The measurement of plantar pressure distribution

Shift in foot load before (left) and after (right) a marathon (Nagel et al. 2008) (platform measuring, Novel

Plantar pressure measurements can be used to assess the loads to which the human body is subjected in normal walking or in sports. Measuring the distribution of force over the sole of the foot is useful as it provides detailed information specific to each region of contact. Especially in patients with diabetic foot syndrome these data are crucial for detecting areas with high loads and for the monitoring of intervention measures.

1 a Heel strike: Small area with high force = high peak pressure; b Mid-stance phase: Large area with slightly reduced force = low peak pressure; c Push-off phase: Small area with high force = high peak pressure Foot ulcers leading to later foot and leg amputations are often due to excessive pressure of the shoe on the foot. The patient then often states: “I only noticed the pressure mark when I saw blood on the shoe”. The patient does not feel any pain due to the neuropathy and so does not notice the pressure damage. The measurement of the plantar pressure distribution objectifies the pressure on the sole of the foot. Third-party payers want this additional service to be covered in the care of diabetics. However, to be able to issue comparable statements on pressure distribution, standardized terminology, biomechanical expertise and clear standards are required.

The human body masters forward movement by transferring forces into the ground. This happens via the sole of the foot during the rolling-off process. This results in a pressure load on different parts of the foot that may change significantly in the presence of pathologies. The measurement of plantar pressure distribution helps give us quantitative and reproducible metrics on pressure load.

Hence, it has a distinct advantage over the blueprint which provides detailed information about the foot load but cannot be adequately reproduced or quantified. Compared to a force measuring plate, no shear forces can be determined in the plantar measurement of pressure distribution; the information is therefore limited to the vertical component of the ground reaction force. However, the force measuring plate does not provide information about the load distribution on the sole of the foot.

Objectives

  1. Assessing foot function during the rolling-off process under dynamic conditions.
  2. Objective and reproducible measurement of plantar foot pressure.
  3. Uniform load distribution, avoiding pressure peaks, ulcerations.


The pressure pattern shows a bimodal curve during the physiological rolling process (Fig. 1). Both peaks occur during heel down (beginning of the stance phase) and at foot press-off (end of the stance phase) when a large force is applied to a small area. In the middle of the stance phase, when the whole foot has ground contact (large area), the pressure is significantly reduced despite only slightly decreasing force induction.

Technology: There are several technologies used to measure pressure. They differ in design and form and thus have different areas of applications. They all share the fact that the sensors employed convert the applied force into an electrical signal which is then retransmitted to the force based on a previous performed calibration.

Common technologies

Capacitive sensors by which two capacitor plates move closer together as a result of the applied force. The change in charge causes a change in voltage (e.g. Novel, Munich) (Fig. 2 a).
Resistive sensors where the contact area of two conductive layers is altered by the force. The change in resistance causes a voltage change (e.g. GeBioM, RS-scan, Medilogic, Fast Scan/Tekscan) (Fig. 2 c to e).

Piezoelectric sensors where the force alters the charge distribution in a crystal lattice. The charge shift causes a change in voltage (e.g. Paromed, Neubeuern) (Fig. 2 b).

2 a Functionality of a capacitive sensor: If the sensor is unloaded (A), the high-frequency signal measured at the receiver is low. This signal corresponds to zero. If the sensor is loaded with increasing pressure (B and C), the distance between the surfaces becomes smaller and the signal at the receiver becomes correspondingly stronger. Thus, this change in the receiver signal is a measure of the pressure produced (Fig. Novel)b Structure of the Parotec sensor: Piezoresistive pressure sensors are embedded in a hydrocell where they measure the fluid pressure (Fig. Paromed). c The pressure transducer of the Fast Scan system consists of resis - tive layers applied between two thin plastic films. The geometric arrangement of the circuit paths defines the size and number of individual lattice-like elements distributed over the film (Fig. Megascan). d The pressure sensors in GeBioM System insole measurement system are all individually wired. (Fig. GeBioM). e In the Medilogic system, the sensors are applied as conductor lines on a flexible sole (Fig. Medilogic)

Parameters of the pressure distribution measurement3 Pressure measurement plate (GeBioM)

  1. Maximum force: Describes the maximum vertical force measured by the sensor over the entire roll-off process.
  2. Contact surface: Describes the area of all sensors that have provided data during the rolling process
  3. Contact time: Describes the duration of ground contact.
  4. Peak pressure: Describes the maximum pressure measured by the sensor over the entire rolling process.
  5. Force-time integral (pulse): Describes the length of time the force is applied to a particular area of the foot.
  6. Gait line: Consists of the force application point of the individual measurements and provides information about the stability of the foot and the temporal distribution of the roll-off process. (Fig. 8).

