Relationships Between Jaw Muscle Cross-Sections and Craniofacial Morphology in Normal Adults, Studied with Magnetic Resonance Imaging

Relationships between jaw muscle cross-sections and craniofacial morphology in normal adults, studied with magnetic resonance imaging

P. H. van Spronsen*, W. A. Weijs**, J. Valk***, B. Prahl-AnderserT, and F. C. van Ginkel* "Department of Orthodontics, Academic Center for Dentistry Amsterdam (ACTA), Louwesweg 1, 1066 EA Amsterdam, "Department of Anatomy and Embryology, Academic Center for Dentistry Amsterdam (ACTA), Meibergdreef 15, 1105 AZ Amsterdam, and "'Department of Radiology and Neuroradiology, Free University Clinics, Amsterdam, The Netherlands

SUMMARY In 32 Caucasian adult males serial MRI scans of the jaw muscles were taken approximately perpendicular to the mean fibre direction of the jaw muscles to determine their cross-sectional areas. These areas are proportional to the maximal isometric strength of a muscle. To describe facial skeletal variation, nine angular and 21 linear cephalometric measurements were

recorded, and statistically reduced by means of multiple regression and principal component Downloaded from analysis. Six components were extracted, rotated, and subsequently correlated with the maximal cross-sectional areas of the jaw elevators and anterior digastric muscle. Positive significant correlations were found between a linear combination of several transversal

skull dimensions on the one hand, and the maximal temporalis and masseter cross-sections on by guest on November 3, 2014 the other. A negative significant correlation was found between the flexure of the cranial base and the temporalis cross-section. Surprisingly, no significant correlations were found between either anterior facial height or posterior facial height and any of the jaw muscles cross-sections. It was concluded that, in adult males with normal skull shape, relationships exist to a limited extent between craniofacial morphology and the cross-sectional areas of the jaw muscles.

Introduction experiments have indicated that removal or repositioning of the jaw muscles may signific- The idea that the muscles of mastication are antly affect craniofacial growth (Washburn, involved in the control mechanism of craniofa- 1947; Horowitz and Shapiro, 1951, 1955; Nanda cial growth is not new. Brodie (1950) has already etal., 1967; Hohl, 1983). described several muscle groups (jaw elevators, Recently, the cross-sectional areas of the jaw post-cervical muscles, supra- and infrahyoidal muscles have been imaged by means of com- muscles) which are involved in maintaining the puted tomography (CT) (Weijs and Hillen, balance of the head on the cervical column \9S4a,b, 1985; Gionhaku and Lowe, 1989) and during growth. Solow and Kreiborg (1977) Magnetic Resonance Imaging (MRI) (Spronsen regarded both muscle activity and soft tissue et al., 1989; Sasaki et al., 1989). It has been stretching as important control factors in shown that mid-belly cross-sections of the jaw - craniofacial growth. Distortion of the muscular muscles as obtained by CT are closely related balance can result in significant distortion of to the anatomically determined physiological craniofacial growth as may be seen in patients cross-section (Weijs and Hillen, 1984a, 1985). with congenital progressive atrophy of the jaw The latter is an indication of the maximal muscles (Kreiborg et al., 1978) and patients isometric strength of a muscle (Maughan et al., with myotonic dystrophy (Gazit et al., 1987). 1983). Therefore, the cross-sectional area of a These patients typically show a long face mor- muscle is probably an indication of its possible phology. Furthermore, the results of animal mechanical influence on the growing craniofa- 352 P. H. VAN SPRONSEN ET AL.

cial skeleton. Using CT scans, relationships sectional area found in the scan series was used have been found between certain cephalometric as data. As in other biomechanical studies variables and the cross-sectional areas (Weijs (Pruim et al. 1978; Osborn and Baragar, 1985; and Hillen, 1986), as well as the volumes (Gion- Koolstra et al., 1990) the temporalis muscle was haku and Lowe, 1989) of the masseter and divided into anterior and posterior parts. Using medial pterygoid muscles. As an imaging tech- the technique of Koolstra et al. (1988) we nique, MRI is preferable to CT since it permits separated its cross-sectional area by a frontal serial scans with different angulations without plane through the tip of the coronoid process, subjecting the patient to radiation and without into a rounded anterior part and a more elong- changing the patient's position. Besides the pos- ated narrow posterior part. sibility of determination of a single mid-belly Conventional cephalometric analysis was per- cross-sectional area, this enables assessment of formed on lateral radiographs taken with a the orientation of the jaw muscles (Spronsen et General Electric system (focus/film distance al., 1988; Sasaki et al., 1989; Koolstra et al., 4 m). The lateral radiographs were traced on 1990). acetate and the cephalometric landmarks (Fig. The aim of this study was to determine 1) were digitized. The following transversal possible associations between the mid-belly dimensions were measured in coronal MRI cross-sectional areas of the jaw elevators and scans: maximal head width (MHW); bizygo- anterior digastric muscle on one hand, and matic width, the largest distance between the normal craniofacial morphology on the other. zygomatic arches (BZW); bigonial width (BGW) and bicondylar width, the distance between the centres of the condyles (BCW) (see Table 2). Downloaded from Subjects and methods Statistical analysis comprised descriptive stat- This study was based on 32 adult male subjects istics and univariate (Pearson) correlations

