Effect of intramuscular local anaesthetics on maximal voluntary activity in human anterior temporal muscles

Arch., oral Bid. Vol. 25, pp. 121 lo 125 F’ergamon Press Ltd 1980. Prmted in Great Britain

LOCAL EFFECT OF INTRAMUSCULAR ANAESTHETICS ON MAXIMAL VOLUNTARY ACTIVITY IN HUMAN ANTERIOR TEMPORAL MUSCLES M. BAKKE, E. MILLER

and 0. C. RASMLJS~EN

Institute of Oral Function, Electromyography, Physiology and Dental Morphology, Royal Dental College, 160 Jagtvej, Copenhagen, DK-2100, Denmark, and TMJ-clinic, Department of Oral Medicine and Oral Surgery, University Hospital, 9 Blegdamsvej, Copenhagen, DK-2100, Denmark

Summary-The electrical activity in the anterior and posterior temporal and masseter muscles was assessed during maximal bite force in the intercuspal position before and at intervals up to 90min after infiltration of 1.5 ml 3 per cent mepivacaine in the anterior temporal muscles. The activity in the anaesthetized muscles was reduced by 6267 per cent 5 min after injection, and a 2635 per cent reduction was still present at 90min. Considering the level of activity in the unanaesthetized muscles, fatigue or other sources of reduction in activity could be excluded. However, interference from these muscles may have hidden an even larger reduction in the anterior temporal muscle. We suggest that the fast and marked decreasing effect on voluntary action was due to blocking of intramuscular nerves and that reduction of activity, relief of pain, and possibly increased blood flow may be obtained by directly blocking the muscle nerve.

INTRODUCTION Infiltration of local anaesthetics is widely recommended for the management of pain and reduced mobility associated with functional disorders of the muscles of mastication. However, the actual mechanism of the treatment is unknown. In contrast, the effect of local anaesthetics applied either by intramuscular infiltration or directly to the muscle nerve is dealt with in detail in neurophysiology. Local anaesthetics abolish spasticity and normal reflexes and impair motor innervation and strength (Bremer and Titeca, 1930; Moller, 1976). Our purpose was to determine the time course of the effect on maximal voluntary activity in normal muscles with the anaesthetic applied as recommended for functional disorders.

MATERIAL

AND

METHODS

Material

The study was made on 6 males 2c-36 years of age with no symptoms or signs of functional disorders of the chewing apparatus.

Intramuscular

injiltration

With a disposable 2 ml Luer syrin e (Henke-Sass Wolf) and a 0.4 mm cannula (Terumo 0 stertle), 1.5 ml of 3 per cent mepivacaine without vasoconstrictor (Carbocain@-dental30 mg/ml, Bofors, Nobel-Pharma) was injected into the right and left anterior temporal muscles, after the skin had been cleansed with 70 per cent isopropanol (Steri-Swab@, Astra-Sjuco AB). The cannula was inserted anterior to the bipolar surface electrode and passed in a posterior-medial direction with its tip below the 2 leads. The local anaesthetic was then injected slowly, after aspiration. Usually, muscular infiltrations were given with the subject sitting upright, but a few times the supine position had to be used to prevent fainting. Procedure

For each subject, all the observations were made in one session. Before injection of the local anaesthetic, muscle activity was assessed as the mean of 4 contractions each lasting 1-2 s (control). Then the effect of the injection was evaluated after 5, 10, 15, 30, 45 and 60 min with 2 contractions at each interval. In 4 of the 6 subjects, the experiment was extended to include muscle activity after 90 min.

Electromyooyraphy

The activity in the anterior and posterior temporal and masseter muscles was recorded bilaterally with bipolar surface electrodes (Moller, 1966) and displayed simultaneously with the mean voltage (Bakke and Moller, 1980). Recordings were obtained during maximal force of biting in the intercuspal position, with the subject sitting upright without head support. The degree of activity during contraction was assessed as the mean level and the peak of the mean voltage (Bakke and Moller, 1980).

Statistical methods

Data were analysed by conventional statistical methods, i.e. arithmetic mean, standard deviation, and standard error of the mean. Differences between paired observations of muscle activity before and after intramuscular infiltration were evaluated by a twotailed Student’s t-test of significance. The individual levels of activity were tested both with respect to absolute differences in PV and to changes in percentages of the level of control. 121

122

M. Bakke, E. Moller and 0. c’. Rasmussen

RAT

MVI LAT RPT

Mvll LPT RMA

hlvlll LMA

lsec

1Z.S

CONTROL

5mm

Fig. 1. The effect of intramuscular anaesthesta: electromyograms from the right and left anterior temporal (RAT, LAT), posterior temporal (RPT, LPT), and masseter (RMA. LMA) muscles and the corresponding mean voltages (MV I, II, III: heavy trace, ht: right muscle; thin trace, tt: left muscle) during maximal bite in the intercuspal position. Recordings obtained before (control) and 5, 30, and 90 min after injection of 1.5 ml per cent mepivacaine without vasoconstrictor into RAT and LAT. There is marked decrease of activity in these muscles at 5 min and still some reduction at 90 min. Note the constant mean voltages of RPT, LPT, RMA, and LMA. Bipolar surface electrodes, copy in Indian ink from ink-jet recording. Subject B.F., male. 36 years old.

