Pain-Pressure Threshold in Painful Jaw MusclesFollowing Trigger Point Injection

Anne S. McMillan BDS, PhD,FDSRCPS

Assistant ProfessorDepartment of Clinical Dental

Sciences

Bruce Blasberg DMDAssociate ProfessorDepartment of Oral, Medical, and

Surgical Sciences

Faculty of Dentistry.University of British Columbia.Vancouver. British ColumbiaCanada

Correspondence to:Dr Anne S. McMillanDepartment of Restorative DentistryThe Dental SchoolUniversity of NewcastleFramlington PlaceNewcastle upon TyneEnglandUnited KingdomNE2 4BW

Pain and tenderness at trigger points and referral sites may he mod-ified in subjects with myofascial pain in the head and neck regionhy injecting local anesthetic into active trigger points, but the effectof injection on jaw muscle pain-pressure thresholds has not heenmeasured. The mechanism by which trigger-point injection affectsmuscle tenderness is also unclear and may be related to the "hyper-stimulation analgesia" induced hy stimulation of an acupuncturepoint. A pressure algometer was used before and after an activetrigger point injection in the masseter to measure the pain-pressurethreshold in the masseter and temporal muscles of 10 subjects withjaw muscle pain of myogenous origin. The pain-pressure thresholdin the masseter and temporal muscles was also measured in amatched control group before and after an acupuncture-point injec-tion in the masseter. The pain-pressure threshold was significantlylower in myofascial pain subjects than in control subjects at allrecording sites. Pain-pressure thresholds increased minimally in themasseter after trigger-point injection, whereas the temporal regionwas relatively unaffected. In the control group, the pain-pressurethreshold increased significantly at all recording sites in the mas-seter after acupuncture-point injection. Although local anestheticinjection acts peripherally at the painful site and centrally wherepain is sustained, pain-pressure thresholds were not dramaticallyincreased in myofascial pain subjects, in contrast to controls. Thissuggests that in subjects with myofascial pain, there was continuedexcitability in peripheral tissues and/or central neural areas whichmay have contrihuted to the persistence of jaw muscle tenderness.J OROFACIAL PAIN 1994;8:384-390.

Jaw muscle tenderness ¡s a common clinical finding in patientswith craniomandibuiar dysfunction.'•' in cases of myofascialpain, focal areas of muscle tenderness, coined "trigger points"(TPs), have been observed clinically in the masseter and tempo-

ral muscles; referred pain associated with active TPs has beenmapped to more distant anatomic sires in the craniofacial region.^''Although TPs are palpable as hyperalgesic spots within the muscle,no specific hisropathologic, biochemical, or electrophysiologic phe-nomena have been attributed to them. However, trauma to deepcraniofacial tissues appears to be the most likely cause.*'

Diminished pain and tenderness at TPs and referral sites haveresulted irom injection of local anesthetic (LA) into active TPs,although the reduction in tenderness has not been quantified.' Themechanism by which LA injection reduces symptoms at referralsites is also unclear. When LA is injected diagnostically, the block-ing effect is presumed to occur at the injection site; however, LAinjected peripherally may reach the centrai nervous system (CNS),

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particularly when no vasoconstrictor is used.'' Forexample, Rowbotham and Fields'" have describedpain relief following intramuscular injections inbody regions not anatomically associated with thepainful area.

The distribution of TPs correlates spatially withacupuncture points in the limbs and jaws."Stimulation of these sites produces gradients ofanalgesia."'" Thus it seems possible that LA injec-tion of an acupuncture point in a normal jaw mus-cle may also generally affect somatosensory thresh-olds in the jaw muscles because of the LA effect onthe CNS or the counteritritation effect of theneedle itself,'"

Measurement of the pain-pressure threshold(PPT) with a pressure algometer is commonly usedto quantify TP sensitivity io the jaw muscles.'"'When variables such as the rare of applied pressure,the size of the algometer recording tip, and thedegree of muscle contraction are controlled, therecording technique is sensitive and rehable,'"'*'"

Tbe LA injection of TPs in painful jaw musclesappears to affect the PPT locally and at more dis-tant sites in the jaws.' Therefore, for this study,PPTs were measured in different regions of themasseter and temporal muscles before and afterthe injection of TPs in painful muscles andacupuncture points in normal muscles. Theauthors postulated that thresholds would increasein painful and normal muscles as a consequence ofLA injection aod that a similar mechanism may beinvolved.

