SCLERAL LENSES have proven an invaluable asset in neutralizing corneal surface astigmatism and irregularities. Residual astigmatism is a critical factor influencing visual outcomes in scleral lens wearers. Lenticular and posterior surface corneal astigmatism contribute to residual astigmatism.
Correcting residual astigmatism is simple with scleral lenses. A spherocylindrical over-refraction is performed and front-surface toricity is incorporated. Typically, this is done as a last step in the fitting process after all other lens parameters have been finalized to ensure that lens rotation is stable. Due to the innovations in technology that are routinely used during fitting, optimal lens fit can be achieved earlier than ever.
Scleral profilometry has the biggest impact, allowing for accurate selection of a toric back-surface lens from available diagnostic sets. Lens rotation should be noted after adequate settling time. Residual ocular astigmatism is unpredictable in irregular corneas; therefore, a diagnostic lens is necessary to determine its presence.
Keratometry or topography over a worn scleral lens can indicate the presence of lens flexure. Although rare in scleral lenses, when present, lens flexure can be corrected by ensuring precise haptic alignment with scleral toricity or by increasing lens thickness. Lens decentration can induce aberrations and astigmatism that can be solved by improving the lens fit. Avoiding these decoys will help to determine whether there is true residual astigmatism that needs to be corrected.
In a stable lens, front toric optics are indicated when there is at least 0.75 DC of residual astigmatism and significant improvement in visual acuity. Clinically, subjective visual improvement should be appreciated by the patient with spherocylindrical over-refraction compared to the spherical equivalent over-refraction. If these factors are present, front-surface toricity can be incorporated into the initial lens order. Achieving optimal vision and a strong “wow” factor early in the fitting process can motivate patients to persist through application and removal training, as well as the intimidating commitment of daily lens handling.
Beyond front-surface optics, wavefront-guided optics target higher-order aberrations (HOAs) present in highly aberrated eyes. Conventional sclerals correct 60% of HOAs (Hastings et al, 2019). For the “20/20 unhappy” patient, the residual HOAs can impact visual satisfaction (Nguyen et al, 2020). Minimizing residual aberrations with wavefront-guided optics can help address visual needs when conventional scleral lens options are insufficient.
The innovations in scleral lens fitting make correction of residual astigmatism with front-surface toric optics easier than ever for obtaining optimal visual acuity and maximizing early patient satisfaction. If there is convincing clinical evidence that incorporating front-surface toric optics is required, incorporating them early will minimize the overall number of lens orders.
References
1. Hastings GD, Applegate RA, Nguyen LC, Kauffman MJ, Hemmati RT, Marsack JD. Comparison of wavefront-guided and best conventional scleral lenses after habituation in eyes with corneal ectasia. Optom Vis Sci. 2019;96(4):238-247. doi:10.1097/OPX.0000000000001365
2. Nguyen LC, Kauffman MJ, Hastings GD, Applegate RA, Marsack JD. Case report: what are we doing for our “20/20 unhappy” scleral lens patients?. Optom Vis Sci. 2020;97(9):826-830. doi:10.1097/OPX.0000000000001563