What Is Corneal Topography?

The computer-aided corneal topography is characterized by its ability to accurately analyze the changes in the shape and curvature of the entire corneal surface, making it possible to analyze corneal properties systematically, objectively, and accurately. The corneal topograph is composed of 3 parts: Placido's disc projection system. Real-time image monitoring system. computer image processing system.

Corneal topography

Computer-assisted corneal topography with its ability to accurately analyze the morphology and

The shape of the front surface of the cornea is similar to the shape of the mountain, and the image of the front surface of the cornea can be easily captured. Therefore, by photographing the front surface of the cornea and analyzing the concentric annular image of the corneal surface obtained, you can understand the corneal surface morphology in detail.

Modern

The corneal topographic map, as a new corneal analysis system, has incomparable advantages compared with previous methods of analyzing the front surface of the cornea, but of course also has its disadvantages.

Various

The corneal topographic map can accurately reflect the entire morphological changes of the corneal surface, enabling the diagnosis of previously unclear diseases, especially the design of contact lenses, corneal refractive surgery options, control of the amount of surgery, and postoperative refractive power. Evaluation of changes and prognosis are of great significance, and even become a necessary means. Understanding the refractive status of the cornea in detail can not only help to understand the pathological and physiological changes of the cornea, but also to diagnose, treat and evaluate the prognosis of corneal lesions (such as keratoconus, limbal corneal degeneration, etc.) that are affected by corneal topography And other aspects are of great significance.

Corneal topography

Corneal topography imaging of keratoconus

Keratoconus is a kind of congenital corneal dysplasia, which is a non-inflammatory progressive thinning of the central cornea and a conical protrusion. In the past, the diagnosis of keratoconus has mainly relied on conventional examinations such as slit lamps. The typical clinical performance of slit lamps is Vogt line, Fleischer ring and corneal scar. If these typical clinical symptoms and signs appear, the diagnosis is easier, but for earlier keratoconus (subclinical stage: asymptomatic, better corrected vision, negative clinical examination), the diagnosis is very difficult.

Radial keratotomy (RK), astigmatic keratotomy (AK), and excimer laser keratotomy (PRK and LASIK, etc.) are contraindicated in patients with keratoconus in recent years; if surgery is performed, it may lead to accelerated disease progression, The surgical effect was obviously poor. The appearance of corneal topography? Early diagnosis of keratoconus provides a more objective basis, so strict screening of keratoconus before surgery is very necessary.

The corneal topography of keratoconus is typical, as shown in Figure 5-7.

The local area becomes steeper, forming a limited cone;

The apex of the zygomatic cone is mostly offset from the center of the visual axis, and its steep area is more common below or below the temporal region;

(3) The main types are the peripheral type where the cone becomes steeper toward the limbus and the central type where the center of the cornea becomes steeper;

From the shape of the cone, divided into? Round, oval and bow tie.

Figure 5-7 Topographic map performance of keratoconus

Main features of corneal topography in early keratoconus

Central corneal refractive power is large;

The lower cornea is significantly steeper than the upper cornea, and the upper cornea is significantly steeper than the lower cornea;

(3) The central corneal refractive error of the same individual is large.

Screening criteria for corneal topography in early keratoconus

From the topographic map of the absolute scale, although the corneal topography of the early keratoconus is more representative? Round, oval, and bow tie, etc., but unlike the normal cornea, its steeper cone area is more limited and more asymmetric. The normal cornea is more symmetry.

A more accurate method of diagnosing early keratoconus is quantitative analysis. Conventional corneal topographic quantitative analysis indicators, such as SRI and SAI, have certain reference value for the diagnosis of early keratoconus, but they have limitations, because the information they provide is only the situation within the 10 rings of the keratoscope. A good understanding of the situation of peripheral keratoconus.

Rabinowitz et al. Used normal corneal control as the reference value and 2 standard deviations of the mean value. It is recommended to use the following subclinical keratoconus diagnostic screening criteria. These years are more common.

Central corneal refractive power> 46.5D;

I-S value> 1.26D;

(3) The difference in corneal refractive power of both eyes of the same patient was> 0.92D.

However, the central corneal power is not a very sensitive index, and the central corneal power of normal eyes can sometimes reach or exceed 50D. Therefore, the diagnosis of subclinical keratoconus cannot be based on only a single indicator? Instead, multiple indicators should be referred to.

False keratoconus refers to a clinical phenomenon in which the topographic map of the cornea behaves similarly to keratoconus due to the pressure of mechanical external forces or human factors.

Possible factors for the formation of false keratoconus are:? Direct contact or metabolic factors of contact lenses (especially rigid contact lenses);? Poor fixation and so on.

Corneal topography refractive surgery

Corneal topography is routinely performed before corneal refractive surgery. In addition to excluding keratoconus, it has important reference values for the design of surgical plans, the prediction of surgical results, and the success of surgery. This test can help surgeons understand the following:

Determining corneal astigmatism and its axial position The corneal topographic map provides accurate and specific information on the refractive status of the entire corneal surface, the amount of corneal astigmatism and its axial position, and reflects the rules of corneal astigmatism. Can it be used for astigmatism correction? Reference and result prediction;

Knowing the corneal refractive power can help determine the surgical area and the amount of surgery; the size of the corneal refractive power also determines the size of the negative pressure suction ring during the operation;

Special cutting of the corneal surface is required for personalized cutting. The center position (eccentric cutting) and cutting amount of the cutting can be designed before the operation, and individualized laser cutting guided by the corneal topography can be performed when necessary.

