Journal of the Anatomical Society of India

Wadhwa, S., Kaul, J.M. and Agarwal, A.K.*
Department of Anatomy, Maulana Azad Medical College, New Delhi.
*Department of E.N.T, Maulana Azad Medical College, New Delhi.

Abstract:

To perform microsurgical maneuvers and manipulations in the limited working space available in the ear, the otologic surgeon needs to be fully conversant with the anatomical details of the middle ear and stapes. In the ossicular chain, the human stapes is strategically located between the middle and internal ear. Its architecture and morphometry have surgical implications on the techniques designed to mobilize it, when affected by the spread of sclerotic bone disease. In India, data on normal anatomical parameters of the middle ear is limited. The present study attempts to provide anatomical detail of the stapes and its relation to the facial nerve in the Indian population, and compare the parameters with those reported from Western population to ascertain any racial differences. These differences in dimensions of the stapes may have a bearing in designing prosthesis more appropriately suited for the Indian population. Seventeen cadaver temporal bones were dissected for qualitative study of the stapes (12), its further evaluation with regard to dimensions (10) and its relationship to the facial nerve by cross - sectioning (5). The measured dimensions of the stapes included: mean anatomical height 3.41± 0.20 mm; average height of the superstructure 3.20 ± 0.17 mm; length 2.97± 0.31 mm and width of the footplate 0.38 ± 0.10 mm; length of the head 0.69±0.21 mm; the length of the neck averaged at 0.47 ± 0.69 mm; length of the posterior crus 1.97 ± 0.15 mm and anterior crus of stapes 1.76 ± 0.16 mm. Our data suggests that the combined dimension of the stapedial head and neck in the Indians is greater than that reported for the Western population. Also the Indian stapedial crura are slender than their Western counterparts, with the thinnest portion being the center of the crura. Concurrent with the advances in instrumentation and technology, a detailed knowledge of the footplate of stapes is essential to optimize the results of reconstructive and prosthetic surgery. Its dimensions influence the choice of the prosthesis used and its placement on the footplate. A normally positioned facial nerve with a well-formed bony facial canal was evident in 60% of the temporal bones. A dehiscent facial canal and an overhanging facial nerve in the area of the footplate were the commonest variations noted in our study. This important finding must be borne in mind during stapedial surgery to avoid injury to the nerve.

Key words: dimensions; stapes; temporal bone; Facial Nerve, Indian population

Introduction:

A WHO survey carried out in 2001 showed that there are at least 250 million people suffering from hearing loss globally, which represents about 4% of the world population. Two-third of these reside in the developing countries. In India, 5.9% of the population, estimated at 60 million, has a disabling hearing impairment and associated moderate or worse hearing impairment in the better ear. It has been estimated that a large segment of the deaf in developing countries requires 32 million hearing aids per year, Kumar (2001).

Reconstruction procedures for sound conduction in the middle ear have advanced substantially since 1950s. A resultant achievement of good hearing following surgery, particularly in the presence of disease or malformations is one of the greatest challenges for the otologists. With advances in instrumentation and technology, newer and better techniques of surgery are available. Increasingly, otologic surgeons are performing surgeries for deafness and vertigo like cochlear implant surgeries, ossicular replacements and ossiculoplasty. The principle of ear surgery involves movement from one known landmark to another, allowing the otologic surgeon to navigate through the maze of the vital structures without damaging them. He thus needs to be fully conversant with the anatomical details of the middle ear and stapes prior to undertaking surgical procedures, which involve mobilization, partial or complete removal and stapedial prosthetic implantation, Tjellström (1977) and Weit et al. (1993). Further, the detailed knowledge of the stapes superstructure is essential in the designing of electromagnetic middle ear implants àWengen et al. (1995). In situations of an eroded or absent incus, an incus replacement ossiculoplasty can be a daunting task and is incumbent upon the otologists to achieve a stable, reliable connection between the tympanic membrane and the mobile stapes to provide good long term results, Wehers (1994). The development of Krauss-modified Schusing ossicle cup prosthesis, used in incus replacement is intimately dependant upon detailed information of the stapes superstructure, Kraus (1993).

