You will be presented with the full text of a biomedical research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
the instrument measures corneal hysteresis ( ch ) and corneal resistance factor ( crf ) as the markers of corneal viscoelastic properties .
technically , ch is the difference between inward and outward applanation pressures created by an air puff and is an indicator of viscous properties of the cornea .
however , crf is a measurement of corneal resistance to deformation and indicates elastic properties of the cornea .
corneal properties other than central corneal thickness ( cct ) have several implications especially in glaucoma and corneal refractive surgery .
corneal viscoelastic properties depend on the structure and organization of collagen fibrils and extracellular matrix ( ecm ) .
collagen fibers are responsible for strength and elasticity of the cornea while the ecm is responsible for the viscous properties .
structural changes with age include an increase in collagen fiber diameter as a result of an increase in number of collagen molecules and expansion of intermolecular space .
ex  vivo measurements of corneal biomechanical properties have shown progressive stiffening of the cornea with age.6 , 7 , 8 , 9 however , the results of in  vivo measurements of corneal biomechanics are inconclusive .
some studies have shown no change by age,11 , 12 , 13 , 14 while some report a decrease in ch and crf.15 , 16 , 17 , 18 , 19 this study was conducted to evaluate the corneal viscoelastic properties in different age groups .
this cross - sectional study was performed at the department of ophthalmology , ahvaz jundishapur university of medical sciences , ahvaz , iran .
the study protocol was approved by the local ethics committee and adhered to the tenets of declaration of helsinki .
informed consent was obtained from all patients . in this observational cross - sectional study ,
exclusion criteria included any history of ocular or systemic diseases including diabetes , hypertension , hyperlipidemia , glaucoma , ocular surgery , keratoconus , ocular or systemic medications including corticosteroids , corneal astigmatism > 2  d , severe dry eye , pregnancy , and contact lens use within 1 month of the study .
after a complete ocular examination , corneal viscoelastic properties were measured using ora and cct measured using an ultrasonic pachymeter ( pachymeter sp 3000 , tomey , nagoya , japan ) . in case of ocular disease , the sound eye and otherwise the right eye was chosen for the study .
individuals were put into 6 groups : group 1 ( 1019 years ) , group 2 ( 2029 years ) , group 3 ( 3039 years ) , group 4 ( 4049 years ) , group 5 ( 5059 years ) , and group 6 ( 6069 years ) .
ora is an air puff tonometer that measures the corneal response to a steady air pulse .
it makes two applanation measurements : a force - in applanation which has been attributed to the dampening effects of the cornea and a force - out applanation that occurs at a lower pressure than the first .
the difference between the two pressures is ch and indicates viscous properties of the cornea , while crf shows the elastic properties of the cornea .
therefore , these parameters may affect intraocular pressure ( iop ) measurements more than cct alone .
the instrument also reports goldman - correlated iop ( iopg ) and its corneal - compensated counterpart ( iopcc ) .
ora parameters including ch , crf , iopg , and iopcc , as well as cct , and spherical equivalent refractive error were considered for analysis .
refractive errors of 1 to 1  d were considered as emmetropia . to obtain a power of 0.8 and   =  0.05 , sample size of 36 cases
in addition , to compare data , we used one way analysis of variance ( anova ) .
to obtain a power of 0.8 and   =  0.05 , sample size of 36 cases was calculated for each age decade .
pearson correlation coefficient and partial correlation were used in analyses . in addition , to compare data , we used one way analysis of variance ( anova ) .
overall , 302 healthy individuals were enrolled in the study . table 1 shows demographic data and participants ' characteristics .
ch and crf were significantly different among the age groups ( p  <  0.001 , anova ) .
group 1 ( 1019 years old ) showed significantly higher ch and crf compared to all other age groups ( p  <  0.001 ) while other groups were not significantly different from each other ( all p values > 0.05 , tukey test ) ( table 1 , fig .  1 ) .
ch and crf were highly correlated ( p  <  0.001 , r  =  0.821 ) .