with a mean age of 31.2 years (s.d., 6.3 years). between muscular and cephalometric variables. by guest on November 3, 2014 All subjects had a normal skull shape, a com- A multiple regression analysis from the SPSSx plete or nearly complete dentition, no serious malocclusions, and no signs or symptoms of functional disorders of the temporomandibular joint. MRI scans (slice thickness: 5 mm; interslice gap: 1.25 mm) were made with a supercon- ducting 0.6 T system (Technicare Teslacon), using a Tl weighted sequence and a 20 cm headcoil. The subjects were scanned in a supine position with the Frankfort Horizontal plan (FH) perpendicular to the floor. On the mid- sagittal scan the range and angulation of three scan series were determined. The temporal muscle was scanned parallel to the FH. The masseter and medial pterygoid muscles were scanned at a 30 degree angle with respect to the FH. The lateral pterygoid muscle scans (slice thickness: 4 mm; interslice gap 1 mm) as well as the anterior digastric muscle scans were taken in a frontal plane. The subjects were instructed to hold their teeth together, but to avoid clenching. The cross-sections of the jaw muscles were traced on acetate from 0.5 times magnified Figure 1 Cephalometric landmarks used in this study. Abbreviations: S = sella turcica; N = nasion; A = subspinale; prints and digitized with a Summagraphic XY B = supramentale; G = gonion; Ba = basion; Ar = articulare; tablet connected to an Olivetti personal com- M = Menton; + 1 = upper incisor; - 1 = lower incisor; +6 = puter. For each jaw muscle the largest cross- 1 upper molar; —6=1 lower molar. JAW MUSCLES AND SKULL SHAPE 353

statistical package (Norusis, 1985) was used to Table 1 Mean and standard deviation (cm2) of the eliminate those variables that were, statistically MRI cross-sectional areas of the jaw muscles (« = speaking, redundant, i.e. almost perfectly pre- 32). dictable from the others (/?>0.98). On the remaining variables a principal component ana- Mean Standard deviation lysis (SPPSx) was applied to establish a reduced TEM R 5.30 0.69 number of mutually uncorrelated new variables TEM L 4.86 0.86 (components), jointly describing the variation TEM A R 2.94 0.68 of the craniofacial measurements. The compon- TEM A L 2.59 0.55 ents were rotated by a Varimax rotation for MAS R 4.67 0.84 MASL 4.47 1.07 ease of interpretation. By using a multiple MPM R 3.24 0.58 regression analysis, the values (scores) of these MPM L 3.23 0.48 components were subsequently correlated with LPM R 4.20 0.48 the mean values of the right and left cross- LPM L 4.20 0.49 sectional areas of the jaw muscles. In this way DIG A R 1.16 0.35 it can be established whether and to what extent DIGAL 1.11 0.33 variation in muscle cross-sectional area is linked Abbreviations: TEM = temporalis (total cross-sectional to variation in skull shape. area); TEM A = anterior temporalis; MAS = masseter; MPM = medial pterygoid muscle; LPM = lateral pterygoid muscle; DIG A = anterior digastric muscle; R = right; L = Results left. MRI scans of the jaw muscles of one subject Downloaded from are shown in Fig. 2I-IV. Note that bone as well as air has a low signal intensity, whereas fat the lateral pterygoid muscle, but no significant

