Innovations in plantar pressure and foot temperature measurements in diabetes
Plantar pressure and temperature measurements in the diabetic foot primarily contribute to identifying abnormal values that increase risk for foot ulceration, and they are becoming increasingly more integrated in clinical practice and daily life of the patient. While plantar pressure measurements have long been present, only recently evidence shows their importance in ulcer prevention, as a data-driven approach to therapeutic footwear provision. The long-term monitoring of plantar pressures with the option to provide feedback, when alarming pressure levels occur, is a promising development in this area, although more technical and clinical validation is required. Shear is considered important in ulcer aetiology but is technically difficult to measure. Innovative research is underway to assess if foot temperature can act as a useful surrogate for shear. Because the skin heats up before it breaks down, frequent monitoring of foot temperature can identify these warning signals. This approach has shown to be effective in preventing foot ulcers. Innovation in diagnostic methods for foot temperature monitoring and evidence on cost effectiveness will likely facilitate implementation. Finally, monitoring of adherence to offloading treatment using temperature-based sensors has proven to be a feasible and relevant method with a wide range of possible research and patient care applications.
These innovations in plantar pressure and temperature measurements illustrate an important transfer in diabetic foot care from subjective to objective evaluation of the high-risk patient. They demonstrate clinical value and a large potential in helping to reduce the patient and economic burden of diabetic foot disease. Copyright © 2016 John Wiley & Sons, Ltd.
Introduction
The measurement of plantar pressure and foot temperature in patients with diabetes has gained substantial, and over the years increased, attention since the diabetic foot became a commonly researched topic in the early 1980s. Both plantar pressure and temperature measurements primarily aim to help in identifying abnormal values that increase the risk of foot complications such as ulceration. Traditionally, these measurements have been applied predominantly within a laboratory and research setting. In recent years, they are becoming used more and more in clinical practice, changing how we evaluate patients from a subjective to a more objective manner. This is important because objective measurements provide more accurate and valuable data to base clinical decisions on in the management of the diabetic foot. Several of the innovative applications of plantar pressure and temperature measurement will be discussed in this article, together with new developments in these areas that may not only further improve our understanding of diabetic foot complications but may also help the patient in self-management of their foot disease.
Plantar pressure measurements
Elevated levels of dynamic plantar pressure while walking have long been identified as significant risk factor in the development of plantar foot ulcers in diabetes (Figure 1). In the early 1960s, Dr. Paul Brand and his colleagues first used plantar pressure measurements in a clinical setting and demonstrated its relevance by showing that foot ulcers in patients with leprosy tend to occur at locations of highest plantar pressure 1, 2. Since these seminal publications, many prospective studies have demonstrated the association between high plantar pressure and foot ulceration in persons with diabetes mellitus 3–6.
Figure 1
Where dynamic plantar pressures are generally shown to be higher at the ulcer or previous ulcer location 7, a plantar pressure threshold that predicts foot ulceration has not been identified to date, despite attempts to do so. Studies show only moderately high sensitivity and specificity values for peak plantar pressure in ulcer development 6, 8. However, these analyses are based only on barefoot plantar pressure, while the biomechanical environment with wearing shoes is different 9, 10. In a more comprehensive analysis of disease-specific, mechanical and behavioural factors in a multivariate model of foot ulcer recurrence, our group found that presence of non-ulcerative lesions, barefoot peak plantar pressure, variation in daily step count and the combination of low in-shoe peak plantar pressure and high adherence to wearing footwear are significant independent factors in non-traumatic plantar foot ulcer recurrence 11. Interestingly, all these parameters can be objectively assessed or identified using plantar pressure, foot temperature or activity monitoring. This demonstrates the clinical relevance of sensor-based and wearable technology, and application thereof, in the diabetic foot field.
Innovations in plantar pressure measurements
An important breakthrough in plantar pressure measurements in recent years is that biomechanical and clinical evidence now supports the use of plantar pressure measurements in the design and evaluation of therapeutic footwear for high-risk diabetic patients 12–14. This evidence is based on the results of two large randomized controlled trials that have progressed our understanding of the role of plantar pressure offloading in ulcer prevention 15, 16. Underlying studies show that the offloading capacity of therapeutic insoles or orthopaedic footwear can be significantly improved when guided by plantar pressure measurement 12–14. Such improved footwear can reduce risk of ulcer recurrence with 46–65%, under the condition that the footwear is worn. This is a major innovation for footwear prescription practice, which has traditionally been more of an art than a science, where footwear was designed and evaluated based on the expertise, skills and experience of the prescribing physician and shoe technician, and efficacy was judged by whether a foot ulcer developed or not. More effective footwear for high-risk diabetic patients can now be obtained when guided by plantar pressure measurements. The use of such a data-driven approach to footwear provision is part of the new 2015 guidance on footwear and offloading of the International Working Group on the Diabetic Foot 17, 18. Such an approach is, however, only useful when implemented. Pressure-based footwear prescription has recently received a claims code in the US Medicare system, to be used in conjunction with a claims code for prescription insoles. In the Netherlands, a project supported by the Dutch Branch Organisation for Orthopaedic Footwear Companies is aimed at implementing the use of in-shoe plantar pressure measurements in therapeutic footwear practice. These examples show that plantar pressure measurement is now recognized by important governing entities as a valuable tool for clinical practice.