age showed a significant negative correlation with ch ( p  <  0.001 , r  =  0.353 ) and crf
female gender had significantly higher ch ( 11.35 vs 10.85 , p  =  0.017 ) and crf ( 10.9 vs 10.4 , p  =  0.019 , independent t - test ) .
after adjustment for gender , cct , iop , and refractive error , age remained significantly correlated with ch and crf ( p  <  0.001 for both , r  =  0.398 and r  =  0.372 , respectively ) .
cct showed a biphasic pattern in which groups 1 , 5 , and 6 had significantly higher ccts compared to other groups ( table 1 , fig .  2 ) .
ch and crf showed significant positive correlation with cct ( p  <  0.001 for both , r  =  0.21 and r  =  0.26 , respectively ) .
iopcc was significantly higher in group 4 compared to group 1 ( p  =  0.045 ) .
iopg was significantly higher in group 1 ( compared to groups 2 and 3 , p  =  0.007 ) and group 4 ( compared to group 3 , p  =  0.007 ) .
iopg and iopcc were significantly correlated ( p  <  0.001 , r  =  0.845 ) and were not significantly different ( p  =  0.148 ) .
iopg ( but not iopcc ) was significantly correlated with cct ( p  =  0.001 , r  =  0.193 ) and crf ( p  <  0.001 , r  =  0.598 ) , while iopcc was negatively correlated with ch ( p  <  0.001 , r  =  0.474 ) .
( p  =  0.055 , anova ) ch and crf showed a significant positive correlation with refractive error ( p  =  0.001 , r  =  0.187 and p  =  0.007 , r  =  0.157 , respectively , fig .  3 ) .
ch and crf were significantly higher in hyperopes compared to emmetropes and myopes ( table 2 ) .
this study showed a significant decrease in corneal response parameters ch and crf measured with the ora with age .
corneal properties including thickness , hydration , elasticity , viscosity , and possibly other unrevealed factors can affect corneal behavior during iop measurements and after refractive surgery .
corneal viscoelastic properties are provided by collagen fibers , and glycosaminoglycans and proteoglycans forming the ground substance .
abnormal corneal viscoelastic properties are seen in keratoconus and post laser keratectasia.2 , 15 aging induces structural changes in the cornea including an increase in collagen fiber diameter as a result of an increase in the number of collagen molecules in collagen fibrils and expansion of intermolecular space due to glycation - induced cross - linking .
ex  vivo studies have shown significant stiffening of the human cornea with age.6 , 7 , 8 , 9 this has been attributed to non - enzymatic cross - linking of the stromal collagen fibrils . however , the relationship between corneal stiffening and age is not linear,6 , 7 and the stiffness increases by a factor of approximately two between the ages of 20 and 100 years .
for example , keratoconus is a disease of youth , the incidence of which decreases with age .
additionally , youth is a risk factor for both post laser keratectasia and keratoconus progression.22 , 23 based on the above - mentioned evidence , an increase in corneal stiffening with in  vivo measurements is expected as well . in  vivo measurement of corneal biomechanics has been possible since 2005 with ora , and for years it was the only commercially available instrument .
ora has shown lower ch and crf in keratoconus and fuchs endothelial dystrophy as expected.2 , 15 however , the results of studies on age - related changes in corneal biomechanics are not consistent .
some studies on children have shown no correlation between age and ch11 , 12 or crf with similar or slightly higher ch and crf values in children compared to adult studies .
on the other hand , several studies have reported a decrease in ch and crf with age.15 , 16 , 17 , 18 , 19 , 24 ortiz et  al in a study on 165 normal eyes showed significantly lower ch in the age group 6080 years compared to age group 914 years .
kotecha et  al reported an iop - independent biomechanical property of the cornea ( corneal constant factor ; ccf ) that increased with thicker cct and decreased with greater age .
their study group consisted of 105 ocular hypertensives and normal individuals with a mean age of 60 years .
some of these studies suffer from a small sample size , limited age range , and mixed study population . in another study on 204 normal eyes with a mean age of 46.7 years ( range : 1989 years ) , a significant decrease in ch and crf
we observed significantly higher ch and crf in the younger age group and a significant negative correlation with age .
