Plantar Pressure Assessment

Measured Variables

The typical components of a system used to measure plantar pressures include the measuring device, which consists of sensors in a platform or insole configuration; a computer for data acquisition, storage, and retrieval for analysis; and a monitor for displaying data. Various software packages are available that allow the clinician to divide the plantar surface of the foot into numerous regions to permit the analysis of data, as is illustrated in Figure 1. The most common variables of interest include peak and average pressure, force, and area. Peak pressure plots represent the highest pressure value recorded by each sensor over the entire stance phase.⁷ Figure 2 depicts a peak pressure matrix obtained during walking for a child with juvenile rheumatoid arthritis (left) and a child of similar age without known pathology (right). The software provides values for pressure and a user-specified color scheme to graphically display the pressures acting on the plantar surface of the foot. In Figure 2, the colors red and purple denote the highest pressures, and the green, blue, and black colors represent the lower pressure values. Peak pressures are often of interest in determining the effectiveness of a cushioned foot orthosis in decreasing pressures under a sensitive metatarsal head, whereas average pressure values provide the clinician with an understanding of typical pressure acting on a specific anatomical region during the walking cycle.

Area refers to the amount of surface contact between the plantar surface of the foot and the sensor. In the peak pressure plot shown in Figure 2, there is a marked variation in the area of contact with the ground between the 2 children. The area beneath the force-time curve as well as the pressure-time curve can also be determined and is referred to as the integral of the curve or impulse (Fig. 3). The impulse can be of use to the clinician in understanding the amount of force or pressure that has been applied over time, in this case the duration of foot contact.¹⁸ Commercially available software also permits the sequential viewing of both pressure and area, beginning at initial contact and ending when the foot leaves the ground, as illustrated in Figure 4. Three-dimensional displays of plantar pressure data can be effective when educating the patient regarding regions of high pressure on the plantar surface of the foot.⁷ Figure 5 illustrates 3-dimensional pressure plots for a child with juvenile rheumatoid arthritis (left plot) and for a child of a similar age without known lower-extremity pathology (right plot). The 3-dimensional plots provide a graphic illustration of the extreme pressures acting under the fourth metatarsal head region of the child with juvenile rheumatoid arthritis in comparison with the other child. Such illustrations can also be effective in the patient education of individuals with diabetes and peripheral neuropathy to assist in their understanding of potential sites of ulceration.

System Specifications

A limitation when using most pressure assessment systems is that the force value measured by the sensor and used in the calculation of pressure is normal force or a force that is perpendicular to the sensor surface. Normal force can be considered vertical force when a platform, fixed to the supporting surface, is used for data collection. When a sensor insole is placed in a shoe, however, normal force may only be considered vertical force during that portion of the stance phase when the entire foot is in contact with the supporting surface.¹⁷ In general, the sensors used for pressure measurement do not measure the fore-aft or medial-lateral shear forces that are obtained using force platforms.¹⁶ This is an important point, because shear forces are considered to be an important factor in the development of plantar ulcers in clients with diabetic neuropathies.⁷ Cornwall and McPoil¹⁹ were able to predict fore-aft or anterior-posterior shear force with a pressure sensor platform using a combination of peak force, time to peak pressure, and stance phase duration. To accomplish this, the authors attached a pressure sensor platform to a force platform and collected data with both systems recording simultaneously at a similar sampling frequency. No researchers to date, however, have been able to determine the medial-lateral shear force component using a pressure sensor platform.

Those specifications that should be considered when selecting a system for measuring pressures include resolution, sampling frequency, reliability, and calibration. Resolution refers to both the size and number of sensors used in the system.⁶ The higher the resolution of the system, the greater the number of sensors. The size of the sensor is also important because different size sensors can alter the pressure reading, as pressure is dependent on both force and area. A force applied to a large sensor will not provide the same pressure reading as the same force applied over a small sensor. This is because the spatial resolution of the system is not high enough. For example, if a vertical force of 50 N were applied to a 1-cm² sensor, the resulting pressure would be 500 kPa. If the same force of 50 N were applied to 4-cm² sensor, however, the resulting pressure would be only 125 kPa. Considering the tremendous anatomical variations in the size of the metatarsal heads, hallux, and toes based on foot size, the resolution of the pressure measurement system becomes an important consideration for the clinician. Resolution becomes even more critical when assessing the plantar pressures in children with small foot sizes.

