United States Patent (19)
`Chubbuck
`
`4,026,276
`(11)
`(45) May 31, 1977
`
`(54)
`(75)
`
`(73)
`
`INTRACRANIAL PRESSURE MONITOR
`Inventor: John G. Chubbuck, Silver Spring,
`Mcd.
`Assignee: The Johns Hopkins University,
`Baltimore, Md.
`Apr. 5, 1976
`Filed:
`Appl. No.: 673,970
`U.S. Cl. ............................ 128/2 P; 128/2. R;
`128/2.05 E; 73/398 C
`int. Cl.'........................................... A61 B 5/02
`Field of Search ................ 128/2 P, 2 R, 2.1 A,
`128/2.05 E, 2.05 D; 73/398 R, 398 C,399
`References Cited
`UNITED STATES PATENTS
`3,504,664 4/1970 Haddad .......................... 128/2.1 R
`73/398 RX
`3,738,356 6/1973 Workman ....
`3,853, 17 12/1974 Murr .............................. 128/2 PX
`3,943,915 3/1976 Severson ............................ 128/2 P
`
`22
`21 )
`(52)
`
`(51)
`(58
`
`56
`
`3,958,558 5/1976 Dunphy et al. .................... 128/2 P
`3,977,391
`8/1976 Fleischmann ............. 128/2.05 E X
`Primary Examiner-Stephen C. Pellegrino
`Attorney, Agent, or Firm-Robert E. Archibald;
`Kenneth E. Darnell
`
`ABSTRACT
`57
`Pressure monitoring apparatus implantable in the cra
`nium to measure intracranial pressure, the apparatus
`comprises a passive resonant circuit having a natural
`frequency influenced by ambient pressure. The reso
`nant circuit has inductance and capacitance capability
`for comparing the local environmental pressure to that
`of a volume of gas trapped inside the apparatus, the
`environmental pressure being measured by observation
`of the frequency at which energy is absorbed from an
`imposed magnetic field located externally of the cra
`1.
`
`35 Claims, 18 Drawing Figures
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`RECORDING AND
`ALARM SYSTEM
`
`- DURA 16
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`Abbott
`Exhibit 1016
`Page 001
`
`

`

`U.S. Patent May 31, 1977
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`Sheet 1 of 10
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`4,026,276
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`SKULL 4
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`RECORDING AND
`ALARM SYSTEM
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`A - DURA 6
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`AF / G. 7
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`Abbott
`Exhibit 1016
`Page 002
`
`

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`U.S. Patent May 31, 1977
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`Sheet 2 of 10
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`4,026,276
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`Abbott
`Exhibit 1016
`Page 003
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`

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`U.S. Patent May 31, 1977
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`Sheet 3 of 10
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`4,026,276
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`Abbott
`Exhibit 1016
`Page 004
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`

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`U.S. Patent May 31, 1977
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`Sheet 4 of 10
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`4,026,276
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`SKULL 4
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`DURA 6
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`AF / G. ed
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`Abbott
`Exhibit 1016
`Page 005
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`

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`U.S. Patent May 31, 1977
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`Sheet 5 of 10
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`4,026,276
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`Abbott
`Exhibit 1016
`Page 006
`
`

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`U.S. Patent May 31, 1977
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`Sheet 6 of 10
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`4,026,276
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`Abbott
`Exhibit 1016
`Page 007
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`

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`U.S. Patent May 31, 1977
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`Sheet 7 of 10
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`4,026,276
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`Abbott
`Exhibit 1016
`Page 008
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`

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`Abbott
`Exhibit 1016
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`Page 009
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`Abbott
`Exhibit 1016
`Page 009
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`U.S. Patent May 31, 1977
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`Abbott
`Exhibit 1016
`Page 010
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`

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`U.S. Patent May 31, 1977
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`Sheet 10 of 10
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`4,026,276
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`Exhibit 1016
`Page 011
`
`

