WO2009093116A1

WO2009093116A1 - Wound dressing system and method

Info

Publication number: WO2009093116A1
Application number: PCT/IB2009/000080
Authority: WO
Inventors: Nicolas Kairinos
Original assignee: The University Of Cape Town
Priority date: 2008-01-25
Filing date: 2009-01-19
Publication date: 2009-07-30

Abstract

A wound treatment system is provided that includes a wound dressing (1 ) suitable for the application of a negative pressure to a wound in relation to which it is applied in use; means (6, 7) for applying a negative pressure to the wound dressing, and means (10) for adjusting the extent of the negative pressure. Sensor means (9) is provided that has an output proportional to a variable selected from one or more of a force measured at the wound- dressing interface, a positive pressure within the tissues immediately adjacent the relevant wound, and perfusion (measured either directly or indirectly) of the tissues. Control means (12) are provided for enabling manual or automatic adjustment of the negative pressure within the dressing system to be made in response to one or more outputs received from said sensor means with an aim of adjusting the value of the variable towards a desired value thereof.

Description

WOUND DRESSING SYSTEM AND METHOD

FIELD OF THE INVENTION

This invention relates to a wound dressing system, and more particularly, to a wound dressing system of the type in which a negative pressure relative to atmospheric pressure is applied to a wound, typically by way of a dressing including a pressure dispersing medium such as a foam or other spongy or porous material, with a view to enhancing the healing process. The invention also relates to a method of enhancing the treatment of a wound as well as a method of enhancing the formation of granulation tissue in appropriate circumstances.

It is to be understood that the term "negative pressure" as used in this specification is intended to mean reduced atmospheric pressure that is negative relative to atmospheric pressure in the sense in which that term is commonly used in the art.

It is also noted that in this specification the term wound dressing relates to a collapsible device, such as a NPWT dressing, as opposed to a rigid device, such as a suction cup.

BACKGROUND TO THE INVENTION

A modality called negative pressure wound therapy (NPWT) has revolutionised wound care in the last 15 years. Although known in the medical literature for over 50 years, it only gained popularity after Morykwas and Argenta et al published seminal papers on the Vacuum-Assisted Closure device (V.A.C.^®) in 1997. Its use has radically changed the way in which many wounds are treated. The relevant dressings include a spongy or porous material, most commonly a suitable foam, placed in or on the wound, and covered by an occlusive adherent film dressing. Tubing, placed in contact with the spongy or porous material, beneath and sealed off by the adherent film, is attached to a source of suction that causes the creation of a negative pressure within the material.

The mechanism of action of these dressings remains largely unknown, although many theories abound. One of the more popular theories is that the negative pressure generated within the underlying wound tissues facilitates the flow of blood to these tissues and in so doing promotes wound healing. In support of this theory many papers have been published, which have demonstrated that blood flow is increase in the tissues beneath and adjacent to the NPWT dressing on application of suction. There has been much conflicting data, however, as to what the optimal suction pressure should be to facilitate wound healing.

Morykwas et al, in numerous porcine studies, demonstrated that a suction pressure of -125 mmHg, not only created the highest increase in blood flow (four fold), but also the largest amount of granulation tissue, both of which are desirable in wound healing. For this reason, applicant understands that a suction pressure of -125 mm Hg is now recommended worldwide. Morykwas et al found that suction pressures higher or lower than -125 mmHg both resulted in lesser beneficial effects in granulation tissue or blood flow than did -125 mmHg. In fact, at suction pressures of -400 mmHg, Morykwas found that blood flow was less than baseline levels, conflicting with the theory that the hypobaric pressure of a NPWT dressing facilitates blood flow toward it.

