Abstract
To study the morphology and anthropometry of human ear ossicles from cadaveric temporal bone and to study the variations of the human ossicles from ossiculoplasty point of view. 38 sets of ear ossicles were collected, each containing malleus, incus and stapes, from temporal bone dissection done in Vasantrao Pawar medical college, Nashik. They were studied under microscope & anthropometrical, morphological evaluation done. Malleus showed variations in handle where 61% were straight and 39% curved. Weight of malleus ranged from 0.03–0.06 gm. The length ranged from 5.5 to 8.2 mm. Incus showed morphological variation in lenticular process which was present in 73% incii. Weight of incus ranged from 0.04 to 0.09 gm, average length of long process 3.5 mm, width of body 4 mm. In stapes variations was seen with head of stapes which was absent in 21% bones. With increasing awareness about postop hearing status, this study will add up in knowledge of morphological and anthropometric variations that exists in Indian population, help otologists to understand middle ear dynamics better which will improve results of ossiculoplasty.
Keywords: Malleus, Incus, Stapes, Ossicles, Ossiculoplasty, Morphology
Introduction
A small organ called the ear performs the special function of hearing, which makes it all the more intriguing that three minor bones, the malleus (hammer), incus (anvil), and stapes (stirrup), are strategically positioned inside this small ear structure to form a semi-rigid structure that acts as a bony cable for transmitting sound in the middle ear [1].
These ossicles connect the tympanic membrane to the fenestra vestibule in a chain across the tympanic cavity and are connected by ligaments to the walls of the middle ear cavity which are held together by articulations. These ossicles enhance and smoothly transport sound waves from the tympanic membrane to the surface of the oval window [2].
Any ear pathology can cause hearing loss by eroding these ossicles [3]. Ossicular chain reconstruction must be carried out in order to restore proper sound transmission [4].
When choosing a material to employ for ossicular reconstruction, several criteria must be taken into account.
The choice of graft becomes challenging because of abundance of grafts available for ossiculoplasty. An ideal graft should have low extrusion rates, good absorption, safety, accessibility, and cost effectiveness. Natural autografts, such as bone and cartilage, have demonstrated good outcomes in terms of improved hearing with a low extrusion rate and inexpensive costs [5].
It is challenging to examine the morphology and anthropometry of middle ear ossicles during surgery. Their variances and proportions can be studied better on cadavers.
The otologists will benefit from having knowledge of these ossicle dimensions, their variances, and their morphometric data when doing reconstructive surgery and creating superior prosthesis.
Materials and Methods
This is an observational study carried out over a period of 6 months from available cadavers for temporal bone dissection workshop organised every 6 months in Dr. Vasantrao Pawar medical college and research centre, Nashik. All temporal bones were included except the temporal bone with any evidence of previous surgeries and those with any previous disease. Temporal bone dissection was carried out meticulously on each temporal bone. All three ossicles were visualized using an optofine microscope and their muscle attachments were separated. With gentle manipulation, ossicles were removed. All three ossicles were washed thoroughly and each set of ossicles was stored in a clean box after labelling it. Following parameters were studied for 3 bones-
Malleus- (1) weight, (2) length, (3) width, (4) length of manubrium, (5) width at neck, (6) curvature of manubrium.
Incus- (1) weight, (2) length of long process, (3) width of body, (4) lenticular process.
Stapes- (1) weight, (2) length, (3) foot plate-length, (4) foot plate-width, (5) anterior crus width, (6) posterior crus width, (7) width of head.
Each ossicle was weighed using a digital electronic weighing machine. Other parameters were measured using dental calliper under optofine microscope as shown in Fig. 1. Photographs were taken from a fixed distance. This study was approved by the Institutional Ethics Committee [IEC_85/2021–22].
Fig. 1.
Instruments used for measurement: a optofine microscope, b digital weighing machine, c dental calliper
Results
38 sets of ear ossicles were collected from temporal bone dissection done on cadavers, each containing malleus, incus and stapes as shown in Fig. 2. They were studied under microscope & anthropometrical, morphological evaluation done.
Fig. 2.
Set of ossicles
Malleus
The weight of malleus in grams ranged from 0.03 to 0.06 with an average of 0.04 gm. Length of malleus in millimetres (mm) ranged from 5.5 to 8.2 with an average of 7.14 mm. Width of malleus ranged from 1.1 to 2.5 mm with an average of 1.92 mm. Length of manubrium ranged from 2.9 to 5.5 mm with an average of 4.02 mm. Width at neck ranged from 0.5 to 1.9 mm with an average of 1.21 mm as shown in Table 1. Malleus showed variations in handle where 61% were straight and 39%curved as shown in Fig. 3. Figure 4 shows specifics of how the measurements were taken on the malleus.
Table 1.
Characteristics of malleus
| Malleus | ||||
|---|---|---|---|---|
| Min | Max | Average | ||
| 1 | Weight (gm) | 0.03 | 0.06 | 0.04 |
| 2 | Length(mm) | 5.5 | 8.2 | 7.14 |
| 3 | Width | 1.1 | 2.5 | 1.92 |
| 4 | Length of manubrium | 2.9 | 5.5 | 4.02 |
| 5 | Width at neck | 0.5 | 1.9 | 1.21 |
| 6 | Curvature of manubrium | Curved 15 | Straight 23 | |
Fig. 3.