4 In-shoe measurement-system (GeBioM)

Implementing the measuring procedure

5 Demonstration of the pressure relief by diabetes- adapted bedding: a) regular shoe b) with diabetes-adapted bedding

  1. Measurements with measuring platforms (Fig. 3): Measuring platforms are used to determine the loading of the foot during walking barefoot. They should, if possible, be set into the ground to ensure an even surface. Measuring platforms can have a very high resolution thanks to several thousand sensors.
  2. Measurements using measuring soles (Fig. 4): Measuring soles are used to carry out measurements in the shoe. They can be used independently of laboratories and make clinically relevant investigation possible. The effect of foot orthoses, in particular, can be checked and demonstrated easily using measuring soles.

 

6 Demonstration of the pressure relief by a soft cushioning foot orthosis a) in a patient with rheumatoid arthritis pressure distribution in a normal shoe b) pressure distribution with foot orthosis (in shoe measurement by Novel). 145_154_Kap_22_Laufbandanalyse_engl_neu 13.04.16 09:30 Seite 1517 Change of the footprint in different deformities: a) Normal foot,b) Splay footc) Flat-splayfoot (platform measure by GeBioM)

Areas of application

Pressure distribution measurements are particularly suited for diagnostics and therapy monitoring in diseases involving the foot. These include, among others:

Diabetes mellitus

Checking for pressure load on the diabetic foot is important because high pressure loads are a proven risk factor for ulcerations that can lead to amputations. Some health insurance contracts in Germany have mandated the use of pressure distribution measurements to check the pressure-relieving effect of diabetesadapted footbedding (Fig. 5).

Rheumatoid arthritis (RA)

Foot and toe deformities occur in over 90% of patients with rheumatoid arthritis. As a result of the deterioration of the foot function, extreme pressure load points occur under the metatarsal heads. The consequences are severe pain and limited mobility. Here, the use of foot ortheses to reduce the pressure can relieve pain and improve mobility (Fig. 6).

Foot and toe deformities

Changes in the foot load due to foot and toe deformities are reflected directly in the pressure load on the individual areas of the foot. As well as an increase in the pressure load in the forefoot area in splayfoot and toe deformities, severe pes planovalgus, splayfeet and flatfoot deformities are associated with an increase in the contact surface in the medial metatarsus area (Fig. 7). On the other hand, the contact surface of the metatarsus is significantly reduced in pes cavus deformity (Fig. 8).

Change of foot load in sport

The pressure distribution measurement can recognize the foot load effectively during and after sports activities such as running. Nagel et al. examined the foot load before and after a marathon in 200 runners and found that the load shifted from the toes to the metatarsal heads, which suggests loss of active toe function due to muscle fatigue (Fig. 9).   

Pressure reduction: Principles and measures

8 Screenshot of foot pressure measurement showing the maximum pressure values in the different areas of the foot and the line of force application points of the individual frames (gait line) (platform measurement by GeBioM)The reduction of foot load plays a crucial role in treatments that rely on pedorthics. The mean peak pressure is approx. 40 – 50 N/cm2 in barefoot walking and 20 – 30 N/cm2 in the shoe at medium walking speed. In diabetes care, a reduction of at least 30% is required by some health insurances. This is proven by measuring the pressure distribution without and with the orthotic device.

It is well known that high peak pressure is not the singular problem in the development of ulcerations. In fact, it depends more on how high the pressure and the exposure time is on the affected area. This effect is determined by the force-time integral or the momentum. However, since the momentum depends directly on body weight and gait velocity, the influence on this parameter is very limited. In addition, gait insecurities generally lead to lower walking speeds and thus increase the duration of force application. This is often the case in patients with peripheral neuropathy.

9 Shift in foot load before (left) and after (right) a marathon (Nagel et al. 2008) (platform measuring, Novel)In treatments relying on pedorthics, it is necessary to provide a good foot bed and shoe modification in order to effectively reduce the peak pressure and momentum. The diabetes-adapted foot bedding can effectively reduce peak pressure depending on the chosen materials; whereas the shoe modification, in the form of a rocker, keeps the load time for the vulnerable area as short as possible. The rocker should be supplemented by a stiff sole to ensure its effectiveness.


Benefits of plantar pressure distribution measurement

  1. Allows assessment of orthotic footwear.
  2. In diagnostics, complements clinical examination.
  3. Helps establish the degree of functional impairment.
  4. Allows outcome evaluation after completion of therapy (conservative/surgical).
  5. Monitors healing process and/or ­disease progression.     

Adress for the authors:

Dr. Arne Nagel, Michael Möller
Möller Orthopädieschuhtechnik
Johann-Krane-Weg 40
48149 Münster


This article is an excerpt from the book “Pedorthics”, written and published bei René Baumgartner, Michael Möller, Hartmut Stinus.