Sampling frequency is an important factor in determining the temporal resolution of the system. Sampling frequency is the number of samples measured by each sensor per second and is recorded in cycles per second or hertz.⁶ Mittlemeier and Morlock²⁰ examined the effect of plantar pressures obtained using 4 different sampling frequencies and reported that pressure data collected between 45 and 100 Hz were adequate for walking. Most commercially available systems (eg, Emed sensor platform, Pedar insole system, F-Scan system) offer sampling rates between 50 and 100 Hz. For higher-speed activities, such as running, sampling frequencies of 200 Hz or greater are often required.²¹

Reliability of the measurements obtained with the pressure sensor are critical for an accurate measurement. Hughes et al²² suggested that using the average of 3 to 5 walking trials enhances the reliability of the pressure measurement, although 100% replicability cannot be expected because of inherent differences in each walking trial. Several researchers^22–24 have evaluated the reliability of measurements obtained for different sensor technologies used for both platform and in-shoe pressure measurement systems. Because they reported variations in the reliability of measurements obtained with the different systems available for measuring plantar pressures, it is critical for the clinician to be aware of the reliability (error) of the system he or she has selected for use.

Calibration is important for establishing the validity of measurements of both force and pressure. Although a system may give consistent repeated measurements, that is, have reliability, the measurements may not provide an accurate representation of the actual force or pressure acting on the plantar surface of the foot because the actual pressure values provided, even though repeatable, may not be accurate. One method for calibrating the sensors in both platform and insole systems is to place either device under a rubber bladder that is then filled with compressed air at several known levels of pressure. The use of this type of air bladder system allows each sensor to be uniformly loaded and permits the generation of a calibration curve for each sensor or group of sensors in a matrix. Having a patient stand on the insole or platform as a method of calibration may not be sufficient because each sensor is not uniformly loaded, and some sensors may not be loaded at all, generating no calibration data for those sensors.

Measurement Technology

Plantar pressures can be measured using a variety of instruments, including force-sensing resistors (FSRs),¹⁷ hydrocells,²¹ microcapsules,²⁵ projection devices,²⁶ pedoscopes,²⁷ and capacitance transducers,²⁸ as well as by critical light deflection.¹⁶ Many of these instruments can be used as discrete sensors or to create a matrix of multiple sensors. It is important for the clinician to be aware of the features associated with these devices prior to using them for data collection.

Discrete measurements utilize individual pressure transducers positioned at specific anatomical locations on the plantar surface of the foot. Thus, the use of discrete transducers requires the clinician to make an a priori decision regarding the appropriate placement for optimal data collection. Once the locations have been determined, the transducers are either held in position on the bottom of the foot with adhesive tape or embedded into an insole of similar material properties. Because only a small number of sensors are used at any one time, a major advantage for using discrete measurements is a higher sampling rate, in some cases up to 200 samples per second.²¹ Thus, discrete measurements are often selected for high-speed activities such as running or various sport movements. Although discrete sensors are usually easy to use and affordable, several issues must be addressed when using this method of pressure data collection. The discrete sensor can act as a foreign body in the shoe, thus acting as an irritant within the shoe as though it were a flat stone.¹⁷ Furthermore, the lack of consistency between the material used to fabricate the sensor and the skin can cause an “edge” effect, which can lead to falsely elevated pressure values. Finally, the discrete sensors can migrate from their original position during dynamic activity secondary to shear stress at the foot-shoe interface.¹⁷ Thus, the actual pressure values recorded may not reflect the desired anatomical landmark originally selected by the clinician.