The facial nerve is one of the most significant and vulnerable structures in the temporal bone. The relationship between the facial nerve and the stapes gets determined by fourth week of fetal life. This nerve is at risk during surgical procedures involving the middle ear and /or the internal ear. An abnormal course of the facial nerve becomes particularly important during surgeries of stapes. The vulnerability of the facial nerve is an ever-present specter that haunts the otologists.

Materials and Methods

Seventeen temporal bones were procured from cadavers available in the Department of Anatomy, Maulana Azad Medical College, New Delhi. The temporal bones were removed en bloc (Fig 1). Of these, ten bones were studied for their gross features while five were processed for cross sectional anatomy to study the relationship of the facial nerve to stapes. By commencing the drilling in the area of the McEwans triangle, the mastoid cortex was unroofed from the posterior canal wall till slightly beyond the sigmoid venous sinus (Fig 2). The posterior canal wall was also thinned forming a reniform cavity to facilitate adequate visualization of the ossicular chain (Fig 3). The ossicles were disarticulated and the malleus and incus were removed. The stapes was visualized in situ. The relationship of the facial nerve to the stapes was noted.

Fig. 1: Diagrammatic representation of the ' Block technique' for removal of temporal bone.	Fig. 3: Landmarks seen within the Tympanic Cavity TMTympanic Membrane; M- Malleus; I --Incus; SStapes; LSC - Lateral Semicircular Canal; Fn- Facial nerve; Ct - Chorda Tympani
Fig. 2: Landmarks found during mastoid antral dissection for exposure of the middle ear cavity from the posterolateral aspect EAM -- External Acoustic Meatus; PCW- Posterior Canal Wall; FI- Fossa Incudis; DR- Digastric Ridge; HSC -- Horizontal Semicircular Canal; TT- Tegmen Tympani; SSSigmoid Sinus	Fig. 4: Diagrammatic representation of the stapes showing the various parameters measured.

With the help of a needle the footplate was freed from the oval window niche to remove the stapes. The stapes was washed and studied using the De Winter Optical Inc. image analyzer (Bio Wizard software) to measure the following parameters (Fig 4):

• height of the stapes,
• height of the stapes to the upper surface of the footplate,
• length of the footplate,
• length of the head perpendicular to the long axis of the footplate,
• length of the head parallel to the long axis of the footplate,
• length of the neck,
• length of the posterior crus,
• width of the posterior crus at the level of the shoulder, center and the footplate,
• length of the anterior crus and
• width of the anterior crus at the level of the shoulder, center and the footplate

To observe the shape of the footplate the stapes was fixed with tissue glue on the slide in an inverted manner and photographed under the image analyzer. Students 't' test was used to statistically compare the measured data.

Five temporal bones were decalcified, blocked in paraffin, sectioned at 20 ìm and stained with hematoxylin and eosin.

Two bones were processed for viewing with scanning electron microscope after critical point drying and sputter coating with gold.

Results
Qualitative Observations

The stapes head displayed a variety of sizes and shapes. One out of ten bones studied had head larger than the rest; while one had a comparatively smaller head .The neck of the stapes was well defined in all but two bones. In these the head seemed to be directly resting on the shoulders. In one specimen the neck was unusually long. The crura showed variations in size and thickness. In all bones studied, the posterior crus were longer than the anterior crus. In six bones the anterior crus was straight as compared to the posterior crus, which was curved. In one stapes, both the crura were equally curved. The obturator foramen showed a variety of shapes. The various shapes observed were: oval in 2 specimens, circular in 3, triangular in 4 and semicircular in 1 (Fig 5a,b).

