Otology & Neurotology 34:1564Y1570 Ó 2013, Otology & Neurotology, Inc.
Anatomic Parameters of the Long Process of Incus for Stapes Surgery *Miklo´s To´th, *Gerhard Moser, *Sebastian Ro¨sch, †Gernot Grabmair, and *Gerd Rasp *Paracelsus Medical University Salzburg, ENT Department, Salzburg; and ÞUpper Austria University of Applied Sciences, Wels, Austria
Hypothesis: The purpose of this study is to offer new anatomic data about the long process of incus (LPI) for stapes surgery. Background: Anatomic study of 50 human macerated incudes to measure different parameters of cross sections of the LPI. Methods: Step-by-step cutting of the LPI perpendicular to its axis starting from its free end next to the lenticular process. The layer thickness was 0.5 mm. The cutting surfaces were documented with Canon EOS 20D camera, and a standard software tool (MATLAB) was used for automated statistical analysis. Results: The LPI had a maximum diameter of 1.15 mm and a minimum diameter of 0.52 mm at the level of 1.5 mm far from the tip of the long process, which is the most common site for
stapes prosthesis attachment. Concerning each cross section, having a long and a short diameter, the average long diameter is 0.9011 mm, and the short diameter is 0.6507 mm. Conclusion: Our anatomic study revealed wide variations of diameters and shape of the LPI. Best possible crimping of stapes prosthesis depends not only on the shape and diameters of the LPI but also on the vertical surface of the LPI as well. To prevent incus necrosis due to compression of the feeding blood vessels, the maximum contact surface of the loop of stapes prosthesis should be about 1.9 mm in length. Key Words: CrimpingVLong process of the incusVOtosclerosisVPistonVStapes surgery. Otol Neurotol 34:1564Y1570, 2013.
Stapes surgery in case of otosclerosis is a highly successful surgical procedure. To achieve excellent hearing results, a firm and stable attachment of the prosthesis on the long process of the incus (LPI) is necessary. The crimping procedure is a decisive step during stapes surgery because loose connection between prosthesis and incus will lead to absorption of sound energy and consequently to conductive hearing loss. One of the main complications is the loose wire syndrome, which causes not only a postoperative air-bone gap above 10 dB, but it could also result in necrosis of the LPI (1). Loose crimping has been postulated to cause microtrauma and/or vascular trauma of the LPI. The blood vessels of the incus and the incudostapedial joint can be compromised and damaged by fierce crimping or circumferential fixation of the piston (2,3). To have a tight attachment of the prosthesis, the surgeon needs detailed information on the anatomy of the incus. Although there are many publications on the macroscopic and histologic morphology of the LPI, clinical
anatomy is poorly mentioned in literature (4Y6). Scanning electron microscopy offers further information about the superficial anatomy of the stapes prosthesis and the audiory ossicles. Unfortunately, the scanning electron microscopy was only used to study the different stapes prostheses but not the bony surface of the incus yet. (7,8) In the present study, our goal was to discover more details on the submacroscopic anatomy of the LPI to improve the efficiency of crimping during stapes surgery. MATERIALS AND METHODS The morphology of the long process of incus was studied. For this purpose, 50 macerated human incudes (11 left and 39 right) were used. The ossicles belong to the bone collection of the Applied and Clinical Anatomy Laboratory in Budapest, Hungary. The sex of the ossicles is unknown. All ossicles were macerated in a thermostat at 56-C, and water was changed daily. After maceration, they were whitened in 3% hydrogen peroxide (H2O2) solution for approximately 1 hour. The remaining H2O2 solution was thoroughly rinsed off with water, and the ossicles were subsequently dried at room temperature. The ossicles had no visible pathology on the surface. Depending on the concentration of the formalin solution, there is great variation of shrinking, changing the real size and form of the specimens. To have exact data about the long process
Address correspondence and reprint requests to Miklo´s To´th, M.D., Ph.D., Department of Otorhinolaryngology, Paracelsus Medical University Salzburg, Mu¨llner HauptstraQe 48, 5020 Salzburg, Austria; E-mail: [email protected] The authors disclose no conflicts of interest. M. T. and G. M. contributed equally to this paper.
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SURGICAL ANATOMY OF THE INCUS FOR STAPEDOTOMY
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FIG. 1. Illustration of the cutting surfaces of the LPI (A-E). The cutting surface is perpendicular to the axis of the LPI. The black arrows demonstrate the axis of the external auditory canal (EAC) by all cross sections. The macroscopic photo (left above) shows the auditory ossicles in the tympanic cavity, and the white circle represents the projection of the LPI from above. m, malleus; i, incus; s, stapes.
of the incus (LPI), we have taken measurements on macerated bones. Additionally, we checked the position of the incus within the tympanic cavity in relation to the position of the axis of the external auditory canal. All specimens were fixed onto a wood block on the superior surface of its body; consequently, the axis of LPI was positioned perpendicular to the surface of the wood block. The LPI was cut step-by-step starting from its free end amounting to the lenticular process. Each cut plane was perpendicular to the axis of the LPI, and distance between the neighboring planes was 0.5 mm (Fig. 1). The cutting surface was documented with a 5:1, 65 mm macro objective, mounted on a Canon EOS 20D camera. The outline of all cross sections was highlighted with a thin black line, and its length was measured in millimeter. An automated analysis tool was invaluable to cope with this number of samples. Therefore, we used MATLAB (The MathWorks, Inc.) as a standard software tool for automated vision analysis. The exact shape of the slices was marked a priori by hand for the reason of robustness.
For these shapes, we calculated the standard vision measures true perimeter, area, convex perimeter, convex area, and the perimeter of an artificial minimum circle, enclosing the outer surface of the LPI on the specific level. The convex perimeter and the perimeter of the artificial circle are the most significant values for surgery tool design (Fig. 2). The artificial minimum circle, enclosing the outer surface of the LPI on the specific level, led to a nonlinear optimization problem from the mathematical point of view. It was solved by an active set strategy with the help of MATLAB (9). The error of the calculated values for all given quantities is far below 1% for rectangular and circular test shapes.
