Pressure algometry over normal muscles. Standard values, validity and reproducibility of pressure threshold

115

Pain, 30 (1987) 115-126 Elsevier

PAI 01056

Pressure algometry over normal muscles. Standard values, validity and reproducibility of pressure threshold Andrew A. Fischer Rehabilitation Medicine Service, Veterans Administration Medical Center, Bronx, NY 10468 (U.S.A.) (Received 20 May 1986, revised received 26 November 1986, accepted 1 December 1986)

Pressure threshold is the minimal pressure (force) which induces pain. The pressure threshold meter (PTM) is a force gauge with a rubber disc of 1 cm2 surface. The instrument has been proven to be useful in clinical practice for quantification of deep muscle tenderness. Trigger points, fibrositis, myalgic spots, activity of arthritis as well as assessment of sensitivity to pain can be diagnosed by PTM. This study therefore established standards for pressure threshold as well as the reproducibility and validity of measurement in 24 male and 26 female normal volunteers at 9 sites. Muscles frequently afflicted by trigger points were examined. The deltoid was chosen as a reference since it is rarely a site for trigger points. Comparison of corresponding muscles on opposite sides failed to demonstrate significant differences (except for 1 muscle in females). These identical results obtained over muscles of opposite sides proved the excellent reproducibility and validity of pressure threshold measurement. Results serve as a reference for clinical diagnosis of abnormal tenderness and for documentation of treatment results. The sensitivity of individual muscles varies. Therefore the results presented should be kept in mind when diagnosis of pathological tenderness by palpation is attempted.

Summpry

Key wordsr Pain; Muscle; Pressure

Introduction Pressure algometry (dolorimetry) has been used for evaluation of sensitivity to pain [7,11,12], and the assessment of pressure perception (Head quoted by Keele [7]). These results were obtained by a pressure (force) gauge attached to a rubber plunger with a OS-cm diameter. Through clinical experience with pressure measurement in myofascial trigger points the author concluded that a plunger surface larger than 0.5 cm diameter (0.196 cm*) was necessary for transmission of the pressure

Correspondence to: Andrew A. Fischer, Rehabilitation Medicine Service, Veterans Administration Medical Center, 130 W. Kingsbridge Road, Bronx, NY 10468, U.S.A. 03063959/87/$03.50

0 1987 Elsevier Science Publishers B.V. (Biomedical Division)

116

(force) into the deep tissues [3]. Therefore the pressure threshold meter has been introduced, consisting of a rubber disk plunger with a l-cm2 surface (diameter = 1.12 cm) and gauge with a range of 11 kg [3]. Pressure algometry proved to be useful in evaluation of fibrositis [2,13,14] and hypersensitive spots, trigger points [3,5,8,15], activity of arthritis [lo] and visceral pain-pressure sensitivity [18]. Pressure threshold is defined as the minimum pressure which induces pain or discomfort. The purpose of the present study was to establish normal values for the pressure threshold meter over muscles which are frequently afflicted by trigger points. The reproducibility and validity of pressure threshold measurements were also evaluated. The results can serve as a reference for clinical diagnosis of tender spots, trigger points, fibrositis, myalgic spots, etc. It was established that pressure sensitivity varies over individual muscles and differs in upper and lower body. These variations in pressure sensitivity are evidently important for the clinical diagnosis of pathological local tenderness on palpation.

Material and methods The pressure threshold meter

The pressure threshold meter (PTM, Fig. l), commercially available through Pain Diagnostics and Thermography, I7 Wooley Lane East, Great Neck, NY 11021, U.S.A.), is a force gauge fitted by a rubber disk with a surface of 1 cm2. A known force (pressure) can be applied on the body through this rubber disk. The gauge is calibrated in Newtons (N) or in kg/cm2, with a range to 11 kg and 100 g divisions. A gauge with identical plunger surface but calibrated to 17 kg and with 200 g divisions is being used for pressure-pain tolerance measurement, i.e., the maximum pressure tolerated [4]. The latter expresses sensitivity to pain [4]. The PTM (Fig. 1) consists of the body, from which emerges a metal rod with a male thread on the end. The rubber disk is screwed onto the rod. Pressure exerted on the rod is transmitted to the body and moves the indicator needle in a clockwise direction. Pressing the zeroing knob returns the indicator needle to zero after each measurement. The indicator needle remains at the achieved force value until the zeroing knob is pressed (maximum hold function). This allows the reading to be made after removal of the meter from the body. Note that on the PTM employed it is not necessary for the indicator to return precisely to zero. Lack of return precisely to zero will not affect the reading on the gauge.

