A method for determining whether the tonometer has reached flattening based on the force applied to the tonometer

Abstract

The present invention discloses a method for determining whether a tonometer indenter reaches flattening based on the force applied to the indenter, comprising: 1) extracting displacement data and force data of the indenter in the direction of insertion during the indentation of the tonometer; 2) derivatizing the force data with respect to the displacement data to obtain a rate of change of the indenter force with respect to the indenter insertion depth, i.e., the displacement; 3) drawing a curve of the rate of change of the indenter force with respect to the indentation depth, wherein the rate of change of the indenter force with respect to the indentation depth is the Y axis and the indentation depth is the X axis; 4) finding the maximum value in the middle of the curve of the rate of change of the indenter force with respect to the indentation depth during the indentation process, i.e., the moment when the indenter reaches flattening. The present invention determines whether the indenter reaches flattening based on the force applied to the indenter by the tonometer, can determine whether the indenter reaches flattening according to the force applied to the indenter, alleviates the difficulty of directly observing the contact image between the indenter and the cornea, and simultaneously improves the accuracy of determination, and has broad prospects in the use of various types of tonometers.

Description

Technical Field

[0001] The present invention relates to the technical field of applanation tonometer use, and in particular to a method for determining whether the tonometer head has reached applanation based on the force applied to the head. Background Art

[0002] To measure intraocular pressure, various tonometers have been developed based on different principles. Among them, the Goldmann applanation tonometer, which offers the most stable and accurate results, requires determining the moment when the indenter reaches applanation in order to obtain a reading.

[0003] Currently, slit lamp microscopy is used to observe whether flattening is achieved in clinical practice. Figure 1 As shown, the tonometer is used in conjunction with a slit lamp. Applanation is achieved when the inner sides of the two semicircular rings are tangent to each other during imaging. Accordingly, the Goldmann applanation tonometer is extremely complex and demanding, from the application of topical anesthesia and the instillation of fluorescent fluid before measurement to the observation under the slit lamp microscope during measurement. Improper operation and inability to accurately determine the moment of applanation can introduce errors into the measurement results. Summary of the Invention

[0004] The purpose of the present invention is to overcome the shortcomings and deficiencies of the prior art and propose a method for determining whether the tonometer head has reached applanation based on the force applied to the tonometer head. The method can use the force information on the tonometer head to determine whether the tonometer head has reached applanation, simplifying the use process of the applanation tonometer, while also reducing the requirements for the instrument user, and achieving more flexible and accurate tonometer measurement.

[0005] To achieve the above-mentioned purpose, the present invention provides a technical solution: a method for determining whether the tonometer indenter has reached flattening based on the force applied to the indenter, comprising the following steps:

[0006] 1) Extract the displacement data and force data of the indenter in the insertion direction during the tonometer insertion process;

[0007] 2) Derivative the force data with respect to the displacement data to obtain the rate of change of the indenter force with respect to the indenter penetration depth, i.e., the displacement;

[0008] 3) Plotting a curve of the rate of change of the indenter force with respect to the indentation depth, with the rate of change of the indenter force with respect to the indentation depth being the Y-axis and the indentation depth being the X-axis. It is known that, based on the contact between the indenter and the cornea during the indentation process, the contact area between the two increases continuously before applanation is achieved and then stops changing after applanation. The rate of change of the indenter force is related to the change in the contact area, and the growth rate of the indenter force reaches its maximum value at the moment of applanation.

[0009] 4) Find the maximum value in the middle of the curve of the rate of change of the indenter force with respect to the indentation depth during the indentation process, which is the moment when the indenter reaches flattening.

[0010] Furthermore, in step 1), the extracted data includes the depth U of the indenter in the direction of indentation at the i-th moment from the start of indentation of the tonometer. i and the force RF on the indenter i , there are N data pairs in total.

[0011] Furthermore, in step 2), the force data is derived from the displacement data to obtain the rate of change K of the force on the indenter with respect to the indentation depth at each moment except the initial moment. i :

[0012]

[0013] Where RF i and U i They are the force and displacement of the indenter in the indentation direction at the current moment, RF i-1 and U i-1 are the force and displacement of the indenter in the pressing direction at the previous moment, respectively.

