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The precision of four commonly used surgical landmarks for locating the facial nerve in anterograde parotidectomy in humans
ARTICLE IN PRESS Annals of Anatomy 192 (2010) 27–32
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RESEARCH ARTICLE
The precision of four commonly used surgical landmarks for locating the facial nerve in anterograde parotidectomy in humans Paul M. Rea a,n, Gerry McGarry b, John Shaw-Dunn a a b
Laboratory of Human Anatomy, Thomson Building, Faculty of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, UK Department of Ear, Nose and Throat Surgery, Glasgow Royal Infirmary, 84 Castle Street, Glasgow G4 0SF, UK
a r t i c l e in fo
abstract
Article history: Received 30 April 2008 Received in revised form 24 September 2009 Accepted 28 September 2009
In addition to using intra-operative facial nerve monitoring in helping to locate the position of the facial nerve in anterograde parotidectomy, numerous soft tissue and bony landmarks have been proposed to assist the surgeon in the early identification of this nerve. There is still dispute within the literature as to the most effective method, if any, of locating the nerve. The purpose of this study was to measure the distance (in twenty-six embalmed cadavers) from four of the most commonly used surgical landmarks to the main trunk of the facial nerve—the posterior belly of digastric muscle (PBDM), the tragal pointer (TP), the junction between the bony and cartilaginous ear canal (EAM) and the tympanomastoid suture (TMS). The main trunk of the facial nerve was found 5.5 7 2.1 mm from the PBDM, 6.9 71.8 mm from the TP, 10.9 71.7 mm from the EAM and 2.5 7 0.4 mm from the TMS. From this, the TMS can be used as a reliable indicator for locating the main trunk of the facial nerve. In addition, this study also demonstrated a statistically significant difference between the sexes in relation to the two bony landmarks used here, the EAM and the TMS, with the facial nerve found further away from those landmarks in females compared to males. With the advent of 3D construction and reformatting of images, these values may come to the forefront in pre-operative planning for locating the facial nerve in anterograde parotidectomy. & 2009 Elsevier GmbH. All rights reserved.
Keywords: Facial nerve Posterior belly of digastric muscle Tragal pointer Tympanomastoid suture Auditory canal Parotidectomy
1. Introduction Parotidectomy is undertaken for a number of reasons, with the most common being for the benign pleomorphic adenoma, previously referred to as a ‘‘mixed’’ tumour due to a number of tissue types being present (Cotran et al., 1999). Since the first known attempt at excision of the parotid gland by Warren of Boston in 1804 (Boswell, 1959), a number of approaches have been developed to allow early intra-operative identification (and thus minimise damage) of the facial nerve (Benedict and Meigs, 1930; McEvedy, 1934; Stein and Geschickter, 1934; McFarland, 1936; Janes, 1940; Rawson et al., 1950). Indeed, nowadays the preferred approach is an anterograde parotidectomy—identification of the main trunk of the facial nerve early in the operative procedure (Marshall and Miles, 1947; Martin, 1952; Patey, 1954; Patey and Thackray, 1958; Beahrs et al., 1960). Although there is continuous monitoring of the facial nerve intra-operatively in an anterograde parotidectomy (Witt, 1998), one area of major concern is identifying the main trunk of the facial nerve early in the procedure (Olsen and Daube, 1994; Terrell et al., 1997; Lowry et al., 2004). Much has been written in the
n
Corresponding author. E-mail address: [email protected] (P.M. Rea).
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literature regarding the ideal landmark, whether that is bony or soft tissue, in helping to locate the main trunk (Britnall et al., 1955; Hogg and Kratz, 1958; Boswell, 1959; Beahrs, 1977; Conley, 1978; Maynard, 1978; Casselman and Mancuso, 1987; Reid, 1989; Nishida and Matsuura, 1993; Holt, 1996; de Ru et al., 2001; Greyling et al., 2007). In addition, there is discrepancy in sex differences that occur using some of the bony landmarks suggested by Conn et al. (1983) and discussed, in brief, by Pather and Osman (2006). Therefore, the purpose of this study is to review four of the most commonly used landmarks in identification of the main trunk of the facial nerve—the posterior belly of digastric muscle (PBDM), the tragal pointer (TP), the junction between the bony and cartilaginous external auditory meatus (EAM) and the tympanomastoid suture (TMS). In addition, this study will clarify any discrepancies that may exist regarding sex differences between these landmarks and their proximity to the main trunk of the facial nerve.