The footplate showed variations with regards to the shape of its superior and inferior borders as well as the relative sizes of its two ends (Fig. 6). The footplate of three bones in the study group showed a process extending from its junction with the posterior crura. While in most of the bones the footplate ended with the crural attachment, in four bones the annular rim of the footplate extended beyond the attachment of the crura. In four bones the posterior end was broader than the anterior; in three bones the anterior end was broader while the remaining had almost equally wide ends. In six bones of the study group, the superior border was convex; one bone had variable degree of concavity in its border and three of them showed a straight border. In five out of ten bones the inferior border was straight. In the remaining bones, the inferior border in two stapes was concave; in two it was convex and one bone showed a sigmoid shape. In one bone the anterior end at its junction with the inferior border showed a process.

Quantative measurements

Fig 5a: Scanning Electron Micrograph of the stapes in the tympanic cavity H-Head; N- Neck; C- Crus; O- Obturator for a men

Fig. 5b: Representative shapes of the stapes head, neck and obturator foramen. H-Head; N- Neck; C- Crus

The height of the stapes (L1) from its head to the undersurface of the stapedial footplate was measured. It showed a range of 3.06 mm to 3.71 mm with a mean of 3.41± 0.20 mm. The height (L2) was measured from the head of the stapes to the upper surface of the footplate. While the minimum height was 2.72 mm, the maximum was 3.29 mm. The average height was 3.20 ± 0.17 mm (Table I).

The length of the footplate ranged between a minimum of 2.64 to a maximum of 3.56 mm and an average of 2.97± 0.31 mm. The width of the footplate was calculated by subtracting L2 from L1. The mean width of the footplate was found to be 0.39± 0.10 mm; ranging from a minimum of 0.19 mm to a maximum of 0.56 mm. The length and width of the footplate are shown in Table I.

The length of the head was measured perpendicular to the footplate. It ranged between 0.53 mm to 1.21 mm with an average length of 0.69± 0.21 mm. The length of the head parallel to the footplate was a minimum of 1.01 mm and a maximum of 1.61 mm; average being 1.24± 0.19 mm (Table II).

Two out of ten bones had no neck, in the remaining the neck length averaged at 0.48± 0.69 mm with a maximum of 2.36 mm (Table II).

The length and breadth of the anterior and posterior crura of the stapes are depicted in Table III and IV. The minimum length of the posterior crus was 1.65 mm and maximum length was 2.11 mm; the average length being 1.97± 0.15 mm. The width at the shoulder ranged between 0.29 mm to 0.56 mm, with an average of 0.44± 0.10 mm. At the center the maximum width was 0.40 mm and a minimum of 0.22mm. The width at the center averaged at 0.30± 0.04 mm. Near the footplate the width averaged at 0.31± 0.07 mm; maximum being 0.39 mm and minimum was 0.17 mm. The anterior crura measured a minimum of 1.46 mm in length and a maximum of 1.96 mm. The mean was 1.77 ± 0.16 mm.

The width at the shoulder varied between 0.32 mm to 0.56 mm; mean was 0.44± 0.07 mm. At the center the minimum width was 0.24 mm and the maximum was 0.38 mm; average was 0.32± 0.05 mm. Close to the footplate the mean thickness was 0.38 ± 0.13 mm (0.22-0.57 mm).

Table I Dimensions of the stapes

Table II: Dimensions of the head and neck of the stapes

Table III: Dimensions of the posterior crura of stapes

Fig. 6: Various shapes of the footplate along with their diagrammatic representation.
	Stapes 1
	Stapes 2
	Stapes 3
	Stapes 4
	Stapes 5

Relationship of the stapes footplate to facial nerve

The close proximity of the facial nerve to the stapes was visualized in gross specimens and cross sections. Of the temporal bones studied, 60% showed a normal positioned facial nerve that traversed lateral to the vestibule above the oval window and below the lateral semicircular canal (Figs. 7, 8). Three temporal bones showed a dehiscent facial nerve (Figs. 9,10), while in 4 bones the facial nerve was overhanging the area of the footplate (Fig. 11).

