Radial Keratotomy

RK has largely been phased out in favour of more sophisticated and predictable laser surgical procedures. Although RK and astigmatic keratotomy are infrequently the primary surgical procedure for myopia and astigmatism, they are still used in conjunction with other refractive and surgical techniques ( ). However, there are many in the contact lens practice who had RK performed within the past 30–40 years ( ) and now they may have presbyopia and suffer from overcorrections ( ), hyperopic shifts ( ) and diurnal variations ( ). According to the prospective evaluation of RK study, a shift of the refractive error in the hyperopic direction continued 10 years after surgery ( ). An understanding of the difficulties in fitting the post-RK cornea is still required for the contact lens management of these patients as it poses far greater challenges than those encountered after other refractive procedures.

There is a common misconception that the mid-peripheral cornea steepens following RK. However, as the anterior cornea displaces to accommodate the gaping incisions, virtually the entire cornea from limbus to limbus flattens. The flattening effect is simply greater in the central cornea than in the periphery, resulting in the false impression of mid-peripheral steepening.

The cornea is malleable and prone to warpage during a period of up to 3 months following RK ( ). It is not advised to commence a contact lens fitting during the first three months unless it is medically necessary.

Phototherapeutic Keratectomy

Phototherapeutic keratectomy is a surgical therapeutic treatment to manage various corneal conditions that uses a medical device called an excimer laser, with the control of a computer to remove a small outer layer of diseased tissue from the cornea. The precision of the procedure leaves the surrounding area with very little trauma. After removing the damaged layer, new tissue is left, which will naturally restore over the newly smoothed surface area. Once the procedure is complete, a contact lens bandage is placed to allow adequate healing and to reduce any pain or discomfort felt. Antibiotics and prescription eye drops will be prescribed and are an important element of the recovery process.

Photorefractive Keratectomy, Laser-Assisted In Situ Keratomileusis and Laser Epithelial Keratomileusis

Irregular astigmatism is a relatively rare finding after either PRK or LASIK. However, it may be induced by the creation of a suboptimal lamellar flap – too thin, irregular, bisected, buttonholed or a free flap, or due to postoperative ectasia. Flap striae are another important and often underrecognized cause ( ). Irregular astigmatism can also arise from a decentred ablation. Patients are often left with varying degrees of uncorrected myopia due to the eccentric position of the treatment zone away from the visual axis. Viewing through the edge of the ablation frequently results in a loss of best corrected visual acuity (BCVA), monocular diplopia and ghosting of distance images, especially under scotopic conditions. The severity of symptoms often correlates with the size of the pupil. A decentred ablation with a large 6–7 mm pupil will produce more serious visual complaints than does a similarly displaced ablation over a small 3–4 mm pupil. Soft contact lenses rarely provide adequate optical correction in this small subset of patients. Optimal visual performance usually requires the use of a rigid lens design.

Refractive surgery techniques are moving forward with the introduction of femtosecond lasers. These create LASIK flaps with better accuracy, uniformity and predictability than do mechanical microkeratomes ( ). The higher-frequency femtosecond platforms elicit less inflammation and provide better visual outcomes. Small-incision lenticule extraction (SMILE) is another procedure made possible by femtosecond lasers and achieves similar safety, efficacy and predictability to LASIK with greater preservation of corneal nerves and biomechanical strength ( ). Hopefully, in the future, advancing diagnostic capabilities and surgical techniques will reduce the need for postrefractive surgery contact lens fitting.

Penetrating Keratoplasty

Full-thickness penetrating KP (PKP), first using mechanically paired blades to produce square grafts but evolving to circular grafts produced with trephines, has been the standard of care in corneal transplantation for more than 50 years. Patch grafts have long been used to seal corneal leaks, but visual results are not important. Corneal transplantation techniques are rapidly evolving ( ). The traditional full-thickness PKP performed with a trephine is becoming less conventional, while LK is becoming a surgery of choice in patients with corneal disease or opacity that spares the endothelium ( ).

Irregular Corneal Topography After Penetrating Keratoplasty

The main optical challenge following traditional PKP [perhaps lessened but not eliminated with the advent of femtosecond laser-assisted corneal transplant (FACT) and other forms of anterior LK (ALK)] has been irregular surface astigmatism ( Fig. 29.6 ). Optical rehabilitation of these distorted corneas remains a principal function of specialty contact lens practice, and PKP remains, despite the growth in the LKs, a common patient presentation. Hence, care of this disease will be discussed in depth in this chapter. It should also be noted that according to : ‘after successful Descemet’s membrane endothelial keratoplasty (DMEK), 23 of 262 eyes (9%) showed subnormal spectacle [corrected vision] and/or monocular diplopia due to corneal scarring, surface irregularities or undetectable optical imperfections that could be managed by contact lens fitting. Prolonged preoperative corneal edema for more than 12 months may be a risk factor for diffuse irregular astigmatism after DMEK …’.

