A Study on Surgical Implications and Variations of Suprameatal Spine and Other Landmarks on the Lateral Surface of Temporal Bone

Abstract

Mastoidectomy forms the main surgical procedure for eradication of disease in chronic otitis media. Such a surgery which contemplates a complete exenteration of all the tympano-mastoid cavities is rightly considered as a difficult one mainly because a large number of vitally important structures are crowded together in the small field of operation and there is always a risk of damaging them. It is therefore, extremely important that the otologist who would undertake this surgery be thorough with the details of the surgical relations of this region. Out of the various structures seen on the lateral surface of temporal bone one very important structure is the suprameatal spine. It is shown to have great variations in shape and size and has been a mystery to researchers due to the implications it is known to have on mastoid region anatomy. In this study we have evaluated the various types of suprameatal spine, prevalence of each type and the effect of each to other landmarks encountered during surgery and also the effect of mastoid pneumatisation on the same.

Introduction

Chronic suppurative otitis media is a major health problem in many populations around the world affecting around 65–330 million people worldwide, mainly in developing countries. A recent WHO study showed the prevalence in Indian population to be as high as 7.8% which is lower than previous estimates that ranged from 16 to 34% due to an improvement in disease understanding and management [1].

Mastoidectomy is the main surgical procedure to eradicate the disease in cases where the medical treatment does not work. In 1873, Hermann Schwartze and Adolf Eysell [2] published an article describing the procedure for mastoidectomy. It was the first systematic description of how and under what conditions the procedure should be performed. He also described some guidelines to locate the antrum and suggested that the shortest pathway to the antrum was from a point beneath the temporal line at the level of the superior wall of the meatus, 7–8 mm behind the suprameatal spine.

The surgical technique today has been presented in 2 steps, first in the lateral surface of the mastoid and then in the medial deeper planes. This stepwise feature makes it mandatory to know the relationships of the surface landmarks with the landmarks in a deeper location especially for the junior otologists and to develop a deep understanding of the temporal bone anatomy.The temporal bone forms the part of the base and the lateral wall of the skull and is situated between the occipital, sphenoidal and parietal bones. It is a result of the fusion and growth of four bones: the squamous, the petrous, the tympanic bones, and the styloid bones. Due to its multiple embryological origins and adverse developmental aspects temporal bone is considered as one of the most complex anatomical structures, followed by cavities and canals that house the audio-vestibular structures and permits the passage of various vessels and nerves. The temporal bone is comprised of four distinct osseous segments including tympanic, mastoid, petrous, and squamous and four surfaces: the lateral, posterior, superior, and inferior surface. Squama makes up for most of the lateral surface extending upwards as a flat bone to cover part of the temporal lobe of the cerebrum. Several important landmarks mark the lateral surface of the temporal bone, These are:

Mastoid Process: It is a cone shaped bony process which is slightly oblique forward and downwards. It serves as a point of attachment for several muscles like the splenius capitis, longissimus capitis, digastric, and the sternocleidomastoid muscles.

Zygomatic Process: Originates above the external auditory canal and leaves the squama and projects anteriorly to unite the zygomatic bone. On the inferior surface of the zygomatic process is the mandibular or glenoid fossa, which accommodates the condyle of the mandible.

The Temporal Line or Supramastoid Crest: The temporal line is a horizontal ridge situated at the upper limit of the mastoid process. It extends behind the posterior root of the zygomatic process and marks the inferior margins of the insertion of the temporal muscle. The temporal line may be a prominent sharp edge or a broad prominence. It is widely accepted that the temporal line is indicative of the inferior level of the middle fossa dura. Hence, during mastoidectomy, it is always recommended to drill along and not above the temporal line in order to avoid inadvertent injury to the dura.

Suprameatal Mac-Ewen’s Triangle: It is a depression on the lateral surface of the mastoid process. It is located just between the anterior end of the temporal line, Henle’s spine, and the posterosuperior quadrant of the external acoustic meatus. It is a very useful anatomic landmark for the surgical access to the antrum.

Suprameatal Henle’s Spine: It is a prominent bony plate seen on the outer surface of the mastoid process. It is situated behind and above the posterosuperior quadrant of the external auditory meatus and serves as a point of attachment to the ligaments fixing the cartilaginous parts of the external acoustic meatus. Henle’s spine is an excellent landmark during mastoidectomy because it indicates the region of the deeply located aditus ad antrum. In addition it may serve as a landmark for middle fossa dura when the temporal line is absent. Due to the variation in its size and shape and its effect on the deeper planes, suprameatal spine has been a subject of many studies. This study aims to objectively evaluate these clinically relevant relationships.

