Factors Affecting Corneal Hysteresis in Taiwanese
Condition: Corneal Hysteresis
Study type: Observational

Summary: 
      The purpose of the study is to investigate the factors affecting corneal hysteresis (CH) in
      Taiwanese.
    

Detailed description: 
      The ocular response analyzer (ORA, Reichert Ophthalmic Instruments, Buffalo, NY, USA)
      measures the biomechanical properties of the cornea in vivo. It provides several measurements
      including CH, corneal resistance factor (CRF), corneal-compensated intraocular pressure
      (IOPcc), and Goldmann-correlated intraocular pressure (IOPg). Corneal hysteresis is described
      as the viscous damping due to the viscoelastic resistance of the cornea to a deformation
      pulse by an air jet of the tonometer. Whereas CH may reflect corneal viscosity, the CRF may
      relate to the elastic properties of the cornea. Both parameters are measured by the ORA using
      a dynamic bidirectional process.

      CH can be affected by various ocular disorders and operation. In previous studies, CH was
      found to be significantly lower in keratoconus, Fuchs' dystrophy, and glaucoma. After laser
      in situ keratomileusis (LASIK) and phacoemulsification, CH permanently or temporally
      decreased. Among normal population, CH was also variable and associated with several factors:
      age, gender, intraocular pressure, central corneal thickness, and corneal curvature. High
      myopic patients with long axial length were found to have lower CH than in normal subjects.
      CH can also affected by races, and CH was largely discussed in Caucasian population. Herein
      we studied normal population of CH and associated factors and in Taiwan.

      We are planned to conduct a retrospective study from January to July, 2009 at Department of
      Ophthalmology, Far Eastern Memorial Hospital, Taipei, Taiwan by chart review. We will enroll
      the patients with bilateral cataract, or unilateral cataract and healthy fellow eye, who not
      having other ocular pathology, prior ocular surgery, or previous trauma. Several ocular
      parameters and demographics of both un-operated eyes in these patients are measured. CRF,
      IOPcc, IOPg, and CH are measured by ORA. Central corneal thickness is measured using the
      ORA's integrated handheld ultrasonic pachymeter. The IOLMaster (Carl Zeiss, Jena, German) is
      used to obtain the ocular biometry including axial length, anterior chamber depth, and
      keratometry. Linear regression models were used to test the correlation of CH and
      quantitative factors. The chi-square test was used to find the significance of correlation in
      categorical values.

      One or two eyes of the patients undergoes uneventful phacoemulsification and in-the-bag
      implantation of posterior chamber intraocular lens (one-piece AcrySof, Alcon lab, Texas, USA)
      by one surgeon (Wang JK). The sutureless corneal wound was 2.75 mm long. CRF, IOPcc, IOPg,
      and CH are measured by ORA preoperatively and 1 day, 1 week, 2 weeks, 3 weeks, 1 month, 2
      months, and 3 months postoperatively. Paired t test was used to compare the changes of these
      parameters following the operation.

      We expect that some factors will correlate with CH, such as axial length, IOP, age, and
      central corneal thickness. Temporary reduction in CH is possibly shortly after
      phacoemulsification, and returning to the baseline following a period of time.
    

Observational model: Case-Control
Time perspective: Retrospective
Sampling method: Probability Sample
Study population: 
        Normal population and patients receiving phacoemulsification
      
Keywords: corneal hysteresis, ocular response analyzer