Pain Medicine 2014; 15: 2095–2104 Wiley Periodicals, Inc.
METHODOLOGY, MECHANISMS & TRANSLATIONAL RESEARCH SECTION Original Research Article Vibration and Rotation During Biaxial Pressure Algometry Is Related with Decreased and Increased Pain Sensations
Djordje Adnadjevic, MSc, and Thomas Graven-Nielsen, DMSc, PhD Laboratory for Musculoskeletal Pain and Motor Control, Center for Sensory-Motor Interaction (SMI), Department of Health Science and Technology, Faculty of Medicine, Aalborg University, Aalborg, Denmark Reprint requests to: Thomas Graven-Nielsen, DMSc, PhD, Laboratory for Musculoskeletal Pain and Motor Control, Center for Sensory-Motor Interaction (SMI), Department of Health Science and Technology, Aalborg University, Fredrik Bajers Vej 7D-3, Aalborg E, DK-9220, Denmark. Tel: 145-9940-9832; Fax: 145-9815-4008; E-mail: [email protected]. Disclosure: The authors report no conflicts of interest.
Abstract Objective. During palpation, the pressure intensity and direction include minor deviations suggesting that standardized variations of the pressure intensity during pressure algometry may optimize the stimulus efficacy. This study examined the perceived pain outcome and reliability of a biaxial (bidirectional) algometer exerting rotational and vibratory stimulation on top of the basic pressure. Methods. In 24 healthy subjects, pressure pain thresholds (PPTs) were recorded with a linear pressure gradient (30 kPa/s) applied by a 1-cm2 probe bilaterally on the tibialis anterior muscle via biaxial
and handheld algometers. During constant pressure stimulation (5 seconds, 75%, 100%, 125% PPT), rotational (45 , 90 , and 180 ), linear vibrational (15, 25, and 50 Hz), and radial vibrational stimulations (5, 15, and 25 Hz) were applied randomly via regular and fanning rounded probes (1 cm2). Subjects rated perceived pain on a 10-cm visual analogue scale on two occasions separated by 1 week period. Results. Repeated measures analysis of variance revealed enhanced effect of rotation angle (P < 0.001), probe (P < 0.001), and radial vibration frequency (P < 0.02), and suppressing effect of axial vibration frequency (P < 0.03) on pain perception, relative to basic pressure alone. PPT reliability of biaxial and handheld algometers showed averaged intraclass correlation coefficient of 0.94 and 0.945, and coefficient of variations of 15.4 and 13.5%, respectively. Conclusions. PPT assessment and multidirectional stimulations can be exerted reliably via biaxial algometer. Linear vibrational stimulation effect on pressure pain perception verified the inhibitory interaction between simultaneous pressure stimulation of low-threshold mechanoreceptors and nociceptors, while radial vibration and rotational stimulation showed facilitatory effects. Key Words. Pressure Algometry; Muscle Stimulation; Pain Perception
Introduction Localized and widespread hyperalgesia together with referred pain is a typical finding in musculoskeletal pain 2095
Adnadjevic and Graven-Nielsen patients [1,2]. The diagnostic approach often includes manual palpation although the interexaminer reliability has been reported as moderate [3,4]. Standardization and quantification of the many variations of manual techniques are one of the problems in manual palpation [5]. Reliable quantitative sensory testing (QST) techniques are therefore needed for assessment of deep tissue hyperalgesia.
ters assessed by the biaxial pressure stimulation setup as well as ordinary stimulation modalities through test–retest procedure. The outcomes could give additional insights as to why manual palpation is a more efficient tissue stimulation method compared with unidirectional assessment, suggesting a shift from standardized unidirectional pressure applicators to multidirectional stimulations. Methods
One approach in the QST battery is pressure algometry involving mechanical stimulation of deep tissue via handheld, actuator-driven, single-point probe or pneumatically driven tourniquet cuff algometers. The handheld pressure algometer is more standardized than manual palpation, but there are some limitations. The examiners need to be skilled in order to get interexaminer repeatability [6–10]. Nussbaum and Downes [7] found that two examiners agreed well on average pain pressure threshold, but there was a large difference between them for individual subjects. Antonaci et al. [6] found interexaminer reliability to be good but lower compared with intraexaminer reliability. State-of-the-art single-point computerized pressure algometry [11–13] offers standardized ways of assessing pressure pain thresholds (PPT) and obtaining stimulus– response (pressure pain) curves in healthy and patient populations contributing to more accurate diagnostics of pain sensitivity by removing examiner variability associated with handheld algometers. The computercontrolled methods assume that unidirectional linear pressure application is the most appropriate technique for assessing pain sensitivity. Current advances in the field of the computer-controlled algometry include biaxial (bidirectional) algometers operating with higher accuracy, at faster speeds, and with higher force precision [14]. Recently repeated biaxial stimulation showed that the inclusion of the second axis (rotation) during musculoskeletal tissue stimulation contributed to more effective methods of pressure stimulation in terms of perceived pain when compared with standalone pressure (Adnadjevic and Graven-Nielsen, Aalborg University, Aalborg, unpublished results). A series of studies also demonstrated that the pressure probe geometry was determining the tissue strain evoked during pressure stimulation [15–17] which is also likely to be important for the biaxial stimulation. In addition, the new biaxial algometry can exert vibration in linear and rotational (radial) directions, thus adding functionality of assessing vibration detection threshold, exploring the efficacy of vibration-induced analgesia, or studying mechanisms behind vibrationevoked deep tissue pain [18–20]. So far, there are no reports on efficacy of computerized biaxial pressure stimulation on pain perception or the reliability of such technique. This study aims to assess 1) if one or several of the novel biaxial pressure stimulation modalities (axial and radial vibration, rotation, probe geometry) induce distinct pain sensation in a healthy population compared with ordinary pressure algometry and 2) the reliability of pain parame2096
