WO2017072028A1

WO2017072028A1 - Implantable support device

Info

Publication number: WO2017072028A1
Application number: PCT/EP2016/075299
Authority: WO
Inventors: Maarten Petrus Joseph KUENEN; Pierre Hermanus Woerlee; Jens MÜHLSTEFF
Original assignee: Koninklijke Philips N.V.
Priority date: 2015-10-27
Filing date: 2016-10-20
Publication date: 2017-05-04

Abstract

There is provided an implantable support device for providing support to an artery of a subject during a tonometry measurement of blood pressure, the implantable support device comprising a first surface that is adapted to support the artery of the subject and a second surface, on an opposite side of the implantable support device to the first surface, that is adapted to interface with a bone of the subject such that movement of the artery relative to the bone during a tonometry measurement of blood pressure is substantially prevented.

Description

Implantable support device

TECHNICAL FIELD OF THE INVENTION

The invention relates to tonometry measurements of blood pressure.

BACKGROUND OF THE INVENTION

High arterial blood pressure (ABP) is the most important risk factor for a wide variety of cardiovascular events, such as stroke, heart attack, etc., that lead to death in many patients. In fact, in its Global Health Risks report (2009), the World Health Organization has attributed over 7 million deaths worldwide to high ABP. As a result, measurement of ABP is of fundamental importance in the prevention of cardiovascular events. This is especially relevant considering that high blood pressure (hypertension) is, in many patients, a chronic problem that requires adequate management for the rest of the patient's lifetime.

Despite decades of research activity, accurate measurement of arterial blood pressure using non-invasive technologies remains a highly challenging subject: the main standard for non-invasive measurement of ABP has not changed significantly since introduction of auscultatory, cuff-based blood pressure measurements by Korotkoff in 1905.

Automatic methods for non-invasive ABP measurement have been proposed based on, e.g., oscillometry, tonometry, and assessment of pulse wave velocity. Despite their popularity, these methods remain hampered by their accuracy.

The general principle of tonometry is shown in Fig. 1 which shows a pressure sensor 10 being pressed over a region of tissue 12 encompassing an artery 13. The artery is shown in cross-section in Fig. 1. The pressure sensor 10 is pressed over the artery, partially occluding it between the pressure sensor 10 and an underlying bone 14. Although the artery is shown contacting the bone 14 in Fig. 1, it will be appreciated that in practice, there may be tissue between the artery 13 and bone 14. In an ideal situation, due to arterial flattening, the effect of the arterial wall is removed from the force balance. The blood pressure acts as a pressure difference between inside and outside the artery (indicated by arrow 15), measured as a force per unit area (the physical unit of pressure). Since the area is equal to that of the surface of the force sensors, the force measured by the pressure sensor 10 can be directly translated to ABP. SUMMARY OF THE INVENTION

Although tonometry is a well -understood measurement principle and is superior to oscillometry since it offers a fully resolved ABP curve suitable for continuous monitoring, the reliability of the measurement relies on the following assumptions: (i) the artery is supported by a bone that provides a flat surface which supports the artery from below, (ii) the arterial cross-section (which under normal conditions is approximately circular) is flattened by the tonometry device, such that the surface of the artery underneath the tonometry device is perfectly flat, (iii) the thickness of the skin and tissue between the artery and the pressure sensor is insignificant compared to the artery diameter, (iv) the pressure sensor is centred on the artery, and (v) the artery is not displaced during the measurement. However, these physical assumptions often cannot be met in practice as in traditional tonometry measurements, the pressure sensor is simply pressed against the surface of the skin at an approximate location of an artery. There is therefore no guarantee that the artery is measured in the optimal position, nor that bone is flat. There is also no way of ensuring that the artery remains steady throughout the measurement and there may also be tissue between the artery and the bone, and/or the artery and the pressure device.

Current approaches to overcome these problems focus on locating an 'optimal' measurement location, e.g. by rotating the tonometry device around the arm until the optimal measurement spot is located. This approach cannot guarantee a proper measurement however, since the underlying bone structure cannot be controlled. Therefore there is a need for an improved tonometry measurement apparatus.

According to a first aspect of the invention, there is provided an implantable support device for providing support to an artery of a subject during a tonometry

measurement of blood pressure, the implantable support device comprising: a first surface that is adapted to support the artery of the subject; and a second surface, on an opposite side of the implantable support device to the first surface, that is adapted to interface with a bone of the subject such that movement of the artery relative to the bone during a tonometry measurement of blood pressure is substantially prevented.

