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

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Relationships Between Jaw Muscle Cross-Sections and Craniofacial Morphology in Normal Adults, Studied with Magnetic Resonance Imaging European Journal of Orthodontics 13 (1991) 351-361 © 1991 European Orthodontics Society 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.
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