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Penetrating Keratoplasty: The Fundamentals
Key Concepts
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Penetrating keratoplasty (PK) plays an important role for corneal disease not suitable for lamellar grafts.
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PK has excellent outcomes in terms of graft clarity, but high levels of postoperative astigmatism or anisometropia remain an issue.
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Risk factors for PK failure include previous graft failure, glaucoma, anterior and posterior synechiae, corneal neovascularization, aphakia or pseudophakia, and long operation time.
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Patients need to be counseled regarding expectations and options regarding postoperative visual acuity and means for correction.
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Surgical steps of PK surgery require expediency and precision to ensure greatest patient safety, a closed graft-host junction, and minimal surgically induced suture-related astigmatism.
Introduction
Penetrating keratoplasty (PK) is a transplant procedure in which full-thickness, host corneal tissue is replaced with donor corneal tissue. Depending on the existing corneal pathology, the objectives of PK may include one or more of the following: (1) to establish a clear central cornea/visual axis, (2) to minimize refractive error in cases of severely distorted corneas, (3) to provide tectonic support, (4) to eliminate infection, and (5) occasionally to alleviate pain.
For most of the past 70 years, PK has been considered the “gold standard” corneal transplant procedure. Over the past several decades, there has been a trend away from PK toward partial-thickness corneal procedures, with the goal of only replacing the diseased corneal layer and maintaining the integrity of any normal corneal anatomy. Endothelial keratoplasty (EK) has overtaken PK as the procedure of choice for symptomatic endothelial cell loss, and, likewise, anterior lamellar keratoplasty is becoming the preferred procedure for advanced keratoconus, stromal dystrophies, and partial-thickness corneal scars when they cannot be treated with excimer laser procedures. ,
In 2018 the Eye Bank Association of America eye banks distributed tissue for 36,028 PKs (compared with 45,821 in 2005, a decrease of 21%), 35,071 EKs (compared with 1429 in 2005, a 25-fold increase), and 2355 anterior lamellar procedures (compared with 869 in 2005, more than a 2.5-fold increase). , The use of the Boston type 1 keratoprosthesis numbered approximately 750 in 2018, compared with 183 in 2005.
PK success rates, defined by grafts remaining clear at 1 year, approach or exceed 90% in numerous series. Thus it is an attractive but mistaken notion to consider this procedure common, simple, and undemanding. Currently, the definition of a successful PK is changing, and surgeons need to strive not only to maintain a clear cornea over time but also to achieve a successful refractive result, with good best corrected and hopefully also uncorrected visual acuity and function. For this purpose, new tools are being developed and used, including postoperative topography-guided excimer laser ablations and femtosecond laser technologies for both corneal trephination and postoperative refractive corneal incisions. Controversially, an Australian registry study indicated that long-term PK outcomes were better than lamellar keratoplasty procedures.
Factors in Patient Selection
Age
One of the advantages of advanced PK recipient age is that, theoretically, the immune system is less likely to mount a graft-destroying rejection. However, advanced age is also an independent risk factor for suprachoroidal hemorrhage, and extra precautions should be taken to prevent it.
Mental Impairment
Mild to moderate mentally challenged individuals can greatly benefit from PK, , and surgeons should not exclude them from receiving treatment. Family support and involvement are extremely important in such cases to increase the likelihood of long-term success.
Coexisting Ocular Disease
Active inflammation or infection increases the risk of graft failure and should be treated prior to transplantation. In conditions such as immune melts, herpetic keratouveitis, or Acanthamoeba , grafts are more likely to succeed in eyes that have been quiet for at least several months.
Ocular surface disease is a leading cause of corneal transplant failure. Control of the ocular surface should be optimized prior to transplantation and tenaciously maintained thereafter. Untreated ocular surface problems that compromise host corneal clarity will probably lead transplanted tissue to a similar or worse fate. Eyes with limbal stem cell deficiency may need stem cell transplantation prior to PK surgery. Patients with dry eye, neurotrophic disease, lash trauma, or exposure keratitis benefit from topical cyclosporine, punctal occlusion, epilation/electrolysis, and tarsorrhaphy in severe cases. , Lid anomalies are best corrected prior to corneal transplantation.
