnitude of the rotation will then be added or subtracted based on the direction of rotation of the original soft toric lens' axis. Now a more appropriate lens based on the rotation intrinsic to the wearer can be determined resulting in the most clear vision possible.
Hamel, JP, "Soft toric lens rotation adjustment strategy" (2002). College of Optometry. 1395.