Matrix measurements use an array of sensors organized in rows and columns rather than an individual discrete sensor. Thus, matrix measurements can assess the distribution of pressures acting over the entire plantar surface of the foot simultaneously. A major advantage when using a matrix measurement is that no a priori decision is required prior to the pressure assessment. In addition, a larger plantar surface area can be assessed at once.

Selection of Platform Versus In-Shoe Data Collection Methods

Although the manufacturers of pressure measurement systems use the various technologies previously discussed for developing both their platform and in-shoe measurement systems, specific issues associated with in-shoe versus platform data collection should be considered by the clinician when determining which method will be selected. The advantages of using a platform device typically include a greater number of sensors, thus a higher resolution, and the fact that the pressure sensors are always positioned parallel to the supporting surface to provide a “true” vertical force measurement. The problems associated with using a platform or mat system for data collection include the large number of steps required to collect data and the targeting of the platform sensor surface by the patient.

The traditional method for collecting data using a platform system has been termed the “mid-gait” technique.³² The mid-gait technique requires that the patient walk across a walkway, usually at least 9 m in length, while pressure data are collected from a single foot contact over the sensor platform. Considering that previous studies²² have indicated that pressure data from at least 3 to 5 steps are required to establish replicability, multiple barefoot walking trials across the walkway are required for data collection. Patients with diabetes and neuropathy could be placed at risk when collecting pressure data using the mid-gait method because the numerous steps required for the mid-gait method can actually increase the possibility of plantar ulceration.⁷ In addition, patients with neurological impairment may have difficulty contacting the platform because of proprioception and coordination problems.⁷

In an attempt to counteract the problems associated with the mid-gait method, Rodgers³³ recommended using pressure data collected using the mid-gait and 1-step methods for patients who are at risk for plantar ulceration. In a later study, Meyers-Rice et al³² described a 2-step method that was superior to the 1-step method in replicating the pattern of plantar pressures attained when using the mid-gait method because the 2-step method provided pressure values that were more similar to those obtained with the mid-gait method.

Another problem associated with platform measurements is the “targeting” of the platform by the patient. The term “targeting” indicates that the patient has altered the walking pattern so that he or she can place the foot in contact with the platform.²³ Unfortunately, targeting may result in alteration of the patient's typical pressure pattern. The use of in-shoe data collection methods is thought to eliminate the problem of targeting, because all the patient is required to do is walk normally. Furthermore, in-shoe data collection provides the clinician with information about pressures occurring within the shoe, at the shoe-foot interface. In-shoe pressure measurement also permits the clinician to study other functional activities, such as dancing, because the pressure-sensing device is located within the shoe.¹⁷ In-shoe pressure measurement is especially important when assessing the effect of specially designed footwear or foot orthoses prescribed to modify the pressures acting on the plantar surface of the foot. Based on the analysis of in-shoe pressure data, the clinician can modify the footwear or orthoses to maximize their benefit to the patient.

Although in-shoe data collection would appear to be a good choice for the clinician, there are several problems associated with this technique. Because the number of sensors that can be incorporated into the pressure sensor insole is less than the number of sensors used in a platform system, the resolution is usually diminished. The sensor insoles are more susceptible to mechanical breakdown because transducer cables connecting the sensors to the computer can be bent or stretched as they exit the shoe.²³ Individual sensors can also be damaged by continuous repetitive loading. In addition, the hot, humid, and usually contoured environment within the shoe can affect the reliability and validity of measurements of the sensor's performance.¹⁷ As the sensor insole can only measure “normal” force because of the position of the insole sensor within the shoe relative to the supporting surface,²³ the measurement of “true” vertical does not occur during the initial and late portions of the walking cycle. Although platform and in-shoe data collection methods both have unique advantages and disadvantages, clinicians should base their selections on the characteristics and functional capabilities of their patients as well as on the desired activity or assistive device they want to study.