Irregular surface astigmatism in a graft, as indicated by the distorted reflex of a perfectly circular flash gun.

Courtesy Desmond Fonn, Bausch & Lomb Slide Library.

documented the prevalence of the various forms of corneal topography following trephine PKP as: ‘prolate’ (30%), ‘oblate’ (30%), ‘mixed’ (20%), ‘asymmetric’ (10%) and ‘steep to flat’ (10%). discussed these corneal topographic outcomes in more clinically descriptive terminology as: ‘nipple’ or steep; ‘proud’, whereby the graft is totally or partially elevated above the host corneal surface ( Figs. 29.7 and 29.8 ); ‘sunken’, whereby the graft is depressed below the host surface; and ‘tilted’ or ‘eccentric’. These outcomes are illustrated in Fig. 29.9 .

Proud graft totally elevated above the host corneal surface in a keratoconic eye.

Courtesy Robert Terry, Bausch & Lomb Slide Library.

Protruding graft post-PK in a keratoconic eye.

Courtesy Melissa Barnett.

Graft profiles.

Adapted from Phillips, A. J. (1997). Postkeratoplasty contact lens fitting. In M. J. Harris (Ed.), Contact lenses for pre- and post-surgery (pp. 97–132). St Louis, MO: Mosby.

suggested that causes of these topographic outcomes include:

• •
  

  suboptimal cutting of the donor, the host or both

  
  
• •
  

  eccentric placement of the graft

  
  
• •
  

  elevation of the graft edge during healing, with poor wound approximation

  
  
• •
  

  loosening of sutures

  
  
• •
  

  localized abnormal healing

  
  
• •
  

  initial corneal irregularity (e.g. keratoconus).

Lamellar Keratoplasty

Epikeratoplasty – a short-lived surgical procedure – was this early LK variant wherein a precut (to a specific optical power) donor cornea was applied to the anterior surface of the intact host stroma (the epithelium but not the stroma was removed) in an effort to address aphakia, keratoconus or high myopia. It was found to result in poor vision (secondary to interface problems) and has been abandoned.

Superficial corneal diseases have long been alternatively treated with ALK wherein, following initial partial-thickness trephination, a blunt blade was used mechanically to separate a deep plane in the tissue to allow replacement of only the diseased superficial cornea. ALK retains the host endothelium to thereby reduce both immunological rejection and physiological failure. ALK optical results were poor, however, were usually reserved for either the corneal periphery or as tectonic procedures later followed by PKP to allow optical rehabilitation.

Several endothelial LK procedures to replace only damaged posterior corneal tissues (as in Fuchs’ dystrophy) have evolved. Deep ALK (DALK) and endothelial KP (EK) are the two main subcategories of LK. The purpose of most types of LK is to maintain as much of healthy, functional tissue of the cornea and remove the diseased portion of the cornea.

DALK is a newer LK that utilizes air, water or a microkeratome to separate the deep stromal fibres from Descemet’s membrane to replace unhealthy stroma, while the recipient corneal endothelium and Descemet’s membrane are retained ( ). DALK is used for corneal conditions that do not involve the corneal endothelium, such as a stromal scar. It is thought that deepening the position of the interface between the replacement and host cornea improves the potential visual results; DALK has gained some popularity owing to its theoretical advantages, in treatment of keratoconus in particular, although its practical superiority to PKP remains in question ( ).

EK has replaced PKP as the preferred surgical treatment for endothelial disorders such as Fuch’s endothelial dystrophy and bullous keratopathy ( ). In EK, the diseased Descemet’s membrane, including the endothelium, is stripped from the recipient posterior stroma ( ). Next, the donor tissue is transferred into the anterior chamber, unfolded and attached to the recipient stroma using an air bubble without the use of sutures ( ). Corneal integrity is improved since the anterior corneal surface is not affected by surface incisions and sutures. Wound-healing issues and suture-related complications are reduced. Faster and more comprehensive visual rehabilitation may occur with spectacle correction only since significant refractive error changes are avoided ( ).