However, a study by Timmers et al, also using laser Doppler, but done on intact skin of human forearms, demonstrated that suction pressures as high as -500 mmHg continued to result in increased tissue perfusion. Others, using porcine models, have found that perfusion is decreased 1 cm from the NPWT dressing (despite suction pressures of -125 mmHg), yet increased a few centimetres away, again conflicting with the theory that NPWT facilitates blood flow toward the dressing and also conflicting with the findings of both Timmers et al and Morykwas et al. Using other animal models, others have found that suction pressures of -80 mmHg are more beneficial and that higher suction pressures may cause tissue damage. To date the optimal suction pressures remain controversial and poorly understood.

Recent studies conducted by the applicant on live human wounds, have demonstrated that the negative pressure wound therapy system paradoxically increases tissue pressure when suction is applied to the dressing, contrary to common perception. This brought into question the findings that NPWT increases underlying tissue perfusion, as an increase in tissue pressure is likely to result in a reduction in perfusion. Therefore further studies were conducted to determine perfusion beneath NPWT dressings, using means other than laser Doppler (used almost universally for this type of research). These large studies found that the immediate effect of applying suction to tissues using NPWT resulted in significant reductions in perfusion in all experiments conducted in these human tissues. This brings into question the appropriateness of a laser Doppler for determining perfusion beneath a NWPT dressing.

In the prior art, there is no discrimination made between a negative atmospheric pressure applied to a wound via a rigid device, such as a suction cup or a collapsible device, such as a NPWT dressing. In fact, in many patents published in the prior art, there are suggestions made that

NPWT dressings can be of a rigid or collapsible variety, implying that the conditions created by these two variations are equivalent. These varieties are perceived by those skilled in the art to create the same physical conditions and result in the same effects on the tissues i.e. a reduction in tissue pressure with a concomitant increase in blood flow. Based on research conducted, it is the applicant's belief that a rigid device and a collapsible device behave in a manner so different to one another, that the conditions created are entirely the opposite. A rigid suction device creates hypobaric tissue pressures, which has been demonstrated to increase perfusion, and is the subject of many published patents and research in the prior art. A collapsible device such as a NWPT dressing, however, creates hyperbaric tissue pressures. These have been demonstrated by the applicant to result in reduced perfusion, contrary to what has been the common perception to date.

A finding more pertinent to the scope of this invention is that the increase in tissue pressure as a result of the application of suction with a NWPT dressing is different in different tissues, despite use of the same suction pressure in each of these wounds. Many variables influence the amount of increased tissue pressure generated, including the consistency of the underlying tissues, the amount of and type of foam/interface dressing used, the configuration of the dressing (circumferential or non-circumferential) and the amount of suction applied. In keeping with this the amount of perfusion reduction observed for the same suction pressure was also different in different individuals. This observation led to the realisation that a specific suction pressure, e.g. -125 mmHg, is unlikely to be ideal for all wounds, as this leads to entirely different levels of tissue pressure and blood flow.

The studies of Morykwas et al have led to the existing practice, as far as applicant is aware, of setting a target suction pressure. For this reason, numerous inventions noted in the prior art have devised systems to ensure that this target suction pressure is adhered to. It is common in the current art, for NPWT systems to have alarms, which would sound should the subatmospheric pressure in the NPWT system be of such a pressure other than that prescribed by the attending physician. This ensures the application of so-called optimal subatmospheric pressures, usually -125 mmHg. Other systems in the prior art have been devised to further keep the subatmospheric pressure in the dressing system constant, even if the ambient atmospheric pressure outside the sealed dressing environment were to change, e.g. if the patient were to be taken to a different altitude during air ambulance travel.

In US Patent Publication no US2008/0041401 Casola et al describe such a system which utilises sensors to determine the amount of differential subatmospheric pressure being applied at the wound and if necessary increasing or decreasing the amount of suction generated by the suction pump. Other sensors mentioned by Casola et al, feed data back to a processor regarding the wound temperature and wound volume, which may change over time, and make appropriate adjustments to subatmospheric pressure to facilitate wound healing. The system of Casola et al, as with those of others in the prior art, concerns itself with conditions within the wound, rather than conditions within the tissues beneath the wound.

