Pressure pain threshold (PPT) is defined as the minimal amount of pressure that produces pain. A simple handheld pressure algometer (PA) with a spring is commonly used, although more sophisticated electrical devices with a strain or pneumatic pressure gauge have been developed. They hold the peak force or pressure (kp (kilopond) = 10 N, Newton = 100 kPa (kilopascal)) until tared, and some may also be connected to a computer and thus have continuous output. PPT measured with a probe 1.6 mm in diameter or larger reflects the tenderness of deep tissues as anesthesia of skin only affects the results of smaller probes (Takahashi et al 2005). The most commonly used surface area of probes is 0.5 or 1 [cm.sup.2]. Rolke et al (2005) compared hand-held spring and electronic PAs and found no significant difference for clinical purposes. The PA is placed perpendicular to the tissue surface and pressure applied steadily at a constant rate. Reported pressure application rates have ranged from 0.05 to 20 N/s (Jensen et al 1986). Higher PPT scores were recorded at higher application rates. Ideally compression should be performed slowly enough to allow the subject time to react when pain is felt. When the subject reports feeling pain the action of pressure is stopped, or to avoid delay by the tester, by pressing a switch on an electronic PA.
PAs have been marketed for diagnostic purposes in clinical practice since neuromuscular conditions are often associated with mechanical hyperalgesia. However, pressure algometry is not a diagnostic tool for differentiating soft tissue pathology from other conditions, as several factors, eg disc prolapse, joint luxation, and bone fracture, may increase the local tenderness of soft tissues. Tenderness may vary greatly in painful body parts and there are often several sensitive sites. Pain may be also referred. Taking numerous measurements over local and referred pain areas would be time consuming. Thus, the PA is not helpful in finding these points. Such points can be located in the clinic simply by manual palpation, as no tools for finding them exist even with recent advances in diagnostic imaging. However, manual palpation is not able to quantify the tenderness of these sites. Pressure algometry may be used to study the amount of tissue tenderness, once the right measurement site has been identified.
Pressure algometry has been recommended for clinical practice on the basis of good repeatability when expressed by intraclass correlation coefficients (ICC) results. However, the ICC lacks sensitivity to systematic changes, such as incremental improvements, or deterioration due to repeated testing. Moreover, ICCs have been shown to range from 0.43 to 0.94 for patient populations and only slightly better correlations have been obtained among healthy subjects (Ylinen et al 2007). The considerable variation in ICCs may depend on several factors such as different measurement sites, small study populations, and the experience of the tester. Fischer (1988) suggested that a compression force equivalent to more than 20 N between a painful site and a corresponding normal site is clinically significant and indicates the presence of hyperalgesia. Equivalent results have been obtained in other studies when analysing intratester measurement error, coefficient of repeatability, and variation (Nussbaum and Downes 1998, Smidt et al 2002, Ylinen et al 2007).
PPT show large inter-individual variability in healthy subjects (Fischer 1988, Rolke et al 2005). Therefore, no normative values have been established outside of which pathology could be identified reliably and case-control studies also have shown inconsistent results (Farasyn and Meeusen 2005, Schenk et al 2007). Pressure algometry has been shown to have good validity when assessed by pain and disability questionnaires (Goolkasian et al 2002, Wlodyka-Demaille et al 2002) and, since it assesses a different aspect of pain, may be warranted in addition to other measures. Tenderness varies greatly at different sites of the same body part also in healthy subjects, but studies have shown no difference in PPT between right and left sites in homologous body regions (Fischer 1987, Prushansky et al 2004). Thus, the healthy side may be used as a normal reference in unilateral painful conditions. Pressure algometry has been claimed to be an objective measure. However, although a quantitative measure, it is nevertheless a subjective measure, as it is based on patient report of pain. Moreover PPT may be influenced subconsciously by the tester while compressing the PA. Thus, blinding is recommended in studies. Caution is especially advised when interpreting the results in clinical practice.
Central Hospital of Central Finland, Finland
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|Title Annotation:||Appraisal: Clinimetrics|
|Publication:||Australian Journal of Physiotherapy|
|Date:||Sep 1, 2007|
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