has a high signal intensity. In the temporalis correlation with the masseter. The masseter by guest on November 3, 2014 scans, the central tendinous plate can clearly be showed a significant positive correlation with distinguished from the muscular tissue (Fig. 21). the anterior digastric muscle. In certain scans of the lateral pterygoid muscle In Table 4, a correlation matrix is presented the superior and inferior heads are clearly visible between the cross-sectional areas of the jaw (Fig. 2III). In Fig. 3a-d the cross-sectional areas muscles and the cephalometric variables. The of the muscles of one subject are presented as correlations are generally low and statistically a function of parallel shift of the scan plane not significant. from origin to insertion. Note that the mid- The variables SNA, SNB, ANB, MP/SN, SP/ belly cross-sections of the masseter, medial MP, and SN/Ar were singled out by means of pterygoid, and anterior digastric muscles are multiple regression analysis as being almost relatively constant over a range of 12 mm. The perfectly predictable by the others. On the means and standard deviations of the jaw remaining variables a principal component ana- muscle cross-sections are listed in Table 1. The lysis was performed which yielded six significant temporalis and masseter showed asymmetries orthogonal components with Eigen values > 1. between left and right muscles. The masseter These components jointly explained 74.7 per muscle tended to be more variable than the cent of the variance of the original cephalo- other muscles. The anterior temporalis cross- metric variables. Based on the factor loading sectional area was approximately 54 per cent of matrix (Table 5) these components were named the total temporalis cross-sectional area. The as follows: means and standard deviations of 9 angular and component 1: anterior total facial height with a 21 linear cephalometric variables are listed in high loading for anterior lower facial height; Table 2. Table 3 lists the correlation matrix component 2: mandibular length; between the cross-sectional areas of the jaw component 3: head and facial width with high muscles. It appeared that the anterior temporalis loadings for maximal head width and inter- muscle showed significant positive correlations with all jaw muscles except for the anterior condylar width; digastric muscle. The medial pterygoid muscle component 4: posterior total facial height with showed a significant positive correlation with high loadings for posterior upper and lower facial height; P. H. VAN SPRONSEN ET AL. Downloaded from by guest on November 3, 2014

Figure 2 (I-IV), each consisting of five scans labelled A-E. Serial MRI scans of the temporalis muscle (I), masseter and medial pterygoid muscle (II), the lateral pterygoid muscle (III), and the anterior digastric muscle (IV). ac = Antrum cyst; c = condyle; dig = anterior belly of the digastric muscle; g = genioglossus muscle; 1 pm = lateral pterygoid muscle (u = upper head, 1 = lower head); mas = masseter; mpm = medial pterygoid muscle; my = mylohyoid muscle; p = parotid gland; pi = platysma; r = ramus; c = central tendinous plate of the temporalis; tem a = anterior temporalis; temp p = posterior temporalis. JAW MUSCLES AND SKULL SHAPE 355 III Downloaded from by guest on November 3, 2014

component 5: sagittal jaw relation (Wits) with means of multiple regression analysis (Table 6). high loadings for overbite and overjet; Significant relationships were found between component 6: nasion/sella/basion angle (cranial the total temporal and anterior temporal cross- base flexure). sectional areas with the cranial base flexure The relationships between the mean cross- (component 6) and furthermore, between sectional areas of the muscles and the scores of anterior temporal muscle cross-sectional area, six principal components were determined by head width, and facial width (component 3). 356 P. H. VAN SPRONSEN ET AL.

Table 2 Mean and standard deviation (SD) of nine Table 3 Pearson correlations between the cross-sec- angular and 19 linear cephalometric measurements tional areas of the jaw muscles. (« = 32). TEM A MAS MPM LPM DIG A Mean SD TEM 0.66"* 0.34 0.50** 0.53** 0.13 Angular measurements (0) TEM A 0.42* 0.46** 0.42* 0.11 S/N/A 83.6 5.0 MAS 0.29 0.33 0.46** S/N/B 80.1 4.1 MPM 0.57** 0.30 A/N/B 3.5 2.4 LPM 0.29 SN/OP 10.8 4.9 DIG A SN/MP 29.1 6.0 SP/MP 23.5 6.1 *P<0.05; **/><0.01; ***/><0.001. Go 126.1 5.5 Abbreviations: see Table 1. S/N/Ba 127.5 5.9 SN/ArGo 85.6 5.7 Discussion Linear measurements (2) wits 2.6 3.5 In recent studies, data on cross-sectional areas overjet 3.2 2.8 and volumes of the human jaw muscles obtained overbite 3.4 1.6 by CT and MRI have been published (Weijs ATFH 128.8 6.6 AUFH 55.5 3.5 and Hillen 1984aA 1985; Spronsen et al., 1989; ALFH 74.9 5.9 Sasaki et al. 1989; Gionhaku and Lowe, 1989; PTFH 85.8 7.1 Hannam and Wood, 1989). Comparison with PUFH 49.3 2.0 the results of this study reveals good agreement Downloaded from PLFH 50.5 3.8 as well as considerable contrasts. Significant Go-Me 77.0 3.6 Ar-Go 53.4 3.4 correlations have been found between both

