Modification of corneal curvature following radial keratotomy in primates

Radial keratotomy was performed on six stumptail monkey eyes (Macaca speciosa) and 22 owl monkey eyes (Aotus trivirgotus). Changes in keratometry, specular microscopy, and tonometry have been studied postoperatively for three to six months. Sixteen radial incisions in stumptail monkey eyes resulted in a significant mean corneal flattening of 2.75 diopters (P less than 0.005). This effect was stable with a mean 2.50 diopters flattening remaining six months after surgery. Sixteen radial incisions in ten owl monkey eyes resulted in a much greater short-term effect, with a mean 10.50 diopters of flattening present at two weeks postoperatively. This flattening deteriorated over the observation period, and no significant effect was noted three months after surgery. A comparison of 8 vs 16 incisions on owl monkey eyes demonstrated that these two procedures are equally effective in initially flattening the cornea. Complications and side effects encountered included perforations, irregular astigmatism, corneal neovascularization, transitory increase in pachometry, and decrease in central corneal endothelial cell density in isolated cases.     

Subjective refraction using power vectors by updating a conventional phoropter with a Stokes lens for continuous astigmatic power generation

Purpose

To implement a pure power vector method for monocular subjective refraction using a regular phoropter with the only modification being the inclusion of a Stokes lens. The proposed methodology was tested with three different Stokes lenses, and the results were compared with conventional clinical refraction procedures.

Methods

Power vector subjective refraction was performed by attaching a Stokes lens to the Risley prism holder. Stokes lenses allow for pure astigmatic compensation in the form of the J₀, J₄₅ components while the spherical lenses in the phoropter allow determination of the spherical component in the form of M (spherical equivalent). The proposed routine is presented step-by-step using three Stokes lenses having different astigmatic powers.

Results

Monocular subjective refraction was performed on 26 healthy subjects with a mean age of 44 ± 16 years, mean spherical equivalent of −0.56 D (range −5.50 to +2.38 D) and refractive astigmatism ≤1.50 D. No differences were found between the results obtained with the conventional technique versus the vector-based procedure for the spherical equivalent (p = 0.28) or astigmatic components (p = 0.34). In addition, visual acuity (VA) was equivalent through the refractions measured with the conventional and vector procedures (p = 0.12). Repeatability coefficients for J₀ and J₄₅ with the new vector methodology were <0.38 D.

Conclusions

The proposed routine could be helpful for cases where it is difficult to get a valid starting point for conventional refraction (e.g., irregular corneas and media opacities), for testing facilities with limited resources/equipment and/or for motivated clinicians who wish to know about alternative methods of refractive error determination.