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Table of Contents
ORIGINAL ARTICLE
Year : 2022  |  Volume : 71  |  Issue : 4  |  Page : 266-271

Anatomical variations of the human ossicular chain in south indian population: morphometry and morphology


Department of Anatomy, ESIC MC and PGIMSR, Bengaluru, Karnataka, India

Date of Submission26-Nov-2021
Date of Decision25-Jul-2022
Date of Acceptance02-Sep-2022
Date of Web Publication01-Dec-2022

Correspondence Address:
Dr. Giridhar Dasegowda
Department of Anatomy, ESIC MC and PGIMSR, Rajajinagar, Bengaluru - 560 010, Karnataka
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jasi.jasi_192_21

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  Abstract 


Introduction: The middle ear consists of the malleus, incus, and stapes. The otologic surgeons need to have a thorough knowledge of anatomical details and variations to provide better operative results and for surgical maneuvers. The present study aimed to analyze the morphological variation and morphometry of the ear ossicles and compare the parameters with those reported in previous studies. Material and Methods: The study was conducted on 28 mallei, 26 incus, and 20 stapes obtained from the cadavers allotted for dissection. Measurements were documented, and the morphological variations were analyzed. Results: The average of the parameters showed that the malleus was 7.59 mm in total length with an angle of 130°; the manubrium was 4.65 mm, the total length of the head and neck was 5.01 mm, and the average weight was 21.50 mg. The incus had a total length and width of 6.37 and 4.89 mm, respectively; a maximal distance of 5.97 mm between the tips with an angle of 101° and weighed an average of 23.81 mg. The stapes had a total length of 3.36 mm, with the stapedial base being 2.83 mm in length and 1.41 mm in width, and weighed an average of 3.16 mg. Discussion and Conclusion: The ossicular chain shows great variations in measurements and morphology. Hence, a thorough anatomical knowledge of the human ossicular chain is required for clinicians for surgical maneuvers and for designing prosthetics to replace the adult middle ear ossicles.

Keywords: Anatomical variation, ear ossicles, incus, malleus, morphology, stapes


How to cite this article:
Dasegowda G, Kadirappa P, Nanjundaraju H, Raja SS, Suresh R. Anatomical variations of the human ossicular chain in south indian population: morphometry and morphology. J Anat Soc India 2022;71:266-71

How to cite this URL:
Dasegowda G, Kadirappa P, Nanjundaraju H, Raja SS, Suresh R. Anatomical variations of the human ossicular chain in south indian population: morphometry and morphology. J Anat Soc India [serial online] 2022 [cited 2023 Feb 4];71:266-71. Available from: https://www.jasi.org.in/text.asp?2022/71/4/266/362550




  Introduction Top


The tympanic cavity of the temporal bone contains three ossicles (auditory ossicles); malleus, incus, and stapes.[1],[2] These ossicles form a chain connecting the tympanic membrane to the fenestra vestibuli (oval window). They have ligamentous connections with the walls of the middle ear cavity and are bound together by articulations. Malleus is the most lateral and is firmly attached to the tympanic membrane; the stapes is the most medial fixed to the fenestra vestibuli and is in direct contact with the perilymph; the intermediate between the two is the incus.[3] This complex ossicular system acts like a bent lever to connect the vibrations of the outer tympanic membrane into intensified thrusts of stapes against the perilymph for a smooth and amplified transmission of sound.[4],[5],[6]

Malleus, shaped like a hammer, consists of anterior and lateral processes, manubrium (handle), neck, and head with which it articulates with incus at the incudomalleolar joint.[7] Incus consists of a body, short and long process. At the tip of the long process lies a medially projecting mass known as the lenticular process, also referred to as the fourth ossicle, due to its incomplete fusion with the long process. The lenticular process articulates with stapes forming the incudo-stapedial joint. Stapes, the smallest bone in the body shaped like a miniature stirrup, consists of small buttons like head, a neck, two crura, and footplate (base).[3],[8]

Many congenital craniofacial, including temporal mandibular disorders, are known to form ossicular anomalies and associated symptoms as their definitive component.[9] Acquired lesions such as chronic suppurative otitis media (CSOM) affect the ossicular chain and form a huge burden of disease in developing countries; thus, there is a higher demand for reconstructive surgeries using the ossicular prosthesis. The present study aims in providing valuable parameters of morphometry and morphological variation in the human ossicular chain.