Discussion:

Earlier reports on anatomy of the stapes in India have provided a qualitative account of the bone with some basic dimensions, while western studies are relatively more quantitative, Dass et al. (1966 a,b;) and àWengen et al. (1995). Different and variable parts of the stapes have been measured which prevents comparisons, thus emphasizing the need for uniform selection of parameters for quantitative evaluation. In the present study the various parameters of stapes have been examined in detail in the Indian population and compared with the data available from the western population.

Fig. 7: Photograph of the tympanic cavity showing a normally positioned facial nerve within an intact bony canal S-stapes; Fn- Facial nerve; LSC-Lateral semicircular canal; I- Incus Pr- Promontory

Fig. 8: Photograph showing the cross sectional anatomy of the temporal bone. The facial nerve is seen within an intact canal (X 6). ME- Middle ear, S- Stapes, FFootplate, Sc- Saccule, U- Utricle, Fn- Facial nerve, IAC -Internal Auditory Canal, SCC-Semicircular canal

Anatomic height of the stapes measured an average of 3.41mm, ranging from a minimum of 3.06 mm to a maximum of 3.71 mm. Similar values have been reported by Urbantschitsch (1876), average - 3.70 mm; min-3.02; max-4.50 mm)), Bast et al. (1949), mean - 3.06 mm ; min-2.50; max-3.78 mm) and Dass et al. (1966 a, mean -3.29 mm ;min-2.80; max-3.93 mm) (Table V).

Clinically, the height of the stapes superstructure is of greater significance in stapedial reconstruction. àWegen et al. (1995) reported this distance to be 3.19 mm (2.91-3.45 mm). In our study it was found to be 3.201 mm (2.72-3.29 mm). There was no significant difference in the length of the stapes in the Western and Indian populations (t = 0.13, p = 0.89) (Table V).

Partial ossicular replacement requires implants which fit onto the head of the stapes alone, thus necessitating the knowledge of the dimensions of the stapedial head Wehers, (1994). The anatomical length of the stapedial head was measured to be an average of 0.69 mm, varying from a minimum of 0.53 mm to a maximum of 1.21 mm. The width of the stapes head varied from 1.01 to 1.61 mm, mean 1.24 mm ±0.19. In the study conducted by àWengen et al. (1995), similar values of width ranging from 0.91 mm to 1.49 mm, with an average of 1.14 mm ± 0.16 were reported. Statistical analysis comparing the values from both the studies did not reveal any significant difference (t =1.20, p = 0.25). (Table V)

The presence of an anatomical neck of the stapes remains controversial Dass et al., (1966a) and àWengen et al. (1995). In our study, eight of the ten bones demonstrated a definite neck with an average length of 0.48 ±0.69 mm, and a maximum of 0.58 mm. In two specimens, the neck was absent, with the head merging smoothly onto the shoulders of the crura, àWengen et al. (1995) reported a minimum length of 0.48 mm and a maximum of 0.88 mm, with a mean stapedial neck length of 0.64±0.13 mm. Comparative analysis of the two studies did not show any significant difference.

(t = 0.74, p = 0.47) (Table V) The attachment of the stapedial muscle may be anomalous or the tendon or the muscle may be altogether absent Dass et al. (1966). The muscular process of the attachment of the stapedial muscle reported by some authors was not identified in any of the ten bones in our study; instead the site of insertion of the stapedius presented a smooth surface.

In our study, the posterior crus were longer than the anterior crus in all the specimens. The posterior crus measured 1.97 mm in length on an average, with a minimum of 1.65 mm and a maximum of 2.11mm, while the anterior crus had a mean length of 1.77 mm (1.46-1.96 mm). These values are similar to those reported by Dass et al (1966a). Urbantschitsch (1876) found a mean posterior crural length of 2.7 mm and a mean anterior crural length of 2.2 mm; while Anson et al (1939) reported an average posterior crural length of 2.4 mm and an average anterior crural length 2.57 mm (Table VI). Due to lack of requisite data (standard deviation), a statistical comparison was not possible, however the measurements show that the stapedial crura in the Indians are shorter in length than in the Western population.

