Anatomic studies of the Human Cochlea: Implications for Cochlear Implantation

Siebenmann (1890) performed corrosion casts of the human labyrinth using Woods metal and described its beautiful anatomy over one hundred years ago1

Despite many anatomical studies of the human cochlea, there are few reports describing its anatomical variations. Such data are of value in cochlear implantation (CI). CI electrodes on the market are designed either for a position along the inner modiolar wall (so-called peri-modiolar electrodes) or along the outer wall of scala tympani. Anatomic variations of the cochlea may influence its final position relative to the cochlea place/frequency map . With less invasive surgical techniques and shorter electrodes the fragile inner ear structures may also be conserved2,3. Preservation of residual hearing is now a goal in all cochlear implant surgery and better knowledge about anatomical variation may limit intra-cochlear damage.

Our collection of plastic human inner ear molds contains 325 specimens (Fig. 1). These are material of un-selected human temporal bones from autopsies. Several studies have been made here in Uppsala by Dimopoulos and Muren4 and Wadin5. The applied method of casting temporal bone specimens was described by Wilbrand et al.6 and Rask-Andersen et al. 1977 7. The silicone and polyester resin material has a shrinkage factor of 0.6 to 1%. Bone specimens are macerated using potassium hydroxide, boiling, hydrogen peroxide (H2O2), and trypsin.

The specimens are placed in a wax form, with orifices of the inner ear canals left open. The casting material, a polyester resin or a silicone rubber material, is poured into the wax form. The specimens are placed in a low-pressure chamber for penetration into the meticulous bony channels of the macerated bones. After hardening, bone is dissolved with hydrochloric acid. The inner ear structures are freed with a fine forceps in the case of the polyester preparations or with scissors in the case of the silicone material.  

Figure 1: Uppsala collection of 325 plastic molds of human inner ear.
Fig.1 Uppsala collection of 325 plastic molds of human inner ear.  

Measurement Procedure

Photographs can be taken of each mold in different positions and width, length, and height of different turns measured8. Relevant anatomic variations such as unusual coiling pattern and asymmetry of individual turns that may influence the introduction of the electrode array into the cochlea can be appreciated. Computer-aided planimetry is performed and a ruler used connected to a computer. To characterize the cochleae we use the midpoint of the long diameter of the round window as reference and starting point for measuring the length of the cochlea. A line is drawn through the central axis of the cochlea to a distant point of the first turn. A line is drawn at right angles to this line through the axis of the cochlea, dividing each turn of the cochlea into quadrants (Fig. 2). Quadrants 1 to 4 constitute the first turn; 5 to 8, the second; and 9 to 12, the third turn. The outer wall length of each quadrant is calculated. Error of measurement was estimated to be approximately 0.08 mm. 

Results and Discussion

Each human cochlea was found to be individually shaped with large variations of the dimensions and coiling characteristics in different cochleae8

Figure 2:  Corrosion cast of a left human cochlea (axial-pyramidal view). The reference points used for estimating the length of the various turns of the cochlea are shown.
Fig. 2. Corrosion cast of a left human cochlea (axial-pyramidal view). The reference points used for estimating the length of the various turns of the cochlea are shown. 

The estimated mean number of turns of the human cochlea was found to be 2.6 with a range from 2.2 to 2.9 (929 degrees; range, 774-1037 degrees). The number of quadrants varied from slightly more than 8 to 12. The outer cochlear wall length ranged from 38.6 to 45.6 mm, with a mean length of 42.0 mm. Maximal radius of the round window (half diameter of round window) was estimated to be 1.1 mm, with a range from 0.3 to 1.6 mm. The mean length of the first turn (quadrants 1-4) was 22.6 mm, with a range from 20.3 to 24.3 mm, representing 53% of the total length (Fig. 3). The mean length of the turns are shown in table. The mean height (diameter) of the cochlea was 3.9 mm, with a range from 3.3 to 4.8 mm. The internal diameter of the first turn varied broadly (1.6-2.6 mm). The mean width of the cochlea (first turn) was 6.8 mm with a range from 5.6 to 8.2 mm. The shape of the first turn of the cochlea seemed to be influenced by the coiling pattern. There were different positions of the central axis of the individual turns. Other authors have therefore used different reference points8.SD indicates standard deviation; n, number of specimens; RW, round window. The total length of the outer wall excluding the basal half of the RW.
 

SD indicates standard deviation; n, number of specimens; RW, roundwindow. The total length of the outer wall excluding the basal half of the RW. 

