Research report
Validity of an electronic pressure algometer

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Abstract

Background

Pressure algometry is one of a number of outcome measures that manual therapy practitioners are able to use in both clinical and research settings. The pressure algometer is used to determine the pressure pain threshold of specific muscle and bone locations.

Objective

To determine the validity of a commonly utilised electronic pressure algometer.

Design

A repeated measures design was used.

Setting

Biomechanics Laboratory, Victoria University, Melbourne.

Methods

A total of 300 measurements were collected from the electronic pressure algometer as it was applied perpendicularly to an AMTI force plate. Thirty (30) measurements were taken at five discrete rates of application; 10 kPa/s, 20 kPa/s, 30 kPa/s, 40 kPa/s and 50 kPa/s on two separate occasions. Maximum data were recorded from both the pressure algometer and force plate and analysed using the Intraclass Correlation Coefficient (ICC) statistic and the test for significant equality.

Results

ICC values between the raw pressure algometer maximum values and the converted force plate data were high (ICC 0.905–0.998) for the two separate testing sessions and for both sessions combined (ICC 0.968–0.994). The test of significant equality revealed however that the data could not be considered as significantly the same even when allowing for a 5% or 10% tolerance.

Conclusion

The results indicate that the electronic pressure algometer utilised in this study demonstrates limited validity in the laboratory setting.

Introduction

Measurement of the subjective pain experience can be performed using verbal or numerical rating scales and visual analogue scales,1 and pain questionnaires such as the McGill Pain Questionnaire.2 Pressure algometry is another method which has been used to measure and quantify an individual's pain experience.3 The pressure algometer quantifies a patient's perceived pain by determining the pressure-pain threshold (PPT) – the point at which a subject perceives pain upon the application of a pressure or force stimulus.3

Normative data for pressure algometry over bony landmarks and muscles has been established by a number of authors.4, 5, 6, 7, 8 Previous research has demonstrated that there is a lower average PPT over bone compared to muscle and the mean PPT has also been demonstrated to increase in the caudal direction.6

A number of methodological issues have been identified in the previously published research in relation to the use of pressure algometry for perceived pain measurement. These issues include the failure to indicate the rate of pressure application8 and the importance of application of pressure at a constant rate,9, 10, 11 using a verbal command from the patient to cease pressure application thus making the result dependent upon the reaction time of the examiner,8, 11 lack of appropriate training in the use of the device,10 and also the sensitization or habituation due to the pressure previously applied when repeated measures are taken.5 Hogeweg et al.5 also suggest that ‘alertness, consciousness and affection to the observer’ may play a role in determining the pressure pain threshold.

Electronic or digital display algometers have been suggested to be more useful than their non-electronic counterparts as they allow the examiner to be guided as to he rate of pressure application and therefore control the rate of pressure application. This is via a built-in display on the electronic algometer that indicates a constant rate of application. This display can also be adjusted to display different rates of pressure application. The algometer also has a subject-operated push button that stops the algometer from further recordings, thus eliminating the reaction time of the examiner as an extraneous variable.8, 11, 12, 13

Electronic algometers are considered to be more superior and produce better reliability due to the non-reliance on investigator reaction time13 and ability to control the rate of pressure application.14 However the validity of these readings has yet to be established against a known standard of force (pressure) measurement. Similarly, these improvements have not prevented the reporting of large standard deviation values with normative data in the published literature.

The aim of this study was to investigate the validity of a commonly utilised electronic pressure algometer against measurements recorded simultaneously on an AMTI force plate (Advanced Medical Technologies Inc., USA).

Section snippets

Pressure algometer

The pressure algometer (Fig. 1) used in this study was a commercially available hand held electronic pressure algometer (Somedic Algometer Type II, Sweden). The electronic display provides a pressure reading in kilopascals (kPa) and also displays the rate of applied pressure in a small LED window. The operator is able to maintain a constant application rate by keeping the ‘+’ sign in the middle of the LED window. The rate of applied pressure can be set between 10 kPa/s and 50 kPa/s in increments

Data management and analysis

The maximum vertical force and pressure values of the force plate and algometer respectively were used for the data analysis. The maximum vertical force output of the force plate was converted to a kPa measurement to allow a direct comparison with the algometer measurements. Using the known conversion 1 N/cm2 = 10 kPa and an area value (algometer probe area) of 1 cm2, the force values were converted to pressure values by simply multiplying each value by 10.

Initially, intra-class correlation

Results

A scatterplot of the raw algometer scores and the converted force plate scores is presented in Fig. 3. Fig. 3 demonstrates that whilst the majority of scores fall along the line of best fit, a number of outliers are present, particularly at the higher end of the raw algometer scores. The ICC values and confidence intervals for each of the two testing sessions are presented in Table 1. The ICC output for session 1 exhibits high correlation values across each of the five pressure application rate

Discussion

The data obtained from this study suggests that the electronic pressure algometer as used satisfies the standard correlational-based statistical tests. However the test for significant equality data is a more stringent measure and indicates the output from the systems cannot be considered to be equal. The data analysis utilised 300 discrete sets, which is typically greater than most studies that investigate the validity of manual therapy examination procedures. As the data sets are not

Conclusion

Whilst the results suggest that the data obtained from both the electronic pressure algometer and force plate are similar according to a standard correlation-based measure, the test for significant equality suggests that the difference between the means of the two data sets is beyond a 10% acceptable difference. Therefore, studies are required to further investigate the validity and reliability of this device.

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