Plantar pressure monitoring and feedback
Current developments move plantar pressure measurements from the lab to the daily life environment of the patient in the form of long-term plantar pressure monitoring and feedback solutions. Such monitoring occurs by using force-sensing insoles or discrete pressure sensors that are embedded in a shoe insert or a sock and are located at high-risk areas. These sensors continuously measure plantar pressure at moderate frequency during everyday activities, and some systems are designed to provide feedback to the patient when above-threshold pressures occur. These warning signals aim to change patient behaviour, for example, reducing or changing activity, changing shoes or checking the shoes for any objects inside, in order to prevent a foot ulcer. As such, these systems act basically as a sensory substitution system for the inability to feel pressure on the foot due to the presence of peripheral neuropathy.
These plantar pressure-monitoring systems may have the potential to become important for ulcer prevention, but their development and application is still in its infancy. Such a system relies on patient-specific plantar pressure thresholds for ulceration, which are currently not available. Some indications for a plantar pressure threshold have been obtained at group level in laboratory studies on barefoot walking or walking in therapeutic footwear 6, 8, 10, 11. In everyday life, activity profiles are quite different with patients turning, shuffling, climbing stairs, or simply standing. Peak plantar pressures during these activities may be significantly correlated to those during level walking 19, but a common plantar pressure threshold for ulceration is unlikely, in particular with standing 20. Setting a plantar pressure threshold may introduce false-positive and false-negative outcomes, where problems may occur unnoticed if thresholds are set to high, or patients may be subjected to unnecessary burden in changing their behaviour if thresholds are set to low. Both affect validity and usability of the system. Systems that rely on pressure sensors in the shoe loose efficacy when the patient walks barefoot. Accuracy of the sensors to measure plantar pressure over time and durability of the sensors may, if not sufficient, also negatively affect the value of such a system. Finally, these systems require clinical validation as a final step to become accepted in diabetic foot care.
Shear
Shear stress is considered important in foot ulcer development because many ulcers occur at locations of hyperkeratosis tissue, which itself is caused by friction (shear) on the foot. Nevertheless, shear has received little attention. Studies that directly show a relationship between shear and foot ulceration do not exist, but some indirect evidence from footwear studies is available. Lavery and colleagues designed insoles that can reduce shear 2.5-fold compared with traditional insole designs 21. They assessed these shear-reducing inserts for efficacy in preventing foot ulcer recurrence and found a positive trend and a 70% effect size, but not a statistically significant reduction, in ulcer incidence compared with traditional insoles 22.
Many of the studies on shear use discrete tri-axial force transducers in the shoe or purpose-built platforms with an array of small force sensors. These studies show that locations of peak shear can correspond with the area of previous ulceration in patients with diabetes 23. However, as is the case with plantar pressure, the association may not be strong. Studies have shown that the location of peak shear corresponds only moderately with the location of peak plantar pressure 24. Therefore, adding shear to the prediction model of ulceration will likely improve predictive value. Unfortunately, because of technical difficulties and maybe also a small potential market, sensor arrays that measure shear inside the shoe are not (commercially) available. Until these technical difficulties are addressed, this area will likely remain a topic of expert opinion, rather than experimental evidence.
Temperature as surrogate for shear
As will be discussed in the next chapter, increases in skin temperature occur as a result of plantar pressure and shear acting on the foot from being ambulatory. Can temperature therefore act as a possible surrogate for shear? A recent study from Yavus et al. measured the skin temperature of patients before and after 10 min of walking on a treadmill and showed that the exercise-induced skin temperature increase was significantly correlated with the measured peak shear but not with the peak resultant shear and plantar pressure 25. Even though in only 23% of cases skin temperature increase could predict the location of peak shear, this may be an initial step in finding clues for using alternative methods as surrogate for the measurement of shear.
Temperature measurements
The reason to measure foot temperature in patients with diabetes who are at risk of developing a foot ulcer is built on the notion that the foot heats up before it breaks down into a foot ulcer. This heating up is due to inflammation and enzymatic autolysis of tissue as a result of mild to moderate repetitive stress on the foot from being ambulatory (Figure 1) 26, 27. The purpose of these measurements is then the early recognition of local temperature increase, so the patient and/or caregiver can take action to normalize temperature values 26. The measurement of foot temperature is easy and can be performed in any setting: clinic, primary care or in the patient’s home, and by any person: healthcare professional, patient or a relative. Foot temperatures are most often assessed in the context of ulcer prevention or treatment adherence.
