What does the LARS principle assist with in contact lens fitting?

The LARS principle is a crucial guideline in contact lens fitting, particularly in relation to managing the axis rotation of soft lenses. LARS stands for "Left Add, Right Subtract," a mnemonic that helps practitioners make precise adjustments when aligning the optical axis of a lens to the patient's visual needs, accounting for any rotational misalignment.

When a contact lens is fitted, it's essential to ensure that the optical center aligns correctly with the patient's line of sight. If there is any rotation of the lens on the cornea—due to factors such as lens design, corneal shape, or fitting position—the LARS principle provides a method to determine the necessary adjustments to the axis. For example, if a lens rotates left of the ideal position, practitioners would use the LARS principle to add degrees to the prescription axis to compensate for that rotation, thereby improving overall vision for the patient.

This principle does not pertain to adjusting lens length, determining lens power, or evaluating lens comfort, as those aspects rely on different considerations and techniques in contact lens fitting.