Aims and Objectives

1. To study the variations in suprameatal spine and the relationship of the various types of suprameatal spine to other surface landmarks and landmarks in the deeper plane.

2. Morphometric analysis of the mastoid bone intraoperatively to find the relation between various landmarks and the various factors influencing the measurements.

Methodology

An explorative study was conducted in the department of otorhinolaryngology, head and neck surgery, JSS Academy of Higher Education and Research, Mysuru for a period of 20 months from July 2018 to Feb 2020. Study included 70 patients who were 18 years or above undergoing cortical mastoidectomy for Chronic otitis media, mucosal type. Patients suffering from Squamosal type of disease were excluded due the effect of disease progression on temporal bone anatomy.

Data Collection

During the study period, patients who were willing to give a written informed consent were enrolled in the trial. HRCT temporal bone was done to assess the status of mastoid pneumatisation. Intraoperative measurements were made to assess the suprameatal spine type and measurements made after a cortical mastoidectomy to study the various relations of surface landmarks on lateral surface of temporal bone. Sterile schirmer’s strip, made of whatman no. 41 filter paper marked with calibrated 1 mm gradations were used for making measurements.

Statistical Analysis

Descriptive statistics were applied and data analysed by proportions and percentages. Relevant preferential statistical tests were applied.

Results

See Tables and Charts.

Discussion

Mastoidectomy as a surgery for eradicating chronic middle ear disease has undergone several transformations to be the procedure known to us today. Before the advent of mastoidectomy as a means of eradicating ear disease patients usually presented to “barber surgeons” with red, warm, and painful abscesses behind the ear that were occasionally drained through an incision. The credit for performing the first mastoidectomy for the removal of purulent secretions went to Jean-Louis Petit, an eighteenth century French surgeon [2]. Unlike his predecessors, Petit clearly established the procedure as a method for drilling the bone using a chisel with deliberate intent of removing pus. The surgery which contemplates a complete exenteration of all the tympano-mastoid cavities is rightly considered as a difficult surgical procedure mainly because a large number of vitally important structures are crowded together in the small field of operation. Any one of these may be easily injured if the surgeon is not thoroughly acquainted with their exact location. Also, the cavities of the tympanum and mastoid, which must be opened and thoroughly cleaned out if these operations are to be successful, are extremely complicated and their character varies widely in different individuals. It is therefore, extremely important that the otologist who would undertake this surgery be thorough with the details of the surgical relations of this region. Out of the various structures seen on the lateral surface of temporal bone one very important structure is the suprameatal triangle, which usually contains a bony spicule or crest in its anterior margin called the suprameatal spine (HS), this spine has been described as serving as an additional attachment point for the ligaments fixing the cartilaginous parts of the external acoustic meatus and temporal fascia and muscle [3]. Suprameatal triangle and spine begin to develop at the end of first year, but traces may be observed at birth. Since the suprameatal triangle and the spine at its lower border nearly complete their development in early adolescence, no changes in these structures have been reported in adult temporal bones [4] and form very consistent landmarks on the lateral surface of temporal bone in ear surgeries. It is shown to have great variations in shape and size and has been a great mystery to researchers due to the implications it is known to have on mastoid region anatomy.

In a study by Anson and Donaldson [5] two types of HS were described: a small and smooth type and a sharp and long type. They also reported mastoid bones with nonexistent HS.

In the study done By Aslan et al. [6] triangular and crest types were found to be 40% and 40% respectively and the HS was absent in 20% however, the study done by Peker et al. [3] showed that the occurrence of the “crest type” of suprameatal spine (77.6% on right and 80% on left side) was significantly greater than the “triangular type” (16% on right and 15.7% on left side) with absent type being the least common(6.4% on right and 4.2% on left side. It is clear that the discrepancy between the above two studies is due to large differences in the number of bones studied. We in our study observed three types of suprameatal spines- crest, triangular and absent type (Figs. 1, 2, 3). Our findings were more consistent with Peker et al. as we found crest type to be most common, in 60% of patients, triangular type was present in 28.6% and absent type in 11.4% of patients (Table1, Fig. 4).

Fig. 1.

Intra operative image showing a crest or prominent type of suprameatal spine

Fig. 2.

Intra operative image showing a triangular type of suprameatal spine

Fig. 3.