The condition with which most systems in the prior art concern themselves relates primarily to the subatmospheric gaseous pressures measured in the wound and/or other parts of the NWPT system from the wound to the pump. None of the prior art, to the best of the applicant's knowledge, focuses on the pressures generated beneath the wound, i.e. within the tissues, these being substantially different to the aforementioned. Regardless of whether wound subatmospheric pressure is kept consistent, e.g. -125 mmHg, these tissue pressures will vary depending on a multitude of other variables discussed earlier. Not only will they vary but, unlike the hypobaric pressures in the wound above them, these pressures measured in the interstitial fluid within the tissues are conversely hyperbaric in nature.

In further studies by the applicant it has also been found that, despite the subatmospheric pressure applied to the wound being kept constant, the observed increased tissue pressures vary over time, usually demonstrating a gradual decline of the increased tissue pressure. This will ultimately affect the observed perfusion within the tissues. These facts have never been described before to the best of applicant's knowledge and it is on this observation that the current invention is based.

OBJECT OF THE INVENTION

It is an object of this invention to provide a wound treatment system aimed at improving the effectiveness of negative pressure wound therapy.

SUMMARY OF THE INVENTION

In accordance with one aspect of this invention there is provided a wound treatment system including a wound dressing suitable for the application of a negative pressure to a wound in relation to which it is applied in use, means for applying a negative pressure to the wound dressing, and means for adjusting the extent of the negative pressure, the wound treatment system being characterised in that sensor means is provided having an output proportional to a variable selected from one or more of a force measured at the wound-dressing interface, a positive pressure within the tissues immediately adjacent the relevant wound, and perfusion (measured either directly or indirectly) of the tissues, and wherein control means are provided for enabling manual or automatic adjustment of the negative pressure within the dressing system to be made in response to one or more outputs received from said sensor means with an aim of adjusting the value of the variable towards a desired value thereof.

Further features of this aspect of the invention provide for the sensor means to be selected from a pressure transducer operatively positioned within the tissue immediately adjacent a wound under treatment, a force sensor placed on top of the wound and under the wound dressing, and a perfusion sensor associated with the tissue immediately adjacent a wound under treatment; and for the control means to be computer or microprocessor operated to automatically control the suction in response to outputs from the sensor means with the suction being regulated until the target value of the variable is achieved.

In accordance with a second aspect of the invention there is provided a method of enhancing the treatment of a wound utilising a negative pressure applied to a wound by way of a suitable wound dressing, the method being characterised in that sensor means is positioned relative to the wound so as to respond to a variable selected from one or more of a force measured at the wound-dressing interface, a pressure within the tissues immediately adjacent the relevant wound, and perfusion (measured either directly or indirectly) of the tissues and to provide an output in response to the value of the variable, communicating the output to control means, and manually or automatically adjusting the negative pressure with an aim of adjusting the value of the variable towards a desired value thereof.

In accordance with a third aspect of the invention there is provided a method of enhancing the formation of granulation tissue within a wound in the human body, such enhancement being carried out utilising a negative atmospheric pressure applied to the wound by way of a suitable wound dressing, the method being characterised in that sensor means is positioned relative to the wound so as to respond to a variable selected from one or more of a force measured at the site-dressing interface, a pressure within the tissues immediately adjacent the relevant site, and perfusion (measured either directly or indirectly) of the tissues and to provide an output in response to the value of the variable, communicating the output to control means, and manually or automatically adjusting the negative pressure with an aim of adjusting the value of the variable towards a desired value thereof.

The invention is based on applicant's understanding that different types of tissues will develop different tissue pressures for the same suction pressure, and as a result thereof different changes in perfusion, and that this means that there cannot be a fixed, ideal suction pressure applicable in all instances. Rather, it is more likely that there is an ideal tissue pressure to which at least particular types of wounds, or wounds in particular patients, will respond most favourably. Such specific tissue pressure is not known at this stage but with further research, this may be discovered in the future. This increased tissue pressure has been demonstrated to reduce perfusion in the tissues adjacent the wound on initial application of NPWT. As NPWT is known to be beneficial to wound healing, it can be extrapolated that a certain level of reduced perfusion in the tissues adjacent to the wound may be desirable.