S-N 75.2 3.5 cross-sectional areas (Hannam and Wood, 1989) by guest on November 3, 2014 S-Ba 51.2 2.7 and volumes (Gionhaku and Lowe, 1989) of AUDH 31.5 3.4 the masseter and medial pterygoid muscle. How- ALDH 45.9 2.9 ever, we cannot confirm these findings. Interest- PUDH 27.4 3.1 PLDH 36.5 2.9 ingly, the mean cephalometric characteristics in MHW 136.8 5.2 our subjects, those of Weijs and Hillen (1986), BZW 128.0 5.8 of Gionhaku and Lowe (1989), and of Hannam BGW 97.7 5.3 and Wood (1989) are fairly comparable. This BCW 99.4 5.2 indicates that differences in techniques rather Abbreviations: S = sella turcica; N = nasion; A = subspin- than variation in skull shape are responsible for ale; B = supramentale; Go = gonion; Ba = basion; Ar = artic- the differences in cross-sectional areas of the ulare; M = Menton; OP = occlusal plane; SP = spina plane jaw muscles. Errors in measuring cross-sectional [anterior nasal spine (ANS)-posterior nasal spine (PNS)]; areas that can amount up to 4.5-12.4 per cent MP = mandibular plane (gonion-menton); ATFH = anterior (Weijs and Hillen, 1984a; Spronsen et al., 1989), total facial height (N-M); AUFH = anterior upper facial height (N-ANS); ALFH = anterior lower facial height the use of different scan planes and probably (ANS-M); PTFH = posterior total facial height (S-Go); the variation in jaw muscle orientation PUFH = posterior upper facial height (S-PNS); PLFH = (Spronsen et al., 1988) might explain the discrep- posterior lower facial height (PNS-Go); AUDH = anterior ancies. upper dental height (incision + 1-SP); ALDH = anterior lower dental height (incision — 1-mandibular plane); The cross-sectional area of the anterior PUDH = posterior upper dental height (mesio-buccal cusp digastric muscle appears to be larger than + 6-SP); PLDH = posterior lower dental height (mesio-buccal cusp -6-MP); MHW = maximal head width; IZW = described by Pruim et al. (1980) for digastric inter-zygomatic width; IGW = inter-gonial width; ICW = muscles of elderly people. Weijs and Hillen inter-condylar width. (1985) also found smaller cross-sections of the jaw elevators in dissection material compared A moderate relationship was found between with CT measurements of young individuals this component and the masseter muscle cross- with complete dentitions. These authors section. No significant relationships were found assumed that the muscles had atrophied as the between either anterior or posterior facial length dissection material lacked almost all natural and any of the jaw muscles. teeth. Their suggestion was supported by JAW MUSCLES AND SKULL SHAPE 357

mas R mas L mpm R mpm L

18 24 30 36 42 48 54 60 mm 18 24 30 36 42 48 54 60 mm Downloaded from by guest on November 3, 2014

6 12 18 24 30 36 42 48 54 60 mm 0 6 12 18 24 30 36 42 48 54 60 mm Figure 3 (a-d) Changes of cross-sectional areas as a result of parallel shift of the scan plane from origin (0 mm) to insertion, a: temporalis muscle; b: masseter and medial pterygoid muscles; c: lateral pterygoid muscle; d: anterior digastric muscle.

Newton et al. (1987) who observed a significant size, shape, or inclination with maxilla or cranial reduction of masseter and medial pterygoid base was significantly related to the size of any cross-sections in CT scans in individuals varying of the jaw muscles. Relationships have been in age from 20 to 90 years. published between the variation in facial length For the sake of comparison with the results and maximum bite force (Sassouni, 1969, Proffit of other studies, Pearson correlation coefficients et al., 1983). This suggests that besides muscle were computed between muscle cross-sectional moment arm (Throckmorton et al., 1980) bite areas and cephalometric variables. The correla- force is also determined by muscle size. tion matrix should be interpreted with caution Recently, significant positive correlations have since the few observed significant correlations been published between the masseter and medial could be the result of chance alone. However, pterygoid cross-sectional area and the maximum the trend of correlations between the width molar bite force (Hannam and Wood, 1989; measurements of the skull and jaw muscle size Spronsen et al., 1989). Nevertheless, we found has been confirmed by others (Weijs and Hillen, only a poor relationship between jaw elevator 1986; Hannam and Wood, 1989). Although cross-sectional areas and facial length. The Weijs and Hillen (1986) found a positive signi- anterior digastric muscle showed only weak ficant correlation between mandibular length negative correlations with posterior total facial (Go-Me) and the masseter and medial pterygoid height and anterior lower facial height. This did muscles cross-sectional areas no such correla- not support the findingso f Bolt and Orchardson tion was found in this study, nor in the one by (1986) who measured maximum mouth opening Hannam and Wood (1989). In fact, none of the forces and found positive significant correla- cephalometric variables expressing mandibular tions with gonial angle and mandibular plane 358 P. H. VAN SPRONSEN ET AL.