the measured values were similar to those reported elsewhere.10 , 11 , 12 most studies evaluating the effect of cct on corneal viscoelastic properties have reported a positive correlation between cct and ch,10 , 11 , 13 , 14 , 16 , 19 , 24 or crf.10 , 11 , 14 , 19 , 24 in the present study , cct showed a biphasic pattern with higher values in the young and elderly and had significant correlation with ch and crf . in group 1 , higher ch and crf are in agreement with higher cct . however , considering structural changes in the elderly , lower ch and higher cct can be explained by changes in ecm and corneal hydration by aging as the elderly have poorer corneal hydration control compared to the young .
thus , while the increase in collagen cross - linking causes corneal stiffening , increased water content of the cornea causes a reduction in ch and crf .
clinically , it is possible to postulate that low ch accompanied by low cct is attributed to collagen weakness like keratoconus , while low ch with high cct is more likely the result of ground substance changes such as hydration
measuring corneal biomechanics by a new ultra - high - speed scheimpflug camera ( oculus corvis st , scheimpflug technology ; wetzlar , germany ) has shown significant corneal stiffening by age manifested in an increase in highest concavity time ( i.e. time from starting until highest concavity is reached ) .
several studies have shown that a negative correlation exists between ch and iop.10 , 11 , 13 , 19 in our study , we observed a significant negative correlation between iopcc and ch , and a significant positive correlation between iopg and crf .
although we observed a significantly higher iopcc in age group 4 compared to group 1 , the difference does not seem to be clinically important .
it has been reported that ch is significantly lower in high myopes14 , 24 , 27 and higher in hyperopes compared to normal individuals .
we observed significantly higher ch and crf in hyperopes compared to myopes and emmetropes as well .
however , by excluding high astigmatism we tried to overcome this limitation . additionally , axial length is correlated with refractive error which was included in the study .
some strengths of our study are the large sample size and that we enrolled individuals with a wide range of age , without previous surgery , ocular pathology or systemic disease , and evaluated corneal viscoelastic changes in 6 decades . in conclusion ,
age - related decrease in ch and crf may be due to increased corneal hydration with age .
iopcc is not correlated to cct and may be a better indicator of real iop .
purposeto determine age - related changes in corneal viscoelastic properties in healthy individuals.methodsthis observational cross - sectional study was performed at the department of ophthalmology , imam khomeini hospital , ahvaz , iran and included 302 healthy individuals in 6 age decades ( range : 1069 years ) . after complete ocular examination , 
 corneal viscoelastic properties were measured by ocular response analyzer and central corneal thickness ( cct ) by an ultrasonic pachymeter . 
 our main outcome measures were corneal viscoelastic properties in different age groups.resultscorneal hysteresis ( ch ) and corneal resistance factor ( crf ) showed a significant negative correlation with age ( p  <  0.001 for both , r  =  0.353 and r  =  0.246 , respectively ) . 
 female gender had significantly higher ch ( p  =  0.017 ) and crf ( p  =  0.019 ) . 
 ch and crf were significantly correlated ( p  <  0.001 , r  =  0.821 ) . 
 cct showed a biphasic pattern with significantly higher thicknesses before 20 and after 50 years of age . 
 ch and crf were significantly correlated with cct ( p  <  0.001 for both , r  =  0.21 and r  =  0.26 , respectively ) and intraocular pressure ( iop ) ( p  <  0.001 for both , r  =  0.474 and r  =  0.598 , respectively ) . 
 corneal - compensated iop ( iopcc ) was significantly higher after age 40 compared to age group < 20 ( p  <  0.045 ) . 
 goldmann - correlated iop ( iopg ) was significantly correlated with cct ( p  =  0.001 , r  =  0.193 ) , while iopcc showed no correlation with cct ( p  =  0.265 , r  =  0.062 ) . 
 ch was significantly higher in hyperopic eyes compared to emmetropic eyes ( p  =  0.009 ) and myopic eye ( p  <  0.001).conclusionsin this study , there was a decrease in ch and crf with an increase in age . 
 hyperopia and female gender are associated with higher ch and crf . 
 cct is higher toward the extremes of life and is significantly correlated with ch and crf .