Figure 3: Diagram showing relative lengths of various quadrants of the human cochlea
 Fig. 3. Diagram showing relative lengths of various quadrants of the human cochlea.  

An unforeseen outcome was the individual design and proportions of the human cochlea. These variations should be taken into account when performing surgery on the cochlea because they may not always be comprehended from a pre-operative computed tomography as performed routinely. Unusual anatomy such as tilting or misalignment of the first and second turns may account for difficulties for the electrode to glide upward and reach the second turn (Fig. 4).The width of the various turns differed greatly between individuals, and the cochlear height varied as much as 1.5 mm, representing one-third of the total height. There was also abrupt turning of the cochlea near the carotid area. In some instances, the carotid canal impinged on the anterior cochlear wall. This condition will influence the force generated by the tip of the electrode on the lateral wall when gliding up the first turn. The estimated mean number of turns of the human cochlea is in accordance with Hardy, who measured the organ of Corti on histological sections. The number of quadrants was in accordance with Kawano et al.9 who investigated 6 human temporal bones and performed sectioning and computer-aided 3-dimensional (3-D) reconstructions. They found the number of turns on average to be 2.69, with a range from 2.63 to 3. Cochleae with up to 3 turns were also described by Tian et al.10 Kawano et al. estimated outer wall length to 40.81 mm, which coincides with our obtained data where we found individual differences of up to 7 mm. We calculated the length of the bony outer wall of the human cochlea, which is longer than the estimated length of the organ of Corti. Corti length corresponds to the extension of the basilar membrane, which is generally around 34 mm. Our estimated values of the width and height of the first turn are in accordance with those reported by Dimopoulos and Muren4. Stakhovskaya et al.11 presented measurements of cochlear width ranging from 6.9 to 8.2 mm, which is slightly more than we found as well as those obtained by Escude´ et al.12Analyses of the molds also confirm the near distance between the upper first turn of the cochlea and the first (labyrinthine) portion of the facial nerve canal. In one specimen, there was virtually no distance between the two structures, indicating that the neural fascicles of the facial nerve lie almost in direct contact with the cochlear soft tissue. This may explain the incident of facial nerve excitation or twitching when stimulating certain electrodes in this region especially in patients with extensive cochlear otospongiosis or with Paget’s disease.  

Figure 4: Plastic mould of a human cochlea demonstrating the height of the various turn. Note skewing of the planes of the first and second turns.
Fig.4. Plastic mould of a human cochlea demonstrating the height of the various turn. Note skewing of the planes of the first and second turns. 

An electrode runs considerably higher up into the cochlea if placed near the modiolus than along the outer wall. This owes to the relatively large diameter of the first turn and small dimensions of the modiolus in the second and third turns. Rosenthal’s canal is well defined only in the first turn of the cochlea. Thus, neurons innervating hair cells in the basal turn coincide fairly well with the location of corresponding hair cells, whereas more apically, neurons merge into a less well-defined canal with less obvious precise place/frequency alignment. Neurons innervating hair cells in the third turn are located more basally (Ariyasu et al. 1989 13.The mean outer wall length of the first turn is of value to estimate in patients undergoing EAS surgery14. An electrode around 21mm will extend one turn if placed laterally. The different dimensions suggest that place/frequency maps vary considerably between individuals especially in the apical portion that is spatially compressed relative to the base15. Maximum insertion depth angle as defined by Xu et al.16 may be a better reference for the position of cochlear implant electrodes because it will not be influenced by the distance of the array to the modiolus.The large variations in cochlear anatomy may question the use of a standard electrode with a fixed shape and favor the idea of using more individually or custom-shaped electrodes. When performing CI surgery in patients intended for combined acoustic and electric hearing, it is necessary to consider the multiple anatomic variations. Despite attempts to preserve hearing after cochlear implantation, a small group of patients seems to lose their residual hearing that become dependent on their implant. A shallow insertion reduces the risk of damage to apical cochlear structures, whereas a deep insertion of the array may improve cochlear implant performance in case residual hearing is lost. A radiological method to pre-operatively predict the required insertion depth to achieve a 360-degree insertion is therefore of value14,17. Although not all factors leading to hearing loss after electrode insertion are known to date, the extensive anatomic variations of this fascinating construction most likely play one important role. 

Fig 5. Corrosion cast of a left human inner ear showing the extension of one turn. Arrow shows distance between the labyrinthine portion of the facial nerve canal and the upper basal part of the cochlea. Sometimes this distance is very small which can explain electric stimulation of the facial nerve in some patients with CI. 
 

Acknowledgments

The authors thank Christer Bäck for skillful photography.   

References

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