Table 4 Pearson correlations between cross-sectional areas and cephalometric variables.

TEM TEM A MAS MPM LPM DIG A

S/N/A 0.27 0.22 0.37* 0.16 0.03 0.00 S/N/B 0.16 0.18 0.28 0.24 0.17 -0.02 A/N/B 0.30 0.15 0.29 -0.11 -0.23 0.23 SN/OP -0.19 -0.13 -0.26 -0.09 -0.10 0.04 SN/MP 0.10 0.04 -0.22 0.07 -0.16 0.04 SP/MP 0.23 0.15 -0.25 0.15 -0.11 -0.06 Go 0.15 0.10 0.10 0.22 0.05 -0.16 S/N/Ba -0.30 -0.30 -0.25 0.28 0.40* 0.18 SN/ArGo -0.07 0.06 -0.12 -0.16 0.50

Wits 0.17 0.04 0.23 -0.12 -0.22 -0.05 overjet 0.12 0.28 0.48** -0.07 -0.09 0.05 overbite -0.15 -0.24 0.10 0.14 0.09 0.21 ATFH 0.13 0.13 -0.17 -0.34 -0.31 -0.18 AUFH -0.13 -0.11 0.07 -0.40* -0.25 0.18 ALFH 0.22 0.21 -0.15 -0.11 -0.22 -0.23 PTFH 0.14 0.23 -0.16 -0.29 -0.13 -0.31 PUFH 0.46** 0.18 0.02 0.05 0.11 -0.22 PLFH 0.09 0.18 -0.11 -0.31 0.14 -0.19 Go-Me 0.05 0.07 0.26 0.11 0.15 -0.07 Ar-Go 0.02 0.18 0.07 -0.08 0.08 -0.21 S-N 0.13 0.05 0.18 -0.15 -0.18 0.00 Downloaded from S-Ba -0.13 -0.16 -0.23 -0.24 -0.10 -0.33 AUDH 0.25 0.31 -0.13 -0.17 -0.21 -0.19 ALDH 0.33 0.39* -0.03 -0.08 -0.20 -0.21

PUDH 0.19 0.27 -0.13 -0.35 -0.15 -0.19 by guest on November 3, 2014 PLDH 0.18 0.20 -0.04 -0.16 0.00 -0.23 MHW 0.21 0.26 0.11 0.34 0.24 -0.08 BZW 0.06 0.13 -0.14 0.30 0.18 -0.15 BGW 0.33 0.42* 0.45* 0.20 0.26 0.00 BCW 0.35 0.46** 0.24 0.30 0.27 -0.12

*P<0.05; **P<0.0\. Abbreviations: see Table 2. angle. They suggested that long-faced people also found significant relationships between a have stronger mouth openers than normal indi- linear combination of transverse cranial and viduals. It is possible that mouth opening force facial dimensions on the one hand, and tem- is determined to a great extent by variation in poralis and masseter cross-sectional areas on moment arm, rather than strength of the digast- the other. ric muscle. The significant negative relationship between Multiple regression analysis revealed signi- the mean temporalis cross-section and cranial ficant relationships only between transverse base flexure is in line with the findingso f Moller skull dimensions (comp. 3) and cranial base (1966) who reported a significant negative cor- flexure (comp. 6) with the temporalis and mas- relation between the EMG activity of the seter. This suggests that the variation in facial anterior temporalis and cranial base flexure,bu t form is only partly determined by the variation it contradicts those of Weijs and Hillen (1986). in cross-sectional area of the jaw muscles. The It should be noted that the cranial base is significant relationship between the transverse formed by activity in the spheno-occipital syn- skull dimensions (component 3) and the mean chondrosis, the spheno-ethmoidal and spheno- temporalis and masseter cross-sectional areas frontal sutures as well as by appositional growth might be explained by the fact that width (Melsen, 1974). In the literature evidence can increase of the face is determined mainly by be found that both genetic and local environ- sutural and appositional growth (Scott, 1957). mental factors may contribute to the ultimate By using factor analysis, Weijs and Hillen (1986) form of the cranial base. Bjork (1972) reported JAW MUSCLES AND SKULL SHAPE 359

Table 5 Rotated factor loading matrix.