`

`10
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`15
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`1
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`4,026,276
`2
`interrogation and recordation apparatus for determin
`ing intracranial pressure. The present implantable pres
`INTRACRANIAL PRESSURE MONITOR
`sure transducer can be permanently placed in a tre
`STATEMENT OF GOVERNMENT INTEREST
`phine or “burr'hole in the skull and operates without
`the need for percutaneous extracranial connections to
`The invention described herein was made in the
`5
`monitoring apparatus.
`course of work under a grant or award from the De
`Measurement of intracranial pressure gradients are
`partment of Health, Education, and Welfare.
`necessary to anticipate and thereby effectively treat
`BACKGROUND OF THE INVENTION
`secondary complications of cerebral insults, such as
`transtentorial herniations, obstructuve hydrocephalus,
`A. Field of the Invention
`and rapidly expanding hematomas. The present inven
`The invention finds utility for monitoring intracranial
`pressure in diagnostic and post-operative situations, the
`tion allows ready determination of specific treatment
`pressure-sensitive apparatus of the invention being
`modalities to reverse these complications as well as
`totally implantable in the cranium.
`providing useful information in the treatment of cere
`bral edema of idiopathic hydrocephalus in children.
`B. Description of the Prior Art
`The need for monitoring intracranial pressure has
`Since individuals in whom intracranial pressure moni
`long been recognized for applications involving intra
`toring is most desirable are those in whom neurosurgi
`cranial hypertension. Although such a need is well
`cal intervention is necessary or anticipated, the un
`identified for hydrocephalic individuals and individuals
`avoidable requirement for a small burr hole through
`who have undergone neurosurgery, other critical situa
`the skull is acceptable. However, unlike previously
`20
`tions involve individuals subject to brain swelling,
`used devices, the present pressure transducer is opera
`edema, obstruction of cerebral spinal fluid pathways,
`ble without the need for electrical circuits or manomet
`or intracranial space-occupying lesions. Accurate mon
`ric conduits which extend through the scalp, both of
`itoring of the intracranial pressure in certain of these
`which offer a portal of entry for infection and compro
`situations allows institution of emergency procedures
`25
`mise patient mobility.
`should pressure rise to dangerous levels.
`The implantable pressure transducer hereby pro
`Common methods for measuring intracranial pres
`vided comprises a non-porous yet complaint enclosure
`sure involve implantation of a pressure transducer hav
`which contains a specific mass of trapped gas and a
`ing wires which pass through the skull and scalp. Mea
`passive r-f resonant circuit (inductance and capaci
`surement of the pressure of the cerebral spinal fluid,
`30
`tance) having a natural frequency which is influenced
`which can be related to the intracranial pressure, has
`by the pressure of the environment of the transducer.
`generally involved lumbar puncture or introduction of
`The capacitance portion of the r-f resonant circuit is
`a catheter into the ventricular spaces. None of these
`comprised of two bellows each with one closed end.
`techniques are suitable for prolonged measurement of
