Which of the following is an anatomic measurement category for GP lens pre-fit?

• A. Eyelash length, tear film breakup time, palpebral aperture height
• B. Corneal thickness, axial length, pupil diameter
• C. Palpebral aperture height/lid position, lid tension, blink rate
• D. Iris diameter, scleral thickness, lid margin position

Answer: (C)

The main idea here is that gas-permeable lens pre-fit relies on lid-driven, anterior eye measurements—the way the eyelids and opening of the eye interact with a GP lens edge and with blinking. Palpebral aperture height tells us how wide the eye opens and how the lid contours over the globe, which affects how the lens edge will sit and whether it will lift or settle. Lid position indicates where the lid sits relative to the cornea during blinking, helping predict edge alignment and potential lens encroachment or edge lift. Lid tension shows how firmly the lids engage the globe, influencing how easily the lens moves with each blink and how stable the fit will be. Blink rate reflects how often tearing and lid motion occur, which impacts tear exchange under the lens and lens settling over time. Together, these lid‑related measurements predict fitting behavior such as centration, movement, and comfort for GP lenses.

Other options mix in measurements that aren’t the primary focus for GP pre-fit. Corneal thickness, axial length, and pupil diameter are more about overall ocular dimensions or refractive power rather than how the lids interact with a GP lens. Tear film breakup time is an ocular surface metric that, while important for lens wearability, isn’t a core GP pre-fit category. Iris diameter or scleral thickness aren’t the standard lid‑interaction measurements used to guide GP edge fit and lid dynamics.