title: Corneal modulus and IOP measurements in canine eyes using Goldmann applanation tonometry and Tono-pen.
abstract: OBJECTIVE
  To experimentally examine the effect of corneal modulus on Goldmann applanation tonometry (GAT) and Tono-pen (Tono-pen XL, Reichert, Inc., Depew, NY) measurements of intraocular pressure (IOP) in a canine eye model.
  
  
  METHODS
  Twenty-one canine globes were recovered from healthy animals. IOP was controlled at 10, 15, 20, 30, and 40 mm Hg and measured by GAT and Tono-pen following standard protocols. The corneas were dissected and uniaxial tensile tests were performed on corneal strips. The correlation between GAT and Tono-pen errors and corneal secant modulus was evaluated using Pearson correlation coefficients. The influence of corneal thickness and the true pressure was also examined.
  
  
  RESULTS
  At a true IOP of 10, 15, 20, 30, and 40 mm Hg, the GAT readings were 1.1 ± 1.0, 5.1 ± 1.5, 9.5 ± 2.0, 17.3 ± 1.6, and 25.3 ± 1.8 mm Hg, respectively. The corresponding Tono-pen readings were 7.8 ± 1.7, 12.4 ± 1.7, 16.1 ± 1.9, 22.5 ± 2.1, and 28.1 ± 2.2 mm Hg, respectively. The mean secant modulus at 1% strain of the canine corneal strips was 1.54 ± 0.43 megapascal (MPa). Corneal secant modulus was significantly correlated with GAT errors when the true IOP was 30 mm Hg (R = 0.49; P < 0.05). No significant correlation was observed between tonometric errors and corneal thickness. Both GAT and Tono-pen errors increased significantly at higher pressures (P < 0.001).
  
  
  CONCLUSIONS
  Both GAT and Tono-pen underestimated IOP in canine eyes. There was preliminary experimental evidence for a correlation between corneal modulus and GAT in the canine eyes and a higher corneal modulus was associated with higher GAT readings at a certain pressure level. The tonometric errors appeared to be pressure-dependent.
authors: Department of Biomedical Engineering, Ohio State University, Columbus, Ohio, USA.
authors: Junhua
authors: J
authors: Tang
authors: Xueliang
authors: X
authors: Pan
authors: Paul A
authors: PA
authors: Weber
authors: Jun
authors: J
authors: Liu
mesh_terms: D000818:Animals; D003315:Cornea / Q000033:anatomy & histology / Q000502:physiology*; D004285:Dogs; D055119:Elastic Modulus / Q000502:physiology*; D007429:Intraocular Pressure / Q000502:physiology*; D023421:Models, Animal; D013718:Tensile Strength; D014065:Tonometry, Ocular / Q000295:instrumentation*