K readings in contact lens fitting are primarily based on which measurement?

K readings, also known as keratometric readings, are specifically related to the curvature of the cornea. When fitting contact lenses, it is essential to understand the shape and curvature of the cornea, as this information helps in selecting the appropriate contact lens design and curvature that will align well with the patient's eye.

The cornea's curvature directly influences how a contact lens fits and performs on the eye. By measuring the steep and flat meridians of the cornea, practitioners can determine the best base curve for the contact lens, ensuring comfort and optimal vision correction. This measurement takes into account the biomechanical characteristics of the eye, which are essential for achieving a correct and effective fit.

The other options focus on aspects that do not contribute directly to the primary function of K readings in contact lens fitting. The measurement of the contact lens itself, the patient's iris diameter, and the thickness of the contact lens, while relevant considerations in fitting and comfort, do not provide the fundamental data that K readings offer about corneal curvature. This distinction is critical for ensuring that contact lenses sit properly on the eye and provide the intended visual correction.