Intra operative image showing an absent type of suprameatal spine

Table 1.

Various types and prevalence of each type of spine of Henle

Type of Henle Spine	Count	%
Triangular	20	28.6%
Crest	42	60.0%
Absent	8	11.4%

Fig. 4.

Types and prevalence of various suprameatal spines observed

Various studies such as that by Goycoolea et al. [7] indicated that the antrum was located 12–15 mm deep to McEven’s triangle and is still the generally accepted norm. It serves as a useful anatomic landmark for surgical access to the antrum. Since the dome of the lateral semi-circular canal (LSSC) is the indication of the deepest plane of the mastoid antrum various studies including ours has measured distance between HS and LSSC. Study by Aslan et al. [6] indicate that the HS–LSSC distance is 15 mm on average consistent with the previously established norm but also found out that the distance is longer in bones with a triangular HS than a crest type HS (16.4 vs. 14.3 mm) and that that otologists should anticipate the mastoid antrum to be found in a deeper location in bones with a triangular HS. Similar findings were observed in our study as the average distance between HS–LSSC was 14.71 mm with Minimum observed value in our being 12 mm and maximum was 19 mm. Mastoids with triangular type of HS had longer HS–LSSC with mean distance 16.2 mm ± 0.95 mm. However, unlike the findings in the study by Aslan et al. [6] we observed mastoids with absent type of HS to have the shortest distance, 12.88 ± 0.83 (Table 2, Fig. 5). This distance was found to not vary significantly with pneumatization (15.3 vs. 14.5 mm) in the study by Aslan et al. as well as our study (Table 3).

Table 2.

Correlation of the type of suprameatal spine and various morphometric measurements made intraoperatively:

	Type Of Henle Spine						p
	Triangular(20)		Crest(42)		Absent(8)
	Mean	SD	Mean	SD	Mean	SD
HS–LTI mm	8.35	0.99	8.14	1.09	8.25	1.04	0.8
HS–LSSC mm	16.20	0.95	14.36	0.66	12.88	0.83	< 0.001
HS–SP mm	12.40	2.70	11.26	2.32	11.50	2.67	0.2
TA–LSSC mm	4.95	0.76	5.31	0.81	4.88	0.83	0.2
HS–SDA mm	13.70	2.30	13.05	1.78	13.25	1.83	0.5

Fig. 5.

Significant co-relation between spine of henle type and depth of antrum

Table 3.

Effect of mastoid pneumatization on morphometric analysis:

	CT Pneumatisation
	Non- Pneumatised		Pneumatised
	Mean	SD	Mean	SD
HS–LTI mm	7.76	0.78	9.29	0.78	< 0.0001
HS–LSSC mm	14.47	1.02	15.29	1.68	0.015
HS–SP mm	10.16	0.96	15.00	1.38	< 0.0001
TA–LSSC mm	4.92	0.70	5.71	0.78	< 0.0001
HS–SDA mm	12.27	0.93	15.57	1.69	< 0.0001

The main landmark on the mastoid surface for the middle cranial fossa dura (MFD) is the linea temporalis inferior (LTI) [7, 8]. It is classical knowledge that the LTI is indicative of the most inferior part of the MFD. Hence it is advised to drill along and not over the LTI to avoid inadvertent injury to the MFD [8]. It has been reported that the LTI might be either a prominent sharp edge or a broad prominence, or sometimes be absent [9]. Although a study by Aslan et al. [6] did not observe any bone with an absent LTI, it suggested that the HS can be utilized for estimation of the level of the MFD. They found that the MFD was situated on average 7.8 mm superior to the HS. No significant difference among different types of HS was observed in their study but it was found to be longer in well-pneumatized bones. Our study was intraoperative and we measured the distance between HS and LTI to avoid risking injury to middle fossa dura. We found the mean distance between HS–LTI to be 8.21 mm with highest recorded distance to be 10 mm and lowest 6 mm.There was no statistically significant difference among different HS types in our study (Table 2), but we found it to vary significantly with mastoid pneumatization with mean distance to be 7.76 ± 0.78 mm in non-pneumatised bones and 9.29 ± 0.78 mm in pneumatised bones (Table 3).