It is known, however, that tissue death will occur if perfusion is reduced to such an extent that there is a severe lack of oxygenation and nutrient delivery to the cells. Therefore the desired reduction in perfusion, which may be beneficial, will conceivably have a limit, beyond which detrimental effects may be observed. It is another object of this invention, therefore to monitor perfusion within the tissues adjacent the wound and regulate the amount of suction in the NPWT system to achieve only the limited reduction in perfusion, which may be beneficial to wound healing. This aspect of the invention will not only ensure a controlled reduction in perfusion to ensure optimal wound healing but will serve as a safety feature to prevent tissue death.

Certain tissues, which have borderline perfusion/blood flow pressures, due to trauma for example, might be compromised if negative pressure wound therapy with unregulated suction pressure is applied. The negative pressure wound therapy and resultant increase in tissue pressure has the potential to completely occlude the blood vessels/vasculature (depending on the amount of suction applied), causing tissue death. Having a sensing means to monitor perfusion in the tissues adjacent to the wound will immediately detect dangerously low levels of perfusion and, using a feedback loop, regulate the subatmospheric pressure to more appropriate levels for these compromised tissues, thereby avoiding tissue death.

In order that the invention may be more fully understood different embodiments thereof will now be described with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:-

Figure 1 is a schematic section taken through a wound to which a negative pressure wound therapy dressing has been applied and illustrating in block format the other components of the system of the invention; and,

Figure 2 is a graph illustrating tissue pressure increments in response to circumferential NPWT.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

Figure 1 illustrates one example of a system according to the invention. In this example a negative pressure wound therapy dressing (1 ) is illustrated as being applied to an open skin wound (2). The wound therapy dressing comprises a body (3) of porous material, in this instance a foam, and an adhesive occlusive film dressing (4) that covers the body (3) of porous material and seals in an airtight manner to the surrounding skin (5). Suction tubing (6) communicates with the body of porous material and connects it to a suction generating device (7). This arrangement is substantially conventional and well known in the art. The suction tubing may enter the airtight environment from beneath the adhesive film where it is attached to the skin or, as in the instance of this example, the suction tubing may be attached to the film dressing via an adhesive flange (8), which ensures an airtight seal between the two. A hole made in the film prior to application of this suction tubing and flange, allows for transmission of the negative pressure to the body of porous material.

As provided by this invention, a sensor (9) is associated with the wound in a position that will depend on the nature of the sensor and the nature of the wound and the positioning and nature of which is further described below. The sensor is adapted to provide an output that is communicated to a control unit in the form of a processor (10) in this instance via a communications cable (11 ). Communication may, of course, be by way of suitable wireless technology.

This feedback is, in use, processed by the processor and utilised to adjust the amount of suction generated by the suction generating device automatically to control the suction in response to outputs from the sensor means with the suction being regulated until the target value of the variable is achieved. The feedback measurements can be displayed on a screen (12), which may be embodied in a single unit as part of the suction device and processing unit. In certain instances, if an automated feedback loop is not utilised, the feedback loop may be carried out manually by a skilled operator, who can then manually adjust the suction pressure according to readings on the display screen.

Reverting to the nature of the sensor, it may measure the force the foam applies to the tissues, pressure within the tissues, tissue perfusion or a combination of any two or all three of these variables. Ideally, this sensor should measure force if it is placed on the wound surface (in between the porous material and the wound), or pressure if placed within the wound soft tissues. In this regard it should be noted that pressure measurement in the space between the porous material and the wound is perceived as being difficult to carry out accurately, as this may indicate the air/fluid pressure (which is likely to be hypobaric and is not indicative of the underlying tissue pressure, which is likely to be hyperbaric).

