If a patient's K reading shows the same diopter value at 180 and 90, what type of lens will be needed?

When the K readings, which represent the curvature of the cornea, show the same diopter value at both 180 and 90 degrees, this indicates that the cornea has a symmetrical curvature. In such cases, the cornea does not have astigmatism, which typically presents with different diopter readings at different meridians.

In this scenario, a spherical lens is required because it provides uniform power in all directions, effectively correcting the patient's vision without any need to account for astigmatism. A spherical lens will provide the appropriate refractive correction by offering a uniform curvature across all meridians, suitable for a patient with equal K readings.

The other types of lenses mentioned—truncated, toric, and prism ballasted—are designed to address specific vision problems typically associated with irregular corneal shapes or astigmatism. Since the patient's readings are equal, these specialized lens technologies are unnecessary. This reinforces that a spherical lens is the most suitable choice for a patient with such symmetrical K readings.