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Patient Safety: Research Report

A Comparison of 4 Airway Devices on Cervical Spine Alignment in Cadaver Models of Global Ligamentous Instability at C1-2

Wendling, Adam L., MD*; Tighe, Patrick J., MD*; Conrad, Bryan P., PhD; Baslanti, Tezcan Ozrazgat, PhD*; Horodyski, MaryBeth, EdD; Rechtine, Glenn R., MD

Author Information
doi: 10.1213/ANE.0b013e318279b37a

Frequently, patients presenting with known or suspected cervical spine disease have other conditions that necessitate rapid advanced airway management to prevent further harm. Neurologic injury may occur in these patients as a consequence of mechanical disruption at the level of instability. However, neurologic injury may also result from a systemic decrease in oxygen delivery as a result of hypoventilation and hypoxemia from a prolonged or failed intubation technique. In patients with upper cervical spine instability requiring intubation, the best technique should be rapid, effective, and be associated with as little mechanical disruption and cardiopulmonary compromise as possible.1–6

The technique most appropriate for achieving successful endotracheal intubation in the patient with upper cervical spine instability is unknown. Techniques that minimize cervical movement, theoretically, offer an advantage.

Previous work on this topic evaluated different techniques of airway management on cervical alignment with a variety of subjects,6 including anesthetized adult patients without cervical spine disease undergoing elective intubation, adult patients presenting for elective surgical management of cervical spine disease, retrospective studies of trauma victims who required intubation, and cadavers with unstable cervical spines.

Our primary hypothesis was that the magnitude of flexion-extension movement during endotracheal intubation in cadavers with a simulated type II odontoid fracture would differ among the airway devices. Cadavers were used similar to other studies.7–21 We created global ligamentous instability at the C1-2 segment (simulated type II odontoid fracture) in fresh cadavers, followed by intubation with 4 airway devices (Airtraq Optical Laryngoscope [AT], King Medical Systems, Newark, DE; Fastrach LMA [intubating laryngeal mask airway {ILMA}], LMA North America, San Diego, CA; Lightwand [LW], Bovie Aaron Medical, St. Petersburg, FL; and Macintosh laryngoscope (MAC), SunMed, Largo, FL) while recording displacement in 3 dimensions with electromagnetic motion analysis. No previous work has evaluated the AT against these other devices with global instability at the C1-2 segment.


In this IRB-exempt, observational cohort trial, we measured the angular movement of 4 different airway devices in cadavers with created global ligamentous instability at the C1-2 segment. Secondary outcomes examined the correlation of movement across all axes, as well as the correlation between the time required for intubation and movement in each separate axis.

Three lightly embalmed cadavers were used. The cadaver subjects underwent the embalming process within 24 hours of death as per institutional requirements for whole body anatomical gifts. This embalming process preserves the tissues without adversely affecting the soft tissue mechanical properties. The stiffness of the joints and tissues of these cadavers was grossly similar to that of fresh cadavers. The light embalming process allowed a longer period of cadaver use before significant tissue degradation occurred and enabled completion of the experimental protocol. The light embalming formula is a mixture of formalin and glycerin, with less formalin than is used in traditional embalming. The cadavers were stored in a cooler to preserve tissue, but testing was done at room temperature. The 3 cadavers were a mean age of 83.3 years (range 79–85 years) and a mean weight of 61.2 kg (range 58.3–63.5 kg). The range of motion before the creation of spinal instability was similar to that reported in normal geriatric adults22–24 (Table 1). An unstable spine was simulated by the surgical creation of a complete segmental injury in the cadavers, resulting in global instability. To standardize the injury, all of the lesions were created at the C1-2 spinal segments by the same surgeon. The lesion involved the creation of a posterior ligamentous resection between C1-2 and a transverse odontoid osteotomy, simulating an unstable type II odontoid fracture. These cadaver subjects were part of a series of investigations of medical interventions on the alignment of the unstable cervical spine.13,14,21

Table 1
Table 1:
Mean and Range of Motion of Cadaver Cervical Spine Before Type II Odontoid Fracture