2. Materials and methods The facial nerve was dissected in 26 adult Caucasian cadavers (mean age at death: 83; range 59–98; 15 female, 11 male; no
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craniofacial abnormalities) that had been embalmed and fixed with formaldehyde in the Laboratory of Human Anatomy, University of Glasgow, carried out in accordance to the Human Tissue (Scotland) Act 2006 (Part 5) and the Anatomy Act 1984. All cadavers had varying degrees of dentition, with none being completely edentulous. In each case, the whole cadaver was used and dissected in the supine position with the head rotated slightly (within the limitations of movement of embalmed material) to the opposite side to expose the full lateral aspect of the face, thus mimicking (as much as possible) the position of the head and neck in an anterograde parotidectomy. A pre-auricular incision was made 4 cm superior to the tragus. It extended in an inferior direction to pass below and behind the lobule, over to the mastoid process and then downwards over the anterior border of the sternocleidomastoid muscle into the lowest skin crease of the neck, to avoid the marginal mandibular and cervical divisions of the facial nerve (Fig. 1). A skin flap was elevated in an anterior direction, the ear reflected superiorly and a skin flap created in the posterior direction. These were held open by using surgical nylon thread (either 4/0 or 5/0) with the distal ends of the thread attached to clamps to maintain separation of the tissues. The anterior border of the sternocleidomastoid muscle was freed from the posterior border of the parotid gland and the muscle retracted in a posterior direction. The ear was retracted as far upwards as possible allowing the cartilage of the floor of the external auditory canal to be identified by its cream coloured appearance. At this point, the TP was clearly noted protruding downward, more prominent than the rest of the tragal cartilage due to its pointed appearance. The parotid gland was then dissected in an antero-inferior direction down to the bony EAM. Dissection continued with a Carl Zeiss operating microscope and an accompanying Eschmann DV 110 high vacuum pump with tap water for suction and drainage to give a clear dissecting field. The periosteum over the mastoid process was elevated in an anterior direction to show the tympanic plate. A V-shaped sulcus was found opening forward—the TMS.
Fig. 1. The position of the incision used (shown by the dashed line) to expose the facial nerve as it enters the parotid gland.
Directly below the TMS, a great deal of fatty tissue was meticulously dissected away, making sure no movement or damage to the main trunk of the facial nerve occurred. After identification of the facial nerve, the field inferior to this was cleaned to identify the full length of the PBDM towards the posterior aspect of the parotid gland. Following identification of the four landmarks (PBDM, TP, EAM and TMS), callipers were used to measure the shortest distance from the edge of the main trunk of the facial nerve closest to each of the landmarks. The point used for the measurements for each of the landmarks was as follows (Figs. 2 and 3):
PBDM: the shortest distance between the main trunk of the facial nerve and the most superior aspect of the muscle belly closest to the facial nerve after it passes the most inferior point on the mastoid process, as this is where the surgeon will encounter this muscle belly first. TP: the shortest distance from the middle of the main trunk of the facial nerve to the most inferior and anterior point of the pointer. EAM: the shortest distance from the main trunk of the facial nerve to the most antero-inferior aspect of the junction between the bony and cartilaginous ear canal, as this is what can be felt in operation, rather than the most inferior part, which is more posterior. TMS: the shortest distance from the main trunk of the facial nerve to the apex of the V-shaped sulcus.
Fig. 2. Lateral view of the left side of the cadaver head showing the main trunk of the facial nerve and each of the landmarks used in this study. VII, facial nerve; PBDM, posterior belly of digastric muscle; TP, tragal pointer; EAM, junction between the bony and cartilaginous auditory canal; TMS, tympanomastoid suture; MP, mastoid process.
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Fig. 3. Diagrammatic representations of the landmarks used in this study, and the point where measurements were undertaken. a = shortest distance between main trunk of the facial nerve and the tragal pointer (TP); b= shortest distance between main trunk of the facial nerve and the posterior belly of digastric muscle (PBDM); c= shortest distance between main trunk of the facial nerve and the tympanomastoid suture (TMS); d= shortest distance between main trunk of the facial nerve and the junction between the bony and cartilaginous auditory canal (EAM); VII= facial nerve, MP= mastoid process, n = point from where measurements were taken.
All measurements were made twice at different times in the day using the same callipers and the average taken for each landmark in each specimen. Statistical analysis was undertaken to determine differences between sides and between the sexes by means of a general ANOVA using the statistical software MinitabTM.
3. Results In all dissected specimens, the main trunk of facial nerve left the skull through the stylomastoid foramen and passed in an antero-lateral direction before entering the posterior surface of the parotid gland. The trunk was never longer than 12 mm before dividing into its two main divisions. The facial nerve was always found superior to the PBDM and inferior to the TP, EAM and TMS (Fig. 2). The range of measurements for the mean distances from the main trunk to each of the four landmarks, female/male and left/right side differences can be seen in Tables 1–3, respectively. For each of the landmarks previously mentioned, they lie at a mean distance from the main trunk of the facial nerve (7SD) of 5.572.1 mm to the PBDM, 6.9 71.8 mm to the TP, 10.9 71.7 mm
to the EAM and 2.570.4 mm to the TMS. In essence, the closest structure to the main trunk of the facial nerve is the TMS, with the junction between the bony and cartilaginous auditory canal (EAM) furthest away. The distance between the main trunk of the facial nerve and each of the landmarks was greater in females (except for the posterior belly of digastric). Indeed, there was actually a statistically significant difference between the sexes in distance of the facial nerve to two of the bony landmarks used here—the EAM (p o0.05), as well as the TMS (p o0.05; Table 2). No statistically significant difference occurred between the sexes for the soft tissue landmarks used here, nor between the left and right sides of the specimens in relation to the distance of the main trunk of the facial nerve from the landmarks.
4. Discussion This study shows that (in the twenty-six embalmed cadavers used) the TMS lies closest to the main trunk of the facial nerve compared to the other landmarks, and also raises the issue of there being a difference between the sexes regarding the distance of the facial nerve from the landmarks used in this study.
