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Understanding Multifocal Designs: The Role of Slab-off Lenses

by Brianna Cuenca

In the field of vision correction, multifocal lenses are essential for managing diverse visual impairments, especially in cases of anisometropia, where there is a significant refractive difference between the two eyes.

What is Anisometropia?

Anisometropia refers to a condition where the refractive error between the two eyes is significantly different. This disparity can arise from various factors, including genetics, trauma, or age-related changes. When one eye has a higher prescription than the other, it can complicate vision correction efforts, particularly with traditional eyeglasses. Medically speaking, the refractive error of one eye differs significantly from that of the other and where each eye contains the same sign. 

Let’s take a look at an example where an anisometropic condition exists. In the illustration below, the right eye is +1.00 sphere and the left eye is +4.00 sphere. Through the distance optical centers, no prism exists. However, a person directing their gaze away from the optical centers will induce unwanted prism. In this example, the patient must look 10 mm away from the optical centers to utilize the bifocals. In doing so, vertical prism is induced in each eye resulting in a total of 3.0 Δ of vertical prismatic imbalance. Bicentric grinding, commonly called slab-off, is needed to correct the vertical imbalance and allow the patient to fuse the two images.

Slab Off Lens

Simply stated, slab-off is a prism ground in the reading portion of a lens. There are two types of slab-off:

  • Conventional Slab-off: Is always ground base up on the back side of the lens with the most minus or least plus power in the 90-degree meridian
  • Reverse Slab-off: As the name suggests, the prism base is oriented base down, or in the reverse direction of the conventional base up orientation. In this case, the slab-off is placed in the front of the lens with the least minus or most plus. 

To simplify matters, it is useful to calculate all slab-off as if it were to be ground in the standard or conventional manner. Then, to use a reverse slab, simply order the lens base down in the opposite eye.

How to Calculate Slab-off

  1. Determine the power of each lens in the 90-degree meridian.
  2. Determine the distance from the optical center of the carrier lens to the reading level.
  3. Apply Prentice’s Rule to calculate the amount of vertical prism induced in each eye at the reading level.
  4. Determine the difference in the amount of vertical prism in each eye. This number is approximately equivalent to the amount of prism to be “slabbed off.”

Example of Calculating Slab-off

Rx: -2.00 -0.50 x 180
      -5.00 -0.75 x 180
                Add: +2.00

Step 1: Determine power in 90-degree meridian for each lens.
R: -2.50
L: -5.75

Step 2: Distance optical center = 10 mm from reading level.

Step 3: Prentice’s Rule Δ = F x dec (mm) ÷ 10
R: -2.50 x 1.0 = 2.50 Δ base down
L: -5.75 x 1.0 = 5.75 Δ base down

Step 4: Difference: the left lens contains 3.25 Δ more base down prism than the right eye.

Conventional slab-off: 3.25 Δ base up on the left eye (most minus lens)
Reverse slab-off: 3.25 Δ base down on the right eye (least minus lens)

The Slab-off Process

The process of creating slab-off lenses involves several steps:

  1. Initial Lens Creation: The lens is initially ground to the prescribed power, ensuring that the front surface is shaped correctly.
  2. Dummy Lens Application: A dummy lens, which matches the base curve of the required lens, is temporarily cemented onto the front surface.
  3. Regrinding: The upper portion of the dummy lens is ground away while the lower portion remains intact. This creates a unique optical axis for both sections of the lens.
  4. Final Adjustments: The back surface of the lens is then finished, and the remaining dummy material is removed from the lower section, resulting in the desired prism effect.

Benefits of Slab-off Lenses

  • Reduced Visual Discomfort: By balancing the prismatic effects between the two eyes, slab-off lenses can alleviate symptoms such as diplopia and eye strain.
  • Enhanced Visual Clarity: Patients often report improved clarity and comfort when transitioning between different visual tasks, such as reading and distance viewing.
  • Versatile Applications: Slab-off lenses can be applied to various lens types, including single vision, bifocals, trifocals, and progressive lenses.