Two attending anesthesiologists and 3 emergency medical technician paramedics were involved in the investigation. Emergency medical technician paramedics are allowed, under New York state law, to provide advanced airway management, including rapid sequence intubation and cricothyrotomy. The providers were instructed by one of the attending anesthesiologists in the technique to be used during this investigation. The providers were then allowed time to practice with each device until they were comfortable with the technique. Prior experience with each device was not recorded. After this training period, each provider intubated each cadaver 3 times with each intubation device, in random order, following the same technique. All intubations were performed from the head of the supine cadavers with a 7-mm internal diameter endotracheal tube (ETT). All 3 cadavers were not able to be intubated by all 5 practitioners due to unavailability. An electromagnetic motion analysis device (Liberty device; Polhemus Inc., Colchester, VT) was used to assess the amount of angular and linear motion during each maneuver as in previous studies.7–19,21 In short, the Liberty device uses electromagnetic fields to establish the 3-dimensional position and orientation of its sensors. It can detect angular motions with a precision of 0.3°. For our current study, sensors were affixed to C1 and C2 posteriorly with custom-made fiberglass mounting brackets, and positioning data were recorded at a sampling rate of 240 Hz. The relative motion produced between C1 and C2 was measured as each intubation technique was applied to the cadavers.

All data were recorded directly from the Liberty device to a laptop computer. The method of calculating angular motion in each axis is described in our previous publications.7–19,21 Angles were calculated using the Tilt-Twist method described by Crawford et al.25

The cadavers were placed in the supine position. Manual in-line immobilization was maintained for all test trials. The procedure for applying manual in-line immobilization was done by a single physician with >20 years of experience in caring for spine-injured patients. The cadaver’s head was held at the temples in a neutral position throughout the intubation attempt. Neither cricoid nor thyroid pressure was used in this study. A flexible fiberoptic bronchoscope was used to confirm intubation.

Regarding the AT, the technique used for intubating the cadavers was based on instructions provided by the AT website.a An AT Regular (Size 3) device was used.

The ILMA used for all intubations was a Fastrach LMA, Size 4 (LMA North America), and the technique used was blind intubation, as delineated in the manufacturer’s instruction manual.b Once the ETT was deemed to be in the trachea, the ETT connector was removed, the ILMA cuff was deflated, and the ILMA was withdrawn over the ETT.

For all intubations, the LW used was the Surch-Lite 15″ (Aaron Medical), and the technique used with this device was as described by Liem.c

A standard Macintosh size 3 blade was used for all MAC intubations. For all intubations with this device, the blade was held in the operator’s left hand, and was placed in the right corner of the cadaver’s mouth whereas the tongue was swept to the left. The blade was advanced along the floor of the mouth, and was lifted in an anterior-caudad direction to expose the glottis to direct vision. Once the glottis was visualized, the ETT was advanced with the right hand into the glottis under direct visualization. If the glottis was not visualized, the blade was slowly withdrawn. If the glottis was still not visualized, the blade was removed and reintroduced, repeating the technique until the glottis was visualized, and the trachea was intubated. A malleable stylet was placed in the lumen of the ETT to assist with all MAC intubations. The technique was concluded when the stylet was removed from the ETT after intubation.

Each technique was attempted up to 3 times per trial, and each trial was repeated for a total of 3 trials per airway device with each practitioner and cadaver. If the above maneuvers still failed, the overall attempt was considered a failure.

A linear mixed model was first applied to compare movement in each axis during intubation. A review of the residual plots of random effects did not demonstrate gross departures from normality. In this model, the axes of flexion-extension, lateral bending, and axial rotation were compared as direct effects on the dependent variable of movement. The cadaver and airway device were considered random effects, and the identity of the person performing the intubation as the subject random effect. The identity of the intubating individual was entered as the subject random effect to test the general random effects according to the intubating individual. The Kenward-Roger method was used to calculate the denominator degrees of freedom due to the unbalanced study design.26 Comparisons of movement in each axis were conducted by the differences among least squares means. Correlations among axes of movement were then examined using a simple Pearson correlation, with normality assumed by the central limit theorem. To examine the association between time required for intubation and movement at C1-2, a Spearman rank correlation was performed between time required and each axis separately and plotted using a locally weighted regression plot.