Subjects This study included 24 healthy subjects (12 females, average age 24 years, weight 68 kg, height 171 cm) with no pain complaints in lower extremities. Written informed consent was obtained from each participant prior to inclusion in the study. The study was conducted in accordance with the Helsinki Declaration and approved by the local ethics committees (N-20120043). Protocol Subjects attended two identical sessions (test–retest paradigm) lasting 2.5 hours, separated by at least 1week time period. They were seated comfortably in the chair with their lower leg positioned directly below the pressure application probe of the biaxial pressure algometer. The leg was resting on the hydraulic bed and fixed via vacuum pillow to stabilize the leg. PPTs were assessed on three sites along the tibialis anterior (TA) muscle: The middle site was situated on the TA muscle belly 2 cm laterally from tibia, and the other two sites were 2.5 cm proximal and distal to it. Stimulation modalities (basic pressure, rotation, radial vibration, and axial vibration) were applied randomly at three PPT-related intensity levels, and subjects rated their pain perception after each stimulation. The contralateral TA muscle received identical stimulation except that the fanning probe (described next) was used instead of the regular probe used on the ipsilateral leg. Stimulations were repeated three times, randomized among intensities and modalities. Thirty-second breaks were held between stimulations within one modality, and subjects rested 5 minutes before the next stimulation paradigm was carried out. Biaxial Pressure Algometer The biaxial pressure algometer consists of the pressure application device, controller, user interface, and a visual analogue scale (VAS) used for pain ratings [14]. The algometer has two degrees of freedom where the linear and rotational axes were controlled independently and synchronously in the force mode. The algometer can apply pressure in the linear (Fz) and circumferential (Mz, rotation) directions where cylindrical piston leaves the housing and makes the contact with the skin as it rotates around the linear axis or vibrates any of the axes. Pressure levels were feedback controlled and recorded by a load cell (8435-5500 Burster, Gernsbach,
Biaxial Pressure Algometry Germany) attached above the probe on the biaxial algometer and sampled at 1,000 Hz (NI PCI-6221, National Instruments, Austin, TX, USA). The 10-cm long electronic VAS was anchored with 0 representing “no pain” and 10 cm representing “worst imaginable pain.” Subjects were instructed that their PPT rests at 5 cm on the VAS, whereas stimulations perceived as more painful should be rated above this value, and those that are not painful but nonetheless give certain pressure sensation should reside in the bottom half of the scale. The VAS signal was sampled at 1,000 Hz rate via data acquisition board (NI PCI-6221). Pressure Stimulations The regular and fanning rounded probes have a skin contact area of 1 cm2 [16]. For the fanning probe, the application tip was misaligned by 0.5 cm with respect to the connection interface of the algometer, thus displacing the tissue more during rotation relative to the regular probe having the tip and the connection interface in line (Figure 1). PPTs were measured via the regular probe at the standard rate of 0.3 kg/s via the computer-controlled algometer followed by the handheld algometer (Somedic AB, €rby, Sweden) assessment at the same rate (1-cm2 Ho probe). The PPT was defined as the moment during the pressure gradient stimulation when innocuous increasing pressure turns into slight pain. Subject indicated the PPT by pressing the button which immediately stopped the increasing pressure and retracted the probe. PPTs were assessed three times per stimulation site, and the average value was used for further analysis and as a basis for the stimulation modality pertinent to the particular stimulation site. For assessment of the different pressure modalities, a fast increasing stimulation (1 kg/s) was applied and maintained for 5 seconds at the required fundamental pressure intensity (75%, 100%, and 125% of the PPT) before it was released. In-between stimuli, a 30-second break was included. Subsequently, rotation, radial vibration, or axial vibration was added onto the basic pressure stimulation in the period when the pressure was constant. The rotation paradigm consisted of the fundamental pressure combined with rotational movements of 45 , 90 , and 180 applied at all three PPT-related intensity levels, where due to the time constraint of 5 seconds, the 45 and 90 stimulations travelled forth and back twice and once respectively at the same speed, and 180 one travelled forth and back once at an increased speed. Radial vibration included the fundamental pressure and a 3 back and forth rotational movements at 5, 15, and 25 Hz delivered. The axial vibration paradigm applied the fundamental pressure as well as vibration at 15, 25, and 50 Hz with 10–20% PPT-related amplitude at the suprathreshold intensity level only (125% PPT). The maximum VAS scores were extracted and analyzed.
Figure 1 Regular (left) and fanning (right) probes are shown along the stimulation arc they describe during rotational and linear (translational) movements. Statistics All values are presented as means and standard error of the mean. The Kolmogorov–Smirnov test was used for normality assessment, and PPTs and VAS scores were normally distributed. Repeated measures analysis of variance (RMANOVA) was used to asses difference in PPTs with the following repeated factors: day (test, retest), method (handheld, biaxial computerized algometry), and side (left, right). In addition, RMANOVA on the maximum VAS scores with respect to the fundamental pressure stimulation (no feature added) was performed with day (test, retest), side (left, right), sites, and pressure intensity level (75%, 100%, 125% PPT). Finally, RMANOVA on the maximum VAS scores with respect to the fundamental pressure stimulation and features added was performed with the following within-group factors per each intensity level (75%, 100%, 125% PPT): day (test, retest), probe (regular, fanning), and stimulation feature (rotational angle [0 , 45 , 80 , 180 ], radial vibration [0, 5, 15, 25 Hz], or axial vibration [0, 15, 25, 50 Hz]). In case of significant factors or interactions, Bonferroni (BON) post hoc test was used to identify pairwise significant differences among different factors. Assessment of repeatability of the newly designed equipment was performed through the normatively used test–retest procedure [21]. The relative reliability was assessed via an intraclass correlation coefficient (ICC [3, k]; reliability is calculated by taking an average of the measurements) test between the two trials with all paradigms put together [22]. Values above 0.8 are considered to be in “perfect agreement” [23]. In addition, Pearson correlation coefficient was examined. Absolute reliability was assessed by examining systematic error of 2097
Adnadjevic and Graven-Nielsen measurement and coefficient of variation (CoV) [24] and by making the Bland–Altman plot depicting limits of agreement (LoA) between test and retest procedures [24,25]. Significance was accepted at P < 0.05.
Table 1 Mean (6 SEM, N 5 24) pressure pain thresholds from the three bilateral sites assessed on the tibialis anterior muscle with the handheld and biaxial pressure algometers
Results Method
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Retest (kPa)
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392.3 6 47.7 369.5 6 41.8 427.1 6 42.3 383.5 6 45.4
400.0 6 46.0 363.1 6 43.7 416.9 6 47.6 371.5 6 50.7
PPTs Right side PPTs were significantly higher than the left side PPTs (Table 1; RMANOVA: F [1, 23] 5 5.86, P < 0.03; PPTright 5 409 6 43.1 kPa, PPTleft 5 371.9 6 42.7 kPa).
Handheld
Pain Induced by Basic Pressure Stimulation Alone
L 5 left; R 5 right; SEM 5 standard error of the mean.