Preoperative glaucoma is a risk factor for graft failure, with a relative risk factor of 2.5 documented by Ing and others. , Elevated intraocular pressure may accelerate endothelial cell loss. , Glaucoma drainage devices are highly associated with graft failure, with few grafts remaining clear at 5-year follow-up. The optimal timing for glaucoma surgery (before, during, or after PK) and the optimal procedure (anterior chamber or pars plana glaucoma drainage device, versus a filtering procedure with or without antimetabolites) are unsettled issues. The development of new glaucoma surgical techniques, especially microinvasive glaucoma surgery (MIGS), brings hope for increasing corneal graft survival in patients with concomitant glaucoma, because these procedures have the advantage of maintaining the anterior segment anatomy. Intraocular pressure control needs to be achieved early on, preferably before transplantation, to best protect the optic nerve and corneal endothelium and especially because uncontrolled glaucoma is also an independent risk factor for intraoperative suprachoroidal hemorrhage. ,
Establish Realistic Expectations
Patient satisfaction is aided by repeated discussions of transplantation goals and realistic expectations. Patients need to be told that after PK it often takes at least 1 year or longer to achieve best vision. They should also be informed that vision in a corneal transplant eye is often compromised by higher than average levels of refractive error, such as nearsightedness or farsightedness, and regular and irregular astigmatism. More than one modality or procedure (e.g., contact lenses, glasses, manual astigmatic keratotomies, femtosecond laser arcuate incisions, topography-guided photorefractive keratectomy) might be required to achieve good corrected visual acuity.
Surgical Setting and Anesthesia
For most patients, PK can be performed safely in an outpatient surgical department under monitored anesthesia care. Retrobulbar, peribulbar, or subtenon ( ) 
anesthetic, and a lid block (Van Lint or Nadbath ), are routinely given, using a relatively long-lasting drug such as bupivacaine 0.5%, combined with a shorter-lasting one such as lidocaine 2%. A general anesthetic may be indicated for patients with ruptured globes, those at increased risk for choroidal hemorrhage, or other circumstances such as youth, mental impairment, deafness, aphasia, or language barriers.
Preoperative Preparations
Infection Control
Preoperative antibiotics may help to reduce the incidence of endophthalmitis associated with intraocular surgery, although the scientific evidence is inconclusive. Problems associated with prophylactic antibiotics include selection of more resistant organisms, the development of resistant bacteria, allergic reactions, and expense. Most surgeons who give preoperative antibiotics use a broad-spectrum, relatively nonsensitizing drug for 1–3 days before surgery.
The most common source of endophthalmitis is the patient’s periocular flora. , Careful attention to the management and elimination of blepharitis preoperatively is important. Likewise, preoperative lid preparation is important, with special attention given to removing any residual lid margin debris, covering the lashes with adherent drapes, and the use of povidone iodine solution. A single application of one drop of 1%−5% povidone-iodine solution on the ocular surface, at the time of the surgical preparation, has been shown to reduce significantly the incidence of endophthalmitis. The solution should be irrigated from the eye before making any incisions to avoid possible intraocular toxicity.
Intraocular Pressure Control
Optimal control of glaucoma should be achieved before surgery. Complete lid and extraocular muscle akinesia is essential to eliminate intraoperative pressure elevations associated with muscle contraction. Further lowering of intraocular pressure using ocular compression just before surgery may help to reduce posterior pressure during the open-sky phase of the surgery and the risk of vitreous loss and choroidal hemorrhage. Bourne demonstrated that preoperative digital massage results in decreased endothelial cell loss in phakic keratoplasty. A Honan balloon or similar device used before the eye preparation, at 30 mm Hg for 30 minutes, is useful. If a Honan or other mechanical device is unavailable, digital pressure can be applied. One should note that laryngeal mask anesthesia does not reduce intraocular pressure.
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