Performing phacoemulsification in human eye-bank models,1,2 animal eyes,3−7 or plastic model eyes8 is essential for acquiring and mastering this modern intraocular surgical technique. However, it is difficult to obtain human cadaver eyes in countries with limitations on human eye-banking and organ donation; plastic models are expensive and not available in developing countries; and experimental animal models with a soft nucleus do not simulate actual human cataract surgery, particularly nuclear phacoemulsification. Many techniques to induce reproducible cataract models with hard nuclei for phacoemulsification training have been reported.2–7 We describe an experimental model in sheep eyes harboring a cataractous human lens nucleus.
This experimental model uses an undamaged human cataractous lens nucleus obtained by extracapsular cataract extraction (ECCE) and kept in a moist chamber or a balanced salt solution at +4°C. A lateral stab incision is made at the corneoscleral limbus of a freshly enucleated sheep eye at the 9 o'clock position by a 3.2 mm slit knife. A capsulorhexis is accomplished with a Utrata forceps under an ophthalmic viscosurgical device (OVD). Phacoemulsification (DORC Harmony 6800 HSP 3) of the central area of the sheep eye lens is done to form a deep cavity for placement of a human lens nucleus. The incision at 9 o'clock is then enlarged to 5.0 to 7.0 mm using a scissors, and a human lens nucleus of the preferred hardness is inserted into the preformed sheep eye lens cavity. The corneal entry site at 9 o'clock is sutured tightly with 10-0 monofilament nylon (Figure 1).
Phacoemulsification training can be started at this point with the model that has a human lens nucleus. A corneal tunnel incision for phacoemulsification practice is performed with a 3.2 mm slit knife at 12 o'clock. A side-port incision is created using a 20-gauge MVR blade at 2 o'clock.
Phacoemulsification by the desired technique and other ECCE techniques including manual nucleus-cracking methods have been performed in the prepared model. After a human lens nucleus has been emulsified, another can be inserted into the same sheep eye by loosening the lateral incision sutures.
Eight bimanual and 2 1-handed phacoemulsification techniques were performed in the eye model. Four freshly enucleated sheep eyes were used by the trainees during practice sessions. The implanted nucleus tended to move to the periphery of the capsule and behind the iris with fluid turbulence or when pushed with the phacoemulsification tip during 1-handed phacoemulsification. This was solved by creating a deeper cavity in the sheep lens to hold the nucleus better.
Manipulation and emulsification of the nucleus were easier to perform with the bimanual technique than with 1-handed phacoemulsification. A chopper inserted through the side-port incision stabilized and chopped the nucleus and moved the fragments toward the phaco tip. The inserted lens nucleus was kept close to the phaco tip with high vacuum levels of about 150 mm Hg.
The practice session was terminated after emulsification of the human lens nucleus. A new nucleus was replaced through the lateral incision when damage to the cornea and/or iris did not prevent appropriate visualization. Three or 4 human lens nuclei were used in each sheep eye.
In 2 trials, the inserted human lens nucleus was divided in the anterior chamber using a vectis and a spatula under OVD protection. Afterward, lens fragments were removed from the eye by the vectis. The manual cracking method could also be performed successfully in this model.
Animal eye models may not simulate actual phacoemulsification surgery since the nuclear material is clear and soft. Thus, many alternatives have been suggested to induce a hard cataractous nucleus; these include chemicals 2–5 or microwave7, a synthetic nucleus,8 organic material such as chestnuts,9 or even implantation of a whole human lens in rabbit eyes.10 We searched for an easy and realistic method using inexpensive and readily available material.
Extracapsular cataract surgery is still performed for senile cataracts in many clinics in developing countries trying to convert to phacoemulsification or in cases of poor zonular support for safety reasons. This means plenty of available human lens nuclei for training purposes, which is the main focus of our technique. We used freshly enucleated sheep eyes as they were readily available; however, other animal eyes can be used in the same manner.
In our study, real cataractous human lens nuclei were used rather than artificially induced cataract nuclei. Thus, the trainees were able to experience the exact hardness fragmentation characteristics, thickness, diameter, and color of human lens nuclei during phacoemulsification practice.
We also had the chance to perform phacoemulsification in 2 human nuclei kept in 10% formalin in our model. We observed that formalin changed the consistency and fragility of the nuclei, possibly by denaturing proteins. Later, we used a moist chamber or a balanced salt solution to keep the human lens nuclei in their natural form.
The corneal structure, thickness, and diameter; the anterior chamber depth and volume; the rigidity and diameter of the capsular bag; and the iris structure are the main characteristics that differ between sheep and human eyes. Implantation of a foldable intraocular lens in the capsular bag through a corneal incision was possible but was not performed because it was not expected to provide realistic training practice for human eyes.
The major advantage of this model is the ability to emulsify human cataractous nuclei of the preferred hardness, which all models have tried to simulate, in the same sheep eye at a low cost.
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