  Material and Methods Top


The study was conducted on a total of 28 Malleus, 26 Incus, and 20 Stapes obtained from the cadavers of both sexes allotted for dissection for undergraduates at ESIC Medical College. IRB was waived for this study on ear ossicles from cadaver. Using the electric bone cutter, the calvaria was removed, exposing the brain. The brain was severed at the level of the medulla oblongata, and the dura mater was stripped to remove the brain. The roof of the middle ear was exposed by removing the tegmen tympani to create a small opening using a chisel and hammer. Malleus was identified by the round head and articulating with incus at the epitympanum; by fine manipulation, with forceps, both the ossicles were removed. A diagonal section through the arcuate eminence was taken on the temporal bone to expose the stapes and removed with forceps after fine manipulation. The ossicles damaged or eroded as a consequence of CSOM, or other diseases were excluded from the study.

The variations in the ossicular chain were observed under the magnification of the simple microscope. The measurements of length were obtained using the Digital Vernier Caliper with the least count of 0.01 mm, and the weight of ossicles was obtained using the Contech Analytical Balance with the least count of 0.01 mg. Photographs were obtained with the help of a stereoscopic microscope, and the measurements of angle were estimated using the photographs. Results were statistically analyzed using the version SPSS - 22 software (IBM Corp., Armonk, N.Y., USA), and P < 0.05 was considered statistically significant. The parameters taken were similar to studies conducted by Unur et al. and Sodhi et al.[1],[4],[10]

The following measurements of Malleus were obtained, as shown in [Figure 1].
Figure 1: Measurements of malleus. (A1): Total length, (A2): Length of manubrium (A3): Length of head and neck

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  1. Total length (A1) in mm: the maximum distance between the top of the head and the end of the manubrium
  2. Length of manubrium (A2) in mm: the maximum distance between the end of the lateral process and the end of the manubrium
  3. Length of the head and neck (A3) in mm: the maximum distance between the top of the head and the end of the lateral process
  4. Angle of malleus: measured between the long axis of the malleus and that of the manubrium
  5. Index: Length of the manubrium ×100/total length.
  6. Weight of the malleus (mg).


The following measurements of incus were obtained, as shown in [Figure 2].
Figure 2: Measurements of Incus. (B1): Total length, (B2): Total width, (B3): Distance between the tips

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  1. Total length (B1) in mm: Maximal distance between the superior edge of the body and the end of the long process
  2. Total width (B2) in mm: Maximal distance between the superior edge of the body and the end of the short process
  3. Maximal distance between the tips of the processes (B3) in mm
  4. Angle of the incus measured between the processes
  5. Index: Total width ×100/total length of the incus
  6. Weight of the incus (mg).


The following measurements of stapes were obtained, as shown in [Figure 3].
Figure 3: Measurements of stapes. (C1): Total height, (C2): Length of footplate, (C3): Width of footplate

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  1. Total height (C1) in mm: Maximum distance between the top of the head and the footplate
  2. Length of footplate (C2) in mm: Maximum length of the long axis of footplate
  3. Width of footplate (C3) in mm: Maximum width of the footplate.
  4. Index: Length of footplate ×100/total height
  5. Weight of the stapes (mg).



  Results Top


The data obtained in this study were determined in two categories: morphologic and morphometric. The morphometrical data obtained are tabulated in [Table 1] for malleus, [Table 2] for incus, and [Table 3] for stapes. The morphological data were based on observation; the distal part of the manubrium (free ends) of the malleus showed variations between a straight line and curved anteriorly, as shown in [Figure 4]. The anterior and lateral process of malleus showed much variability in length. The inferior border of the short process of incus showed variation between a notch and a straight course (without notch), as shown in [Figure 5]. A wide angle and narrow-angle variation were noted with a well-developed corpus and less developed corpus, respectively, in incus, as shown in [Figure 6]. The variations of stapes noted were with respect to the length of the neck, growth on the footplate, and variations in the obturator foramen. The different shapes observed in stapes in the foramen were a circular, semicircular, tunnel, and square, as shown in [Figure 7]. Two stapes were noted with growth on the footplate, as shown in [Figure 8].
Table 1: Morphometric analysis of malleus (n=28)