`The closed ends, lying in close proximity to each other,
`these pressures. Danger of infection, patient discom
`35
`form a capacitance directly proportional to their areas
`fort, and the certain need for a second operation to
`and inversely proportional to their spacing. Increasing
`remove the measuring device are negative aspects of all
`intracranial pressure elongates both bellows diminish
`of these prior art techniques. Certain of these tech
`ing the spacing of the closed ends and, thereby, lower
`niques actually cause leakage and blockage of the hy
`ing the natural frequency of the resonant circuit. An
`draulic system within the cranium and directly affect
`40
`equivalent embodiment of this principal is achieved
`pressure measurements.
`with one bellows having its closed end lying in close
`A number of intracranial pressure measurement sys
`proximity to a fixed conductive surface. In effect, the
`tems have been postulated and even tested in recent
`transducer acts to compare the surrounding environ
`years. Virtually all of these systems involved placement
`mental pressure to that of the gas trapped inside the
`of a transducer within the cranium with wires passing
`45
`transducer.
`through the scalp to a recordation sub-system. Use of
`The resonant frequency of the transducer is sensed
`these systems posed a constant risk of infection and
`by monitoring apparatus external of the cranium by
`required constant adjustments to compensate for
`determining the natural frequency at which the trans
`changes in the position of the patient. Such systems
`ducer absorbs energy from an electromagnetic field.
`were necessarily short-term in use. Attempts were
`50
`This measured frequency is then converted directly to
`made by Atkinson et al and Olson et al. in 1967 and
`a pressure reading.
`w
`1968 respectively to implant a variable capacitor
`The implantable transducer is characterized by the
`mounted on two sides of an air-filled tambour, the
`following features:
`resonant frequency of the variable tuned circuit then
`1. sufficient elastic compliance so that measurable
`being read by imposing a radio wave thereon through
`55
`deformations result from small changes in intracranial
`the intact scalp. The devices thus proposed were sub
`pressure;
`ject to extreme fragility and needed to be constantly
`2. non-porosity of the enclosure so that the mass of
`recalibrated for temperature and atmospheric pressure
`gas trapped within the device does not change mean
`changes. Further, error was prevalent in the use of
`ingfully during the useful life of the device; and,
`these devices due to drift in the zero reading, i. e.,
`60
`3 electrical non-conductivity so that the resonant
`"baseline drift'.
`circuit in the device is not shielded from an external
`monitoring radio-frequency magnetic field.
`SUMMARY OF THE INVENTION
`These characteristics are produced primarily by utiliza
`The invention is a system for monitoring either con
`tion of a combination of ceramic (non-porous and
`tinuously or intermittently the intracranial pressure,
`65
`non-conductive) and metallic (non-porous and compli
`the uses of which have been described hereinabove. In
`ant) material to form the chamber enclosing the
`particular, the invention provides a passive implantable
`trapped volume of gas within the device.
`pressure transducer useful in association with external
`
`Abbott
`Exhibit 1016
`Page 012
`
`