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Table 1 Distances from the main trunk of the facial nerve to each of the landmarks (n= 52).
Mean (mm) Min (mm) Max (mm) 7SD (mm)
Posterior belly of digastric muscle (PBDM)
Tragal pointer (TP)
Junction between bony and cartilaginous auditory canal (EAM)
Tympanomastoid suture (TMS)
5.5 2.5 9.2 2.1
6.9 3.4 11.3 1.8
10.9 8.2 13.7 1.7
2.5 2.1 3.6 0.4
Table 2 Distances from the main trunk of the facial nerve to each of the landmarks comparing females (F) and males (M) (n =15 females; n =11 males).
TP EAM TMS PBDM
Mean (mm)
7 SD (mm)
Max (mm)
Min (mm)
F
M
F
M
F
M
F
M
7.1 11.5 2.6 5.3
6.7 10.2 2.3 5.9
1.6 1.8 0.5 1.7
2.0 1.3 0.1 2.6
11.3 15.3 3.4 9.2
11.3 13.3 3.3 13.2
4.1 8.3 2.1 2.4
3.4 8.2 2.1 2.5
Table 3 Distances from the main trunk of the facial nerve to each of the landmarks comparing (L) and right (R) sides (n= 26 left, n= 26 right).
TP EAM TMS PBDM
Mean (mm)
7 SD (mm)
Max (mm)
Min (mm)
L
R
L
R
L
R
L
R
7.0 11.0 2.38 5.4
6.9 10.8 2.6 5.7
1.9 1.9 0.3 2.1
1.7 1.4 0.5 2.2
11.3 15.3 3.3 13.2
11.3 13.4 3.6 11.7
3.7 8.2 2.1 2.5
3.4 3.4 2.1 2.4
In essence, it shows that although the absolute values are small overall, it demonstrates that the TMS is simply closer to the main trunk of the facial nerve and when the surgeon locates this landmark, they are indeed very close to the facial nerve. The use of these absolute values is not advocated in day to day surgical practice, but highlights the close proximity and reliability of this landmark to a very important structure, the facial nerve. Indeed, damage to this nerve not only leads to paralysis of the muscles of facial expression but can lead to corneal drying, ulceration and possible blindness (Moser and Oberascher, 1997), but also oral incompetence and lack of execution of mastication, fluid retention and difficulty in verbal communication and emotional expression (de Swart et al., 2003). Therefore, all options should be used, including anatomical landmarks, in locating this structure to minimise damage to it. This study does, however, have some limitations. As the cadavers were formaldehyde fixed, this alters the texture and pliability of the tissue, especially for the soft tissue landmarks namely the TP and PBDM. In addition, the studies were conducted with the ear reflected and the sternocleidomastoid muscle removed, clearly not representing the operative situation, and without as much rotation of the head as would be found in surgery. There were an uneven number of male and female specimens, many of which were elderly at the time of death. This is in contrast to the typical age range of patients affected by pleomorphic adenoma (the most common parotid pathology for which anterograde parotidectomy is performed), which is in the
fifth decade of life (Woods et al., 1977). Also, the population used in this study were all of Caucasian origin, therefore not taking into account any racial differences. Against these limitations there are considerable strengths. As this study was carried out in the laboratory setting, it was possible to spend as long as required to accurately identify all surrounding structures. The regional anatomy could easily be demonstrated in a bloodless field, unlike the operative situation where bleeding can easily obscure the operative field. The variability was minimised by using twenty six cadavers (fifty two half heads) with each measurement taken twice at different times in the day. In addition, the same callipers were used throughout the course of this study to prevent variability occurring in the use of different equipment. In our study, the landmark furthest away from the main trunk of the facial nerve was the EAM. Here, the mean distance ( 7SD) from the main trunk to this landmark was 10.971.7 mm. Also, it is well known that cartilaginous structures are softer, and it may be that the fixed position in the embalmed cadaver did not represent the true position in life (de Ru et al., 2001). As this landmark is furthest away and may be subject to movement, it cannot be counted as a reliable landmark. The TP was the second furthest structure away from the main trunk of the facial nerve in this study, lying a mean ( 7SD) distance away of 6.9 71.8 mm. Trible (1975) argued that the TP is not reliable in use for identifying the main trunk of the facial nerve as it is flexible and subject to retraction. de Ru et al. (2001) echoed this by showing that it was asymmetrical, blunt, mobile, and had a very irregular tip. In addition, they discussed that it may not even ‘‘point’’ in the direction of the facial nerve, with confusion amongst various observers existing regarding the exact position of this landmark. Indeed, on dissection, there was considerable variability in its size and shape, and frequently did not ‘‘point’’ in the direction of the facial nerve, therefore making it an unreliable landmark to use. The PBDM was closer than the previous two landmarks, with a mean ( 7SD) of 5.5 72.1 mm, but it is a soft tissue landmark subject to varying positions. Boswell (1959) found the facial nerve 1.5 cm antero-cranial to the upper border of PBDM and the site of attachment to the mastoid process. Holt (1996) noted the main trunk of the facial nerve to be 9 mm from the anterior border of this muscle. In addition, the attachment of the muscle may be more anterior in its insertion to the mastoid process in some people, and more posterior in others (de Ru et al., 2001) thus altering the relation of it to the main trunk of the facial nerve. Therefore, although close to the facial nerve, this landmark is not ideal for locating the main trunk of the facial nerve in an anterograde parotidectomy. The landmark found closest to the main trunk of the facial nerve was the TMS, with a mean (7SD) distance from it of 2.570.4 mm. This study is in agreement with others who also advocate that this suture is closest to the main trunk of the facial nerve with its position being invariable, related to the nerve as it