In advanced corneal disease, such as long-standing corneal oedema and significant anterior stromal scarring, visual outcomes may be affected by surface irregularities after EK such as subepithelial stromal scarring, collagen disorganization and subepithelial fibrosis ( ). Contact lenses may be used to correct reduced vision and reduce monocular diplopia and ghost images ( ).

The rate of EK surgery has continued to rise in the United States. In the past 10 years, EK procedures have increased from 1308 in 2010 to 30,650 in 2019 ( ). After the corneal graft is performed and the cornea is clear, corneal topography or corneal tomography and spectacle refraction will help guide the choice of contact lens design.

Subcategories of EK include deep lamellar EK (DLEK) ( ), Descemet’s stripping membrane EK (DSEK) ( ), or an automated version using a microkeratome called Descemet’s stripping automated membrane EK (DSAEK) ( ) and DMEK ( ), wherein only the endothelial layer is replaced ( Fig. 29.10 ). DSAEK is currently the most prevalent procedure where the donor tissue is comprised of Descemet’s membrane, endothelium and an added thin layer of posterior stroma ( ). In general, visual acuity with DSAEK is better than PKP; however, the layer of attached stroma in a DSAEK procedure may reduce visual acuity, so there are fewer cases that obtain vision better than 20/25 ( ). In DMEK, the goal is to leave the host anterior corneal surface smooth and intact so that patients may not require contact lens correction ( ). DMEK in particular mitigates the two principal liabilities of KP: immunological graft reactions ( ) and secondary glaucoma from prolonged topical corticosteroid use ( ).

A cornea post-Descemet’s stripping endothelial keratoplasty (for treatment of the combination of iris naevus or Cogan–Reese syndrome; Chandler’s syndrome; and essential iris atrophy, called ICE syndrome). Note increased corneal thickness extending into the anterior chamber seen in the optical section superiorly.

DLEK, DSEK, DSAEK and DMEK, each more technically demanding than its predecessor, theoretically also offer rapid healing, more acceptable refractive outcomes and better retention of corneal strength, compared with PKP – but surgery is technically very demanding and donor adherence may be challenging. With increasing optical success of DSEK, DMEK and DALK, and selectively targeting diseased corneal tissue with lamellar surgery, PKP has lost its position as the sole dominant KP technique over the past decade ( ). The discussion about which procedure may prove optimal for many patients continues ( ).

No modern discussion of KP, however, would be complete without a discussion of artificial corneas. Though viable artificial cornea materials and surgical procedures have long been sought so as to address insufficient human transplant material, immunology and religious concerns, only recently has this become a practical reality ( ); several competitive designs may shortly be available ( ).

IntraLase-Enabled Penetrating Keratoplasty

Development of the IntraLase femtosecond laser allows corneal surgeons to create complex, shaped PKP incisions (e.g. ‘top hat’, ‘mushroom’, ‘zigzag’) that are identical in both host and graft tissue. Called IntraLase-enabled KP (IEK or ‘FACT’), this procedure creates PKPs with customized ‘interlocking’ – edge designs that it is hoped will minimize induced astigmatism and enhance wound strength. The risk for displacement of the graft may also be reduced. Fewer sutures are required, improved wound-healing time and decreased regular and irregular astigmatism will hopefully enhance vision and expedite contact lens fitting after femtosecond laser-based full-thickness KP than with previous techniques. A study of 116 keratoconus patients found that the 56 patients treated with IEK had significantly better vision improvement than did those receiving traditional trephine PKP: 3 months postoperatively, significantly more IEK patients were reported to have better best spectacle-corrected visual acuity ( P = .001) and visual recovery 20/40 or better ( P <.001) and lower topographic astigmatism ( P = .001) ( ). An earlier large study had reported a lower (30%) improvement in 2800 patients treated with traditional trephine PKP ( ).

Conventional Soft Lenses

Soft contact lenses emerged in the early 1970s with the creation of cross-linked hydrogel polymers and the introduction of hydrogel contact lenses ( ). Conventional hydrogel lenses fail to meet the Holden and Mertz criteria (87.0 Dk / t ) to provide the cornea with enough oxygen to avoid excess corneal swelling during overnight wear ( ). Some conventional hydrogel lenses do not provide the cornea with enough oxygen (24.1 Dk / t ) to avoid excess corneal swelling during daily wear ( ). Hypoxia complications include corneal oedema, corneal endothelial cell damage, corneal neovascularization and bulbar injection ( ). Silicone hydrogel contact lenses have reduced likelihood corneal hypoxia ( ).

