Contact force ratio: A new parameter to assess foot ...

Prosthetics and Orthotics International, 2004, 28, 167-174

Contact force ratio: a new parameter to assess foot arch function

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A.K.L. LEUNG*, J.C.Y. CHENG**, M. ZHANG*, Y. FAN*** and X. DONG***

*Jockey Club Rehabilitation Engineering Centre, The Hong Kong Polytechnic University, Hong Kong, China **Department of Orthopaedics and Traumatology, The Chinese University of Hong Kong, Hong Kong, China ***Laboratory of Biomechanical Engineering, Department of Applied Mechanics, Sichuan University, Chengdu, China

Angle (Clarke, 1933), Footprint Index (Irwin, 1937), Contact Index I (Qaura et al., 1980), Staheli's Arch Index (Staheli et al, 1987), Arch Index (Cavanagh and Rodgers, 1987), Contact Index II (Didia et al., 1987), Chippaux-Smirak Index (Forriol and Pascual, 1990), Arch Length Index and Truncated Arch Index (Hawes et al., 1992). Some investigators reported footprint measurements are unreliable for prediction of arch height (Cobey and Sella, 1981; Hamill et al, 1989; Hawes et al, 1992). On the other hand, McCrory et al (1997) suggested the Arch Index as a simple quantitative mean of assessing arch height. With the introduction of pedobarograph and electronic pressure sensing mat (Abboud and Rowley, 1996) a dynamic footprint with timing and pressure information can be collected during gait. Chu et al. (1995) also reported that the pedobarograph collected Arch Index and Modified Arch Index, which incorporated pressure data correlated well with arch height. The Modified Arch Index, compared to other footprint parameters, had the highest correlation coefficient with arch height (Shiang et al, 1998). The conflicting results could be caused by the inaccuracy of data collection and the variations of the weightbearing conditions under which the footprints were collected. These weight-bearing situations included single leg static standing, double legs static standing, during the initial step, the second step or mid-gait (Meyers-Rice et al, 1994; Wearing et al., 1999). At least three measurements were suggested for comparison between groups of different subjects (Hughes etal, 1991).

Abstract Static footprint parameters have been used to quantify arch height with conflicting results. This could be caused by the inherent inaccuracy and variations of the methodology used. Since the foot is a dynamic structure that undergoes changes during a step, it is more desirable to capture and analyse the dynamic footprint at an instant during the gait cycle that can most closely reflect the weight-bearing foot function. Forty (40) volunteer subjects were recruited for the reliability test of a new parameter, the Contact Force Ratio (CFR), derived from dynamic footprint. This is a measure of midfoot loading during gait. The mid-gait dynamic footprints were collected using a pressure sensing mat. Results of ICC tests showed that the CFR had good intratester (0.918) and intertester (0.909) reliability. The validity of the method was examined by correlating the parameter to the functional change in arch height, i.e. the Navicular Drop between the nonweight-bearing and weight-bearing conditions. Introduction and objective Investigators have worked on inked static footprints to quantify arch height. These parameters include the Footprint Angle (Schwartz et al., 1928; Cureton, 1935), Arch All correspondence to be addressed to Dr. A.K.L. Leung, Jockey Club Rehabilitation Engineering Centre, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China. Tel: (+852) 2766-7676; Fax: (+852) 2362-4365: E-Mail: [email protected] 167


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A.K.L. Leung, J.C.Y. Cheng, M. Zhang, Y. Fan andX. Dong

