title: Residual bed thickness and corneal forward shift after laser in situ keratomileusis.
abstract: OBJECTIVE
  To prospectively assess the forward shift of the cornea after laser in situ keratomileusis (LASIK) in relation to the residual corneal bed thickness.
  
  
  METHODS
  Miyata Eye Hospital, Miyazaki, Japan.
  
  
  METHODS
  Laser in situ keratomileusis was performed in 164 eyes of 85 patients with a mean myopic refractive error of -5.6 diopters (D) +/- 2.8 (SD) (range -1.25 to -14.5 D). Corneal topography of the posterior corneal surface was obtained using a scanning-slit topography system before and 1 month after surgery. Similar measurements were performed in 20 eyes of 10 normal subjects at an interval of 1 month. The amount of anteroposterior movement of the posterior corneal surface was determined. Multiple regression analysis was used to assess the factors that affected the forward shift of the corneal back surface.
  
  
  RESULTS
  The mean residual corneal bed thickness after laser ablation was 388.0 +/- 35.9 microm (range 308 to 489 microm). After surgery, the posterior corneal surface showed a mean forward shift of 46.4 +/- 27.9 microm, which was significantly larger than the absolute difference of 2 measurements obtained in normal subjects, 2.6 +/- 5.7 microm (P<.0001, Student t test). Variables relevant to the forward shift of the corneal posterior surface were, in order of magnitude of influence, the amount of laser ablation (partial regression coefficient B = 0.736, P<.0001) and the preoperative corneal thickness (B = -0.198, P<.0001). The residual corneal bed thickness was not relevant to the forward shift of the cornea.
  
  
  CONCLUSIONS
  Even if a residual corneal bed of 300 microm or thicker is preserved, anterior bulging of the cornea after LASIK can occur. Eyes with thin corneas and high myopia requiring greater laser ablation are more predisposed to an anterior shift of the cornea.
authors.affiliation: Miyata Eye Hospital, Miyasaki, Japan.
authors.forename: Kazunori
authors.initials: K
authors.lastname: Miyata
authors.forename: Tadatoshi
authors.initials: T
authors.lastname: Tokunaga
authors.forename: Masaaki
authors.initials: M
authors.lastname: Nakahara
authors.forename: Shinichiro
authors.initials: S
authors.lastname: Ohtani
authors.forename: Ryohei
authors.initials: R
authors.lastname: Nejima
authors.forename: Takahiro
authors.initials: T
authors.lastname: Kiuchi
authors.forename: Yuichi
authors.initials: Y
authors.lastname: Kaji
authors.forename: Tetsuro
authors.initials: T
authors.lastname: Oshika
mesh_terms: D000328:Adult; D003315:Cornea / Q000473:pathology*; D003316:Corneal Diseases / Q000209:etiology*; D019781:Corneal Topography; D004108:Dilatation, Pathologic / Q000209:etiology; D005260:Female; D006801:Humans; D007429:Intraocular Pressure; D020731:Keratomileusis, Laser In Situ / Q000009:adverse effects*; D008297:Male; D008875:Middle Aged; D009216:Myopia / Q000601:surgery*; D011183:Postoperative Complications; D011446:Prospective Studies; D012044:Regression Analysis