title: Comparison of residual stromal bed thickness measurement among very high-frequency digital ultrasound, intraoperative handheld ultrasound, and optical coherence tomography.
abstract: OBJECTIVE
  To compare residual stromal thickness (RST) measurement by three methods: very high-frequency digital ultrasound (VHFDU), intraoperative handheld ultrasound (US), and optical coherence tomography (OCT).
  
  
  METHODS
  This was a retrospective analysis of 93 eyes (55 patients) that underwent LASIK retreatment by flap lift where VHFDU and OCT measurements were obtained before the retreatment and intraoperative US RST measurement was performed after lifting the flap. Corneal vertex RST measurements for intraoperative US and OCT were compared with the VHFDU measurement. Linear regression and Bland-Altman analysis were performed.
  
  
  RESULTS
  Mean corneal vertex RST was 327±53 μm for VHFDU, 338±54 μm for intraoperative US, and 326±43 μm for OCT. Mean difference between VHFDU and intraoperative US was 11±11 μm (range: -12 to 46 μm). Mean difference between VHFDU and OCT was -2±23 μm (range: -53 to 57 μm). The coefficient of determination (R(2)) was 0.96 between VHFDU and intraoperative US and 0.82 between VHFDU and OCT. Intraoperative US was on average 11 μm thicker than VHFDU independent of RST value. Optical coherence tomography was on average thicker than VHFDU for low RST values (250 to 300 μm) and thinner than VHFDU for high RST values (>350 μm). The RST was thicker than VHFDU by >10 μm in 50% and >20 μm in 21% of eyes measured with intraoperative US and by >10 μm in 32% and >20 μm in 18% of eyes measured with OCT.
  
  
  CONCLUSIONS
  Residual stromal thickness measured by both intraoperative US and OCT was thicker than VHFDU in a significant number of eyes, which could have resulted in retreatment being performed in an eye with insufficient RST.
authors.affiliation: London Vision Clinic, London, United Kingdom. dzr@londonvisionclinic@com
authors.forename: Dan Z
authors.initials: DZ
authors.lastname: Reinstein
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Archer
authors.forename: Marine
authors.initials: M
authors.lastname: Gobbe
mesh_terms: D000328:Adult; D000368:Aged; D003319:Corneal Stroma / Q000000981:diagnostic imaging / Q000473:pathology*; D003941:Diagnostic Techniques, Ophthalmological / Q000295:instrumentation*; D006801:Humans; D020731:Keratomileusis, Laser In Situ; D054018:Lasers, Excimer / Q000627:therapeutic use; D008875:Middle Aged; D016343:Monitoring, Intraoperative; D009216:Myopia / Q000601:surgery; D019233:Retreatment; D012189:Retrospective Studies; D013524:Surgical Flaps; D041623:Tomography, Optical Coherence / Q000295:instrumentation*; D014463:Ultrasonography / Q000295:instrumentation*; D055815:Young Adult