The primary outcome, the effect of the airway device on movement at the C1-2 level, was then examined using a mixed models approach. Each axis was treated as a separate outcome. The airway device was entered into the model as a fixed effect, and the person intubating as the subject random effect. The cadaver and the interaction between airway device and cadaver were entered as general random effects. Denominator degrees of freedom were calculated using the Kenward-Roger method due to the unbalanced study design. Summary statistics of movement along each axis were reported for each airway device, as was the statistical significance of the airway device’s contribution to movement along each axis. Results were reported as means with 95% confidence intervals (CIs) for all continuous data, along with mean differences with adjusted 95% CIs, using least square means methods for comparisons where appropriate. The Tukey-Kramer method was used to adjust for multiple comparisons, and adjusted P values were reported. For all analyses, α was designated as 0.05. Data were analyzed using SAS 9.2 (SAS Institute, Cary, NC).


Overall, 153 intubation trials were conducted for each of the 3 axes of movement. Our providers failed to intubate with the ILMA on 3 of 39 attempts. All other intubations were successful. Motion data were analyzed only with successful intubation. Table 2 details the number of intubations performed by each provider.

Table 2
Table 2:
Number of Intubations by Each Practitioner

First, we assessed the overall impact of intubation on movement. Overall, the most significant movement was noted in the flexion-extension axis. Movement in the flexion-extension axis was greater than both lateral bending (mean difference 3 [95% CI, 2.4–3.5], P < 0.0001) and axial rotation (mean difference 3.1 [95% CI, 2.5–3.6], P < 0.0001), with no evidence of a significant difference between lateral bending and axial rotation (mean difference 0.1 [95% CI, −0.4 to 0.7], P = 0.86; Fig. 1). Movements during intubation significantly correlated with all measured directions of movement (flexion-extension by lateral bending Spearman r = 0.5 [95% CI, 0.4–0.6], P < 0.0001; flexion-extension by axial rotation Spearman r = 0.7 [95% CI, 0.6–0.8], P < 0.0001; axial rotation by lateral bending Spearman r = 0.5 [95% CI, 0.4–0.6], P < 0.0001). The time required for intubation is significantly correlated with movement for all axes (r = 0.6, P < 0.0001; Fig. 2).27

Figure 1
Figure 1:
Overall distribution of movement of all devices. Motion in flexion-extension was significantly greater than axial rotation or lateral bending. No difference was found between axial rotation and lateral bending.
Figure 2
Figure 2:
Locally weighted regression plots demonstrating the correlation between time to intubate and increased motion in each axis.53 The solid line is obtained by first degree local regression using smoothing parameters 0.67 for flexion-extension, 0.62 for lateral bending, and 0.76 for axial rotation; these were automatically determined minimizing the corrected Akaike information criterion. Shaded region represents a 95% confidence interval based on the mean predicted value for each observation. Spearman rank correlation coefficients are also reported to demonstrate a correlation between time to intubate and increased motion in each axis.

Summary statistics are listed in Table 3 that describe the movement along each axis by the airway device. Overall, the airway device had a statistically significant effect on motion in flexion-extension (P = 0.002) and axial rotation (P = 0.007), but not on lateral bending (P = 0.10) axes of movement. For flexion-extension and axial rotation, the LW technique resulted in significantly less movement than with the ILMA (mean difference in flexion-extension: 3.2° [95% CI, 0.9°–5.5°], P = 0.003; mean difference in axial rotation 1.6° [95% CI, 0.3°–2.8°], P = 0.01) and MAC (mean difference in flexion-extension 3.1° [95% CI, 0.8°–5.4°], P = 0.005; mean difference in axial rotation 1.4° [95% CI, 0.1°–2.6°], P = 0.03). Tables 4 to 6 depict the statistical comparisons between each device in the 3 degrees of motion evaluated.