Subjects examined for normal values included vohrnteers recruited from hospital employees, volunteer workers and students in the Veterans Administration Medical Center at Bronx, NY. On screening all subjects stated that they were healthy and had no complaints of pain in any part of the body at the time of the examination or in the preceding period. Twenty-four males (average age 35.9 years, range 22-63)

Fig. 1. Pressure threshold meter.

and 26 females (average age 28.6 years, range 21-57) were evaluated for control values. Eight areas frequently afflicted by trigger points were selected for measurement of normal values. Fig. 2 shows the sites of measurements which included mm. teres major, upper trapezius, levator scapulae, supraspinatus, infraspinatus, pectoralis, gluteus medius and paraspinals at the L4 level, 2 and 4 cm from the midline. The middle deltoid muscle was measured for normal reference since usually it is not affected in patients with neck or lower back pain. Method

Each subject was asked to sign a standard consent form. All testing was done during the hours of 8.00 a.m.-4.30 p.m. All measurements were done with the subject prone, the only exception being the m. pectoralis major which was performed in sitting position. The measurement consisted of 3 steps. First the subject was instructed, “say ‘yes’ when you start feeling pain or discomfort.” Then the rubber tip of the PTM was placed on the point to be examined, the shaft vertical to the examined surface. Pressure was increased at approximately 100 g/set continuously. When the subject said, ‘yes,’ the pressure was stopped and the meter was removed from the skin.

4 finger width from midline, L4 level

Gluteus medius

Fig. 2. Sites of pressure

threshold

measurement

in normal

control

subjects.

Since all subjects were right-handed, there was no need to distinguish according to dexterity.

Result.9 Pressure threshold in normal subjects The results are presented in Tables I-IV. The measurements were performed by a pressure gauge calibrated in Newtons. The results were converted into kilograms according to the formula kg = (N X 0.102 kg)/N. Since the recently available meters are calibrated more conveniently in kilograms, the main data are presented in metric units. Table I shows mean values and standard deviation of pressure threshold over examined locations in males and females. The figures are average values obtained from the right and left sides. The presentation of bilateral results in one figure was decided upon because the side-to-side contrast for females and males (Table II) showed that only 1 muscle, the m. infraspinatus, showed a significant difference (in females, P < 0.05).

119 TABLE I PRESSURE THRESHOLD IN NORMAL PERSONS (kg/cm*)

Upper trapezius Pectoralis major Levator scapulae Teres major Supraspinatus Gluteus medius Infrasp~atus Middle deltoid ParaspinaIs L 2 cm Paraspinais L 4 cm Force in kg = * P c 0.05. ** PiO.01.

Males

Females

Muscle

Mean

SD.

Mean

S.D.

3.7

1.9

4.6 4.2 4.6 6.5 5.4 5.1 6.1 6.8

1.9 1.5 2.2 2.8 2.8 2.3 2.4 3.0

5.4 5.4 5.6 6.4 6.7 6.8 7.3 7.7 8.8 9.0

2.8 2.4 2.2 2.3 3.0 2.7 2.8 2.7 2.4 2.7

Difference between males and females 2.5 * 2.2 * 3.9 ** 2.8 * * O.SNS 2.4 * 3.6 ** 3.9 ** 2.6 *

Newton X 0.102 kg Newton .

