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Underestimation of intraocular pressure after photorefractive keratectomy: a biomechanical analysis

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Abstract

Excimer laser surgery, to correct corneal refraction, induces changes in corneal thickness and curvature. Both factors can cause measurement errors when determining intraocular pressure (IOP). This study evaluates effects of photorefractive keratectomy (PRK) on IOP measurements, using Goldmann applanation tonometry (GAT) and Applanation resonance tonometry (ART), in an in vitro model. Six porcine eyes was enucleated and pressurised to a constant IOP = 30 mmHg. After removal of the epithelium, the eyes were PRK-treated for a total of 25 dioptres. The measured IOP decreased 13.2 mmHg for GAT and 9.0 mmHg for ART. The total underestimation by GAT was larger than for ART, and a part of the ART underestimation (3.5 mmHg) was assigned to sensitivity to the change in corneal surface structure resulting from the removal of epithelium. The flat contact probe of GAT, as compared with the convex tip of ART, provided explanation for the difference in IOP measurement error after PRK.

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Abbreviations

GAT:

Goldmann applanation tonometry

ART:

Applanation resonance tonometry

PTK:

Photo therapeutic keratectomy

PRK:

Photo refractive keratectomy

PZT:

Lead zirconate titanate

IOP:

Intraocular pressure (mmHg)

IOPGAT :

IOP measured with GAT (mmHg)

IOPART :

IOP measured with ART (mmHg)

IOPVC :

IOP in the vitreous chamber set by a saline column (mmHg)

CCT:

Central corneal thickness (μm)

CCTPach-Pen :

CCT measured with Pach-pen (μm)

ΔCCT:

The difference between CCTPach-Pen and CCT estimated from the expected ablanation (μm)

A :

Contact area between cornea and sensor tip (mm2)

D :

Dioptres (m−1)

Q :

Refractive power (D)

R :

Corneal curvature (mm)

F C :

Contact force (mN)

F Rigidity :

Force related to corneal rigidity

F Surface tension :

Capillary forces related to the surface tension

f :

Frequency (Hz)

β :

Coefficients in the ART model

L :

Applanation depth (mm)

L f :

Applanation depth related to “flowing” (mm)

dF C :

Change in contact force (mN)

df :

Change in resonance frequency (Hz)

dL :

Change in applanation depth (mm)

dA :

Change in contact area (mm2)

ρ :

Radius of sensor tip curvature (mm)

λ :

Proportionality constant between df/dL and dA/dL

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Authors and Affiliations

  1. Department of Biomedical Engineering and Informatics, University Hospital, Umeå, 901 85, Sweden

    Per Hallberg & Anders Eklund

  2. Department of Clinical Science, Ophthalmology, Umeå University, Umeå, 901 87, Sweden

    Kenneth Santala & Christina Lindén

  3. Koskela Eye Clinic, Box 47, Skolgatan 73 A, Umeå, 901 02, Sweden

    Timo Koskela

  4. Department of Applied Physics and Electronics, Umeå University, Umeå, 901 87, Sweden

    Olof Lindahl

Authors
  1. Per Hallberg
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  2. Anders Eklund
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  3. Kenneth Santala
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  4. Timo Koskela
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  5. Olof Lindahl
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  6. Christina Lindén
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Correspondence to Per Hallberg.

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Hallberg, P., Eklund, A., Santala, K. et al. Underestimation of intraocular pressure after photorefractive keratectomy: a biomechanical analysis. Med Bio Eng Comput 44, 609–618 (2006). https://doi.org/10.1007/s11517-006-0093-6

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  • DOI: https://doi.org/10.1007/s11517-006-0093-6

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