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may be difficult or impossible to fit with contact lenses because of the altered shape of the cornea.

Surgical Correction of Astigmatism

Astigmatism may originate in the cornea or the lens; it may be physiological or follow surgery or other trauma. Spectacles do not adequately correct irregular astigmatism or high degrees of regular astigmatism (cf. Chapter 10, p. 115). These are best corrected by rigid contact lenses or surgery. The surgical correction of astigmatism is often unpredictable, and success is sometimes measured by the post-operative tolerance of spectacles or contact lenses to correct the residual refractive error rather than by its complete elimination.

Post-keratoplasty astigmatism (see below) is frequently irregular and its correction presents a challenge: the meridia of maximum and minimum curvature may not be at right angles to each other. Furthermore, zones of maximum and minimum curvature may be randomly distributed around the circumference of the corneal graft. Continuous sutures, interrupted sutures, a combination of both or double continuous sutures all have potential advantages in reducing graft astigmatism. Keratoscopy allows a certain amount of adjustment to be carried out during surgery to leave the cornea roughly spherical. Later adjustment may be made by redistributing tension along a continuous suture or by selectively removing interrupted sutures in the steeper meridia. Such manipulations are more exact if guided by computerised analysis of the corneal topography. When astigmatism is not amenable to further suture manipulation, other surgical or laser techniques outlined below may be used.

Relaxing Incisions – Transverse and Arcuate Keratotomy

The incision of the cornea causes it to bulge at that site. This reduces the surface curvature of the central cornea in the meridian in which the incision is made and induces increased curvature in the meridian at 90° to it (Fig. 17.2).

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Fig. 17.2

A corneal incision

(a) causes flattening of surface curvature

in the same meridian as the incision

(c) and steepening of the meridian 90° away (b).

This is often used after corneal grafting and also to control astigmatism in healthy corneas during cataract surgery. For example, the placement of the incision for small incision cataract surgery can be used to reduce pre-existing corneal astigmatism if the incision is made in the steepest meridian of the cornea.

Astigmatism persisting after all corneal graft sutures have been removed may be corrected by incising the graft–host junction over 60–90° where it is crossed by the meridian of steepest corneal curvature. An improvement of this technique is to place the incisions wholly within the graft. This conserves the wound and has a more predictable effect. The incisions are made tangential or curvilinear to the central zone (Fig. 17.3). An arcuate incision has the theoretical advantage that it is cut in cornea of uniform thickness and it is also concentric with the visual axis and likely to produce a more regular effect. Linear incisions pass through cornea of varying thickness, and parts of the incision are closer to the visual axis than

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Fig. 17.3

Configurations for corneal incisions to reduce astigmatism, alone (left, middle) or in combination with radial sutures (right).

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others. Longer incisions or incisions closer to the visual axis have a greater effect. Nomograms are available which relate the astigmatic correction from different combinations of clear zone diameter, incision arc and the number of incisions.

Compressive Techniques – Wedge Resection

High degrees (more than 10.0 D) of astigmatism after penetrating keratoplasty may be corrected by removing a deep arcuate wedge measuring 60–90° from the graft–host junction in the flattest meridian. The effect is the reverse of a relaxing keratotomy. The wound is sutured with nonabsorbable mersilene to shorten the cornea and steepen the curvature in that meridian.

Compression Sutures

A tight suture placed across the graft–host junction in the flattest meridian increases the curvature of the cornea and reduces astigmatism (Fig. 17.4). The topography will change if the sutures are removed for overcorrection or if they degenerate.

Fig. 17.4

A tight corneal suture flattens the cornea adjacent to it and displaces the corneal apex away from it, causing the cornea to bulge away.

Relaxing incisions may be combined with compression sutures placed 90° away from them to reduce large degrees of corneal astigmatism (Fig. 17.3).

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