Method and device for monitoring biomechanical properties of the eye

Abstract

A contact lens shaped measuring device ( 1 ) includes sensor ( 3 ) having a protrusion ( 14 ) towards the cornea ( 22 ). The measuring device ( 1 ) is flexible to a degree that it is flattened by a closing eye lid and the protrusion creates an indentation of the cornea. The occurring force on the protrusion is measured by the sensor. Applying a constant lid acceleration/deceleration model to the movement of the lid and a mechanical model to the cornea, the tension of the cornea is determined and deduced from the force measured with the lid closed, yielding the true intraocular pressure. In an alternative, the protrusion is characterized by a discontinuity in its shape, or the sensor is subdivided, each subsensor being characterized by a protrusion of different shape. With the values obtained as extrema and at the discontinuity or with different protrusions, the cornea tension can be obtained by a (linear) extrapolation.

What is claimed is: 
     
         1 . A method for monitoring biomechanical properties of the eye, using a measuring arrangement which comprises a measuring device designed to be born in the manner of a contact lens, the measuring device comprising a force detector in operative engagement with a protrusion directed towards the cornea of an eye once the measuring device is applied to the eye, the measuring device being flexible in order to be pressed against the cornea by a lid of the eye moving over the measuring device, wherein the method comprises the following steps:
 measuring and storing force values and indication of time when the force values occurred;   determining the time t start  when the force values start to be significantly greater than zero, time t stop  at which the force values after t start  are no more substantially greater than zero, equivalent to the times when the protrusion first gets in contact with the cornea respectively disconnects therefrom;   determining a maximum force value between t start  and t stop  in order to determine the phases of eye closing, eye being closed, and eye opening;   deriving from the force values and the corresponding time values between t start  and t End  a value indicative of the tension of the cornea.   
     
     
         2 . The method of  claim 1 , wherein the indentation of the cornea by the protrusion is derived from the position of the eye lid while closing or opening. 
     
     
         3 . The method of  claim 2 , wherein the position of the lid is determined in applying a movement of constant acceleration or deceleration, the acceleration and deceleration values being selected in a set of predetermined values with the length of the time between either t start  or t End  and the time where the maximum force occurs used as the selection criterion. 
     
     
         4 . The method of  claim 1 , wherein a maximum force occurring between t start  and t stop  is determined, a period of time t IOP0  to t IOP1  comprising the maximum force value is determined, in which the force values do not deviate more than 10%, preferably not more than 5% and even more preferably not more than 2% from the average of the force values in the period, and to determine an uncompensated intraocular pressure as the average of force values in the period, and to determine the cornea tension value using the measured force-time values with the values of the period t IOP0  to t IOP1  excised. 
     
     
         5 . The method of  claim 4 , wherein the sensor comprises a rigid part which is capable to abut the cornea if the lid of the eye is closed so that the indentation of the cornea is given by the height H knob,max  of the protrusion. 
     
     
         6 . The method of  claim 1 , wherein the force values and the corresponding time values either directly or respectively derived values are inserted a set of equations furnished by a model of the behaviour of the cornea, solving the system of equations by varying parameters of the model, and computing the tension of the cornea using the parameters. 
     
     
         7 . The method of  claim 1 , wherein the shape of the protrusion is provided with at least one discontinuity so that at least one significant step in the force values is observed, with the step being attributable to an indentation depth predefined by the discontinuity. 
     
     
         8 . The method of  claim 7 , wherein an extrapolation, preferably a linear extrapolation towards indentation depth  0 , or a non-linear fit is performed using at least two of the measured value pairs:
 maximum force and corresponding indentation depth; and   the pairs of force values and adhering indentation depths defined by the at least one discontinuity.   
     
     
         9 . The method of  claim 8 , wherein the discontinuity comprises a transition to a protrusion of significantly larger extension transversal to its height. 
     
     
         10 . The method of  claim 8 , wherein the discontinuity comprises a presence of protrusions of different height. 
     
     
         11 . A device for measuring properties of the eye substantially shaped as a contact lens, wherein the device comprises a sensor of annular shape and peripherally arranged and having at least one protrusion directed towards the eye when applied to an eye, the device having such an overall flexibility that a lid closing over the device can deform it sufficiently to indent the cornea by the protrusion, 
       wherein the protrusion has at least two portions of different height. 
     
     
         12 . A device for measuring properties of the eye substantially shaped as a contact lens, wherein the device comprises a sensor of annular shape and peripherally arranged and having a protrusion directed towards an eye when applied to the eye, the device having such an overall flexibility that a lid closing over the device can deform it sufficiently to indent the cornea by the protrusion,
 wherein the protrusion has flanks which comprises at least one step creating a transition zone from a larger protrusion to a smaller protrusion.   
     
     
         13 . A device for measuring properties of the eye substantially shaped as a contact lens, wherein the device comprises a sensor of annular shape and peripherally arranged and having a protrusion directed towards an eye when applied to the eye, the device having such an overall flexibility that a lid closing over the device can deform it sufficiently to indent the cornea by the protrusion,
 wherein the sensor is subdivided in at least to subsensors, the subsensor having protrusions of different size, so that the subsensors timely shifted touch the cornea when the lid closes over the device.