`

`10
`
`15
`
`35
`
`20
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`4,026,276
`4
`3
`FIG. 13 is an elevation in partial section of a third
`In addition to other "absolute pressure' embodi
`embodiment of the invention;
`ments of the invention, a "gauge pressure' embodi
`FIG. 14 is a top view in section of the embodiment of
`ment of the invention is also provided in which the
`FIG. 13; and,
`difference in pressure between the intracranial pres
`FIG. 15 is an elevation in partial section of a gauge
`sure and the pressure immediately beneath the scalp,
`pressure embodiment of the invention.
`which pressure is an approximation of barometric pres
`sure, is measured.
`DESCRIPTION OF THE PREFERRED
`Accordingly, it is a primary object of the invention to
`EMBODMENTS
`provide a system for monitoring intracranial pressure
`The invention particularly provides an intracranial
`without the need for percutaneous extracranial electri
`pressure sensing implant, referred to hereinafter as the
`cal connections or manometric conduits.
`transducer, which contains a passive radio-frequency
`It is a further object of the invention to provide an
`resonant circuit having a natural frequency influenced
`implantable pressure transducer which passively pro
`by the pressure of the transducer's environment. The
`vides an indication of intracranial pressure to extracra
`transducer is configured so that a comparison is contin
`nial monitoring apparatus on interrogation of the trans
`uously made between the environmental pressure and
`ducer.
`the pressure of a fixed mass of gas entrapped inside the
`It is yet another object of the invention to provide an
`transducer. The environmental pressure is effectively
`implantable pressure transducer having sufficient elas
`measured by observing the frequency at which the
`tic compliance to be measurably deformed by changing
`transducer absorbs energy from an externally imposed
`intracranial pressure while being sufficiently non-por
`electromagnetic field. The invention further provides
`ous to prevent loss of entrapped gas from the trans
`apparatus for imposing an electromagnetic field on the
`ducer or leakage of body fluids into the transducer.
`transducer and for remotely measuring the frequency
`It is yet another object of the invention to provide am
`at which energy is absorbed by the transducer.
`implantable pressure transducer which provides a mea
`Referring now to FIGS. 1 and 2, the present implant
`25
`sure of the difference between the intracranial pressure
`able transducer is seen at 10 to be positioned within a
`and the pressure immediately beneath the scalp, which
`burr hole 12 in the skull 14 of an individual who has
`pressure is an approximation of atmospheric, and thus
`need for monitoring of the intracranial pressure. The
`barometric, pressure.
`lowerface of the transducer 10 is seen to be positioned
`Further objects and advantages of the invention will
`against the dura 16, a membrane which lies beneath the
`30
`become more readily apparent in light of the following
`skull 14 and above the subarachnoid space, i.e., the
`detailed description of the preferred embodiments of
`space between the skull 14 and the brain. The trans
`the invention.
`ducer 10 can be mounted as will be described in detail
`hereinafter to bear against the dura 16 without deflect
`BRIEF DESCRIPTION OF THE DRAWINGS
`ing the dura downwardly enough so as to obliterate the
`FIG. 1 is a schematic illustrating the environment and
`subarachnoid space below the transducer. The dura 16
`general operation of the invention;
`may therefore act to transmit subdural cerebral spinal
`FIG. 2 is a detailed elevation in partial section of an
`fluid pressure to the transducer 10. Pressure sensed by
`implanted transducer according to one embodiment of
`the transducer 10 is measured by an external detector
`the invention;
`18 which is positioned externally of the scalp over the
`40
`FIGS. 3a, 3b, and 3d are assembly views in perspec
`implanted transducer 10, the detector 18 being electri
`tive illustrating the several parts of one embodiment of
`cally connected to a recording and alarm monitor 20.
`the implantable pressure transducer in various stages of
`Through use of the monitor 20 in a manner to be de
`assembly;
`scribed hereinafter, a continuous record of intracranial
`FIG. 4 is a perspective in partial section of one em
`pressure can be made. Additionally, a visual or audio
`45
`bodiment of the implantable pressure transducer;
`alarm can be integrated into the monitor 20 to provide
`FIG. 5 is a schematic illustrating in part the manner
`a signal when intracranial pressures reach dangerously
`in which the transducer is mounted in the cranium;
`high (or low) levels.
`FIG. 6 is a graph illustrating the resonant frequency
