Contact Lens Fitting and Management
Core concepts
Understand the parameters that govern a fit: base curve, diameter, sagittal depth, and material properties (Dk, Dk/t, water content, modulus). Know the difference between soft spherical, toric, and multifocal designs, and rigid gas-permeable lens fitting philosophies (alignment, apical bearing, edge lift). Be able to interpret fluorescein patterns and identify steep versus flat fits. Understand replacement schedules, solution compatibility, and the signs of complications: corneal neovascularization, infiltrative keratitis, giant papillary conjunctivitis, and tight-lens syndrome.
Traps
Questions often conflate Dk (oxygen permeability) with Dk/t (transmissibility). Higher water content does not always mean more oxygen to the cornea; silicone hydrogel materials achieve high Dk at lower water content. Do not confuse a steep fit (central pooling, mid-peripheral bearing) with a flat fit (central bearing, edge pooling). Also remember that a toric lens must not rotate excessively; stabilization design matters.
Worked rule of thumb
When evaluating a soft toric fit, check rotation first: if the lens rotates more than 10 degrees, the cylinder axis will be misaligned. Adjust the axis by the observed rotation direction (compensate on the order form). For RGP lenses, use the "three-point touch" concept: slight central clearance with mid-peripheral bearing and adequate edge lift.