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leads to the stylomastoid foramen and also allowing the nerve to be located close to this foramen where it is least likely to be displaced (Britnall et al., 1955; Hogg and Kratz, 1958; Maynard, 1978; Reid, 1989; de Ru et al., 2001). Specifically, our study is in agreement with de Ru et al. (2001) who showed the main trunk of the facial nerve to lie 2.7 mm from the TMS. Indeed, Reid (1989) highlighted that the main trunk of the facial nerve was only 3–4 mm deep to the suture, demonstrating the proximity of this landmark to the nerve. However, Tabb et al. (1970) thought that the facial nerve was 6–8 mm distal to the suture but had no anatomical dissections and were merely reviewing earlier work by Purcelli (1963) who stated that the facial nerve was 7.2 mm from the inferior lateral margin of the TMS. Indeed, further variation in the distance of the facial nerve to the TMS was demonstrated by Pather and Osman (2006). They highlighted that the distance of the nerve to the suture was from 4.9–18.6 mm in a sample of 40 cadavers. Our study, however, shows that the TMS lies closest to the main trunk of the facial nerve (mean of 2.4 mm), and as such can easily be used to locate the main trunk of the facial nerve. The other very interesting finding that emerged from this study is a sex difference for the distance of the main trunk of the facial nerve to the two bony landmarks used in this study. Recently, Pather and Osman (2006) certainly discussed a sex difference in the facial nerve to one of the landmarks they used in their study—the angle of the mandible. However, they compared the differences in distance in relation to a study by Conn et al. (1983), who were not examining the angle of the mandible, but rather the centre of the most posterior point of the ramus of the mandible. In addition, the landmark that Pather and Osman (2006) advocated as the most reliable in locating the main trunk of the facial nerve – the transverse process of the axis – is a landmark not traditionally used by surgeons in locating the nerve in performing anterograde parotidectomy, with no sex differences occurring with that point. To our knowledge, the previous literature does not highlight specific sex differences in the distance of the facial nerve to commonly used surgical landmarks, which seems unusual considering obvious sex differences in the skull. Interestingly, the statistically significant differences noted between male and female specimens are in relation to the two bony landmarks used here—the EAM and the TMS. It may be postulated that these differences could be accounted for by the greater robusticity of the male skull, compared to the female, with males having a larger mastoid process, deeper manibular ramus, larger nuchal crest rigidity and more rugose muscle attachments (Nafte, 2000; White and Folkens, 2005). As the bony landmarks involve the mastoid process, or the immediate surrounding area, these sex differences identified in this study could be accounted for by these established sex differences between male and female skulls. However, interestingly, the distance of the main trunk of the facial nerve to these two landmarks was greater in female specimens compared to male specimens. Indeed, this is certainly an area which needs further investigation as has not been discussed, to our knowledge, in detail within the literature. However, what cannot be determined from this study is if differences between edentulous and non-edentulous mandibles affect the distance of the main trunk of the facial nerve to any of the four landmarks used here (as all had varying degrees of dentition, though none were completely edentulous). Indeed, Pather and Osman (2006) used both edentulous and nonedentulous specimens in their study and did not highlight any differences between the two groups. In addition, it should be noted that although the TMS may be used as an indicator of the closeness to the main trunk of the facial nerve, locating this structure is fraught with difficulties especially when the tumour
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in the parotid gland is large. A bulky infiltrating tumour may distort the intra-parotid facial plexus and scarring may exist from previous operative procedures at this site thus creating a difficult operating area and location of these landmarks (Conley, 1978), highlighting the importance of intra-operative facial nerve monitoring which is routinely undertaken in parotidectomy (Terrell et al., 1997; Lowry et al., 2004; Olsen and Daube, 1994). However, there has been a growing interest in pre-operative imaging and accurate localisation of the facial nerve using other specific landmarks, and its relation to the tumour in planning anterograde parotidectomy. Izzo et al. (2004) revealed that ECHO colour/power Doppler scanning played a very important role in the diagnosis and surgical management of parotid gland pathology. Yeow et al. (2000) demonstrated that using ultrasound guidance, pre-operative localisation of the intra-parotid facial nerve could be related to the intra-parotid vessels. El-Hakim et al. (2003) extended this with a comprehensive study showing that the most accurate structure to use on both CT and MRI scanning for locating parotid lesions in relation to the facial nerve was the retromandibular vein. Indeed, with a move in some fields of surgery for high resolution imagery and 3D construction and reformatted imaging, these values may come to the forefront as highly relevant as we move to smaller fields of view with higher field strengths (Li et al., 1993; Held et al., 1997; Lane et al., 2004).