Materials and Methods

Twenty female subjects aged 21 to 54 years par-ticipated io tbe study. Subjects were matched forgender and age because PPTs in cranicfacial mus-cles are affected by these variables," Ten suhjectswere recruited from patients referred to theTMJ/facial pain clinic at the University Hospital,Vancouver, BC, for the diagnosis and managementof temporomandibular disorders and craniofacialpain. The subjects were classified as having cra-oiofacial pain of myogenous origin. Diagnosticcriteria for subject selection was based on theInternational Headache Society's Classification ofmyofascial pain," namely continuous, dull pain inone or more jaw muscles, localized tenderness mfirm bands of muscle, and palpation of specifictender areas ("active" TPs) that lead to changes inpatterns of pain referral. There was no radiograph-ie evidence of TMJ pathosis io the subjects withmuscle pain.

The other 10 "control" subjects were dentalhygiene students who had complete natural denti-tions and reported no history of jaw dysfunction,

PPT Recording

Each subject was seated in an upright position in adental chair. The masseter muscle was palpated todetermine the anterior and posterior limits of itssuperficial part. The central point of site Ml waslocated 10 mm posterior to the anterior horder ofthe muscle and 10 mm superior to the inferior bor-der of the mandible. Site M3 was located 10 mmposterior to the anterior border of the muscle and10 mm inferior to tbe lowest point on the zygo-matic buttress. Site M2 was located 10 mm poste-rior ro the anterior border of the muscle, equidis-tant from sites Ml and M3, Site M4 was located10 mm anterior to the posterior border of the mus-cle, equidistant from sites M2 and M3. Site M5was located 10 mm anterior to the posterior bor-der of the muscle, equidistant from sites Ml andM2(Figl).

The anterior temporal muscle was palpated todetermine its anterior border. Site Tl was located10 mm posterior to tbe anterior border of the mus-cle and 10 mm superior to the highest point on thezygomatic buttress. Site T2 was located 10 mmposterior to the anterior border of rhe muscle and15 mm superior to the ceotral point of site Tl, SiteT3 was 15 mm superior to site T2, Site T4 waslocated 10 mm superior to the highest point on thezygomatic buttress and 15 mm posterior to Tl.Site T5 was located 15 mm posterior to site T4

(Fig !)•A pressure algometer with a recording tip

10 mm in diameter (Model PTH-AF2, PainDiagnostics and Thermography Corp, Great Neck,NY) was used to measure PPTs, The instrumenthas been described by Reeves et al,"* Before datawere collected, tbe operatot was calibrated using astopwatch to ensure that a controlled rate of pres-sure (0.5 kg/cm2/s) was applied perpendicular tothe skin overlying the recording sites,'"

The PPT was determined as the point at whichthe pressure stimulus applied to the skin changedfrom a sensation of pressure to pain." Each subjectwas asked to raise her right hand when the pres-sure applied to the recording site changed from asensation of pressure to pain. Then the algometerwas removed from the recording site.

The experiment took place in a dental officewith only the operator and subject present tominimize distraction from extraneous sources.The subject sat upright in a dental cbair witb her

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McMillan

Fig 1 The location of PPT measurement sites in theright masserer (Ml to M5) and temporal (Tl to T5)muscles.

arms resting on her lap. The subject fixed herattention on the test stimulus (algometer),because a change in attention has been shown tomodulate cutaneous sensitivity.^' Pain-pressurethresholds were measured in the right masseterand temporal muscles of control subjects. Inmyofascia! pain subjects, PPTs were measured inthe masseter and temporal muscles on the sideassociated with an active TP. Two PPT measure-ments were made at each recording site. DuringPPT measurement, the operator used his or herhand to exert counterpressure on the contralater-al side of the subject's head.