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Table 2: Morphometric analysis of incus (n=26)

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Table 3: Morphometric analysis of stapes (n=20)

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Figure 4: Free end of manubrium of malleus showing straight course (a) and curved anteriorly (b)

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Figure 5: Straight (a): And notch (b): Course on inferior border of incus

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Figure 6: Narrow (a): And wide angle (b): Between the processes in incus

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Figure 7: Morphological variations of the foramen of stapes. (a): Circular, (b): Square, (c): Semi-circular, (d): Tunnel

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Figure 8: Growth on footplate on stapes

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  Discussion Top


Vesalius de described malleus and incus in his monumental work “De humani corporis fabrica” in 1543.[5]

Eustachius and Ingrassia were the first to describe stapes in 1546.[11]

Lempert and Wolff described several unique features of the ear ossicles such as its embryological development, position in the tympanic cavity, and enveloping mucosa.[12]

The present morphometric data of malleus, incus, and stapes obtained from the present study have been compared to the studies conducted previously, as shown in [Table 4].
Table 4: Comparison of morphometric data of ear ossicles with previous studies

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The parameters of malleus and incus were similar to the study conducted by Unur et al., while the parameters of stapes were similar to the study by Farahani and Nooranipour.[1],[16] Among the ossicles, stapes was found to be the most variable. Similar results have been obtained from the previous studies.[7],[17],[18],[19],[20] Stapes has also shown variation in its positioning in the fossa of the oval window.[21] In the present study, malleus showed variation in the course of manubrium, straight or a curved course. This goes in conjunction with the previous studies.[1],[8],[18] A study conducted by Todd and Creighton observed a malleus with an absent lateral process and inflected manubrium.[22] Such unusual findings were not obtained in the present study.

The morphological variation observed in the incus included a narrow or wide angle between the processes and the notch observed in the inferior border of the short process. The early development of the posterior ligament of the incus, which connects the fossa incudis to the short process, may result in the formation of the notch.[8]

The global burden of hearing loss is at least 250 million people, according to a survey conducted by WHO in 2001. In India, close to 60 million population suffer from disabling hearing impairment. Conditions like cholesteatoma, Otitis media, an inflammatory condition of the middle ear cavity, and congenital malformations like incudo-stapedial anomalies, congenital absence of incus may require ossicular reconstruction surgeries.[23],[24] The two types of prosthesis used are partial ossicular replacement prosthesis and total ossicular replacement prosthesis.[25] Homograft ossicular prosthesis obtained from cadavers has gained acceptance and has been found to provide superior audiologic results when compared to autograft.[15] Another study reported that angiogenesis was predominant in allogeneic ossicles, whereas autogenetic ossicles showed angiogenetic and Appositional osteogenesis.[26] Several studies have been conducted to describe fetal ossicles and its use as prostheses or homografts.[6],[27] However, few studies stated that custom manufacturing of prostheses for each patient could optimize prosthesis fit as large soft-tissue structural variations prevent a single middle ear model from being applicable to all ears.[2],[28]

Middle ear ossicles have also been found to have anthropological importance. In lower animals, malleus and incus have been found to form a single unit called the maleo-incal complex.[29] A study on the comparison of Neanderthal and modern humans stated that variation in ear ossicles is due to convergent brain expansion.[30] Thus, these morphological variations provide anthropological evidence in evolution.


  Conclusion Top


Auditory ossicles have been extensively studied since their discovery. The ossicles show great variation in their arrangement, morphology, and morphometry. Ossicles can be used as homograft; hence, there is a need to study and preserve them in ossicular banks. The morphometrical measurements and the morphological variations of ear ossicles observed have been reported in the present study. A thorough anatomical knowledge of the human ossicular chain and its variation is essential for clinicians to perform surgical maneuvers and to design prosthetics for ossicular replacement. The morphological differences observed are explained by the embryological variation in the development.

Acknowledgment

Our deepest gratitude is expressed to the following persons for their contribution in making the study possible. Dr. Simant Baliarsingh, Department of Biochemistry, for the assistance in data collection, and the staff of the Department of Anatomy for the advice and guidance.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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Farahani RM, Nooranipour M. Anatomy and anthropometry of human stapes. Am J Otolaryngol 2008;29:42-7.  Back to cited text no. 16
    
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]



 

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