Table IV: Comparison of parameters of stapes

Table V: Comparison of parameters of stapes

Notably the anatomical length of the stapes was similar in the various studies; hence the finding of shorter crural lengths in our study suggests the possibility of either a longer head - neck or increased width of the stapes footplates, àWengen et al. (1995), in their study measured the combined length of the head and neck as well as the height of the stapes superstructure, but did not measure the crural lengths separately. Comparing the measurements of combined head and neck length with their data, these dimensions in the Indian population were greater.

Knowledge of the crural thickness has significant bearing on stapedial mobilization and stapedectomy as it determines the site of fracture of the crura, Dass et al. (1966a). Compared to the data reported by àWengen et al. (1995) we found that the thickness of the crura was significantly less in the Indian population (Table VI).

Table VI: Comparison of parameters of stapes

In the present study, the thinnest portion of both crura was at the center in majority of the cases (70%). This is in contrast to earlier reports, both Indian and Western, wherein the distal third of the crura is thinnest. None of the studies mention about the dimensions of the crura in its middle third Dass et al. (1966a) and àWengen et al., (1995). Of the two crura, the posterior crus were thicker at the shoulder region, while the anterior crus were thicker in its middle and distal third. Contrastingly both the Indian and Western literature report the anterior crus to be uniformly thinner than the posterior crus Dass et al. (1966a) and àWengen et al. (1995).

The stapedial footplate forms a vital link between the middle and the internal ear. During reconstructive surgery, replacement prostheses (Fig. 12) are used to bridge the gap between the tympanic membrane, malleus or the incus and stapes footplate. The positioning of the prosthetic shaft on the stapes footplate has a significant bearing on the eventual outcome of the surgery. Colletti et al. (1984) reported the anterior placement to be better than the central and posterior sites, while other investigators found the anterior and central sites favorable Asai et al. (1999). From an anatomic point of view the anterior footplate location for prosthetic placement might be preferable as the annular ligament is thinner and wider Bruner (1954). Although other investigators have found the anterior end to be broader in majority of the bones, in our study, both the anterior and posterior ends of the stapes footplate were equally broad. The relevance of this finding on prosthesis placement needs to be validated with clinical studies in the Indian context

Fig 12: Photograph of a prosthesis used to bridge the gap between incus and the stapedial footplate

The greater part of stapes develops from the second branchial arch and the facial nerve is the nerve of this branchial arch; consequently they share an intimate relation. Both the second branchial arch and the otic capsule play a role in the development of the facial canal. The cartilage contributes to the bony wall of the segment of the tympanic part of the canal while the groove or sulcus on the otic capsule accommodates the facial nerve and also contributes to its bony cover. A displaced facial nerve or absence of a bony canal increases the risk of nerve injury by an unwary surgeon. In our study a well-formed bony facial canal was evident in six bones. A dehiscent facial canal as seen in three bones is believed to be a normal variant rather than an anomaly by most authors Boone et al. (2002). The occurrence of these variants has a direct impact on the course of the surgery and is also critical for reconstructing the conduction apparatus, Jahrsdoerfer (1981) and Kapur (1991).

Conclusion

Although no significant difference in the length of the stapes in the Western and Indian populations was appreciable, the combined dimension of the stapedial head and neck was found to be greater in the Indian population suggesting shorter crural lengths. In addition the Indian stapedial crura are significantly slender than their Western counterparts, with the thinnest portion being the center of the crura.

These differences in dimensions of the stapes in different racial populations have surgical implications in reconstructive endaural surgeries and have an impact on the design and dimensions of indigenously manufactured prosthesis to suit the Indian population.

Acknowledgements

I am grateful to Dr Sabita Mishra, Department of Anatomy, MAMC New Delhi for teaching and aiding me in the use of the image analyzer in this project. I am deeply indebted to Dr Anoop Sabharwal, Department of E.N.T, MAMC New Delhi for helping and teaching me the nuances of temporal bone dissection. I thank the Department of Anatomy and E.N.T, MAMC as well as SAIF- EM Facility, New Delhi for their technical support.

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