Original Research—Otology and Neurotology
Feasibility of the Ultrasonic Bone Aspirator in Retrosigmoid Vestibular Schwannoma Removal
Otolaryngology– Head and Neck Surgery 2015, Vol. 153(3) 427–432 Ó American Academy of Otolaryngology—Head and Neck Surgery Foundation 2015 Reprints and permission: sagepub.com/journalsPermissions.nav DOI: 10.1177/0194599815587485 http://otojournal.org
Justin S. Golub, MD1, Jon D. Weber, MD1, James L. Leach, MD2, Natalie R. Pottschmidt1, Mario Zuccarello, MD3, Myles L. Pensak, MD1, and Ravi N. Samy, MD1
Sponsorships or competing interests that may be relevant to content are disclosed at the end of this article.
Abstract Objective: Postoperative headache is an undesirable consequence of retrosigmoid vestibular schwannoma (VS) removal. An ultrasonic bone aspirator (UBA) may reduce headache by minimizing subarachnoid bone dust dispersion. The feasibility of removing internal auditory canal (IAC) bone with a UBA is unknown. This study assessed volume and duration of IAC bone removal in clinical and laboratory settings. Study Design: (1) Retrospective review of radiologic data and intraoperative videos. (2) Cadaveric temporal bone model. Setting: (1) Tertiary care medical center. (2) Laboratory. Subjects Methods: We calculated the volume of IAC bone drilled during retrosigmoid VS removal using postoperative computed tomography scans. We then measured the time spent actively drilling IAC bone by analyzing operative videos. Finally, we measured bone ablation rates in a cadaveric temporal bone model using a drill and UBA. Results: The mean 6 SD volume of IAC bone removed during surgery was 0.32 6 0.17 mL (n = 9). The time spent actively removing IAC bone with a drill was only 10.4 6 3.5 minutes, less than a third of the total IAC opening time of 34.2 6 13.1 minutes (n = 5). On cadaveric specimens, the UBA removed bone at 0.21 6 0.03 or 0.35 6 0.07 mL/s at 15% or 50% power, respectively (n = 4). This extrapolates to 15.0 6 3.0 to 25.0 6 3.9 minutes to remove the same 0.32 mL from surgery. Conclusions: The volume and duration of IAC bone removal during retrosigmoid VS surgery are small. Using a UBA at low power instead of a drill would extend the length of surgery by 5 to 15 minutes, with the theoretical potential for reducing headache.
Keywords ultrasonic bone aspirator, Sonopet, bone dust, headache, retrosigmoid, vestibular schwannoma, acoustic neuroma
Received February 2, 2015; revised April 13, 2015; accepted April 28, 2015.
H
eadache is one of the most common complications of retrosigmoid vestibular schwannoma (VS) removal, affecting 17% to 80% of patients.1-3 Several factors may contribute to headache, including the size and location of the craniotomy, reconstruction of the defect, cerebellar retraction, and interruption of suboccipital musculature.3,4 Bone dust contamination of the subarachnoid space during internal auditory canal (IAC) drilling is one of the leading etiologic theories. Subarachnoid bone dust may then lead to a chemical meningitis and subsequent headache.2 We recently demonstrated in a cadaveric laboratory model that an ultrasonic bone aspirator (UBA) disperses up to 25 times less bone dust than that of a standard otologic drill.4 This is likely due to the close proximity between the integrated suction port and the ablation surface (approximately 1 mm, depending on the tip). Use of a UBA in lieu of a standard otologic drill for removal of IAC bone may therefore lower the incidence of postoperative headache through reduced bone dust dispersion. Before the UBA can be widely implemented in the operating room for this purpose, its feasibility needs to be assessed. The UBA is designed for fine, highly controlled bone ablation. Previously published uses have included removal of the round window overhang,5 stapedotomy,6 and middle cranial fossa facial nerve decompression.7 A chief 1
Department of Otolaryngology–Head and Neck Surgery, University of Cincinnati College of Medicine, and the Neurosensory Disorders Center at UC Neuroscience Institute, Cincinnati, Ohio 2 Department of Radiology, Cincinnati Children’s Hospital Medical Center, Cincinnati, Ohio 3 Department of Neurosurgery, University of Cincinnati College of Medicine, and Mayfield Clinic, Cincinnati, Ohio Corresponding Author: Justin S. Golub, MD, Neurotology Fellow, Department of Otolaryngology– Head and Neck Surgery, University of Cincinnati, Cincinnati Children’s Hospital Medical Center, PO Box 670528, 231 Albert Sabin Way, Cincinnati OH 45267-0528, USA. Email: [email protected]
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Figure 1. Post-retrosigmoid computed tomography. (A) Reconstruction of bone removed during internal auditory canal drilling (yellow); native temporal bone is blue. (B) Axial, (C) sagittal, (D) coronal views; yellow highlighting represents the estimated region of bone removed.
concern, therefore, is that IAC bone removal may be significantly slower than that with an otologic drill. In complex multidisciplinary skull base tumor removal cases, minimizing the approach time is important. In this report, we measure the volume and timing of bone removal during IAC drilling in patients who underwent retrosigmoid VS removal. We then directly compare bone ablation time in a cadaveric model between the UBA and a standard otologic drill. Ablation rates are extrapolated to predict how long IAC bone removal using the UBA would take in the operating room.
Operative IAC Bone Removal Technique and Devices
Methods Patient Selection and Tumor Characterization
Calculation of Operative IAC Bone Removal Volume
Ten consecutive patients undergoing retrosigmoid VS resection from December 2013 to January 2015 were retrospectively included in the study. Inclusion criteria for the IAC bone removal volume experiment were (1) availability of preoperative magnetic resonance imaging and (2) postoperative computed tomography (CT; slice thickness 1 mm). Exclusion criterion was revision VS surgery. Of the 10 retrosigmoid surgeries, 5 were video recorded for educational purposes and included in the IAC bone removal timing experiment. Preoperative tumor measurements were made with gadolinium-enhanced T1 images (slice thickness, 3 mm; McKesson Radiology Station, version 12; McKesson, San Francisco, California).