Significance of corneal refractive surgery after corneal topography is of great clinical significance for the evaluation of surgical effects and dynamic observation of corneal healing. Its main functions are to evaluate the effect of surgery, to dynamically observe wound healing after surgery, and to follow-up observation of refractive regression.

(I) Evaluation of the effect of surgery

The uniformity of laser cutting can be roughly divided into the following types.

The uniform central cutting type cutting area is concentric, the center is flat, and the edges are stepwise. The naked eye has the best vision, and the patient's satisfaction is also the best, as shown in Figure 5-8.

Figure 5-8 Uniform center cutting

Bow tie-shaped corneal cutting areas still have bow-shaped changes, indicating that corneal astigmatism still exists after surgery, as shown in Figure 5-9;

Figure 5-9 Bow shape after cutting

(3) The semi-circular cutting type cutting area is semi-circular, that is, there is an area greater than 1 mm and a range of <180 ° around the cutting area, and the refractive power is more than 1D smaller than other areas;

The keyhole type cutting area is a keyhole type, that is, the area around the cutting area is larger than 1mm, and the range is> 180? The refractive power is 1D less than that of other areas;

The shape of the irregular cutting area is irregular, the power of each quadrant is different, and there is no law to follow;

Central island type is unique to excimer laser surgery, that is, the corneal topography in the central area of> 1mm range of corneal refractive power greater than the adjacent tissue above the island-like area is called "central island".

The position of the cutting center theoretically the best cutting position should be the cutting center and the visual center coincide. However, the clinical anastomosis is relative, and the deviation between the cutting center and the visual center often occurs, which is called eccentric cutting, as shown in Figure 5-10. If the degree of eccentricity is less than 0.5mm, it will rarely affect the visual function after surgery, so the range allowed for surgery should be less than 0.5mm.

Figure 5-10 Center offset after cutting

The size of the cutting area The size of the diameter of the central cutting area (S) refers to the diameter (unit, mm) of the area within the range from the flatness of the central refractive power to the steepening 1.5D on the corneal topographic map. When the pupil is normal, if S> 5mm, there is generally no complaint of glare; if S <3mm, obvious glare may appear.

The amount of cutting can be obtained by subtracting the corneal topographic maps before and after the operation. Generally, corneal topographic mappers have a differential map for direct observation.

(2) Postoperative dynamic observation is mainly used for the dynamic observation of refractive regression after PRK and LASIK. Corneal topographic maps can be followed up regularly after PRK (that is, the results of this examination are compared with the previous results). If there is a regression, it is mainly related to the wound healing response after PRK. ^[4]

Corneal topography contact lens

The contact lens test is equipped with a crucial parameter, namely corneal curvature. In recent years, the corneal topography can be used to better evaluate the morphology of the cornea in order to select a suitable contact lens. Typical corneal distortion topographic charts are available:? Irregular astigmatism in the center of the cornea;? Axial change in astigmatism;? Radial asymmetry;? Gradually flattening from the periphery to the central cornea as opposed to normal cornea;? If the contact lens is eccentric Phenomenon, the corneal area where the contact lens often stays relatively flat; the cornea outside the contact lens area becomes relatively steep.

The corneal topographic map can be used to select pre-fitting parameters for the contact lens wearer and dynamic evaluation of corneal changes after wearing.

Corneal topography other eye diseases

Common eye trauma and postoperative corneal surface irregularities and large changes in refractive distribution. Some people have reported changes in corneal topography related to refractive changes after eye trauma and surgery. Therefore, corneal topography can be used to analyze the type of postoperative astigmatism and determine the location of astigmatism to prevent and correct postoperative astigmatism.

Corneal topography and refractive power often change after penetrating corneal transplantation or lamellar corneal transplantation. Corneal topographic maps are used to selectively remove sutures, adjust continuous sutures, and perform astigmatic keratotomies to improve postoperative astigmatism.

One of the main reasons that affect the naked eye vision after cataract surgery is postoperative corneal astigmatism. The degree and nature of postoperative astigmatism can be understood by observing corneal morphological changes through a corneal topographic map, which has important clinical significance for the evaluation of postoperative cataract. Relevant literature reports show that small incisions of cataracts have smaller changes in corneal morphology than large incisions and recover faster. Generally, they return to the preoperative state 5 to 6 weeks after surgery. The smaller the incision, the faster and stable vision recovery. It was suggested that the corneal morphology changed slightly in the early postoperative period, but recovered quickly and remained stable. Corneal topographic maps are used to evaluate the changes in astigmatism after cataract surgery, which can vividly reflect the changes in corneal morphology, and can quantitatively analyze the changes in corneal morphology and curvature, make accurate judgments of the anterior corneal surface morphology before and after surgery. Great guiding significance. ^[5]