The order of measurement of PPT recordingsites was randomized." During a rest period of atleast 30 seconds between each measurement, thesubject relaxed her jaw. All sites were measuredwith the subject clenching lightly in the intercuspalposition (approximately 107o maximum voluntarycontraction) because variations in motor activitycan affect sensory thresholds.''-^'

Local Anesthesia

In normal subjects, the acupuncture p- -• "So(Jiache) was located in the right masseti-r Usinganatomic landmarks, the surface marking of thepoint was located on the skin, 15 mm anterior andsuperior to the angle of the mandible. The locationof the point was then confirmed using an acupunc-ture point locator (Model 4, Joanco MedicalElectronics, North Vancouver, BC).

In subjects with myofascial pain, the massetermuscles were palpated to locate an active TP. Thespot was determined on the basis of local andreferred symptoms intensifying on firm palpation.*Its location was marked on the skin overlying themuscle.

After baseline PPT measurements of both con-trol and pain subjects, 0.5 mL of procame (1%)LA without vasoconstrictor was injected percuta-neously into the designated acupuncture point ofcontrol subjects by means of a 25-gauge hypoder-mic needle and disposable syringe. In the myofas-cial pain subjects, the active TP was injected usingthe same protocol. In both groups, the PPT mea-surement procedure was repeated after 4 minutes.

Data Analysis

Mean PPTs obtained from the two stimulus trials ateach recording site, before and after injection, wereused for data analysis. A four-factor analysis ofvariance (ANOVA) was used to compare PPTs fromboth masseter and temporal muscles before andafter injection of LA in an acupuncture or activetrigger point. A two-factor ANOVA was then usedto compare PPTs by region in eacb muscle beforeand after injection of LA in an acupuncture point oractive TP. An independent two-sample í test wasused to compare PPTs in control and myofascialpain subjects prior to injections. A 5% level of sig-nificance was used for all tests. Using confidenceintervals, PPT data from control subjects (beforeinjection) were calculated to permit pairwise region-al comparisons of PPTs (confidence limits, 95%).

Results

In control subjects, there were regional differencesin PPTs in both muscles (P < .0001). Before injec-tion, pairwise regional comparisons revealed thatthresholds at recording areas Ml, M2, and M3;M4 and M5; and T2, T4, and T5 were similar.The magnitude of the PPT measured at eachrecording site increased after injection of LA in the

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Fig 2 Location of TP injection sites ¡n = 10) in themasseter muscles of myofasciai pain subjects.

acupuncture point, and there were statistically sig-nificant differences at all recording sites in themasseter {P < .001), Thresholds in the temporalregion were consistently higher rhan in the mas-serer muscle before and after injection (P < ,0001].Mean PPT values (kg) measured at recording sitesin the right masseter and temporal muscles of con-trol subjects, before and after injection of LA inacupuncture point S6, are shown in Table 1,

PPTs were significantly lower in myofascial painsubjects than controls at all recording sites (P < .0001),The magnitude of the PPT measured at eachrecording site in the masseter increased slightlyafter LA injection in the TP. However, in the tem-poral region only the PPT at site T4 Increased,There were statistically significant differenceshetween PPTs before and after injection at sitesM2 and M3 {,05 >P> .01). PPT data from rightand left masseter and temporal muscles in myofas-cial pain subjects, before and after injection of LAin TPs, are shown in Table 2, Data from right andleft sides were pooled because PPTs obtained bilat-erally in the jaws have been shown to be corre-lated," The distriburion of TP injection sites in themasseter are depicted in Fig 2.

Discussion

PPTs in Painful Muscles

Measurement of the PPT is used as an adjunct inthe diagnosis of myogenic pain and to assess theeffectiveness of clinical treatments such as passivemuscie stretch and intramuscular injections.'' Inthe present study, the PPTs in subjects wirh musclepain were markedly lower than those in normal

Table 1 Pain-Pressure Threshold (PPT, mean ± 1SD) m the Right iVIasseter (M) and Temporal (T)Muscles of Control Subjects (n = 10) Before andAfter Acupimcture-Point Injection

Recurdingsite

M l

U 2M3M4

rvi5

Tl

T2T3T4T5

Before

1,8 ± 0 41.7 ± 0.41.8 ± 0.42 1 + 0 52,1 ± 0.5

2,5 ±0.52 8 ± 0.63.2 ± 0.72.7 ±0.429 + 05

PPT (kg)

After

2,2 ±0,3 '2.1 ±0,4'2,2 ± 0.5'2.5 ±0 5'2.7 ±04*

2.6 ± 0.42.9 ±0.53.5 ±0,82.9 ± 0.43 1 ± 0.6

l lest. ANOVA, P < .001.

