Development and validation of a new intraocular pressure estimate for patients with soft corneas

doi:10.1016/j.jcrs.2019.04.004

Journal of Cataract & Refractive Surgery

Volume 45, Issue 9, September 2019, Pages 1316-1323

$Journal of Cataract & Refractive Surgery$

https://doi.org/10.1016/j.jcrs.2019.04.004 Get rights and content

Purpose

To introduce and clinically validate a new method of estimating intraocular pressure (IOP) in patients with keratoconus and soft corneas with the aim of significantly reducing dependence on corneal biomechanics.

Setting

Vincieye Clinic, Milan, Italy, and Rio de Janeiro Corneal Tomography and Biomechanics Study Group, Brazil.

Design

Retrospective case series.

Method

This study comprised participants enrolled at two hospitals on two continents. Numerical analysis based on the finite element method was performed to simulate the effect of tonometric air pressure of the Corvis ST dynamic Scheimpflug analyzer on eye globes with wide variations in thickness, geometry, and tissue. The numerical predictions of ocular behavior were used to develop a new algorithm to produce predictions of the biomechanically corrected IOP (bIOP) in eyes with a soft cornea (bIOP_s). Predictions of the bIOP_s were assessed in the keratoconic clinical datasets (because on average these corneas are softer) and compared with the previously developed bIOP algorithm predictions obtained for normal healthy eyes.

Results

The study comprised 722 eyes (722 participants). The main outcome was the absence of a significant difference in IOP between healthy eyes and keratoconic eyes when the bIOP and bIOP_s algorithms were used (P > .05). There was, however, a significant difference with the uncorrected Scheimpflug analyzer IOP in both groups (P < .001). Furthermore, the bIOP_s predictions were significantly less affected by corneal thickness and patient age than the Scheimpflug analyzer IOP.

Conclusion

The bIOP_s algorithm was more reliable at estimating the IOP in eyes with a soft cornea and was validated for use in eyes with keratoconus.

Section snippets

Biomechanically Corrected Intraocular Pressure Algorithm Development

The bIOP_s algorithm was developed using numerical simulation by the Biomechanical Engineering Group, University of Liverpool, United Kingdom, following the same procedure used for the bIOP algorithm.11, 12 In brief, the Corvis ST air-puff effect was simulated on numerical models of whole eye globes in which true IOP was varied between 10 mm Hg and 35 mm Hg. The 3-dimensional anterior and posterior topographies of the keratoconic eyes included in the clinical validation part of this study were

Dataset 1 (Milan)

Dataset 1 included 315 participants (164 healthy; 151 with keratoconus). The mean age was 35 ± 13 years old (range 14 to 73 years) in the group with normal corneas and 33 ± 12 years (range 14 to 73 years) in the group with keratoconic corneas; the difference was not statistically significant (P = .507). In contrast, the mean CCT was significantly greater in normal eyes (543 ± 32 μm; range 458 to 635 μm) than in soft eyes (482 ± 45 μm; range 239 to 595 μm) (P < .001).

The mean bIOP in normal eyes

Discussion

The reliable measurement of IOP in patients with soft corneas, including those with keratoconus, has always been a challenge.⁶ Soft corneas are thinner, steeper, softer, and less regular than healthy tissue; thus, the IOP in soft corneas is usually systematically underestimated.⁸ This can cause problems when using eyedrops (eg, steroids) or having procedures that can induce a rise in IOP, making it difficult to evaluate whether a borderline IOP measurement represents a clear abnormality.

