15 Optical Coherence Tomography Analysis of Wound Architecture in Sub-1-mm Cataract Surgery (700-µm Cataract Surgery)
Clear corneal incisions are commonly used in cataract surgeries because of their self-sealing behavior and not requiring sutures. This self-sealing wound acts as a barrier for the ingress of microbes, thereby reducing the chance of postoperative infection. Clinically, cataract wounds are observed by the gold standard slit-lamp biomicroscopy. The nature of corneal wounds, however, has been well studied only by high-resolution imaging such as optical coherence tomography (OCT). 1 Wound architecture of a clear corneal wound includes the incision length or size, angle, epithelial alignment, stromal thickness, coaptation, and endothelial apposition. Microphakonit, a sub-1-mm cataract surgery, involves a clear corneal incision of less than 1 mm. 2 , 3 The smallest cataract incisions, reported in 2005, using 700-µm cataract surgical instruments has been termed microphakonit 2 , 3 , 4 , 5 to differentiate it from 0.9-mm phakonit. 6 , 7 In this chapter, we highlight the salient features and observations of cataract wound in microphakonit.
15.1 Microphakonit: 700-µm Cataract Surgery
With the patient under local anesthesia, a clear corneal side-port incision is made using a 0.8-mm keratome, and an ophthalmic viscosurgical device is injected. The main clear corneal phacoemulsification (phaco) incision is also made using a 0.8-mm microphakonit keratome at the intended position. A 5- to 6-mm capsulorhexis is performed with the 26-gauge needle bent to form a cystitome. A 25-gauge rhexis forceps (MicroSurgical Technology [MST], Redmond, WA) may also be used by those more accustomed to performing the rhexis using the capsulorhexis forceps. In the nondominant hand, a globe stabilization rod can be used to control the eye movements.
Cortical cleaving hydrodissection is performed, and the fluid wave under the nucleus and the rotation of the nucleus are checked. The advantage of microphakonit is that one can do hydrodissection from both incisions so that even the subincisional region can be easily hydrodissected. It is noteworthy, however, that because there is little escape of fluid, one should be careful during hydrodissection. If too much fluid is passed into the eye, a complication such as a posterior capsular rent may occur; therefore, it is necessary to decompress the anterior chamber during this maneuver by applying slight posterior pressure on the scleral lip while doing hydromaneuvers. The 22-gauge (0.7-mm) irrigating chopper (MST) is connected to the infusion line of the phaco machine and introduced using the foot pedal on position 1. The phaco probe is connected to the aspiration line, and the 0.7-mm phaco tip (without an infusion sleeve) is introduced through the clear corneal incision. Using the phaco tip and moderate ultrasound power, the center of the nucleus is directly embedded, starting from the superior edge of the rhexis, with the phaco probe directed obliquely downward toward the vitreous (Fig. 15.1a). The settings at this stage are as follows: phaco power, 50%; aspiration flow rate, 20 cc/minute; and vacuum, 100 to 200 mm Hg. Using the karate-chop technique, the nucleus is chopped and removed. Cortical washup is then accomplished using the bimanual irrigation/aspiration (0.7-mm set) technique (Fig. 15.1b).
One of the limitations in bimanual minimally invasive cataract surgery is destabilization of the anterior chamber during the surgery. This limitation has been overcome by the introduction of gas-forced infusion and the use of an antichamber collapser, 8 , 9 which injects air into the infusion bottle, pushing more fluid into the eye through the irrigating chopper and also preventing a surge. Thus, the use of 20/21-gauge irrigating chopper solves the problem of destabilization of the anterior chamber during surgery. Now, using a 22-gauge (0.7-mm) irrigating chopper, it is extremely essential that gas forced infusion, also called external gas forced infusion, be used in the surgery. When the surgeon uses the air pump contained in the same phaco machine, it is called internal gas forced infusion. Transmission of infective microbes is prevented by the use of a Millipore filter connected to the machine. The Stellaris machine made by Bausch & Lomb (Rochester, NY) has an inbuilt air pump to produce pressurized infusion.
15.1.1 Anterior Segment OCT Evaluation of Microphakonit Wound
Direct visualization of the wound can be achieved using anterior-segment (AS) OCT. Examination of postoperative wound morphology can be done using the prototype AS OCT (Carl Zeiss Meditec, Inc, Dublin, CA) Visante of 1310-nm wavelength. The method is noncontact and noninvasive. Corneal high-resolution (10 × 3 mm) single scan mode is preferred for wound examination. The axial resolution of AS OCT used is 18 µm, and transverse resolution is 60 µm. This noncontact method of imaging provides micrometer-scale cross-sectional images of the tissue. The patient is seated comfortably in the chair with the head positioned. The forehead and the chin of the patient are positioned in the headrest and the dual chin rest with automated right and left sensors, and the patient is advised to view the target through the examination eye. Then the examiner starts the scanning process by focusing with the video screen. Once the corneal vertex is observed on the screen, the clear corneal wound is traced. Usually, the wound will be seen as small opaque line across the cross-sectional cornea (Fig. 15.2).