33rd Annual Scientific Meeting proceedings

Corneal surgery in the horse is a commonly encountered but challenging endeavor as the equine globe is prone to trauma and ocular surface tumors. The purpose of this presentation is to review different and unique aspects of corneal microsurgery and provide an overview of the most common corneal surgical techniques in horses paying special attention to tissue and instrument handling to improve the surgical outcome.

The cornea is composed of four functional layers:  tear film, the epithelium, the stroma, and the innermost endothelium. The equine cornea as measured by ultra-biomicroscopy is 854±61μm, with the stroma representing 90% of its thickness.¹

INSTRUMENTATION
Corneo-conjunctival surgical procedures are microsurgical techniques which require magnification as they involve the use of fine suture material from 7-0 to 9-0 in horses. The choice of appropriate surgical technique and instrumentation determines the outcome and efficiency of surgery, and forceps in particular are key to handling ocular tissues appropriately and helping with tissue dissection and incision.   Fixation forceps are designed to grasp and stabilize ocular tissues and are marked in different sizes (0.12,0.3 and 0.5). To handle and manipulate the cornea, 0.12 Castroviejo or Colibri forceps should be used whereas the conjunctiva should be grasped with 0.3 fixation forceps such as Bishop-Harmon. Ophthalmic forceps, have stops in the inner platform of the handles to help control the force applied. Excessive digital pressure on the instrument handle may cause splaying of the tips, resulting in loss of control and poor fixation which leads to frustration for the surgeon.²

Corneal incision/dissection can be achieved with different instruments including a 64-beaver blade, crescent corneal knife, biopsy punch, or a corneal trephine. Tenotomy scissors (Stevens and Wescott) are essential for dissecting conjunctiva from Tenon´s capsule in conjunctival graft procedures which is the key to the surgical success of this technique.^2,3

SURGICAL CORNEAL TECHNIQUES
Corneal lacerations_ Corneal suture
Repair should occur as soon as possible to reduce the risk of infection. Extremely careful and gentle handling during the ocular exam and preparation for surgery  must be used to avoid further damage to the eye. Corneal lacerations can be partial or full-thickness. Performing a Seidel test and evaluating the sclera in lacerations that extend to the limbus are important to fully assess the damage and formulate a plan. Sutures should be passed at 90% stromal depth as shallow sutures result in internal wound gape whereas a full-depth suture can become a track for microorganisms.³

The first suture should be placed near the center of the laceration with the next two sutures placed halfway between the central suture and each of the two ends of the wound. This will help in reforming the anterior chamber. If there is a scleral laceration, the first suture should be placed at the limbus. If an avulsed piece of viable corneal tissue is present, it should be preserved.⁵

For corneal suturing, the needle should enter the cornea perpendicular to its surface. The easiest and most practical way to do this is to hold one side of the corneal wound at a 45-degree angle and penetrate the tissue with the needle at 45 degrees using minimal wrist motion to follow the needle curvature which helps to emerge from the other side of the wound perpendicular to the tissue. Tying the sutures with surgeon’s knots is highly recommended in equine corneal surgery.³ When deciding the distance for the placement of a second suture, it is important to remember the compression zones. Different lengths of suture bites result in different zones of compression.  Wound leakage occurs when there is an insufficient overlap of compression zones which allows wound gape and leakage.^5,6 (Fig 1)  Another important concept is the tension or compression created by the suture on the tissue. When overtightened, the inner aspect of the wound may gape, creating a fistula and the opposite intended effect, forcing the surgeon to place extraneous sutures to keep the wound watertight.^5,6

Superficial Keratectomy K)_ Corneal dissection
SK is the surgical removal of the corneal epithelium and anterior stroma. Clinical indications are ocular surface neoplasia and non-neoplastic processes including dermoid, indolent ulcers, and trauma/infectious keratitis such as fungal or immuno-mediated keratitis. This procedure is performed in two steps. The first step is the incision (which can be partial or complete) and definition of the correct depth followed by lamellar dissection using a Martinez corneal dissector, a crescent blade, or 64 beaver blade.  In the partial incision, an isolated corneal incision is made to the appropriate depth, adjacent to the affected corneal tissue. In the complete incision, the area to be removed is outlined first using a dermal biopsy punch, microsurgical blade, or corneal trephine. Finally, the lamellar dissection is performed, to keep the dissection in a single plane the dissecting instrument should be kept constantly parallel to the corneal tissue using a sweeping movement while applying forward pressure.^2,3 

For SK deeper than half of the corneal thickness, placement of some form of graft or flap is indicated to provide physical support, or the use of biomaterials such as Vetrix© BioSIS, Acell© or conjunctival and amnion graft.