RESULTS Taking our measurements of LPI, the average long and short diameters were continuously wider toward the body of incus (Fig. 3; Table 1). Observing all investigated levels
FIG. 2. Drawings of the cross section of the LPI showing the measured parameters: perimeter (A), convex perimeter (B), and enclosed circle perimeter (C). Otology & Neurotology, Vol. 34, No. 9, 2013
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of LPI, the average long diameter varies from 0.8091 to 1.2478 mm, and the average short diameter varies from 0.6367 to 0.8126 mm. Results of the quantitative analysis
of the incus dimensions at each level are shown in Table 1. The long process of incus had a maximum diameter of 1.1505 mm and a minimum diameter of 0.5213 mm at
FIG. 3. Cross sections of the LPI made perpendicular to its axis of the LPI at the levels of 0.5, 1.0, and 1.5 mm far from the tip of the LPI. The arrows in the middle row indicate the axis of the external auditory canal in all 50 ossicles. Arrows positioned above the cross section indicate incus of the left side. Arrows below the cross section indicate incus of the right side. Otology & Neurotology, Vol. 34, No. 9, 2013
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SURGICAL ANATOMY OF THE INCUS FOR STAPEDOTOMY TABLE 1.
The short and the long diameters of the cross sections of long process of the incus were measured at the levels of 0.5, 1.0, 1.5, 2.0, and 2.5 mm far from the tip of the long process of the incus 0.5 mm
Average Maximum Minimum
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1.0 mm
1.5 mm
2.0 mm
2.5 mm
LD
SD
LD
SD
LD
SD
LD
SD
LD
SD
0.8272 1.0516 0.555
0.6367 0.8134 0.4606
0.0891 0.9528 0.6494
0.6374 0.7573 0.528
0.9011 1.1505 0.6764
0.6507 0.8179 0.5213
1.0539 1.3865 0.7393
0.7078 0.8741 0.5528
1.2478 1.982 0.9146
0.8126 1.2382 0.6292
All data is measured in millimeters. LD indicates long diameter; SD, short diameter.
level of 1.5 mm far from the tip of LPI, which is the most common site for stapes prosthesis attachment. At this cross section, the average long diameter is 0.9011 mm and the short diameter 0.6507 mm. The short diameter of the LPI demonstrates a lower variation than the long diameter measured in all levels. The average short diameter varied between 0.6367 and 0.8126 mm, and the greatest difference can be found between the cross sections of level 2.0 and 2.5 mm (Table 1). For each cross section of the 5 levels of the 50 ossicles, the following parameters were measured: true perimeter, area, convex perimeter, convex area, and artificial circle perimeter (5) consequently (Fig. 2; Table 2). Similar to the results of the short and long diameters, the average true perimeter was shortest in the cross sections at level of 1.0 mm with 2.5325 mm and increased continuously toward the body of the incus. At the level of 1.5 mm, the average true perimeter was 2.7025 mm. The average convex perimeter was shorter than the average true perimeter in all 5 cross sections of the LPI because of all the excavations artificially being covered with a straight line, simulating the position of the stapes prosthesis loop. The shortest average convex perimeter was
also in cross sections at level of 1.0 mm (2.4748 mm). At the level of 1.5 mm, the average convex perimeter was 2.6757 mm. The smallest circle, which can cover the cross section of the LPI, the so-called artificial circle perimeter, was also measured. Its average length was slightly longer than the average convex perimeter at all 5 levels. The exact length in millimeter can be seen in Table 2. The cross sections at level of 1.5 mm had an average length of 2.9251 mm. The cross sections of LPI demonstrate 3 different kinds of shape: oval, nearly round shaped, or irregular. In 25 of the investigated 50 incudes, we found an oval shape: 15 had nearly a round shape, and 10 showed an irregular shape. In all specimens, we could identify a long and a short axis of each cross section. In 29 specimens, the axis of the external auditory canal was parallel to the long axis of the cross section. Only in 5 cases, it was parallel to the short axis. In 16 specimens, neither the long nor the short axis was parallel to the axis of the external auditory canal. The tip of the LPI is usually wider than the thinnest portion of the LPI, which can be found just above the lenticular process. Cranially to the lenticular process, the
TABLE 2. Further geometric parameters (perimeter, area, convex perimeter, convex area, and enclosed circle perimeter) of the cross sections of the long process of the incus were measured at the levels of 0.5, 1.0, 1.5, 2.0, and 2.5 mm far from the tip of the long process of the incus Perimeter
Area
Convex perimeter
Minimum Maximum Average
1.8962 3.7726 2.8262
0.2476 0.6793 0.4247
1.8916 3.2044 2.5714
Minimum Maximum Average
2.1420 3.1479 2.5325
0.2926 0.5392 0.4151
2.1159 2.8487 2.4748
Minimum Maximum Average
2.2392 3.3324 2.7025
0.3319 0.7029 0.4810
2.2392 3.3197 2.6757
Minimum Maximum Average
2.3737 5.0418 3.2165
0.3898 0.9848 0.6213
2.3718 3.9310 3.0782
Minimum Maximum Average
2.7292 5.3450 3.7296
0.4816 1.6279 0.8526
2.6852 5.3008 3.5979
Convex area
Enclosed circle perimeter
0.2499 0.7146 0.4549
1.9473 3.4092 2.7414
0.3122 0.5627 0.4252
2.1479 3.0920 2.6306
0.3430 0.7171 0.4888
2.3115 3.7281 2.9251
0.3978 0.9985 0.6384
2.4261 4.4718 3.4299
0.4941 1.6787 0.8716
2.9555 6.3911 4.0415
0.5 mm
1.0 mm
1.5 mm
2.0 mm
2.5 mm
All data was measured in millimeters. Otology & Neurotology, Vol. 34, No. 9, 2013
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FIG. 4. The segment of the LPI between the levels 1.0 and 1.5 mm far from the tip of the LPI was cut twiceVonce along the long diameter and once along the short diameter in all fifty incudes. We defined and measured the angle (>) between the lateral wall of the LPI and the cross section, represented by the greater long diameter and the greater short diameter. Depending on the position of the greater long and short diameter, we could define the shape of the cross section as a truncated cone (B, D) or an inverted truncated cone (A, C) in the segment between 1.0 and 1.5 mm far from the tip.