Curvature: a straight, b curved
Fig. 4.
Measurements of malleus
Incus
Weight of incus ranged from 0.04 to 0.09 gm and average being 0.06 gms. Length of long process ranged from 2.5 to 5 mm and average being 3.47 mm. Width of body ranged from 2.1 to 5 mm and average being 4.04 mm as shown in Table 2. Incus showed morphological variation in lenticular process which was present in 73% incii as shown in Fig. 5. Figure 6 shows specifics of how the measurements were taken on the incus.
Table 2.
Characteristics of incus
| Incus | ||||
|---|---|---|---|---|
| Min | Max | Average | ||
| 1 | Weight (gm) | 0.04 | 0.09 | 0.06 |
| 2 | Length of LP(mm) | 2.5 | 5 | 3.47 |
| 3 | Width of body | 2.1 | 5 | 4.04 |
| 4 | Lenticular process | Present 27 | Absent 10 | |
Fig. 5.
Lenticular process: a absent, b present
Fig. 6.
Measurements of incus
Stapes
Weight of stapes ranged from 0.01 to 0.03 gms and with an average of 0.02 gms. Length of stapes ranged from 2.5 to 4 mm with an average of 2.92 gms. Foot plate length ranged from 2 to 3 mm with an average of 2.3 mm. Foot plate width ranged from 1 to 1.9 mm with an average of 1.36 mm. Anterior crus width ranged from 0.3 to 0.7 mm with an average of 0.56 mm. Posterior crus width ranged from 0.5 to 0.8 mm with an average of 0.77 mm. Width of head ranged from 0.5 to 1.6 mm with an average of 1.08 mm as shown in Table 3. Stapes variations was seen with head of stapes which was absent in 21% bones as seen in Fig. 7. Out of 10 sets of ossicles 56% incii were having absent lenticular process and stapes head was absent in 44% bones as seen in Fig. 8. Among the three bones incus was heaviest ranging from 0.04 to 0.09 gm followed by malleus ranging from 0.03 to 0.06 gm and the lightest being stapes ranging from 0.01 to 0.03 gm. Figure 9 shows specifics of how the measurements were taken on the stapes. Variations seen in malleus, incus and stapes are plotted on graph as shown in Fig. 10
Table 3.
Characteristics of stapes
| Stapes | ||||
|---|---|---|---|---|
| Min | Max | Average | ||
| 1 | Weight (gm) | 0.01 | 0.03 | 0.02 |
| 2 | Length (mm) | 2.5 | 4 | 2.92 |
| 3 | FP-length | 2 | 3 | 2.30 |
| 4 | FP-width | 1 | 1.9 | 1.36 |
| 5 | Anterior crus width | 0.3 | 0.7 | 0.56 |
| 6 | Posterior crus width | 0.5 | 0.8 | 0.77 |
| 7 | Width of head | 0.5 | 1.6 | 1.08 |
Fig. 7.
Stapes with absent head
Fig. 8.
Absent lenticular process with absent stapes head
Fig. 9.
Measurements of stapes
Fig. 10.
Variations of malleus, incus and stapes: a malleus showed variations in handle where 61% were straight and 39% curved. b Incus showed morphological variation in lenticular process which was present in 73% incii. c Stapes variations was seen with head of stapes which was absent in 21% bones
Discussion
The extraordinary engineering architecture of the human hearing system is composed of intricate geometries. There are just a few morphometrical studies despite the fact that the ossicular chain has been understood for 500 years [6]. Conductive hearing loss is frequently caused by ossicular discontinuity. Hearing outcomes have been greatly improved by using ossicular graft material in ossicular chain repair [7]. The first ossicular reconstruction was carried out by Hall and Rytzner in 1957 using autologous incus, which has since become the most used autograft material [8]. By using the notched incus with short process hearing results are superior to any other method [9]. Autograft prostheses have a very low extrusion rate, little danger of disease transmission, are biocompatible, and don’t require reconstitution [10]. The number of surgeries for CSOM (chronic suppurative otitis media), ankylosis of the stapes, TORP (total ossicular replacement prosthesis), PORP (partial ossicular replacement prosthesis), and other conditions has increased, making the morphology and morphometry of the middle ear ossicles much more significant [11]. Our study’s morphometric analysis of the malleus, incus, and stapes were comparable with other studies while variations were seen with shape of manubrium of malleus, lenticular process of incus and head of stapes. In order to make the necessary and anticipated modifications surgeons should be aware of the middle ear dimensions and potential variations. Ossicles are crucial for reconstructive procedures and a thorough evaluation of this area should be done during ossiculoplasty.
Conclusion
This study evaluates the potential morphological and anthropometric variation that can exist in cadavers with a goal to further our understanding of middle ear dynamics in light of the rapidly increasing demand for ossiculoplasty.
We aim of enhancing current knowledge of ossicles with new data and promote temporal bone dissections so as to preserve ossicles in ossicular banks by using the proper safeguarding and sterilization techniques for upcoming use as homograft in ossiculoplasty.
This study would extend our anticipated understanding that there would be a possibility of absent stapes head with absent lenticular process of incus, with studies revealing incus to be the most frequently used autologous graft.
Funding
No financial funding received.
Declarations
Conflict of Interest
Authors declare that they have no conflict of interests.
Ethical Approval
This study was approved by the Institutional Ethics Committee [IEC_85/2021–22].
Footnotes
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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