[0014] Further, in step 3), according to the pressure head force RF i About the pressing depth U i The rate of change K i To reverse the change of the contact area CAREA between the indenter and the cornea; before the indenter reaches flattening, as U i Increase, CAREA gradually increases, RF i The increasing rate K i After the pressure head reaches flatness, CAREA reaches its maximum value and no longer changes. Correspondingly, RF i The rate of increase begins to decrease, so K i The maximum value is achieved when flattening.

[0015] Further, in step 4), according to the pressure head force RF i With the pressing depth U i The rate of change K i The correlation between the contact area CAREA of the indenter and the cornea and the rate of change curve K of the indenter force with respect to the indentation depth drawn in step 3) are i -U i The position where the maximum value is obtained in the middle is the position where the tonometer head reaches flattening.

[0016] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0017] 1. The present invention adopts for the first time a method of determining whether the tonometer indenter has reached flattening based on the force applied to the indenter, thus overcoming the difficulty in observing the contact image between the indenter and the cornea.

[0018] 2. Compared with the original judgment standard, the present invention improves the judgment accuracy.

[0019] 3. The method of the present invention is simple to operate and has strong adaptability, and has broad application prospects in the use of various types of tonometers. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The present invention is a schematic diagram of an existing device and method for determining whether a tonometer has reached applanation.

[0021] Figure 2 This is a diagram of the eyeball simulation model used in the present invention.

[0022] Figure 3 This is a diagram showing the contact between the indenter and the cornea during the insertion of the tonometer (left: before insertion, center: just flattened, right: continuing to press down).

[0023] Figure 4 This is a graph showing the change in the contact area CAREA between the indenter and the cornea as the tonometer is pushed into the cornea (the contact area remains unchanged after flattening).

[0024] Figure 5 The contact range between the indenter and the cornea changes as the tonometer is pressed in (the contact range remains unchanged after flattening)

[0025] Figure 6 This is a table showing the numbers and parameters of the 27 eyeball models.

[0026] Figure 7 is the rate of change of the indenter force RF with the indentation displacement U Comparison of the relationship between the indenter and corneal contact area CAREA and the indentation displacement U (corneal elastic modulus is 0.9 MPa, thickness is 0.44 mm, and radius of curvature is 7.8 mm; the results determined using the contact area and the new method are compared, and the determination times are exactly the same for both).

[0027] Figure 8 is the rate of change of the indenter force RF with the indentation displacement U Comparison of the relationship between the indenter and corneal contact area CAREA and the indentation displacement U (multiple models within the common corneal parameter range, the picture numbers correspond to Figure 6 Model parameters in ; the vertical line indicates the position where the tonometer reaches applanation; all calculation results show a high degree of agreement between the two discrimination methods). DETAILED DESCRIPTION

[0028] The present invention will be described in further detail below with reference to the embodiments and drawings, but the embodiments of the present invention are not limited thereto.

[0029] This embodiment discloses a method for determining whether a tonometer head has reached flattening based on the force applied to the tonometer head, comprising the following steps:

[0030] 1) Extract the displacement data and force data of the indenter in the insertion direction during the tonometer insertion process, as follows:

[0031] The extracted data includes the depth U of the indenter at the time of indentation, the indenter direction at the time of indentation, and the indenter depth at the time of indentation. i and the force RF on the indenter i , there are N data pairs in total.

[0032] 2) Derivative the force data with respect to the displacement data to obtain the rate of change of the indenter force with respect to the indenter penetration depth, i.e., displacement; wherein, derivative the force data with respect to the displacement data to obtain the rate of change K of the indenter force with respect to the indentation depth at each moment except the initial moment. i :

[0033]

[0034] Where RF i and U i They are the force and displacement of the indenter in the indentation direction at the current moment, RF i-1 and U i-1 are the force and displacement of the indenter in the pressing direction at the previous moment, respectively.