The footprint analyses described above were all based on the accumulated plantar images during a complete step or in static standing. These footprints displayed the peak pressure values recorded by individual sensors. With an electronic pedobarograph or pressure sensing mat, it is now possible to capture dynamic plantar pressure at different instants of the stance phase. Since the foot is a dynamic structure that involves many changes within a step, it is more desirable to analyse the dynamic footprint at an instant that can most closely reflect the foot function. Navicular Drop is a functional change upon weight-bearing and is defined as the difference in vertical distance between the original positions of the navicular tuberosity in Subtalar Joint Neutral Position to the final position of the tuberosity in Resting Stance Calcaneal Position (Mueller et al., 1993). Normal Navicular Drop in adult is approximately 10mm. Measures above 15mm are considered abnormal (Brody, 1982). The test was more reliable than the open kinetic chain method (Sell et al., 1994). However the reliability was low when the test was performed by inexperienced testers (Picciano et al, 1993). The objective of this study is to derive a dynamic footprint parameter to reflect arch function. The validity of the method was examined by correlating the parameters to the Navicular Drop measured by an experienced orthotist specialized in foot orthotics. Materials and methods Subjects selection criteria Forty (40) subjects, aged 20 to 23 years (30 male and 10 female) were recruited to participate in this experiment. All subjects reported no foot pain or discomfort in normal weight-bearing activities. Equipment The Tekscan HR portable pressure-sensing mat (Tekscan, Inc., South Boston, MA, USA), which consisted of an ultra-thin flexible printed circuit with conductive ink, was used for the collection of dynamic footprints (Fig. 1). It provides foot-floor contact area and pressure information at different instants of the stance phase, as well as the peak pressure values of each sensor location. It is 49.5 x 45cm in size. The sensor spatial resolution is 4 sensors

Fig. 1. The Tekscan pressure-sensing mat. per cm2. The total number of sensors is 8352. A compressive testing fixture consisting of an inflatable rubber bladder filled with compressed air was used to apply a temporally and spatially constant load on the whole mat for calibration. Before taking each measurement, the pressuresensing mat was calibrated for subject weight. Since the limited capability of the system's absolute measurement was noted (McPoil et al., 1995; Woodburn and Helliwell, 1996; Quesada and Rash, 2000), the pressure data collected were used as relative measurements for calculation of ratios. A height gauge was used to measure the Navicular Drop. An electronic balance and a Laser Posture Device (Fig. 2) (Otto Bock Orthopaedic Industry, Germany)

Laser Posture Device

Height Gauge

Fig. 2. The Laser Posture Device. The device projects a laser line on the body of the subject which intersects the position of the centre of gravity.

Contact force ratio and arch function


were used to monitor the weight-bearing conditions during which Navicular Drop was measured. Measurement of contact force ratio (CFR) The mid-gait footprint collection method (Wearing et ah, 1999) was used. The pressuresensing mat was placed at the middle of a 10metre long walk path. Each subject was asked to walk barefoot at his/her normal speed and land the foot on the pressure mat naturally. When the subject's foot failed to land completely on the mat, the data collection trial was repeated. Data were collected at a frequency of 50Hz. A noise threshold was set to 2 (a value between 0 and 255 raw units, which is the recording range of the system) to reduce the presence of noise on the recordings. Digital output values that were equal to or below the threshold would be set to be zero by the software, thereby filtering out unwanted low level force readings (noise). The stance phase occupies about 62% of the time of a gait cycle (Sutherland, 1980). Noticeable events within the stance phase include heel strike, foot flat, mid-stance, heel off and toe off. Before heel strike, the subtalar joint is slightly inverted. At heel strike, it then begins

Foot axis

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to evert until heel off (McPoil and Cornwall, 1994; Cornwall and McPoil, 1999aband 2002). Thus the footprint at the instant just before heel off was selected for analysis as it reflected the maximum eversion of the tarsal mechanism consisting primarily of the subtalar and talocalcaneonavicular joints. The instant was identified by the obvious change of contour of the heel region of the image. A dynamic footprint parameter was developed by adopting the Arch Index (Cavanagh and Rodgers, 1987) approach. Mid-gait dynamic footprints during a step were collected by the Tekscan pressure mat. On the "peak footprint" (Fig. 3), which has all the peak pressure values recorded at each sensor location the tip of the second toe and the centre of the heel were marked. A foot axis was drawn by connecting and crossing these two points. A third point was marked on the foot axis at the most posterior position of the footprint. With the toes ignored, the encased length of the foot axis within the main body of the footprint was divided into three equal lengths to divide the fore, mid and rear foot. Knowing the area that had been activated and the pressure recorded by individual pressure sensor the load being applied on the midfoot and

Heel centre

Most posterior position of the foot axis Tip of second toe Fig. 3. A "peak footprint" collected by the pressure-sensing mat.