Factor 1 Factor 2 Factor 3 Factor 4 Factor 5 Factor 6

Pet of Var 20.6 16.3 14.6 9.5 7.1 6.6

ATFH 0.90 ALFH 0.82 ALDH 0.80 PLDH 0.78 Go-Me 0.82 Se-N 0.74 PUDH 0.57 0.69 MHW 0.84 BCW 0.74 BZW 0.58 BGW 0.58 PTFH 0.79 PUFH 0.68 PLFH 0.65 Go< -0.57 Wits 0.78 OJ 0.75 OB 0.65 N/Se/Ba 0.94 OP/SeN 0.57 Downloaded from

Abbreviations: see Table 1; Pet of Var = percentage of variance.

Table 6 Matrix of /? (beta) weights of principal component scores and the mean of right and left by guest on November 3, 2014 muscular cross-sectional areas.

R2 Com 1 Com 2 Com 3 Com 4 Com 5 Com 6 P P P P P P TEM 0.28 0.13 0.13 0.26 -0.11 0.00 -0.40** TEM A 0.48** 0.18 0.05 0.45*** -0.17 -0.10 -0.45*** MAS 0.26 -0.06 0.10 0.34* -0.14 0.18 -0.26 MPM 0.18 -0.10 -0.10 0.31 -0.18 -0.13 0.10 LPM 0.11 -0.04 0.07 0.28 -0.06 0.02 0.16 DIG A 0.07 -0.15 0.04 -0.05 -0.17 0.01 0.14

