## Table of Contents

### What is Keratometer?

A keratometer is an optical instrument designed for measuring the curvature of the anterior surface of the cornea, specifically across a fixed chord length typically ranging from 2 to 3 millimeters. The term “keratometry” is derived from “**kerato**,” referring to the cornea, and “**metry**,” indicating measurement.

### Principle of Keratometer:

- Keratometer identify curvature of cornea based on the 1
^{st}purkinje image formed by anterior surface of the cornea. - From the size of the image (1st purkinje image) formed by the anterior Cornea, radius of curvature of anterior Cornea can be calculated using the formula: r = 2i/o.
- Then, dioptric power of anterior Cornea is detected by using the formula: D = N1 -N2/r.
- Let’s see details mechanism of how Keratometer identify corneal curvature & corneal Power:

#### 1. Reflection from Anterior Cornea:

- Despite being transparent, the anterior cornea reflects a small portion of incident light rays.
- This reflection occurs due to the difference in refractive indices between the cornea and the surrounding media.
- Consequently, the cornea behaves like a convex mirror, forming an image of objects.

#### 2. Relationship between Curvature and Image Size:

- The curvature of the cornea influences the size of the image formed by the reflected light rays.
- As the curvature increases, the size of the image decreases, and vice versa.
- This relationship between curvature and image size serves as the basis for corneal curvature measurement using a keratometer.

#### 3. Calculation of Radius of Curvature:

- The radius of curvature of the anterior cornea can be calculated using the formula r = 2i/o

Here

- r = Represents the radius of curvature of cornea.
- i = Denotes the size of the image formed by the cornea.
- o = Represents the size of the object projected by keratometer.

#### 4. Determination of Dioptric Power:

- Once the radius of curvature is known, the dioptric power of the anterior cornea can be calculated using the formula: D = N1 -N2/r,

Here,

- D = Represents the dioptric power,
- N1 = is the refractive index of the keratometer (1.3375),
- N2 = is the refractive index of air (1.000), and
- r = is the radius of curvature obtained from the previous step.

So,

- D = (1.33375 – 1.00) X 1000/r (Multiply by 1000 to find the answer in Dioptric value)
- D = 0.3375 X 1000/r
- D = 337.5/r

Thus, Dioptric value of cornea can be easily identify dividing 337.5 by the radius of curvature or cornea (r).

**Types of Keratometer:**

- There are two main types of keratometers, each with distinct characteristics and methods of operation:
- a. Manual Keratometer:
- b. Automated Keratometer:

#### a. Manual Keratometer:

- In a manual keratometer, the corneal power is determined through the manual adjustment of mires by the examiner.
- This method requires the examiner to have specific skills to perform the test accurately.
- There are two subtypes of manual keratometers:
- One Position Manual Keratometer.
- Two Position Manual Keratometer.

##### One Position:

- This type allows for the measurement of both horizontal and vertical meridians while keeping the keratometer in the same position.
- An example of this type is the Bausch & Lomb Keratometer.

##### Two Position:

- With this type, the keratometer needs to be repositioned after measuring one meridian to measure another.
- An example is the Javal & Schiotz Keratometer.

#### b. Automated Keratometer:

- An automated keratometer automatically measures corneal power, eliminating the need for manual adjustment by the examiner.
- This type offers high accuracy and ease of use.
- Examples of automated keratometers include the Humphrey autokeratometer and the Canon Autokeratometer.
- Additionally, when an autorefractometer and keratometer are combined into a single instrument, it is referred to as an Auto Kerato-refractometer.

**Procedure of Manual Keratometry:**

- The procedure of using a keratometer involves several steps to ensure accurate measurements. Here’s a precise explanation based on the provided information:

#### 1. Instrument Calibration:

- Begin by calibrating the keratometer using a steel ball of known curvature.
- Match the keratometer reading with the known curvature of the steel ball to ensure accurate measurements.

#### 2. Patient Preparation:

- Request the patient to sit comfortably, positioning their chin in the chin rest and resting their forehead in the headrest.
- Non testing eye should be occulded.
- Instruct the patient to keep the eyes wide open while blinking normally.

