title: Influence of axial length on thickness measurements using spectral-domain optical coherence tomography.
abstract: OBJECTIVE
  The purpose of this study was to assess the influence of axial length on spectral-domain optical coherence tomography (SD-OCT) thickness measurements in patients with subretinal visual implants.
  
  
  METHODS
  Data from eight emmetropic pseudophakic eyes of eight patients with subretinal visual implants were analyzed retrospectively. These patients participated in the monocentric part of a multicenter trial. The axial length was measured in three short (<22.5 mm), three medium (22.51-25.50 mm), and two long (>25.52 mm) eyes. Using Heidelberg Spectralis, the known thickness of a subretinal implant microchip (70 μm) was measured on 15 images per eye with SD-OCT, using the software calipers.
  
  
  RESULTS
  The mean axial length was 20.8 ± 0.8 mm in short eyes, 23.3 ± 0.4 mm in medium eyes, and 26.3 ± 0.5 mm in long eyes. We found in short eyes, in medium eyes, and in long eyes a mean value of microchip thickness measurements from SD-OCT of 82.9 ± 1.4 μm, 70.5 ± 1.3 μm, and 64.2 ± 1.3 μm, respectively. The thickness measurements decreased in SD-OCT measurements with longer axial lengths significantly (P < 0.0001).
  
  
  CONCLUSIONS
  Axial length influences SD-OCT thickness measurements. Our findings demonstrate accuracy of the scaling in SD-OCT thickness measurements in emmetropic medium eyes. Caution is recommended when comparing the measured values of short and long eyes with the normative database of the instrument. There is a need for larger sample-size studies to confirm our results. (ClinicalTrials.gov number, NCT01024803.).
authors.affiliation: Centre for Ophthalmology, University of Tübingen, Tübingen, Germany.
authors.forename: Tobias
authors.initials: T
authors.lastname: Röck
authors.affiliation: Centre for Ophthalmology, University of Tübingen, Tübingen, Germany.
authors.forename: Karl Ulrich
authors.initials: KU
authors.lastname: Bartz-Schmidt
authors.affiliation: Department of General Medicine, Ruhr-University Bochum, Bochum, Germany.
authors.forename: Matthias
authors.initials: M
authors.lastname: Bramkamp
authors.affiliation: Centre for Ophthalmology, University of Tübingen, Tübingen, Germany.
authors.forename: Daniel
authors.initials: D
authors.lastname: Röck
mesh_terms: D000328:Adult; D000368:Aged; D058452:Axial Length, Eye; D001766:Blindness / Q000473:pathology* / Q000503:physiopathology / Q000601:surgery; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D008875:Middle Aged; D012189:Retrospective Studies; D041623:Tomography, Optical Coherence / Q000379:methods*; D014792:Visual Acuity; D057488:Visual Prosthesis
keywords: Heidelberg Spectralis; axial length; in vivo measurement; spectral-domain optical coherence tomography (SD-OCT); subretinal visual implant