Patent Publication Number: US-2004059248-A1

Title: Implant for determining intra-ocular pressure

Description:
[0001] The present invention relates to an implant for determining the pressure of the aqueous humour in an eye according to the preamble of claim 1.  
       [0002] All over the world, glaucoma is a major cause of blindness. The basic cause is an increased interior pressure of the eye which in most cases results from a reduced drain of aqueous humour. In order to select appropriate drugs or to suggest an operation, it is necessary to continuously record the interior eye pressure which may vary significantly during the day.  
       [0003] The present methods of recording the interior eye pressure in a non-invasive way use applanation tonometry. When applying this method, the cornea is deformed from outside, and the force required for this is correlated with the interior eye pressure. This method has several disadvantages: On the one hand, the result of the measurements is affected by non-adjustable and individually varying disturbances such as the rigidity of the cornea. Another drawback lies in the use of the tonometer. In most cases, only well instructed staff is capable of picking up the interior eye pressure in a discontinuous manner. The recording of series of measuring values showing the course of changes of the interior eye pressure over one or several days normally requires in-patient treatment in a hospital. This means that the course of changes of the interior eye pressure during normal daily life can hardly be determined. Furthermore, it is impossible to record the course of changes of the interior eye pressure in very short time intervals over a longer period.  
       [0004] DE 198 58 172 A1 teaches an intra-ocular lens which includes a measuring unit for determining the intra-ocular pressure. The measuring unit in the form of a telemetric endo-system comprises a pressure sensor element for measuring the intra-ocular pressure, a data processing unit for the production of a pressure controlled output signal and a range system with a micro coil. The micro coil serves for the receipt of feed, control and data signals and can transmit measuring and data signals. The measuring unit is disposed on a thin support film with conducting tracks which are in electric connection with the individual components. A drawback of this arrangement resides in that accurate detection of the intra-ocular pressure relative to the surroundings is not possible.  
       [0005] DE 197 28 069 C1 describes an implant for continuous determination of the intra-ocular pressure. The implant comprises a pressure sensor element for determination of the intra-ocular pressure, a data processing unit for conversion of the sensor signals into wirelessly transmitted information, a data logger and a transmit-receive unit. The pressure sensor element comprises a micro-mechanical pressure sensor. The measuring data continuously supplied by the pressure sensor can be stored in the data logger. Accurate determination of the intra-ocular pressure relative to the direct surroundings of the front chamber is not possible, the pressure sensor only measuring absolute pressure.  
       [0006] It is therefore an object of the present invention to provide an implant which allows continuous recording of the pressure of the aqueous humour.  
       [0007] This object is achieved by the features specified in the characterizing part of claim 1. The crux of the invention is to provide two pressure sensor elements on an implant. One pressure sensor element measures the interior pressure of the aqueous humour. A second pressure sensor element is provided, measuring the ambient pressure. The overpressure of the aqueous humour is determined by the data of the two pressure sensor elements.  
       [0008] Other advantageous embodiments of the invention will become apparent from the dependent claims. 
     
    
    
     [0009] Additional advantages and details of the invention will become apparent from the description of three example embodiments with reference to the attached drawings in which:  
     [0010]FIG. 1 shows an implant inserted into an eye according to the present invention in accordance with a first embodiment;  
     [0011]FIG. 2 shows a cross-section through the implant;  
     [0012]FIG. 3 shows a cross-section through the implant along the line III-III in FIG. 2;  
     [0013]FIG. 4 shows a schematic circuit diagram of the implant illustrated in FIG. 1;  
     [0014]FIG. 5 shows a schematic circuit diagram of an implant in accordance with a second embodiment;  
     [0015]FIG. 6 shows a plane view of an implant in accordance with a third embodiment; and  
     [0016]FIG. 7 shows a cross-section through the implant along the line VII-VII in FIG. 6. 
    