[0035] 3) Plot a curve of the rate of change of the indenter force with respect to the indentation depth, with the rate of change of the indenter force with respect to the indentation depth being the Y-axis and the indentation depth being the X-axis. It is known from the contact between the indenter and the cornea during the indentation process that the contact area between the two increases continuously before applanation is reached and then stops changing after applanation. The rate of change of the indenter force is related to the change in the contact area. At the moment of applanation, the growth rate of the indenter force reaches its maximum value, as shown below:

[0036] According to the pressure head force RF i About the pressing depth U i The rate of change K i To reverse the change of the contact area CAREA between the indenter and the cornea; before the indenter reaches flattening, as U i Increase, CAREA gradually increases, RF i The increasing rate K i After the pressure head reaches flatness, CAREA reaches its maximum value and no longer changes. Correspondingly, RF iThe rate of increase begins to decrease, so K i The maximum value is achieved when flattening.

[0037] 4) According to the pressure head force RF i With the pressing depth U i The rate of change K i The correlation between the contact area CAREA of the indenter and the cornea and the rate of change curve K of the indenter force with respect to the indentation depth drawn in step 3) are i -U i The position where the maximum value is obtained in the middle is the position where the tonometer head reaches flattening.

[0038] The above-mentioned method for determining whether the tonometer head reaches flattening based on the force applied to the tonometer head in this embodiment is based on a finite element model of the eyeball, and the specific situation is as follows:

[0039] 1) Establish a finite element model of the eyeball

[0040] The present invention uses finite element software to establish a model of the eyeball based on the existing eyeball parameters. In this modeling, the geometric parameters adopted are as follows: the maximum cross-section of the cornea is a circle with a diameter of 11mm, the initial curvature radius of the front surface is 7.8mm, and the initial thickness of the middle is 0.52mm; the sclera is a shell structure with uniform thickness, the outer surface curvature radius is 12mm, and the thickness is 0.8mm; the limbus is responsible for connecting the cornea and the sclera, and the thickness is 0.67mm. The cornea, limbus, and sclera constitute the wall of the eyeball, and the inside of the eyeball wall is filled with uniform pressure, which is the intraocular pressure we are concerned about. This pressure can simulate the pressure of the real aqueous humor. When the volume shrinks after being compressed, the pressure will increase accordingly. In the calculation model of the present invention, since the eyeball has the property of axisymmetry, in order to save calculation time, a quarter-symmetrical model is adopted. The overall model is as follows Figure 2 From top to bottom, they are the tonometer tip, cornea, limbus, and sclera. According to the actual parameters of the Goldmann applanation tonometer, the radius of the indenter is 1.53 mm.

[0041] 2) Simulate tonometer insertion behavior

[0042] During the simulation of intraocular pressure measurement, the tonometer indenter will slowly press a distance perpendicular to the cornea. During this process, the indenter displacement increases continuously, and the contact area between the tonometer indenter and the cornea also increases from zero until it is in complete contact with the cornea. The contact surface remains unchanged with a radius of 1.53mm. Figure 3As shown in the figure, during the entire insertion process, the cornea deforms increasingly, the initial contact area between the indenter and the cornea gradually increases, and the force exerted on the indenter also gradually increases. However, once the indenter plane is in full contact with the cornea, as the indenter's insertion distance continues to increase, the contact area reaches its maximum and no longer increases. Therefore, at the moment of applanation, there will be an inflection point on the curve corresponding to the indenter pressure-indentation displacement. Finding this inflection point can determine when the tonometer has completely applanated the cornea.

[0043] Before finding this inflection point, the moment of flattening can also be determined by directly observing the change in the contact area between the tonometer head and the cornea. The curve of the change in output contact area with indentation displacement shows that, Figure 4 As shown in the figure, when the pressure is 0.39mm, the contact area CAREA reaches its maximum, which is the moment when the tonometer just completely flattens the cornea. In addition, it can also be found by directly observing the contact range between the cornea and the indenter, such as Figure 5 As shown in the figure, the contact range gradually increases before the 0.39mm insertion, but after 0.39mm, the contact range stabilizes. In other words, the 0.39mm insertion is the point at which the tonometer completely flattens the cornea. The results obtained by the two methods are consistent, ensuring the correct determination of the applanation moment. If the applanation moments determined by the two methods above are the same as those determined by the new method, then the new method's determination criteria are valid.