The VAS scores following basic pressure stimulations at 75%, 100%, and 125% of the individual PPT within the rotational paradigm were 4.0 6 0.3, 4.7 6 0.2, and 5.4 6 0.2 cm, respectively, and all are significantly different (RMANOVA: F [2, 22] 5 57.31, P < 0.001). Similarly, within the radial vibration paradigm scores were 3.9 6 0.3, 4.6 6 0.2, and 5.4 6 0.2 cm (RMANOVA: F [2, 22] 5 36.32, P < 0.001). Basic 125% PPT stimulation for axial vibration resulted in VAS of 5.9 6 0.3 cm. Pain Induced by Pressure and Simultaneous Rotation For VAS scores at the 75% PPT intensity level, analysis revealed probe 3 feature (RMANOVA: F [3, 21] 5 3.76, P < 0.03, BON: P < 0.04) interaction effect; 100% PPT intensity level analysis resulted in a feature (RMANOVA: F [3, 21] 5 37.07, P < 0.001) main effect; lastly, 125% PPT intensity level analysis revealed a probe 3 feature (RMANOVA: F [3, 21] 5 5.64, P < 0.01, BON: P < 0.02) interaction effect. Post hoc analysis of the probe 3 feature interaction effects on VAS scores is shown in Figure 2 (a– c) and show generally increased VAS scores following rotations compared with the basic pressure stimulation. Pain Induced by Pressure and Simultaneous Radial Vibration For VAS scores at the 75% and 100% PPT intensity level, analysis revealed a feature (RMANOVA: F [3, 21] 6.27, P < 0.02) main effect, whereas the 125% PPT intensity level analysis resulted in a probe 3 feature (RMANOVA: F [3, 21] 5 3.77, P < 0.03, BON: P < 0.02) interaction effect. Post hoc analysis of the probe 3 feature interaction effect on VAS scores is shown in Figure 2 (d–f) where the VAS scores after pressure stimulations with radial vibration are typically increased compared with the basic stimulations. Pain Induced by Pressure and Simultaneous Axial Vibration For VAS scores at the 125% PPT intensity level, the RMANOVA resulted in a main significant effect of the feature parameter (RMANOVA: F [1, 23] 5 3.96, P < 0.03). All frequencies (15, 25, 50 Hz) applied on top 2098
Biaxial
of the suprathreshold pressure stimulation resulted in significantly lower VAS scores than the basic pressure stimulation (basic: 5.9 6 0.2 cm, 15 Hz: 5.5 6 0.3 cm, 25 Hz: 5.5 6 0.3 cm, 50 Hz: 5.5 6 0.3 cm). Relative Reliability of PPT and VAS Scores Panels a, c, and e in Figure 3 show scatter plots of test–retest VAS scores and PPT values giving impression about relative reliability. The PPT reliability analysis of the biaxial algometer and handheld algometers revealed a high-averaged (left and right TA) intraclass as well as Pearson’s correlation coefficient ICChandheld 5 0.95; Rbiaxial 5 0.89; (ICCbiaxial 5 0.94; Rhandheld 5 0.89). Further breakdown of the relative reliability parameters to the left and right TA muscles is contained in Table 2. VAS scores ICC and Pearson’s correlation coefficients were 0.77 and 0.64, respectively. Absolute Reliability of PPT and VAS Scores Panels b, d, and f show Bland–Altman plots of VAS scores and PPT values obtained during the two test sessions, providing graphical impression of absolute reliability measures. The biaxial PPT algometry revealed an averaged (left and right) 95% confidence interval LoAbiaxial 5 2182.1 to 160.0 kPa, an absolute, averaged BIASabs, biaxial 5 11.1 kPa, and a CoVbiaxial 5 15.4%. Similarly, handheld PPT algometry showed an averaged (left and right) 95% confidence interval LoAhandheld 5 2157.2 to 158.2 kPa, an absolute, averaged BIASabs, handheld 5 7 kPa, and a CoVhandheld 5 13.5%. Further separation of the absolute PPT reliability parameters into the left and right PPT values is contained in Table 2. VAS score absolute reliability is reflected in the sparse Bland–Altman plot with LoAVAS 5 22.8 to 2.7 cm, bias of BIASVAS 5 20.054 cm, and CoVVAS 5 16.3%. Other important absolute reliability findings are outlined in Table 2. Discussion The present study showed for the first time that biaxial algometry exerting basic pressure stimulation simultaneously
Biaxial Pressure Algometry
Figure 2 Mean (6 standard error of the mean [SEM], N 5 24) visual analogue scale (VAS) scores after pressure stimulation with rotational (a–c) and radial vibration (d–f) paradigms at 75%, 100%, and 125% pressure pain threshold (PPT) intensities for the regular and fanning probes. Significantly different VAS scores (Bonferroni [BON]: P < 0.05) compared with basic (*), basic and 45 /5 Hz (䉬), basic, 45 /5 Hz, and 90 /15 Hz (#), between probes ($), and 25 from 15 Hz (¶) are illustrated. Some of the illustrated post hoc findings are based on a main effect of feature (i.e., no probe effects) but illustrated identically for each probe. with linear vibrational stimulation evokes inhibitory effect on pressure pain perception while radial vibration and especially rotational stimulation manifest in increased pain perception. Axial Vibration Stimulation Axial vibration on top of the basic suprathreshold stimulation reduced the pain perception compared with the basic stimulation alone, regardless of the frequency used. This is in line with previous findings [26] where it was shown that vibration modulates nociceptive afferent input from
the laser stimulation. A more recent study showed among other findings that 80-Hz vibration combined with pressure compared with pressure alone reduced soreness in the triceps surea muscle [27]. The presence of pain relief by vibration is confirmed in the current data, which is consistent with the gate control theory [28]. Rotational Stimulation The larger tissue displacements by rotation corresponded to a higher pain perception. The increase in 2099
Adnadjevic and Graven-Nielsen Bland Altman Plot
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Figure 3 Test–retest Scatter and Bland–Altman plots for visual analogue scale (VAS) (a, b) and Biaxial pressure pain thresholds (PPT) (right leg, c, and d; left leg, e, and f). Panels on the left show scores at day 1 and day 2 for all the individual test paradigms in 24 subjects (1,248 paired VAS scores [a]; 24 paired PPT measurements [c and e]). Test–retest points would coincide perfectly if they did not deviate from the 45 line in panels a, c, and e. Panels on the right show Bland–Altman plots for VAS (b) and PPT (d and f) defined by absolute reliability parameters outlined in Table 2. 2100
0.94 0.89 0.94 0.89
pain ratings with the increase in rotational stimulation angle could be attributed to spatial summation phenomenon. Recruitment of nociceptors of the deep tissue seems to be more effective at larger angles regardless of the probe geometry (regular, fanning). Although conventional spatial summation [29] is not admittedly happening here, it may be a comparable paradigm as a larger area of tissue is stimulated over a relatively short duration (5 seconds), especially with the fanning probe.
0.93 0.87
PPT Left: Handheld PPT Right: Handheld PPT Left: Biaxial PPT Right: Biaxial
It is difficult to separate the skin and deep tissue nociceptive input and clearly state which factors make rotational stimulation more effective due to possible convergence of somatic and cutaneous nociceptive afferents on the same dorsal horn neuron [30]. Even though skin sensitivity could influence pain perception induced by pressure stimulation [31], another study showed that mechanical pain sensations could be elicited with full skin anesthesia, suggesting that the pain most likely originated from the deep musculoskeletal tissue [32].
ICC 5 intraclass correlation coefficient; PPT 5 pressure pain threshold; VAS 5 visual analogue scale.