Ulcer prevention
The value of infrared thermometry in foot ulcer prevention has been assessed in several randomized controlled trials 28–30. The technique uses a simple handheld infrared thermometer, with which skin temperature is measured on a daily basis at regions at risk on both feet. The aim is to recognize, if present, local temperature increase as pre-sign of foot ulceration. Absolute temperature values are not a good indicator for this nor are comparisons between regions in one foot. A 2.2 °C difference between corresponding regions on both feet is used as threshold for the patient to take action and reduce ambulatory activity. When temperature differences remain, further diagnosis by the healthcare professional is indicated and, if needed, (change in) offloading treatment. Using this approach, foot skin temperature monitoring has shown to be effective in preventing a first or a recurrent foot ulcer in high-risk patients, with effect sizes showing to be between 65% and 75% compared with usual care. How well the patient adheres to monitoring the foot temperature shows to be a strong contributor to outcome. Despite this proven efficacy, this method has not been implemented in daily foot care. The reasons for this are not really clear, but may have to do with reimbursement issues, a lack of confirmation of the results of these trials in other geographical settings, and/or with patient barriers in the daily use of the thermometer.
Innovative approaches may overcome some of the technical and usability aspects with the current handheld infrared thermometers. Automated procedures, such as with guidance in measurements, storage of data, calculation of temperature differences, signalling in case of above-threshold differences, and transfer of data to a smart device or the treating physician, are a few of the possible features for a next generation of skin thermometers. Temperature scales or mats placed on the bathroom floor may overcome some of the usability limitations with handheld thermography, although such devices may have their own limitations such as with the foot size specific placement of temperature sensors. Incorporating temperature sensors in socks adds the option of continuous monitoring and feedback, which can warn patients when abnormal local foot temperatures occur. However, is such frequent monitoring necessary, given the fact that a once per day measurement of foot temperature already shows to be efficacious in ulcer prevention. Future studies will have to provide the answer.
Advanced imaging
With advanced infrared imaging using high-resolution infrared cameras, temperature profiles of the foot can be studied in more detail than with handheld thermography 31, 32. More precise identification of a pre-sign or sign of foot ulceration is possible with such imaging. In the future, such identification may be automated, reducing the effort by the clinician or professional to assess the images. Additionally, better differentiation between foot complications that would allow more specific referral for treatment may be possible with more accurate temperature profiles. Such systems are currently too expensive for home-based temperature monitoring, but they can bring an intelligent telemedicine system for non-invasive automated analysis one step closer.
Adherence monitoring
As a new objective tool for monitoring adherence to offloading treatment, temperature-based sensors have been designed to fit in prescribed footwear or offloading devices 33. For this purpose, a small sensor is placed either on the inner side of the shoe or device upper, close to the ankle/leg, or embedded in the shoe insert, under the top layer of the insert. The first method shows good accuracy for identifying when the shoes are on or off, and therefore time spent wearing prescription shoes 33. The second option of embedding sensors in the insert has the advantage that the patient can be blinded to the adherence measurement. To further increase relevance, these sensors can be synchronized with body-worn sensors that monitor patient activity, in order to determine adherence with the patient is ambulatory and is supposed to wear protective footwear.
The application range for such sensor technology is large 34. It can be used to objectively assess treatment adherence in individual patients or patient groups. It can be used to explain (lack of) treatment efficacy: if therapeutic footwear has been shown to be worn, but is not effective, solutions for better footwear should be sought for. However, when the patient shows to be non-adherent and develops a foot ulcer, adherence and not shoe quality should be primarily focused on. Furthermore, these measurements can be used to explore reasons for non-adherence, to evaluate the effect of interventions that aim to improve adherence and to individualize type and frequency of footwear prescription. They can even be used as a quality assurance method for footwear prescription practice, such as with benchmarking of centres, where those centres that show high treatment adherence with their patients may be considered to deliver better foot care.
These adherence measurements may, however, introduce some potential ethical dilemmas when used in patient care, which are related to ownership of and access to monitored data 35. An adherence reminder system that signals the patient when not wearing prescribed footwear while being active will likely not lead to any ethical discussion. But who else has access to the data? The treating physician or shoe technician may have good reasons to use the data to improve patient care or services. But what about the insurance company, who may want to have access to data for benchmarking, but who may also be interested to use the data to prevent reimbursement of expensive footwear that is not worn by the patient. These ethical and legal issues should be resolved to define the breadth of the application range of adherence monitoring.
Conclusions
Plantar pressure measurements have a demonstrated clinical value to help improve therapeutic footwear and to reduce risk of foot ulcer recurrence in diabetes. This innovates footwear practice to a more data-driven approach. New developments in long-term plantar pressure monitoring and patient feedback are promising but require further technical and clinical validation. Shear proves difficult to measure. This area requires more development or may require alternative solutions that can act as surrogate for shear. Daily monitoring of foot temperature can effectively identify local increases in skin temperature as pre-sign of foot ulceration, which can significantly reduce the incidence of foot ulcers in high-risk patients if acted properly on. Evidence for cost effectiveness of this approach and innovation in diagnostic methods may improve implementation. Finally, temperature-based sensors have been used successfully in the objective monitoring of adherence to offloading treatment. This provides many opportunities for applications in research and patient care. These applications and innovations illustrate an important transfer from subjective to objective evaluation of the high-risk patient, and they demonstrate the clinical value and large potential of plantar pressure and temperature measurements to help in reducing the patient and economic burden of diabetic foot disease.
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