The implantable support device enables a more reliable measurement of the blood pressure in an artery to be made by placing such an implantable support device between the artery and bone of the subject. As the first surface supports the artery and the second surface is adapted to interface with the bone, the implantable support device provides a stable platform against which the artery can be partially occluded during the blood pressure measurement.

In some embodiments the second surface is curved so as to follow the shape of the bone. This helps to reduce movement of the implantable device relative to the bone.

In some embodiments the first surface is curved so as to support the artery along a part of its length. A curved first surface helps prevent the artery from moving on the first surface. In some embodiments, the implantable device can be rigidly attached to the bone. In some embodiments the implantable support device is made of a biocompatible material.

In some embodiments the implantable support device is made of a compliant material that becomes rigid after being implanted into the body. This allows the implantable device to be moulded to the precise contours of the bone, increasing the stability of the implantable device on the bone.

In some embodiments the implantable support device reflects ultrasound and/or electromagnetic waves. This allows the location of the implantable device to be detected when it is implanted.

In some embodiments the implantable support device further comprises at least one magnet, portion of paramagnetic material or portion of ferromagnetic material. This helps the implantable device to be detected and also helps ensure the correct placement of an external pressure sensor in a tonometry measurement.

According to a second aspect of the invention, there is provided an apparatus for performing a tonometry measurement of blood pressure in an artery, the apparatus comprising: an implantable support device as described above that comprises at least one magnet, portion of paramagnetic material or portion of ferromagnetic material; and a pressure device comprising at least one electromagnet, magnet, portion of paramagnetic material or portion of ferromagnetic material that is adapted to attract the pressure device towards the at least one magnet, portion of paramagnetic material or portion of ferromagnetic material in the implantable support device and apply pressure to the artery; and a pressure sensor for measuring the blood pressure in the artery.

According to a third aspect of the invention, there is provided an apparatus for performing a tonometry measurement of blood pressure in an artery, the apparatus comprising: an implantable support device as in any of the embodiments or aspects described above; and a pressure device that is adapted to apply pressure to the artery, the pressure device comprising a pressure sensor for measuring the blood pressure in the artery. According to a fourth aspect of the invention, there is provided a method of performing a tonometry measurement, the method comprising: pressing an artery between a pressure sensor and an implanted support device as described in any embodiment or aspect above, so as to partially occlude the artery; and measuring the blood pressure in the artery using the pressure sensor.

According to a fifth aspect of the invention, there is provided an apparatus for performing a tonometry measurement of blood pressure in an artery, the apparatus comprising: an implantable support device for providing support to the artery during a tonometry measurement; and a pressure device comprising a pressure sensor for measuring the blood pressure in the artery; wherein the pressure device can be selectively magnetically attracted to the implantable support device in order to partially occlude the artery between the implantable support device and the pressure sensor when a blood pressure measurement is to be made.

In some embodiments the implantable support device and the pressure device are magnetically attracted to each other by means of a first electromagnet on one of the implantable support device and the pressure device and a corresponding electromagnet, magnet, portion of paramagnetic material or portion of ferromagnetic material on the other one of the implantable support device and the pressure device.

In some embodiments the apparatus further comprises a controller configured to adjust the strength of the electromagnetic field generated by the first electromagnet based on a pressure measured by the pressure sensor.

In some embodiments the pressure device comprises a plurality of pressure sensors. In these embodiments the apparatus can further comprise: at least a second electromagnet on the one of the implantable support device and the pressure device; and a controller that is configured to process measurements of pressure from the plurality of pressure sensors and to adjust the strength of the electromagnetic fields generated by the first electromagnet and/or the second electromagnet to obtain substantially uniform pressure measurements from the plurality of pressure sensors.

In some embodiments the pressure device is implantable into the body. In other embodiments the pressure device is for use outside the body.

According to a sixth aspect, there is an implantable support device for providing support to an artery of a subject during a tonometry measurement of blood pressure in which the artery is pressed towards a bone, the implantable support device comprising: a first surface that is adapted to support the artery of the subject to prevent or substantially prevent movement of the artery relative to the implantable support device when pressure is applied to the artery during the tonometry measurement; and a second surface, on an opposite side of the implantable support device to the first surface, that is adapted to interface with a bone of the subject such that movement of the implantable support device relative to the bone during the tonometry measurement of blood pressure is prevented or substantially prevented.

In some embodiments, the first surface is shaped so as to provide a generally flat or near-flat surface against which the artery can be pressed during the tonometry measurement.