0.96 0.91 0.77 0.64 ICC (3, k) Pearson’s R
VAS Relative Reliability Parameters
Confidence interval limits of agreement Limits of agreement residuals Coefficient of variation
26.3 kPa 21.8 kPa 253.1 to 40.4 kPa 2171.9 to 159.2 kPa 37.3 kPa 2251.9 to 79.2 kPa 291.9 to 239.2 kPa 2207 to 207 kPa 14.1% 27.7 kPa 19.8 kPa 234.8 to 50.1 kPa 2142.5 to 157.8 kPa 33.8 kPa 2215.0 to 85.3 kPa 269.9 to 230.4 kPa 2187.7 to 187.7 kPa 12.8% 212.0 kPa 23.5 kPa 262.4 to 38.3 kPa 2190.3 to 166.2 kPa 40.2 kPa 2276.4 to 80.1 kPa 2104.2 to 252.4 kPa 2217.6 to 217.6 kPa 17.9% 210.1 kPa 21.6 kPa 256.3 to 36.2 kPa 2173.8 to 153.6 kPa 36.9 kPa 2252.9 to 74.5 kPa 294.7 to 232.8 kPa 2198.9 to 198.9 kPa 12.8% Bias Standard error bias Confidence interval bias Limits of agreement Standard error limits of agreement
20.054 cm 0.0397 cm 20.132 to 0.024 cm 22.8 to 2.7 cm 0.068 cm 22.9 to 2.6 cm 22.7 to 2.8 cm 23.4 to 3.4 cm 16.3%
PPT Left: Handheld PPT Right: Handheld PPT Left: Biaxial PPT Right: Biaxial VAS Absolute Reliability Parameters
Table 2
Absolute and relative reliability parameters of VAS and PPT (PPT: left and right, biaxial vs handheld algometry) scores
Biaxial Pressure Algometry
Moreover, it was shown that deep tissues are more effectively stimulated via strain when the rounded probes are used [15], and it is assumed that this roundness reduces the probe–skin contact friction during rotation as opposed to the conventional flat probes. Thus, it is speculated that the increase in rotational travel angle alongside the rounded probe is more effectively increasing strain on the deep structures. Such mechanism is perhaps what is underlying the effectiveness of manual finger palpation performed by the physicians [3,5,33,34], where the tissue is stimulated multidirectionally with varying intensity. The rotational stimulation paradigm could represent the next step in computerized pressure algometry to become more comparable with standardized palpatory movements. Radial Vibration Stimulation During radial vibration stimulation, fanning probe showed to be just as painful as the regular probe for the same stimulation intensity, indicating less pronounced probe geometry effect relative to rotational stimulation paradigm. Due to the limited angle of vibration, spatial summation is probably less efficient compared with the larger angle rotations. Regardless of the PPT-related stimulation intensity, basic stimulation is generally perceived as less painful than the 5- and 15-Hz stimulation frequency and as painful as 25-Hz stimulation. In contrast to radial stimulation at the suprathreshold level, it seems that linear (axial) vibration at the same intensity is perceived as less painful relative to its respective baseline. This contrasting finding between radial and axial vibration could be due to the fact that radial vibration is effectively a mixture of rotation with a small travel angle (5 ) and vibration due to its stimulation frequency (5, 15, 25 Hz). Therefore, it is possible that the phenomenon of spatial summation is present (rotation), as well as the 2101
Adnadjevic and Graven-Nielsen competing effect of an inhibitory mechanism due to vibration (down pressure). Reliability of PPT Assessments Prior studies have investigated handheld PPT test–retest reliability in different muscles and joints over longer periods of time; PPTs were highly consistent and repeatable over the 4 days of testing with ICC > 0.94 [35]; in knee osteoarthritis patients, a reliability study showed that this method was the most reliable tool in QST battery [36]. PPTs assessed via the biaxial pressure algometer yielded a high ICC. Unidirectional (linear) computerized algometers using indentation rather than a forcefeedback drive mechanism as in this study were reported to have somewhat lower relative reliabilities relative to the handheld ones used in the same study [37]. Similar to a previous study investigating absolute reliability [37] where handheld algometry exhibited 5.3% lower CoV relative to the unidirectional computerized algometry (15.6%), in this present study, CoV of the handheld algometer was 1.9% lower than that of the biaxial algometer (15.4%). Past studies have additionally used LoA to assess absolute reliability for QST between two test occasions [38]. Other studies used LoA to make comparisons among handheld algometer, pneumatic one, and gear-driven, computer-controlled algometer to assess whether the new method is up to par with the previous ones [39]. A recent study [4] confirmed that handheld pressure algometry shows a small trend for test–retest differences to become greater with higher PPT measurements, suggesting that measures may be less precise in the higher range of the PPT values, which could be an inherent feature of pressure algometry. Additionally, handheld intrarater reliability reflected by LoA rested in the 2100 to 100 kPa range, whereas in this study, it averaged 2157.2 to 158.2 kPa for left and right leg. The difference could be attributed to a test–retest time difference (same day vs at least 1 week). Biaxial and handheld PPT LoA have comparable 95% confidence intervals and systematic biases suggesting validity of the clinically accepted method (handheld) and the repeatability of the challenging method (biaxial). However, it is worth noting that the average of three measurements per three sites per muscle was taken, therefore potentially contributing to the comparability and thus the stability for each of two methods used in terms of the outcome measure. Reliability of VAS Scores The ICC of 0.77 for the averaged VAS scores represents a “substantial agreement” [23]. Low systematic bias suggests that the combined pain responses could be considered reliable. Whether the LoA interval is clinically/research-wise acceptable is to be determined by 2102
the intended use of the new biaxial algometer in the future experimental procedures. A certain amount of caution needs to be exercised when stating that 95% of the differences lie in this interval as the finding is pertinent to this particular experimental design. Conclusions This study presented a novel biaxial pressure algometer that may be used to quantify tenderness in the musculoskeletal structures via standardized pressure stimulation with simultaneous linear and rotational stimulations; it has also revealed a good relative and absolute reliability of this methodology via test–retest paradigm. Different kinds of pressure stimulations onto superficial and deep structures were demonstrated to elicit more pronounced (radial vibration, rotation) or reduced (axial vibration) pain perceptions compared with ordinary pressure stimulation. The novel pressure stimulations with higher efficacy could supplement musculoskeletal QST methods contributing to a more standardized assessment techniques and more knowledge on basic musculoskeletal pain mechanisms. Stimulation parameters examined in this study such as fanning probe rotation and precise control of the stimulus duration, intensity, and frequency could be pivotal in optimal assessment of pain hypersensitivity. Further, these stimulation parameters could complement methods used for detecting tender spots in muscular tissue in a more robust and standardized manner, which is important for quantifying the level of muscle soreness during rehabilitation for instance, thus aiding in patient treatment.