Compared to hydrogel lenses, silicone hydrogel lenses are not associated with improved comfort ( ) nor reduced frequency of microbial keratitis ( ). Limited well-controlled studies related to silicone hydrogel materials being used specifically for medical purposes are present in the literature (Jacobs et al., 2001).

Bandage Lenses

Bandage lenses are used in the immediate postoperative period to relieve pain and to assist in re-epithelization following PRK and LASEK. These procedures involve removal of the corneal epithelium. The corneal surface needs between 2 and 4 days to regenerate, so lenses are typically worn continuously for 3–5 days following surgery to mitigate pain and promote reepithelialization ( ). Managing the wound-healing response is critical for a successful outcome to ensure re-epithelization occurs quickly, thereby reducing stromal haze ( ). This is best achieved with lenses of high Dk / t , for increased corneal oxygenation. Lens fit and corneal shape impacted comfort for managing postoperative pain in PRK. In a prospective study eyes were fit with a silicone hydrogel lens in two base curves: 8.4 and 8.8 mm ( ). Corneas with a postoperative keratometry (K) value of less than 38 dioptres had better pain outcomes with the 8.8 mm base curve. Steeper corneas with a postop K value greater than 42 dioptres did better with the 8.4-mm lens. Lens fit, in contrast to lens material or Dk, is important in bandage contact lens selection after PRK.

Many studies have been carried out to assess the relative benefits of different silicone hydrogel lens materials and designs when used as bandage lenses ( ). All found no major clinical difference between the lenses, although patients reported greater comfort with senofilcon A lenses. concluded that factors other than oxygen permeability affect pain and epithelial healing.

Bandage hydrogel contact lenses are also sometimes used as drug delivery devices or to treat nonhealing epithelial defects in the immediate postoperative period and for wound dehiscence at any time ( ) ( Fig. 29.14 ). Bandage soft lenses are also used both for protection (e.g. dehydration) and for optical ‘smoothing’ over the Dohlman KPro artificial cornea ( ).

A bandage hydrogel lens used 1 day after surgery to cover a graft that has been secured with interrupted sutures.

Drug Delivery

Soft contact lenses for ophthalmic drug delivery may be used to allow extended wear and increased bioavailability. The use of contact lenses as ocular drug delivery systems has been considered intuitive for decades ( ). At the time of publication, a single approved product became available. Johnson & Johnson Vision Acuvue Theravision with Ketotifen contains ketotifen, an H1 histamine receptor antagonist for the prevention of itch associated with eye allergies as the first and only vision correction contact lens that relieves allergic eye itch ( ).

Following Refractive Surgery

With myopic ablations, the mid-peripheral cornea (beyond the central 6.0–7.0 mm) remains unchanged. Therefore the major concern in fitting soft or corneal contact lenses is the relative difference between the flatter central cornea and the steeper (normal) mid-peripheral cornea. This difference creates few problems for patients who had low to moderate myopia prior to surgery. In such cases, the small amount of tissue ablated does not noticeably affect the contact lens fit or the on-eye lens dynamics. Most of the currently available daily disposable or frequent-replacement soft lenses are viable options for the postlaser correction. However, after surgery a complex relationship exists between visual acuity, defocus (refractive error), diffraction and optical aberrations. The loss of postoperative central corneal asphericity can result in a form of spherical aberration ( ). Patients with large pupils may be symptomatic. Improved optical correction can be achieved with soft lens designs that incorporate aberration-correcting anterior aspheric optics.

Radial Keratotomy

In principle, the fitting of a soft contact lens to a cornea that has undergone RK is very similar to fitting a normal cornea. The back optic zone radius (BOZR) will usually need to be approximately 0.5 mm greater than the flattest keratometry reading. However, in most cases a BOZR of greater than 9 mm will be required to achieve satisfactory alignment with the post-RK cornea, therefore the soft lens will need to be custom made. Disposable lenses will be a possibility only when the amount of central flattening (and subsequent negative asphericity) is of a relatively low degree ( ). It must be remembered that even toric soft lenses will not neutralize the irregular corneal astigmatism that often occurs as a result of RK.

Incisional neovascularization is a common complication associated with the use of soft contact lenses following RK, especially when the incisions reach the limbus ( ) ( Fig. 29.15 ). Its incidence can be minimized through the use of high-oxygen-permeability silicone hydrogel lenses used on a daily-wear basis.

Incisional neovascularization secondary to the wearing of a low-water-content soft contact lens.

