Rigid gas permeable (RGP) materials possess characteristics, including oxygen permeability, surface chemistry, hardness/modulus, and specific gravity, that are crucial to patient comfort and satisfaction.

Here’s how these characteristics may influence lens selection.  

Oxygen permeability
Dk is defined as the product of the diffusion coefficient of oxygen in a material (D) and the solubility coefficient of oxygen in a material (k). It is a property intrinsic to the polymer matrix and thickness of a lens. Thickness, however, determines the transmissibility (Dk/t) of different lenses fabricated from the same material.

Lens selection tip: The power of the lens may affect your choice of material.¹ Therefore, consider using a higher-Dk material for higher-plus powers (e.g., ≥ 3.00 D) and higher-minus powers (e.g.,≥ - 6.00 D). These prescription ranges, respectively, are thicker in the center and thicker at the edge. Higher-Dk materials optimize oxygen transmissibility.

Surface chemistry
Wettability is defined by contact angle. The lower the wetting angle, the more hydrophilic the lens surface.

The wettability of a contact lens is determined by its surface chemistry; advances in surface chemistry optimize how easily liquid (observed through interaction with the tears) spreads over the lens surface.

Laboratories employ a variety of techniques to enhance surface wettability. These may include oxygen plasma cleaning, with or without laser, and application of a polyethylene glycol.

Lens selection tip: Clinically, a more highly wettable lens surface reduces friction between the anterior surface of the lens and the eyelids, increasing comfort, and reducing lens deposits.² This makes this type of lens surface ideal for patients who have ocular surface disease. Patients who have a compromised tear film should be managed with appropriate concurrent dry eye disease treatment to achieve best wear results.

Hardness/modulus
Methods for measuring the hardness or stiffness of plastics (including contact lenses) involve measuring the resistance to indentation from various weighted loads. One such device, the Shore durometer, can also provide a general indication of modulus. ODs generally expect better dur-ability from a higher-modulus material, although this does not necessarily predict strength or scratch resistance.

Lens selection tip: The higher the Dk, the greater the possibilty of lens flexure, so balancing permeability and modulus is key in patients who have higher (> 2.00 D) degrees of corneal toricity.³

Specific gravity
Specific gravity (SG) is the ratio of the density of a solid, to the density of equal volume of water (SG = 1.00) at the same temperature. Thus, contact lenses designed from materials that have a higher SG will have a greater mass.

Lens selection tip: High-plus lenses will have a greater mass due to increased lens thickness. Reducing the SG can help offset this increase, leading to better lens centration.

A caveat: Changing the SG can affect the lens mass by up to 20%.⁴ This may affect achieving a lid-attached fit or otherwise troubleshooting a low-riding lens.⁵

Next issue, I present cases highlighting the considerations of these material properties in designing RGP lenses. OM

References:

1. Gardner HP, Fink BA, Mitchell LG, Hill RM. The effects of high-Dk rigid contact lens center thickness, material permeability, and blinking on the oxygen uptake of the human cornea. Optom Vis Sci. 2005;82(6):459-466. doi:10.1097/01.opx.0000168562.64251.66

2. Costa D, De Matteis V, Treso F, et al. Impact of the physical properties of contact lens materials on the discomfort: role of the coefficient of friction. Colloids Surf B Biointerfaces. 2024;233:113630. doi:10.1016/j.colsurfb.2023.113630

3. Stevenson RW. Young's modulus measurements of gas permeable contact lens materials. Optom Vis Sci. 1991;68(2):142-145. doi:10.1097/00006324-199102000-00010

4. Rigid Gas Permeable. Nursing and Health Professions. (Accessed Nov. 18, 2024) https://www.sciencedirect.com/topics/nursing-and-health-professions/rigid-gas-permeable

5. ScienceDirect. International Contact Lens Clinic Volume 19, Issues 3–4, March–April 1992, Pages 84-88. Controlling rigid lens centration through specific gravity. 

Susan A. Resnick, OD, FAAO, is the “Contact Lenses” columnist for Optometric Management and president and managing partner at Drs. Farkas, Kassalow, Resnick, and Associates, a contact lens and anterior segment specialty practice in New York. She is a diplomate in the Cornea, Contact Lens, and Refractive Technology section of the AAO, a diplomate of the ABO, and a fellow of the Scleral Lens Education Society.