According to a seventh aspect of the invention, there is provided an apparatus for performing a tonometry measurement of blood pressure in an artery, the apparatus comprising: an implantable support device as described above in respect of the sixth aspect that comprises at least one magnet, portion of paramagnetic material or portion of ferromagnetic material; and a pressure device comprising at least one electromagnet, magnet, portion of paramagnetic material or portion of ferromagnetic material that is adapted to attract the pressure device towards the at least one magnet, portion of paramagnetic material or portion of ferromagnetic material in the implantable support device and apply pressure to the artery; and a pressure sensor for measuring the blood pressure in the artery.

According to an eighth aspect of the invention, there is provided an apparatus for performing a tonometry measurement of blood pressure in an artery, the apparatus comprising: an implantable support device as described above in respect of the sixth aspect; and a pressure device that is adapted to apply pressure to the artery, the pressure device comprising a pressure sensor for measuring the blood pressure in the artery.

According to a ninth aspect of the invention, there is provided a method of performing a tonometry measurement, the method comprising: pressing an artery between a pressure sensor and an implanted support device as described above in respect of the sixth aspect, so as to partially occlude the artery; and measuring the blood pressure in the artery using the pressure sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which:

Fig. 1 illustrates the basic concept of a tonometry measurement; Fig. 2 shows a cross-section of a subject's limb with an implantable device according to an embodiment of the invention;

Fig. 3 shows an example of an apparatus according to an embodiment of the invention;

Fig. 4 shows a flow chart of a method according an embodiment of the invention.

Fig. 5 shows an apparatus for performing a tonometry measurement of the blood pressure according to an alternative aspect of the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fig. 2 shows an implantable support device 20, implanted in a limb of a subject, such as an arm. The implantable support device 20 is positioned in the tissue 12 of the limb between an artery 13 and a bone 14. The implantable support device 20 has a first surface 22 that generally faces an artery 13 or other blood vessel and a second surface 24, which is on the opposite side of the implantable support device 20 to the first surface 22, that generally faces a bone 14.

The first surface 22 is adapted to support the artery 13 of the subject and the second surface 24 is adapted to interface with a bone 14 of the subject such that movement of the artery 13 relative to the bone 14 during a tonometry measurement of blood pressure is prevented or substantially prevented (e.g. very little movement of the artery 13 relative to the bone 14 is possible or permitted). That is, the support device 20 provides support for the artery 13 as the artery 13 is pressed against the first surface 22 of the support device 20.

The first surface 22 can be adapted to support the artery 13 of the subject to prevent or substantially prevent movement of the artery 13 relative to the support device 20 when pressure is applied to the artery 13 during the tonometry measurement.

The second surface 24 can be adapted to interface with a bone 14 of the subject such that movement of the support device 20 relative to the bone 14 during the tonometry measurement of blood pressure is prevented or substantially prevented.

When the support device 20 is implanted in a subject's limb as shown in Fig.

2, a tonometry measurement of the blood pressure can be made using an external tonometry device comprising a pressure sensor that is placed over the implanted support device 20. The artery 13 is partially occluded between the tonometry device and the support device 20 and the pressure sensor on the tonometry device can then be used to measure the blood pressure.

The first surface 22 is shaped so as to support the artery 13 along part of its length, constraining the artery to reduce or substantially prevent significant movement (e.g. lateral movement) of the artery relative to the bone 14 during tonometric measurements. In addition to being shaped to constrain the artery 13 against movement relative to the bone 14, the first surface 22 can also be shaped so as to provide a generally flat or near-flat surface against which the artery 13 can be pressed during a tonometry measurement which increases the consistency of the compression of the artery 13 during different tonometry measurements.

The second surface 24 is shaped so as to keep the support device 20 in generally the same position relative to the bone 14 during a tonometry measurement of blood pressure (i.e. as pressure is applied to press an artery against the first surface 22. Thus, the second surface 24 can be shaped to follow the contours of the particular bone 14 that it is to be used with the measurement.

In the illustrated embodiment, both the first and second surfaces 22, 24 are generally curved or arch- shaped, with the apexes of the arches curving towards one another. However, it will be appreciated that the first and/or second surfaces can have a different shape to that shown.

As the first surface 22 is curved or arched, the first surface 22 supports the artery along part of its length, constraining the artery position towards the apex of the curve or arch during a tonometry measurement and thus reduces or substantially prevents significant movement of the artery. The curve or arch of the second surface 24 interfaces with the bone of the subject to hold the implantable device steady against the bone. Thus, in use, the implantable support device 20 forms a stable platform on which to make a tonometry measurement of the blood pressure.

