From the Guidelines
For a steep cornea, a base curve that is approximately 0.2 to 0.3 mm flatter than the flattest keratometry reading (K reading) of the cornea is generally recommended for contact lens fitting. This recommendation is based on the most recent study available, which suggests that a flatter base curve allows the lens to align properly with the corneal surface without creating excessive pressure on the apex of the steep cornea 1. The goal is to achieve good centration, adequate movement with blinking, and even weight distribution across the corneal surface. Some key considerations for selecting the appropriate base curve include:
- Corneal topography: The shape and curvature of the cornea will influence the choice of base curve.
- Lens material: Different lens materials may require slightly different base curves to achieve optimal fit and comfort.
- Patient's specific needs: The patient's lifestyle, visual requirements, and comfort level should all be taken into account when selecting the base curve. It's worth noting that the study by 1 found that flat-fitting contact lenses provided worse visual acuity than steep-fitting contact lenses, highlighting the importance of selecting the correct base curve for optimal visual outcomes. However, an excessively flat fit can cause edge lift and lens instability, so the exact base curve selection should be individualized based on the patient's specific needs.
From the Research
Base Curve Recommendation for Steep Cornea
- The recommended base curve for a steep cornea can be determined using various methods, including axial and instantaneous videokeratographic data 2.
- Studies have shown that axial steep simulated keratometry (SIM-K) can best represent the RGP base curve for the entire group of patients with keratoconus 2.
- In patients with dry eye syndrome and keratoconus, the base curve of the final scleral lens prescribed correlated with the steep keratometric power, the flat keratometric power, and the reference sphere 3.
- A study found that the base curve (BC) of rigid gas permeable (RGP) contact lenses can be predicted using the following equation: BC = 2.455 + 0.280 (steep keratometry) + 0.368 (flat keratometry) + 0.047 (corneal astigmatism) 4.
- For patients with pellucid marginal corneal degeneration (PMCD), a nomogram for spherical RGP contact lens fitting can be used, with a base curve of 6.0 to 7.0 mm and a diameter of 8.0 to 9.5 mm for patients with astigmatism of less than 10D, and a base curve of 7.0 to 7.5 mm and a diameter of 10.0 to 10.5 mm for patients with astigmatism of more than 10D 5.
- Short-term scleral lens wear in keratoconic patients may cause flattening of the anterior cornea, with a decrease in Ksteep, Kmax, and CCA 6.