The IAC bone was removed in 10 patients who underwent the retrosigmoid approach for VS removal. Bone removal was performed by 1 of 3 neurotologists or by a neurotology fellow under direct supervision. Bone was removed beginning with a 4-mm diamond bur on a standard pneumatic otologic drill (Midas Rex Legend MR7; Medtronic, Minneapolis, Minnesota) or an electric otologic drill (S2; Stryker, Kalamazoo, Michigan). Successively smaller diamond burs were used as appropriate. Maximum drill speed was approximately 75,000 revolutions per minute (RPM).
The volume of IAC bone removed was measured in 9 patients (1 of the original 10 patients did not have a postoperative CT scan available). On postoperative day 1, routine CT studies of the head were obtained per the usual clinical protocol at our institution. These studies were retroactively reformatted into contiguous slice (0.625 to 1 mm) temporal bone studies. Morphometric analysis was then performed in Brainlab iPlan version 3.0 (Brainlab AG, Feldkirchen, Germany). Resection boundaries were traced in the axial, coronal, and sagittal planes by a board-certified neuroradiologist (J.L.L.) in concert with a neurotology fellow (J.S.G.; Figure 1). Because CT scanning is not routinely performed before retrosigmoid VS removal, preoperative CT scans were unavailable for tracing the resection boundary. Therefore, we used the contralateral nonoperated side and the nearly flat plane of the ipsilateral petrous bone surface
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429 on bone and the percentage of bone-drilling time spent with the drill on bone.
Comparison of Bone Removal Rates between Drill and UBA
Figure 2. Timing variables during internal auditory canal (IAC) opening.
to approximate the extent of bone removal in the postoperative scan. Three-dimensional volume-rendered images of the temporal bone surface and the resection region were reviewed in each case (Figure 1A) to ensure correct boundaries compared with operative findings. Interpolation was performed in iPlan, and estimated volumes of resection were computed. The results among the subjects were averaged.
Measurement of Operative IAC Bone Removal Timing To obtain an approximate baseline standard of care, we measured the amount of time that it took to remove IAC bone with a standard otologic drill in 5 of the 10 patients who underwent retrosigmoid VS removal. Video output from the operative microscope was recorded. Timing measurements were made by viewing the recorded video in a VLC Media Player (VideoLAN, Paris, France). Three time measurements were then calculated (Figure 2): Total continuous duration of IAC opening: Timing began with application of bipolar cautery to the dura overlying the IAC bone (before drilling) and ended immediately after opening of the dura overlying the IAC contents (just prior to IAC tumor removal). Total continuous duration of IAC bone drilling: Timing began with the first application of the drill to the IAC bone and ended immediately after all IAC drilling had been performed. Total cumulative duration with drill on bone (active IAC bone removal): This cumulative time of active bone removal was defined as the bur contacting the bone of the IAC and actively rotating; the total cumulative time of active bone removal was calculated. On the basis of these measurements, we also calculated the percentage of the IAC opening time spent with the drill
Blocks of cadaveric squamous temporal bone (approximately 2 3 3 cm) were weighed dry. A region of 0.5 3 1 3 1 cm within the block was then ablated without interruption via either a UBA or a drill. The duration of bone removal was recorded. The dried bone block was then weighed again. The weight of bone ablated was calculated as the difference between the pre- and postablation weights. The rate of bone ablation was then calculated and expressed in mg/s. Bone removal was performed by technicians trained and supervised by a neurotology fellow experienced with the retrosigmoid approach. The bone ablation technique (ie, pressure applied by the instrument tip, amplitude of strokes) was designed to mimic actual surgical technique. Technicians practiced until their technique was appropriate and timing data were consistent. The drill used was the CORE Saber (Stryker) with a 3-mmdiameter diamond bur. The UBA used was the Sonopet (Stryker) with a Spetzler Long Micro Claw Tip, which contains an ablation surface of 1.25 3 2 mm. UBA power settings were varied. The UBA suction was kept constant at 100%, and irrigation ranged from 15 to 40 mL/min. Burs and tips were replaced after several uses to ensure that they were not becoming dull.
Bone Density Calculation Four rectangular blocks of temporal bone squamosa were harvested from cadaveric specimens. The blocks lacked air cells and appeared similar to the density of the petrous bone overlying the IAC. The volume was calculated as length 3 width 3 height. The bones were weighed dry. Density was calculated as mass/volume. Using this density measurement, we converted the volume of bone removed from the patients who underwent retrosigmoid surgery into a mass, which was in turn used to calculate the time to remove this amount of bone given a device’s particular bone ablation rate (mg/s).
Statistics and Regulatory Approval Statistical analysis was performed in SPSS 19.0 (IBM, Armonk, New York). Means between groups were compared with an independent-samples t test or analysis of variance, as appropriate. Significance was defined as P \ .05. Values are presented mean 6 standard deviation. The University of Cincinnati Institutional Review Board approved the retrospective use of human subjects data for this study.
Results Tumor Characteristics and Operative IAC Bone Removal Volume In the 10 patients with VS, the mean maximum tumor dimension was 19.8 6 5.8 mm. The mean dimension of the
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Table 1. Patients, Tumor Characteristics, and IAC Bone Removal Volume. Tumor Extent, mma
Tumor Subject 1 2 3 4 5 6 7 8 9 10 Mean 6 SD
Age, y 49 71 58 57 42 60 59 61 76 65 61 6 10
Size, mmb
Sex Side M M F F M F M M M F
L R R R R R L L R R
16 16 29 14 14 15 12 24 26 23
3 12 3 10 3 10 3 9 3 19 3 22 3938 3 23 3 17 3 12 3 11 3 10 3 11 3 14 3 13 3 19 3 16 3 16 3 15
Maximum Dimension, mm
IAC
CPA
IAC Bone Removal Volume, mL
16 16 29 14 23 15 12 24 26 23 19.8 6 5.8
4.9 9.0 12.7 7.8 3.1 5.9 6.9 12.5 11.0 12.5 8.6 6 3.5
10.8 6.6 16.7 5.8 12.8 9.1 5.6 11.5 15.0 9.6 10.4 6 3.8
N/A 0.33 0.36 0.18 0.03 0.25 0.30 0.38 0.45 0.61 0.32 6 0.17
Abbreviations: CPA, cerebellopontine angle; F, female; IAC, internal auditory canal; L, left; M, male; N/A, not available; R, right. a IAC and CPA extent refer to lateral-medial dimensions. b Length 3 width 3 height: Tumor length refers to largest lateral-medial dimension; width refers to largest anterior-posterior dimension; and height refers to largest superior-inferior dimension.