    
     [0017] Now a first embodiment of the invention will be described with reference to FIGS. 1 through 4. An implant  1  for measuring the pressure of the aqueous humour in the front ventricle of an eye comprises a hollow needle  2  enclosing a feeding channel  3 . Said feeding channel  3  is open to the outside on one end  4  of said hollow needle  2 . On the opposite end of said hollow needle  2 , the latter is connected to a pressure sensor unit  5 . Said pressure sensor unit  5  comprises a housing  6  having a circular bottom  7  formed as supporting body and an annular cylindrical wall  8  projecting upwards therefrom. Said bottom  7  comprises a central aperture  9  with said hollow needle  2  being joined with said bottom  7  along the periphery of said aperture  9 . Thus, said channel  3  opens into the interior space  10  of said housing  6 . Said housing  6  is closed with a top lid  11  comprising an annular cylindrical edge  12  which surrounds said wall  8  in a sealing manner and is connected with it by bonding or locking. Said lid  11  comprises a central aperture  13  through which the atmospheric pressure acts on said interior space  10 . In said interior space  10 , a first pressure sensor element  14  and a second pressure sensor element  15  are arranged with the second element being bonded sheet-like onto the first element and with both elements being embedded in a plastic matrix  16 . Said plastic matrix  16  is essentially shaped as a flat cylinder. Said plastic matrix  16  is supported by a flat annular seal  17  on said bottom  7  of said housing  6 , wherein a measuring chamber  18  is formed between said matrix  16  and said bottom  7  with said annular seal  17  forming the peripheral border of said measuring chamber  18  and with said measuring chamber  18  being connected to said channel  3 . The diameter D M  of said measuring chamber  18  is larger than the inner diameter D K  of said hollow needle  2 .  
     [0018] Now the structure of said pressure sensor elements  14  and  15  will be described in greater detail. Said pressure sensor element  14  comprises a circular substrate  19  on the centre of which an array of one or more, e.g. 3 time 3, micromechanical pressure sensors  20  is provided. These are micromechanical absolute pressure sensors common in the market which detect pressure capacitively or piezo-resistively. Two measuring memories  21 , a controller  22  and a central data processing unit or CPU  23  are provided adjacent and electrically connected to said membranes  20 . Along the edge of said substrate  19 , an annular transmitter coil  24  is provided which ends up in said controller  22 . Said membranes  20  measure the ambient pressure absolutely, i.e. compared with a known pressure present behind said membranes  20 . Beneath said substrate  19 , another disk-shaped substrate  25  is provided which comprises in its centre a number of sensor membranes  26  projecting downwards and designed as micromechanical absolute pressure sensors for measuring the pressure of a liquid supplied through said channel  3 . As the diameter D M  of said measuring chamber  18  is larger than the diameter D K  of said channel  3 , a comparatively large number of sensor membranes  26  can be arranged while said hollow needle  2  is kept as thin as possible. Said sensor membranes  26 , too, measure the pressure compared with a known pressure present behind them, i.e. the absolute pressure. Said CPU  23  subtracts the absolute pressures measured by said membranes  20  and  26  from one another in order to determine the pressure of the liquid in said hollow needle  2 , i.e. the overpressure of the aqueous humour, compared with the ambient pressure.  
     [0019] Now the data processing in said pressure sensor unit  5  and the communication with the environment will be described in greater detail with reference to FIG. 4. An external control device  27  is located outside of said implant  1  and comprises a program transmission unit  28 , a power transmission unit  29 , and a measuring data transmission unit  30  which units are connected via a controller  31  with a transmitter coil  32 . Within said implant  1 , said transmitter coil  24  allocated to said transmitter coil  32  for the telemetric transmission of data and power is connected with said controller  22  which on its part is connected with a program transmission unit  33 , a power transmission unit  34 , and a measuring data transmission unit  35 . Said power transmission unit  34  is connected by means of lines  36  via a power-storing unit  37  to said CPU  23  to provide a power supply to the latter. Said program transmission unit  33  is connected by means of a line  38  to said measuring program memory  39  and a measurement control unit  40 , which units pick up data in time intervals Δt from the sensor signal unit  41  and determine on their basis, in a downstream measuring data processing unit  42 , the relative pressure Δp and store it in a measuring value memory  43  and/or  21 , which memory is connected by means of a line  44  to said measuring data transmission unit  35 .  
     [0020] Now the implantation of said implant  1  into the eye with reference to FIG. 1. The space between the cornea  45  and the iris plane is designated as front ventricle  48  which contains aqueous humour. The rear ventricle is located between the iris plane and the vitreous body  49  located behind the lens  47 . The rear ventricle, too, is filled with aqueous humour. Said lens  47  is connected with the ciliary body  51  by means of zonula fibres  50 . Said implant  1  is inserted into the edge or limbal portion  52  of the eye, wherein said hollow needle  2  penetrates the limbus from outside so that aqueous humour can flow from the front ventricle  48  of the eye through said channel  3  to said membranes  26 . Said pressure sensor unit  5  is located outside the front chamber  48  in the episcleral tissue beneath the conjunctiva.  
     [0021] Now the operation of said implant  1  will be described. The aqueous humour of the front ventricle of the eye  48  flows through said channel  3  to said membranes  26 . There the pressure is measured compared with a known pressure. At the same time, said membranes  20  measure the ambient pressure compared with a known pressure. Said measuring data processing unit  42  calculates from these signals the medically relevant pressure difference. Said implant  1  is a long-term implant. For power supply and data transmission, said control device  27  is placed near said implant  1 . This can, for example, be accomplished by accommodating said control device  27  in spectacles. Said power transmission unit  29 , said controller  31 , said transmitter coils  32  and  24 , said controller  22 , and said power transmission unit  34  co-operate to charge said power store  37 . The capacity of said power store  37  is chosen so that the power supply of the implant is guaranteed for a longer period of time and that the time interval for recharging the power