Measuring and testing – Orthopedic pressure distribution
Reexamination Certificate
1996-06-06
2001-04-24
Williams, Hezron (Department: 2856)
Measuring and testing
Orthopedic pressure distribution
C073S866400, C073S774000, C073S781000
Reexamination Certificate
active
06220088
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to method and equipment for assessment of patient support systems.
Pressure on the human skin leads to triaxial distortion of the skin and the underlying tissue and comparison and distortion of tissues against the bony skeletons. As a result, the blood vessels which are in these tissues become distorted and this leads to a diminution of the blood supply causing ischaemia with resultant tissue necrosis. This is manifest by the development of pressure sores which, if not carefully treated, do not heal but become chronic and more and more extensive. The problem is particularly prevalent in patients who are elderly or disabled and confined for long periods to bed or to a wheelchair.
Frequently, patients admitted to hospital develop pressure sores during their treatment, and the additional nursing cost of treating pressure sores is considerable.
In one recent study (see A. B. Ward, Prescriber's Journal, 30(6) 1990), it was stated that between 3 and 11% of all patients admitted to hospital develop a pressure sore, and the average duration of treatment for those with significant lesions is 51 days at an estimated cost of £26,000. It has been estimated (see Turner, Wound Management, 1(1), April 1991) that the care of patients suffering from pressure sores is costing the National Health Service as much as £200,000,000 per annum.
A variety of patient support devices and equipment have been developed in an effort to prevent, and/or assist the recovery from, pressure sores. The cost of such equipment varies widely and various claims are made for their effectiveness by the manufacturers and distributors. However, there is no proper basis for assessing the value of these claims or the effectiveness of the equipment, since there are no generally accepted protocols or methods for the evaluation and comparison of patients support systems.
While clinical evaluation has been carried out on behalf of individual manufacturers of equipment and by researchers, the number of patients observed in these tests have been too small for proper statistical evaluation. Another approach has been to directly measure the interface pressure between a support surface and the patient, but such measurements are confounded by experimental and repeatability errors, e.g. because of the difficulty of accurately placing a pressure transducer at precisely the same point when testing two patients support surfaces with a given patient. Another difficulty is that measurements can vary as a result of changes in temperature and moisture levels. Further, the use of patients and volunteers makes comparison of various workers findings virtually impossible since the viscoelastic properties of patients and volunteers' tissues of various ages is variable. The same groups of patients and volunteers cannot be available at all times world-wide which would be required for accurate comparisons.
SUMMARY OF THE INVENTION
The present invention arises from a different approach to the problem of assessing support surfaces and is based on the recognition that the tissue of the patient's body and the surface on which he is supported are both distorted by the act of making measurements of interfacial pressure.
According to one aspect of the present invention, there is provided a method of comparing the suitability of two surfaces for supporting a person thereon, which comprises applying a load to each of said surfaces, with a first indenter having a given profile and a substantially rigid form, and measuring the resulting interface pressure, applying the same load to each of said surfaces using a second indenter having the same profile as said first indenter, but having a viscoelastic form and measuring the interface pressure and deriving information as to the characteristics of the supporting surfaces by comparing the interfacial pressure measurements.
A large discrepancy in the pressures measured for a given surface using the two indenters, indicates that interfacial pressure was reduced by the deformation of the viscoelastic indenter. This may be assumed to be translated in practice to a high degree of local distortion of the patient's skin, thereby providing unsatisfactory support conditions. On the other hand, a closer conformity in pressure between the two measurements indicates that the deformable indenter has generally maintained its shape during the measurement, thus suggesting that the support system was capable under the conditions of loading of taking up a form approximating to that of the deformable indenter.
Thus, by comparing interfacial pressure measurements for the two indenters over a range of applied loads and conditions, a more reliable picture can be developed of the relative performance of two support surfaces.
At this stage, it is not believed to be essential that the deformable indenter has a degree of deformability which conforms accurately to that of parts of the human body. However, it is believed that the best results will be obtained by attempting to mimic the deformability of parts of the human body, such as the buttocks, which contact a support surface such as a bed or a chair.
Materials exist at the present time which have been designed as tissue prostheses. For example, silicone polymers are used extensively for manufacture of artificial breast prostheses. Such materials can be produced to provide a variety of different degrees of deformability, depending upon the polymerisation mix and polymerisation conditions. One supplier of such materials is Amoena (UK) Limited of Chandlers Ford, Eastleigh, Hampshire, England.
In a more sophisticated development of the concept of the present invention, an anatomical phantom can be constructed which can be provided with interchangeable rigid and viscoelastic parts, which will represent rear profiles of one or more of the head, shoulders, arms, buttocks, legs and heel components of a person. The parts of the phantom may be articulated to mimic at least a part of the movements of a human skeleton so that measurements can be taken of interfacial pressures at points along the contacting surfaces between the phantom and the support structure at different angular positions, both of the support surface (if appropriate) and/or of the phantom. Thus, for example, a bed which is designed to be used in the supine position and in the sitting position can be tested for its suitability for nursing patients with a predilection towards pressure sores, in the two main angular positions of the bed.
While current investigations suggest that the support characteristics of beds and cushions can be determined most readily by the method described above, using both rigid and deformable indenters, particularly but not exclusively in the case of the fully developed human phantom it may be possible to measure the deformation of the critical parts or the pressure applied to them, such as the buttocks, by direct imaging techniques or by incorporating pressure transducers or load sensors within the deformable parts. For example, direct imaging can be achieved by radiographic techniques. One way in which radiographic techniques can be employed is to attach metal (eg lead) foil strips to the surface of the deformable part or within the part and, by using radiography, to record the change on its profile under load as a stereo image.
Another feature of the present invention is that the human phantom can be provided with temperature controlled sweating characteristics. This will enable the heat and water vapour transfer values of the support surface to be measured in a situation which mimics the conditions at the surface in use.
According to a further aspect of the present invention, therefore, there is provided a human phantom comprising mutually articulated parts, having components manufactured from viscoelastic materials.
Preferably, these viscoelastic materials are interchangeable with rigid materials of similar profile. The phantom is conveniently open at the upper side to provide convenient points for supporting and/or ar
Bain Duncan Shirreffs
Scales John Tracey
Loo Dennis
RAFT Trustees Limited
Sughrue Mion Zinn Macpeak & Seas, PLLC
Williams Hezron
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