The support device 20 can be made of any solid biocompatible material that has appropriate viscoelastic properties (i.e., a high stiffness) for supporting the artery 13 as pressure is applied to the artery 13. Alternatively, in some embodiments, the support device 20 (or at least the part of the support device 20 facing the bone 14) can be made of a compliant (e.g. mouldable) biocompatible material that hardens out (e.g. becomes rigid) after implantation to form a shape according to the specific anatomy of the subject (i.e. the shape of the bone 14 at the measurement site). Biocompatible materials include metals, polymers, ceramics and composite materials. In some embodiments, polymeric biomaterials are used.

In some embodiments, to improve the fit of the implantable support device 20 to the bone prior to implantation, the bone surface can be imaged (e.g. using ultrasound) so that the implantable device can be tailored to fit the individual bone structure. Once the three dimensional contour of the bone is known, a bespoke support device 20 can be created, for example using 3D printing techniques.

The support device 20 can be printed directly using a 3D printer. Alternatively, a 3D representation of the bone can be printed to form a mould that is used to shape a compliant biocompatible material to the contours of the bone. The biocompatible material can then harden out on the mould before being implanted into the subject. In this way, biocompatible material does not have to be moulded directly onto the subject's bone.

The support device 20 is preferably sufficiently small in order to allow unimpaired movement of the artery and limb when a tonometry measurement is not being made. In some embodiments, the support device 20 can be generally free to move with respect to the bone 14 when a tonometry measurement is not being made. In alternative embodiments, the support device 20 can be rigidly attached to the bone so that it is fixed in place to further stabilise the position of the artery 13 during the tonometry measurement. For example, the support device 20 may be screwed or pinned into the bone, or attached via an adhesive. As another example, the support device 20 could be clamped around the bone 14.

In some embodiments, the implantable support device 20 is made of a material that reflects ultrasound waves and/or electromagnetic waves, such as optical light (or particular wavelengths of optical light). This enables optical or ultrasound imaging to be used to reliably detect the position of the support device 20 in the limb and thus determine the correct placement of the external tonometry device for the tonometry measurement. In some embodiments, the implantable support device 20 may comprise specific optical and/or ultrasound markers that can be detected by an external imaging device (or by the external tonometry device if the imaging components are integrated into that device). More generally, the implantable support device 20 may be made of any material that has a different acoustic and/or optical properties (e.g. reflectance) compared with the surrounding tissue 12, such that the support device 20 can be clearly distinguished from the surrounding tissue when acoustic and/or optical imaging is used to determine its position.

The optical and/or ultrasound markers on the implantable support device also allow the arterial position and arterial flow profile to be detected. The markers are designed to be visible using ultrasound (by having a hard reflecting surface) and/or optical (e.g. by having a very large absorption at specific wavelengths, such as (near) infrared) imaging. Since the markers are always around the artery, and the proposed implantable device has a rigid reflecting surface beneath the artery, this creates the optimal circumstances for ultrasound/optical imaging of the artery that is right in front of the support device 20. The arterial flow profile can thus be measured using ultrasound Doppler techniques or, if the artery is sufficiently superficial, with laser Doppler techniques. In some embodiments detecting a flow profile can be used to validate the artery position.

In further embodiments, the implantable support device 20 may further comprise at least one magnet, portion of paramagnetic material or portion of ferromagnetic material (i.e. a material that is influenced by a magnetic field). This enables the location of the implantable device to be easily located using an electromagnet, a magnet, portion of paramagnetic material or a portion of ferromagnetic material on or in the external tonometry device. Furthermore, the magnet, portion of paramagnetic material or portion of ferromagnetic material can be used to support correct alignment of the external tonometry device and the implantable support device 20. In some embodiments, the support device 20 can be completely formed of a magnetic, paramagnetic or ferromagnetic material.

Fig. 3 shows an example of a tonometry blood pressure measurement being made using an apparatus 30 according to an embodiment of the invention. The apparatus comprises an implantable support device 20 and a pressure device 32 that is used externally (i.e. outside the body of the subject). The support device 20 can be as described above with reference to Fig. 2. The pressure device comprises a pressure sensor on the underside of the pressure device 32 which makes contact with the surface of the skin/tissue 12. The implantable support device 20 generally holds the artery in place using the first surface 22 of the device 20 during a tonometry measurement. In this embodiment the first surface 22 is shown as having a channel in which the artery 13 sits. In this embodiment, the channel is formed by means of side walls or protrusions 34.

