Anatomical Study of the Innervation of the Masseter Muscle and Its Correlation with Myofascial Trigger Points

Figures & data

Figure 1 Dimensions of the masseter muscle in relation to segments: (A) superior transverse; (B) lower transverse; (C) longitudinal line of the masseter muscle; and (D) middle transverse line.

Note: Figure courtesy of Marcos Vinicius de Oliveira.

Figure 2 (A) Left side view of the face. Orientation of the abscissa (x) and ordinate (y) of the left masseter. By convention: upper anterior quadrant (UAQ) with positive ordinate and abscissa; lower posterior quadrant (LPQ) with negative ordinate and abscissa; superior posterior quadrant (SPQ) with negative and positive ordinate abscissa; and lower anterior quadrant (LAQ) with positive and negative ordinate abscissa; (B) Left side view of the face. Sextant of the left masseter muscle for localization of the penetration points of the masseteric nerve. D: The middle transverse line separates the upper and lower areas divided into three equally sized segments. The areas are numbered from I–VI, where I–III are superior and IV–VI are inferior in the posterior to anterior direction, respectively; (C) Left side view of the face. 1) zygomatic bone; 2) jaw; 3) temporal muscle; 4) folded masseter muscle. A) Masseteric nerve indicated in yellow; B, C, D and E indicate the branches of the masseteric nerve entering the masseter muscle in white.

Table 1 Description of the Characteristics of Cadavers

Variables	Description
	(N=16)
Age
Mean±SD	78.9±14
Median (min; max)	83 (43; 95)
Gender, n (%)
Female	10 (62.5)
Male	6 (37.5)
Race, n (%)
White	15 (93.8)
Yellow	1 (6.3)
Height (m)*
Mean±SD	1.71±0.11
Median (min; max)	1.7 (1.54; 1.9)
Weight (kg)*
Mean±SD	65.2±13.9
Median (min; max)	68 (45; 90)
BMI (kg/m^2)*
Mean±SD	22.1±4
Median (min; max)	21.6 (17.3; 31.2)

Note: *Only 10 cadavers have information available.

Abbreviations: SD, standard deviation; BMI, body mass index.

Table 2 Description of Masseter Measurements by Sex

Variables	Gender		Total	P
	Female (N=10)	Male (N=6)	(N=16)
Right masseter muscle
A				0.237
Mean±SD	4.39±0.89	3.87±0.68	4.19±0.84
Median (min.; max.)	4.55 (2.72; 5.5)	3.73 (3.13; 5)	4.5 (2.72; 5.5)
B				0.262
Mean±SD	3.69±0.43	3.33±0.8	3.55±0.6
Median (min.; max.)	3.5 (3.22; 4.5)	3.36 (1.98; 4.5)	3.5 (1.98; 4.5)
C				0.100
Mean±SD	5.38±0.99	6.2±0.71	5.68±0.96
Median (min.; max.)	5.21 (4.2; 7)	6.31 (4.95; 7)	5.63 (4.2; 7)
D				0.174
Mean±SD	3.95±0.65	3.62±0.28	3.83±0.55
Median (min.; max.)	4.2 (2.84; 4.5)	3.49 (3.39; 4.1)	3.8 (2.84; 4.5)
Left masseter muscle
A				0.656
Mean±SD	3.73±0.84	3.53±0.87	3.65±0.83
Median (min.; max.)	4 (2.16; 4.5)	3.37 (2.6; 5)	3.9 (2.16; 5)
B				0.208
Mean±SD	3.63±0.75	3.15±0.63	3.45±0.72
Median (min.; max.)	3.75 (2.58; 4.6)	3.35 (1.98; 3.6)	3.54 (1.98; 4.6)
C				0.242
Mean±SD	5.28±0.92	5.81±0.69	5.48±0.86
Median (min.; max.)	5.25 (3.7; 7)	5.9 (4.9; 6.81)	5.59 (3.7; 7)
D				0.71
Mean±SD	3.64±0.68	3.52±0.43	3.6±0.58
Median (min.; max.)	3.76 (2.13; 4.5)	3.52 (3.05; 4)	3.56 (2.13; 4.5)

Notes: Dimensions of the masseter muscle in relation to segments: A, superior transverse; B, lower transverse; C, longitudinal line of the masseter muscle; D, middle transverse line.

Abbreviation: SD, standard deviation.

Table 3 Description of the Entry Points of the Masseteric Nerve Branches in the Masseter Muscle According to Sex and Side

Variables	Gender		Total	P
	Female (N=10)	Male (N=6)	(N=16)
Total masseter muscle				0.492
mean±SD	11±4.42	8.83±2.71	10.19±3.92
median (min; max)	12 (6; 16)	8 (6; 14)	8.5 (6; 16)
Right masseter muscle				0.22
mean±SD	5.5±2.27	4±1.26	4.94±2.05
median (min; max)	5.5 (3; 8)	3.5 (3; 6)	4 (3; 8)
Left masseter muscle				0.792
mean±SD	5.5±2.22	4.83±1.72	5.25±2.02
median (min; max)	6.5 (3; 8)	4.5 (3; 8)	5 (3; 8)
P*			0.165

Note: Mann–Whitney Test; *Paired Wilcoxon Test.

Abbreviation: SD, standard deviation.

Table 4 Description of the Number of Points of Entry of the Masseteric Nerve Branches According to Sextants

Sextant	Mean±SD	Median (Min; Max)	P
1	1.16±1.14	1 (0; 4)	<0.001
2	1.81±1.26	2 (0; 4)
3	0.72±0.96	0 (0; 3)
4	0.34±0.65	0 (0; 2)
5	0.59±0.8	0 (0; 3)
6	0.47±0.84	0 (0; 3)

Note: GEE with Poisson distribution and identity binding function.

Abbreviation: SD, standard deviation.

Figure 3 Sextant left masseter muscle. Percentage of nerve entry points by area. Areas numbered from I–VI, where I–III are superior and IV–VI are inferior in the posterior to anterior direction, respectively.