The Importance of Prism in Vision Correction

Prism plays a crucial role in vision correction for individuals with anisometropia. When light passes through a lens, it can bend or deviate from its original path, leading to image displacement. This effect can be particularly pronounced when the eyes are not aligned perfectly.

Understanding Prismatic Effects

  1. No Prism Through Optical Center: When gazing directly through the optical center of a lens, no prismatic effect is experienced.
  2. Image Displacement: As the gaze shifts away from the optical center, unwanted prism is induced, leading to visual disturbances.
  3. Vertical Prism Imbalance: In cases of significant anisometropia, excessive vertical prism imbalance can occur, complicating the visual experience.

Indications for Slab-off Lenses

Not all patients with anisometropia require slab-off lenses. Several factors influence the decision to utilize this technique, including the severity of symptoms and the degree of vertical imbalance.

Identifying Candidates for Slab-off

  1. Degree of Anisometropia: Patients with a significant difference in refractive power, typically greater than 2 diopters, may benefit from slab-off lenses.
  2. Symptoms of Diplopia: Individuals experiencing double vision or difficulty fusing images may find relief with slab-off lenses.
  3. Presbyopia Considerations: Older patients requiring multifocal lenses often experience increased symptoms due to the need for clear vision at varying distances.

Assessing Visual Comfort

  • Patient History: A thorough examination of the patient’s visual history and symptoms is essential in determining the need for slab-off lenses.
  • Trial Lenses: Utilizing trial lenses during the fitting process can help assess the effectiveness of slab-off in alleviating visual discomfort.

The Role of Digital Technology in Slab-off Lens Production

Advancements in digital lens manufacturing have revolutionized the way slab-off lenses are produced. The transition from traditional grinding techniques to digital technology has enhanced precision and efficiency.

Benefits of Digital Slab-off

  1. Precision Manufacturing: Digital surfacing allows for more accurate lens designs, resulting in improved optical quality.
  2. Smoother Transitions: The ability to create gradual changes in lens power enhances the overall visual experience.
  3. Thinner Lenses: Digital technology often enables the production of thinner and lighter lenses, improving both aesthetics and comfort.

Collaborating with Optical Labs

When ordering slab-off lenses, it is essential to collaborate closely with optical labs to ensure the desired specifications are met. Providing clear instructions regarding slab line placement and lens design can significantly impact the final product.

Ordering Slab-Off Lenses: A Step-by-Step Guide

To successfully order slab-off lenses, practitioners should follow a structured approach to ensure optimal outcomes for their patients.

Recommended Steps for Ordering

  1. Assess Vertical Imbalance: Calculate the vertical imbalance induced by the distance lenses to determine the need for slab-off.
  2. Select Appropriate Lens: Refer to a lens selection table to determine where to apply conventional slab-off or reverse slab-off based on the lens combination.
  3. Determine Slab Line Placement: Specify the slab line placement based on the multifocal style being used, ensuring proper alignment with the reading segment.
  4. Consult with the Lab: Engage with the optical lab to confirm their capabilities for producing digital slab-off lenses and to discuss any specific requirements.

Example of Slab-Off Application

Lens Type Slab-Off Application Reverse Slab-Off Application
Bifocal Base-up prism in the lower segment Base-down prism in the lower segment
Trifocal Base-up prism in the reading area Base-down prism in the reading area
Progressive Consult lab for recommendations Consult lab for recommendations

OTI Key Takeaways for Slab-off 

Slab-off lenses represent a vital solution for individuals grappling with anisometropia and associated visual challenges. By understanding the intricacies of this lens design, practitioners can provide tailored solutions that enhance patient comfort and clarity. As advancements in digital technology continue to shape the optical industry, the future of slab-off lenses looks promising, offering improved outcomes for those in need of specialized vision correction.

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