Section 3.2: Decentration & Vertical Imbalance

Section 3.2: Decentration & Vertical Imbalance

Spectacle frame selection and lens fitting are critical steps in translating a prescription into functional eyewear. A primary objective is to align the optical center (OC) of each lens with the patient's pupil. However, the geometric center of the frame and the patient's pupillary distance (PD) rarely coincide. Consequently, the optician must calculate the required decentration to position the lenses correctly. Furthermore, in cases of unequal refractive errors (anisometropia), looking through the reading area of the lenses induces vertical prismatic differences. This section covers decentration calculations, the clinical challenge of vertical imbalance at near, and the laboratory techniques used to correct it.

Lens Decentration Calculations

When a spectacle frame is selected, its lenses are held in place by the eyewire. The frame has its own geometric properties, defined by the Box System of Measurement:

• A-measurement: The horizontal width of the lens box.
• DBL (Distance Between Lenses): The width of the frame bridge.

The Frame PD (also called the Geometric Center Distance or GCD) is the distance between the geometric centers of the right and left lens openings. It is calculated using the formula: $\text{Frame PD} = A + DBL$

If the patient's pupillary distance (Patient PD) is smaller than the Frame PD, the lens optical centers must be moved inward (nasally) from the frame's geometric centers to align with the pupils. This process is called decentration.

The formula to calculate the total horizontal decentration for both eyes combined is: $\text{Total Decentration} = \text{Frame PD} - \text{Patient PD}$

Because this decentration is divided equally between both eyes to maintain symmetrical cosmetics, the decentration per eye ($c$) is: $\text{Decentration per eye } (c) = \frac{\text{Frame PD} - \text{Patient PD}}{2}$

Directing the Decentration

• Inward (Nasal) Decentration: Required when the Patient PD is smaller than the Frame PD. This is the most common clinical scenario.
• Outward (Temporal) Decentration: Required when the Patient PD is larger than the Frame PD.

Calculation Example

A frame has an A-measurement of 52 mm and a DBL of 20 mm. The patient's distance PD is 64 mm. Calculate the decentration per eye.

1. Calculate the Frame PD: $\text{Frame PD} = 52\text{ mm} + 20\text{ mm} = 72\text{ mm}$.
2. Calculate the total decentration: $\text{Total Decentration} = 72\text{ mm} - 64\text{ mm} = 8\text{ mm}$.
3. Calculate decentration per eye: $\text{Decentration per eye} = 8\text{ mm} / 2 = 4\text{ mm}$ nasal (in) per eye.

Proper decentration ensures that the patient looks directly through the optical centers of the lenses when looking straight ahead, avoiding any unwanted horizontal induced prism.

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Vertical Imbalance at Near

When a patient looks straight ahead, their pupils line up with the optical centers of their spectacles. However, when reading or performing close tasks, the patient lowers their eyes. They typically look through a point located 8 to 10 mm below the optical center of the lens.

If the patient has a similar prescription in both eyes, looking down induces the same amount of vertical prism in both eyes (e.g., $2.0\Delta$ Base Down in both eyes). Because these vertical prisms are in the same direction, they cancel each other out, resulting in no vertical imbalance.

However, if the patient has anisometropia—a significant difference in refractive power between the two eyes in the vertical meridian—looking down will induce different amounts of vertical prism. The difference between these induced vertical prismatic forces is called vertical imbalance at near.

The Clinical Significance Threshold

The human visual system is highly adaptable to horizontal disparities (as we have strong convergence and divergence muscles), but it has extremely limited vertical compensation abilities (vertical fusional amplitude is typically only $1\Delta$ to $2\Delta$).

• Vertical imbalance becomes clinically significant when it reaches $1.50\Delta$ or greater.
• Uncorrected vertical imbalance causes symptoms such as diplopia (double vision), asthenopia (eyestrain), letters running together, losing place while reading, and severe headaches.

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Calculating Vertical Imbalance

To calculate the vertical imbalance, we determine the induced prism in each eye at the reading level using Prentice's Rule, and then find the difference between them.

Direction of Induced Vertical Prism When Looking Down

• Plus Lens: Looking down below the optical center is equivalent to decentering the lens up relative to the eye. For a plus lens, the induced prism is in the same direction as decentration (Up). Thus, looking down through a plus lens induces Base Up (BU) prism.
• Minus Lens: Looking down below the optical center is equivalent to decentering the lens up. For a minus lens, the induced prism is in the opposite direction of decentration (Down). Thus, looking down through a minus lens induces Base Down (BD) prism.

Step-by-Step Calculation Example

A patient is prescribed the following spectacles, with a reading level 10 mm below the optical centers:

• OD: $-4.50$ D sphere
• OS: $-1.50$ D sphere

1. Convert the reading drop distance to centimeters: $c = 10\text{ mm} = 1.0\text{ cm}$.
2. Calculate induced prism for the right eye (OD): $P_{OD} = 1.0\text{ cm} \times 4.50\text{ D} = 4.5\Delta$ Since OD is a minus lens, looking down induces Base Down (BD) prism. So, $P_{OD} = 4.5\Delta$ BD.
3. Calculate induced prism for the left eye (OS): $P_{OS} = 1.0\text{ cm} \times 1.50\text{ D} = 1.5\Delta$ Since OS is a minus lens, looking down induces Base Down (BD) prism. So, $P_{OS} = 1.5\Delta$ BD.
4. Calculate the net vertical imbalance: Since both induced prisms are Base Down (same direction), they are subtractive: $\text{Vertical Imbalance} = 4.5\Delta\text{ BD} - 1.5\Delta\text{ BD} = 3.0\Delta$ The right eye has $3.0\Delta$ more Base Down prism than the left eye. Because $3.0\Delta \ge 1.5\Delta$, this patient will experience severe vertical imbalance and requires correction.

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Slab-Off Prism (Bicentric Grinding)

To resolve vertical imbalance at near, laboratories use a process called slab-off prism (historically called bicentric grinding).

Slab-off is a laboratory procedure where a horizontal line is created across the lower half of one lens (usually matching the top of the reading segment). The laboratory grinds Base Up (BU) prism onto the lower half of the lens to neutralize the excess Base Down prism (or lack of Base Up prism) in that eye.

Clinical Selection Rules

To determine which lens receives the slab-off prism, use the following rules:

• Slab-off is always ground on the lens with the MOST MINUS (or LEAST PLUS) vertical power.
• Slab-off induces Base Up (BU) prism.

Reverse Slab-off

In modern optical laboratories, a popular alternative is reverse slab-off. Instead of grinding Base Up prism on the most minus lens, reverse slab-off uses pre-cast lenses that add Base Down (BD) prism to the most plus (or least minus) lens. This is cosmetically identical and much more cost-effective for modern plastic and polycarbonate lenses.

Slab-off Selection Decision Matrix