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Annals of Anatomy journal homepage: www.elsevier.de/aanat
RESEARCH ARTICLE
The precision of four commonly used surgical landmarks for locating the facial nerve in anterograde parotidectomy in humans Paul M. Rea a,n, Gerry McGarry b, John Shaw-Dunn a a b
Laboratory of Human Anatomy, Thomson Building, Faculty of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, UK Department of Ear, Nose and Throat Surgery, Glasgow Royal Infirmary, 84 Castle Street, Glasgow G4 0SF, UK
a r t i c l e in fo
abstract
Article history: Received 30 April 2008 Received in revised form 24 September 2009 Accepted 28 September 2009
In addition to using intra-operative facial nerve monitoring in helping to locate the position of the facial nerve in anterograde parotidectomy, numerous soft tissue and bony landmarks have been proposed to assist the surgeon in the early identification of this nerve. There is still dispute within the literature as to the most effective method, if any, of locating the nerve. The purpose of this study was to measure the distance (in twenty-six embalmed cadavers) from four of the most commonly used surgical landmarks to the main trunk of the facial nerve—the posterior belly of digastric muscle (PBDM), the tragal pointer (TP), the junction between the bony and cartilaginous ear canal (EAM) and the tympanomastoid suture (TMS). The main trunk of the facial nerve was found 5.5 7 2.1 mm from the PBDM, 6.9 71.8 mm from the TP, 10.9 71.7 mm from the EAM and 2.5 7 0.4 mm from the TMS. From this, the TMS can be used as a reliable indicator for locating the main trunk of the facial nerve. In addition, this study also demonstrated a statistically significant difference between the sexes in relation to the two bony landmarks used here, the EAM and the TMS, with the facial nerve found further away from those landmarks in females compared to males. With the advent of 3D construction and reformatting of images, these values may come to the forefront in pre-operative planning for locating the facial nerve in anterograde parotidectomy. & 2009 Elsevier GmbH. All rights reserved.
Keywords: Facial nerve Posterior belly of digastric muscle Tragal pointer Tympanomastoid suture Auditory canal Parotidectomy
1. Introduction Parotidectomy is undertaken for a number of reasons, with the most common being for the benign pleomorphic adenoma, previously referred to as a ‘‘mixed’’ tumour due to a number of tissue types being present (Cotran et al., 1999). Since the first known attempt at excision of the parotid gland by Warren of Boston in 1804 (Boswell, 1959), a number of approaches have been developed to allow early intra-operative identification (and thus minimise damage) of the facial nerve (Benedict and Meigs, 1930; McEvedy, 1934; Stein and Geschickter, 1934; McFarland, 1936; Janes, 1940; Rawson et al., 1950). Indeed, nowadays the preferred approach is an anterograde parotidectomy—identification of the main trunk of the facial nerve early in the operative procedure (Marshall and Miles, 1947; Martin, 1952; Patey, 1954; Patey and Thackray, 1958; Beahrs et al., 1960). Although there is continuous monitoring of the facial nerve intra-operatively in an anterograde parotidectomy (Witt, 1998), one area of major concern is identifying the main trunk of the facial nerve early in the procedure (Olsen and Daube, 1994; Terrell et al., 1997; Lowry et al., 2004). Much has been written in the
n
Corresponding author. E-mail address: [email protected] (P.M. Rea).
0940-9602/$ - see front matter & 2009 Elsevier GmbH. All rights reserved. doi:10.1016/j.aanat.2009.09.005
literature regarding the ideal landmark, whether that is bony or soft tissue, in helping to locate the main trunk (Britnall et al., 1955; Hogg and Kratz, 1958; Boswell, 1959; Beahrs, 1977; Conley, 1978; Maynard, 1978; Casselman and Mancuso, 1987; Reid, 1989; Nishida and Matsuura, 1993; Holt, 1996; de Ru et al., 2001; Greyling et al., 2007). In addition, there is discrepancy in sex differences that occur using some of the bony landmarks suggested by Conn et al. (1983) and discussed, in brief, by Pather and Osman (2006). Therefore, the purpose of this study is to review four of the most commonly used landmarks in identification of the main trunk of the facial nerve—the posterior belly of digastric muscle (PBDM), the tragal pointer (TP), the junction between the bony and cartilaginous external auditory meatus (EAM) and the tympanomastoid suture (TMS). In addition, this study will clarify any discrepancies that may exist regarding sex differences between these landmarks and their proximity to the main trunk of the facial nerve.