Table III shows the differences between individual muscles in females and males respectively. Significant differences include the following: the lumbar paraspinals, both lateral and medial, mm. glutei and to a lesser degree the deltoid and infraspinatus muscles were dist~guished by a higher pressure threshold than the other muscles tested. This difference can be expected from clinical experience. Most of the muscles examined, however, fall in the same range of pressure threshold. Figs. 3 and 4 demonstrate the incidence of identical findings on the y-axis and the obtained values on the x-axis. Only 2 muscles - the supraspinatus in females TABLE II PRESSURE THRESHOLD (kg/cm2) OVER RIGHT AND LEFT CORRESPONDING AND STATISTICAL SIGNIFICANCE OF THE DIFFERENCE Muscle

Upper trapezius Pectoralis major Levator scapulae Teres major Supraspinatus Gluteus medius Infraspinatus Middle deltoid Paraspinals L 2 cm Pamspinals L 4 cm (All) * P < 0.05.

Males

MUSCLES

Females

Right

Left

5.3 5.3 5.5 6.3 6.5 6.7 1.2 1.6 8.6 8.8

5.0 5.4 5.8 6.4 6.5 7.0 7.0 1.6 7.8 8.4

(6.8)

(6.7)

1

Right

Left

t

0.92 NS 0.92 NS 1.66 NS 0.71 NS 0.00 NS 1.83 NS 0.94 NS 0.04 NS 0.26 NS 1.84 NS

3.6 4.1 4.5 _

3.7 3.9 4.5

0.06 NS 1.34 NS 0.02 NS

4.5 6.4 5.3 5.1 6.0 6.1

4.5 6.0 4.8 4.7 5.1 6.4

0.16 NS 1.62 NS 2.14* 1.38 NS 1.16 NS 1.28 NS

(5.1)

(4.9)

III

SIGNIFICANCE

F

** _

** ** ** _

NS NS NS NS

NS NS NS NS NS _

NS NS * **

medius

NS NS NS ** * * ** **

F

trapezius

M

Gluteus

NS NS NS NS NS * ** ** **

M

* P < 0.05. ** P
Upper trapezhs Pectoralis major Levator scapulae Teres major Supraspinatus Gluteus me&us Infraspinatus Middle deltoid Paraspinals L 2 cm Paraspinals L 4 cm

Upper trapezius Pectoralis major Levator scapulae Teres major Supraspinatus Gluteus me&us Infraspinatus Middle deltoid Paraspinals L 2 cm Paraspinals L 4 cm

OF DIFFERENCES

n = 26.

Upper

M = males, n = 24; F = females,

STATISTICAL (RIGHT)

TABLE

NS * * NS NS NS

NS NS NS NS

*

* * * NS NS NS _

F

_ _

_ _

_ _ _

F

major

M

Infraspinatus

NS NS NS NS NS NS * ** ** **

M

Pectoralis

IN PRESSURE

NS NS **

NS NS NS * ** *I **

NS NS NS NS NS _ NS *

NS NS

_

*

F

** NS NS NS NS _

** **

M

Middle deltoid

NS NS * **

F NS _ _

NS NS _

scapulae M M

NS

* NS _

** ** ** *

NS

NS NS NS

_ * ** *

**

* NS NS _

** ** ** ** ** **

M ** **

L4cm

NS NS NS ** **

NS NS NS NS _

L2cm F

NS NS ** **

NS **

NS _

NS _

SIDE

NS _

NS NS *

_ ** ** **

**

F

NS NS * **

NS NS _ **

NS _

F

OF THE

Supraspinatus

ON ONE

M

Paraspinals

NS NS NS NS ** **

NS NS NS _

F

MUSCLES

Teres major

OF INDIVIDUAL

M

Levator

THRESHOLD

BODY

g

121 SUPRASPINATUS-FEMALES

IO

oRlGHT

.LEFT

8

2-

L

I

20

40

PRESSURE

60

I

1

80

I

1

100

THRESHOLD-NEWTONS/Sq

I

120

cm

Fig. 3. Pressure threshold distribution over the supraspinatus muscle in females.

UPPER TRAF’EZIUS-MALES

IO -

*LEFT

oRlGHT

6w ii6 % 4 4-

I

I

20

40 PRESSURE

60

SO

100

THRESHOLD-NEWTONSlSq

I20

140

cm

Fig, 4. Pressure threshold distribution over the upper trapezius muscle in males.

