Specific Gravity of Contact Lenses

According to Wikipedia, “Specific gravity is defined as the ratio of the density of a given solid or liquid substance to the density of water at a specific temperature and pressure, typically at 4°C (39°F) and 1 atm (760.00 mmHg)”.

Why is specific gravity important in contact lenses

Gravity is an important factor in how the lens fits and moves on the eye.  It is therefore important to consider the affect of lens mass and the specific gravity of the contact lens.

Work has been done looking at the specific gravity of contact lens materials, and how well they bind to other materials.  In 1991, Ghormley suggested that the specific gravity of a contact lens material may be related to the “frequency of adherence”. In clinical trials, he saw that contact lenses made from materials of higher specific gravity tended to bind more frequently than materials of low specific gravity.  In 1992 a study looked at rigid lens specific gravity and whether it played a role in controlling vertical lens positioning.  The conclusions of that study showed some cases where there was a level of control of lens position through specific gravity or contact lens thickness.

RGP contact lenses, or rigid gas permeable, are usually designed with a curvature that is specifically designed to match the irregularities of the wearer’s corner. This keeps the lens in position because it actually fits the cornea. In this instance, the wearer need not be concerned about specific gravity on RGP contact lenses. Specific gravity on RGP contact lenses is not what keeps the contacts in place. Rather, RGP contacts are designed to mold to the eye and stay in place because of their shape.

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