2. Materials and methods The facial nerve was dissected in 26 adult Caucasian cadavers (mean age at death: 83; range 59–98; 15 female, 11 male; no
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craniofacial abnormalities) that had been embalmed and fixed with formaldehyde in the Laboratory of Human Anatomy, University of Glasgow, carried out in accordance to the Human Tissue (Scotland) Act 2006 (Part 5) and the Anatomy Act 1984. All cadavers had varying degrees of dentition, with none being completely edentulous. In each case, the whole cadaver was used and dissected in the supine position with the head rotated slightly (within the limitations of movement of embalmed material) to the opposite side to expose the full lateral aspect of the face, thus mimicking (as much as possible) the position of the head and neck in an anterograde parotidectomy. A pre-auricular incision was made 4 cm superior to the tragus. It extended in an inferior direction to pass below and behind the lobule, over to the mastoid process and then downwards over the anterior border of the sternocleidomastoid muscle into the lowest skin crease of the neck, to avoid the marginal mandibular and cervical divisions of the facial nerve (Fig. 1). A skin flap was elevated in an anterior direction, the ear reflected superiorly and a skin flap created in the posterior direction. These were held open by using surgical nylon thread (either 4/0 or 5/0) with the distal ends of the thread attached to clamps to maintain separation of the tissues. The anterior border of the sternocleidomastoid muscle was freed from the posterior border of the parotid gland and the muscle retracted in a posterior direction. The ear was retracted as far upwards as possible allowing the cartilage of the floor of the external auditory canal to be identified by its cream coloured appearance. At this point, the TP was clearly noted protruding downward, more prominent than the rest of the tragal cartilage due to its pointed appearance. The parotid gland was then dissected in an antero-inferior direction down to the bony EAM. Dissection continued with a Carl Zeiss operating microscope and an accompanying Eschmann DV 110 high vacuum pump with tap water for suction and drainage to give a clear dissecting field. The periosteum over the mastoid process was elevated in an anterior direction to show the tympanic plate. A V-shaped sulcus was found opening forward—the TMS.
Fig. 1. The position of the incision used (shown by the dashed line) to expose the facial nerve as it enters the parotid gland.
Directly below the TMS, a great deal of fatty tissue was meticulously dissected away, making sure no movement or damage to the main trunk of the facial nerve occurred. After identification of the facial nerve, the field inferior to this was cleaned to identify the full length of the PBDM towards the posterior aspect of the parotid gland. Following identification of the four landmarks (PBDM, TP, EAM and TMS), callipers were used to measure the shortest distance from the edge of the main trunk of the facial nerve closest to each of the landmarks. The point used for the measurements for each of the landmarks was as follows (Figs. 2 and 3):
PBDM: the shortest distance between the main trunk of the facial nerve and the most superior aspect of the muscle belly closest to the facial nerve after it passes the most inferior point on the mastoid process, as this is where the surgeon will encounter this muscle belly first. TP: the shortest distance from the middle of the main trunk of the facial nerve to the most inferior and anterior point of the pointer. EAM: the shortest distance from the main trunk of the facial nerve to the most antero-inferior aspect of the junction between the bony and cartilaginous ear canal, as this is what can be felt in operation, rather than the most inferior part, which is more posterior. TMS: the shortest distance from the main trunk of the facial nerve to the apex of the V-shaped sulcus.
Fig. 2. Lateral view of the left side of the cadaver head showing the main trunk of the facial nerve and each of the landmarks used in this study. VII, facial nerve; PBDM, posterior belly of digastric muscle; TP, tragal pointer; EAM, junction between the bony and cartilaginous auditory canal; TMS, tympanomastoid suture; MP, mastoid process.
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Fig. 3. Diagrammatic representations of the landmarks used in this study, and the point where measurements were undertaken. a = shortest distance between main trunk of the facial nerve and the tragal pointer (TP); b= shortest distance between main trunk of the facial nerve and the posterior belly of digastric muscle (PBDM); c= shortest distance between main trunk of the facial nerve and the tympanomastoid suture (TMS); d= shortest distance between main trunk of the facial nerve and the junction between the bony and cartilaginous auditory canal (EAM); VII= facial nerve, MP= mastoid process, n = point from where measurements were taken.
All measurements were made twice at different times in the day using the same callipers and the average taken for each landmark in each specimen. Statistical analysis was undertaken to determine differences between sides and between the sexes by means of a general ANOVA using the statistical software MinitabTM.
3. Results In all dissected specimens, the main trunk of facial nerve left the skull through the stylomastoid foramen and passed in an antero-lateral direction before entering the posterior surface of the parotid gland. The trunk was never longer than 12 mm before dividing into its two main divisions. The facial nerve was always found superior to the PBDM and inferior to the TP, EAM and TMS (Fig. 2). The range of measurements for the mean distances from the main trunk to each of the four landmarks, female/male and left/right side differences can be seen in Tables 1–3, respectively. For each of the landmarks previously mentioned, they lie at a mean distance from the main trunk of the facial nerve (7SD) of 5.572.1 mm to the PBDM, 6.9 71.8 mm to the TP, 10.9 71.7 mm
to the EAM and 2.570.4 mm to the TMS. In essence, the closest structure to the main trunk of the facial nerve is the TMS, with the junction between the bony and cartilaginous auditory canal (EAM) furthest away. The distance between the main trunk of the facial nerve and each of the landmarks was greater in females (except for the posterior belly of digastric). Indeed, there was actually a statistically significant difference between the sexes in distance of the facial nerve to two of the bony landmarks used here—the EAM (p o0.05), as well as the TMS (p o0.05; Table 2). No statistically significant difference occurred between the sexes for the soft tissue landmarks used here, nor between the left and right sides of the specimens in relation to the distance of the main trunk of the facial nerve from the landmarks.
4. Discussion This study shows that (in the twenty-six embalmed cadavers used) the TMS lies closest to the main trunk of the facial nerve compared to the other landmarks, and also raises the issue of there being a difference between the sexes regarding the distance of the facial nerve from the landmarks used in this study.