Table 3
Table 3:
Summary Statistics for Movement at the C1-2 Level
Table 4
Table 4:
Differences Among Airway Devices in Flexion-Extension at C1-2
Table 5
Table 5:
Differences Among Airway Devices in Lateral Bending at C1-2
Table 6
Table 6:
Differences Among Airway Devices in Axial Rotation at C1-2


Management of the airway during elective intubation in the patient with upper cervical spine disease affords the practitioner several options, including awake flexible fiberoptic intubation with awake positioning and serial neurologic assessments. However, fiberoptic intubation is impractical due to the longer time required for intubation in emergency situations.28 In addition, patient cooperation is usually required for awake fiberoptic intubation, and, as such, patients requiring emergency intubation may offer far fewer alternatives for the safe management of the airway.

Occasionally, rapid and effective advanced airway management must be completed in the patient with known or suspected upper cervical spine instability in the prehospital setting. Neurologic injury may occur due to mechanical disruption at the unstable segments, but care must also be taken to not compromise global perfusion and oxygen delivery with multiple prolonged intubation attempts or failed intubation, which could also exacerbate neurologic injuries. Therefore, consideration for speed and success of intubation must be weighed heavily due to the likely harm from failed intubation, with resultant hypoxia, hypercarbia, risk of aspiration, and unrecognized esophageal intubation.1–6

We have shown that the LW technique created the least flexion-extension and rotational motion at C1-2 during advanced airway management with in-line stabilization whereas the ILMA and MAC produced the most movement. Progression of neurologic injury in patients with upper cervical spine disease is common, but the exact cause of the deterioration remains unclear. Expert opinion, in general, recommends minimizing cervical spine movement. Previous investigations considered a 10% to 30% reduction in motion to be a clinically significant result.29,30 In cadavers with a simulated type II odontoid fracture, the LW technique resulted in >3° less extension than with the ILMA and MAC, which is more than a 40% reduction in extension over the ILMA and MAC techniques. Whether just over 1° less rotation is clinically relevant between the LW and the ILMA and MAC is less certain. The correlations between the time required and magnitude of movement were low and of questionable clinical relevance.

Direct visualization from the oropharynx to the glottis with techniques such as the MAC typically requires the patient to assume the “sniffing position,” with extension of the upper cervical spine and flexion of the lower cervical spine.31 Therefore, there are theoretical benefits to devices that facilitate intubation without direct visualization, such as the LW, AT, and ILMA. A number of studies have evaluated cervical spine motion using various techniques. The reported extent to which each cervical segment is displaced is variable, depending on which segment is measured, how the displacement is measured, and which population is studied. In the published literature, the range of reported cervical spine extension created by direct visualization with techniques such as the MAC at the C1-2 segment is as low as 4° and as high as 13.5°.20,32–39 Techniques such as the LW have been reported to produce approximately 2° of extension at the C1-2 segment.37 With indirect visualization techniques, such as the AT, Bullard, and GlideScope, the range of reported cervical spine extension at C1-2 is 3° to 10.4° with the AT, 2.6° to 4.5° with the Bullard, and 4° to 7° with the GlideScope.35–40 The impact of the ILMA technique on cervical spine extension at C1-2 has also been reported from <1° to 5°.41–43

