Amit Gefen, PhD

Disclosure: This work was supported by grant no. 6418-6 from the Chief Scientist’s Office of the Ministry of Health, Israel.

Pressure-related chronic wounds, such as diabetic neuropathic foot ulcers and pressure ulcers, are an important health concern that affect millions of patients and costs billions annually.1 Studies conducted at multiple centers in the United States indicate that ulcers in the feet of patients with diabetic neuropathy account for $150 million (US) of the direct annual costs of type 2 diabetes. Deep tissue damage requiring amputation costs about $47,000 per individual case.1 Likewise, the database of the US Centers for Disease Control and Prevention (CDC) indicates that the annual cost of treating pressure ulcers in spinal cord injury (SCI) patients is $1.2 billion in the US alone.2 Pressure-related chronic wounds may occur when soft tissues are compressed between bony prominences— eg, the metatarsal heads in the foot or the ischial tuberosities (IT) in the buttocks and a supporting surface (a shoe insole or a wheelchair sitting surface). Injury occurs when the magnitude of the applied mechanical load or time of exposure to the load, or their combination, exceeds the tissue’s tolerance, which is commonly referred to as “injury threshold.” In such cases, cell death occurs in paths of mechanical breakdown,or ischemic necrosis, or both.3 Pressurerelated chronic wounds rarely develop spontaneously in animals, sometimes limiting utilization of animal models for etiological studies.4 Accordingly, much of our understanding of these wounds is based on clinical experience5 that emphasizes a need for objective, quantitative means of measurements of the conditions under which pressure-related chronic wounds develop in humans. Interface pressure data are measures of the spatial and temporal compressive forces per unit area that act on soft tissues contacting a support surface (eg, wheelchair, shoe, etc.). They are basic engineering tools for evaluating the susceptibility of an individual to suffer a pressurerelated wound. Though interface pressure measurements cannot reveal all risk factors for a chronic pressure-related wound (eg, existence of a peripheral vascular disease) they do reflect important biomechanical risk factors, such as loss of tissue thickness that causes higher intensity pressure near bony prominences, foot deformities, regions of callus in plantar tissue, and nonenzymatic glycosylation of collagen that induces stiffening of connective tissues in the plantar foot.6

current techniques for body-support interface pressure measurements with focus on foot and sitting pressures, 2) list the pressure value ranges measured under the foot in standing and walking, under the buttocks in sitting with particular emphasis on abnormal alterations in foot pressures as result of diabetic neuropathy, and alterations in sitting pressures among paralyzed patients, and 3) discuss clinical utilization of interface pressure measurements in the fitting of diabetic footwear and wheelchair cushions.

Role of Interface Pressure Measurements in Prevention of Diabetic Foot Ulcerations and Pressure Ulcers

Diabetic foot ulcers are a common and potentially severe complication of diabetes that affect up to 68 per 1000 patients with diabetes per year in the US.5 More than half of these patients develop infection while 20% require some form of amputation to remove necrotic tissue.5 The key risk factors for diabetic foot ulceration include peripheral neuropathy, foot deformity, loss of plantar pad thickness, abnormal stiffening of the foot sole resulting from changes in collagen fiber architecture, and repetitive focal mechanical loads causing micro tears in the plantar pad.6 It is likely that the prevalence of complications and amputations can be lowered substantially if those patients at risk can be identified and equipped with protective measures, such as customdesigned foot wear. While examinations of the level of sensation in the foot (eg, Semmes-Weinstein filaments) are able to quantify foot neuropathy, they cannot identify specific areas of the foot that are susceptible to ulceration (ie, the filaments are merely used to assess loss of sensation).

Conversely, identifying the specific regions in a patient’s foot that are at risk is an initial, basic step before any footwear modification can be made. Foot pressure measurements are able to reveal the highly loaded regions on the foot during static as well as dynamic activities, which then permits a patient-specific footwear prescription so that high focal pressures are relieved. An example of this is if an elevated pressure region is detected below one of the metatarsal heads, a plug made of soft material can be inserted into the shoe midsole at that site causing the shoe to deform more at the region of the plug. This increases the foot-shoe local contact area at the problematic region, which in turn, alleviates the high focal pressures.7 Interface pressure measurements have the potential of being a highly useful, practical tool for helping to protect diabetic neuropathic patients from ulceration.