In embodiments where the implantable support device 20 comprises at least one magnet, portion of paramagnetic material or portion of ferromagnetic material, the at least one magnet portion of paramagnetic material or portion of ferromagnetic material can be in side walls 34. The external pressure device 32 will further comprise at least one corresponding magnet, portion of paramagnetic material, portion of ferromagnetic material or electromagnet that is positioned in the external pressure device 32 that draws the external pressure device 32 towards the support device 20 so as to assist in obtaining the correct alignment of the external pressure device 32 with the support device 20 (and thus the artery 13).

During a tonometry measurement, the pressure device 32 is pressed over the artery 13 and the implanted support device 20 so as to partially occlude the artery. The pressure sensor is then used to measure the arterial blood pressure based on the pressure exerted by the artery on the pressure sensor.

A method 400 of performing a blood pressure measurement is shown in Fig. 4. The method comprises the steps of pressing an artery between a pressure sensor and an implanted support device 20, so as to partially occlude the artery (step 402); and making a blood pressure measurement using the pressure sensor (step 404). The support device 20 can be as described above with reference to Figs. 2 and 3. Both steps 402 and 404 can be performed by a pressure device 32 as described above with reference to Fig. 3.

Figure 5 shows an apparatus 50 according to another aspect of the invention, comprising a support device 52 for providing support to the artery 13 during a tonometry measurement and a pressure device 54 comprising a pressure sensor 56 for measuring the blood pressure of an artery 13. In this aspect, rather than the support device 52 providing a support on the bone 14 against which the artery can be pressed during a tonometry measurement, the support device 52 can be selectively magnetically attracted towards the pressure device 54. This can be achieved by means of electromagnets 58 on the pressure device 54 and magnets or portions of paramagnetic or ferromagnetic material 59 on the implantable device 52, for example. It will be appreciated that the positions of the electromagnets 58 and portions of paramagnetic or ferromagnetic material 59 can be interchanged such that the electromagnets 58 are part of the support device 52 and the paramagnetic or ferromagnetic portions are part of the pressure device 54. In particular, when a tonometry measurement is to be performed, the electromagnet(s) in or on the pressure device 54 are operated to generate a magnetic field and attract the support device 52 towards the skin, thereby partially occluding the artery 13.

In the embodiment shown in Fig. 5 the pressure device 54 is used externally (i.e. outside the body), and the pressure device 54 is placed on the skin so that it magnetically attracts the implanted support device 52 (and thus the artery 13 too) towards the pressure device 54. In an alternative to the embodiment shown in Fig. 5, both the pressure device 54 and the support device 52 may be implanted into the body and arranged generally around an artery 13. In this embodiment, the pressure device 54 and support device 52 can be arranged so that they do not occlude the artery 13 when a tonometry measurement is not being made (e.g. the electromagnets are not used to generate a magnetic field), and when a tonometry measurement is to be performed the electromagnet(s) can be activated (for example in response to a control signal from a device that is outside the subject's body, or at a predetermined blood pressure measurement time) to draw the pressure device 54 and support device 52 together, thereby occluding the artery and enabling a pressure measurement to be made.

An advantage of both the pressure device 54 and support device 52 being implanted into the body is that the amount of tissue between the artery and the pressure sensor becomes less significant compared to the artery diameter, resulting in more reliable tonometry measurements. Another advantage of this aspect of the invention (regardless of whether the pressure device 54 is also implanted in the subject's body) is that it is not necessary for the support device 52 to be attached to a bone, which simplifies the placement of the support device 52 in the body and enables the tonometry measurement to be performed in more locations on the body (e.g. locations away from a bone). The implantable support device 52 may have a first surface 60 that generally faces an artery 13 or other blood vessel.

As in the aspect described above with respect to Figs. 2 and 3, the first surface 60 is adapted to support the artery 13 of the subject during a tonometry measurement such that movement of the artery 13 relative to the pressure device 54 is substantially prevented. That is, the support device 52 provides support for the artery 13 as the artery 13 is pressed against the first surface 60 of the support device 52.

The first surface 60 can be shaped so as to support the artery 13 along part of its length, constraining the artery to reduce or substantially prevent significant movement of the artery relative to the bone 14 during tonometric measurements. The first surface 60 can also be shaped so as to provide a generally flat or near-flat surface against which the artery 13 can be pressed during a tonometry measurement which increases the consistency of the compression of the artery 13 during different tonometry measurements.