Table 2 Pain-Pressure Threshold (PPT, mean ± 1SD) in Masseter (M) and Temporal (T) Muscles ofMyofascial Pain Subjects Before and After Trigger-Point (TP) Injection

Recordingsite

MlM2M3M4M5

TlT3T3T4T5

Before

0,5 ± 0,20.4 ±0,10.5 ± 0.20.5 ±0.20.6 ± 0 2

0,7 ± 0,2O.S ± 0,40 8 = 0.207 ± 0 30.8 ± 0 3

PPT (kg)

After

0.6 + 0.20 5 ± 0.2'0.6 ±0 3 '0 7 ± 0,40.7 ± 0,3

0,7 ±0,30,8 ± 0.3O 7 ± O 3o a ±0.308 ±0.4

'Stalistical lesf ANOVA. 05 > P> .01

jaw muscles, Ohrbach and Gale"" and Reeves et al'"have described similar findings in subjects withmyofascial pain affecting the jaws. The reason forthe decrease in pain threshold is still unclear, butwhen muscles suffer mechanical or chemical trau-ma, pressure stimuli of innocuous intensity appliedover the affected site result in muscle pain.*-'"' Thehyperalgesia is thought to be a consequence of sen-sitization of muscle nociceptors and other low-threshold mechanoreceptive endings so rhat weakstimuli elicit pam. The sensitization is most likelydue ro the release of endogenous substances, suchas prostaglandin H2 and hradykinin, from dam-aged tissue. This release lowers the mechanicalthreshold of nociceptivc endings into the innocu-

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Otis range.'' Any increase in tnput from peripheraltissues SLich as tnuscles appears to increase theexcitability of trigeminal and spinal nociccptiveneurons."-' These netirons respond to both deepand cutaneous afferent input. Hu et al' haveobserved that stimulation of rat muscle afférentsby the small-fiber irritant mustard oil appears tofacilitate cutaneous afferent input ro nociccptivebrain stem neurons. It also causes an expansion ofcutaneous mechanoreceptive fields with the poten-tial effect that a stimulus applied to a hyperalgesicarea is likely to result in greater afferent input tothe hrain stem than a comparable stimulus appliedto a normal area. This mechanism may account, atleast in part, for the tenderness and pam referralpatterns that were associated with the myofascialpain subject group in the present study.

the combined activation of low-threshold iirimaryafférents and small-diameter nociceptive primaryafférents, which provide a type of counterirritattonand thus inhibit the central transmission of noci-ceptive information (reviewed in Bushneil et aP).Buíihnell et al'" have shown that the pain thresholdcan be altered significantiy in musculoskeletal tis-sues by acupuncturelike transcutaneous electricalnerve stimulation. It is also possible that the LAinjection had a nonspecific pain-teducing effectbecause of its peripheral and central effects on sen-sory-discriminative pathways."• '•"

The location of active TPs in the masseter didnot appear to be closely correlated with the dis-position of known acupuncture points; however,the sample size was small compared with previousstudies."•'-

Effect of LA on PPTs

The LA injection of TPs has been cited as a meansof reducing muscle tenderness locally and at refer-ral sites.''" A minor increase in the PPT throughoutthe masseter muscles of myofascial pain subjectsafter injection was found in the present study, burthe PPT was relatively unaffected in the temporalregion. There are a number of reasons why therewas only a minimal change in the PPT at moredistant anatomical sites. Travell and Simons' havedescribed how referred pain may, in some in-stances, become more intense when the TP is pene-trated by a needle; during the injection itself, theincrease in pain may activate "latent" TPs, result-ing in a decrease in the PPT at referral sites. It isalso possible, although unlikely, rhat the TP wastnissed during injecrion, leading to a diminishedLA effect.' Local anesthetic is used to treat musclepain because of its peripheral action ar the painfulsite and because it can act centrally where the painis thought to be sustained.-'-' However, it has beenshown in the limbs that LA administered at theinjury site does not necessarily decrease theexcitability of the spinal cord to preinjury levels."Thus it is possible that in the subjects of thepresent study, although the local site was blocked,there may have been continued excitability intrigeminal neurons in the brain stem, which led topersistence of the pain.