(Fig. 3) and the upper trapezius in males (Fig. 4) - are presented in order to demonstrate the character of distribution. Similar distribution patterns were characteristic for all muscles measured. The important feature of the observed distribuTABLE IV PRESSURE THRESHOLD IN NORMAL PERSONS (kg/cm2) Muscle

Teres major Upper trapezius Levator scapulae Supraspinatus Infraspinatus Middle deltoid Pectoralis major Gluteus medius Paraspinals 2 cm Force in kg =

Average value

84.1%

Male

Female

Male

Female

Male

Female

Male

Female

6.0 4.8 5.2 6.0 6.9 1.3 5.1 6.4 8.0

4.0 3.3 4.2 4.2 4.8 4.8 5.9 5.7

4.1 2.9 3.6 3.9 4.6 5.1 3.3 4.3 5.6

2.7 2.0 2.7 2.8 3.0 3.1 3.7 3.8

2.9. 1.8 2.5 2.6 3.1 3.6 2.5 2.8 3.9

1.8 1.3 1.8 1.8 1.8 2.0 2.4 2.5

2.0 1.1 1.7 1.7 2.0 2.5 1.4 1.9 2.7

1.2 0.8 1.2 1.2 1.1 1.3 1.5 1.7

Newton x 0.102 kg Newton

97.7%

99.5%

122

tion is that the largest number of values fall in the middle of the scale - between 30 and 60 N with lower incidence at higher values. This distribution does not follow a bell shape since some measurements spread into higher levels, shifting the average value upward. Because of this uneven distribution, standard deviation from the average cannot be used to establish the lowest cutoff point for normal limits. In order to eliminate the error induced by the uneven spread of distribution favoring higher pressures, natural logarithmic values were calculated with standard deviations. The average log~t~c values and standard deviations were then converted back to a linear scale which is usable for clinical reference. Table IV presents the average pressure threshold in kg/cm2 obtained by the above method. The 84.1% seems to be the proper secure cutoff for a clinical use, indicating the lowest normal pressure threshold. Therefore the column under 84.1% should be used for clinic& norms.

Discussion Nomenclature and use of pressure gauges Pressure gauges used for evaluating pain were called dolorimeters [lo], algometers f7], algesiometers, etc. (Dorland’s medical dictionary). Dolorimeter or algometer implies the use for measurement of pain, The author considers ‘pressure threshold meter’ and pressure threshold measurement more accurate descriptions for the following reasons: (1) The point of measurement is defined exactly. Threshold expresses the minimum stimulus inducing pain. Tolerance refers to the maximum intensity of stimulus tolerated. (2) The expressions dolori- and algometer not only fail to specify the point of measurement but also the kind (heat, electric stimulus, mechanical, etc.). Pressure threshold or tolerance describes exactly the modality used as well as the diagnostic criterion, The device can also be used for the measurement of pressure tolerance, i.e., the maximum pressure tolerated: however, a higher-range gauge, extending to 17 kg, is often needed for this [S]. Patients also resent the use of higher pressure in hypersensitive areas because it induces intense pain. Pressure threshold over tender spots was found to be more accurate, more reproducible and more informative than pressure tolerance [3]. Therefore for all practical reasons ‘pressure threshold measurement’ seems to be the best description of the method.

Discussion of results The results are interesting from several points of views. (a) ~e~j~ilj~ of pressure t~res~ald rne~~r~rnent. Analysis of the results obtained from contralateral ~rrespon~ng spots showed that only 1 muscle, the m. infraspinatus in females, showed a significant difference and at the P -c 0.05 level only. Excellent reproducibility and reliability of the pressure threshold measurement were