`. The structure of the transducer 10 can best be seen in
`response of a typical transducer versus the sensed intra
`FIGS. 3a through 3d and 4 to comprise two disc-like
`SO
`cranial pressure;
`sections 22 and 24 which are bonded together to form
`FIG. 7 is an elevation in partial section of a portion of
`a hollow circular cylinder. Each section has a central
`a transducer illustrating the sealing thereof;
`aperture 26 formed in its planar face, the apertures 26
`FIG. 8 is an idealized assembly perspective illustrat
`respectively receiving bellows 28 and 30 thereinto. The
`ing the arrangement of elements used to implant the
`sections 22 and 24 are formed of a ceramic or other
`non-porous material and are suitably bonded together,
`transducer;
`FIG. 9 is an idealized perspective illustrating the
`such as with glass frit or with epoxy cement. The bel
`method employed to position the transducer properly
`lows 28 and 30 are preferably formed of gold-plated
`nickel. The bellows 28 and 30 are each formed with an
`in the cranium;
`FIG. 10 is a graph illustrating the pressure response
`annular flange 32 surrounding the open end thereof, .
`60
`of the transducer at varying implantation depths in the
`the flange 32 being bonded, such as by epoxy, to the
`peripheral portion of the planar face of each section 22
`cranium;
`FIG. 11 is a block diagram illustrating the electrical
`or 24 which surrounds each of the apertures 26. The
`components of the transducer, the external detector,
`planar closed end of each of the bellows 28 and 30 is
`and the external monitoring apparatus used to detect
`disposed toward the interior of the enclosed chamber
`34 formed on bonding together of the sections 22 and
`and monitor the resonant frequency of the transducer;
`24, the said closed ends extending into the chamber
`FIG. 12 is an elevation in partial section of a second
`from opposite sides thereof and being spaced apart by
`embodiment of the transducer,
`
`55
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`65
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`Abbott
`Exhibit 1016
`Page 013
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`4,026,276
`6
`5
`the sections 22 and 24 would be sufficiently compliant
`a finite, pre-determined spacing as will be described in
`and non-conductive but prove to be too porous. The
`greater detail hereinafter.
`use of non-porous and non-conductive glass proves to
`The hollow circular cylindrical chamber 34 defined
`be unsatisfactory due to the non-compliant nature of
`by the sections 22 and 24, when fully closed by bonding
`the material in structurally adequate wall thicknesses.
`of the bellows 28 and 30 into respective apertures 26,
`The requirement for low porosity, low electrical con
`forms a reference pressure enclosure in which is en
`ductivity, and relatively high elastic compliance are
`trapped a given mass of a gas (or other compressible
`met in the transducer 10 by forming the major portions
`non-toxic fluid), the pressure of which mass of gas is
`of the planar faces the enclosure (formed by the sec
`used as a reference pressure relative to the pressure of
`tions 22 and 24 with central apertures 26) of non-por
`the transducer's environment, i.e., within the skull 14
`ous and non-conductive ceramic material and by form
`in which the transducer 10 is disposed. The gas prefer
`ingreduced portions of the planar faces with metal, i.e.,
`ably takes the form of pure nitrogen having condens
`the bellows 28 and 30, the metal being non-porous and
`able gases such as water vapor removed therefrom. The
`compliant. Since the metal is also electrically conduc
`gas within the chamber 34 is sealed therein at standard
`tive, a portion of the magnetic field absorption cross
`atmospheric pressure. Leakage of gas from the cham
`15
`section of the transducer 10 is shielded. However, by
`ber 34 or seepage of other fluids into the chamber 34 is
`controlling the percentage of the surface area of the
`to be prevented. Bonding of the flanges 32 of the bel
`planar faces of the transducer 10 which is subtended by
`lows 28 and 30 to the sections 22 and 24 must therefore
`the metal bellows 28 and 30, the percentage of the
`be accomplished so as to minimize changes in this ref
`magnetic field absorption cross-section which is
`erence pressure gas volume. A metallization ring can
`20
`shielded to the externally imposed electromagnetic
`be formed around each of the apertures 36 and flange
`field is readily held within acceptable limits. The area
`26 soldered thereto. Alternatively, the flanges 26 can
`of the metal portion forming the planar faces of the
`be bonded to the ceramic material with epoxy. Al
`transducer 10 is preferably held to approximately 20%
`though epoxy is a more porous material than solder, the