The

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To investigate these lateral perturbations, we designed an experiment to sequentially record the corneal profiles when two consecutive air-puffs were applied at the center of the same cornea within a short period. Moreover, we used modal decomposition to decompose anterior surface profiles into symmetric and antisymmetric modes to comprehensively analyze the asymmetric characteristics. To extract mechanical properties, we utilized high-pass frequency analysis (>250 Hz) to filter out noise and errors.
Symmetric modes between the two consecutive air-puffs exhibited major similarities during vibration; however, antisymmetric modes exhibited minor differences in lateral perturbations of asymmetric vibration. The antisymmetric modes might be related to air-puff misalignment and mechanical properties. Through applying frequency analysis, the mechanical properties could be proven at high frequencies and misalignment shown at low frequencies. Furthermore, we compared the corneal vibration profiles of 259 healthy participants and 50 patients with keratoconus. Their properties showed that the antisymmetric modes of the keratoconus group exhibited a completely opposite direction of deformation compared to that in the healthy group.
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Detection of Keratoconus With a New Corvis Biomechanical Index Optimized for Chinese Populations
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The aim of this study was to introduce an optimized version of the Corvis Biomechanical Index for Chinese populations (cCBI).
Retrospective, multicenter clinical validity enhancement study.
Patients were included from 7 clinics in Beijing, Shenyang, Guangzhou, Shanghai, Wenzhou, Chongqing, and Tianjin, China. Logistic regression was used to optimize the values of the constants of the CBI, based on database 1 as the development dataset (6 of 7 clinics), to create a new version of the index named cCBI. The factors of the CBI (A1Velocity, ARTh, Stiffness Parameter-A, DARatio2mm, and Inverse Integrated Radius) and the cutoff value were kept the same (0.5). With the formation of cCBI determined, it was validated on database 2 (1 of the 7 clinics).
Two thousand four hundred seventy-three patients (healthy and keratoconus) were included. In database 2, the area under the curve of the cCBI was 0.985 with 93.4% specificity and 95.5% sensitivity. In the same dataset, the original CBI produced an area under the curve of 0.978 with 68.1% specificity and 97.7% sensitivity. There was a statistically significant difference between the receiver operating characteristic curve of cCBI and CBI (De Long P = .0009)
The new cCBI for Chinese patients was shown to be statistically significantly better when compared with CBI to separate healthy from keratoconic eyes. The presence of an external validation dataset confirms this finding and suggests the use of cCBI in everyday clinical practice to aid in the diagnosis of keratoconus in patients who are of Chinese ethnicity.
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It is limiting to use conventional methods when characterising material properties of complex biological tissues with inhomogeneous and anisotropic structure, such as the anterior cruciate ligament (ACL) in the knee joint. This study aims to develop and utilise a three-dimensional digital image correlation method (3D DIC) for the purpose of determining material properties of femur-ACL-tibia complex across the surface without any contact between the tissue and the loading equipment. A full-field (360° view) 3D DIC test setup consisting of six digital single-lens reflex cameras was developed and ACL specimens from skeletally mature dog knee joints were tested. The six cameras were arranged into three pairs and the cameras within each pair were positioned with 25° in between to obtain the desired stereovision output. The test setup was calibrated twice: first to obtain the intrinsic and extrinsic parameters within camera pairs, and second to align the 3D surfaces from each camera pair in order to generate the full view of the ACLs. Using the undeformed 3D surfaces of the ligaments, ACL-specific finite element models were generated. Longitudinal deformation of ligaments under tensile loads obtained from the 3D DIC, and this was analysed to serve as input for the inverse finite element analysis. As a result, hyperelastic coefficients from the first-order Ogden model that characterise ACL behaviour were determined with a marginal error of <1.5%. This test setup and methodology provides a means to accurately determine inhomogeneous and anisotropic material properties of ACL. The methodology described in this study could be adopted to investigate other biological and cultured tissues with complex structure.
Determining the material properties of soft tissues with complex anatomical structure, such as the anterior cruciate ligament (ACL), is important to better understand their contribution to musculoskeletal biomechanics. Current conventional methods for characterising material properties of the ACL are often limited to a contact measurement approach, however an improved understanding of the mechanics of this complex tissue is vital in terms of preventing injury and developing novel therapies. This article reports the development and utilisation of non-contact optical methodology involving full-field three-dimensional digital image correlation and finite element analysis to accurately investigate material properties of the ACL, in a controlled environment. This technique reduces inaccuracies due to specimen clamping and more importantly considers the inhomogeneous nature of the examined tissue.
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2020, Archivos de la Sociedad Espanola de Oftalmologia
Estudiar el comportamiento de las propiedades biomecánicas de la córnea en la DMEK (Descemet membrane endothelial keratoplasty).
Estudio observacional longitudinal prospectivo; 21 ojos pre- y post-DMEK. Seguimiento de 3 meses con el analizador Scheimpflug dinámico (Corvis ST, Oculus; Wetzlar, Alemania). Se midieron los siguientes parámetros: PIOnc: presión intraocular (PIO) no corregida, PIOb: PIO corregida mediante biomecánica, amplitud de deflexión máxima (DefA), CCT: paquimetría corneal central, velocidad de aplanación 1 y 2, peak distance, HC deformation amplitude: amplitud de deformación en máxima concavidad, radio cóncavo inverso integrado, DAR 1 y 2: amplitud de deformación a 1 mm y 2 mm del ápex corneal, respectivamente. ARTh: relación paquimétrica de Ambrosio, SP1: parámetro de rigidez, CBI: corvis biomechanical index y el radio máximo inverso. Se realizaron las medidas de forma preoperatoria y postoperatoria con un seguimiento de 3 meses y se compararon con una prueba t para muestras emparejada.
Se produjo una disminución significativa de la PIOnc de 1,54 ± 3 mmHg (p < 0,05), aumento significativo de la PIOb posquirúrgica de 3,79 ± 3,18 mmHg (p < 0,001), una disminución significativa paquimétrica de 164,4 ± 91,35 μm (p < 0,001) tras la intervención. Todos los parámetros dinámicos del analizador Scheimpflug cambiaron significativamente después de la cirugía (p < 0,05), excepto las variable ARTh y CBI.
Las variables indican un descenso en la resistencia de la córnea post-DMEK, con un aumento de la PIOb, al menos los primeros 3 meses tras la cirugía. Este hallazgo es especialmente relevante en el seguimiento de los pacientes con coexistencia de glaucoma.
To study the behaviour of the biomechanical properties of the cornea in DMEK (Descemet membrane endothelial keratoplasty).
Prospective longitudinal observational study. 21 pre and post-DMEK eyes. 3-month follow-up with the dynamic Scheimpflug Analyzer (Corvis ST, Oculus; Wetzlar, Germany). The following parameters were measured: IOPnc: non-corrected intraocular pressure (IOP), IOPb: IOP corrected by biomechanics, maximum deflection amplitude (DefA), CCT: central corneal pachymetry, flattening speed 1 and 2, peak distance, HC deformation amplitude: deformation amplitude in maximum concavity, integrated inverse concave radius, DAR 1 and 2: deformation amplitude at 1 mm and 2 mm from the corneal apex, respectively. ARTh: Ambrosial pachymetric ratio, SP1: stiffness parameter, CBI: Corvis Biomechanical index and the maximum inverse radius. Pre-operative and post-operative measurements were performed with a 3-month follow-up and compared with a paired sample t-test.
There was a significant decrease in the IOPnc of 1.54 ± 3 mmHg (p < 0.05), a significant increase in the post-surgical IOPb of 3.79 ± 3.18 mmHg (p < 0.001), a significant pachymetric decrease of 164.4 ± 91.35 μm (p < 0.001) after the intervention. All dynamic parameters of the Scheimpflug analyzer changed significantly after surgery (p < 0.05), except the ARTh and IWC variables.
Variables indicate a decrease in corneal strength post-DMEK, with an increase in IOPb, at least the first 3 months after surgery. This finding is especially relevant in the follow-up of patients with coexisting glaucoma.