There was a generalized increase in the PPTafter the LA injection of an acupuncture point inthe masseter muscles of normal subjects. This wasnot unexpected because stimulation of acupunc-ture points by needling can produce sustainedchanges in pain thresholds." Such "hyperstimula-tion analgesia" is thought to be a consequence of

General Obsetvations on PPTs

During isometric exercises such as clenching of theteeth, somatosensory thresholds may be altered ifpain is associated with the motor task."-'*However, in the current study, control subjectsreported no discernible discomfort during lightclenching, and myofascial pain subjects reportedno additional discomfort as a consequence of thetask. Such innocuous conditioning stimuli do notappear to inhibit pain in humans."

Pain-pressure thresholds in rhe temporal regionwere higher than in the masseter muscles of nor-mal and painful muscles, irrespective of theanatomic location ofthe recording site. These find-ings concur with previous observations in thejaws'^'' and may be due in part to the differentdensity of connective tissue tendon in the anteriortemporal muscle compared with rhe masseter. " Incontrol suhjects, there were regional differences inthresholds in both muscles, which has been shownpreviously by Ohrbach and Gale'' and McMillanand Lawson,'' although there was some variationm the distribution of anatomic regions with similarthresholds. In the current study, values for PPTs inthe temporal region were similar to those mea-sured by McMillan and Law son; however, PPTs inthe masseter were lower than those measured hyMcMillan and Lawson." These results may havebeen due to gender differences in craniofacial mus-cle PPTs.' The PPTs found in the current studywere consistently lower than those described byOhrbach and Gale"; although they measured PPTsin both males and females, and it is uncertainwhether they controlled the level of muscle con-traction during recordings. Both of these variablescould account for the differences in PPTs.

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Acknowledgments

The authors wish to thank Mr Ping Ma of ihe StatisticalConsulting Service at the University of British Columbia. Thissrudy was supported by the Medical Research Council ofCanada.

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Obrbach R, Gale EN. Pressure pain thresholds in normalmuscles: Reliability, measurement effects, and topographicdifferences. Pam 1989b;37:257-263.Graff-Radford SB, Reeves JL, Baker RL, Chiu D. Effectsof transcutaneous electrical stimulation on myofascialpain and trigget point sensitivity. Pain 1 9 8 9 Í 3 7 : 1 - 5 .McMillan AS, Lawson E. Effect of tooth clenching andjaw opening on pain-pressure thresholds in the human jawmuscles. J Orofacial Pain 1994 ;8T250-257 .Jensen R, Rasmussen BK, Pedersen B, Lous I, Olesen J.Cephalic muscle tenderness and pressure pain threshold ina general population. Pain 1992;48:197-203.American Academy of Craniomandibuiar Disorders.McNeill C (ed). Craniomandibuiar Disorders; Guidelinesfor Evaluation, Diagnosis, and Management. Chicago:Quintessence, 1990:18.

List T, Helkimo M, Karlsson R. Influence of pressure rateson the reliability of a pressure threshold meter. JCraniomandib Disord Facial Oral Pain 199];5:I73-178.Bushneli MC, Duncan GH, Dubner R, Jones RL, MaixnerW. Attentional influences on noxious and innocuous cura-neous heat detection in humans and monkeys. J Neurosci1985;5:11O3-1I1O.