`in order to retain sufficient elastic compliance while
`epoxy joints have a high ratio of path length to diffusion
`limiting shielding as aforesaid. This shielding dimin
`cross-section compared to that of the ceramic portion
`ishes the coefficient of coupling between the trans
`of the transducer 10. Simple epoxy bonding is therefore
`ducer 10 and external monitoring apparatus to be de
`considered to be adequate and economically attractive
`scribed, thereby reducing slightly the implantation
`since base-line drift, i.e., change in the reference pres
`depth at which the resonant frequency of the trans
`sure, is sufficiently slight with such bonding as to be
`30
`ducer 10 can be measured.
`tolerable.
`The transducer 10 is enclosed within a casing 38
`The assembled sections 22 and 24 further have an
`formed of two cylindrical half-sections 40 and 42, the
`inductance coil 36 wound thereabout, the ends of the
`half-sections 40 and 42 being boned together to form
`coil 36 being respectively soldered to the bellows 28
`the closed cylindrical casing 38. The casing 38 is pref
`and 30 to form a passive resonant circuit in which the
`erably formed of a suitable bio-compatible plastic such
`closed ends of the bellows lie in proximity to each other
`as Lexan, a product of General Electric Inc., or Poly
`to form the capacitive portion of the circuit. The induc
`sulfone. The half section 40 is seen to be formed with
`tance coil 36 preferably is formed with eight turns. The
`threading 44 over the cylindrical surface thereof and
`coil 36 can be formed of silver, platinum, copper, or
`with slots 46 in the upper planar face 43 thereof, the
`other highly conductive material. The conductive ma
`40
`slots 46 not extending through the face. The half-sec
`terial should have low electrical resistance in order to
`tion 40 is bonded to the half-section 42 using a suitable
`provide a high Q to the resonant circuit of which the
`solvent and plastic grit of the type of plastic employed
`coil 36 forms a part. Silver and platinum would be
`as the material of which the half-sections are formed.
`preferred due to the less toxic properties of these con
`The lower planar face 45 of the half-section 42 is typi
`ductors relative to copper. Platinum is least toxic of the
`45
`cally made less thick than the upper planar face 43, the
`three conductors mentioned, although the relatively
`face 45 being as thin as is structurally practical in order
`higher resistance of the platinum causes some loss of Q.
`to aid in pressure transfer across said lower face. For
`An increase in pressure externally of the transducer 10
`example, if the half-sections 40 and 42 are formed of
`acts to elongate both of the bellows 28 and 30, thereby
`polysulfone, a grit of polysulfone dissolved in dichloro
`bringing the closed ends thereof closer together to
`50
`ethane is used to cement the half-sections 40 and 42
`increase the capacitance and to lower the resonant
`together. Mating inner and outer annular shoulders 48
`frequency of the circuit. Pressure changes can thus be
`and 50 form a lap-joint 52 which seals the casing 38
`monitored externally of the transducer 10 by measure
`more securely.
`ment of the frequency at which the circuit absorbs
`The spacing internally of the casing 38 and externally
`energy from an externally imposed electromagnetic
`of the transducer 10 is filled with a suitable fluid 54,
`field.
`such as medical grade silicon liquid, which transmits
`As discussed briefly hereinabove, the transducer 10
`external pressure on the casing 38 to the bellows 28
`must exhibit sufficient elastic compliance such that a
`and 30. The fluid 54, in addition to its ability to trans
`measurable deformation will result from relatively
`mit pressure, should be chosen to have a low dielectric
`small changes in intracranial pressure. At the same
`60
`constant in order to minimize stray capacitance, to
`time, the enclosure for the entrapped gas must be ex
`have a low ability to absorb moisture, and to be non
`tremely non-porous so that the mass of gas entrapped
`toxic. The casing 38 acts at its most basic level to iso
`will not change appreciably during the useful life of the
`late the circuit formed by the bellows and the coil 36
`transducer 10. Further, the transducer 10 must be elec
`from conductive body fluids which would short-circuit
`trically non-conductive so that the resonant circuit
`the bellows capacitance.
`formed by the bellows (28 and 30) and the inductance
`Since a number of the transducers 10 would be man
`coil 36 will be shielded from the externally imposed
`ufactured in order to meet the clinical needs mentioned
`electromagnetic field. The use of plastic material for
`
`35
`
`55
`
`65
`
`Abbott
`Exhibit 1016
`Page 014
`
`

