Introduction

Frontalis is a paired muscle of the forehead that extends from the supraorbital region to approximately the level of the coronal suture line. The muscle originates from the galea aponeurotica and inserts along the eyebrow. Part of the muscle is attached to the nasal bone and the angular process of the frontal bone. The muscle is flat and quadrilateral in shape, with the fibres orientated upwards and outwards along the forehead. The galea splits to enclose the muscle and has a thin superficial component and a well-defined, deeper component. The galea continues as superficial temporal fascia laterally and attaches to the occipitalis posteriorly. The frontalis is firmly attached to the superficial fascia and dermis of the skin via multiple fibrous septa. The blood supply of the muscle comes from the frontal branch of the superficial temporal artery laterally and from the supratrochlear and supraorbital arteries medially. The frontal branch is the terminal branch of the superficial temporal artery that anastomoses with the supraorbital and supratrochlear arteries within the muscles. The latter vessels exit through the foramen and notch of the orbital rim, pass through the corrugators and supply the frontalis muscle. The temporal branch of the facial nerve innervates the muscle. The nerve travels within the innominate fascia as it crosses the zygomatic arch and runs in the superficial temporal fascia to supply the frontalis muscle.

The glabellar complex: At the eyebrow area, the fibres of the frontalis intertwine with the procerus, corrugator supercilii, depressor supercilii and orbicularis oculi to form the glabellar complex. The corrugators are bilateral muscles arising from the medial supraorbital bone and insert into the middle eyebrow skin. The fibres lie deep to the medial fibres of the frontalis and superficial to the periosteum. The procerus is a vertical flat muscle arising from the nose and interdigitates with the frontalis in the middle. Similarly, the depressor supercilii and the medial fibres of the orbicularis oculi interdigitate with the frontalis medially and altogether form the glabellar complex muscles.

The clinical significance of the frontalis is especially related to facial aesthetics and pathological conditions involving soft tissue defects and motor imbalance of the forehead. The frontalis plays an important role in the dynamic equilibrium of the eyebrows. The frontalis act as the elevating muscle of the upper face, and the resting tone of the muscle has a significant effect on the position of the eyebrows. Forehead rhytids represent one of the most common causes of aesthetic consultation and are often treated with neurotoxin injections. Understanding the intricate interaction of this muscle with the glabellar complex is important to achieve an optimal outcome in these clinical situations. The muscle can be selectively used for reconstructive purposes, and the applications include frontalis myo-fascial flaps for cranialisation of the frontal sinus and for soft tissue cover of the median and para-median forehead defects [1,2,3].

The morphological anatomy of the frontalis was studied on the backdrop of various clinical situations. David Knize in 1996 analysed the mechanical forces acting on the eyebrows and the age-related changes of the eyebrows [4]. The presence of midline dehiscence was described by Spegial et al., who highlighted the importance of selective injection of the neurotoxins [5]. Further studies explored the anatomy of the muscles in relation to blepharoptosis and asymmetry of the eyebrows [6, 7]. Three-dimensional analyses of the forehead wrinkles of cadaveric specimens provided important information regarding the plane of dissection during forehead lift [8].

This dissection study was designed to evaluate the variation of the morphology of the frontalis muscle, and the results are analysed against ethnic and gender variation.

Materials and Method

The study was conducted at the Department of Anatomy, at the Imperial College, London, UK and at the University of Colombo, Sri Lanka. Eleven formalin-fixed cadavers of Caucasian origin and 17 cadavers from South East Asian origin were used for the study. One specimen from each group was disqualified due to the presence of previous surgery involving the forehead leading to the altered anatomy of the muscle. The final numbers were 16 Southeast Asian origin specimens and 10 Caucasian origin specimens. Amongst these 14 were adult females and 12 were adult males.

The dissection was carried out by the authors. The skin flap of the forehead was elevated by sharp dissection using magnification to preserve the muscle fibres intact. The flaps were dissected from the supraorbital region up to the vertex superiorly and to the temporal and zygomatic regions laterally. The muscle bellies were exposed completely in order to study the morphology. The topographic anatomy was analysed, and the pattern of interdigitation between two muscle bellies was noted. The following measurements were taken on each specimen.

  • The length and width of the muscle bellies on the right and left side.

  • The gap between the two bellies of the muscle at the base, middle and at the insertion.

  • The width of muscle insertion to the eyebrows.

Results

The average length of the muscle is 10.9 cm in males and 9.1 cm in females. The average width of the muscle in females was 4 cm at the origin and 6.5 cm at the eyebrow insertion. In males, the average width of the muscle at the origin is 4.4 cm and 5.8 cm at the eyebrow insertion. The comparison between the length and width of the muscle between the two ethnic groups does not reveal any statistically significant differences.

The muscle fibres of the frontalis had an upward and outward slope from the base. The two muscle bellies were found to have interdigitation at various levels along the medial border. Beyond the level of the interdigitation, the medial border of the muscle was found to have a smooth outline. The classification of the muscle was based on the distance between the two bellies and the extent of interdigitation on the midline.

Type 1: The two bellies of the muscle decussate only at the base and gradually diverge upwards and outwards (Fig. 1).