#### 3. Focusing Mires:

- For the horizontal meridian, adjust the horizontal knob until the plus (+) signs of the central and left images are superimposed.
- For the vertical meridian, adjust the vertical knob until the minus (-) signs of the central and upper images are superimposed.
- If astigmatism is present, the plus (+) or minus (-) signs may not initially be superimposed. In such cases, rotate the instrument until they are aligned properly.

**Interpretation of Manual Keratometry:**

- Interpretation of manual keratometry involves assessing the shape and power of the cornea based on the appearance of the mires.

#### 1. Spherical Cornea:

- When the cornea is spherical, there is no difference in power between the two meridians.
- The mires are observed as perfectly circular or spherical, indicating uniform curvature across the cornea.

#### 2. Astigmatism:

- Astigmatism is characterized by a difference in power between the two meridians of the cornea.
- In “With the rule astigmatism,” the mires appear horizontally oval, indicating greater curvature in the vertical meridian.
- Conversely, in “Against the rule astigmatism,” the mires appear vertically oval, indicating greater curvature in the horizontal meridian.

#### 3. Irregular Cornea (Keratoconus):

- In cases of irregular cornea, such as keratoconus, the shape of the mires will be irregular rather than perfectly spherical.
- The irregularity in mire shape suggests an uneven or distorted corneal surface, characteristic of conditions like keratoconus.

**Extended Keratometer:**

- Extended keratometry expands the measurement range of a manual keratometer beyond its standard range of 36D to 52D.
- Here’s a precise explanation of how extended keratometry works:

**Standard Range:**

- A manual keratometer typically measures corneal power in the range of 36D to 52D without additional lenses.

**Extension for Higher Powers:**

- If the corneal power in any meridian exceeds 52D, an extra lens is added to the keratometer to extend its range.
- A +1.25D lens is placed over the eyepiece, allowing measurement up to 61D.
- The final keratometry (K) reading is adjusted by multiplying it by 1.185 after placing the +1.25D lens.

**Example Calculation for Higher Powers:**

- If the K reading after adding the +1.25D lens is 48D, the final corneal power is calculated as 48 * 1.185 = 56.88D.

**Extension for Lower Powers:**

- Similarly, if the corneal power in any meridian is less than 36D, a -1.00D lens is added to the keratometer.
- This allows measurement down to 30D.
- The final K reading is adjusted by multiplying it by 0.840 after placing the -1.00D lens.

**Example Calculation for Lower Powers:**

- If the K reading after adding the -1.00D lens is 36D, the final corneal power is calculated as 36 * 0.840 = 30.24D.

**Indication of Keratometry:**

- Keratometry serves various indications in ophthalmic practice, offering valuable insights into corneal characteristics and aiding in diagnostic and treatment decisions.

#### 1. Estimate Radius of Curvature & Dioptric Power of Anterior Cornea**:

- Keratometry enables the estimation of the radius of curvature and dioptric power of the anterior cornea, providing essential information for refractive assessment and contact lens fitting.

#### 2. Measurement of Corneal Astigmatism:

- If different between K1 & K2 is equal to cylinder power revealed in retinoscopy or autorefractometer then its corneal astigmatism.
- If cylinder power revealed in retinoscopy or autorefractometer is more than the difference in K1 & K2 then its internal (Lenticular or retinal) astigmatism.

#### 3. Detection of Irregular Astigmatism:

- Keratometry aids in identifying irregular astigmatism by observing distorted mires in multiple directions, contrasting with the uniform oval pattern seen in regular astigmatism.

#### 4. Detection of Progressive Myopia:

- Keratometry can detect progressive myopia by monitoring changes in K1 and K2 values over time, with increasing values indicating curvature myopia progression.

#### 5. IOL Power Calculation:

- K1 and K2 values play a crucial role in calculating intraocular lens (IOL) power, serving as important parameters alongside axial length for accurate IOL selection in cataract surgery.

#### 6. Pre- & Post-Surgical Astigmatism:

- Keratometry assists in evaluating pre-existing astigmatism and monitoring post-surgical astigmatic changes, guiding treatment planning and follow-up care.

#### 7. Non-Contact Tear Film Breakup Time Test:

- Fluorescein dye is applied, and patient is asked to not blink.
- With the break of tear film mires get distorted which is break point.
- Break point less 10sec is considered dry eye suspect.

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