What are slab-off prism?
Slab-off prisms, also known as bi-centric grinding, are a technique used in optometry to correct vertical imbalance in patients with significant differences in the prescription power of their two eyes, a condition known as anisometropia
What is a Slab-off Prism lens?
A slab-off prism is a lens that has been ground in such a way that it has two different centers of curvature. This is achieved by grinding the lower half of the lens to induce a vertical prismatic effect. The purpose of this is to offset the prismatic effect caused by the difference in lens power between the two eyes
How Does a Slab-off Prism Work?
Every lens has inherent prismatic effects. When an object is viewed through the center of a lens, there is no shift in the image. However, as you move your gaze from the center of the lens to the lower part, the image shifts its position. The amount of shift depends on the power of the lens.
A slab-off prism works by shifting this image back into position. This is achieved by adding base up (BU) prism to the most minus or least plus lens in the vertical meridian to offset excessive base down (BD) induced by the opposite lens2. This effectively brings the images seen through both lenses back onto the same level, alleviating problems such as double vision and headaches.
When is a Slab-off Prism Prescribed?
A slab-off prism is generally recommended when anisometropia causes problems due to different prismatic effects. These effects come into play when the wearer of bifocal lenses or progressives performs eye movements up and down, for example, to switch from distance vision into reading distance.
Typical problems that indicate a need for a slab-off prism include:
- Double vision when looking down.
- Extremely uncomfortable vision while reading.
- Persistent discomfort that does not go away even after five minutes of reading1.
- If these symptoms are present and applying a prism lens to the testing frame compensates for the prismatic effect and leads to noticeably more comfort, then a slab-off prism should be applied.
A slab-off prism is generally prescribed in the following scenarios:
- Anisometropia: This is a condition where there is a significant difference in the refractive power of the two eyes. The prismatic effect caused by this difference can lead to double vision or other visual discomforts when looking down or reading.
- Reading Discomfort: If a patient experiences persistent discomfort that does not go away even after five minutes of reading, a slab-off prism may be prescribed.
- Double Vision: If a patient experiences double vision when looking down, a slab-off prism may be recommended.
How to prescribe slab-off prism: case study
- Patient: John Doe
- Diagnosis: Anisometropia
- Right Eye (OD): -4.00 DS
- Left Eye (OS): -1.00 DS
- Add: Slab-off prism on the right lens
In this example, the patient has a 3 diopter difference between the two eyes, which is causing visual discomfort. The slab-off prism is added to the more minus or least plus lens (in this case, the right lens) to counteract the prismatic effect caused by the difference in power between the two lenses.
Can I wear a slab-off prism with contact lenses?
Contact lenses tend to be more stationary on the eye, moving with it so that the prismatic effect is almost nonexistent1. Therefore, the slab-off technique, which is used to correct diplopia (double vision) by grinding a base-up prism on half the lens, is typically not applied to contact lenses. However, it’s always best to consult with your eye care professional for personalized advice based on your specific needs and circumstances.
Slab-off prisms are an effective solution for correcting vertical imbalance caused by anisometropia. By understanding how they work and when they are needed, optometrists can provide better care for their patients and improve their visual comfort and quality of life.
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