A force sensor, placed on top of the wound surface however, is unlikely to be significantly influenced by the ambient air/fluid pressure and will register an increased mechanical force when suction is applied to the body of porous material, as the porous material pushes down on the sensor. The sensor may also incorporate the ability to assess perfusion, either directly, for example by way of photometry of red blood cells, or indirectly, for example by assessing tissue oxygen saturation. This perfusion-sensing sensor may be placed onto or into the tissues.

The amount of suction generated by the suction device (7) will vary according to the readings of the sensor (9). Should the sensor measure forces/pressures higher than the desired predetermined setting, then the suction pressure will be reduced by the feedback mechanism and vice versa for forces/pressures that are measured to be lower than the desired predetermined settings. This adjustment will continue until the desired settings are achieved.

Should the sensor be the type that measures perfusion, the suction device will, in a similar manner (using the feedback mechanism), be regulated to decrease suction if perfusion drops below the predetermined level and increase suction if the opposite is required. Alarms and/or suction trip switches for safety purposes, may be incorporated into the software and hardware of the system.

In another embodiment of the system, there may be multiple sensors within the one dressing system. The regulating software may be configured to respond to these sensors' inputs in a variety of ways. The software may be configured to regulate the amount of suction based on an average of the various sensors' measurements.

Another potential configuration would have the software respond to the sensor which measures the highest or lowest measurement of multiple sensors used. This latter configuration may be particularly useful when one dressing is likely to generate different forces/pressures in different areas beneath the same dressing. This may occur for example, if placed over various tissue types or tissues with different consistencies or different contours. The pressure increase in each type may be different for the same amount of suction pressure and therefore may be advisable to have a sensor over or in each different tissue area. In such a scenario, the software could be configured to allow the sensor experiencing the greatest force or pressure or drop in perfusion to dictate the feedback to the suction generating device. Should forces or pressures or drop in perfusion reach undesirable levels at a specific sensor, this latter configuration would then allow for adjustment of the suction pressure until the force, pressure or perfusion reaches a more desirable level for the area within the dressing experiencing the extreme forces. This will prevent the risk of tissue necrosis in parts of the wound.

However, the system should allow for a physician to manually override any specific sensor's feedback. This may be necessary where the physician is aware that a particular sensor is likely to experience extremes of measurement but which may not necessarily be to the detriment of the patient. If part of the dressing is placed directly over exposed bone, for example, it may experience greater forces in this area when suction is applied than in areas of softer tissue consistencies. However, it may be that bone is fairly resilient to these forces as they are unlikely to affect the perfusion within the bone, particularly if the bone is denuded of periosteum (the only component of bone to potentially suffer ill effects if exposed to large forces/pressures). More Specific Example

In a typical example of such a system, although not intended to be exhaustive or to limit the invention, the negative pressure wound therapy dressing may be the type manufactured by Kinetic Concepts, lnc (KCI^®), USA. The KCI^® dressing is comprised of a reticulated, open cell, polyurethane foam. The communicating nature of the pores (400 - 600 μm diameter) of the foam allows for even distribution of the suction pressure throughout the foam. The foam is placed onto a wound and is then covered by an adhesive plastic film. The film, which seals off the wound and foam, allows the wound and foam environment to become airtight. A hole is then made in the film. Suction tubing, with an adhesive sealing flange at one end, is then attached around the hole to create an airtight connection. When suction pressure is applied via the suction tubing, the dressing collapses and creates a vacuum like seal. The manufacturer recommends dressing changes every 48 hours; however this may vary depending on clinical needs.