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METHODOLOGY, MECHANISMS & TRANSLATIONAL RESEARCH SECTION Original Research Article Vibration and Rotation During Biaxial Pressure Algometry Is Related with Decreased and Increased Pain Sensations
Djordje Adnadjevic, MSc, and Thomas Graven-Nielsen, DMSc, PhD Laboratory for Musculoskeletal Pain and Motor Control, Center for Sensory-Motor Interaction (SMI), Department of Health Science and Technology, Faculty of Medicine, Aalborg University, Aalborg, Denmark Reprint requests to: Thomas Graven-Nielsen, DMSc, PhD, Laboratory for Musculoskeletal Pain and Motor Control, Center for Sensory-Motor Interaction (SMI), Department of Health Science and Technology, Aalborg University, Fredrik Bajers Vej 7D-3, Aalborg E, DK-9220, Denmark. Tel: 145-9940-9832; Fax: 145-9815-4008; E-mail: [email protected]. Disclosure: The authors report no conflicts of interest.
Abstract Objective. During palpation, the pressure intensity and direction include minor deviations suggesting that standardized variations of the pressure intensity during pressure algometry may optimize the stimulus efficacy. This study examined the perceived pain outcome and reliability of a biaxial (bidirectional) algometer exerting rotational and vibratory stimulation on top of the basic pressure. Methods. In 24 healthy subjects, pressure pain thresholds (PPTs) were recorded with a linear pressure gradient (30 kPa/s) applied by a 1-cm2 probe bilaterally on the tibialis anterior muscle via biaxial
and handheld algometers. During constant pressure stimulation (5 seconds, 75%, 100%, 125% PPT), rotational (45 , 90 , and 180 ), linear vibrational (15, 25, and 50 Hz), and radial vibrational stimulations (5, 15, and 25 Hz) were applied randomly via regular and fanning rounded probes (1 cm2). Subjects rated perceived pain on a 10-cm visual analogue scale on two occasions separated by 1 week period. Results. Repeated measures analysis of variance revealed enhanced effect of rotation angle (P < 0.001), probe (P < 0.001), and radial vibration frequency (P < 0.02), and suppressing effect of axial vibration frequency (P < 0.03) on pain perception, relative to basic pressure alone. PPT reliability of biaxial and handheld algometers showed averaged intraclass correlation coefficient of 0.94 and 0.945, and coefficient of variations of 15.4 and 13.5%, respectively. Conclusions. PPT assessment and multidirectional stimulations can be exerted reliably via biaxial algometer. Linear vibrational stimulation effect on pressure pain perception verified the inhibitory interaction between simultaneous pressure stimulation of low-threshold mechanoreceptors and nociceptors, while radial vibration and rotational stimulation showed facilitatory effects. Key Words. Pressure Algometry; Muscle Stimulation; Pain Perception
Introduction Localized and widespread hyperalgesia together with referred pain is a typical finding in musculoskeletal pain 2095
Adnadjevic and Graven-Nielsen patients [1,2]. The diagnostic approach often includes manual palpation although the interexaminer reliability has been reported as moderate [3,4]. Standardization and quantification of the many variations of manual techniques are one of the problems in manual palpation [5]. Reliable quantitative sensory testing (QST) techniques are therefore needed for assessment of deep tissue hyperalgesia.
ters assessed by the biaxial pressure stimulation setup as well as ordinary stimulation modalities through test–retest procedure. The outcomes could give additional insights as to why manual palpation is a more efficient tissue stimulation method compared with unidirectional assessment, suggesting a shift from standardized unidirectional pressure applicators to multidirectional stimulations. Methods
One approach in the QST battery is pressure algometry involving mechanical stimulation of deep tissue via handheld, actuator-driven, single-point probe or pneumatically driven tourniquet cuff algometers. The handheld pressure algometer is more standardized than manual palpation, but there are some limitations. The examiners need to be skilled in order to get interexaminer repeatability [6–10]. Nussbaum and Downes [7] found that two examiners agreed well on average pain pressure threshold, but there was a large difference between them for individual subjects. Antonaci et al. [6] found interexaminer reliability to be good but lower compared with intraexaminer reliability. State-of-the-art single-point computerized pressure algometry [11–13] offers standardized ways of assessing pressure pain thresholds (PPT) and obtaining stimulus– response (pressure pain) curves in healthy and patient populations contributing to more accurate diagnostics of pain sensitivity by removing examiner variability associated with handheld algometers. The computercontrolled methods assume that unidirectional linear pressure application is the most appropriate technique for assessing pain sensitivity. Current advances in the field of the computer-controlled algometry include biaxial (bidirectional) algometers operating with higher accuracy, at faster speeds, and with higher force precision [14]. Recently repeated biaxial stimulation showed that the inclusion of the second axis (rotation) during musculoskeletal tissue stimulation contributed to more effective methods of pressure stimulation in terms of perceived pain when compared with standalone pressure (Adnadjevic and Graven-Nielsen, Aalborg University, Aalborg, unpublished results). A series of studies also demonstrated that the pressure probe geometry was determining the tissue strain evoked during pressure stimulation [15–17] which is also likely to be important for the biaxial stimulation. In addition, the new biaxial algometry can exert vibration in linear and rotational (radial) directions, thus adding functionality of assessing vibration detection threshold, exploring the efficacy of vibration-induced analgesia, or studying mechanisms behind vibrationevoked deep tissue pain [18–20]. So far, there are no reports on efficacy of computerized biaxial pressure stimulation on pain perception or the reliability of such technique. This study aims to assess 1) if one or several of the novel biaxial pressure stimulation modalities (axial and radial vibration, rotation, probe geometry) induce distinct pain sensation in a healthy population compared with ordinary pressure algometry and 2) the reliability of pain parame2096
