Numerical study on human cornea and modified multiparametric correction equation for Goldmann applanation tonometer

Results of First In Vivo Trial of an Acoustic Self-Tonometer

Purpose

Glaucoma is the world's most common cause of irreversible blindness, which makes early diagnosis, with the goal of preserving vision, essential. The current medical intervention is to reduce intraocular pressure (IOP) to slow down progression of the disease. The main goal of this study was to test a novel handheld acoustic self-tonometer on humans.

Methods

A sound pressure pulse generated by a loudspeaker causes the eye to vibrate. A pressure chamber is placed on the human orbit to form a coupled system comprised of the patient's eye, the enclosed air, and the loudspeaker. A displacement sensor in front of the loudspeaker membrane allows the dynamic behavior of the entire system to be detected.

Results

For this clinical trial series, a prototype of the acoustic self-tonometer principle was applied. The resulting membrane oscillation data showed sensitivity of patient IOP, but direct allocation of the measured damping and frequency to the IOP was not significant. For this reason, an artificial neural network was used to find relationships among the subjects’ biometric eye parameters in combination with the self-tonometer data for the IOP reference. An expanded measurement uncertainty (k_p = 2) equal to 6.53 mm Hg was determined for the self-tonometer in a Bland–Altman analysis using Goldmann applanation tonometer reference measurements.

Conclusions

The usability and success rate of producing valid measurement values with the device during self-measurements by test subjects was nearly 92%. The cross-sensitivities observed require compensation in a possible redesign phase to reduce the measurement uncertainty by at least 25% to the maximum of 5 mm Hg required to seek medical device approval.

Translational Relevance

Building on successful laboratory experiments with pig eyes, this article reports the results of testing the acoustic tonometer on humans.

Novel dynamic corneal response parameters in a practice use: a critical review

Background

Non-contact tonometers based on the method using air puff and Scheimpflug’s fast camera are one of the latest devices allowing the measurement of intraocular pressure and additional biomechanical parameters of the cornea. Biomechanical features significantly affect changes in intraocular pressure values, as well as their changes, may indicate the possibility of corneal ectasia. This work presents the latest and already known biomechanical parameters available in the new offered software. The authors focused on their practical application and the diagnostic credibility indicated in the literature.

Discussion

An overview of available literature indicates the importance of new dynamic corneal parameters. The latest parameters developed on the basis of biomechanics analysis of corneal deformation process, available in non-contact tonometers using Scheimpflug’s fast camera, are used in the evaluation of laser refractive surgery procedures, e.g. LASIK procedure. In addition, the assessment of changes in biomechanically corrected intraocular pressure confirms its independence from changes in the corneal biomechanics which may allow an intraocular pressure real assessment. The newly developed Corvis Biomechanical Index combined with the corneal tomography and topography assessment is an important aid in the classification of patients with keratoconus.

Conclusion

New parameters characterising corneal deformation, including Corvis Biomechanical Index and biomechanical compensated intraocular pressure, significantly extend the diagnostic capabilities of this device and may be helpful in assessing corneal diseases of the eye. Nevertheless, further research is needed to confirm their diagnostic pertinence.

Dependence of dynamic contour and Goldmann applanation tonometries on peripheral corneal thickness

AIM

To determine the effects of peripheral corneal thickness (PCT) on dynamic contour tonometry(DCT) and Goldmann applanation tonometry (GAT).

METHODS

A cross-sectional study. We created a software which calculates the corneal contour (CC) as a function of the radius from the corneal apex to each pixel of the contour. The software generates a central circumference with a radius of 1 mm and the remainder of the cornea is segmented in 5 rings concentric with corneal apex being its diameter not constant around the corneal circumference as a consequence of the irregular CC but keeping constant the diameter of each ring in each direction of the contour. PCT was determined as the mean thickness of the most eccentric ring. Locally weighted scatterplot smoothing (LOWESS) regression was used to determine the pattern of the relationship between PCT and both DCT and GAT respectively. Thereafter, two multivariable linear regression models were constructed. In each of them, the dependant variable was intraocular pressure (IOP) as determined using GAT and DCT respectively. In both of the models the predictive variable was PCT though LOWESS regression pattern was used to model the relationship between the dependant variables and the predictor one. Age and sex were also introduced control variables along with their first-degree interactions with PCT. Main outcome measures include amount of IOP variation explained through regression models (R²) and regression coefficients (B).

RESULTS

Subjects included 109 eyes of 109 healthy individuals. LOWESS regression suggested that a 2^nd-degree polynomial would be suitable to model the relationship between both DCT and GAT with PCT. Hence PCT was introduced in both models as a linear and quadratic term. Neither age nor sex nor interactions were statistically significant in both models. For GAT model, R² was 17.14% (F=9.02; P=0.0002), PCT linear term B was -1.163 (95% CI: -1.163, -0.617). PCT quadratic term B was 0.00081 (95% CI: 0.00043, 0.00118). For DCT model R² was 14.28% (F=9.29; P=0.0002), PCT linear term B was -0.712 (95% CI: -1.052, -0.372), PCT quadratic term was B=0.0005 (95% CI: 0.0003, 0.0007).

CONCLUSION

DCT and GAT measurements are conditioned by PCT though this effect, rather than linear, follows a 2^nd-degree polynomial pattern.

Is Multiple Sclerosis Associated With a Lower Intraocular Pressure?

OBJECTIVE

To determine if multiple sclerosis (MS) is associated with lower intraocular pressure (IOP) compared with individuals without MS.

METHODS

Thirty patients with clinically definite MS were identified and a retrospective chart review was conducted. Each patient with MS underwent IOP recording by a single investigator using kinetic applanation tonometry. Measurement of central corneal thickness (CCT) also was obtained. Similarly, 30 study controls were identified and kinetic applanation tonometry and CCT were recorded. Univariate analysis of covariance was conducted to determine a statistically significant difference between IOP between MS and control groups, controlling for age.

RESULTS

Analyses were adjusted for age and 2 subjects were excluded because of steroid use. The average IOP in MS group was 12.3 mm Hg (right eye = 12.3 mm Hg, left eye = 12.2 mm Hg) and in the control group was 17 mm Hg (right eye = 16.9 mm Hg, left eye = 17 mm Hg). There was a significant effect of presence of MS on IOP accounting for 53% variability in mean IOP (F(1,55) = 60.7; P < 0.001) when compared with the control group.

CONCLUSIONS

This study demonstrated that IOP was significantly lower in patients with MS compared with controls. A more in-depth prospective study design is required, along with further investigation of possible etiologies. Identifying the mechanism of decreased IOP in patients with MS might allow development of new-targeted therapies for the treatment of glaucoma.

Corneal biomechanics: a review

Biomechanics is often defined as 'mechanics applied to biology'. Due to the variety and complexity of the behaviour of biological structures and materials, biomechanics is better defined as the development, extension and application of mechanics for a better understanding of physiology and physiopathology and consequently for a better diagnosis and treatment of disease and injury. Different methods for the characterisation of corneal biomechanics are reviewed in detail, including those that are currently commercially available (Ocular Response Analyzer and CorVis ST). The clinical applicability of the parameters provided by these devices are discussed, especially in the fields of glaucoma, detection of ectatic disorders and orthokeratology. Likewise, other methods are also reviewed, such as Brillouin microscopy or dynamic optical coherence tomography and others with potential application to clinical practice but not validated for in vivo measurements, such as ultrasonic elastography. Advantages and disadvantages of all these techniques are described. Finally, the concept of biomechanical modelling is revised as well as the requirements for developing biomechanical models, with special emphasis on finite element modelling.