When the current invention is utilised to compliment a dressing, such as the one described above, the sensor/s are placed onto or within the wound tissues (depending on the type of sensor used) prior to application of the foam. Similarly, the foam could conceivably be pre-manufactured to contain the required sensors in its wound-facing base. The wires of the sensors could exit the airtight environment in a manner similar to that used for the suction tubing, or could simply exit beneath the adhesive film stuck to the skin (although this may increase the risk of losing an airtight seal). Wires could, alternatively, even traverse tissues to exit at a site remote from the wound site, as indicated by numeral (13) in Figure 1. The benefits of wireless technology for these sensors to transmit their information becomes immediately obvious in this scenario, not only to prevent loss of a seal, but also to make the implementation of the invention practically easier for a clinician. Although the ideal force, tissue pressure or perfusion range to promote wound healing is not known at this stage, many studies using a suction pressure of -125 mmHg have been conducted and found this to be the suction pressure most likely to promote wound healing. Although there are many inherent flaws in these studies, they have resulted in most people the world over using this suction pressure and most wounds seem to respond favourably to this amount of suction. It can therefore be postulated that the force generated by a negative pressure wound therapy dressing undergoing suction at -125 mmHg is likely to be close to the force range that is ideal to promote wound healing.

In an experiment to test what force a 5x5x3 cm negative pressure wound therapy dressing generated on the thigh of a healthy individual, it was found that at a suction pressure of -125 mmHg, the force on the tissues consistently ranged between 1.0 and 1.1 N. This offers only a very rough idea of the order of magnitude that could be expected of the ideal force. On tissue of a different consistency or with a dressing foam of different size or shape, the suction pressures required to produce the same force are likely to be different. It must be stressed that extensive clinical research needs to be conducted to ascertain the force range that tissues respond to most favourably and what factors affect these forces and also how these forces relate to tissue pressures and perfusion and as a result to healing or granulation tissue formation.

Healthy tissues may respond better to a different amount of force than poorly perfused tissues could tolerate, for example. Even once these values are ascertained, it can be seen that due to the fact that many of these variables may influence wound healing individually, it would be ideal to have a sensing system as provided by this invention which has the ability to monitor force, tissue pressure and perfusion simultaneously and respond to these individual sensor outputs intelligently, based on adequate clinical research data. Similar research will need to be conducted to ascertain forces, tissue pressures and levels of perfusion at which tissue damage is likely to occur. Tissue damage is influenced by a variety of variables in this scenario, such as amount of pressure applied and length of time, amount of hypoperfusion as a result of this pressure, type of tissue (nerve, muscle, fat etc.). Although tissue damage as a direct result of the increased pressure is unlikely with these types of dressings, the amount of pressure exerted is enough to decrease perfusion to the tissues, as demonstrated in prior research. This research has shown that circumferential NPWT dressings, commonly used for burn wounds of hands, generate the highest tissue pressures. Capillary perfusion pressure ranges between 10 and 50 mm Hg. Studies on capillary perfusion pressure in the fingertips of humans have been shown to be in the order of about 35 mm Hg. In studies conducted by the applicant on circumferential NPWT dressings, tissue pressures have been shown to exceed pressures of 25 mm Hg at suction pressures as low as -75 mm Hg (the minimum degree of suction pressure usually recommended) (Fig. 2). Should a particular patient have compromised perfusion, whereby the capillary perfusion pressure is, for example only 20 mm Hg, it can be seen how increasing tissue pressures by 25 mmHg may lead to collapse of these capillaries. In instances where suction pressures of -450 mm Hg were used, tissue pressures reached pressures as high as 90 mm Hg. At the commonly used suction pressure of -125 mm Hg, one patient's hand tissue pressures were seen to rise in excess of 30 mm Hg. In this specific patient the experiment had to be abandoned as she complained of aching of her fingertips, a symptom indicative of lack of blood flow. This ache was immediately alleviated when suction was reduced to -75 mmHg.

In another study conducted on healthy individuals to ascertain the amount of reduction in perfusion in hands due to NPWT, it was found that, at the commonly used suction pressure of -125 mm Hg, there were reductions in excess of 30% in some cases (Table 1 below). This demonstrates that even at this relatively low suction pressure the amount of tissue pressure generated and the concomitant reduction in perfusion may be to the detriment of some patients and for this very reason needs to be monitored via the sensing means referred to above, in order to regulate the suction pressure.

