title: The roles of cornea and axial length in corneal hysteresis among emmetropes and high myopes: a pilot study.
abstract: OBJECTIVE
  To observe the roles of cornea and axial length (AL) in corneal hysteresis (CH) among emmetropes and high myopes.
  
  
  METHODS
  The CH, corneal resistance factor (CRF) and corneal-compensated intraocular pressure (IOPcc) of 40 young emmetropes (spherical equivalent +/-0.50 D) and 40 age-matched high myopes (-6D or more) were measured by the Ocular Response Analyzer. AL was measured using a partial coherence interferometer. The mean keratometry reading (meanK) and corneal volume (CV) in 4 zones were measured using the Pentacam. Full stromal thickness (FST) and epithelial and Bowman's thicknesses (Epi + BT) were evaluated using confocal microscopy. Valid confocal microscopic results from the more emmetropic eyes of 30 emmetropes and the more myopic eyes of 27 high myopes were studied.
  
  
  RESULTS
  High myopes exhibited significantly longer AL (27.34 ± 0.90 mm versus 23.71 ± 0.79 mm), lower CH (10.17 ± 1.38 mmHg versus 11.11 ± 1.25 mmHg), and higher IOPcc (16.50 ± 3.05 mmHg versus 13.91 ± 2.49 mmHg) compared with emmetropes. The 2 groups showed similar CRF, CVs, meanK, FST and Epi + BT. By combining the results of both groups, CH and CRF were significantly correlated with FST (r(2)= 0.368 and r(2 )= 0.408, respectively), CVs (r(2)= 0.251-0.391 and r(2)= 0.201-0.346, respectively), but not with Epi + BT or meanK. Furthermore, AL and IOPcc were significantly correlated with CH (r(2)= 0.112 and r(2)= 0.161, respectively) but not with CRF. The multivariate analysis indicated that FST (standardized coefficients, β = 0.591) and CV (β = 0.575) had stronger association than IOPcc and AL for CH.
  
  
  CONCLUSIONS
  High myopes exhibited lower CH than emmetropes. Although CH had stronger correlation with FST than AL, it might contain the biomechanical properties of components other than the cornea.
authors.affiliation: School of Optometry, The Hong Kong Polytechnic University , Hong Kong SAR , China.
authors.forename: Yin-Zhi
authors.initials: YZ
authors.lastname: Wong
authors.forename: Andrew K C
authors.initials: AK
authors.lastname: Lam
mesh_terms: D000293:Adolescent; D000328:Adult; D058452:Axial Length, Eye / Q000473:pathology*; D003315:Cornea / Q000502:physiology*; D004548:Elasticity / Q000502:physiology*; D058453:Emmetropia / Q000502:physiology*; D005260:Female; D006801:Humans; D007368:Interferometry; D007429:Intraocular Pressure / Q000502:physiology; D008297:Male; D018613:Microscopy, Confocal; D047728:Myopia, Degenerative / Q000503:physiopathology*; D010865:Pilot Projects; D014065:Tonometry, Ocular; D055815:Young Adult
keywords: Axial length; corneal hysteresis; corneal stromal thickness; corneal volume; myopia