Subjects This study included 24 healthy subjects (12 females, average age 24 years, weight 68 kg, height 171 cm) with no pain complaints in lower extremities. Written informed consent was obtained from each participant prior to inclusion in the study. The study was conducted in accordance with the Helsinki Declaration and approved by the local ethics committees (N-20120043). Protocol Subjects attended two identical sessions (test–retest paradigm) lasting 2.5 hours, separated by at least 1week time period. They were seated comfortably in the chair with their lower leg positioned directly below the pressure application probe of the biaxial pressure algometer. The leg was resting on the hydraulic bed and fixed via vacuum pillow to stabilize the leg. PPTs were assessed on three sites along the tibialis anterior (TA) muscle: The middle site was situated on the TA muscle belly 2 cm laterally from tibia, and the other two sites were 2.5 cm proximal and distal to it. Stimulation modalities (basic pressure, rotation, radial vibration, and axial vibration) were applied randomly at three PPT-related intensity levels, and subjects rated their pain perception after each stimulation. The contralateral TA muscle received identical stimulation except that the fanning probe (described next) was used instead of the regular probe used on the ipsilateral leg. Stimulations were repeated three times, randomized among intensities and modalities. Thirty-second breaks were held between stimulations within one modality, and subjects rested 5 minutes before the next stimulation paradigm was carried out. Biaxial Pressure Algometer The biaxial pressure algometer consists of the pressure application device, controller, user interface, and a visual analogue scale (VAS) used for pain ratings [14]. The algometer has two degrees of freedom where the linear and rotational axes were controlled independently and synchronously in the force mode. The algometer can apply pressure in the linear (Fz) and circumferential (Mz, rotation) directions where cylindrical piston leaves the housing and makes the contact with the skin as it rotates around the linear axis or vibrates any of the axes. Pressure levels were feedback controlled and recorded by a load cell (8435-5500 Burster, Gernsbach,
Biaxial Pressure Algometry Germany) attached above the probe on the biaxial algometer and sampled at 1,000 Hz (NI PCI-6221, National Instruments, Austin, TX, USA). The 10-cm long electronic VAS was anchored with 0 representing “no pain” and 10 cm representing “worst imaginable pain.” Subjects were instructed that their PPT rests at 5 cm on the VAS, whereas stimulations perceived as more painful should be rated above this value, and those that are not painful but nonetheless give certain pressure sensation should reside in the bottom half of the scale. The VAS signal was sampled at 1,000 Hz rate via data acquisition board (NI PCI-6221). Pressure Stimulations The regular and fanning rounded probes have a skin contact area of 1 cm2 [16]. For the fanning probe, the application tip was misaligned by 0.5 cm with respect to the connection interface of the algometer, thus displacing the tissue more during rotation relative to the regular probe having the tip and the connection interface in line (Figure 1). PPTs were measured via the regular probe at the standard rate of 0.3 kg/s via the computer-controlled algometer followed by the handheld algometer (Somedic AB, €rby, Sweden) assessment at the same rate (1-cm2 Ho probe). The PPT was defined as the moment during the pressure gradient stimulation when innocuous increasing pressure turns into slight pain. Subject indicated the PPT by pressing the button which immediately stopped the increasing pressure and retracted the probe. PPTs were assessed three times per stimulation site, and the average value was used for further analysis and as a basis for the stimulation modality pertinent to the particular stimulation site. For assessment of the different pressure modalities, a fast increasing stimulation (1 kg/s) was applied and maintained for 5 seconds at the required fundamental pressure intensity (75%, 100%, and 125% of the PPT) before it was released. In-between stimuli, a 30-second break was included. Subsequently, rotation, radial vibration, or axial vibration was added onto the basic pressure stimulation in the period when the pressure was constant. The rotation paradigm consisted of the fundamental pressure combined with rotational movements of 45 , 90 , and 180 applied at all three PPT-related intensity levels, where due to the time constraint of 5 seconds, the 45 and 90 stimulations travelled forth and back twice and once respectively at the same speed, and 180 one travelled forth and back once at an increased speed. Radial vibration included the fundamental pressure and a 3 back and forth rotational movements at 5, 15, and 25 Hz delivered. The axial vibration paradigm applied the fundamental pressure as well as vibration at 15, 25, and 50 Hz with 10–20% PPT-related amplitude at the suprathreshold intensity level only (125% PPT). The maximum VAS scores were extracted and analyzed.
Figure 1 Regular (left) and fanning (right) probes are shown along the stimulation arc they describe during rotational and linear (translational) movements. Statistics All values are presented as means and standard error of the mean. The Kolmogorov–Smirnov test was used for normality assessment, and PPTs and VAS scores were normally distributed. Repeated measures analysis of variance (RMANOVA) was used to asses difference in PPTs with the following repeated factors: day (test, retest), method (handheld, biaxial computerized algometry), and side (left, right). In addition, RMANOVA on the maximum VAS scores with respect to the fundamental pressure stimulation (no feature added) was performed with day (test, retest), side (left, right), sites, and pressure intensity level (75%, 100%, 125% PPT). Finally, RMANOVA on the maximum VAS scores with respect to the fundamental pressure stimulation and features added was performed with the following within-group factors per each intensity level (75%, 100%, 125% PPT): day (test, retest), probe (regular, fanning), and stimulation feature (rotational angle [0 , 45 , 80 , 180 ], radial vibration [0, 5, 15, 25 Hz], or axial vibration [0, 15, 25, 50 Hz]). In case of significant factors or interactions, Bonferroni (BON) post hoc test was used to identify pairwise significant differences among different factors. Assessment of repeatability of the newly designed equipment was performed through the normatively used test–retest procedure [21]. The relative reliability was assessed via an intraclass correlation coefficient (ICC [3, k]; reliability is calculated by taking an average of the measurements) test between the two trials with all paradigms put together [22]. Values above 0.8 are considered to be in “perfect agreement” [23]. In addition, Pearson correlation coefficient was examined. Absolute reliability was assessed by examining systematic error of 2097
Adnadjevic and Graven-Nielsen measurement and coefficient of variation (CoV) [24] and by making the Bland–Altman plot depicting limits of agreement (LoA) between test and retest procedures [24,25]. Significance was accepted at P < 0.05.
Table 1 Mean (6 SEM, N 5 24) pressure pain thresholds from the three bilateral sites assessed on the tibialis anterior muscle with the handheld and biaxial pressure algometers
Results Method
Side
Test (kPa)
Retest (kPa)
R L R L
392.3 6 47.7 369.5 6 41.8 427.1 6 42.3 383.5 6 45.4
400.0 6 46.0 363.1 6 43.7 416.9 6 47.6 371.5 6 50.7
PPTs Right side PPTs were significantly higher than the left side PPTs (Table 1; RMANOVA: F [1, 23] 5 5.86, P < 0.03; PPTright 5 409 6 43.1 kPa, PPTleft 5 371.9 6 42.7 kPa).
Handheld
Pain Induced by Basic Pressure Stimulation Alone
L 5 left; R 5 right; SEM 5 standard error of the mean.
