Intubation biomechanics: validation of a finite element model of cervical spine motion during endotracheal intubation in intact and injured conditions

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OBJECTIVE

Because of limitations inherent to cadaver models of endotracheal intubation, the authors’ group developed a finite element (FE) model of the human cervical spine and spinal cord. Their aims were to 1) compare FE model predictions of intervertebral motion during intubation with intervertebral motion measured in patients with intact cervical spines and in cadavers with spine injuries at C-2 and C3–4 and 2) estimate spinal cord strains during intubation under these conditions.

METHODS

The FE model was designed to replicate the properties of an intact (stable) spine in patients, C-2 injury (Type II odontoid fracture), and a severe C3–4 distractive-flexion injury from prior cadaver studies. The authors recorded the laryngoscope force values from 2 different laryngoscopes (Macintosh, high intubation force; Airtraq, low intubation force) used during the patient and cadaver intubation studies. FE-modeled motion was compared with experimentally measured motion, and corresponding cord strain values were calculated.

RESULTS

FE model predictions of intact intervertebral motions were comparable to motions measured in patients and in cadavers at occiput–C2. In intact subaxial segments, the FE model more closely predicted patient intervertebral motions than did cadavers. With C-2 injury, FE-predicted motions did not differ from cadaver measurements. With C3–4 injury, however, the FE model predicted greater motions than were measured in cadavers. FE model cord strains during intubation were greater for the Macintosh laryngoscope than the Airtraq laryngoscope but were comparable among the 3 conditions (intact, C-2 injury, and C3–4 injury).

CONCLUSIONS

The FE model is comparable to patients and cadaver models in estimating occiput–C2 motion during intubation in both intact and injured conditions. The FE model may be superior to cadavers in predicting motions of subaxial segments in intact and injured conditions.

ABBREVIATIONS FE = finite element; MD = mean difference; Oc = occiput.

Article Information

Correspondence Bradley J. Hindman, Department of Anesthesia, University of Iowa Hospitals and Clinics, 200 Hawkins Dr., Iowa City, IA 52242. email: brad-hindman@uiowa.edu.

INCLUDE WHEN CITING Published online October 20, 2017; DOI: 10.3171/2017.5.SPINE17189.

Disclosures The authors report the following. Dr. Todd: has received research funding from Karl Storz Endoscopy. Dr. Fontes: consultant for Medtronic and Stryker.

© AANS, except where prohibited by US copyright law.

Headings

Figures

  • View in gallery

    Anterior (coronal, A), lateral (sagittal, B), and axial (C) views of the Oc–C7 cervical spine finite element model. Only the caudal portion of the occiput is shown. The spinal cord can be most easily visualized in the axial view. D: For Macintosh intubations, the intubation force vector (FINT) originated from the midpoint of the anterior face of the C-3 vertebral body at an angle of 70° from the coronal plane and consisted of anterior and inferior vector components. E: For Airtraq intubations, the intubation force vector originated from the anterior face of the C-2 vertebral body (11 mm cephalad to the Macintosh force point) at an angle 90° from the coronal plane. The inferior surface of the C-7 vertebral body was fixed in all directions. The occiput was allowed to rotate around the sagittal (X) axis for all simulations and translate in the axial (Z) direction.

  • View in gallery

    Macintosh (upper) and Airtraq (lower) values for segmental intervertebral extension in patients, FE model, and cadavers. Values are mean ± SD. Each group comprises 14 subjects. With each laryngoscope, total force is comparable among the 3 groups.

  • View in gallery

    Finite element model strain field maps of minimum principal strain (compression) of the spinal cord during endotracheal intubation in the presence of an intact (stable) spine (A–D), Type II odontoid fracture (E–H), and severe C3–4 distractive-flexion injury (I–L). For these maps, the model was loaded with the normal (clinical average) Macintosh force profile (48.8 N) and the normal (clinical average) Airtraq force profile (10.4 N). Panels in the upper row show peak minimum principal strain in the midline sagittal plane. Panels in the lower row show peak minimum principal strain values in the transverse plane corresponding to the level of the cord with greatest minimum principal strain. The lower boundary of the scale (−1.00e−01 [−10%], red) is limited to enhance contrast among regions with varying strain. Black indicates minimum principal strain values exceeding −10%.

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