Contact Lens Water Content: High vs. Low and Orientation Differences

The water content of contact lenses and their orientation: horizontal or vertical?

Hello Doctor: Recently, I noticed that my contact lens prescription seems insufficient, so I wanted to get a new pair of contact lenses.
However, when I visited the optical store, the so-called "professionals" gave me conflicting information.
1.
Some told me that a higher water content in contact lenses (e.g., 55%) is better, while others said that a lower water content (e.g., 38%) is preferable, which has left me more confused.
2.
I heard that our contact lenses (toric lenses) come in horizontal or vertical orientations.
Could you please explain the differences between them? Which one is better? Thank you for addressing these two questions!

SAM, 20~29 year old female. Ask Date: 2004/08/01

Dr. Lin Zhengyi reply Ophthalmology

1.
Soft contact lenses do not move as noticeably with blinking as rigid gas permeable lenses, which prevents the tear film underneath from circulating.
As a result, most of the oxygen required by the cornea must penetrate through the soft contact lenses to reach the surface of the cornea.
The material and thickness of the lenses affect the rate of oxygen transmission.
Generally, a higher water content leads to better oxygen permeability; however, due to the softer material and lower refractive index, soft contact lenses must be made thicker, which can actually reduce their oxygen permeability and make them more prone to debris accumulation.
Therefore, high water content lenses have both advantages and disadvantages.
For users who wear contact lenses for extended periods, it is recommended to choose materials with medium to high water content (which offer higher oxygen permeability).
2.
In eyes with astigmatism, the surface of the cornea is shaped like a rugby ball, with different curvature along the long axis and the short axis.
The terms horizontal or vertical simply indicate the orientation of the axis and do not relate to the severity of the astigmatism.

Reply Date: 2004/08/01

More Info

When it comes to choosing contact lenses, understanding the water content and orientation differences is crucial for optimal eye health and comfort. Let's break down your questions regarding high vs. low water content in contact lenses and the differences between horizontal and vertical orientations.

1. Water Content in Contact Lenses
The water content of contact lenses refers to the percentage of water present in the lens material. Generally, contact lenses can be categorized into two types based on their water content: high-water-content lenses (typically above 50%) and low-water-content lenses (around 38% to 45%).

High Water Content Lenses:
- Advantages: High-water-content lenses tend to be more comfortable for many wearers because they can provide better hydration to the eye. They also have higher oxygen permeability, which is essential for maintaining corneal health. The increased water content can help the lens remain moist, reducing dryness and discomfort during wear.

- Disadvantages: However, these lenses can also be thicker, which may reduce their oxygen transmission despite the high water content. Additionally, they are more prone to protein deposits and other debris, which can lead to discomfort and increased risk of eye infections if not properly cleaned and maintained.

Low Water Content Lenses:
- Advantages: Low-water-content lenses are often thinner and can provide better oxygen permeability due to their material properties. They are less likely to attract deposits, making them easier to maintain and potentially more suitable for long-term wear.

- Disadvantages: Some wearers may find these lenses less comfortable, especially if they have dry eyes or wear them for extended periods. The lower water content may not provide the same level of hydration, leading to dryness and irritation.

Ultimately, the choice between high and low water content lenses depends on individual preferences, eye health, and lifestyle. If you wear lenses for long hours or have a history of dry eyes, a medium to high water content lens may be beneficial. Conversely, if you experience discomfort with high water content lenses, you might want to explore lower water content options.

2. Orientation Differences: Horizontal vs. Vertical
The orientation of toric lenses, which are designed to correct astigmatism, can be either horizontal (with the axis of the lens oriented horizontally) or vertical (with the axis oriented vertically). The choice between these orientations is primarily based on the specific characteristics of your astigmatism.

- Horizontal Orientation: This is typically used for patients whose astigmatism is more pronounced in the horizontal meridian. These lenses are designed to stabilize on the eye in a way that aligns with the natural position of the eye, providing clearer vision for those with horizontal astigmatism.

- Vertical Orientation: Conversely, vertical orientation lenses are used for astigmatism that is more pronounced in the vertical meridian. These lenses are designed to provide stability and clarity for patients with vertical astigmatism.

The effectiveness of either orientation depends on the individual’s eye shape and the specific nature of their astigmatism. An eye care professional will conduct a thorough examination to determine the best orientation for your lenses based on your unique prescription and visual needs.

Conclusion
In summary, when selecting contact lenses, consider both the water content and the orientation of the lenses. High-water-content lenses may offer comfort and hydration but require diligent cleaning to prevent deposits. Low-water-content lenses can be easier to maintain but may not provide the same level of comfort for all wearers. Additionally, the orientation of toric lenses should be tailored to your specific astigmatism needs. It is always best to consult with an eye care professional who can provide personalized recommendations based on your eye health and lifestyle.

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