The VAS scores following basic pressure stimulations at 75%, 100%, and 125% of the individual PPT within the rotational paradigm were 4.0 6 0.3, 4.7 6 0.2, and 5.4 6 0.2 cm, respectively, and all are significantly different (RMANOVA: F [2, 22] 5 57.31, P < 0.001). Similarly, within the radial vibration paradigm scores were 3.9 6 0.3, 4.6 6 0.2, and 5.4 6 0.2 cm (RMANOVA: F [2, 22] 5 36.32, P < 0.001). Basic 125% PPT stimulation for axial vibration resulted in VAS of 5.9 6 0.3 cm. Pain Induced by Pressure and Simultaneous Rotation For VAS scores at the 75% PPT intensity level, analysis revealed probe 3 feature (RMANOVA: F [3, 21] 5 3.76, P < 0.03, BON: P < 0.04) interaction effect; 100% PPT intensity level analysis resulted in a feature (RMANOVA: F [3, 21] 5 37.07, P < 0.001) main effect; lastly, 125% PPT intensity level analysis revealed a probe 3 feature (RMANOVA: F [3, 21] 5 5.64, P < 0.01, BON: P < 0.02) interaction effect. Post hoc analysis of the probe 3 feature interaction effects on VAS scores is shown in Figure 2 (a– c) and show generally increased VAS scores following rotations compared with the basic pressure stimulation. Pain Induced by Pressure and Simultaneous Radial Vibration For VAS scores at the 75% and 100% PPT intensity level, analysis revealed a feature (RMANOVA: F [3, 21] 6.27, P < 0.02) main effect, whereas the 125% PPT intensity level analysis resulted in a probe 3 feature (RMANOVA: F [3, 21] 5 3.77, P < 0.03, BON: P < 0.02) interaction effect. Post hoc analysis of the probe 3 feature interaction effect on VAS scores is shown in Figure 2 (d–f) where the VAS scores after pressure stimulations with radial vibration are typically increased compared with the basic stimulations. Pain Induced by Pressure and Simultaneous Axial Vibration For VAS scores at the 125% PPT intensity level, the RMANOVA resulted in a main significant effect of the feature parameter (RMANOVA: F [1, 23] 5 3.96, P < 0.03). All frequencies (15, 25, 50 Hz) applied on top 2098
Biaxial
of the suprathreshold pressure stimulation resulted in significantly lower VAS scores than the basic pressure stimulation (basic: 5.9 6 0.2 cm, 15 Hz: 5.5 6 0.3 cm, 25 Hz: 5.5 6 0.3 cm, 50 Hz: 5.5 6 0.3 cm). Relative Reliability of PPT and VAS Scores Panels a, c, and e in Figure 3 show scatter plots of test–retest VAS scores and PPT values giving impression about relative reliability. The PPT reliability analysis of the biaxial algometer and handheld algometers revealed a high-averaged (left and right TA) intraclass as well as Pearson’s correlation coefficient ICChandheld 5 0.95; Rbiaxial 5 0.89; (ICCbiaxial 5 0.94; Rhandheld 5 0.89). Further breakdown of the relative reliability parameters to the left and right TA muscles is contained in Table 2. VAS scores ICC and Pearson’s correlation coefficients were 0.77 and 0.64, respectively. Absolute Reliability of PPT and VAS Scores Panels b, d, and f show Bland–Altman plots of VAS scores and PPT values obtained during the two test sessions, providing graphical impression of absolute reliability measures. The biaxial PPT algometry revealed an averaged (left and right) 95% confidence interval LoAbiaxial 5 2182.1 to 160.0 kPa, an absolute, averaged BIASabs, biaxial 5 11.1 kPa, and a CoVbiaxial 5 15.4%. Similarly, handheld PPT algometry showed an averaged (left and right) 95% confidence interval LoAhandheld 5 2157.2 to 158.2 kPa, an absolute, averaged BIASabs, handheld 5 7 kPa, and a CoVhandheld 5 13.5%. Further separation of the absolute PPT reliability parameters into the left and right PPT values is contained in Table 2. VAS score absolute reliability is reflected in the sparse Bland–Altman plot with LoAVAS 5 22.8 to 2.7 cm, bias of BIASVAS 5 20.054 cm, and CoVVAS 5 16.3%. Other important absolute reliability findings are outlined in Table 2. Discussion The present study showed for the first time that biaxial algometry exerting basic pressure stimulation simultaneously
Biaxial Pressure Algometry
Figure 2 Mean (6 standard error of the mean [SEM], N 5 24) visual analogue scale (VAS) scores after pressure stimulation with rotational (a–c) and radial vibration (d–f) paradigms at 75%, 100%, and 125% pressure pain threshold (PPT) intensities for the regular and fanning probes. Significantly different VAS scores (Bonferroni [BON]: P < 0.05) compared with basic (*), basic and 45 /5 Hz (䉬), basic, 45 /5 Hz, and 90 /15 Hz (#), between probes ($), and 25 from 15 Hz (¶) are illustrated. Some of the illustrated post hoc findings are based on a main effect of feature (i.e., no probe effects) but illustrated identically for each probe. with linear vibrational stimulation evokes inhibitory effect on pressure pain perception while radial vibration and especially rotational stimulation manifest in increased pain perception. Axial Vibration Stimulation Axial vibration on top of the basic suprathreshold stimulation reduced the pain perception compared with the basic stimulation alone, regardless of the frequency used. This is in line with previous findings [26] where it was shown that vibration modulates nociceptive afferent input from
the laser stimulation. A more recent study showed among other findings that 80-Hz vibration combined with pressure compared with pressure alone reduced soreness in the triceps surea muscle [27]. The presence of pain relief by vibration is confirmed in the current data, which is consistent with the gate control theory [28]. Rotational Stimulation The larger tissue displacements by rotation corresponded to a higher pain perception. The increase in 2099
Adnadjevic and Graven-Nielsen Bland Altman Plot
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Figure 3 Test–retest Scatter and Bland–Altman plots for visual analogue scale (VAS) (a, b) and Biaxial pressure pain thresholds (PPT) (right leg, c, and d; left leg, e, and f). Panels on the left show scores at day 1 and day 2 for all the individual test paradigms in 24 subjects (1,248 paired VAS scores [a]; 24 paired PPT measurements [c and e]). Test–retest points would coincide perfectly if they did not deviate from the 45 line in panels a, c, and e. Panels on the right show Bland–Altman plots for VAS (b) and PPT (d and f) defined by absolute reliability parameters outlined in Table 2. 2100
0.94 0.89 0.94 0.89
pain ratings with the increase in rotational stimulation angle could be attributed to spatial summation phenomenon. Recruitment of nociceptors of the deep tissue seems to be more effective at larger angles regardless of the probe geometry (regular, fanning). Although conventional spatial summation [29] is not admittedly happening here, it may be a comparable paradigm as a larger area of tissue is stimulated over a relatively short duration (5 seconds), especially with the fanning probe.
0.93 0.87
PPT Left: Handheld PPT Right: Handheld PPT Left: Biaxial PPT Right: Biaxial
It is difficult to separate the skin and deep tissue nociceptive input and clearly state which factors make rotational stimulation more effective due to possible convergence of somatic and cutaneous nociceptive afferents on the same dorsal horn neuron [30]. Even though skin sensitivity could influence pain perception induced by pressure stimulation [31], another study showed that mechanical pain sensations could be elicited with full skin anesthesia, suggesting that the pain most likely originated from the deep musculoskeletal tissue [32].
ICC 5 intraclass correlation coefficient; PPT 5 pressure pain threshold; VAS 5 visual analogue scale.