Fig. 1
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Type 1: The two muscle bellies decussate at the eyebrow attachment in the midline and gradually diverge upwards and outwards

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Type 2: The two bellies of the muscle decussate up to the mid-length of the muscle and diverge thereafter (Fig. 2).

Fig. 2
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Type 2: The two muscle bellies decussate up to the mid length of the muscle and diverge thereafter

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Type 3: There is no interdigitation between the two muscle bellies at any level (Fig. 3).

Fig. 3
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Type 3: There is no interdigitation between the two muscle bellies at any level

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Analysis of the extent of the attachment of the muscle to the eyebrows revealed two main patterns. In six specimens, the muscle was attached only up to the medial two-thirds of the eyebrows (Fig. 4). Amongst these, five specimens were of Caucasians origin. In the rest of the specimens, the muscles were attached along the full length of the eyebrow (Fig. 5).

Fig. 4
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The frontalis muscle is attached only up to the medial two-thirds of the eyebrow

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Fig. 5
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The frontalis muscle is attached to the full length of the eyebrow

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Discussion

This study provides a comprehensive analysis of the topographic anatomy of the frontalis muscle. Based on our findings, the muscle can be classified into three main groups in relation to the anatomical configuration. The variation is based on the extent of interdigitation between the two muscle bellies in the midline. It is interesting to observe the parallel between the configuration of the frontalis and platysma muscles, which were subject to a similar anatomical classification. The study also notes the variations in attachment of the muscle to the eyebrows.

The anatomical study done by Spegial et al. highlighted the presence of midline dehiscence of the frontalis on the forehead and discussed the importance of the selective placement of toxins for forehead wrinkles. The dissection was done via the coronal approach at the subperiosteal plane. The flap was lifted inferiorly to analyse the pattern of the muscle. Topographic measurement of the muscle, midline attenuation point, angulations of the muscle bellies and the distance between the two bellies were analysed. The authors also noticed a staircase and ‘W’ pattern of the medial border of the muscle that had a gender variation. This study provided important information regarding the anatomy of the muscle. Whilst the ingeniosity of preserving the frontalis muscles on the skin flap allows for easier dissection, the subsequent contour analysis of the muscle anatomy is no longer in its anatomical position. Thus, the measurements and to a lesser extent morphological description on a non-anatomical position of the muscle may introduce errors related to the impossibility of restoring the 3D shape of the muscles.

In our study, we took great care to preserve and maintain the normal anatomy of the muscle in order to evaluate the variations in the morphology. This flat muscles on either side extended from the supraorbital region up to just proximal to the coronal suture line. In our study, none of the specimens showed an irregular staircase or ‘W’ pattern on the midline.

The important observation was related to the interdigitation at various levels of the muscle. The medial border of the muscle bellies was smooth cranial to the interdigitation. The average lengths and widths of the muscle in this study were comparable between both ethnic groups. The length of the muscle is comparatively longer in males and this correlates with the longer forehead in men. Regarding the distribution of the type of the muscle, there was no significant variation between the genders from both ethnic groups. The most common pattern that was observed in both ethnic groups was the Type 2 pattern (46%) where the muscle bellies were decussating up to the mid-forehead. Types 1 and 3 were distributed in equal percentages (27%) in both groups.

Understanding the anatomical variation of the frontalis muscle assists in the successful application of neurotoxins for the treatment of forehead rhytids. During the assessment of the patient, many factors are taken into consideration on deciding the total dose and pattern of the toxin injection. The anatomical features such as shape of the eyebrows, presence of brow ptosis and the characteristics of the frontalis especially in relation to the shape, height, width and muscle mass are important determinants. The static and dynamic evaluation caused by the complex interplay between the glabellar muscles is also taken into account during planning. The variation in the facial features amongst the genders can have an impact on the treatment protocol as men usually have a large forehead and greater muscle mass compared to their female counterparts. The thickness, shape and position of the eyebrows are different amongst the genders and the patient expectation and aesthetic outcome varies between both groups.

The morphological variation and the varied pattern of attachment of the muscle along the eyebrows observed in this study may have an impact on the characteristics of the forehead rhytids and ageing patterns. Appreciating these differences is imperative in deciding the total dose and eventual placement of the toxin. Gentle palpation of the forehead during rest and dynamic movement of the eyebrows may help in identifying the type of the muscle during the initial evaluation. The injection sites and depth are taken into consideration when treating the horizontal forehead rhytids especially in type 1 and type 3, where the decussation of the muscle bellies is scarce in the midline (Figs. 6, 7 and 8). Observation of these morphological variations reinforces the finding of the previous paper as the application of toxins in the non-muscle tissues may result in alteration in the diffusion distances with undesirable results [5]. In situations where the muscle is attached along the whole length of the eyebrows, the standard application of neurotoxins can provide the anticipated elevation of the lateral eyebrow. Whereas it is more difficult to produce a satisfactory lateral brow elevation using neurotoxins, where the muscle has no attachment to the lateral eyebrow. These patients usually present with a lateral slanting eyebrow and should be advised about the limitations of the treatment with the neurotoxins and alternate options may be considered to achieve the lateral eyebrow lift (Figs. 4 and 9c).