LPI becomes continuously thicker and runs into the incus body without any clearly visible border. Stapes prostheses are usually crimped at about 1.4 mm far from the tip of the LPI. We measured the steepness of the conical bony surface between the levels 1.0 and 1.5 mm far from the tip of the LPI (Fig. 4), to provide further anatomic details of the contact area for stapes prosthesis loop. At the level of 1.0 mm, the average long diameter was 0.81 and 0.9 mm for the level of 1.5 mm. The average short diameter was 0.64 mm at level of 1.0 and 0.65 mm for the level of 1.5 mm (Table 1). Finally, we investigated the shape of the planes between the level of 1.0 and 1.5 mm, defined through the short and the long diameter of the cross sections. The planes showed inverted truncated shape or truncated shape, depending on the level of the greater long diameter and the greater short diameter (Fig. 4). The planes defined through the long diameter on both levels showed inverted truncated shape in 43 cases. Trun-
TABLE 3.
The steepness of the long process of the incus was measured in all 50 incudes Long diameter
No. Average Maximum Minimum
Short diameter
ITC
TC
ITC
TC
43 83.0989.8771.72-
7 84.8389.3577.58-
25 85.529079.31-
25 87.2389.7481.06-
In most of the cases, the long diameter of the cross sections between 1.0 and 1.5 mm far from the tip was bigger cranially. The short diameter was found to be bigger cranially in the half of the cases and smaller in the remaining half. The angle (>) between the lateral wall and the greater long diameter or the greater short diameter of each of the both cross sections between 1.0 and 1.5 mm is measured in degrees (see also Fig. 4). ITC indicates inverted truncated cone; TC, truncated cone.
cated shape was found in 7 cases. The planes defined through the short diameter on both levels presented controversial findings. There was an equal amount of inverted truncated shape and truncated shape (Table 3). To describe the steepness of the lateral wall between the 2 levels, we measured the angle between the lateral wall and the cross section, represented by the greater long diameter and the greater short diameter. We selected this specific angle (>) for measurement to compare the steepness independently from the shape of the plane. The average angle, represented by the greater long diameter and the lateral wall, was 83.09 degrees (maximum, 89.87 degrees; minimum, 71.72 degrees). In 7 of 50, the long diameter was cranially shorter, demonstrating a truncated cone. The average angle was 84.83 degrees (maximum, 89.35 degrees; minimum, 77.58 degrees). Measuring the short diameter, we received completely different results. In 25 cases, a truncated cone could be observed and also in 25 cases an inverted one. If the shorter diameter was greater cranially, the average angle was 85.52 degrees (maximum, 90 degrees; minimum, 79.31 degrees). If the LPI showed a truncated cone, the average angle was 87.23 degrees (maximum, 89.74 degrees; minimum, 81.06 degrees).
DISCUSSION The most critical points of stapedotomy are the perforation of the footplate and the crimping of the stapes prosthesis. The perforation can be performed with a manual perforator, laser, or microdrill, depending on the surgical situation, but the crimping remains more difficult. There is no adequate instrument, which could ensure a stable attachment of the stapes prosthesis loop on the LPI. According to the literature, the McGee and the straight
Otology & Neurotology, Vol. 34, No. 9, 2013
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SURGICAL ANATOMY OF THE INCUS FOR STAPEDOTOMY alligator forceps are the 2 instruments usually used for crimping. Hu¨ttenbrink and Beutner (10) developed a new crimping tool for better anchoring of the prosthesis on the LPI. The new tool is equipped with a retraining spring handle, and its tips are arcuated to have better fitting around the bone. Since May 2007, a new titanium stapes prosthesis, the Soft-Clip Piston, is available. This is a self-retaining clip prosthesis and does not need crimping. Consequently, it should minimize the intraoperative and postoperative damage of the ossicle chain. The short-term hearing results are very good, but this prosthesis cannot be used in case of very thin or very thick LPI (11). We found a great variability of diameters of the LPI. The remaining diameter of a closed clip prosthesis is approximately 0.5 mm, with an optimum expansion of 0.9 mm. In cases of a minimum diameter of 0.7 mm of the LPI at the level of 1.5 mm, fitting to the bone surface could be too loose because of a reduced expansion of the prosthesis loop. With an extreme diameter of 1.15 mm, the expansion of the prosthesis loop exceeds its given diameter more than 2 times. This could lead to either impossibility of adaption of the prosthesis to the bone or to a lack of remaining force to the bone, administered by the loop, because of overexpansion. Furthermore, great size of the LPI leads to greater forces needed to adapt the prosthesis, resulting in possible incus luxation. Another possibility to minimize the risk of manual malcrimping or incus luxation is the use of a self-crimping shape memory alloy Nitinol stapes piston. Short- and long-term follow-up studies revealed good hearing results with this prosthesis, but the crimping depends on the anatomy of the LPI, similar to the self-retaining clip prosthesis (12). A study of 190 cases using heat-activated crimping prostheses demonstrated 21 patients who had initial good hearing results but required revision because of lateral displacement of the prosthesis out of the vestibule and/or incus in the further course of time (13). Similar to the soft clip prosthesis, optimum fitting depends on the diameter and form of the LPI. Again, in cases of extreme diameters like mentioned previously, fitting to the bone could be restricted by the given properties of the prosthesis, optimized for a certain diameter. The most commonly used site for prosthesis crimping is between 1.0 and 1.5 mm far from the tip of the LPI. According to Kwok et al. (14), the average level of prosthesis attachment is on a level of 1.4 T 0.28 mm far from the tip of the LPI. The diameter on this level demonstrates a wide range. Kwok et al. found a minimum diameter of 0.66 T 0.05 mm and a maximum diameter of 0.81 T 0.1 mm after examination of 11 incudes. Our study of 50 incudes showed a much greater variability ranging from 0.52 to 1.15 mm. If we look on the anatomic properties of LPI to find influencing factors for the crimping results, we have to look on the crimping technique itself as well (15). Using the McGee or the straight alligator forceps, the crimping occurs at the ventrolateral and dorsomedial surface because the instruments enter the tympanic cavity approximately parallel to the external auditory canal. In more than half of the observed specimens in our collection
