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presbyopia is a progressive loss of crystalline lens accommodation that results in an inability to focus at near vision , which occurs with aging .
the large number of potential patients who might seek permanent spectacle - free correction of this condition have stimulated the increasing interest in refractive surgery . in recent years
, various surgical solutions [ 38 ] have been developed to effectively treat this condition . however
, none has emerged as the final and optimum solution for presbyopia because of complications , invasiveness , and lack of predictability and stability .
high intensity focused ultrasound ( hifu ) is a noninvasive or minimally invasive medical procedure which has shown considerable potential for a variety of therapeutic applications .
therapeutic ultrasound , although less well known than ultrasound for diagnostic imaging , has become a topic of growing interest in ophthalmology . in recent years
, several experimental and clinical studies for the treatment of glaucoma have been conducted [ 1015 ] .
although several studies have addressed the effects of hifu on the cornea in the 1980s and 1990s [ 16 , 17 ] , nowadays hifu has more advantages , such as controlled accuracy , accuracy of positioning in the treatment area , and smaller focal zones [ 1113 ] .
furthermore , a higher operating frequency allows for a steeper transition between the focal zone and the untreated area , thus reducing the risk of heating the neighboring healthy tissue .
therefore , the potential therapeutic applications of hifu might encompass many present uses of laser therapy in ophthalmology and also offer new methods of treating ocular problems .
the primary goal of the present study was to establish a new technique for correcting presbyopia .
so , we evaluate the histologic characteristics of corneal stroma lesions induced by hifu in animals .
we also report the change in dioptric power on rabbit eyes in vitro after hifu corneal circular treatment .
an approval of animal use was granted by the animal care and use committee at chongqing medical university .
whole animal management followed the arvo statement for the use of animals in ophthalmic and visual research ( arvo animal policy ) .
enucleated eyes from adult new zealand white rabbits ( 5 - 6 months old , 22.5  kg ) were provided by laboratory animal center of chongqing medical university .
before all animals were sacrificed using venous air embolism , they were anaesthetized using sumianxin ii intramuscular injection ( 0.1  ml / kg ) and all efforts were made to minimize suffering .
the fat and muscle were removed from all eyes with scissors after enucleation . within 1 hour of enucleation
, the eyes were stored in isotonic saline on ice until used , not later than 24 hours after enucleation .
hifu keratoplasty treatments were delivered with a prototype , which was designed by chongqing hifu technology co , ltd . ,
continuous therapeutic energy was emitted from a focal ultrasound transducer ( focal length , 5  mm ; focal zone , 65  m ) with a working frequency of 10.2  mhz , and therapy power was set at 1  w ( group 1 , 6 eyes ) , 2  w ( group 2 , 6 eyes ) , and 3  w ( group 3 , 6 eyes ) for an exposure time of 6 seconds .
intact rabbit eye globes were placed in a transparent plexiglas observation cell , which was put on the rotary motor with uniform velocity ( figure 1(a ) ) .
the ultrasound transducer placed perpendicular to the corneal surface and both had no direct contact with each other .
the focal length of the transducer was adjusted by vernier caliper to focus on the same depth of rabbit corneal stroma .
a stromal treatment ring ( 8  mm ) was produced within the peripheral corneal stroma after hifu radiation ( figures 1(b ) and 1(c ) ) .
the sirius scheimpflug camera ( cso , firenze , italy ) was used for corneal topographic imaging and characterized the treatment effects of the eye before and after hifu keratoplasty application immediately .
the rabbit eye was fixed with a fixed bracket , and it positioned in the front of the sirius scheimpflug camera .
the simulated keratoscope readings ( sim - k ) in central 3  mm zone of the rabbit cornea were analyzed .
for histologic examination , the corneas were fixed in 4% buffered formaldehyde , dehydrated in alcohol solutions of increasing concentration , cleared in xylene , embedded in paraffin , and sectioned into 5  m thick sections .
the sections were stained with hematoxylin - eosin and examined under a light microscope ( lm ) .
digimizer software ( version 3.1.1.0 ) was used for measuring the maximum width and length of the focal region . for tem examination ,
the corneas were placed into a glutaraldehyde 2.5% in 0.1  mol / l cacodylate buffer ( ph 7.3 ) at 4c , for at least 24 hours , and then postfixed in 1% osmium tetroxide in 0.1  mol / l cacodylate buffer ( ph 7.3 ) at 4c for 1 hour .
group 1 ( 1  w ) and group 3 ( 3  w ) were measured before and after hifu keratoplasty application by using the sirius scheimpflug camera . the mean keratometric power ( sim - k value ) for corneas before treatment was 49.42  0.30  d and 48.00  1.95  d in groups 1 and 3 , respectively . the mean keratometric power ( sim - k value ) after treatment was 51.37  1.11  d and 57.00  1.84  d in groups 1 and 3 , respectively .  
figure 2 shows the corneal topographic variation in the tangential corneal curvature induced by hifu treatment in a rabbit eye . as shown in figure 3 , the stroma cornea
the mean major axis of the oblique elliptical shape in cornea of group 1 was 284  18  m , 431  7  m in group 2 , and 532  33  m in group 3 .
the mean minor axis of the oblique elliptical shape in cornea of group 1 was 122    24  m , 245    33  m in group 2 , and 274    68  m in group 3 .
meanwhile , the major axis and minor axis of the focal region got longer when the powers of the hifu got increased ; the difference was statistically significant ( p < 0.05 ) .
tem of hifu - treated zones demonstrated that the morphologic appearance of the keratocytes was close to normal ( figure 4(b ) ) . within the hifu - affected area ,
thus the morphology and the structure of keratocytes would be affected by the crumpled collagen layers .