TABLE 1

Volunteer Sex Age Suction Perfusion Perfusion

No Pressure Reduction (%) Reduction (%)

Left Test Hand Right Test Hand

1 M 35 400mmHg 59.3 48.0

2 M 35 400mmHg 47.8 40.5

3 M 36 400mmHg 40.1 36.0

4 F 31 400mmHg 45.8 26.1

5 F 29 400mmHg 37.1 18.9

6 M 34 125mmHg 29.0 9.1

7 F 64 125mmHg 14.4 19.8

8 M 31 125mmHg 30.3 19.0

9 M 42 125mmHg 1.9 22.0

10 F 36 125mmHg 19.5 15.1

At the other end of the spectrum, further clinical research could also be used to ascertain the amount of force or tissue pressure that is so inadequate that it renders the negative pressure wound therapy useless. This would naturally indicate the level at which the present invention should trigger the feedback mechanism to increase the amount of suction pressure.

With adequate research data, the addition of the system of this invention to a conventional negative pressure wound therapy dressing could significantly enhance wound healing and prevent complications, such as tissue necrosis. Using the feedback system of the invention, it should be possible to attain a specific force on the tissues or pressure within the tissues or level of perfusion within the tissues. This system will facilitate achieving the optimum tissue pressure or force or perfusion, which maximally enhances wound healing and in addition, prevents the negative pressure wound therapy system from causing tissue necrosis due to overt tissue pressure and/or hypoperfusion.

Claims

CLAIMS:

1. A wound treatment system including a wound dressing (1 ) suitable for the application of a negative pressure to a wound in relation to which it is applied in use, means (6, 7) for applying a negative pressure to the wound dressing, and means (10) for adjusting the extent of the negative pressure, the wound treatment system being characterised in that sensor means (9) is provided having an output proportional to a variable selected from one or more of a force measured at the wound- dressing interface, a positive pressure within the tissues immediately adjacent the relevant wound, and perfusion (measured either directly or indirectly) of the tissues, and wherein control means (12) are provided for enabling manual or automatic adjustment of the negative pressure within the dressing system to be made in response to one or more outputs received from said sensor means with an aim of adjusting the value of the variable towards a desired value thereof.

2. A wound treatment system as claimed in 1 in which the sensor means is selected from a pressure transducer operatively positioned within the tissue immediately adjacent a wound under treatment, a force sensor placed on top of the wound and under the wound dressing, and a perfusion sensor associated with the tissue immediately adjacent a wound under treatment.

3. A wound treatment system as claimed in either one of claims 1 or 2 in which the control means is computer or microprocessor operated to automatically control the suction in response to outputs from the sensor means with the suction being regulated to cause the value of the variable to approach a target value of the variable until such target value is achieved.

4. A wound treatment system as claimed in any one of the preceding claims in which wires (13) traverse tissues to exit at a site remote from the wound site.

5 A wound treatment system as claimed in any one of the preceding claims in which the target value is a tissue pressure of between 1 and 50 mm Hg.

6. A method of enhancing the treatment of a wound utilising a negative pressure applied to a wound by way of a suitable wound dressing, the method being characterised in that sensor means is positioned relative to the wound so as to respond to a variable selected from one or more of a force measured at the wound-dressing interface, a pressure within the tissues immediately adjacent the relevant wound, and perfusion (measured either directly or indirectly) of the tissues and to provide an output in response to the value of the variable, communicating the output to control means, and manually or automatically adjusting the negative pressure with an aim of adjusting the value of the variable towards a desired value thereof.

7. A method of enhancing the formation of granulation tissue within a wound in the human body, such enhancement being carried out utilising a negative atmospheric pressure applied to the wound by way of a suitable wound dressing, the method being characterised in that sensor means is positioned relative to the wound so as to respond to a variable selected from one or more of a force measured at the site- dressing interface, a pressure within the tissues immediately adjacent the relevant site, and perfusion (measured either directly or indirectly) of the tissues and to provide an output in response to the value of the variable, communicating the output to control means, and manually or automatically adjusting the negative pressure with an aim of adjusting the value of the variable towards a desired value thereof.