Table 2. Time Analysis of IAC Opening and Drilling Based on Surgical Videos. Total Continuous Duration, min
Time with Drill on Bone, %
Subject
IAC Opening
Bone Drilling
Total Cumulative Duration with Drill on Bone,a min
IAC Opening
Bone Drilling
1 2 8 9 10 Mean 6 SD
21.4 28.7 35.2 29.7 55.8 34.2 6 13.1
14.4 20.9 34.0 25.1 34.5 25.8 6 8.6
4.8 9.9 14.0 10.5 12.8 10.4 6 3.5
23 35 40 35 23 31 6 8
33 47 41 42 37 40 6 5
Abbreviation: IAC, internal auditory canal. a Active IAC bone removal.
axial IAC and CPA components of the tumor were 8.6 6 3.5 and 10.4 6 3.8 mm, respectively (Table 1). The volume of ablated IAC bone was measured using postoperative CT after 9 retrosigmoid VS resections (Figure 1). (Of note, subject 1 did not have a fine-cut postoperative CT scan and thus was excluded from the bone volumetric analysis.) The mean bone volume removed was 0.32 6 0.17 mL (Table 1). The density of temporal bone was calculated at 1.83 6 0.14 g/mL. Based on the formula ‘‘density = mass/volume,’’ the mean mass of IAC bone removed was 586 6 304 mg.
before IAC tumor removal began) was 34.2 6 13.1 minutes. The mean total continuous duration of IAC bone drilling (from first to last use of the drill) was 25.8 6 8.6 minutes. In contrast, the mean total cumulative duration with drill-on-bone (active IAC bone removal) was only 10.4 6 3.5 minutes.
Operative IAC Bone Removal Timing
Thus, only 31% 6 8% of the total IAC opening effort was spent actively removing IAC bone with a drill. Furthermore, only 40% 6 5% of the IAC drilling effort was spent with the drill actively on bone.
The components of IAC opening (Figure 2) were temporally analyzed in detail for 5 of the original 10 patients (Table 2):
Comparison of Bone Removal Rates Between Drill and UBA
The mean total continuous duration of IAC opening (from bipolaring petrous ridge dura to the moment
Blocks of cadaveric temporal bone were ablated with either a standard otologic drill with a 3-mm diamond bur or a UBA with a Micro Claw Tip (with an ablation surface of
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Table 3. Calculated Time to Remove IAC Bone Using Various Instruments and Settings.a Bone Ablation Rate Location
Device
OR (n = 5) Lab (n = 4) Lab (n = 3) Lab (n = 4) Lab (n = 4)
Drill Drill UBA UBA UBA
Tip
Power (%) or Speed
mg/s
4, 3, 2 diamonds 75,000 RPM 0.94 3 diamond 60,000 RPM 2.35 6 0.53 Long Micro Claw 100% 1.26 6 0.21 Long Micro Claw 50% 0.65 6 0.13 Long Micro Claw 15% 0.39 6 0.06
mL/s 0.51 1.28 6 0.29 0.69 6 0.11 0.35 6 0.07 0.21 6 0.03
Time to Remove IAC Bone, min Calculated Device Estimated Time Measured on Bonec Difference, mind Drill on Boneb 10.4 6 3.5 4.2 6 0.9 7.8 6 1.3 15.0 6 3.0 25.0 6 3.9
0 –6.2 –2.6 14.6 114.6
Abbreviations: Lab, laboratory; OR, operating room; RPM, revolutions per minute; UBA, ultrasonic bone aspirator. a Values are presented as mean 6 SD. b Includes cumulative duration of time that the drill tip is on the bone (drill-on-bone time). c Based on post-retrosigmoid computed tomography scans (bone removed, 0.32 mL; n = 9) and calculated density of temporal bone (1.83 g/mL). d From current operative protocol. Negative numbers indicate net faster times, positive numbers indicate net slower times.
1.25 3 2 mm). The drill at 60,000 RPM was the fastest condition, ablating bone at 2.35 6 0.53 mg/s (95% confidence interval, 1.83 to 2.88). The UBA with Micro Claw Tip at 100% power ablated bone at 1.26 6 0.21 mg/s (95% confidence interval, 1.01 to 1.50; P \ .05). This was 54% of the ablation rate of the drill. At 15% power, the UBA ablated bone at 0.39 6 0.06 mg/s (95% confidence interval, 0.33 to 0.46). This was 17% the ablation rate of the full-speed drill (Table 3). The known mean volume (0.32 mL) and, in turn, mass (586 mg) of IAC bone ablated after retrosigmoid surgery was used to estimate how long bone ablation would take using the drill or UBA at various settings. With the drill at 60,000 RPM and by employing a 3-mm diamond bur the entire time, IAC bone could be removed in only 4.2 6 0.9 minutes. (This simplified model does not does not take into account using different-sized burs or slowing the RPM, nor does it factor interruptions in drilling.) In comparison, with the UBA at 100% power with a Micro Claw Tip (1.25 3 2 mm), the IAC bone could be removed in 7.8 6 1.3 minutes. Reducing the UBA to 50% power would result in IAC bone removal in 15.0 6 3.0 minutes. Reducing the UBA further to 15% power would result in IAC bone removal in 25.0 6 3.9 minutes (Table 3).