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A.K.L. Leung, J.C.Y. Cheng, M. Zhang, Y. Fan and X. Dong surface of the balance but the balance reading 7T! '3*'' * was less than 1 kg. There was difficulty in monitoring the orientation of the ankle-foot complex during the single leg stance condition. Therefore the half body weight-bearing condition was selected for the measurement of the weight-bearing navicular height. The condition was monitored through the use of the Laser Posture device. Measurement was recorded when the subject stood on the platform of the device with both limbs at shoulder width and when the laser line projected on the natal cleft of the body (Fig. 2). Navicular Drop was Fig. 4. Using an electronic balance to monitor the the difference between the two measurements of weight-bearing condition. the same subject. All the measurements were done by the same orthotist (Tester 1). other area can be calculated. The Contact Force Ratio is the ratio of the midfoot loading to the Statistical analysis total loading of the contacted foot with the toes The ICC (3, 1) and ICC (2, 1) tests were ignored. Using Visual Basic language, a performed to examine the intratester reliability software programme was written for the division and intertester reliability of the CFR method. of the footprint into the fore, mid and rear The coefficient of correlation between the CFR portions and the calculation of the CFR. and the Navicular Drop was calculated. Measurement ofNavicular Drop To measure the Navicular Drop (Brody, 1982; Mueller et al., 1993), the height of the navicular tuberosity from the supporting surface was firstly measured in the non-weight-bearing condition. This was done by asking the subject to sit on a chair and put the foot on an electronic balance (Fig. 4). The measurement was recorded when the foot fully contacts the top

Results The CFR values for the 40 subjects were recorded and are shown in Table 1. The data under Tester la and Tester lb are measurements of individual subjects by Tester 1 on two occasions. The navicular height in non-weightbearing and weight-bearing conditions, and the resultant Navicular Drop of each subject is also shown in the table.

0.25

Fig. 5. Scattergram of Navicular Drop versus CFR.


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Table 1. Contact force ratio and navicular drop of 40 subjects. Subject

Sex

Age

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

M F M M M M F F

20 20 21 23 22 22 22 21 23 20 20 21 23 20 21 22 22 21 23 22 23 21 22 22 21 21 23 22 21 21 22 21 21 22 22 23 21 22 21 22

M F F M F M F

M M M

M M F M M M M M M M M M F M F M M M F M M M

Contact Force Ratio Tester la Tester lb Tester 2 0.192 0.205 0.183 0.185 0.215 0.157 0.149 0.144 0.211 0.175 0.203 0.191 0.204 0.136 0.185 0.122 0.153 0.195 0.209 0.192 0.156 0.207 0.206 0.185 0.205 0.193 0.211 0.148 0.176 0.203 0.207 0.184 0.216 0.225 0.195 0.152 0.177 0.186 0.172 0.211

The reliability of the CFR method was examined. The ICC (3, 1) intratester reliability for tester 1 was 0.918 (95% CI 0.8508-0.9559). The ICC (2, 1) for intertester reliability between

0.184 0.213 0.191 0.196 0.223 0.148 0.156 0.153 0.218 0.187 0.192 0.198 0.195 0.128 0.196 0.132 0.162 0.208 0.218 0.183 0.166 0.219 0.195 0.191 0.216 0.199 0.223 0.158 0.168 0.240 0.218 0.192 0.208 0.214 0.207 0.163 0.164 0.197 0.182 0.203

0.182 0.217 0.198 0.202 0.228 0.169 0.162 0.157 0.223 0.189 0.190 0.207 0.192 0.135 0.196 0.134 0.145 0.206 0.221 0.206 0.168 0.221 0.217 0.196 0.22 0.205 0.223 0.161 0.165 0.194 0.220 0.197 0.207 0.212 0.213 0.166 0.162 0.201 0.184 0.201

Navicular Drop (mm) Non-Wt- Wt-Bearing Drop Bearing 61 58 59 62 57 57 41 66 58 53 49 58 56 61 57 62 55 46 62 52 53 60 54 55 53 58 61 52 55 54 48 57 58 56 49 53 52 55 49 52

51 47 49 53 45 50 34 60 45 45 37 48 47 55 47 55 48 36 50 40 45 50 44 46 44 49 50 45 47 46 38 48 46 43 39 46 45 47 40 40

10 11 10 9 12 7 7 6 13 8 12 10 9 6 10 7 7 10 12 12 8 10 10 9 9 9 11 7 8 8 10 9 12 13 ' 10 7 7 8 9 12

tester 1 and tester 2 was 0.8796 (95% CI 0.6850 to 0.9460). It was noted that significant correlation (r = 0.785) existed between the CFR and Navicular Drop (Fig. 5).