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(29 of 50), the long diameter of the LPI was parallel to the axis of the external auditory canal. In these cases, the crimping occurs parallel to the long axis of the LPI, that is, on the larger surface of the circumference of the LPI. If the long diameter of the LPI is perpendicular to the axis of the external auditory canal (5 of 50), there is only a small surface for crimping. There is a general meaning on the shape of the LPI being an inverted truncated cone, widening from caudal to cranial. In our investigations, we could prove that widening occurs in a very variable amount and could even vary between the different surfaces of the same segment between 2 levels. We focused on the segment between levels of 1.0 and 1.5 mm, which is the typical site of prosthesis attachment. The measured angle (>) was generally smaller with reference to the vertical planes defined through the long diameters, than the angle (>) for the vertical planes of the short diameters. A bigger angle (>) provides a larger vertical surface for the attachment of the loop to the bone. Some stapes prostheses (e.g., Platinum-Teflon, KPiston, and Titanium a`Wengen) have a band-shaped loop. If the LPI shows a truncated cone, the band cannot be attached to the whole bony surface, despite a spiral shaped band. The true surface for crimping depends not only on the horizontal course of the surface of the LPI but on the vertical course of the surface as well. The horizontal course of the surface depends on the position of the long diameter in relation to the axis of the external auditory canal. Although there might be the optimum situation of the long diameter being parallel to the axis of the external auditory canal, we do not see better hearing results than in cases of the long diameter being perpendicular to the axis. The vertical course of the surface is defined through the angle (>). The angle (>) has great influence on the results of crimping. We want to emphasize that the entire bony surface of the LPI, being crucial for crimping results, depends on both horizontal and vertical surface properties. If the long diameter of the LPI lies parallel to the axis of the external auditory canal, it represents a longer course of the horizontal surface. Interestingly, in all of our cases with this situation, we found a smaller angle (>), leading to a smaller surface in the vertical course of LPI. Taking the opposite situation with a shorter course of the horizontal surface because of the long diameter being perpendicular to the axis of the external auditory canal, there was found a larger angle (>). This leads to equalized surface properties in both situations, affording equal results in most of the cases using a bandshape prosthesis loop. According to Nadol (16), the most common histopathologic correlate of conductive hearing loss after stapedectomy is resorptive osteitis of the incus at the site of prosthesis attachment (64%). Necrosis of the LPI can occur after malcrimped stapes prosthesis or due to compromised blood supply of the incus. Gerlinger et al. (17) observed the bony foramina of the feeding blood vessels of the LPI of 100 incudes and found up to 4 nutritive foramina on the LPI. The foramina were located anteromedially, mostly on the middle and cranial third of the LPI. The anteromedial Otology & Neurotology, Vol. 34, No. 9, 2013
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surface of the LPI is always covered with the stapes prosthesis after crimping because the loop of all stapes prostheses is longer than the circumference of the thickest LPI. To prevent incus necrosis because of compression of the feeding blood vessels, the loop of stapes prosthesis should be shorter, covering only three-fourths of the circumference. In our anatomic collection, the average convex perimeter was 2.47 mm at the level of 1.0 and 2.68 mm at the level of 1.5 mm far from the tip of the LPI. An ideal prosthesis loop for stapedotomy should have a length of approximately 1.9 mm, keeping the anteromedial quarter of the LPI uncovered. Another possibility to prevent compression of the feeding blood vessels is to develop a new instrument, allowing best possible crimping of a shorter prosthesis and keeping the anteromedial quarter of the LPI free.
CONCLUSION The purpose of this study is to provide new anatomic information on the long process of incus with special regard to the area of crimping in case of stapes prosthesis. We investigated 50 macerated incudes, measured the variability of diameters and shape, and established an additional parameter for the description of the vertical surface of the crimping area, the angle (>). With the help of those parameters taken together, we could describe a possible explanation for generally good hearing results, independently from the choice of the prosthesis. Furthermore, we could show limitations for the indication of specific prosthesis because of a great variation of diameters, not mentioned in literature before. Finally, we want to emphasize on the anteromedial surface of the LPI, being relevant for the blood supply of the bone. We suggest to keep this area uncovered by the prosthesis loop after crimping, to prevent possible incus necrosis.