123

demonstrated by the fact that there was no significant difference with the contralateral findings. The results are consistent with previous reports, in which other areas of the body have been examined [lo-12,151. The reliability of the pressure algometer between different observers was assessed in schizophrenics by Merskey et al. [ll] and in normal subjects by Merskey and Spear [12]. The latter study showed that the pressure at which a subject first describes awareness of pain (threshold) correlates well between different observers, occasions and sites [12]. The study concluded that pressure pain has a degree of reliability which makes possible its quantitative use for the investigation of emotional states, anesthesia and analgesia in spite of the fact that the pain threshold report depends a great deal upon such factors as attitude, sex and cultural role. This study noted that the reliability is particularly good where each individual serves as his own control, since the scatter of readings taken in each person tends to be less than that of readings taken in several people [12]. Reeves et al. [15], using the same pressure threshold meter employed in this study, demonstrated a high degree of validity and reliability of the instrument in the detection of myofascial trigger points in temporo-mandibular muscles. (b) Differences in pressure threshold between males and females. Females demonstrated lower pressure thresholds than males in every muscle (Table I). The differences between the sexes were significant in all muscles except the m. gluteus medius. In addition, the relative sensitivity of some muscles differs in the two sexes. For example, the difference between females and males in the upper trapezius muscle was minimal (2.7 and 2.9 kg) while in the m. teres major females showed significantly lower values: 2.7 kg vs. 4.1 kg in males. (c) Differences in pressure thresholds of individual muscles. The pressure threshold shows some significant differences between individual muscles. The muscles examined in this study fall into 3 groups according to their sensitivity. The most sensitive muscle is the upper trapezius, as shown in Table I. The rest of the examined muscles fall into 2 groups: the shoulder blade muscles with an average sensitivity of 2.7 kg in females and 3.9 kg (3.6-4.1) in males include the mm. levator scapulae, supraspinatus and teres major. The lower back muscles (lumbar paraspinals, m. gluteus medius with 3.7 kg in females and 5.6 and 4.3 kg respectively in males) represent a group of less sensitive muscles. Close to this group are the lowest shoulder blade muscle, the infraspinatus (3.0 kg in females and 4.6 kg in males) and the deltoid (3.7 kg in females and 5.1 kg in males). In summary, the lumbar paraspinals and the gluteus medius muscle have the highest pressure threshold in both sexes. As a general rule muscles situated in lower parts of the body are less sensitive and are characterized by a higher pressure threshold. The shoulder blade muscles are more sensitive, except for the m. infraspinatus, which is closer to the average of the lower back muscles than to that of the shoulder blade area. The described differences in pressure threshold, the expression of the pressure-pain sensitivity of individual muscles, are significant. This information should be kept in mind when assessing the pathological degree of tenderness. This is equally true when using pressure threshold measurements or when relying on the subjective method of palpation.

124

Clinical use of pressure algometry

Several authors used pressure (force) gauges attached to a plunger for the measurement of general pain sensitivity in normal tissues. Head employed algometry in studying the thalamic syndrome (quoted by Keele [7]). Keele [7] established normal values over the forehead. He used a plunger of 0.5 cm diameter and the gauge was calibrated to 7.7 kg. Fischer [3,4] introduced the pressure tolerance meter with a rubber disk of 1 cm2 calibrated to 17 kg and established normal values over the shin bone and deltoid muscle. Normal muscle tolerance was found to be higher than that of bone [4]. A reversal of the muscle-bone tolerance ratio may be indicative of diffuse muscle tenderness. Such findings raise a suspicion of a muscle disorder frequently caused by endocrine (thyroid or estrogen) deficiency. When the pressure threshold is low over all muscles, the muscle-bone tolerance ratio is helpful in diagnosing generalized muscle hypersensitivity [4]. The use of pressure algometers described above was focused on the assessment of general sensitivity to pain in normal tissues. The goal of this investigation was to establish normal values to be used for the diagnosis of hypersensitive spots located in muscles. McCarthy et al. [lo] used a lo-lb gauge for determining the inflammatory activity of rheumatoid arthritis in hand joints by quantification of articular tenderness (pain threshold). They found that the instrument provided precise information for computing Lansbury’s ‘articular index’ [9] when used to quantitate tenderness in subcutaneous small joints. The authors were able to determine more quantitatively the initial degree of involvement in arthritis and subsequent spread of disease activity [lo]. Pressure threshold has been used successfully for evaluations of inflammation in animals. Hyperalgesia showed a high negative correlation with logarithm of antiinflammatory medication (r = 0.983) [l]. Similarly, pressure threshold correlated well with sedimentation rate and mood in arthritics [13]. Tenderness assessed by pressure threshold was found to be the most reliable parameter for grading inflammation in arthritis [9]. Campbell et al. [2] used pressure threshold measurement for evaluating fibrositic tender spots. Measurement of deep visceral tenderness was used successfully in patients with abdominal pain due to pancreatitis, cholecystopathy and duodenal ulcer [18]. A gauge calibrated to 3.6 kg was used. The diseased side showed a lower pressure threshold by 1.0 kg. This improved to 0.2 kg difference as compared to the sound side when pain was controlled by paraspinal anesthesia. Similarly, paravertebral tender points showed a pressure threshold 0.2 kg lower than the healthy side when pain was present. The difference between the two sides decreased to 0.05 kg when the pain subsided [18]. Clinical use of the pressure threshold meter

