title: Comparison of laser interferometry and ultrasound A-scan in the measurement of axial length.
abstract: OBJECTIVE
  Laser interferometry is a new, non-contact technique for the measurement of axial length. In this study we compared measurements of axial length obtained with this technique with those obtained with ultrasound (A-scan). The reproducibility and examiner-dependency of the two methods were also analysed.
  
  
  METHODS
  Patients presenting at the cataract assessment clinic were invited to participate in the study. Axial length measurements were obtained both by contact ultrasound (A-scan) and by non-contact laser interferometry (IOLMasterTM V1.1, Carl Zeiss, Jena, Germany). Intraocular lens powers were calculated using both sets of measurements. The coefficient of variation served as a measure of reproducibility.
  
  
  RESULTS
  A total of 100 eyes in 100 patients were evaluated after informed consent had been obtained. Although estimates of axial length obtained with the two techniques were highly correlated, axial lengths obtained with the contact method (mean 23.35 mm, SD 1.81 mm) were consistently lower than those obtained with the non-contact method (mean 23.55 mm, SD 1.76 mm) and the difference was statistically significant (p < 0.001). The coefficient of variation was lower with laser interferometry (0.1%) than with the ultrasound technique (0.49%).
  
  
  CONCLUSIONS
  Different estimates of axial length are obtained using contact and non-contact techniques, with the latter producing consistently higher measurements than the former. Laser interferometry provides more reproducible results that should improve the accuracy of measurements of axial length in the clinical setting.
authors.affiliation: Department of Ophthalmology, University Hospital of Wales, Heath Park, Cardiff CF4 4XW, Wales, UK.
authors.forename: R
authors.initials: R
authors.lastname: Goyal
authors.forename: R V
authors.initials: RV
authors.lastname: North
authors.forename: J E
authors.initials: JE
authors.lastname: Morgan
mesh_terms: D000368:Aged; D001699:Biometry / Q000379:methods; D003941:Diagnostic Techniques, Ophthalmological; D005123:Eye / Q000033:anatomy & histology*; D005260:Female; D006801:Humans; D007368:Interferometry / Q000379:methods*; D007834:Lasers; D007910:Lenses, Intraocular; D008297:Male; D055095:Optics and Photonics; D011446:Prospective Studies; D015203:Reproducibility of Results; D014463:Ultrasonography / Q000379:methods