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Discussion The walking speed of a subject can affect the pressure data collected. Rosenbaum et al. (1994) used an EMED-SF system (225x455 mm, 2016 sensors, two sensors per cm2, 70 Hz sampling frequency) to investigate the effect of walking speed on plantar pressure patterns. They reported that the peak pressure significantly increased under the heel and the medial part of the forefoot and decreased under the midfoot and lateral forefoot with an increase in speed. Harrison and Folland (1997) also reported that there were minimal differences in the mean values of peak pressures and peak pressure time integrals of seven discrete areas of the foot under different walking conditions. In this study, each subject was required to walk at his/her own comfortable speed. Hystersis is defined as the difference of a particular pressure point between the output readings while increasing the pressure and the output readings while decreasing the pressure (Liptak, 1982). This is clinically relevant because locomotion requires weight transfer (Polliack et al., 2000). In this study, data was acquired before heel off from the midfoot and the forefoot as the load transmitted is gradually increasing and from the hindfoot while the load transmitted there was gradually decreasing. Further study should include the effect of hysteresis on the output data. The sampling frequency, the sizes of the sensors, the resolution and overall size of the pressure sensing equipment could also affect the results. Mittlemeier and Morlock (1993) examined the effect of sampling frequency on plantar pressure obtained and reported that a frequency of 45 to lOOHz frequency was adequate for measurement of pressures during normal walking. In this study, it was expected that the 50 Hz frequency was able to detect the plantar pressure distribution just before heel off. Hawes et al. (1992) demonstrated a high reliability coefficient (over 0.90) for their measured footprint parameters (arch angle, footprint index, arch index, arch length index and truncated arch index) by the correlation of their data measured on two different occasions by the same person. The objectivity coefficient of the parameters was between 0.70 and 0.91 by computing the correlation of the data measured by 2 different persons. The correlation between the footprint parameters and arch height was

between -0.39 and 0.39. With the exception of the footprint index, all other parameters correlated higher than 0.80 with each other. The authors suggested that the parameters studied were invalid as a basis for prediction of arch height. However McCrory et al. (1997) found the correlation coefficient between Arch Index and normalized navicular height to be r=0.71. Chu et al. (1995), reported significant correlation between the pedobarograph collected arch index and arch height (r=0.70), as well as between Modified Arch Index and arch height (r=0.71). Among the parameters, Modified Arch Index had a better ability (r=0.739) to classify foot arch (Shiang et al., 1998). The reliability coefficients of arch angle, footprint index, arch index, arch length index and truncated arch index were 0.91, 0.94, 0.93, 0.96 and 0.96. The corresponding objectivity coefficients were 0.79, 0.86, 0.91, 0.70 and 0.91 (Hawes et al., 1992). The coefficients of variation of arch angle, footprint index, arch length index, truncated arch index, Staheli index, Chippaux-Smirak index, arch index and modified arch index were 21.08%, 32.82%, 25.36%, 38.14%, 29.58%, 30.94%, 31.90% and 44.93% respectively (Shiang etal., 1998). In this study, the Contact Force Ratio has an intratester reliability (ICC 3,1) of 0.918 and an intertester reliability (ICC 2,1) of 0.8796. The Contact Force Ratio has significant correlation (r=0.785) to Navicular Drop. Flexible flat foot is one of the most common lower limb conditions in children. There is a lack of basic understanding of the natural growth and development of the child's foot and definition of normality. There are difficulties in differentiating the abnormal foot from the normal variance. Consequently there is controversy in the clinical management of the condition. The parameter and experimental procedures used in this study can be further developed to examine natural arch growth in children, to identify the abnormal arch and to evaluate the effect of interventions. Conclusion A new parameter, the Contact Force Ratio (CFR), was derived from the dynamic footprint to reflect foot function. The parameter was based on the normalization of midfoot loading to the total loading to the foot contact area, with toes ignored, just before the heel off instant. Forty


(40) subjects were recruited to participate in the experiment for measurement of CFR and Navicular Drop. Results of ICC tests showed that the CFR had good intratester (0.918) and intertester (0.909) reliability. The CFR had significant correlation (r = 0.785) to the Navicular Drop.

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