Our results lie within these ranges for all techniques, except for the ILMA, with which we found a mean degree of extension of 7.43°. This difference highlights the strengths and weaknesses of our study methods. We used electromagnetic motion analysis to assess motion at the unstable segment and were able to directly and simultaneously track displacement in 3 dimensions. The previously cited studies only used lateral fluoroscopic images to measure motion in 2 dimensions. Fluoroscopy has a number of significant limitations, most notably the interobserver variation in measurements. In addition, the Polhemus device is extremely accurate and able to detect differences in angular motion to 0.3°. We used cadavers with a created global instability at C1-2 whereas most other investigations used normal adult patients undergoing elective surgical procedures. The use of cadavers allowed us to repeatedly study the worst possible instability under the same patient conditions, thus limiting the variability in motion due to the variable airway anatomy among patients. Although the use of cadavers may be considered a weakness due to tissue degradation and rigor, we used only lightly embalmed cadavers to attempt to make the situation as close to in vivo as possible. The range of motion before the creation of instability was similar to that reported in normal geriatric adults. Another limitation of our study is the variability introduced by the heterogeneity in the experience and background of our practitioners. We included 5 providers, 2 attending anesthesiologists, and 3 emergency medical technician paramedics who used the same techniques to intubate. These groups have significant differences in levels of academic accomplishment, training, and experience; however, these practitioners may be representative of those called upon to provide advanced airway support to the patient with a known or suspected unstable cervical spine. Practitioners will have broad differences in experience with different techniques. Furthermore, the prehospital use of these and other similar airway devices, such as the AT, GlideScope, C-MAC, LW, and supraglottic airways like the ILMA, used by emergency medical technicians and other types of providers has been documented.44–53 Therefore, we believe the results recorded in our study may reflect the care provided to a patient with a known or suspected unstable spine. The variance imparted by the practitioners’ differing backgrounds in airway management and cadavers in an unbalanced, repeated-measures design was managed through the use of mixed models. Unfortunately, we discarded the data of 3 unsuccessful intubations with the ILMA before analysis. The data analyzed in this study were only from successful intubations.

This is the first study to evaluate the AT in subjects with C1-2 instability and compare the use of the AT with other devices in a controlled, reproducible manner. These 4 devices were selected for study because they are amenable for use in a wide array of settings, including the prehospital setting, because they are simple, inexpensive, and portable. Each technique was shown to have some advantages over the others in terms of speed, success rate, and cervical spine motion. It must be recognized that although the ILMA required the most time to complete by our practitioners, intermittent ventilation could continue during the procedure, and, although it had a lower success rate for our providers when using the blind technique, an improved success rate would be expected if a flexible fiberoptic scope was used to facilitate ILMA intubation, although that was not tested.

In conclusion, during intubation of the cadavers with created global instability at C1-2 (simulated type II odontoid fracture), the LW technique resulted in significantly less extension and axial rotation at C1-2 than with the ILMA and MAC techniques.


Name: Adam L. Wendling, MD.

Contribution: This author helped design the study, conduct the study, write the manuscript, and collect the data.

Attestation: Adam L. Wendling has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.

Name: Patrick J. Tighe, MD.

Contribution: This author helped analyze the data and write the manuscript.

Attestation: Patrick J. Tighe reviewed the analysis of the data and approved the final manuscript.

Name: Bryan P. Conrad, PhD.

Contribution: This author helped design the study, conduct the study, write the manuscript, and collect the data.

Attestation: Bryan P. Conrad has seen the original study data and approved the final manuscript.

Name: Tezcan Ozrazgat Baslanti, PhD.

Contribution: This author helped analyze the data and write the manuscript.

Attestation: Tezcan Ozrazgat Baslanti reviewed the analysis of the data and approved the final manuscript.

Name: MaryBeth Horodyski, EdD.

Contribution: This author helped design the study, conduct the study, and write the manuscript.

Attestation: MaryBeth Horodyski has seen the original study data and approved the final manuscript.

Name: Glenn R. Rechtine, MD.

Contribution: This author helped design the study, conduct the study, and write the manuscript.

Attestation: Glenn R. Rechtine has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

This manuscript was handled by: Sorin J. Brull, MD, FCARCSI (Hon).


a Airtraq® Guided Video Intubation. Available at: Accessed September 15, 2011.
Cited Here

b Instruction Manual, LMA Fastrach Reusable & LMA Fastrach Single Use, revised 2006. San Diego, CA: LMA North America; Revised 2006. Available at: Accessed September 15, 2011.
Cited Here

c Liem EB. Lighted Stylet Intubation. The Virtual Airway Device. Intubation Techniques and Tutorials. Gainesville, FL: The University of Florida Center for Simulation, Advanced Learning and Technology. Available at: Accessed September 15, 2011.
Cited Here


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