`

`P
`P =
`
`O
`
`15
`
`25
`
`35
`
`P=18.5 (T-98.6)
`
`The distance (X in milli-inches) of the surface of the
`bellows 28 and 30 from the midpoint between the bel
`lows can be given in terms of the resonant R-F fre
`quency (co):
`
`a o LK
`1 - of LC,
`
`P
`P + P = T =y
`ar
`
`f: -
`f?
`(1 - Blf) (1 - 3 f)
`
`where:
`P = barometric pressure (mm HO);
`PA = applied pressure (mm HO);
`T = temperature (F. degrees absolute)
`
`k
`k
`2 A- Al
`2 L
`22r)
`
`t
`where P and T are respectively the closure pressure
`and temperature at the time the vent hole 56 in the
`transducer 10 is sealed. The ideal value for these last
`two parameters are 10,336 mm HO (760 mm Hg) and
`558R (body temperature, absolute). Hence the tem
`perature sensitivity of the transducer 10 is 18.5 mm
`HO/F. and the formula giving the pressure correction
`(P) for body temperature (T) is:
`
`4,026,276
`8
`7
`Since the volume change caused by bellows deforma
`hereinabove. it is believed necessary to discuss calibra
`tion is negligible, the internal pressure P can be ex
`tion and manufacturing control considerations which
`pressed in terms of temperature as follows:
`are useful to a reasonable practice of the invention.
`Calibration of the transducer 10 is essentially a matter
`of control of the spacing of the closed ends of the bel
`lows 28 and 30. At atmospheric pressure and at usual
`body temperature, the closed ends of the bellows are to
`be spaced 0.004 inch apart. Control of the spacing
`between the closed ends of the bellows is principally a
`matter of controlling the thickness of the epoxy bond
`between the two ceramic sections 22 and 24. It is
`deemed preferable to epxoy bond the sections 22 and
`24 together rather than to fire a joint therebetween
`using glass frit, since epoxy bonding can be done after
`the bellows 28 and 30 are mounted on the sections 22
`and 24 whereas firing of the glass frit requires that the
`heat-treatable bellows be mounted after the bonding
`between the sections is accomplished. The spacing
`uncertainty introduced by the bonding of the bellows
`28 and 30 to the ceramic sections 22 and 24 can be
`determined electrically and eliminated by honing the
`ceramic sections on a diamond dust-impregnated cop
`per flat. Honing is performed on the surfaces of the
`sections 22 and 24 which are later bonded together.
`Control of the reference pressure within the sealed
`transducer 10 requires the epoxy bonds between the
`L = inductance (1.4 X 10 henries);
`ceramic sections 22 and 24 to be formed and cured,
`such as by baking, before final closure of the chamber
`K = a constant (4.039); and,
`C, F stay capacitance (pupuF).
`34. A small vent hole 56 formed in one of the sections,
`The general equation can now be written:
`30
`such as in the section 22, is used to seal the chamber 34
`in a manner to be described more fully hereinafter.
`Compensation for error in spacing between the closed
`ends of the bellows 28 and 30 requires a bias in the
`closure temperature at the rate of 10.8°C per milli-inch
`of spacing error. The integrity of the bonded joints
`must also be insured since even a minute rate of leak
`age into or from the chamber 34 results in an unaccept
`able rate of drift of the reference pressure. Thus, the
`chamber 34 is leak tested after closure of the vent hole
`56 and allowance for the entrapped gas to reach room
`40
`temperature. As the entrapped gas in the sealed cham
`ber 34 cools to room temperature, the resonant fre
`quency of the circuit diminishes approximately 8.4
`= 2nt)* LCs; and
`MHz. If any gross leaks are present, the frequency will
`f, = frequency understandard conditions of pressure
`drift back to the body temperature value. If the room
`45
`(10,366 mm HO) and temperature.
`temperature frequency remains stable, indicating no
`The last three numbers y, g, and f, are sufficient to
`gross leaks, the transducer 10 is then fine leak tested by
`describe completely the performance of the transducer
`soaking the transducer in pressurized helium for a pe
`10. The value forf, is measured directly and the values
`riod of time to allow helium to enter through any mi
`for y on 6 are determined from two frequency readings
`croscopic leak which might exist. The transducer 10 is
`50
`f, and f at two different pressure readings P, and P.
`then placed in a helium leak tester vacuum chamber to
`made at a known temperature and barometric pressure.
`detect escape of helium for the chamber 34. If no he
`The frequency response of the transducer 10 versus the
`lium is detected, the transducer 10 is considered to be
`pressure acting on the transducer 10 is shown graphi
`leak free.
`cally in FIG. 7.
`The operation of the transducer 10 can be described
`Referring now to FIGS. 1, 2, and 5, the transducer 10
`by the following mathematical relationships, the pres
`encased in the casing 38, which will be referred to
`sure P external of the transducer being related to the
`hereinafter as the “encased transducer 10”, is seen to
`pressure P by the following:
`be held within a cranial burr hole 12 by means of a
`collar 58. Implantation of the transducer 10 can be
`made under local anesthesia through a curvilinear inci
`sion in the scalp 64. The burr hole 2 is made in the
`skull 14, usually as the result of treatment for a siuta
`tion which incidentally requires monitoring of intracra
`nial pressure, according to known procedures: Typi
`cally, an air-driven trephine or brace trephine is used to
`make the hole 12, the cavity in the skull 14 being
`trimmed with a curette to expose a circular area of
`
`55
`
`60
`
`P. = P + (X, -X)
`
`where:
`k = the spring constant of the bellows 28 and 30;
`A = cross-sectional area of the bellows closed end;
`X = neutral (unstressed) position of the bellows
`(half the separation of the bellows); and,
`x = the stressed position of the bellows.
`
`65
`
`Abbott
`Exhibit 1016
`Page 015
`
`