according to figure 4(d ) , the collagen layers crumpled significantly within the hifu treatment area . besides the typical structure of stromal collagen , microfibrillar aggregations can also be observed throughout the area of crumpled collagen layers .
in the present study , the histologic changes and the variation in dioptric power induced by hifu treatment in rabbit corneas were evaluated . since 1898 , using heat to change the morphology of the cornea has been employed for different therapeutic and surgical objectives .
the best known of these techniques is probably thermokeratoplasty ( tkp ) , which is based on the principle that heating corneal tissue causes collagen fibers to shrink and hence changes the corneal curvature . to date , 3 main tkp technologies have emerged : laser thermokeratoplasty ( ltk ) , conductive keratoplasty ( ck ) , and microwave keratoplasty . however , due to complications , invasiveness , and lack of predictability and stability , none has emerged as the optimum solution for presbyopia .
hifu thermal treatment is a novel , minimally invasive medical procedure which has shown considerable potential for a variety of therapeutic applications .
the conception of hifu was firstly brought forward by lynn and putnam in 1944 . in 1960 , w. j. fry and f. j. fry   applied hifu technology to experimentally treat nervous system diseases and suggested the potential of hifu in surgical operations . in ophthalmology ,
hifu for the treatment of glaucoma and ultrasonic drug delivery are the two main areas of research and potential clinical application .
nevertheless , hifu keratoplasty was first described in 1990 by rutzen et al .   as a theoretical option for inducing collagen shrinkage in corneas .
they commented that hifu could be used to heat the peripheral cornea with great precision because of the small focal zone and the excellent aiming capabilities of hifu .
however , with the development of high - frequency miniaturized transducer , hifu enables the creation of smaller focal zones ( 65  m ) that better target the treatment areas , particularly for small organs such as clear cornea .
the higher operating frequency ( 10.2  mhz ) also allows for a steeper transition between the focal zone and the untreated area .
in addition , hifu also enables a defined and adjustable tissue volume to be heated and treated at any depth or location within the eye .
our group evaluated the histologic characteristics of corneal stroma lesions induced by hifu in rabbits and also report the change in dioptric power of the procedure . in the present study , our results demonstrated that hifu keratoplasty increased the power of the rabbit corneas from 49.42  0.30  d to 51.37  1.11  d in group 1 and from 48.00  1.95  d to 57.00  1.84  d in group 3 .
the results indicated that the variations of curvature of the rabbit corneas got steeper when the powers of the hifu got increased .
like other thermal techniques ( i.e. , ltk , ck ) , hifu thermal treatment is applied in the periphery of the cornea , it causes peripheral flattening and corresponding steepening in the center ( figures 2(b ) and 2(d ) ) .
in addition , our results show a protective effect of epithelium and basement membrane within the hifu - treated area ( figure 3 ) .
moreover , as this study showed , because of the epithelium intactness , the topography examination is possible immediately after surgery . in the clinic
it is well known that postoperative dry eye can be commonly seen after excimer laser - based refractive surgery .
however , hifu thermal treatment requires no cutting of the cornea ; therefore , the problem of postoperative dry eye and infection would not likely occur .
our findings demonstrated that hifu thermal treatment induced annular elliptical treatment zone in the anterior stroma ( figure 3 ) .
although the shape of hifu thermal lesion differs from ck , which creates a thermal footprint that is uniformly cylindrical , the effect seems to be similar to ck .
a full circular elliptical treatment ring of hifu keratoplasty applied to the peripheral cornea produces a cinching effect that increases the curvature of the central cornea .
for this reason , further studies are necessary to determine whether there is regression of treatment effect .
our long - term goal is to use ultrasound to produce permanent alterations in corneal curvature .
in addition , the time and dose of the effect on collagen fibers as well as matrix changes will have to be fully elucidated in in vivo experiment .
our group will continue to develop hifu keratoplasty to improve repeatability and will compare its performance to existing methods for correcting presbyopia .
the development of treatment parameters will be necessary to determine the optimum depth , diameter of treatment ring , and power of treatment to produce specific refractive changes . in conclusion
, the results in our study suggest that hifu as a noninvasive method for applying in the peripheral cornea could steepen the central cornea .
we have initially exploited hifu to establish a new technique for changing the refractive index of the intermediate cornea to correct presbyopia .
there will be a good application prospect for hifu keratoplasty in the treatment of presbyopia .
purpose . to evaluate high intensity focused ultrasound ( hifu ) as an innovation and noninvasive technique to correct presbyopia by altering corneal curvature in the rabbit eye . 
 methods . 
 eighteen enucleated rabbit eyes were treated with a prototype hifu keratoplasty . according to the therapy power , 
 these eyes were divided three groups : group 1 ( 1  w ) , group 2 ( 2  w ) , and group 3 ( 3  w ) . 
 the change in corneal power was quantified by a sirius scheimpflug camera . 
 light microscopy ( lm ) and transmission electron microscopy ( tem ) were performed to determine the effect on the corneal stroma . 
 results . in the treated eyes , 
 the corneal curvature increases from 49.42  0.30 diopters ( d ) and 48.00  1.95  d before procedure to 51.37  1.11  d and 57.00  1.84  d after hifu keratoplasty application in groups 1 and 3 , respectively . 
 the major axis and minor axis of the focal region got longer when the powers of the hifu got increased ; the difference was statistically significant ( p < 0.05 ) . 
 lm and tem showed hifu - induced shrinkage of corneal stromal collagen with little disturbance to the underlying epithelium . conclusions . 
 we have preliminarily exploited hifu to establish a new technique for correcting presbyopia . 
 hifu keratoplasty will be a good application prospect for treating presbyopia .