Discussion Our data demonstrated that the volume of IAC bone removed with a drill after retrosigmoid VS resection is extremely small, at less than a third of a milliliter. Accordingly, the amount of time spent actively removing IAC bone (ie, drill-on-bone time) is small at 10.4 minutes. This occupied less than one-third of the entire IAC opening effort. Furthermore, less than one-half of time between drill appearance and drill cessation was spent with the drill actively removing bone. Bone ablation rate of the chosen IAC bone removal tool is therefore responsible for a small fraction of the overall retrosigmoid surgery duration. Surgeon efficiency, such as keeping the ablation device on the bone, may represent an
even more important factor than what particular device is used for IAC ablation. In an earlier study by our group,4 we illustrated in a cadaveric temporal bone model that the dissemination of bone dust was greatly reduced when using the UBA compared to a standard otologic drill with external suctionirrigator. Because bone dust spread into the subarachnoid space is a leading etiologic theory to explain post-retrosigmoid headache, this finding has potential clinical relevance. For example, the UBA at 15% power resulted in 25 times less bone dust spread compared to that of a standard otologic drill and adjacent suction-irrigator. Even at a moderate power setting of 50%, the UBA dispersed .6 times less bone dust.4 These differences are due to a suction port built into the tip of the UBA, located only 1 mm away from the ablation surface. While reducing UBA power decreased bone dust dispersion, it also decreased the ablation rate. For example, at the highest UBA power setting (100% power with Micro Claw Tip), the bone ablation rate was approximately half that of the otologic drill (at 60,000 RPM with 3-mm diamond bur). At a low UBA power setting (15% power), the bone ablation rate was only 17% that of the drill. Despite these seemingly large differences, in the real-life operative scenario, we believe that using the UBA at a lower power setting would not meaningfully lengthen the operative time. First, the 3-mm diamond on an otologic drill is not typically used for the entire duration of IAC bone removal. Rather, 4 mm may be used initially, followed by smaller (and slower) burs as the IAC dura is approached. In contrast, the Micro Claw Tip for the UBA is a more delicate instrument, measuring only 1.25 3 2 mm. Thus, it could be used for the entire duration of bone removal. Second, the actual amount of time spent actively ablating bone is actually a minority of the IAC opening procedure. Based on our analysis of surgical videos, only 10 minutes were spent with the drill actively removing bone. Considering the ablation rate of the UBA at 50% power, it would take 15 minutes to remove IAC bone. This is a difference of only 5 additional
432 minutes when compared with that of the drill. At the very low power setting of 15%, the UBA would extend surgery 15 minutes. These figures would vary depending on how much the tumor widened the IAC. We studied the UBA Long Micro Claw Tip because its length and size are practical for lateral skull base work. The Micro Claw Tip, however, is smaller than a 2-mm diamond bur. Thus, if we take into account its small size, the UBA ablation rate is actually fairly rapid. This suggests that the UBA technology may be capable of much higher ablation rates. We are currently investigating alternative tips and measuring their ablation rates and bone dissemination properties. Our study had several limitations. The sample sizes for the IAC bone removal volume and timing measurements in the retrospective study were relatively small. Thus, it is possible that these values may under- or overestimate the IAC bone removal effort. Another limitation is that preoperative CT scans were not available to precisely identify the volume of bone removed. Instead, the contralateral undrilled temporal bone was used as a proxy. Because the anatomy, including aeration pattern, of the temporal bone is not always symmetric, this may have reduced accuracy. The technicians removing bone during our laboratory experiments were not blinded to the hypothesis. Although the bone ablation technique attempted to closely resemble that in the operating room, a possible bias may have minimized the differences in ablation rates between the drill and the UBA. A final limitation is that laboratory bone removal was performed on temporal bone blocks rather than whole heads. While this was necessary to design a system to accurately capture bone dust and measure ablation rates, it does reduce realism. In conclusion, the volume and duration of active IAC bone removal during retrosigmoid VS surgery are relatively small. While the UBA with Micro Claw Tip has a lower ablation rate than a 3-mm diamond drill, its use would only marginally extend the length of surgery. The UBA is a feasible alternative to the drill for IAC bone removal, for which its previously demonstrated ability to greatly reduce bone dust spread also carries a theoretical possibility of reducing post-retrosigmoid headache. We hope that this and other preliminary studies pave the way for prospective clinical trials comparing the UBA to the otologic drill for retrosigmoid VS removal. Acknowledgments The authors thank Alison A. Weiss, PhD, Jaye A. Ward, Daniel I. Choo, MD, and Karen Gallegos Villalobos, PhD, for their generosity in sharing laboratory equipment, as well as Mary Kemper for manuscript review and Jessica Keller for Institutional Review Board assistance.
Otolaryngology–Head and Neck Surgery 153(3) Author Contributions Justin S. Golub, study conception, data analysis, drafting, final approval, accountability for all aspects of work; Jon D. Weber, methods, data analysis, drafting, final approval, accountability for all aspects of work; James L. Leach, data analysis, radiologic data analysis, drafting, final approval, accountability for all aspects of work; Natalie R. Pottschmidt, methods, data analysis, drafting, final approval, accountability for all aspects of work; Mario Zuccarello, data analysis, drafting, final approval, accountability for all aspects of work; Myles L. Pensak, data analysis, drafting, final approval, accountability for all aspects of work; Ravi N. Samy, data analysis, drafting, final approval, accountability for all aspects of work.
Disclosures Competing interests: Justin S. Golub, Stryker: funded an educational/mission trip unrelated to this project; Plural Publishing: book royalties, free books for consulting; Cochlear: educational trip; Ravi N. Samy, MedEl: surgical advisory board, research; Cochlear Americas: surgical advisory board; Stryker: speaker, funded an educational/mission trip unrelated to this project. Sponsorships: Stryker: lent the ultrasonic bone aspirator used for experiments but was not involved in study design or manuscript review. Funding source: None.
References 1. Ansari SF, Terry C, Cohen-Gadol AA. Surgery for vestibular schwannomas: a systematic review of complications by approach. Neurosurg Focus. 2012;33:E14. 2. Catalano PJ, Jacobowitz O, Post KD. Prevention of headache after retrosigmoid removal of acoustic tumors. Am J Otol. 1996; 17:904-908. 3. Teo MK, Eljamel MS. Role of craniotomy repair in reducing postoperative headaches after a retrosigmoid approach. Neurosurgery. 2010;67:1286-1292. 4. Weber JD, Samy RN, Nahata A, Zuccarello M, Pensak ML, Golub JS. Reduction of bone dust with ultrasonic bone aspiration: implications for retrosigmoid vestibular schwannoma removal [published online March 9, 2015]. Otolaryngol Head Neck Surg. 5. Cuda D, Murri A, Tinelli N. Piezoelectric round window osteoplasty for Vibrant Soundbridge implant. Otol Neurotol. 2009; 30:782-786. 6. Salami A, Vercellotti T, Mora R, Dellepiane M. Piezoelectric bone surgery in otologic surgery. Otolaryngol Head Neck Surg. 2007;136:484-485. 7. Samy RN, Krishnamoorthy K, Pensak ML. Use of a novel ultrasonic surgical system for decompression of the facial nerve. Laryngoscope. 2007;117:872-875.