Fig. 6
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Impact of the anatomical variation of the muscle in administering neurotoxin

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Fig. 7
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Considerations in administering neurotoxins in type 1—decussation only at the eyebrows with non-muscle tissue in the mid forehead

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Fig. 8
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Considerations in administering neurotoxins in type 3—no interdigitation between the two muscle bellies with non-muscle tissue in the mid forehead

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Fig. 9
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Variation in the position of the eyebrows—a Medial peak, b Lateral elevation, c and d Lateral slanting

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The frontalis muscle attachment to the eyebrows was extensively studied by David Knize. His anatomical dissection and clinical studies provided an in depth analysis of the mechanical forces related to the ptosis of the eyebrows. He observed a consistent relationship between the lateral border of the frontalis muscle to the temporal fusion line of the skull. The muscle either ended or attenuated lateral to the attachment to the temporal fusion line. He also observed that the position of the temporal fusion line varied amongst the specimens, from the middle third of the eyebrow to just lateral to the eyebrow. The histological study associated with the dissections described the relationship of the frontalis muscle to the orbicularis oculi and corrugator supercilia and the galeal plans. Knize concluded that the ptosis of the lateral eyebrow is greatly influenced by the gravitational descent of the unsupported soft tissue mass over the temporalis fascial plane lateral to the temporal fusion line. The lateral descent is further influenced by the mobile lateral part of the galeal fat pad and the action of transverse band of the corrugator and lateral portion of the orbicularis oculi muscles. His descriptions of the ptosis of the lateral eyebrows were mostly correlated with the ageing process.

The dissection finding in our study highlights two main variations of the attachment of the muscle to the eyebrows. In the majority of the specimens, the lateral part of the frontalis origin expanded the full length of the eyebrows, and in a minority of specimens (6 out of 24), it was reaching only up to the junction between the medial two-thirds and lateral third of the eyebrows. In our clinical practice, we have observed a number of young patients present with a noticeable downward slanting of the lateral segment of the eyebrows. This can also be described as the medial position of the “peak” of the eyebrows, between the medial third and lateral two thirds of the eyebrow (Fig. 9a), with the majority of patients presenting with a lateral eyebrow “peak” at the junction between the medial two-thirds and the lateral third (Fig. 9b). This observation is made also on young individuals who often have aesthetic concerns related to these ‘sad eyebrows’ that are often “over-plucked” laterally (Fig. 9c, d). The causes of this anatomical variation cannot be attributed to the dynamic imbalance of the eyebrows caused by the ageing process, and when asked to raise the eyebrows, these patients often display a deficit of lateral elevation. Typically, in these patients, it is more difficult to produce a satisfactory lateral brow elevation using neurotoxin.

Six of the specimens have shown this pattern of muscle attachment and the variation was found to be in higher percentages in the Caucasians. David Knize describes a similar situation mostly observed during the ageing process. It is however unlikely that this represents a symmetric dehiscence of the muscle insertion and more likely that it represents a normal variation and can be a distinct deformity in young individuals. Attention to this variation of the attachment of the frontalis muscle and modification of the approach through surgical and non-surgical treatments is likely to impact on the outcomes.

Based on the dissection studies, the key steps for the forehead lift caused by the ageing process were recommended by David Knize. These include lateral eyebrow segment re-suspension by releasing and cephaloid transpositioning of the zone of fixation and suture stabilisation to the temporalis fascia and superficial temporal fascia of the scalp. Resection of the galeal fat pad and hyperactive corrugator supercilii muscles were proposed as additional procedures to reinforce the lateral eyebrow lift in these cases. From our clinical observations, the young patients presenting with slanting eyebrows possibly caused by the deficient lateral attachment of frontalis may have limitations in achieving lateral brow lift using neurotoxins. In the surgical interventions, this patient population is more likely to benefit from a strong superolateral vector of brow elevation (Fig. 10).

Fig. 10
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Direction of forces acting on the eyebrows with deficient muscle fibres laterally

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Clinical testing to some extent can assess the anatomical variation of the frontalis muscle. The muscle can be tested during rest and animation by gentle palpation. Ultrasonographic assessment of the muscles may be helpful in providing more information about the morphology. A recent publication by Volk et al. demonstrated quantitative ultrasonographic assessment of the facial muscles. The authors describe the usefulness of the ultrasound in defining the facial muscles in relation to the visibility, separation from the adjacent muscles, and the reliability of the landmarks of the facial muscles. Six facial muscles including frontalis could be clearly defined by this method [9]. Selective use of this mode of investigation may be useful in planning and modifying the aesthetic procedures in this group of patients.

Conclusion

An anatomical dissection was carried out to analyse the morphological variation of the frontalis muscle. A classification of the muscle is presented based on the extent of the interdigitation between the medial borders of the muscle bellies. The prevalence of the normal variation amongst two ethnic groups and the gender variation is analysed. Variable attachment of the muscle to the eyebrows helps to understand the dynamic balance in patients with the downward slanting eyebrows. Preoperative investigation and in-depth analysis will help to formulate the appropriate treatment in these patients.