REFERENCES 1. McGee TM. The loose wire syndrome. Laryngoscope 1981;91: 1478Y83. 2. Alberti PW. The blood supply of the incudostapedial joint and the lenticular process. Laryngoscope 1963;73:605Y28. 3. Alberti PW. The blood supply of the long process of the incus and the head and neck of stapes. J Laryngol Otol 1965;79:966Y70. 4. Schwalbe GA. Lehrbuch der Anatomie der Sinnesorgane. E. Besold, 1887:487Y92. 5. Skinner M, Honrado C, Prasad M, et al. The incudostapedial joint angle: implications for stapes surgery prosthesis selection and criming. Laryngoscope 2003;113:647Y53. 6. Gulya AJ, Schuknecht HF. Anatomy of the Temporal Bone with Surgical Implications. New York, NY: Parthenon Publishing Group, 1995:63Y8. 7. Kwok P, Fisch U, Strutz J, et al. Comparative electron microscopic study of the surface structure of gold, Teflon, and titanium stapes prostheses. Otol Neurotol 2001;22:608Y13. 8. Fontana M, Ferri E, Lora L, et al. A scanning electron microscopic study of crimping of stapedial prostheses. Auris Nasus Larynx 2012; 39:461Y8. 9. D´ Errico J. A suite of minimal bounding objects. 3 Apple Hill Drive, Natick, Massachusetts 01760 USA: The MathWorks, Inc. Available at: http://www.mathworks.com/matlabcentral/fileexchange/34767a-suite-of-minimal-bounding-objects. Accessed January 25, 2012. 10. Huettenbrink KB, Beutner D. A new crimping device for stapedectomy prostheses. Laryngoscope 2005;115:2065Y7. 11. Bast F, Schrom T. First experiences with the new soft-clip piston as an alloplastic prosthesis during stapedotomy [in German]. Laryngorhinootologie 2009;88:304Y8. 12. Huber AM, Veraguth D, Schmid S, et al. Tight stapes prosthesis fixation leads to better functional results in otosclerosis surgery. Otol Neurotol 2008;29:893Y9. 13. Ying YL, Hillman TA, Chen DA. Patterns of failure in heatactivated crimping prosthesis in stapedotomy. Otol Neurotol 2011; 32:21Y8. 14. Kwok P, Fisch U, Gleich O, et al. Stapes surgery: the diameter of the long process of the incus. Otol Neurotol 2006;27:469Y77. 15. Kwok P, Fisch U, Strutz J, et al. Stapes surgery: how precisely do different prostheses attach to the long process of the incus with different instruments and different surgeons? Otol Neurotol 2002;23: 289Y95. 16. Nadol JB Jr. Histopathology of residual and recurrent conductive hearing loss after stapedectomy. Otol Neurotol 2001;22:162Y9. 17. Gerlinger I, Toth M, Lujber L, et al. Necrosis of the long process of the incus following stapes surgery: new anatomical observations. Laryngoscope 2009;119:721Y6.
Otology & Neurotology, Vol. 34, No. 9, 2013
Copyright © 2013 Otology & Neurotology, Inc. Unauthorized reproduction of this article is prohibited.
Anatomic Parameters of the Long Process of Incus for Stapes Surgery *Miklo´s To´th, *Gerhard Moser, *Sebastian Ro¨sch, †Gernot Grabmair, and *Gerd Rasp *Paracelsus Medical University Salzburg, ENT Department, Salzburg; and ÞUpper Austria University of Applied Sciences, Wels, Austria
Hypothesis: The purpose of this study is to offer new anatomic data about the long process of incus (LPI) for stapes surgery. Background: Anatomic study of 50 human macerated incudes to measure different parameters of cross sections of the LPI. Methods: Step-by-step cutting of the LPI perpendicular to its axis starting from its free end next to the lenticular process. The layer thickness was 0.5 mm. The cutting surfaces were documented with Canon EOS 20D camera, and a standard software tool (MATLAB) was used for automated statistical analysis. Results: The LPI had a maximum diameter of 1.15 mm and a minimum diameter of 0.52 mm at the level of 1.5 mm far from the tip of the long process, which is the most common site for
stapes prosthesis attachment. Concerning each cross section, having a long and a short diameter, the average long diameter is 0.9011 mm, and the short diameter is 0.6507 mm. Conclusion: Our anatomic study revealed wide variations of diameters and shape of the LPI. Best possible crimping of stapes prosthesis depends not only on the shape and diameters of the LPI but also on the vertical surface of the LPI as well. To prevent incus necrosis due to compression of the feeding blood vessels, the maximum contact surface of the loop of stapes prosthesis should be about 1.9 mm in length. Key Words: CrimpingVLong process of the incusVOtosclerosisVPistonVStapes surgery. Otol Neurotol 34:1564Y1570, 2013.
Stapes surgery in case of otosclerosis is a highly successful surgical procedure. To achieve excellent hearing results, a firm and stable attachment of the prosthesis on the long process of the incus (LPI) is necessary. The crimping procedure is a decisive step during stapes surgery because loose connection between prosthesis and incus will lead to absorption of sound energy and consequently to conductive hearing loss. One of the main complications is the loose wire syndrome, which causes not only a postoperative air-bone gap above 10 dB, but it could also result in necrosis of the LPI (1). Loose crimping has been postulated to cause microtrauma and/or vascular trauma of the LPI. The blood vessels of the incus and the incudostapedial joint can be compromised and damaged by fierce crimping or circumferential fixation of the piston (2,3). To have a tight attachment of the prosthesis, the surgeon needs detailed information on the anatomy of the incus. Although there are many publications on the macroscopic and histologic morphology of the LPI, clinical
anatomy is poorly mentioned in literature (4Y6). Scanning electron microscopy offers further information about the superficial anatomy of the stapes prosthesis and the audiory ossicles. Unfortunately, the scanning electron microscopy was only used to study the different stapes prostheses but not the bony surface of the incus yet. (7,8) In the present study, our goal was to discover more details on the submacroscopic anatomy of the LPI to improve the efficiency of crimping during stapes surgery. MATERIALS AND METHODS The morphology of the long process of incus was studied. For this purpose, 50 macerated human incudes (11 left and 39 right) were used. The ossicles belong to the bone collection of the Applied and Clinical Anatomy Laboratory in Budapest, Hungary. The sex of the ossicles is unknown. All ossicles were macerated in a thermostat at 56-C, and water was changed daily. After maceration, they were whitened in 3% hydrogen peroxide (H2O2) solution for approximately 1 hour. The remaining H2O2 solution was thoroughly rinsed off with water, and the ossicles were subsequently dried at room temperature. The ossicles had no visible pathology on the surface. Depending on the concentration of the formalin solution, there is great variation of shrinking, changing the real size and form of the specimens. To have exact data about the long process
Address correspondence and reprint requests to Miklo´s To´th, M.D., Ph.D., Department of Otorhinolaryngology, Paracelsus Medical University Salzburg, Mu¨llner HauptstraQe 48, 5020 Salzburg, Austria; E-mail: [email protected] The authors disclose no conflicts of interest. M. T. and G. M. contributed equally to this paper.
