Jackson Cross Cylinder

Excerpt

Numerous studies have shown refractive errors to be the largest cause of visual impairment worldwide. Myopia has recently become an important public health condition in children. In 1887, Edward Jackson proposed using fixed-powered stoke lenses to determine the power of a cylindrical lens. These lenses were required to correct ocular astigmatism. Two decades later, Jackson discovered that the cross-cylinder could also be used to discover the axis of the astigmatic correction. Since then, this procedure has become popular and is still used in modern times by ophthalmologists, optometrists, and orthoptists to refine the astigmatic correction of an eye.

The Jackson cross cylinder (JCC) is a combination of cylindrical lenses (sphero-cylindrical lenses) with a handle (See Jackson cross cylinder image). The components have opposite signs, such as +0.25 DS combined with -0.50 DC; this results in a net meridional refractive power of +0.25 DC in one principal meridian and -0.25 DC in the other. The JCC combines two equal but opposite cylindrical lenses; its spherical component is half that of the cylindrical component. Its principal meridian is marked by the red and black or red and white line. The handle of the JCC bisects the principal axes by 45 degrees. This orientation helps to present different power before the eye by rotating. The JCC comes in +0.25/-0.25 or +0.50/-0.50 formats. The commonly used JCCs are +0.25/ -0.50 (or -0.25/ +0.50) and +0.50/ -1.0 (or -0.50/ +1.0).

Astigmatism

To understand the JCC and its principle, it is important to take a brief look at the concept of astigmatism. The word "astigmatism" comes from a combination of the English article "a," which in this case means "without," and the greek word "stigma," which means "point." In combining the words, the definition indicates the concept of "without a point."

Astigmatism results from a difference in curvature of the key components of the eye's refractive system, which include the cornea and the lens. In astigmatism, the rays of light do not have a point focus on the retina, thus creating a blurry image. There are two foci that, in principle, do not fall at the same location, which gives rise to different forms of astigmatism. The distance between the foci is termed the "conoid of Sturm" (See image).

Before the JCC can be effectively used, the circle of least confusion must be positioned on the retina, which is the smallest point that can be formed by a lens system, and the closest an astigmatic eye can come to achieving a clear image. The center of least confusion is referred to as the "center of the conoid of Sturm."

Astigmatism can be divided into the following five types, which are based on the location of the foci:

1. Simple myopic astigmatism (See image): is where one of the foci is in front of the retina while the other focus is on the retina.

2. Simple hyperopic astigmatism (See image): in this case, one of the foci is behind the retina at a virtual location while the other focus is on the retina.

3. Compound myopic astigmatism: this type of astigmatism is present when both foci are in front of the retina and are at different locations.

4. Compound hyperopic astigmatism: this condition is present when both foci are at different locations behind the eye.

5. Mixed astigmatism (See image): this astigmatism results when the refractive media creates two foci. One focus is positioned in front of the retina, and the other is behind the retina.

Astigmatism can also be categorized based on the following causes (i.e., the refractive media):

1. Corneal astigmatism: this is when the cornea is not evenly shaped, thus resulting in a difference in the refraction of light from the meridians.

2. Lenticular astigmatism: the refractive surface of the lens is not equal in this type of astigmatism.