Interface pressure measurements are also important in protecting permanent wheelchair users from pressure ulcers. Current pressure ulcer rates are unacceptably high in at-risk populations as up to 23% of all nursing home residents and 60% of persons with SCI develop pressure ulcers.8 The US Agency for Healthcare Research and Quality (AHRQ) reports a generally lower incidence rate for inpatients (> 11%).9 Comparably, 54% of these pressure ulcers occur in 70- to 89-yearold hospitalized patients.9 Surgical repair of buttock ulcers may be more than $70,000 per case, hence, between $3.5 and $7 billion are spent each year in the US on this malady.8

The ischial tuberosities, the sacral coccygeal area, and the greater and lesser trochanters and intertrochanteric crests support most sitting pressures. Prolonged sitting by patients with neurological impairment and physical disability can lead to pressure ulcers at these sites since the soft tissues deformed between the bones and wheelchair support are deprived of oxygen and nutrients because of blood vessel obstruction or occlusion. Moreover, muscle atrophy in paralyzed patients reduces the natural cushioning abilities of the buttock’s soft tissues, which increases the buttocksseat interface pressures. Similarly to diabetic foot ulcers, the prevalence andseverity of pressure ulcers in permanent wheelchair users can be substantially decreased if patients at risk are equipped with tailormade protective means. For example, addition of foams or creating cutouts of cushion material at regions of high focal sitting pressures can redistribute the mechanical loads and improve capillary blood flow in the skin and subdermal tissues.10 Overall, interface pressure measurements are useful for determining the anatomical sites that require pressure redistribution and for quantifying the extent of improvements achieved by optional interventions.

Technological Concepts

Units. In most of the clinical and engineering literature, pressures are specified using standard international (SI) units so that pressure is the force in Newtons acting over the contact area in square meters.This is the definition of the Pascal (Pa) unit. Foot and sitting pressures are usually reported in kilopascals (kPa, 1 kPa = 1000 Pa). Other units found in the literature for reporting pressures are kg/cm2, where 100 kPa are approximately 1 kg/cm2, and millimeters of mercury (mmHg) where 100 kPa are approximately 750 mmHg. All data in this review are provided in kPa as well as in mmHg for standardization and for convenience.

Device configurations. The pressure-sensing devices on the market vary in sensor configuration to meet different applications. For monitoring foot pressures, devices can be generally classified as pressure distribution platforms or in-shoe systems. Pressure distribution platforms can be used for static or dynamic (standing or walking) studies and are made of a flat rigid array of pressure sensing elements arranged in a matrix configuration. Since these matrix sensor configurations need to be embedded in the floor for a natural gait, use of pressure distribution platforms is generally restricted to gait laboratories. For the subject to walk naturally on the pressure platform, and for the foot to hit the center of the pressure-sensing area so that optimal measurements can be taken, training the subjects is usually required. Additionally, pressure distribution platforms can only measure the interaction of the barefoot with a rigid ground, since footwear will cause the platform to measure the pressures between the shoe and ground, not the foot and the shoe. For evaluation of footwear, a commonality in examining patients with diabetes, an in-shoe sensor configuration is needed. This configuration is needed after the regions of high pressures during barefoot gait are identified with a pressure distribution platform. The in-shoe sensors are thin, flexible, and embedded in a thin insole that is inserted under the foot while walking with shoes. Unlike the pressure distribution platforms, which record pressures during a single step, inshoe systems record several subsequent steps and, therefore, statistical analysis of pressure data is more powerful at a shorter study time. In-shoe systems may act at a wireless configuration and/or may be connected to a portable small data-logger that allows monitoring of footshoe pressure distributions at the patient’s daily environment while wearing his/her own shoes or some alternative therapeutic shoes. Thus, in-shoe systems can be effective for prescribing the most suitable footwear solution for a patient with diabetes. However, the spatial resolution of pressure data obtained with inshoe systems is generally lower than when pressure distribution platforms are employed, as fewer sensors are used in the inshoe devices.

Similarly to the foot pressure-sensing devices, sitting pressure measurements can be made with a pressure distribution sitting platform that determines the pressures between the buttocks and a rigid foundation—ie, the platform. Pressures between the patient’s buttocks and his/her own wheelchair or wheelchair cushion can be measured with a thin, flexible array of sensors (a pressure mat), which covers the entire sitting area. While the first (rigid) type is used mostly for research purposes— eg, to determine the effect of a pathology (such as SCI) on the sitting pressure distributions in a standard repeatable configuration—the flexible mat is useful in the clinic for evaluating the relative pressure relieving effects of different wheelchair or cushion modifications.