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FIG. 1. Illustration of the cutting surfaces of the LPI (A-E). The cutting surface is perpendicular to the axis of the LPI. The black arrows demonstrate the axis of the external auditory canal (EAC) by all cross sections. The macroscopic photo (left above) shows the auditory ossicles in the tympanic cavity, and the white circle represents the projection of the LPI from above. m, malleus; i, incus; s, stapes.
of the incus (LPI), we have taken measurements on macerated bones. Additionally, we checked the position of the incus within the tympanic cavity in relation to the position of the axis of the external auditory canal. All specimens were fixed onto a wood block on the superior surface of its body; consequently, the axis of LPI was positioned perpendicular to the surface of the wood block. The LPI was cut step-by-step starting from its free end amounting to the lenticular process. Each cut plane was perpendicular to the axis of the LPI, and distance between the neighboring planes was 0.5 mm (Fig. 1). The cutting surface was documented with a 5:1, 65 mm macro objective, mounted on a Canon EOS 20D camera. The outline of all cross sections was highlighted with a thin black line, and its length was measured in millimeter. An automated analysis tool was invaluable to cope with this number of samples. Therefore, we used MATLAB (The MathWorks, Inc.) as a standard software tool for automated vision analysis. The exact shape of the slices was marked a priori by hand for the reason of robustness.
For these shapes, we calculated the standard vision measures true perimeter, area, convex perimeter, convex area, and the perimeter of an artificial minimum circle, enclosing the outer surface of the LPI on the specific level. The convex perimeter and the perimeter of the artificial circle are the most significant values for surgery tool design (Fig. 2). The artificial minimum circle, enclosing the outer surface of the LPI on the specific level, led to a nonlinear optimization problem from the mathematical point of view. It was solved by an active set strategy with the help of MATLAB (9). The error of the calculated values for all given quantities is far below 1% for rectangular and circular test shapes.
RESULTS Taking our measurements of LPI, the average long and short diameters were continuously wider toward the body of incus (Fig. 3; Table 1). Observing all investigated levels
FIG. 2. Drawings of the cross section of the LPI showing the measured parameters: perimeter (A), convex perimeter (B), and enclosed circle perimeter (C). Otology & Neurotology, Vol. 34, No. 9, 2013
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of LPI, the average long diameter varies from 0.8091 to 1.2478 mm, and the average short diameter varies from 0.6367 to 0.8126 mm. Results of the quantitative analysis
of the incus dimensions at each level are shown in Table 1. The long process of incus had a maximum diameter of 1.1505 mm and a minimum diameter of 0.5213 mm at
FIG. 3. Cross sections of the LPI made perpendicular to its axis of the LPI at the levels of 0.5, 1.0, and 1.5 mm far from the tip of the LPI. The arrows in the middle row indicate the axis of the external auditory canal in all 50 ossicles. Arrows positioned above the cross section indicate incus of the left side. Arrows below the cross section indicate incus of the right side. Otology & Neurotology, Vol. 34, No. 9, 2013
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SURGICAL ANATOMY OF THE INCUS FOR STAPEDOTOMY TABLE 1.
The short and the long diameters of the cross sections of long process of the incus were measured at the levels of 0.5, 1.0, 1.5, 2.0, and 2.5 mm far from the tip of the long process of the incus 0.5 mm
Average Maximum Minimum
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1.0 mm
1.5 mm
2.0 mm
2.5 mm
LD
SD
LD
SD
LD
SD
LD
SD
LD
SD
0.8272 1.0516 0.555
0.6367 0.8134 0.4606
0.0891 0.9528 0.6494
0.6374 0.7573 0.528
0.9011 1.1505 0.6764
0.6507 0.8179 0.5213
1.0539 1.3865 0.7393
0.7078 0.8741 0.5528
1.2478 1.982 0.9146
0.8126 1.2382 0.6292
All data is measured in millimeters. LD indicates long diameter; SD, short diameter.
level of 1.5 mm far from the tip of LPI, which is the most common site for stapes prosthesis attachment. At this cross section, the average long diameter is 0.9011 mm and the short diameter 0.6507 mm. The short diameter of the LPI demonstrates a lower variation than the long diameter measured in all levels. The average short diameter varied between 0.6367 and 0.8126 mm, and the greatest difference can be found between the cross sections of level 2.0 and 2.5 mm (Table 1). For each cross section of the 5 levels of the 50 ossicles, the following parameters were measured: true perimeter, area, convex perimeter, convex area, and artificial circle perimeter (5) consequently (Fig. 2; Table 2). Similar to the results of the short and long diameters, the average true perimeter was shortest in the cross sections at level of 1.0 mm with 2.5325 mm and increased continuously toward the body of the incus. At the level of 1.5 mm, the average true perimeter was 2.7025 mm. The average convex perimeter was shorter than the average true perimeter in all 5 cross sections of the LPI because of all the excavations artificially being covered with a straight line, simulating the position of the stapes prosthesis loop. The shortest average convex perimeter was
also in cross sections at level of 1.0 mm (2.4748 mm). At the level of 1.5 mm, the average convex perimeter was 2.6757 mm. The smallest circle, which can cover the cross section of the LPI, the so-called artificial circle perimeter, was also measured. Its average length was slightly longer than the average convex perimeter at all 5 levels. The exact length in millimeter can be seen in Table 2. The cross sections at level of 1.5 mm had an average length of 2.9251 mm. The cross sections of LPI demonstrate 3 different kinds of shape: oval, nearly round shaped, or irregular. In 25 of the investigated 50 incudes, we found an oval shape: 15 had nearly a round shape, and 10 showed an irregular shape. In all specimens, we could identify a long and a short axis of each cross section. In 29 specimens, the axis of the external auditory canal was parallel to the long axis of the cross section. Only in 5 cases, it was parallel to the short axis. In 16 specimens, neither the long nor the short axis was parallel to the axis of the external auditory canal. The tip of the LPI is usually wider than the thinnest portion of the LPI, which can be found just above the lenticular process. Cranially to the lenticular process, the
TABLE 2. Further geometric parameters (perimeter, area, convex perimeter, convex area, and enclosed circle perimeter) of the cross sections of the long process of the incus were measured at the levels of 0.5, 1.0, 1.5, 2.0, and 2.5 mm far from the tip of the long process of the incus Perimeter
Area
Convex perimeter
Minimum Maximum Average
1.8962 3.7726 2.8262
0.2476 0.6793 0.4247
1.8916 3.2044 2.5714
Minimum Maximum Average
2.1420 3.1479 2.5325
0.2926 0.5392 0.4151
2.1159 2.8487 2.4748
Minimum Maximum Average
2.2392 3.3324 2.7025
0.3319 0.7029 0.4810
2.2392 3.3197 2.6757
Minimum Maximum Average
2.3737 5.0418 3.2165
0.3898 0.9848 0.6213
2.3718 3.9310 3.0782
Minimum Maximum Average
2.7292 5.3450 3.7296
0.4816 1.6279 0.8526
2.6852 5.3008 3.5979
Convex area
Enclosed circle perimeter
0.2499 0.7146 0.4549
1.9473 3.4092 2.7414
0.3122 0.5627 0.4252
2.1479 3.0920 2.6306
0.3430 0.7171 0.4888
2.3115 3.7281 2.9251
0.3978 0.9985 0.6384
2.4261 4.4718 3.4299
0.4941 1.6787 0.8716
2.9555 6.3911 4.0415
0.5 mm
1.0 mm
1.5 mm
2.0 mm
2.5 mm
All data was measured in millimeters. Otology & Neurotology, Vol. 34, No. 9, 2013
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FIG. 4. The segment of the LPI between the levels 1.0 and 1.5 mm far from the tip of the LPI was cut twiceVonce along the long diameter and once along the short diameter in all fifty incudes. We defined and measured the angle (>) between the lateral wall of the LPI and the cross section, represented by the greater long diameter and the greater short diameter. Depending on the position of the greater long and short diameter, we could define the shape of the cross section as a truncated cone (B, D) or an inverted truncated cone (A, C) in the segment between 1.0 and 1.5 mm far from the tip.