*/><0.08; **/><0.05; ***/»<0.01. remarkable differences in the cranial base flexure condylar growth strongly determine the shape of different ethnic groups. But it has also been of the mandible as well as the anterior facial shown that extension of the head as the result height. The lack of correlation between the of long-term obstruction of the nasopharyngeal components of facial and mandibular length airway can significantly influence cranial base with jaw muscle cross-sections suggests that flexion (Solow and Tallgren, 1976; Solow and adult mandibular dimensions are determined Greve, 1979). Besides considerable changes in less by local environmental factors such as jaw craniofacial morphology, the postural EMG muscle strength. This hypothesis is supported activity of neck and masticatory muscles is by the findings of Horowitz et al. (I960). In a significantly changed following extension and study of adult twins, they found a high compon- flexion of the head (Forsberg et al., 1985) or ent of genetic variability for mandibular length, experimental obstruction of the nasal airway anterior total facial height and anterior lower (Hellsing et al., 1986). facial height. Lundstrom and McWilliam (1987, Bjork's implant studies (1963, 1969) have 1988) compared vertical and horizontal cephalo- shown that the direction and magnitude of metric variables with regard to heritability in 360 P. H. VAN SPRONSEN ET AL. adult twins and established the highest herit- References ability values for vertical facial dimensions. On Atchley WR^ Plummer A A, Riska B 1985 Genetics of the other hand, Atchley et al. (1985) emphasized mandible form in the mouse. Genetics 111: 555-577 the role of environmental factors as responsible Beecher R M, Corruccini R S 1981 Effects of dietary consist- for the correlations between muscle function ency on craniofacial occlusal development in the rat. and mandibular form. Low level masticatory Angle Orthodontist 51: 61-69 function by feeding soft diet induced more Bjork A 1963 Variations in the growth pattern of the human vertically orientated craniofacial growth in mandible: longitudinal radiographic study by the implant growing rats (Kiliaridis et al., 1985; Engstrom method. Journal of Dental Research 42: 400-411 et al., 1986) and a decrease in the transverse Bjork A 1969 Prediction of mandibular growth rotation. width of the maxilla (Watt and Williams, 1951; American Journal of Orthodontics 55: 585-599 Bjork A 1972 The role of genetic and local environmental Moore, 1965; Beecher and Corruccini, 1981). factors in normal and abnormal morphogenisis. Acta Ingervall and Bitsanis (1987) showed that train- Morphologica Neerlando Scandinavica 10: 49-58 ing of the jaw muscles by daily chewing on Bolt K J, Orchardson R 1986 Relationship between mouth- tough material had a significant positive effect opening force and facial skeleton dimensions in human on the maximal molar bite force, and EMG females. Archives of Oral Biology 31: 789-793 activity of the anterior temporalis and masseter Brodie AG 1950 Anatomy and physiology of head and in long-faced children. These authors observed neck musculature. American Journal of Orthodontics 36: a decrease of posterior rotation of the mandible 831-844 Engstrom C, Kiliaridis S, Thilander B 1986 The relationship after 1 year chewing exercises. between masticatory function and craniofacial morpho- In this study few relationships were found logy. II A histological study in the growing rat fed a soft between muscle size and skull shape. However, diet. European Journal of Orthodontics 8: 271-279 Downloaded from Forsberg C M, Hellsing E, Linder-Aronson S, Sheik- our observations should be interpreted with holeslam A 1985 EMG activity in neck and masticatory caution since the influence of the entire soft muscles in relation to extension and flexion of the head. tissue layer including the postural activity of European Journal of Orthodontics 7: 177-184 by guest on November 3, 2014 facial and lingual muscles has not been consid- Gazit E, Bornstein N, Lieberman M, Serfaty V, Gross M, ered as potential environmental factor in shap- Korczyn A D 1987 The stomatognathic system in ing the face. Furthermore, it has already been myotonic dystrophy. European Journal of Orthodontics emphasized that errors, both in the determina- 9: 160-164 Gionhaku N, Lowe A A 1989 Relationship between jaw tion of craniofacial measurements and cross- muscle volume and craniofacial form. Journal of Dental sections may have a detrimental effect upon the Research 68: 805-809 correlation coefficients. Hannam A G, Wood W W 1989 Relationships between the Further research needs to be focused on the size and spatial morphology of human masseter and medial pterygoid muscles, the craniofacial skeleton and relationship between jaw muscle cross-sections jaw biomechanics. American Journal of Physical Anthro- and skull shape in subjects with extreme long pology (submitted for publication) or short faces. Hellsing E, Forsberg C M, Linder-Aronson S, Sheik- holeslamA 1986 Changes in postural EMG activity in the neck and masticatory muscles following obstruction of the nasal airways. European Journal of Orthodontics Address for correspondence 8: 247-253 Dr P. H. van Spronsen Hohl TH 1983 Masticatory muscle transposition in prim- ates: effects on craniofacial growth. Journal of Maxillo- Department of Orthodontics Facial Surgery 11: 149-156 Academic Centre for Dentistry Amsterdam Horowitz S L, Shapiro H H 1951 Modification of mandib- (ACTA) ular architecture following removal of the temporalis de Boelelaan 1115 muscle in the rat. Journal of Dental Research 30: 276-280 1081 HV Amsterdam Horowitz S L, Shapiro H H 1955 Modification of skull and The Netherlands jaw architecture following removal of the masseter muscle in the rat. American Journal of Physical Anthropology 13: 301-308 Horowitz S L, Osborne R H, DeGeorge F V 1960 A cephal- Acknowledgements ometric study of craniofacial variation in adult twins. Angle Orthodontics 30: 1-5 We thank all subjects who volunteered for this Ingervall B, Bitsanis E 1987 A pilot study of the effect of study. We also thank Mrs K. van der Vegt for masticatory muscle training on facial growth in long-face making the excellent MRI scans. children. European Journal of Orthodontics 9: 15-23 JAW MUSCLES AND SKULL SHAPE 361