Data and Analysis

Visualization of pressure data is commonly done by means of color-coded diagrams, which show the area of contact (under the foot or buttocks) with regions of high pressure marked using “warm” colors (red or yellow), and regions of low pressure marked using “cold” colors (blue or green). Examples are provided in Figures 1–3. It is also possible to connect points of equal pressure over the contact region by lines, which yields an “isobaric” pressure diagram. Some systems use 3- dimensional topographic maps to visualize pressures, although this format is less popular. The center of pressure, defined as the time-dependent location where the normal force between the body and the supporting surface acts, is shown on the pressure diagram (Figure 3). For presentation and comparison of statistical data from groups ofsubjects, it is generally accepted to use bar graphs plotted at anatomical regions of interest on a scheme of the contact area (eg, the foot, Figure 1A). Based upon the pressure diagrams, some parameters can be further calculated by pressure analysis software codes. These standard parameters typically include the contact area, the vertical compression force (summation of pressure measured at each sensor multiplied by the sensor’s area), the peak pressure, the contact area exposed to pressures exceeding a certain threshold, the average pressure in a circular area of predefined size (under the ischial tuberosity), and the pressure-time integral (area bounded under the pressuretime plot, [Figure 2A]).11–13 Drerup et al13 affirmed that a drawback of such analyses is that they require the evaluation of one parameter at a time, which complicates comparative analyses. They suggested use of “pressure dose” graphic representation, which is a rectangle with height proportional to the mean pressure and width proportional to the relative loading time during the stance phase of walking.13 The area of such square, therefore, indicates the pressure dose applied to the foot during a single step.

All pressure parameters can be calculated for the whole body-support contact area or for specific regions of interest (Figure 1A). Commercial software packages for analysis of foot pressure patterns (eg, the Research Foot software, Tekscan Co., Boston, Mass) or a comparable Novel Co. (Munich, Germany) product employ automatic algorithms for subdividing the foot into anatomical regions of interest for which local pressure parameters are calculat-ed separately. Such parametric analyses are useful for quantitative assessment of pressure data from a patient through a follow-up period or for statistical analyses of pressure data from groups of subjects.

Measurement Techniques

Foot and sitting pressure data are both spatial (pressures are nonuniformly distributed over the contact area),and temporal (local pressures are timedependent). As emphasized in this review, pressure data are most useful for clinical decision-making if both the spatial and temporal characteristics are acquired simultaneously,and if objective quantification of the pressure magnitudes and exposure times is possible. Simple low-cost pressure measurement methods, such as those that employ ink (the Harris projection

with fluorescent lights (podoscopes), usually do not allow simultaneous acquisition of spatial and temporal pressure data or do not allow the quantification and storage of the data.14,15 For example, an ink print of the foot obtained from one stance phase (Harris projection) shows the regions subjected to maximal plantar pressures, yet it cannot provide the exposure times. It is also difficult to quantify the pressure distribution.14,15 The simultaneous and quantitative acquisition of spatial and temporal pressure data generally requires the utilization of electronic or coupled optical-electronic methods. This review focuses on such methods. Typically, an electronic pressure measurement device includes the pressure sensors embedded in a platform, a sitting mat or an insole configuration, a computer (for data acquisition, display, storage, and analysis), and wired or wireless interface between the sensors and computer. The most common electronic sensor technologies used in body pressure measurements employ capacitance sensors, resistive sensors, piezoelectric and piezoresistive sensors. All of these sensors are able to provide an electronic signal (voltage, current) that is proportional to the pressure applied on the sensor facilitating realtime computerized production of a pressure-time response (Figure 2A). Accumulation of data from multiple sensors allows production of a pressuretime curve from an entire contact surface (eg, under the whole foot or some regions of interest under the buttocks [Figure 2B]).

Capacitance sensors consist of 2 thin, conductive, and electrically charged plates that are separated by an insulating “dielectric” elastic layer. When pressure is applied to the sensor, the dielectric elastic layer deforms, which shortens the distance between the plates and results in a voltage change proportional to the pressure magnitude.14,15 This technology is currently employed by Novel Co. in its EMED® foot pressure platform, the Pliance® sitting pressure mat, and the Pedar® insole system (Novel Co.)

foam encapsulated between 2 electrodes. The electrical current through the resistive sensor increases as the conductive layer deforms under pressure. Force-sensing resistors (FSR), a variation of this technology, are made of a piezoresistive conductive polymer that changes resistance in a predictable manner when subjected to force. The polymer contains both electrically conducting and nonconducting particles (with sizes in the order of fraction of microns) that are suspended in matrix. Applying a force or pressure causes conductive particles to touch each other and the electrodes, thereby increasing the current through the sensor.14,15 The FSR technology is employed by Tekscan Co. (Boston, Mass) in its MatScan® foot pressure platform, its body pressure measurement system (BPMS™) sitting mat, and the FScan® in-shoe system (Tekscan Co.)

Fig. 3. Vertebral bodies were dissected and potted in polymethylmethacrylate using a level to keep the end plate surface flat.