Feasibility of the Ultrasonic Bone Aspirator in Retrosigmoid Vestibular Schwannoma Removal
Otolaryngology– Head and Neck Surgery 2015, Vol. 153(3) 427–432 Ó American Academy of Otolaryngology—Head and Neck Surgery Foundation 2015 Reprints and permission: sagepub.com/journalsPermissions.nav DOI: 10.1177/0194599815587485 http://otojournal.org
Justin S. Golub, MD1, Jon D. Weber, MD1, James L. Leach, MD2, Natalie R. Pottschmidt1, Mario Zuccarello, MD3, Myles L. Pensak, MD1, and Ravi N. Samy, MD1
Sponsorships or competing interests that may be relevant to content are disclosed at the end of this article.
Abstract Objective: Postoperative headache is an undesirable consequence of retrosigmoid vestibular schwannoma (VS) removal. An ultrasonic bone aspirator (UBA) may reduce headache by minimizing subarachnoid bone dust dispersion. The feasibility of removing internal auditory canal (IAC) bone with a UBA is unknown. This study assessed volume and duration of IAC bone removal in clinical and laboratory settings. Study Design: (1) Retrospective review of radiologic data and intraoperative videos. (2) Cadaveric temporal bone model. Setting: (1) Tertiary care medical center. (2) Laboratory. Subjects Methods: We calculated the volume of IAC bone drilled during retrosigmoid VS removal using postoperative computed tomography scans. We then measured the time spent actively drilling IAC bone by analyzing operative videos. Finally, we measured bone ablation rates in a cadaveric temporal bone model using a drill and UBA. Results: The mean 6 SD volume of IAC bone removed during surgery was 0.32 6 0.17 mL (n = 9). The time spent actively removing IAC bone with a drill was only 10.4 6 3.5 minutes, less than a third of the total IAC opening time of 34.2 6 13.1 minutes (n = 5). On cadaveric specimens, the UBA removed bone at 0.21 6 0.03 or 0.35 6 0.07 mL/s at 15% or 50% power, respectively (n = 4). This extrapolates to 15.0 6 3.0 to 25.0 6 3.9 minutes to remove the same 0.32 mL from surgery. Conclusions: The volume and duration of IAC bone removal during retrosigmoid VS surgery are small. Using a UBA at low power instead of a drill would extend the length of surgery by 5 to 15 minutes, with the theoretical potential for reducing headache.
Keywords ultrasonic bone aspirator, Sonopet, bone dust, headache, retrosigmoid, vestibular schwannoma, acoustic neuroma
Received February 2, 2015; revised April 13, 2015; accepted April 28, 2015.
H
eadache is one of the most common complications of retrosigmoid vestibular schwannoma (VS) removal, affecting 17% to 80% of patients.1-3 Several factors may contribute to headache, including the size and location of the craniotomy, reconstruction of the defect, cerebellar retraction, and interruption of suboccipital musculature.3,4 Bone dust contamination of the subarachnoid space during internal auditory canal (IAC) drilling is one of the leading etiologic theories. Subarachnoid bone dust may then lead to a chemical meningitis and subsequent headache.2 We recently demonstrated in a cadaveric laboratory model that an ultrasonic bone aspirator (UBA) disperses up to 25 times less bone dust than that of a standard otologic drill.4 This is likely due to the close proximity between the integrated suction port and the ablation surface (approximately 1 mm, depending on the tip). Use of a UBA in lieu of a standard otologic drill for removal of IAC bone may therefore lower the incidence of postoperative headache through reduced bone dust dispersion. Before the UBA can be widely implemented in the operating room for this purpose, its feasibility needs to be assessed. The UBA is designed for fine, highly controlled bone ablation. Previously published uses have included removal of the round window overhang,5 stapedotomy,6 and middle cranial fossa facial nerve decompression.7 A chief 1
Department of Otolaryngology–Head and Neck Surgery, University of Cincinnati College of Medicine, and the Neurosensory Disorders Center at UC Neuroscience Institute, Cincinnati, Ohio 2 Department of Radiology, Cincinnati Children’s Hospital Medical Center, Cincinnati, Ohio 3 Department of Neurosurgery, University of Cincinnati College of Medicine, and Mayfield Clinic, Cincinnati, Ohio Corresponding Author: Justin S. Golub, MD, Neurotology Fellow, Department of Otolaryngology– Head and Neck Surgery, University of Cincinnati, Cincinnati Children’s Hospital Medical Center, PO Box 670528, 231 Albert Sabin Way, Cincinnati OH 45267-0528, USA. Email: [email protected]
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Figure 1. Post-retrosigmoid computed tomography. (A) Reconstruction of bone removed during internal auditory canal drilling (yellow); native temporal bone is blue. (B) Axial, (C) sagittal, (D) coronal views; yellow highlighting represents the estimated region of bone removed.
concern, therefore, is that IAC bone removal may be significantly slower than that with an otologic drill. In complex multidisciplinary skull base tumor removal cases, minimizing the approach time is important. In this report, we measure the volume and timing of bone removal during IAC drilling in patients who underwent retrosigmoid VS removal. We then directly compare bone ablation time in a cadaveric model between the UBA and a standard otologic drill. Ablation rates are extrapolated to predict how long IAC bone removal using the UBA would take in the operating room.