Kiliaridis S, Engstrom C, Thilander B 1985 The relationship Pruim G J, De Jongh H J, Ten Bosch J J 1980 Forces acting between masticatory function and craniofacial morpho- on the mandible during bilateral static bite at different logy. I. A cephalometric longitudinal analysis in the bite force levels. Journal of Biomechanics 13: 755-763 growing rat fed a soft diet. European Journal of Ortho- Sasaki K, Hannam A G, Wood WW 1989 Relationships dontics 7: 273-283 between the size, position, and angulation of human jaw Koolstra J H, Van Eijden T G M J, Weijs W A, Naeye M muscles and unilateral first molar bite force. Journal of 1988 A three-dimensional mathematical model of the Dental Research 68: 499-503 human masticatory system predicting maximum possible Sassouni V 1969 A classification of skeletal facial types. bite forces. Journal of Biomechanics 21: 563-576 American Journal of Orthodontics 55: 109-123 Koolstra J H, Van Eijden TGMJ, Van Spronsen P H, Scott J H 1957 The growth in width of the facial skeleton. Weijs W A, Valk J 1990 Computer assisted estimation of American Journal of Orthodontics 43: 366-371 lines of action of human masiticatory muscles recon- structed in-vivo by means of MRI parallel sections. Arch- Solow B, Tallgren A 1976 Head posture and craniofacial ives of Oral Biology 35: 549-556 morphology. American Journal of Physical Anthropology 44: 417-435 Kxeiborg S, Leth Jensen B, Moller E, Bjork A 1978 Crani- ofacial growth in a case of congenital muscular dystrophy: Solow B, Kreiborg S 1977 Soft-tissue stretching: a possible a roentgencephalometric and electromyographic investi- control factor in craniofacial morphogenesis. Scandinav- gation. American Journal of Orthodontics 74: 207-215 ian Journal of Dental Research 85: 505-507 Lundstrom A, McWilliam J 1987 A comparison of vertical Solow B, Greve E 1979 Craniocervical angulation and nasal and horizontal cephalometric variables with regard to respiratory resistance. In: McNamara J A (ed.) Naso- heritability. European Journal of Orthodontics 9: respiratory function and craniofacial growth. Monograph 104-108 no. 9, Craniofacial Growth Series, Center for Human Growth and Development. University of Michigan, Ann Lundstrom A, McWilliam J 1988 Comparison of some Arbor 87-119 cephalometric distances and corresponding facial propor- tions with regard to heritability. European Journal of Spronsen van P H, Prahl-Andersen B, Weijs W A, Valk J Downloaded from Orthodontics 10: 27-29 1988 Assessment of jaw muscle orientation by means of Magnetic Resonance Imaging (MRI). Journal of Dental Maughan R J, Watson J S, Weir J 1983 Strength and cross- Research 67: 430 sectional area of human skeletal muscle. Journal of Physiology 338: 37-49 Spronsen van P H, Weijs W A, Prahl-Andersen B, Valk J, by guest on November 3, 2014 Ginkel van F 1989 Comparison of jaw muscle bite force Melsen B 1974 The cranial base. Acta Odontologica Scandi- cross-sections obtained by means of magnetic resonance navica 32 Suppl. 62 imaging and high resolution CT-scanning. Journal of Moller E 1966 The chewing apparatus: an electromyo- Dental Research 68: 1765-1770 graphic study of the action of the muscles of mastication Throckmorton G S, Finn R A, Bell W H 1980 Biomechanics and its correlation to facial morphology. Acta Physiolog- of differences in lower face height. American Journal of ica Scandinavica 69: 1-229 Orthodontics 77: 410-420 Moore WJ 1965 Masticatory function and skull growth. Washburn S L 1947 The relation of the temporal muscle to Journal of Zoology 146: 123-131 the form of the skull. Anatomical Record 99: 239-248 Nanda S K, Merow WW, Sassouni V 1967 Repositioning Watt D G, Williams C H M 1951 The effects of the physical of the masseter muscle and its effect on skeletal form and consistence of food on the growth and development of structure. American Journal of Orthodontics 37: 304-308 the mandible and the maxilla of the rat. American Journal Newton J P, Abel R W, Robertson EM, Yemm R 1987 of Orthodontics 37: 895-928 Changes in human masseter and medial pterygoid muscles Weijs W A, Hillen B 1984a Relationship between the physio- with age: a study by computed tomography. Gerodontics logical cross-section of the human jaw muscles and their 3: 151-154 cross-sectional area in computer tomograms. Acta Anato- Norusis M J 1985 Balancing on beams, multivariate analysis mica 118: 129-138 of variance. In: SPSS", Advanced Statistics Guide, Weijs W A, Hillen B 19846 Relationship between masticat- McGraw-Hill Book Company, Chicago ory muscle cross-section and skull shape. Journal of Osborn J W, Baragar F A 1985 Predicted pattern of human Dental Research 63: 1154-1157 muscle activity during clenching derived from a computer Weijs W A, Hillen B 1985 Physiological cross-section of the assisted model: symmetric vertical bite forces. Journal of human jaw muscles. Acta Anatomica 121: 31-35 Biomechanics 18: 599-612 Weijs W A, Hillen B 1986 Correlations between the cross- Promt W R, Fields H W, Nixon W L 1983 Occlusal forces sectional area of the jaw muscles and craniofacial size in normal and long-face adults. Journal of Dental and shape. American Journal of Physical Anthropology Research 62: 566-571 70: 423-431