store can be made as long as possible. Said program transmission unit  28 , said controller  31 , said transmitter coils  32  and  24 , said controller  22 , and said program transmission unit  33  can be used to alter said measuring program memory  39 . In this manner, the time interval Δt in which the measuring values are recorded can be changed from the outside. The pressure difference determined by said measuring data processing unit  42  is stored in the measuring value memory  43 . When a telemetric connection is established between said control device  27  and said implant  1 , the data are transmitted from said measuring value memory  43  via said measuring value transmission unit  35 , said controller  22 , said transmitter coils  24  and  32 , said controller  31  to said measuring data transmission unit  30  where they can be read and medically used. Said measuring value memory  43  is designed so that in case of a memory overflow the data stored first will be erased first. Should the time interval for reading be exceeded, the latest course of eye pressure changes will be retained. The use of an optical or acoustic signal transmitter makes it possible to inform the patient about a pathological increase of the interior eye pressure, so that appropriate therapeutic action can be taken without delay.  
     [0022] Now a second embodiment of the present invention will be described with reference to FIG. 5. Identical components are given the same reference numerals as in the first embodiment, to which reference is made here. Functionally identical but structurally differing components are given the same reference numerals followed by an inverted comma. The main difference compared with the first embodiment is that said units  33 ,  34 , and  35  are not connected to said transmitter coil  24  via a common controller  22  but that every unit comprises a transmitter coil of its own. In the area of a first coil unit  53 , the telemetric program transmission is accomplished from an external programming device  54  to said measuring program memory  39 . In the area of a second coil unit  55 , the telemetric power transmission is accomplished from an external power supply unit  56  to said power store  37 . A third coil unit  57  accomplishes the reading of data from said measuring value memory  43  and the data transmission to an external measuring data collecting unit  58 . The advantage of this arrangement is that, compared with the first embodiment, no controller  22  is required. As a drawback, a number of transmitter coils  32  are required within the implant so that it has to be larger.  
     [0023] Now a third embodiment of the present invention will be described with reference to FIGS. 6 and 7. Structurally identical components are given the same reference numerals as in the first embodiment, to which reference is made here. Functionally identical but structurally differing components are given the same reference numerals followed by two inverted commas. The essential difference compared with the first embodiment relates to the design of the housing  6 ″ and in particular to the fact that all electronic components are provided on a conducting film  59 . The implant  1 ″ comprises as a supporting body a conducting film  59  comprising a flat, essentially rounded rectangular main portion  60  and a web-shaped front ventricle portion  61  projecting outward. Said conducting film  59  is encapsulated in a one-piece plastic housing  6 ″ made of biologically compatible material. Said housing  6 ″ comprises a main housing  64  surrounding said main portion  60  of said conducting film  59  and a housing arm  65  extending therefrom at an angle a of approximately 120° downwards and including said portion  61  of said conducting film  59 , wherein said arm  65  comprises a pointed outer end  62 . Said arm  65  ends in a tip at its outer end  62  in order to facilitate the pushing of said arm  65  through the sclera  63  of the eye. The length L H  of said main housing  64  essentially corresponds with the length L A  of said housing arm  65 . However, other dimensions are possible as well.  
     [0024] Various electronic elements are formed on said conducting film  59  by means of known microtechnical structuring such as the so-called flip chip technology. On said main portion  60 , said measuring value memory  21 , said controller  22 , said central data processing unit  23  as well as a first pressure sensor element  14 ″ with sensor membranes  20 ″ are provided. Near said elements  20 ″,  21 ,  22 , and  23 , the transmitter coil  24 ″ is placed on the opposite side of said conducting film  59 . Immediately above said sensor membranes  20 ″, said housing  6 ″ comprises an area  66  having a lesser thickness. This area provided for transmitting the ambient pressure to said sensor membranes  20 ″ is chosen so that said membranes  20 ″ are sufficiently protected from the surrounding tissue and tissue fluid while, on the other hand, the ambient pressure is passed on to said membranes  20 ″ essentially unchanged to be measured there. It is also possible to provide the other electronic elements known from the first and second embodiment on said conducting film  59 . The second pressure sensor element  15 ″ with its sensor membranes  26 ″ is provided on said portion  61  of said conducting film  59 . Here, too, a pressure transmission area  67  of lesser thickness in said housing  6 ″ is provided adjacent to said membranes  26 ″ so that said membranes  26 ″ are one the one hand sufficiently protected from the surrounding tissue and the aqueous humour while on the other hand the interior pressure of the aqueous humour can be measured as flawlessly as possible. The pressure element  15 ″ and the other elements  20 ″,  21 ,  22 ,  23  on the main portion  60  of said conducting film  59  are connected with each other by conducting tracks  68  on said conducting film  59  to transmit data and to maintain the power supply. The length L A  of said housing arm  65  is chosen so that the outer third in the front ventricle of the eye  48  is immersed in aqueous humour. Said pressure sensor element  15 ″ is located in this outer third. Said arm  65  has in its central third a thickness D M  larger than the thickness D A  of said arm  65  in the area of said main housing  64 . In this manner, said arm  65  is prevented from slipping out of said front ventricle of the eye  48 .  
     [0025] A particular advantage of the design of the third example embodiment is the fact that all electronic elements are provided on one conducting film  59 . This enables miniaturisation and mass manufacturing without problems, as known techniques of microelectronics and especially flip chip techniques may be used. Pressures in two different spaces, i.e. the front ventricle of the eye  48  and the limbal space  52 , can be measured. The physiologically relevant overpressure of the aqueous humour in the front ventricle of the eye can be determined compared with the surroundings of the front ventricle of the eye.