LPI becomes continuously thicker and runs into the incus body without any clearly visible border. Stapes prostheses are usually crimped at about 1.4 mm far from the tip of the LPI. We measured the steepness of the conical bony surface between the levels 1.0 and 1.5 mm far from the tip of the LPI (Fig. 4), to provide further anatomic details of the contact area for stapes prosthesis loop. At the level of 1.0 mm, the average long diameter was 0.81 and 0.9 mm for the level of 1.5 mm. The average short diameter was 0.64 mm at level of 1.0 and 0.65 mm for the level of 1.5 mm (Table 1). Finally, we investigated the shape of the planes between the level of 1.0 and 1.5 mm, defined through the short and the long diameter of the cross sections. The planes showed inverted truncated shape or truncated shape, depending on the level of the greater long diameter and the greater short diameter (Fig. 4). The planes defined through the long diameter on both levels showed inverted truncated shape in 43 cases. Trun-
TABLE 3.
The steepness of the long process of the incus was measured in all 50 incudes Long diameter
No. Average Maximum Minimum
Short diameter
ITC
TC
ITC
TC
43 83.0989.8771.72-
7 84.8389.3577.58-
25 85.529079.31-
25 87.2389.7481.06-
In most of the cases, the long diameter of the cross sections between 1.0 and 1.5 mm far from the tip was bigger cranially. The short diameter was found to be bigger cranially in the half of the cases and smaller in the remaining half. The angle (>) between the lateral wall and the greater long diameter or the greater short diameter of each of the both cross sections between 1.0 and 1.5 mm is measured in degrees (see also Fig. 4). ITC indicates inverted truncated cone; TC, truncated cone.
cated shape was found in 7 cases. The planes defined through the short diameter on both levels presented controversial findings. There was an equal amount of inverted truncated shape and truncated shape (Table 3). To describe the steepness of the lateral wall between the 2 levels, we measured the angle between the lateral wall and the cross section, represented by the greater long diameter and the greater short diameter. We selected this specific angle (>) for measurement to compare the steepness independently from the shape of the plane. The average angle, represented by the greater long diameter and the lateral wall, was 83.09 degrees (maximum, 89.87 degrees; minimum, 71.72 degrees). In 7 of 50, the long diameter was cranially shorter, demonstrating a truncated cone. The average angle was 84.83 degrees (maximum, 89.35 degrees; minimum, 77.58 degrees). Measuring the short diameter, we received completely different results. In 25 cases, a truncated cone could be observed and also in 25 cases an inverted one. If the shorter diameter was greater cranially, the average angle was 85.52 degrees (maximum, 90 degrees; minimum, 79.31 degrees). If the LPI showed a truncated cone, the average angle was 87.23 degrees (maximum, 89.74 degrees; minimum, 81.06 degrees).