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ArticleDevelopment and validation of a new intraocular pressure estimate for patients with soft corneas

Purpose

Setting

Design

Method

Results

Conclusion

Section snippets

Biomechanically Corrected Intraocular Pressure Algorithm Development

Dataset 1 (Milan)

Discussion

Surv Ophthalmol

J Cataract Refract Surg

Exp Eye Res

Exp Eye Res

Exp Eye Res

Med Eng Phys

Ophthalmology

Ophthalmology

Am J Ophthalmol

J Optom

Evaluating the material parameters of the human cornea in a numerical model

Acta Bioeng Biomech

Applanation tonometry and central corneal thickness

Acta Ophthalmol (Copenh)

Assessment of the biomechanical properties of the cornea with the Ocular Response Analyzer in normal and keratoconic eyes

Invest Ophthalmol Vis Sci

Which tonometry in eyes with keratoconus?

Eye

Comparison of dynamic contour tonometry and Goldmann applanation tonometry in keratoconus

Cornea

Intraocular pressure values obtained by Ocular Response Analyzer, dynamic contour tonometry, and Goldmann tonometry in keratokonic corneas

J Glaucoma

Goldmann applanation tonometry versus ocular response analyzer for intraocular pressure measurements in keratoconic eyes

Cornea

Detection of keratoconus with a new biomechanical index

J Refract Surg

Article
Development and validation of a new intraocular pressure estimate for patients with soft corneas