Several authors confirmed the usefulness of the pressure threshold meter in clinical practice and research. Fischer [3] and Fischer and Chang [S] described the use of PTM for the diagnosis of trigger points and hypersensitive spots and for the evaluation of treatment effects, such as injections, physiotherapy and coolant spray. Kraus [8] also found the PTM useful in the clinical management of trigger points.

125

This expert employs the instrument in every physical examination for the diagnosis of trigger points and their reaction to treatment. Reeves et al. [15] confirmed that the PTM employed in this study is ‘very well suited for assessing trigger point sensitivity in the muscles of the shoulder girdle and upper back.’ The study also established that the PTM is a valid and reliable tool, accurate in diagnosing tender spots and in quantifying their relative tenderness. The clinical uses of the results of this study include the following: Normal pressure threshold values serve to diagnose the presence of pathological tenderness in muscles. Hypersensitive spots are the main diagnostic criterion of myofascial and other trigger points [8,16,17] and of tender spots characteristic of fibrositis [2,13,14]. Standards for differences between contralateral areas made it possible to establish additional criteria of abnormal tenderness. Clinical experience showed that a side-to-side difference exceeding 2 kg can be considered abnormal. When the sound side is used as a reference, the patient serves as his/her own control. Comparison with the normal opposite side presents a more precise and reliable criterion of abnormal tenderness than would be gained by comparison with standards obtained from control subjects. In general a muscle pressure threshold equaling 3 kg or less can be considered abnormally low [2,3]. In addition to diagnosing hypersensitive spots and their location the degree of their sensitivity can be established using PTM [3,15]. Pressure threshold expresses the sensitivity and activity of trigger points [15], fibrositic tender spots [2,13,14], arthritis [lo] and inflammation [l]. The course of development of these conditions can be followed. The reaction to therapeutic interventions including analgesic and antiinflammatory medication, physiotherapy, injection and manipulation, can be quantified and documented in clinical practice or research [3]. Exacerbation or recurrence of a painful condition can be diagnosed [3]. Patients’ complaints of pain caused by soft tissue pathology can be corroborated or clarified by pressure threshold measurements. Trigger points are exquisitely tender spots which spontaneously or on stimulation - by pressure, movement or injection - often shoot pain into a remote area called a referred pain zone [16,17]. Each trigger point has a specific referred pain zone. Patients sometimes perceive pain in the referred pain zone but not in the trigger point where the pain originates [16,17]. This fact should be kept in mind when complaints of pain are correlated with pressure threshold values. The knowledge of referred pain zones is therefore important in locating the trigger point which is causing the pain. Corroboration of pressure threshold findings by objective methods

The results of pressure threshold measurement are corroborated by other objective methods. Thermography has objectively demonstrated the presence of trigger points in the form of diskoid-shaped hot spots [3,5]. The pressure threshold measured at these hot spots is significantly lower than over corresponding contralateral normal tissue [5]. Tissue compliance measurement is another extension of physical examination. It objectively and quantitatively documents changes in soft tissue consistency [3].

126

Increased resistance (decreased softness or compliance) in the form of a ‘ taut band is recognized as diagnostic of myofascial trigger points [l&17]. Changes in muscle tone or consistency which are diagnosed by palpation can be documented yuantitatively and objectively by tissue compliance measurement [3]. The tissue compliance meter is a hand-held mechanical instrument which records the depth of penetration achieved by a unit of force applied. Unlike pressure threshold measurement, tissue compliance does not require a patient’s subjective reaction The trigger points manifesting a ‘taut band’ or other alterations in tissue consistency can be corroborated objectively by tissue compliance recordings.

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