Operative IAC Bone Removal Technique and Devices
Methods Patient Selection and Tumor Characterization
Calculation of Operative IAC Bone Removal Volume
Ten consecutive patients undergoing retrosigmoid VS resection from December 2013 to January 2015 were retrospectively included in the study. Inclusion criteria for the IAC bone removal volume experiment were (1) availability of preoperative magnetic resonance imaging and (2) postoperative computed tomography (CT; slice thickness 1 mm). Exclusion criterion was revision VS surgery. Of the 10 retrosigmoid surgeries, 5 were video recorded for educational purposes and included in the IAC bone removal timing experiment. Preoperative tumor measurements were made with gadolinium-enhanced T1 images (slice thickness, 3 mm; McKesson Radiology Station, version 12; McKesson, San Francisco, California).
The IAC bone was removed in 10 patients who underwent the retrosigmoid approach for VS removal. Bone removal was performed by 1 of 3 neurotologists or by a neurotology fellow under direct supervision. Bone was removed beginning with a 4-mm diamond bur on a standard pneumatic otologic drill (Midas Rex Legend MR7; Medtronic, Minneapolis, Minnesota) or an electric otologic drill (S2; Stryker, Kalamazoo, Michigan). Successively smaller diamond burs were used as appropriate. Maximum drill speed was approximately 75,000 revolutions per minute (RPM).
The volume of IAC bone removed was measured in 9 patients (1 of the original 10 patients did not have a postoperative CT scan available). On postoperative day 1, routine CT studies of the head were obtained per the usual clinical protocol at our institution. These studies were retroactively reformatted into contiguous slice (0.625 to 1 mm) temporal bone studies. Morphometric analysis was then performed in Brainlab iPlan version 3.0 (Brainlab AG, Feldkirchen, Germany). Resection boundaries were traced in the axial, coronal, and sagittal planes by a board-certified neuroradiologist (J.L.L.) in concert with a neurotology fellow (J.S.G.; Figure 1). Because CT scanning is not routinely performed before retrosigmoid VS removal, preoperative CT scans were unavailable for tracing the resection boundary. Therefore, we used the contralateral nonoperated side and the nearly flat plane of the ipsilateral petrous bone surface
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429 on bone and the percentage of bone-drilling time spent with the drill on bone.
Comparison of Bone Removal Rates between Drill and UBA
Figure 2. Timing variables during internal auditory canal (IAC) opening.
to approximate the extent of bone removal in the postoperative scan. Three-dimensional volume-rendered images of the temporal bone surface and the resection region were reviewed in each case (Figure 1A) to ensure correct boundaries compared with operative findings. Interpolation was performed in iPlan, and estimated volumes of resection were computed. The results among the subjects were averaged.
Measurement of Operative IAC Bone Removal Timing To obtain an approximate baseline standard of care, we measured the amount of time that it took to remove IAC bone with a standard otologic drill in 5 of the 10 patients who underwent retrosigmoid VS removal. Video output from the operative microscope was recorded. Timing measurements were made by viewing the recorded video in a VLC Media Player (VideoLAN, Paris, France). Three time measurements were then calculated (Figure 2): Total continuous duration of IAC opening: Timing began with application of bipolar cautery to the dura overlying the IAC bone (before drilling) and ended immediately after opening of the dura overlying the IAC contents (just prior to IAC tumor removal). Total continuous duration of IAC bone drilling: Timing began with the first application of the drill to the IAC bone and ended immediately after all IAC drilling had been performed. Total cumulative duration with drill on bone (active IAC bone removal): This cumulative time of active bone removal was defined as the bur contacting the bone of the IAC and actively rotating; the total cumulative time of active bone removal was calculated. On the basis of these measurements, we also calculated the percentage of the IAC opening time spent with the drill
Blocks of cadaveric squamous temporal bone (approximately 2 3 3 cm) were weighed dry. A region of 0.5 3 1 3 1 cm within the block was then ablated without interruption via either a UBA or a drill. The duration of bone removal was recorded. The dried bone block was then weighed again. The weight of bone ablated was calculated as the difference between the pre- and postablation weights. The rate of bone ablation was then calculated and expressed in mg/s. Bone removal was performed by technicians trained and supervised by a neurotology fellow experienced with the retrosigmoid approach. The bone ablation technique (ie, pressure applied by the instrument tip, amplitude of strokes) was designed to mimic actual surgical technique. Technicians practiced until their technique was appropriate and timing data were consistent. The drill used was the CORE Saber (Stryker) with a 3-mmdiameter diamond bur. The UBA used was the Sonopet (Stryker) with a Spetzler Long Micro Claw Tip, which contains an ablation surface of 1.25 3 2 mm. UBA power settings were varied. The UBA suction was kept constant at 100%, and irrigation ranged from 15 to 40 mL/min. Burs and tips were replaced after several uses to ensure that they were not becoming dull.
Bone Density Calculation Four rectangular blocks of temporal bone squamosa were harvested from cadaveric specimens. The blocks lacked air cells and appeared similar to the density of the petrous bone overlying the IAC. The volume was calculated as length 3 width 3 height. The bones were weighed dry. Density was calculated as mass/volume. Using this density measurement, we converted the volume of bone removed from the patients who underwent retrosigmoid surgery into a mass, which was in turn used to calculate the time to remove this amount of bone given a device’s particular bone ablation rate (mg/s).
Statistics and Regulatory Approval Statistical analysis was performed in SPSS 19.0 (IBM, Armonk, New York). Means between groups were compared with an independent-samples t test or analysis of variance, as appropriate. Significance was defined as P \ .05. Values are presented mean 6 standard deviation. The University of Cincinnati Institutional Review Board approved the retrospective use of human subjects data for this study.