It is known that the MFD does not have a straight course. The MFD–LSSC distance was measured as 5.6 mm in the study conducted by Aslan et al. [6]. It was found to be longer in bones with a crest type of HS than in bones with a triangular type of HS (6.9 vs. 4.4 mm). No significant difference was observed with the alteration in pneumatization of mastoid bone, they came to a conclusion that when a bone with a triangular HS is encountered, the surgeon should anticipate the shorter distance between MFD and LSSC leading to more difficult removal of disease at that location. In our study we measured the distance between LSSC and tegmen antri and found the mean distance to be 5.17 mm However, unlike the study by Aslan et al. [6] we did not find any statistically significant difference among different HS types (Table 2). It was however found to vary with pneumatisation with pneumatised bones have longer measurements, 5.71 ± 0.78 mm as compared to non-pneumatised bones having 4.92 ± 0.70 mm (Table 3).

The sigmoid sinus plate (SP) is generally described as the thin bone separating the sigmoid sinus from adjacent structures ie. The mastoid air cells. Classic anatomic descriptions define the sigmoid sinus as the posterior limit of both the mastoid cavity and Trautmann's triangle [10].

The anatomy of the sigmoid sinus (SS) is highly variable, and it is of utmost importance to have detailed knowledge about these variations while performing mastoid surgeries, cochlear implantations, approaches to cerebellopontine angle and other similar surgeries. The variable course of the lateral sinus has been recognized previously by several authors. Meltzer [11] as well as Tumarkin [12] studied the interrelationship between pneumatization and the lateral sinus from a surgical viewpoint. A study done by Singh et al. in 2019 [13] classified sigmoid sinus into three grades with grade I being most posterior that is greatest distance from external auditory canal and grade III being the most anteriorly lying ie. shortest distance from external auditory canal and found that more pneumatised bones were related to grade I and sclerotic mastoids were related to grade III sigmoid sinus. They also found that grade I had the highest mastoid volume as compared to grade III which had the lowest mastoid volume. Similarly, a study by Shatz and Sa´de [14] measured the distance from the lateral sinus to the external auditory canal and found it to be significantly smaller in patients with sclerotic mastoids, a finding that they interpreted as supporting a hereditary theory for chronic otitis media. The mean distance measured for the 150 well pneumatised bones in their study was 13.5 ± 2.8 mm (range, 8–22 mm) whereas for the sclerotic mastoids was 7.8 ± 1.7 mm (range, 7–11 mm). There was a 42% difference between the means of the two groups, which was statistically significant, However the study done by Shatz and Sade [14] was based on radiological measurements. A similar study by Turgut and Tos based [15] on 60 temporal bones measured the shortest distance between sigmoid sinus and external auditory canal as a parameter of the SS location in the mastoid and compared this measurement with the degree of mastoid pneumatization and mastoid length. However contrary to the results obtained by various other studies there was no significant effect of pneumatization on the location of the SS. A similar study by Pedros et al. [16] measured the distance between the spine of Henle and midpoint of sigmoid sinus in 96 temporal bones and found the distance to be shorter in sclerotic mastoids. They also found type II sigmoid sinus ie. Intermediate type to be the most common type which is similar to the findings by Singh et al. [13]. We in our study measured the shortest distance between HS and sigmoid sinus and findings were similar to studies done by Singh et al. [13] and Pedros et al. [16] Mean distance in our study was 11.6 mm with distance being longer in pneumatised mastoids, 15 ± 1.38 as compared to non-pneumatised mastoids with distance being 10.16 ± 0.96 mm. We also measured the distance between HS and sinodural angle (SDA) and found it to be longer in well pneumatised bones (Table 3). Years of research has established that sigmoid sinus is the only constant structure in the mastoid that is not subject to modification by pathological processes. Understanding the possible anatomic variations of the SS can be very useful for an experienced surgeon and can also serve as a great landmark for preventing otologic complications.

Conclusion

Utilizing the data collected in this study, we have aimed to establish some principles involved in mastoid surgery which will be particularly useful to junior otologists.

Mastoid surgery should involve identification of the surface landmarks first especially the spine of Henle and its variations, followed by the landmarks in the deeper plane. As the distance between HS–LSSC is found to vary with HS type, the surgeon can be alerted beforehand whether to expect a superficial or deep lying antrum and prevent major complications. Also, recording mastoid pneumatisation should form an important preoperative assessment as various distances vary with the status of mastoid pneumatisation. This can be particularly useful in avoiding sigmoid sinus injuries.

Declaration

Conflicts of interest

All the authors declare they have no conflict of interest and have not received any funding.

Ethical Approval

All procedures performed in this study were in accordance with the ethical standards of the institute.