0.96 0.91 0.77 0.64 ICC (3, k) Pearson’s R
VAS Relative Reliability Parameters
Confidence interval limits of agreement Limits of agreement residuals Coefficient of variation
26.3 kPa 21.8 kPa 253.1 to 40.4 kPa 2171.9 to 159.2 kPa 37.3 kPa 2251.9 to 79.2 kPa 291.9 to 239.2 kPa 2207 to 207 kPa 14.1% 27.7 kPa 19.8 kPa 234.8 to 50.1 kPa 2142.5 to 157.8 kPa 33.8 kPa 2215.0 to 85.3 kPa 269.9 to 230.4 kPa 2187.7 to 187.7 kPa 12.8% 212.0 kPa 23.5 kPa 262.4 to 38.3 kPa 2190.3 to 166.2 kPa 40.2 kPa 2276.4 to 80.1 kPa 2104.2 to 252.4 kPa 2217.6 to 217.6 kPa 17.9% 210.1 kPa 21.6 kPa 256.3 to 36.2 kPa 2173.8 to 153.6 kPa 36.9 kPa 2252.9 to 74.5 kPa 294.7 to 232.8 kPa 2198.9 to 198.9 kPa 12.8% Bias Standard error bias Confidence interval bias Limits of agreement Standard error limits of agreement
20.054 cm 0.0397 cm 20.132 to 0.024 cm 22.8 to 2.7 cm 0.068 cm 22.9 to 2.6 cm 22.7 to 2.8 cm 23.4 to 3.4 cm 16.3%
PPT Left: Handheld PPT Right: Handheld PPT Left: Biaxial PPT Right: Biaxial VAS Absolute Reliability Parameters
Table 2
Absolute and relative reliability parameters of VAS and PPT (PPT: left and right, biaxial vs handheld algometry) scores
Biaxial Pressure Algometry
Moreover, it was shown that deep tissues are more effectively stimulated via strain when the rounded probes are used [15], and it is assumed that this roundness reduces the probe–skin contact friction during rotation as opposed to the conventional flat probes. Thus, it is speculated that the increase in rotational travel angle alongside the rounded probe is more effectively increasing strain on the deep structures. Such mechanism is perhaps what is underlying the effectiveness of manual finger palpation performed by the physicians [3,5,33,34], where the tissue is stimulated multidirectionally with varying intensity. The rotational stimulation paradigm could represent the next step in computerized pressure algometry to become more comparable with standardized palpatory movements. Radial Vibration Stimulation During radial vibration stimulation, fanning probe showed to be just as painful as the regular probe for the same stimulation intensity, indicating less pronounced probe geometry effect relative to rotational stimulation paradigm. Due to the limited angle of vibration, spatial summation is probably less efficient compared with the larger angle rotations. Regardless of the PPT-related stimulation intensity, basic stimulation is generally perceived as less painful than the 5- and 15-Hz stimulation frequency and as painful as 25-Hz stimulation. In contrast to radial stimulation at the suprathreshold level, it seems that linear (axial) vibration at the same intensity is perceived as less painful relative to its respective baseline. This contrasting finding between radial and axial vibration could be due to the fact that radial vibration is effectively a mixture of rotation with a small travel angle (5 ) and vibration due to its stimulation frequency (5, 15, 25 Hz). Therefore, it is possible that the phenomenon of spatial summation is present (rotation), as well as the 2101
Adnadjevic and Graven-Nielsen competing effect of an inhibitory mechanism due to vibration (down pressure). Reliability of PPT Assessments Prior studies have investigated handheld PPT test–retest reliability in different muscles and joints over longer periods of time; PPTs were highly consistent and repeatable over the 4 days of testing with ICC > 0.94 [35]; in knee osteoarthritis patients, a reliability study showed that this method was the most reliable tool in QST battery [36]. PPTs assessed via the biaxial pressure algometer yielded a high ICC. Unidirectional (linear) computerized algometers using indentation rather than a forcefeedback drive mechanism as in this study were reported to have somewhat lower relative reliabilities relative to the handheld ones used in the same study [37]. Similar to a previous study investigating absolute reliability [37] where handheld algometry exhibited 5.3% lower CoV relative to the unidirectional computerized algometry (15.6%), in this present study, CoV of the handheld algometer was 1.9% lower than that of the biaxial algometer (15.4%). Past studies have additionally used LoA to assess absolute reliability for QST between two test occasions [38]. Other studies used LoA to make comparisons among handheld algometer, pneumatic one, and gear-driven, computer-controlled algometer to assess whether the new method is up to par with the previous ones [39]. A recent study [4] confirmed that handheld pressure algometry shows a small trend for test–retest differences to become greater with higher PPT measurements, suggesting that measures may be less precise in the higher range of the PPT values, which could be an inherent feature of pressure algometry. Additionally, handheld intrarater reliability reflected by LoA rested in the 2100 to 100 kPa range, whereas in this study, it averaged 2157.2 to 158.2 kPa for left and right leg. The difference could be attributed to a test–retest time difference (same day vs at least 1 week). Biaxial and handheld PPT LoA have comparable 95% confidence intervals and systematic biases suggesting validity of the clinically accepted method (handheld) and the repeatability of the challenging method (biaxial). However, it is worth noting that the average of three measurements per three sites per muscle was taken, therefore potentially contributing to the comparability and thus the stability for each of two methods used in terms of the outcome measure. Reliability of VAS Scores The ICC of 0.77 for the averaged VAS scores represents a “substantial agreement” [23]. Low systematic bias suggests that the combined pain responses could be considered reliable. Whether the LoA interval is clinically/research-wise acceptable is to be determined by 2102
the intended use of the new biaxial algometer in the future experimental procedures. A certain amount of caution needs to be exercised when stating that 95% of the differences lie in this interval as the finding is pertinent to this particular experimental design. Conclusions This study presented a novel biaxial pressure algometer that may be used to quantify tenderness in the musculoskeletal structures via standardized pressure stimulation with simultaneous linear and rotational stimulations; it has also revealed a good relative and absolute reliability of this methodology via test–retest paradigm. Different kinds of pressure stimulations onto superficial and deep structures were demonstrated to elicit more pronounced (radial vibration, rotation) or reduced (axial vibration) pain perceptions compared with ordinary pressure stimulation. The novel pressure stimulations with higher efficacy could supplement musculoskeletal QST methods contributing to a more standardized assessment techniques and more knowledge on basic musculoskeletal pain mechanisms. Stimulation parameters examined in this study such as fanning probe rotation and precise control of the stimulus duration, intensity, and frequency could be pivotal in optimal assessment of pain hypersensitivity. Further, these stimulation parameters could complement methods used for detecting tender spots in muscular tissue in a more robust and standardized manner, which is important for quantifying the level of muscle soreness during rehabilitation for instance, thus aiding in patient treatment.
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