Results Tumor Characteristics and Operative IAC Bone Removal Volume In the 10 patients with VS, the mean maximum tumor dimension was 19.8 6 5.8 mm. The mean dimension of the
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Otolaryngology–Head and Neck Surgery 153(3)
Table 1. Patients, Tumor Characteristics, and IAC Bone Removal Volume. Tumor Extent, mma
Tumor Subject 1 2 3 4 5 6 7 8 9 10 Mean 6 SD
Age, y 49 71 58 57 42 60 59 61 76 65 61 6 10
Size, mmb
Sex Side M M F F M F M M M F
L R R R R R L L R R
16 16 29 14 14 15 12 24 26 23
3 12 3 10 3 10 3 9 3 19 3 22 3938 3 23 3 17 3 12 3 11 3 10 3 11 3 14 3 13 3 19 3 16 3 16 3 15
Maximum Dimension, mm
IAC
CPA
IAC Bone Removal Volume, mL
16 16 29 14 23 15 12 24 26 23 19.8 6 5.8
4.9 9.0 12.7 7.8 3.1 5.9 6.9 12.5 11.0 12.5 8.6 6 3.5
10.8 6.6 16.7 5.8 12.8 9.1 5.6 11.5 15.0 9.6 10.4 6 3.8
N/A 0.33 0.36 0.18 0.03 0.25 0.30 0.38 0.45 0.61 0.32 6 0.17
Abbreviations: CPA, cerebellopontine angle; F, female; IAC, internal auditory canal; L, left; M, male; N/A, not available; R, right. a IAC and CPA extent refer to lateral-medial dimensions. b Length 3 width 3 height: Tumor length refers to largest lateral-medial dimension; width refers to largest anterior-posterior dimension; and height refers to largest superior-inferior dimension.
Table 2. Time Analysis of IAC Opening and Drilling Based on Surgical Videos. Total Continuous Duration, min
Time with Drill on Bone, %
Subject
IAC Opening
Bone Drilling
Total Cumulative Duration with Drill on Bone,a min
IAC Opening
Bone Drilling
1 2 8 9 10 Mean 6 SD
21.4 28.7 35.2 29.7 55.8 34.2 6 13.1
14.4 20.9 34.0 25.1 34.5 25.8 6 8.6
4.8 9.9 14.0 10.5 12.8 10.4 6 3.5
23 35 40 35 23 31 6 8
33 47 41 42 37 40 6 5
Abbreviation: IAC, internal auditory canal. a Active IAC bone removal.
axial IAC and CPA components of the tumor were 8.6 6 3.5 and 10.4 6 3.8 mm, respectively (Table 1). The volume of ablated IAC bone was measured using postoperative CT after 9 retrosigmoid VS resections (Figure 1). (Of note, subject 1 did not have a fine-cut postoperative CT scan and thus was excluded from the bone volumetric analysis.) The mean bone volume removed was 0.32 6 0.17 mL (Table 1). The density of temporal bone was calculated at 1.83 6 0.14 g/mL. Based on the formula ‘‘density = mass/volume,’’ the mean mass of IAC bone removed was 586 6 304 mg.
before IAC tumor removal began) was 34.2 6 13.1 minutes. The mean total continuous duration of IAC bone drilling (from first to last use of the drill) was 25.8 6 8.6 minutes. In contrast, the mean total cumulative duration with drill-on-bone (active IAC bone removal) was only 10.4 6 3.5 minutes.
Operative IAC Bone Removal Timing
Thus, only 31% 6 8% of the total IAC opening effort was spent actively removing IAC bone with a drill. Furthermore, only 40% 6 5% of the IAC drilling effort was spent with the drill actively on bone.
The components of IAC opening (Figure 2) were temporally analyzed in detail for 5 of the original 10 patients (Table 2):
Comparison of Bone Removal Rates Between Drill and UBA
The mean total continuous duration of IAC opening (from bipolaring petrous ridge dura to the moment
Blocks of cadaveric temporal bone were ablated with either a standard otologic drill with a 3-mm diamond bur or a UBA with a Micro Claw Tip (with an ablation surface of
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Table 3. Calculated Time to Remove IAC Bone Using Various Instruments and Settings.a Bone Ablation Rate Location
Device
OR (n = 5) Lab (n = 4) Lab (n = 3) Lab (n = 4) Lab (n = 4)
Drill Drill UBA UBA UBA
Tip
Power (%) or Speed
mg/s
4, 3, 2 diamonds 75,000 RPM 0.94 3 diamond 60,000 RPM 2.35 6 0.53 Long Micro Claw 100% 1.26 6 0.21 Long Micro Claw 50% 0.65 6 0.13 Long Micro Claw 15% 0.39 6 0.06
mL/s 0.51 1.28 6 0.29 0.69 6 0.11 0.35 6 0.07 0.21 6 0.03
Time to Remove IAC Bone, min Calculated Device Estimated Time Measured on Bonec Difference, mind Drill on Boneb 10.4 6 3.5 4.2 6 0.9 7.8 6 1.3 15.0 6 3.0 25.0 6 3.9
0 –6.2 –2.6 14.6 114.6
Abbreviations: Lab, laboratory; OR, operating room; RPM, revolutions per minute; UBA, ultrasonic bone aspirator. a Values are presented as mean 6 SD. b Includes cumulative duration of time that the drill tip is on the bone (drill-on-bone time). c Based on post-retrosigmoid computed tomography scans (bone removed, 0.32 mL; n = 9) and calculated density of temporal bone (1.83 g/mL). d From current operative protocol. Negative numbers indicate net faster times, positive numbers indicate net slower times.
1.25 3 2 mm). The drill at 60,000 RPM was the fastest condition, ablating bone at 2.35 6 0.53 mg/s (95% confidence interval, 1.83 to 2.88). The UBA with Micro Claw Tip at 100% power ablated bone at 1.26 6 0.21 mg/s (95% confidence interval, 1.01 to 1.50; P \ .05). This was 54% of the ablation rate of the drill. At 15% power, the UBA ablated bone at 0.39 6 0.06 mg/s (95% confidence interval, 0.33 to 0.46). This was 17% the ablation rate of the full-speed drill (Table 3). The known mean volume (0.32 mL) and, in turn, mass (586 mg) of IAC bone ablated after retrosigmoid surgery was used to estimate how long bone ablation would take using the drill or UBA at various settings. With the drill at 60,000 RPM and by employing a 3-mm diamond bur the entire time, IAC bone could be removed in only 4.2 6 0.9 minutes. (This simplified model does not does not take into account using different-sized burs or slowing the RPM, nor does it factor interruptions in drilling.) In comparison, with the UBA at 100% power with a Micro Claw Tip (1.25 3 2 mm), the IAC bone could be removed in 7.8 6 1.3 minutes. Reducing the UBA to 50% power would result in IAC bone removal in 15.0 6 3.0 minutes. Reducing the UBA further to 15% power would result in IAC bone removal in 25.0 6 3.9 minutes (Table 3).