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Resumen

El umbral de presión-dolor en los músculos mandibularesdolorosos luego de una inyección en puntos deestimulación

El dolor y ia sensibilidad en los punios de estimulación y en iossitios de reFerencia pueden ser modificados en ias personascon dolor miofaciai en la cabeza y la región cervical por mediode la inyección de anestésico local en puntos de estimulaciónactivos, pero no se ha determinado el efecto de lal inyecciónsobre los umbrales de presión-dolor de los músculos mandibu-lares. El mecanismo por ei cual la inyección en los puntos deeslimuiación afecta la sensibiiidad muscular no esta ciaro ypuede ser relacionado a ia 'analgesia por hiperestimulaciór,"inducida por la estimuiación de un punto de acupuntura. Seuliiizó un algómetro de presión antes y después de reaiizar unainyección en un punto de estimuiación activo en ei maseteropara medir ei umbrai de presión-doior de tai múscuio y de lostemporales, en 10 sujetos afectados por dolor muscuiarmandibular de origen miógeno El umbrai de presión-dolor en elmúsGuio masetero y en los temporaies también fue medido enun grupo de control correspondiente, antes y después de lainyección en un punió de acupuntura del masetero. El umbral depresión-dolor fue significativamente menor en el grupo quepadecía doior miofacial que en ei grupo de controi En todos lossitios de registro, los umbrales de doior aumentaron como míni-mo en ei masetero iuego de la inyección en ios puntos deestimuiación, mientras que la región del temporal ro fue afec-tada relativamente. En ei grupo de controi, ei umbrai de presión-doior aumentó significativamente en todos ios sitios de registrodespués de la inyección en un punto de acupuntura Aunque iainyección con anestésico locai aclúa periféricamente en ei sitiodoioroso y centraimente donde ei doior es aiimentado, iosumbraies de presión-dolor no fueron aumentados dramática-mente en ei grupo experimental en comparación con el grupode control. Esto indica que en ios sujetos con dolor miofacial,existía una excitabilidad continua en los tejidos periféricos y/oen ias áreas nerviosas centraies las cuales pueden inaber con-tribuido a la persistencia de la sensibiiidad de ios músculosmandibulares.

Zusammenfassung

Druckschmerzschwelle schmerzhafter Kiefermusi^elnnacb Injektion im Triggerpunkt

Schmerz und Empfindliciikeit in Triggeipunkten und Arealen mitübertragenem Schmer; können wohi bei Personen mitmyofasiiaiem Schmerz im Kopf und in der Nackenregion durchinjektion von Lokalanastheükum m aktive Triggerpunkte gelin-dert werden, aber die Auswirkung einer injektion auf dieDrucksobmerïschwelle der Kaumusiieln ist nicfit untersuchtworden. Der Mechanismus, durch den eine Triggerpunktinjek-tion die Empfindlichkeit der Muskuiatur beeinfiusst, ist unifiarund könnte verwandt sein mit der " iHyperstimulations-Anaigesie," weiche bei Stimulation eines Akupunkturpunktsinduiiert wird. Die Druckschmerzschwelle der Mm Masseterenund Temporales von 10 Patienten mit myofasziaienKaumuskeischmerzen wurde mit einem Druckdoiorimeter vorund naoh injektion in einem aktiven Triggerpunkt im Míviasseter gemessen, Dre Drucksohmerzsohwelie in den Mm,Masseteren und Temporales wurde auch in einer paraileiisiertenKontrollgruppe vor und nach einer injektion in einenAkupunkturpunkt im M Masseter gemessen. DieDruckschmerzEChweile war bei den Patienten mit muskuiärenSchmerzen signifikant tiefer als bei den Kontroiipersonen, Beiailen Messsteilen stiegen die Druckschmerzschwelien im M,Masseter nach Triggerpunktinjektion leicht an, währenddem sichin der Temporalregion kaum eine Auswirkung zeigte, in derKontroilgruppe stieg die Drucksohmerzschwelie signifikant beiallen Messstelien nach einer Akupunkturpunktinjektion Obwohieine injektion von Lokaianästhetikum peripher bei derschmerzhaften Steiie wirkt und zentrai, wo der Sciimerz erhal-ten bleibt, wurde bei den Patienten mit myofasziaien Schmerzenim Gegensatz zur Kontroligruppe keine dramatische Erhöhungder Druckschmerzschweile erzieit Dies legt nahe, dass beiPersonen mit myofasziaien Schmerzen eine bleibendeErregbarkeit der peripheren Gewebe und/oder der zentraienneuraien Regionen bestenhen bleibt, weiche zur bieibendenEmpfindlichkeit der Kiefermuskein beigetragen haben konnte

390 Volume 8, Number 4, 1994