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A step towards preventing diabetic foot ulceration

Naomi Green
BSc(Hons) MChS
RSscan Lab Ltd

The rate of lower limb amputations in people with diabetes is 15 times higher than in people without, and 15% of foot ulcers will result in amputation.(1,2) According to NICE, approximately 5% of those with diabetes (or around 90,000 people) develop a foot ulcer each year.(3)
Once ulceration has occurred, there is a significant impact on the patient and on healthcare resources.(4)  Wilson quoted the national average cost of lower limb amputation to be £4,835.(5)
However, diabetic foot ulceration is a preventable complication and, as such, initiating a preventive treatment approach should be a key responsibility for all of the multidisciplinary team in any healthcare setting.(6) Studies (not randomised controlled) have shown that the rate of amputation may be reduced by 40% or more through screening patients, providing education and developing multidisciplinary diabetes footcare teams.(7) Singh et al found substantial evidence to support the screening of all patients with diabetes to identify those at risk of foot ulceration.(8)

What is foot screening?
Diabetic foot screening involves testing for nerve damage (neuropathy) and vascular disease in the lower limb. However, in the at-risk foot, foot structure and function are highly influential in ulcer formation and should play an important part of the screening process. Reduced joint mobility, foot deformity or bony prominences result in an increased loading in patients with diabetic neuropathy and have been shown to significantly increase dynamic plantar foot pressure.(9) A significant aetiological factor in foot ulceration is high plantar pressure - when the areas of the sole of the foot are in contact with the ground and pressure is distributed across the sole (plantar surface) of the foot. As different areas of the plantar aspect of the foot make contact with the ground, the pressure of the ground beneath the foot may not be evenly distributed, resulting in some areas having higher pressure.(6,10) For this reason, plantar foot pressure measurement in diabetic foot screening is crucial.(11) Pham et al found that, in foot screening, best specificity for a single ulcerogenic factor was offered by foot pressures.(6)
Dynamic pressure measurement systems measure the pressure under the foot while the patient is walking and can provide almost instant biomechanical information to the clinician in a format that is easy to interpret with a small amount of training.
Pressure is an applied force over a specific area, with the main forces that act on the lower limbs during walk being applied from the ground. Analysis of pressure information under the foot can give the clinician valuable information on the forces being applied to the foot and lower limbs of the body. For this reason, increasing numbers of practitioners are using pressure measurement to assist them with the assessment and diagnosis of actual and potential problems in the patient with diabetes.

Result frequency and resolution
A key factor in measuring plantar pressure is the sampling frequency (Hz) of the system - that is, the number of frames (or pictures) that the system records each second. A higher sampling frequency means a more detailed analysis of foot function, as the subtle and fast movements during the stance phase are captured only by faster systems. An example of this can be seen in  Figure 1, showing the same foot captured at 125 (left) and 500 (right) frames a second. Each black dot on the images shows the centre of pressure each time a frame was recorded, with a clear difference seen in the amount of information captured at the two different speeds.
Linked in with the sampling frequency is the second key factor - the resolution of the system. It relates to the size of the sensors that are recording the pressure. If a system has a low resolution, then once again the detailed analysis of foot function will be compromised. Larger sensors will result in lower values for peak pressures, loading rates and impulses, which in patients with diabetes could lead to problems.
Both pressure and contact time have been found to be important parameters.(12) In the neuropathic diabetic foot, rear foot and forefoot pressures are increased, and high peak pressures have been found to be associated with a significantly increased risk of foot ulceration.(13,10) As diabetic neuropathy progresses, pressure-loading patterns and force-time curves become increasingly abnormal, and the duration of plantar metatarsal loading (how long the area of the foot was in contact with the ground and how long it loaded the body's weight) increases. When the loading rate or loading time (impulse), over a particular area, is increased, these areas are likely to show ischaemic changes, and tissue damage resulting in ulceration may occur.(14) Systems with a high sampling frequency (500Hz) and resolution mean that data provided by the software package are of great significance as they replicate the pressure distribution during the stance phase accurately.

Benefits of screening
Using a pressure measurement system to identify people with high plantar pressures could not only increase the chances of preventing the incidence of ulceration, but also reduce the personal and economic burden of ulceration.(15)
One of the requirements of the standards of the National Service Framework for Diabetes was "empowering people with diabetes". The introduction of a pressure system to a clinic can be beneficial to both the health professional and the patient. From the patient's perspective, this visual representation of peak pressures and foot position gives a greater understanding of the problem and an improvement in patient compliance. Early identification of an at-risk foot should then be followed by interventions to reduce the risk of ulceration. These include the use of orthotic devices, pressure relief and appropriate footwear.
Foot orthoses produced using dynamic data allow more confident correction of biomechanical anomalies, as they relate to the moving, weightbearing structure. Traditionally, foot orthoses have been prescribed based on static, nonweightbearing measurements aimed at changing the position of the foot in relation to the ground and, in diabetes, in deflecting pressure from areas that have ulcerated or are believed to be at risk of ulceration. Using dynamic data to prescribe orthoses means that the insoles are based on how the patient walks and can therefore be more confidently designed to correct or control the movements of the foot that are occurring during walking, while also deflecting pressure from areas of high pressure or load for long periods of time. A recent study by Dr Sue Barnett, using pressure measurement to assemble specific orthotics that correct biomechanical abnormalities and redistribute foot pressure, resulted in 11 patients out of 14 recovering from ulceration and not reulcerating (see Figure 2).(16)
The implications for cost reduction to the NHS if this could be replicated across the country would be massive, and could provide numerous patients with an improved quality of life.



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