Rebound Tonometry Viscoelastic Parameter Extraction
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Solution Overview
Problem
Rebound tonometers primarily measure intraocular pressure (IOP) without accounting for viscoelastic forces from corneal tissue, leading to measurement errors due to biomechanical properties of the cornea, and fail to derive additional useful information from the measured voltage signal.
Innovation Solution
Calculating parameters such as Lost Energy Ratio (LER), Time Shift (TS), damping parameter (σ), and elastic parameter (η) from the velocity signal of a rebound tonometer, which provide insights into viscoelastic properties of the cornea and help in assessing conditions beyond IOP, including glaucoma progression, and adjusting IOP measurements to reduce errors caused by viscous forces.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the probe deceleration is measured over the entire contact period (from tin to tout), then the measurement process is simple, but the IOP measurement accuracy deteriorates due to viscoelastic forces from corneal tissue
Solution Approach 1:
The patent segments the corneal deformation process into multiple phases: pre-applanation (before corneal contact), applanation (during contact when cornea is flattened), and post-applanation (after contact). By analyzing the voltage signal specifically during the applanation phase and comparing it with pre- and post-applanation phases, the method isolates the period where viscous forces are minimized, thereby improving IOP measurement accuracy without significantly increasing device complexity
Solution Approach 2:
The patent performs preliminary analysis of the voltage signal to identify the applanation phase characteristics before calculating IOP. By detecting the onset and offset of corneal contact through signal analysis, the system prepares the data processing to focus on the relevant time window, ensuring accurate separation of elastic and viscous force contributions before final IOP calculation
2Device complexity
If only IOP is derived from the voltage signal, then the measurement process is straightforward, but useful information about corneal biomechanics is lost
Solution Approach 1:
The patent extends the single voltage signal measurement to serve multiple diagnostic functions. By analyzing the same voltage signal for both IOP determination and corneal biomechanical properties (hysteresis, stiffness, viscoelasticity), the system achieves multi-functionality without requiring additional sensors or measurement devices, thus avoiding increased device complexity while preventing information loss
Solution Approach 2:
The patent transforms the one-dimensional voltage signal analysis into multi-dimensional interpretation by extracting multiple parameters (IOP, corneal hysteresis, stiffness, viscoelastic properties) from the same signal. This dimensional expansion of data interpretation allows comprehensive assessment of both pressure and tissue characteristics without adding physical measurement dimensions
3Ease of operation
If the analysis includes the entire probe contact period, then the measurement is easier to implement, but viscous forces from corneal tissue contaminate the IOP measurement
Solution Approach 1:
The patent automatically segments the voltage signal into distinct phases (pre-applanation, applanation, post-applanation) based on signal characteristics. By identifying the applanation phase boundaries through algorithmic analysis, the system isolates the measurement window where viscous forces are minimal, maintaining ease of operation through automated phase detection while improving precision by excluding contaminated data portions
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
These parameters allow for more accurate IOP measurements by accounting for corneal biomechanics, improving the assessment of glaucoma progression and providing additional information useful for ophthalmic diagnostics and refractive surgery, while reducing measurement errors associated with viscous forces.
Implementation Method 1
The coil may be energized momentarily to propel the probe toward the cornea by electromagnetic force
Implementation Method 2
after energizing current to the coil is shut off, a current may be induced in the coil by the moving probe to provide a detectable voltage signal representing velocity of the probe
Data Source
AI summary
Viscoelastic properties of the cornea are derived from an ophthalmic measurement signal representing velocity as a function of time of a contact probe rebounded by the eye. The viscoelastic properties include a “Lost Energy Ratio” (LER), a “Time Shift” (TS), a damping parameter (σ), and an elastic parameter (η). An improved method for determining intra-ocular pressure from the measurement signal is also disclosed, wherein a first derivative of the measurement signal at a moment in time when velocity of the probe is zero due to contact of the probe with the cornea is calculated and correlated to an intra-ocular pressure value.