Although not shown in Fig. 5, the apparatus 50 can further comprise a controller that is able to control the signal provided to the electromagnet(s) 58 so as to vary the strength of the magnetic field generated by electromagnets 58. The controller may be part of pressure device 54 or part of a separate device.

In some embodiments, the controller can be configured to adjust the strength of the electromagnetic field generated by the electromagnet(s) 58 based on a pressure measured by pressure sensor 56. For example, before and/or during a tonometry

measurement, the controller can receive pressure measurements from pressure sensor 56 and adjust the electromagnetic field generated by electromagnet(s) 58 in an iterative manner in order to keep the average pressure measured by the pressure device substantially constant. In another example, before and/or during a tonometry measurement, the controller may receive pressure measurements from pressure sensor 56 and adjust the electromagnetic field generated by electromagnet(s) 58 in order to increase or decrease the pressure applied by the pressure device to sufficiently occlude the artery 13 and/or to obtain an optimal pressure value for making a tonometry measurement of the blood pressure.

In a further embodiment, the apparatus 50 may comprise a plurality of pressure sensors arranged in an array so as to measure pressure at different locations across the artery. In this embodiment, the blood pressure measurement can be obtained as an average of the blood pressure measurements made by one or more of the plurality of pressure sensors 56. Furthermore, it is possible to use an array of pressure sensors to verify whether a correct tonometric measurement has been made as if the artery is sufficiently flattened, the measured pressure should be substantially constant across the array. The apparatus 50 may therefore further comprise a controller (or the controller mentioned above) that is configured to adjust the relative strengths of the electromagnetic fields of a number of electromagnets 58 so as to adjust the pressure distribution across the artery. In particular the electromagnetic field strengths can be adjusted so as to obtain a substantially uniform pressure measurement across the pressure sensors in the array.

As noted above, the invention provides a number of advantages, and in particular it allows for a reliable continuous or discontinuous measurement of ABP based on the tonometry principle. The invention also allows for measurement of the ABP waveform (contrary to oscillometric and auscultatory methods).

Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. An implantable support device for providing support to an artery of a subject during a tonometry measurement of blood pressure in which the artery is pressed towards a bone, the implantable support device comprising:

a first surface that is adapted to support the artery of the subject to prevent or substantially prevent movement of the artery relative to the implantable support device when pressure is applied to the artery during the tonometry measurement; and

a second surface, on an opposite side of the implantable support device to the first surface, that is adapted to interface with a bone of the subject such that movement of the implantable support device relative to the bone during the tonometry measurement of blood pressure is prevented or substantially prevented.

2. An implantable support device as in claim 1 wherein the second surface is curved so as to follow the shape of the bone.

3. An implantable support device as in claim 1 or 2 wherein the first surface is curved so as to support the artery along a part of its length.

4. An implantable support device as in claim 1, 2 or 3, wherein the first surface is shaped so as to provide a generally flat or near-flat surface against which the artery can be pressed during the tonometry measurement.

5. An implantable support device as in any one of the preceding claims wherein the implantable device is configured to be rigidly attached to the bone.

6. An implantable support device as in any one of the preceding claims wherein the implantable support device is made of a biocompatible material.

7. An implantable support device as in any one of the preceding claims wherein the implantable support device is made of a compliant material that becomes rigid after being implanted into the body.

8. An implantable support device as in any one of the preceding claims wherein the implantable support device reflects ultrasound and/or electromagnetic waves.

9. An implantable support device as in any one of the preceding claims wherein the implantable support device further comprises at least one magnet, portion of

paramagnetic material or portion of ferromagnetic material.

10. An apparatus for performing a tonometry measurement of blood pressure in an artery, the apparatus comprising:

an implantable support device as claimed in claim 9; and

a pressure device comprising:

at least one electromagnet, magnet, portion of paramagnetic material or portion of ferromagnetic material that is adapted to attract the pressure device towards the at least one magnet, portion of paramagnetic material or portion of ferromagnetic material in the implantable support device and apply pressure to the artery; and

a pressure sensor for measuring the blood pressure in the artery.

11. An apparatus for performing a tonometry measurement of blood pressure in an artery, the apparatus comprising:

an implantable support device as claimed in any of claims 1 to 8; and a pressure device that is adapted to apply pressure to the artery, the pressure device comprising a pressure sensor for measuring the blood pressure in the artery.

12. A method of performing a tonometry measurement, the method comprising:

pressing an artery between a pressure sensor and an implanted support device as claimed in any of claims 1 to 9 so as to partially occlude the artery; and

measuring the blood pressure in the artery using the pressure sensor.