Discussion Our data demonstrated that the volume of IAC bone removed with a drill after retrosigmoid VS resection is extremely small, at less than a third of a milliliter. Accordingly, the amount of time spent actively removing IAC bone (ie, drill-on-bone time) is small at 10.4 minutes. This occupied less than one-third of the entire IAC opening effort. Furthermore, less than one-half of time between drill appearance and drill cessation was spent with the drill actively removing bone. Bone ablation rate of the chosen IAC bone removal tool is therefore responsible for a small fraction of the overall retrosigmoid surgery duration. Surgeon efficiency, such as keeping the ablation device on the bone, may represent an
even more important factor than what particular device is used for IAC ablation. In an earlier study by our group,4 we illustrated in a cadaveric temporal bone model that the dissemination of bone dust was greatly reduced when using the UBA compared to a standard otologic drill with external suctionirrigator. Because bone dust spread into the subarachnoid space is a leading etiologic theory to explain post-retrosigmoid headache, this finding has potential clinical relevance. For example, the UBA at 15% power resulted in 25 times less bone dust spread compared to that of a standard otologic drill and adjacent suction-irrigator. Even at a moderate power setting of 50%, the UBA dispersed .6 times less bone dust.4 These differences are due to a suction port built into the tip of the UBA, located only 1 mm away from the ablation surface. While reducing UBA power decreased bone dust dispersion, it also decreased the ablation rate. For example, at the highest UBA power setting (100% power with Micro Claw Tip), the bone ablation rate was approximately half that of the otologic drill (at 60,000 RPM with 3-mm diamond bur). At a low UBA power setting (15% power), the bone ablation rate was only 17% that of the drill. Despite these seemingly large differences, in the real-life operative scenario, we believe that using the UBA at a lower power setting would not meaningfully lengthen the operative time. First, the 3-mm diamond on an otologic drill is not typically used for the entire duration of IAC bone removal. Rather, 4 mm may be used initially, followed by smaller (and slower) burs as the IAC dura is approached. In contrast, the Micro Claw Tip for the UBA is a more delicate instrument, measuring only 1.25 3 2 mm. Thus, it could be used for the entire duration of bone removal. Second, the actual amount of time spent actively ablating bone is actually a minority of the IAC opening procedure. Based on our analysis of surgical videos, only 10 minutes were spent with the drill actively removing bone. Considering the ablation rate of the UBA at 50% power, it would take 15 minutes to remove IAC bone. This is a difference of only 5 additional
432 minutes when compared with that of the drill. At the very low power setting of 15%, the UBA would extend surgery 15 minutes. These figures would vary depending on how much the tumor widened the IAC. We studied the UBA Long Micro Claw Tip because its length and size are practical for lateral skull base work. The Micro Claw Tip, however, is smaller than a 2-mm diamond bur. Thus, if we take into account its small size, the UBA ablation rate is actually fairly rapid. This suggests that the UBA technology may be capable of much higher ablation rates. We are currently investigating alternative tips and measuring their ablation rates and bone dissemination properties. Our study had several limitations. The sample sizes for the IAC bone removal volume and timing measurements in the retrospective study were relatively small. Thus, it is possible that these values may under- or overestimate the IAC bone removal effort. Another limitation is that preoperative CT scans were not available to precisely identify the volume of bone removed. Instead, the contralateral undrilled temporal bone was used as a proxy. Because the anatomy, including aeration pattern, of the temporal bone is not always symmetric, this may have reduced accuracy. The technicians removing bone during our laboratory experiments were not blinded to the hypothesis. Although the bone ablation technique attempted to closely resemble that in the operating room, a possible bias may have minimized the differences in ablation rates between the drill and the UBA. A final limitation is that laboratory bone removal was performed on temporal bone blocks rather than whole heads. While this was necessary to design a system to accurately capture bone dust and measure ablation rates, it does reduce realism. In conclusion, the volume and duration of active IAC bone removal during retrosigmoid VS surgery are relatively small. While the UBA with Micro Claw Tip has a lower ablation rate than a 3-mm diamond drill, its use would only marginally extend the length of surgery. The UBA is a feasible alternative to the drill for IAC bone removal, for which its previously demonstrated ability to greatly reduce bone dust spread also carries a theoretical possibility of reducing post-retrosigmoid headache. We hope that this and other preliminary studies pave the way for prospective clinical trials comparing the UBA to the otologic drill for retrosigmoid VS removal. Acknowledgments The authors thank Alison A. Weiss, PhD, Jaye A. Ward, Daniel I. Choo, MD, and Karen Gallegos Villalobos, PhD, for their generosity in sharing laboratory equipment, as well as Mary Kemper for manuscript review and Jessica Keller for Institutional Review Board assistance.
Otolaryngology–Head and Neck Surgery 153(3) Author Contributions Justin S. Golub, study conception, data analysis, drafting, final approval, accountability for all aspects of work; Jon D. Weber, methods, data analysis, drafting, final approval, accountability for all aspects of work; James L. Leach, data analysis, radiologic data analysis, drafting, final approval, accountability for all aspects of work; Natalie R. Pottschmidt, methods, data analysis, drafting, final approval, accountability for all aspects of work; Mario Zuccarello, data analysis, drafting, final approval, accountability for all aspects of work; Myles L. Pensak, data analysis, drafting, final approval, accountability for all aspects of work; Ravi N. Samy, data analysis, drafting, final approval, accountability for all aspects of work.
Disclosures Competing interests: Justin S. Golub, Stryker: funded an educational/mission trip unrelated to this project; Plural Publishing: book royalties, free books for consulting; Cochlear: educational trip; Ravi N. Samy, MedEl: surgical advisory board, research; Cochlear Americas: surgical advisory board; Stryker: speaker, funded an educational/mission trip unrelated to this project. Sponsorships: Stryker: lent the ultrasonic bone aspirator used for experiments but was not involved in study design or manuscript review. Funding source: None.
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