[0044] 3) Calculation results and processing

[0045] According to the existing common corneal parameter ranges, the common corneal elastic modulus E range is 0.1kPa to 0.9kPa, the thickness CCT range is 0.44mm to 0.69mm, and the corneal anterior surface curvature radius R range is 7mm to 8.6mm. A total of 27 combinations of corneal elastic modulus of 0.1kPa, 0.5kPa and 0.9kPa, corneal thickness of 0.44mm, 0.56mm and 0.69mm, and corneal anterior surface curvature radius of 7mm, 7.8mm and 8.6mm were selected for modeling. The numbers of each model are as follows: Figure 6 As shown. Further, taking the model with a corneal elastic modulus of 0.9 MPa, a thickness of 0.44 mm, and a curvature radius of 7.8 mm as an example, the change rate of the RF-U curve is calculated, and the calculation results are as follows: Figure 7 As shown. The entire curve presents a "W" shape, and at the moment of flattening, the curve reaches the middle peak. That is to say, during the entire indentation process, RF is constantly increasing. However, before the indenter completely flattens the cornea, the rate of increase of the indenter pressure RF is gradually increasing; and after the indenter is in full contact with the cornea, the rate of increase of the indenter pressure RF begins to decrease because the contact area no longer increases. Using this feature, it is possible to accurately identify, Figure 7 The peak in the middle of the "W" in the curve is the moment when the flattening is achieved. In addition, the same pattern is also shown in the data of the other 26 models. The calculation results of the 27 models are as follows: Figure 8 shown.

[0046] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for determining whether a tonometer head reaches flattening based on the force applied to the tonometer head, characterized in that: The following steps are involved: 1) Extract the depth data and force data of the indenter in the insertion direction during the tonometer insertion process; The extracted data includes the depth U of the indenter at the time of indentation, the indenter direction at the time of indentation, and the indenter depth at the time of indentation. i and the force RF on the indenter i , there are N data pairs in total; 2) Derivative the force data with respect to the depth data to obtain the rate of change of the indenter force with respect to the indenter penetration depth; The force data is derived from the depth data to obtain the rate of change K of the indenter force with respect to the indentation depth at each moment except the initial moment. i : Where RF i and U i They are the force and depth of the indenter in the indentation direction at the current moment, RF i-1 and U i-1 are the force and depth of the indenter in the indentation direction at the previous moment respectively; 3) Plotting a curve of the rate of change of the indenter force with respect to the indentation depth, with the rate of change of the indenter force with respect to the indentation depth being the Y-axis and the indentation depth being the X-axis. It is known that, based on the contact between the indenter and the cornea during the indentation process, the contact area between the two increases continuously before applanation is achieved and then stops changing after applanation. The rate of change of the indenter force is related to the change in the contact area, and the growth rate of the indenter force reaches its maximum value at the moment of applanation. According to the pressure head force RF i About the pressing depth U i The rate of change K i To reverse the change of the contact area CAREA between the indenter and the cornea; before the indenter reaches flattening, as U i Increase, CAREA gradually increases, RF i The increasing rate K i After the pressure head reaches flatness, CAREA reaches its maximum value and no longer changes. Correspondingly, RF i The rate of increase begins to decrease, so K i The maximum value is achieved during flattening; 4) Find the maximum value in the curve of the rate of change of the indenter force with respect to the indentation depth during the indentation process, which is the moment when the indenter reaches flatness; According to the pressure head force RF i With the pressing depth U i The rate of change K i The correlation between the contact area CAREA of the indenter and the cornea and the rate of change curve K of the indenter force with respect to the indentation depth drawn in step 3) are i -U i The position where the maximum value is obtained is the position where the tonometer head reaches flattening.

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