`

`15
`
`25
`
`4,026,276
`10
`9
`itself. Alternatively, corrections can manually be made
`dura. Bone wax is used to stem bleeding from the walls
`in response to barometric pressure changes.
`of the burr hole 12 and a bipolar coagulator is used to
`Prior to implantation, the encased transducer 10 (as
`stop any bleeding which may exist on the surface of the
`well as the collar 58) is to be sterilized internally as well
`exposed dura 16. The collar 58 is then attached to the
`as externally. The transducer 10 cannot be autoclaved
`skull 14, an annular flange 60 on the upper end of the
`because the great heat would rupture the bellows 28
`collar resting on the table of the skull 14. The flange 60
`has peripherally spaced apertures (not shown) therein
`and 30. While the encased transducer 10 can be
`brought to a temperature of 120° C and held at that
`which allows suturing of the collar 58 to the periosteum
`temperature for an extended period of time, steriliza
`or bone. In the event the periosteum is not available,
`tion by radiation appears to be the best procedure.
`the galea tissue is turned over the flange 60 as a flap
`10
`Referring now to FIG. 7, a preferred method for
`and secured with interrupted sutures. The scalp mar
`sealing the transducer 10 is shown. As aforesaid, a vent
`gins are approximated for subsequent wound closure.
`hole 56 is formed in one of the ceramic sections 22 or
`An annular cylindrical neck 62 which forms the re
`24. After assembly of the bellows 28 and 30 to the
`maining portion of the collar 58 extends into the burr
`sections and bonding together of the sections, the spe
`hole 12 and is threaded internally at 63 to receive the
`cific mass of gas is finally entrapped within the cham
`threads 44 on the outer cylindrical surface of the casing
`ber 34 by sealing of the vent hole 56. The vent hole 56
`38 of the encased transducer 10. The encased trans
`is best sealed by providing a thin annular metal insert
`ducer 10 can thereby be screwed into the neck 62 of
`57 within the hole 56. The insert 57 has sloping inner
`the collar 58 to a depth sufficient to cause the lower
`cavity walls and may preferably be formed of brass.
`planar face 45 of the casing 38 to bear against the dura
`20
`The insert 57 can be simply coated with soft solder
`16 as seen clearly in FIG. 2. The encased transducer 10
`while leaving the hole 56 open. The transducer 10
`is rotated within the collar 58 by means of a two
`would then be brought to body temperatu