Diagnostic techniques that directly evaluate biomechanics before refractive surgery, ide­ ally to help differentiate between healthy and abnormal corneas, are available and in de­ velopment. Devices currently available rely on bidirectional applanation tonometry or corneal deformation mea­sure­ment with Scheimpflug imaging.

With noncontact bidirectional applanation, a focused air jet is used to indent the cor­ nea and rec­ord 2 pressure mea­sure­ments: the pressure at which the cornea moves inward, reaching the first applanation, and the pressure at which the cornea recovers from a slight concavity as the air pump decreases pressure at an inverse rate (the second applanation). The pressure at the second applanation is lower than that at the first; this difference is called corneal hysteresis (CH), which represents the viscous (time-­dependent) dissipation of energy in the cornea (Fig 1-19, left). Low CH has been associated with keratoconus (Fig 1-19, right) and identified as an in­de­pen­dent risk ­factor for progressive visual field loss in glaucoma. The mean of ­these 2 pressures is the Goldmann-­correlated IOP (IOPg). The corneal-­compensated IOP (IOPcc) is a pressure mea­sure­ment that takes into account the biomechanical properties and is in­de­pen­dent of the central corneal thickness. The corneal re­sis­tance ­factor (CRF) is an indicator of the overall re­sis­tance to the air puff that is weighted more ­toward the instantaneous (elastic) properties of the cornea.

The corneal deformation method uses an ultra-­high-­speed Scheimpflug camera to mon­ itor the deformation of the cornea from a noncontact tonometer air-­puff indentation.

This method provides detailed information about the biomechanical characteristics of the cornea by determining velocity, length, and time lapse during dif­fer­ent phases of applanation.

Dupps WJ Jr, Wilson SE. Biomechanics and wound healing in the cornea. Exp Eye Res. 2006;83(4):709–720.

Garcia-­Porta N, Fernandes P, Queiros A, Salgado-­Borges J, Parafita-­Mato M, González-­ Méijome JM. Corneal biomechanical properties in dif­fer­ent ocular conditions and new mea­sure­ment techniques. ISRN Ophthalmol. 2014;2014:724546.

Corneal biomechanical properties are of major importance in keratorefractive surgeries. The vari­ous procedures differ in the biomechanical response they produce, depending on the biomechanical load of the ablated or dissected cornea. In LASIK, a corneal flap is created with a microkeratome or femtosecond ­laser, followed by thinning of the stromal bed with excimer ­laser ablation. In photorefractive keratectomy (PRK), the ­laser is ap­ plied directly to the anterior corneal stroma without creating a flap. In small-­incision lenticule extraction (SMILE), a femtosecond ­laser is used to create a 3-­dimensional len­ ticule, which is then extracted through a small corneal incision ranging from 2 to 5 mm. ­Because no flap is created and the most anterior stromal lamellae remain intact (except in the region of the small incision), this procedure may provide better biomechanical stability. Some studies have demonstrated better CH and CRF measurements following SMILE compared with LASIK or ­laser subepithelial keratomileusis (LASEK), possibly ­because the stiffer anterior stroma is preserved. However, ectasia has been reported ­after SMILE, as well.

Chen M, Yu M, Dai J. Comparison of biomechanical effects of small incision lenticule extraction and laser-­assisted subepithelial keratomileusis. Acta Ophthalmol. 2016;94(7):​ e586–­e591.

Ganesh S, Brar S, Arra RR. Refractive lenticule extraction small incision lenticule extraction: a new refractive surgery paradigm. Indian J Ophthalmol. 2018;66(1):10–19.

All keratorefractive procedures induce refractive changes by altering corneal curva­ ture; however, the method by which the alteration is accomplished varies by the pro­ cedure and the refractive error. Treatment of myopia requires a flattening, or decrease, in central corneal curvature, whereas treatment of hyperopia requires a steepening, or increase, in central corneal curvature. Keratorefractive surgery can be performed using a variety of techniques, including incisional, tissue addition or subtraction, alloplastic material addition, and ­laser ablation (see the section ­Laser Biophysics for discussion of ­laser ablation).

Overall patient satisfaction ­after the surgery depends largely on the successful correc­ tion of refractive error and creation of a corneal shape that maximizes visual quality. As discussed previously, the normal prolate corneal shape produces an aspheric optical sys­ tem, while oblate corneas increase sphere aberration and visual symptoms such as glare, halos, and reduced contrast sensitivity. (See the section “Effect of excimer ­laser ablation on higher-­order aberrations.”)