Conjunctival grafts_ Conjunctival dissection and suture
While conjunctival grafts are used frequently in equine ophthalmology to manage deep/melting corneal ulcers, and descemetoceles, they are associated with some scarring.² Conjunctival grafts (360º, hood, pedicle, or bridge) give vascular and physical support to the weakened area. If aqueous humor leakage is identified, before performing a conjunctival graft, the repair of the perforation using either biomaterials, amnion, or cornea, is recommended in order to avoid post-surgical premature leakage and conjunctival graft retraction.³

Conjunctival pedicle graft (CPG) is the most commonly performed.  Key points in creating a CPG include choosing the proper site and size, being free of Tenon´s capsule, and keeping CPG´s blood supply without corneal epithelial entrapment.⁷ The most common pitfalls/problems are:  inadequate corneal recipient bed preparation, poor Tenon´s capsule dissection from the graft, and deficient suturing technique.^2,3,5

The first step is to clean all the necrotic tissue from the ulcer bed and to debride the normal epithelium around it to help create adhesions and prevent epithelial downgrowth.³ Bulbar conjunctiva is grasped 1-2 mm from the limbus using 0.3 mm fixation forceps, and a snip incision is made using Wescott´s tenotomy scissors.³ Then a tunnel is created using blunt dissection and maintaining the plane beneath the bulbar conjunctiva without including Tenon´s capsule.  Once the tunnel is created, the CPG is formed by making a parallel incision along the limbus and a second parallel incision made several millimeters posterior to the first incision. The flap is rotated over the defect and sutured in place with 7-0 to 9-0 absorbable suture material starting with cardinal sutures always introduced from the conjunctival to the corneal tissue. Any unintended conjunctival tears during graft dissection should be sutured.^3,7

Corneoconjunctival transpositions (CCT)_ Conjunctival/corneal dissection and suture
The CCT is an autologous partial-thickness (lamellar) corneal graft in which the peripheral cornea is grafted into the axial cornea, providing tectonic support while maintaining its conjunctival vascular attachments.³

Figure 1

References

1. Knickelbein KE, Lassaline ME, Kim S, Scharbrough MS, Thomasy SM. Corneal thickness and anterior chamber depth of the normal adult horse as measured by ultrasound biomicroscopy. Vet Ophthalmol. 2022 May;25 Suppl 1(Suppl 1):17-24. doi: 10.1111/vop.12971. Epub 2022 Jan 27. PMID: 35084084; PMCID: PMC9246829.
2. Herring IP. Corneal Surgery: Instrumentation, Patient Considerations and Surgical Principles. Clinical Techniques in Small Animal Practice 2003; 18:3; 152-167.
3. Brooks DE, Plummer CE. Diseases of the equine cornea. In: Veterinary Ophthalmology. 4^th Edition. (B. Gilger, ed.), Wiley & Sons, Inc., Hoboken, NJ, USA, pp. 253-440, 2022.
4. Legault GL, Kumar B. Corneal Laceration. [Updated 2023 Jan 2]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-.
5. Pagano, L.; Shah, H.; Al Ibrahim, O.; Gadhvi, K.A.; Coco, G.; Lee, J.W.; Kaye, S.B.; Levis, H.J.; Hamill, K.J.; Semeraro, F.; Romano, V. Update on Suture Techniques in Corneal Transplantation: A Systematic Review. J. Clin. Med. 2022, 11, 1078. https://doi.org/10.3390/jcm11041078 
6. Macsai MS, Fontes BM. Trauma suturing techniques. In: Ophthalmic Microsurgical Suturing Techniques. 1st Edition. (M. S. Macsai (Ed.), Springer-Verlag Berlin Heidelberg, pp. 61 - 70, 2007.
7. Zemba M, Stamate AC, Tataru CP, Branisteanu DC, Balta F. Conjunctival flap surgery in the management of ocular surface disease (Review). Exp Ther Med. 2020 Oct;20(4):3412-3416. doi: 10.3892/etm.2020.8964. Epub 2020 Jul 1. PMID: 32905115; PMCID: PMC7465514

FIGURES

1. Compression zones, Ophthalmic Microsurgical Suturing Techniques. Macsai 2007