Studies On Real-Time Intraocular Pressure During Small Incision Lenticule Extration And Effects On Centration Of Treatment Zone By Changes Of Body Positions And Shifts Of Cornea Vertex Postoperatively

PurposeTo investigate Real-Time intraocular pressure(IOP)during small incision lenticule extraction(SMILE)for myopia correction.To evaluate the pupil center shifts and pupil diameter size in the seated position,supine position,right and left lateral position.To investigate effect of angle kappa on ablation centration following SMILE surgery.To evaluate the shift of the cornea vertex postoperatively and factors associated with the shift.Methods1.Twelve female New Zealand white rabbits(body weight 2.22 ± 0.23 kg)were divided into two groups according to the induced refractive spherical correction in the right eyes,-2.00 D group(n=6)and-6.00 D group(n=6).Oxybuprocaine was applied on the surgical eye peroperatively for topical anesthesia,the rabbits were also anesthetized with intramuscularly administered xylazine(2 mg/kg body weight)and intravenous chloral hydrate(50 mg/kg body weight)for the general anesthesia.IOP was measured under the two kinds of anesthetisia mentioned above with a tonometer Then,the corneal thickness was measured by an anterior segment optical coherence tomography(ASOCT).During SMILE,Real-Time IOP was measured by an optic fiber pressure sensor(OFPS)in the vitreous cavity of rabbit eyes via the nasal pars plana.2.21 subjects(42 eyes)aged from 18 to 38 years were captured eye images by a smartphone with a novel Application(EyeTurn)at 40cm in four body positions respectively(seated,supine,right lateral and left lateral)under similar photopic conditions.During taking images,subjects were asked to focus on the camera light,and EyeTurn App(EyeNexo,Boston,MA)was used to guide positioning and minimize head tilt.From the images,the centration and diameter of the corneal limbus and the pupil were computed,therefore,the pupil center shifts and pupil diameter changes can be analyzed.3.A total of 43 consecutive patients(86 eyes)underwent the SMILE surgery at a single institution were included in the present study.The location of pupil center and corneal vertex was measured using a Scheimpflug camera preoperatively and at 1,3,and 6 months postoperatively.The distance between the corneal vertex and pupil center was defined as the deviation(D),while the difference vector of cornea vertex between preoperative and postoperative was defined as AD.The correlations between AD and spherical equivalent(SE)/changes of pupil diameter were analyzed.Results1.The IOP measured After topical anesthesia was(11.77±2.21)mmHg,higher than the IOP measured after general anesthesia(9.97±1.98)mmHg(P<0.001).The average central corneal thickness(CCT)of 12 rabbits was(318.68±29.01)μm,having positive correlation with the IOP measured after topical anesthesia(r=0.235,P=0.009).No correlation was found between IOP measured after general anesthesia,Real-Time preoperative baseline IOP and CCT(r=0.530,0.7704).During SMILE procedure,the preoperative baseline IOP was measured as 15.29±3.06mmHg for-2.00 D group and 15.04±1.75mmHg for-6.00 D group,the IOP before suction was measured as 44.31±14.86mmHg for-2.00 D group and 46.81±14.82mmHg for-6.00 D group,the IOP at suction on was 83.94±23.87 mmHg for-2.00 D group and 89.17±22.66 mmHg for-6.00 D group,the IOP during the lenticule creation was 74.81±20.64 mmHg for-2.00 D group and 76.94±27.43 mmHg for-6.00 D group,the IOP at suction off was 37.79±11.08 mmHg for-2.00 D group and 38.37±17.81 mmHg for-6.00 D group,the postoperative baseline IOP was 15.36±2.38mmHg for-2.00 D group and 15.69±1.32mmHg for-6.00 D group.There were not significant differences between two groups at each step of SMILE procedure(P<0.05).The mean IOP during the lenticule separation/extraction was 32.26±2.91mmHg,the duration of IOP increase was 123.57±33.29 s.Taking the step of lenticule creation into account,the duration of IOP increase during the whole procedure was about 166.05s(no more than 3minutes).2.Pupil center located slightly nasal and superior in the seated position relative to limbus center,the shift deviations were(0.20±0.10)mm for right eyes and(0.20±0.07)mm for left eyes.However,it shifted more nasally and superiorly in the supine position,the shift deviations were(0.55±0.11)mm for right eyes and(0.55±0.11)mm for left eyes,significant differences were found when compared to the seated position(P<0.001,0.001).In right lateral position,both of pupil centers shifted to right,but the shift deviation of left eye(0.33±0.14)mm was larger than the right eye(0.18±0.13)mm(P=0.005).In left lateral position,both of pupil centers shifted to left,but the shift deviation of right eye(0.30±0.19)mm was larger than the left eye(0.21±0.11)mm(P=0.046).When switching from the seated position to the supine position,the pupil diameter became smaller,from(3.43±0.72)mm to(2.82±0.42)mm for right eye,and from(3.45±0.72)mm to(2.81±0.43)mm for left eye(P=0.09,0.08).No significant changes of pupil diameter were found about between seated position and lateral positons(P<0.05).The x-coordinates of the cornea vertex were(0.020±0.103)mm for right eyes and-(0.023±0.112)mm for left eyes preoperatively.At 6 months postoperatively,there were no differences with the preoperative level(P=0.312,0.071).The y-coordinates of the cornea vertex were(0.072±0.104)mm for right eyes and(0.058±0.104)mm for left eyes preoperatively.At 1 month postoperatively,they increased,(0.129±0.139)mm for right eyes and(0.093±0.122)mm for left eyes,no significant changes were found at 3 months and 6 months postoperatively(P>0.05).There were no significant changes in Difference vectors of cornea vertex(AD)at 1 month,3 months,and 6 months after surgery(P>0.05),about 0.1 mm.A significant correlation was observed between AD and SE(r=-0.408,P<0.001).Conclusions1.The IOP fluctuations in the vitreous cavity using an OFPS in a rabbit model during SMILE showed that Real-Time IOP significantly was increased during Pre-suction,Suction on,Cutting,suction off and lenticule separation/extraction compared to baseline IOP,although,peaked at Suction on(averaged about 80 mmHg).Neither the degree of myopic correction nor central corneal thickness significantly affected these changes in IOP.2.The pupil center shifted towards nasally and superiorly when the body position changed from seated to supine with the pupillary constriction simultaneously.Therefore,the pupil center shifts with the change of body position should be in consideration during the corneal refractive surgery since the centration might be affected.Because of no automatic central position system applied during SMILE,more attentions should be paied to the influence of body position changes on the pupil center when surgens determine the center of treatment zone according to the pupil center3.The cornea vertex shifted after SMILE,mainly shifted superiorly about 0.1 mm postoperatively,which may be larger with the myopic refractive error becoming higher.The shift of cornea vertex postoperatively may cause measurement errors leading to the misjudgment of decentration or introduction of reference axis-related aberrations measured by corneal topographers.