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Table 1 Distances from the main trunk of the facial nerve to each of the landmarks (n= 52).
Mean (mm) Min (mm) Max (mm) 7SD (mm)
Posterior belly of digastric muscle (PBDM)
Tragal pointer (TP)
Junction between bony and cartilaginous auditory canal (EAM)
Tympanomastoid suture (TMS)
5.5 2.5 9.2 2.1
6.9 3.4 11.3 1.8
10.9 8.2 13.7 1.7
2.5 2.1 3.6 0.4
Table 2 Distances from the main trunk of the facial nerve to each of the landmarks comparing females (F) and males (M) (n =15 females; n =11 males).
TP EAM TMS PBDM
Mean (mm)
7 SD (mm)
Max (mm)
Min (mm)
F
M
F
M
F
M
F
M
7.1 11.5 2.6 5.3
6.7 10.2 2.3 5.9
1.6 1.8 0.5 1.7
2.0 1.3 0.1 2.6
11.3 15.3 3.4 9.2
11.3 13.3 3.3 13.2
4.1 8.3 2.1 2.4
3.4 8.2 2.1 2.5
Table 3 Distances from the main trunk of the facial nerve to each of the landmarks comparing (L) and right (R) sides (n= 26 left, n= 26 right).
TP EAM TMS PBDM
Mean (mm)
7 SD (mm)
Max (mm)
Min (mm)
L
R
L
R
L
R
L
R
7.0 11.0 2.38 5.4
6.9 10.8 2.6 5.7
1.9 1.9 0.3 2.1
1.7 1.4 0.5 2.2
11.3 15.3 3.3 13.2
11.3 13.4 3.6 11.7
3.7 8.2 2.1 2.5
3.4 3.4 2.1 2.4
In essence, it shows that although the absolute values are small overall, it demonstrates that the TMS is simply closer to the main trunk of the facial nerve and when the surgeon locates this landmark, they are indeed very close to the facial nerve. The use of these absolute values is not advocated in day to day surgical practice, but highlights the close proximity and reliability of this landmark to a very important structure, the facial nerve. Indeed, damage to this nerve not only leads to paralysis of the muscles of facial expression but can lead to corneal drying, ulceration and possible blindness (Moser and Oberascher, 1997), but also oral incompetence and lack of execution of mastication, fluid retention and difficulty in verbal communication and emotional expression (de Swart et al., 2003). Therefore, all options should be used, including anatomical landmarks, in locating this structure to minimise damage to it. This study does, however, have some limitations. As the cadavers were formaldehyde fixed, this alters the texture and pliability of the tissue, especially for the soft tissue landmarks namely the TP and PBDM. In addition, the studies were conducted with the ear reflected and the sternocleidomastoid muscle removed, clearly not representing the operative situation, and without as much rotation of the head as would be found in surgery. There were an uneven number of male and female specimens, many of which were elderly at the time of death. This is in contrast to the typical age range of patients affected by pleomorphic adenoma (the most common parotid pathology for which anterograde parotidectomy is performed), which is in the
fifth decade of life (Woods et al., 1977). Also, the population used in this study were all of Caucasian origin, therefore not taking into account any racial differences. Against these limitations there are considerable strengths. As this study was carried out in the laboratory setting, it was possible to spend as long as required to accurately identify all surrounding structures. The regional anatomy could easily be demonstrated in a bloodless field, unlike the operative situation where bleeding can easily obscure the operative field. The variability was minimised by using twenty six cadavers (fifty two half heads) with each measurement taken twice at different times in the day. In addition, the same callipers were used throughout the course of this study to prevent variability occurring in the use of different equipment. In our study, the landmark furthest away from the main trunk of the facial nerve was the EAM. Here, the mean distance ( 7SD) from the main trunk to this landmark was 10.971.7 mm. Also, it is well known that cartilaginous structures are softer, and it may be that the fixed position in the embalmed cadaver did not represent the true position in life (de Ru et al., 2001). As this landmark is furthest away and may be subject to movement, it cannot be counted as a reliable landmark. The TP was the second furthest structure away from the main trunk of the facial nerve in this study, lying a mean ( 7SD) distance away of 6.9 71.8 mm. Trible (1975) argued that the TP is not reliable in use for identifying the main trunk of the facial nerve as it is flexible and subject to retraction. de Ru et al. (2001) echoed this by showing that it was asymmetrical, blunt, mobile, and had a very irregular tip. In addition, they discussed that it may not even ‘‘point’’ in the direction of the facial nerve, with confusion amongst various observers existing regarding the exact position of this landmark. Indeed, on dissection, there was considerable variability in its size and shape, and frequently did not ‘‘point’’ in the direction of the facial nerve, therefore making it an unreliable landmark to use. The PBDM was closer than the previous two landmarks, with a mean ( 7SD) of 5.5 72.1 mm, but it is a soft tissue landmark subject to varying positions. Boswell (1959) found the facial nerve 1.5 cm antero-cranial to the upper border of PBDM and the site of attachment to the mastoid process. Holt (1996) noted the main trunk of the facial nerve to be 9 mm from the anterior border of this muscle. In addition, the attachment of the muscle may be more anterior in its insertion to the mastoid process in some people, and more posterior in others (de Ru et al., 2001) thus altering the relation of it to the main trunk of the facial nerve. Therefore, although close to the facial nerve, this landmark is not ideal for locating the main trunk of the facial nerve in an anterograde parotidectomy. The landmark found closest to the main trunk of the facial nerve was the TMS, with a mean (7SD) distance from it of 2.570.4 mm. This study is in agreement with others who also advocate that this suture is closest to the main trunk of the facial nerve with its position being invariable, related to the nerve as it
