contact lens case reports
Fitting Soft Lenses Post Surgery
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
While GPs are often the lens of choice for post-surgical eyes, a number of factors can preclude their use in some patients. In such cases, a wide range of soft lens materials and designs have been successfully employed following refractive corneal surgery.
Soft Lens Choices
If we're considering a standard soft lens design, we begin by selecting a lens diameter that is 2.0mm larger than the patient's HVID. We select a base curve that is 0.70mm flatter than the pre-operative mean K. For example, a patient with a pre-operative mean K of 43.00D (7.85mm) and a corneal diameter of 12.0mm would require a lens with a base curve radius of approximately 8.55mm and a diameter of 14.0mm.
Advise post-refractive surgery patients that visual acuity may fluctuate when wearing traditional soft lenses due to a number of factors that can include blink-induced lens flexure across the flatter central cornea; uncorrected refractive error; soft lens dehydration; and in the case of RK, diurnal fluctuations in corneal shape and refraction. Additionally, the lack of post-operative asphericity can create aberrations in a number of patients, especially those who have large pupils.
Clinical experience has shown that lenses that incorporate anterior aspheric optics can help lessen patient symptoms of flare, glare and ghosting.
One of the primary concerns with fitting soft lenses post-refractive surgery is excessive apical vaulting over the flattened central cornea. You can address this issue by using a custom soft lens that incorporates a reverse geometry configuration — flatter central radius of curvature and a steeper peripheral radius (e.g. 8.3mm, 8.6mm or 8.9mm). In specific cases, you can further enhance lens performance by increasing center thickness to help mask irregular astigmatism and by adding anterior aspheric optics to address some of the surgically induced aberrations.
A 45-year-old female underwent RK surgery in the early 1990s (Figure 1). She was successfully wearing a Frequency 55 Aspheric (CooperVision) lens in her right eye but required a custom reverse geometry lens for her left eye.
Figure 1. Corneal topographies of the patient's right and left eye. Note the increased corneal flattening present on the left eye.
We selected the initial central base curve by adding 0.50mm to the post-operative mean K. For example, her mean post-operative K was 36.50D (9.25mm); 0.50mm was then added, resulting in a central base curve of approximately 9.0mm. We selected a peripheral radius of 8.6mm. You can independently adjust the two posterior lens radii and overall diameter to optimize fit. We empirically selected the power of the initial diagnostic lens based on the prescription at the plane of the cornea (Figure 2). Ultimately, the patient's VA was 20/25 with a comfortable wearing time of 14 hours a day. CLS
Figure 2. Parallelepiped image of the 0.35mm thick soft lens on the patient's left eye.
Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Paragon Vision Sciences. Mark André is an associate professor of optometry at Pacific University. He is also a consultant for CooperVision.