DISCUSSION The most critical points of stapedotomy are the perforation of the footplate and the crimping of the stapes prosthesis. The perforation can be performed with a manual perforator, laser, or microdrill, depending on the surgical situation, but the crimping remains more difficult. There is no adequate instrument, which could ensure a stable attachment of the stapes prosthesis loop on the LPI. According to the literature, the McGee and the straight
Otology & Neurotology, Vol. 34, No. 9, 2013
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SURGICAL ANATOMY OF THE INCUS FOR STAPEDOTOMY alligator forceps are the 2 instruments usually used for crimping. Hu¨ttenbrink and Beutner (10) developed a new crimping tool for better anchoring of the prosthesis on the LPI. The new tool is equipped with a retraining spring handle, and its tips are arcuated to have better fitting around the bone. Since May 2007, a new titanium stapes prosthesis, the Soft-Clip Piston, is available. This is a self-retaining clip prosthesis and does not need crimping. Consequently, it should minimize the intraoperative and postoperative damage of the ossicle chain. The short-term hearing results are very good, but this prosthesis cannot be used in case of very thin or very thick LPI (11). We found a great variability of diameters of the LPI. The remaining diameter of a closed clip prosthesis is approximately 0.5 mm, with an optimum expansion of 0.9 mm. In cases of a minimum diameter of 0.7 mm of the LPI at the level of 1.5 mm, fitting to the bone surface could be too loose because of a reduced expansion of the prosthesis loop. With an extreme diameter of 1.15 mm, the expansion of the prosthesis loop exceeds its given diameter more than 2 times. This could lead to either impossibility of adaption of the prosthesis to the bone or to a lack of remaining force to the bone, administered by the loop, because of overexpansion. Furthermore, great size of the LPI leads to greater forces needed to adapt the prosthesis, resulting in possible incus luxation. Another possibility to minimize the risk of manual malcrimping or incus luxation is the use of a self-crimping shape memory alloy Nitinol stapes piston. Short- and long-term follow-up studies revealed good hearing results with this prosthesis, but the crimping depends on the anatomy of the LPI, similar to the self-retaining clip prosthesis (12). A study of 190 cases using heat-activated crimping prostheses demonstrated 21 patients who had initial good hearing results but required revision because of lateral displacement of the prosthesis out of the vestibule and/or incus in the further course of time (13). Similar to the soft clip prosthesis, optimum fitting depends on the diameter and form of the LPI. Again, in cases of extreme diameters like mentioned previously, fitting to the bone could be restricted by the given properties of the prosthesis, optimized for a certain diameter. The most commonly used site for prosthesis crimping is between 1.0 and 1.5 mm far from the tip of the LPI. According to Kwok et al. (14), the average level of prosthesis attachment is on a level of 1.4 T 0.28 mm far from the tip of the LPI. The diameter on this level demonstrates a wide range. Kwok et al. found a minimum diameter of 0.66 T 0.05 mm and a maximum diameter of 0.81 T 0.1 mm after examination of 11 incudes. Our study of 50 incudes showed a much greater variability ranging from 0.52 to 1.15 mm. If we look on the anatomic properties of LPI to find influencing factors for the crimping results, we have to look on the crimping technique itself as well (15). Using the McGee or the straight alligator forceps, the crimping occurs at the ventrolateral and dorsomedial surface because the instruments enter the tympanic cavity approximately parallel to the external auditory canal. In more than half of the observed specimens in our collection
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(29 of 50), the long diameter of the LPI was parallel to the axis of the external auditory canal. In these cases, the crimping occurs parallel to the long axis of the LPI, that is, on the larger surface of the circumference of the LPI. If the long diameter of the LPI is perpendicular to the axis of the external auditory canal (5 of 50), there is only a small surface for crimping. There is a general meaning on the shape of the LPI being an inverted truncated cone, widening from caudal to cranial. In our investigations, we could prove that widening occurs in a very variable amount and could even vary between the different surfaces of the same segment between 2 levels. We focused on the segment between levels of 1.0 and 1.5 mm, which is the typical site of prosthesis attachment. The measured angle (>) was generally smaller with reference to the vertical planes defined through the long diameters, than the angle (>) for the vertical planes of the short diameters. A bigger angle (>) provides a larger vertical surface for the attachment of the loop to the bone. Some stapes prostheses (e.g., Platinum-Teflon, KPiston, and Titanium a`Wengen) have a band-shaped loop. If the LPI shows a truncated cone, the band cannot be attached to the whole bony surface, despite a spiral shaped band. The true surface for crimping depends not only on the horizontal course of the surface of the LPI but on the vertical course of the surface as well. The horizontal course of the surface depends on the position of the long diameter in relation to the axis of the external auditory canal. Although there might be the optimum situation of the long diameter being parallel to the axis of the external auditory canal, we do not see better hearing results than in cases of the long diameter being perpendicular to the axis. The vertical course of the surface is defined through the angle (>). The angle (>) has great influence on the results of crimping. We want to emphasize that the entire bony surface of the LPI, being crucial for crimping results, depends on both horizontal and vertical surface properties. If the long diameter of the LPI lies parallel to the axis of the external auditory canal, it represents a longer course of the horizontal surface. Interestingly, in all of our cases with this situation, we found a smaller angle (>), leading to a smaller surface in the vertical course of LPI. Taking the opposite situation with a shorter course of the horizontal surface because of the long diameter being perpendicular to the axis of the external auditory canal, there was found a larger angle (>). This leads to equalized surface properties in both situations, affording equal results in most of the cases using a bandshape prosthesis loop. According to Nadol (16), the most common histopathologic correlate of conductive hearing loss after stapedectomy is resorptive osteitis of the incus at the site of prosthesis attachment (64%). Necrosis of the LPI can occur after malcrimped stapes prosthesis or due to compromised blood supply of the incus. Gerlinger et al. (17) observed the bony foramina of the feeding blood vessels of the LPI of 100 incudes and found up to 4 nutritive foramina on the LPI. The foramina were located anteromedially, mostly on the middle and cranial third of the LPI. The anteromedial Otology & Neurotology, Vol. 34, No. 9, 2013
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surface of the LPI is always covered with the stapes prosthesis after crimping because the loop of all stapes prostheses is longer than the circumference of the thickest LPI. To prevent incus necrosis because of compression of the feeding blood vessels, the loop of stapes prosthesis should be shorter, covering only three-fourths of the circumference. In our anatomic collection, the average convex perimeter was 2.47 mm at the level of 1.0 and 2.68 mm at the level of 1.5 mm far from the tip of the LPI. An ideal prosthesis loop for stapedotomy should have a length of approximately 1.9 mm, keeping the anteromedial quarter of the LPI uncovered. Another possibility to prevent compression of the feeding blood vessels is to develop a new instrument, allowing best possible crimping of a shorter prosthesis and keeping the anteromedial quarter of the LPI free.
CONCLUSION The purpose of this study is to provide new anatomic information on the long process of incus with special regard to the area of crimping in case of stapes prosthesis. We investigated 50 macerated incudes, measured the variability of diameters and shape, and established an additional parameter for the description of the vertical surface of the crimping area, the angle (>). With the help of those parameters taken together, we could describe a possible explanation for generally good hearing results, independently from the choice of the prosthesis. Furthermore, we could show limitations for the indication of specific prosthesis because of a great variation of diameters, not mentioned in literature before. Finally, we want to emphasize on the anteromedial surface of the LPI, being relevant for the blood supply of the bone. We suggest to keep this area uncovered by the prosthesis loop after crimping, to prevent possible incus necrosis.
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