AT

100

PT

---_

----

50 -

O-

100

----

50 0 Smin.

10min.

----

---_

_--_--mll_mI 6

MA

----

*

NS

15min.

NS ----

30min.

NS

NS

NS

NS

NS

NS

NS

NS ----

45min.

60min.

0

NS

90min.

Fig. 2. The effect of injection wtth 1.5ml 3 per cent mepivacaine without vasoconstrictor in the anterior temporal muscles (AT) on the mean voltage of the anaesthetized muscles, and of the posterior temporal (PT) and masseter muscles (MA) during maximal bite in the intercuspal position. Changes of the average level (A, light shaded columns) and the peak (P. dark shaded columns) of the mean voltage are given in per cent of pre-injection values (100 per cent). Mean of the activity in the right and left muscle of each pair from 6 (5--60 min) and 4 (90 min) subjects, The probability p of an observed change occurring by chance: NS: p > 0.10. ” : p < 0.10, * : p < 0.05, ** : p < 0.01. *** : p < 0.001.

Average level Peak Average level Peak Average level Peak

-170 + 22***

-237 + 30*** -34 * 22 ns

-50 & 26” -36 + 18”

-70 + 34”

252 f 19

358 + 27 206 f 23

299 + 32 321 f 39

501 f 66

5 min M k SE

-53 + 50ns

-45 * 29 ns -17 f 31 ns

-241 f 21*** -37 + 22 ns

-171 * 19***

-57 + 43ns

-53 + 25” -20 + 26 ns

-221 + 27*** -32 + 21 ns

-163 + 20***

i-3 k 26ns

-44 f 21” +29 f 13*

-178 k 25*** -26 + 18 ns

-140 + 18***

+22 * 32 ns

-9 * 21 ns +26 k 20ns

-98 + 27** -1 f 12ns

-88 & 19***

90 min M + SE

+48 f 32 ns

+4 + 16ns +29 k 17ns

-89 f 24** -2 k 12ns

-83 + 15***

11 (5560 min) and 7 (90 min).

-27 + 47ns

-10 + 13ns fl + 27ns

-126 + 31** -7 + 11 ns

-98 f 23**

between paired observations of mean voltage before and during intramuscular anaesthesia of anterior temporal muscles: time after injection/@ 60 min 10 min 15 min 30 min 45 min M f SE M f SE M + SE M + SE M + SE

Differences

before intramuscular anaesthesia (control) and the Mean of activity in the right and left muscles in 6

M: mean; SE: standard error. M: ns, p > 0.10; M”: p < 0.10; M*: p < 0.05; M**: p < 0.01; M***: p < 0.001; two tailed t-test, degrees of freedom: Negative differences: reduction relative to control; positive differences: increase relative to control.

Masseter

Posterior temporal

Anterior temporal

Muscle

Parameter of mean voltage

Control (before injection) IJV M 1_ SE

Table 1. Mean voltage in the elevator muscles of the mandible during maximal bite in the intercuspal position change followed by infiltration of 1.5 ml 3 per cent mepivacaine in the right and left anterior temporal muscles. (control and 5-60 min) and 4 subjects (90 min)

; cz 8 A P B s. 0

E 2 E B L Ir: T =. G: ‘j

5 Q

124

M. Bakke, E. Maller and 0. C. Rasmussen RESULTS

Five minutes after intramuscular infiltration, the amplitude of the electromyograms from the anterior temporal muscles during maximal bite decreased markedly, and the level of activity was still reduced at 30 and 90 min (Fig. 1). Simultaneous recordings from the unanaesthetized posterior temporal and masseter muscles remained unchanged. As the levels of activity in the right and left of each of the 3 pairs of muscles did not differ significantly, data from the 2 sides were pooled. Local anaesthesia of the anterior temporal muscles, was followed by a highly significant reduction of the mean voltage both in percentage of the level of control (Fig. 2) and in absolute nV (Table 1). The reduction was 6267 per cent during the first 15 min and still 26-35 per cent at 90 min. The mean voltage in the posterior temporal muscles tended to be reduced during the first 5- 15 min both in absolute values of nV (Table 1) and in percentage of control (Fig. 2). The decrease was most pronounced at 15 min (peak values, Fig. 2). During anaesthesia of the anterior temporal muscles, maximal activity in the masseter muscles remained close to the level of control apart from an increase at 30 min (mean level, Table 1 and Fig. 2). However, throughout the experiment the mean voltage tended to increase systematically from slightly below to slightly over the level of control ((71 Fig. 2). DISCUSSION