Soft Lenses Over Keratoprosthesis

The function of a contact lenses over KPro is to protect the corneal tissue that surrounds to the anterior plate of the KPro, maintain hydration to prevent desiccation, avert epithelial breakdown, dellen formation and corneal melt ( ). In a multivariate analysis, eyes that had soft contact lenses removed or lost without replacement were more likely to develop complications ( ). Surface deposits on soft contact lenses may impair BCVA ( ). Hybrid or large diameter rigid lenses may help decrease the amount of surface deposits ( ).

Radial Keratotomy

Following RK, the cornea may exhibit significant corneal flattening with only minimal mid-peripheral flattening (approximately 0.1–0.2 mm flatter than its preoperative curvature). Therefore in the fitting of a corneal lens, a BOZR should be selected to align with the ‘more normal’ mid- peripheral cornea, approximately 4.0 mm from the centre along the horizontal meridian.

The radius of the postoperative mid-peripheral cornea can be determined through corneal mapping. Alternatively, a diagnostic lens can be selected with a BOZR that is 0.1–0.2 mm flatter than the preoperative flat K-reading and the fit can be evaluated. The appropriate BOZR should result in a fluorescein pattern that displays apical clearance over the flatter central cornea and a zone of mid-peripheral bearing at the 3 and 9 o’clock locations ( Fig. 29.16 ). The lens should display unobstructed movement along the vertical meridian.

Fluorescein pattern postradial keratotomy. The cornea has a central flat K of 8.76 mm and a mid-peripheral curvature (4 mm temporal) of 7.94 mm. The lens has a back optic zone radius of 7.94 mm.

Lens decentration is a common problem following RK. It is often the result of uneven wound healing, which creates geographic surface elevations on which the lens pivots ( Fig. 29.17 ). Lens decentration is best resolved by increasing the overall lens diameter to 10.5 mm or larger or by moving to a scleral lens design. Final lens power is best determined by performing a sphero-cylinder refraction over a well-centred diagnostic lens.

Pivot point created by uneven wound apposition, seen here at the top right corner of the tissue section (cat eye).

Photorefractive Keratectomy, Laser-Assisted In Situ Keratomileusis and Laser Epithelial Keratomileusis

With both PRK and LASIK, corneal lens fitting is best delayed until approximately 8–12 weeks after surgery. At this point, the refraction and topography have stabilized. At 3 months postsurgery, the integrity of the flap interface is usually sufficient to withstand the minor trauma associated with lens insertion and removal, as well as normal on-eye movements that occurs with blinking.

Many patients who have undergone PRK or LASIK can be successfully fitted with traditional spherical or aspheric corneal lenses. However, suggest that patients fitted with corneal lenses can display mild to moderate lens instability and decentration after PRK. As with soft lens fitting, the extent of tissue removal will influence the ease of fitting. For example, a patient with a preoperative refractive error of −4.00 D might end up postoperatively −1.00 D undercorrected. The 3.00-D myopic reduction was produced by an ablation of approximately 36 μm of tissue (less than the thickness of the human epithelium). The minimal difference between the central and mid-peripheral cornea creates few fitting and/or optical problems for a traditional corneal lens design. These individuals are often best fitted with a BOZR designed to align the mid-peripheral corneal topography 4.0 mm from the centre of the cornea ( Fig. 29.18 ).

Simulated fluorescein pattern of a postlaser-assisted in situ keratomileusis eye with a −3.00 D correction.

In contrast, the corneal shape following hyperopic laser correction is steeper, with a positive value for eccentricity often exceeding 0.5–0.7 ( ). It may be necessary to consider a lens design usually reserved for keratoconus.

Reverse-Geometry Corneal Lens Designs

PRK, LASIK and LASEK are all tissue subtraction procedures and the amount of corneal tissue removed will play an important role in the postsurgical management of the patient.

Tissue removal is determined by the Munnerlyn formula:

<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='Depthofablation=(opticzonediameter)2×(refractiveerror)/3′>Depthofablation=((opticzonediameter)2×(refractiveerror))/3Depthofablation=(opticzonediameter)2×(refractiveerror)/3
Depthofablation=(opticzonediameter)2×(refractiveerror)/3

Example of myopia correction for a refractive error of −6.00 D and performing an optical zone diameter of 6.00 mm:

<SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='Depthofablation=6.002×6.00/3′>Depthofablation=(6.002×(6.00))/3Depthofablation=6.002×6.00/3
Depthofablation=6.002×6.00/3

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