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leads to the stylomastoid foramen and also allowing the nerve to be located close to this foramen where it is least likely to be displaced (Britnall et al., 1955; Hogg and Kratz, 1958; Maynard, 1978; Reid, 1989; de Ru et al., 2001). Specifically, our study is in agreement with de Ru et al. (2001) who showed the main trunk of the facial nerve to lie 2.7 mm from the TMS. Indeed, Reid (1989) highlighted that the main trunk of the facial nerve was only 3–4 mm deep to the suture, demonstrating the proximity of this landmark to the nerve. However, Tabb et al. (1970) thought that the facial nerve was 6–8 mm distal to the suture but had no anatomical dissections and were merely reviewing earlier work by Purcelli (1963) who stated that the facial nerve was 7.2 mm from the inferior lateral margin of the TMS. Indeed, further variation in the distance of the facial nerve to the TMS was demonstrated by Pather and Osman (2006). They highlighted that the distance of the nerve to the suture was from 4.9–18.6 mm in a sample of 40 cadavers. Our study, however, shows that the TMS lies closest to the main trunk of the facial nerve (mean of 2.4 mm), and as such can easily be used to locate the main trunk of the facial nerve. The other very interesting finding that emerged from this study is a sex difference for the distance of the main trunk of the facial nerve to the two bony landmarks used in this study. Recently, Pather and Osman (2006) certainly discussed a sex difference in the facial nerve to one of the landmarks they used in their study—the angle of the mandible. However, they compared the differences in distance in relation to a study by Conn et al. (1983), who were not examining the angle of the mandible, but rather the centre of the most posterior point of the ramus of the mandible. In addition, the landmark that Pather and Osman (2006) advocated as the most reliable in locating the main trunk of the facial nerve – the transverse process of the axis – is a landmark not traditionally used by surgeons in locating the nerve in performing anterograde parotidectomy, with no sex differences occurring with that point. To our knowledge, the previous literature does not highlight specific sex differences in the distance of the facial nerve to commonly used surgical landmarks, which seems unusual considering obvious sex differences in the skull. Interestingly, the statistically significant differences noted between male and female specimens are in relation to the two bony landmarks used here—the EAM and the TMS. It may be postulated that these differences could be accounted for by the greater robusticity of the male skull, compared to the female, with males having a larger mastoid process, deeper manibular ramus, larger nuchal crest rigidity and more rugose muscle attachments (Nafte, 2000; White and Folkens, 2005). As the bony landmarks involve the mastoid process, or the immediate surrounding area, these sex differences identified in this study could be accounted for by these established sex differences between male and female skulls. However, interestingly, the distance of the main trunk of the facial nerve to these two landmarks was greater in female specimens compared to male specimens. Indeed, this is certainly an area which needs further investigation as has not been discussed, to our knowledge, in detail within the literature. However, what cannot be determined from this study is if differences between edentulous and non-edentulous mandibles affect the distance of the main trunk of the facial nerve to any of the four landmarks used here (as all had varying degrees of dentition, though none were completely edentulous). Indeed, Pather and Osman (2006) used both edentulous and nonedentulous specimens in their study and did not highlight any differences between the two groups. In addition, it should be noted that although the TMS may be used as an indicator of the closeness to the main trunk of the facial nerve, locating this structure is fraught with difficulties especially when the tumour
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in the parotid gland is large. A bulky infiltrating tumour may distort the intra-parotid facial plexus and scarring may exist from previous operative procedures at this site thus creating a difficult operating area and location of these landmarks (Conley, 1978), highlighting the importance of intra-operative facial nerve monitoring which is routinely undertaken in parotidectomy (Terrell et al., 1997; Lowry et al., 2004; Olsen and Daube, 1994). However, there has been a growing interest in pre-operative imaging and accurate localisation of the facial nerve using other specific landmarks, and its relation to the tumour in planning anterograde parotidectomy. Izzo et al. (2004) revealed that ECHO colour/power Doppler scanning played a very important role in the diagnosis and surgical management of parotid gland pathology. Yeow et al. (2000) demonstrated that using ultrasound guidance, pre-operative localisation of the intra-parotid facial nerve could be related to the intra-parotid vessels. El-Hakim et al. (2003) extended this with a comprehensive study showing that the most accurate structure to use on both CT and MRI scanning for locating parotid lesions in relation to the facial nerve was the retromandibular vein. Indeed, with a move in some fields of surgery for high resolution imagery and 3D construction and reformatted imaging, these values may come to the forefront as highly relevant as we move to smaller fields of view with higher field strengths (Li et al., 1993; Held et al., 1997; Lane et al., 2004).
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