With the mandible at rest, Moller (1976) found that injection of 1 ml 3 per cent mepivacaine into the anterior temporal muscles results in reduced postural activity in these muscles and lowering of the mandible. We found similar blocking in injected muscles, whereas activity in the unanaesthetized muscles only tended to decrease. Therefore, the marked reduction of activity in the anterior temporal muscle could hardly be due to fatigue or pain caused by the trauma following injection as suggested by Seibel r? al. (1978) and Fort, Yagiela and Benoit (1979). With intramuscular infiltration, local anaesthetics may prevent contraction by blocking intramuscular nerves (Matthews and Rushworth, 1957; Gassel and Diamantopoulus. 1964) transmission at the motor endplates (Deguchi and Narahashi, 1971; Katz and Miledi, 1975; Matsuo et al., 1978), and the release of Ca’+-ions (Thorpe and Seeman, 1971; Almers and Best, 1976, Caputo, Vergara and Bezanilla, 1979). An additional site of action is the cell membrane of the muscle fibres by blocking propagation of the muscle action potentials. However, interaction between potentials of adjacent muscle fibres (Buchthal, Guld and Rosenfalck, 1955) would counteract such an effect. Early reports on infiltration with local anaesthetics deal with procaine in small concentrations (e.g. Liljestrand and Magnus. 1919, 0.5-I ml of 1 per cent in the triceps muscle of decerebrate cat; Walshe. 1924. 8-25 ml of 1 per cent in the muscles of the upper extremities in patients with paralysis agitans; Bremer and Titeca. 1930, 0.75 ml of 1 per cent in leg muscles of decerebrate cats). All these workers describe reduced or abolished rigidity and reflexes, with nor-

ma1 response to nerve stimulation as well as direct faradic stimulation of the muscle (Liljestrand and Magnus. 1919). and normal voluntary power (Walshe, 1924). Using larger doses (4-8 ml), Liljestrand and Magnus (1919) prevented the response to nerve stimulation, but not to direct stimulation of muscle. Hence, these studies point more to an effect of the anaesthetic on intramuscular nerve fibres than directly on muscle fibres. Direct blocking of motor nerves supports this suggestion: Matthews and Rushworth (1957) found that local application of 0.2 per cent procaine to the common nerve of soleus and gastrocnemius of decerebrate cat abolished the stretch reflex within about 5 min. whereas the reaction to tetanic stimulation either remained normal or decreased with a delay of about 20 min. In man, injection of 4-10 ml 1 per cent procaine with adrenaline into the sciatic nerve abolished the ankle jerk and the H-reflex (recorded below the nerve block) in less than 30 min (Gassel and Diamantopoulus. 1964). The direct motor response decreased 60-70 per cent after 30 min, and it took two and a half hours to maximal reduction (8(X90 per cent, their Fig. 2). We observed a pronounced effect on maximal voluntary activity. In addition, muscle activity was impaired much faster than was after blocking of the muscle nerve by Matthews and Rushworth (1957) and Gassel and Diamantopoulus (1964). Because blocking of motor nerves counteracts tetanic stimulation and voluntary contraction more effectively than infiltration, we assume that the reduction of activity we found was obtained by blocking intramuscular nerves. We ascribe the fast and marked effect on muscle activity to the larger potency of 3 per cent mepivacaine as compared to the 0.2-l per cent procaine used by the workers just mentioned. The tendency for a decrease of activity in the posterior temporal muscles might have been due to spread of the anaesthetic. However, it is probably due to reduced recordings from the anaesthetized anterior part of the muscle. As impedances of electrodes are unstable and vary widely, the rejection of extraneous activity (e.g. from the anterior to the posterior part of the temporal muscle and vice versa) demands a high input impedance of the amplifier compared with electrode impedance (Moller, 1966). In spite of such a precaution, activity from facial muscles may interfere with recordings from the elevators (Moller, 1975, Fig. 5). A similar mutual interference between the 2 parts of the temporal muscle could easily account for a 10-I 5 per cent reduction of maximal activity in its posterior part. In addition, spread of activity from the posterior part (and from the masseters) may have interfered with the recordings from the anaesthetized anterior temporal muscle. implying that the actual reduction in fact exceeded the 62.-67 per cent, we observed. The insignificant. but systematic change of maximal activity in the massetcr muscles, from a little below to slightly above control level (cf Fig. 2) was probably a combined effect of reduced interference from the anterior part of the temporal muscle (S-15 min) and facilitation due to repetitive contractions (3(X90 mint Our study has demonstrated that infiltration of 3 per cent mepivacaine causes a fast and marked de-

Intramuscular anaesthetics and muscle activity crease of muscle activity probably due to blocking of intramuscular nerves. Full strength of contraction was not regained after 90 min. Gassel and Diamantopoulus (1964) observed an increase of muscle temperature following blocking of the muscle nerve, probably from enhanced blood flow. Hence, to add an increase of blood flow to the relaxing and painrelieving effect of the local anaesthetic, directly blocking the muscle nerve would be preferable to intramuscular infiltration. REFERENCES

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