Occipital and external acoustic meatus to axis angle: a useful predictor of oropharyngeal space in rheumatoid arthritis patients with atlantoaxial subluxation

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OBJECTIVE

Dyspnea and/or dysphagia is a life-threatening complication after occipitocervical fusion. The occiput-C2 angle (O-C2a) is useful for preventing dyspnea and/or dysphagia because O-C2a affects the oropharyngeal space. However, O-C2a is unreliable in atlantoaxial subluxation (AAS) because it does not reflect the translational motion of the cranium to C2, another factor affecting oropharyngeal area in patients with rheumatoid arthritis (RA) who have reducible AAS. The authors previously proposed the occipital and external acoustic meatus to axis angle (O-EAa; i.e., the angle made by McGregor’s line and a line joining the external auditory canal and the middle point of the endplate of the axis [EA line]) as a novel, useful, and powerful predictor of the anterior-posterior narrowest oropharyngeal airway space (nPAS) distance in healthy subjects. The aim of the present study was to elucidate the validity of O-EAa as an indicator of oropharyngeal airway space in RA patients with AAS.

METHODS

The authors investigated 64 patients with RA. The authors collected lateral cervical radiographs at neutral position, flexion, extension, protrusion, and retraction and measured the O-C2a, C2-C6, O-EAa, anterior atlantodental interval (AADI), and nPAS. Patients were classified into 2 groups according to the presence of AAS and its mobility: group N, patients without AAS; and group R, patients with reducible AAS during dynamic cervical movement.

RESULTS

Group N had a significantly lower AADI and O-EAa than group R in all but the extension position. The O-EAa was a better predictor for nPAS than O-C2a according to the mixed-effects models in both groups (marginal R2: 0.510 and 0.575 for the O-C2a and O-EAa models in group N, and 0.250 and 0.390 for the same models, respectively, in group R).

CONCLUSIONS

O-EAa was superior to O-C2a in predicting nPAS, especially in the case of AAS, because it affects both O-C2a and cranial translational motion. O-EAa would be a useful parameter for surgeons performing occipitocervical fusion in patients with AAS.

ABBREVIATIONS AADI = anterior atlantodental interval; AAS = atlantoaxial subluxation; BMI = body mass index; CGH = center of gravity of the head; C2Ta = C2 tilting angle; C2-C6a = C2-C6 angle; nPAS = narrowest oropharyngeal airway space; O-C2a = occiput-C2 angle; OCF = occipitocervical fusion; O-EAa = occipital and external acoustic meatus to axis angle; RA = rheumatoid arthritis; SVA = sagittal vertical axis; TMJ = temporomandibular joint.

Article Information

Correspondence Kazuaki Morizane: Graduate School of Medicine, Kyoto University, Kyoto, Japan. morizane@kuhp.kyoto-u.ac.jp.

INCLUDE WHEN CITING Published online June 21, 2019; DOI: 10.3171/2019.3.SPINE181390.

Disclosures Dr. Neo is a consultant for DePuy Synthes Products, LLC.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    Compared to no reduction (A), reduction of AAS is accompanied by translational motion of the cranium, mandible, and C1, and decreased AADI and nPAS, despite a constant O-C2a (B). The gray single-headed arrow indicates translational motion and the black and red double-headed arrows indicate nPAS and AADI, respectively. Figure is available in color online only.

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    Study flowchart.

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    Radiographic measurements. O-EAa represents the angle of McGregor’s line and the line connecting the external acoustic meatus and the midpoint of the caudal endplate of C2 (i.e., EA line). C2Ta represents the angle of the caudal endplate of C2 and the EA line. O-C2a represents the angle between McGregor’s line and the caudal endplate of C2. C2-C6a represents the angle of the caudal endplates of C2 and C6. nPAS represents the narrowest anterior-posterior distance from the posterior pharyngeal wall to the back of the tongue between the levels of the uvula tip and the epiglottis tip. AADI represents the anterior atlantodental interval. O-C2a, C2-C6a, AADI, and nPAS were measured as previously reported (Izeki et al., 2013). Figure is available in color online only.

  • View in gallery

    Cervical radiographs of a 57-year-old woman with reducible AAS. A: Cervical flexion radiograph shows nPAS: 22.4 mm, O-C2a: −5.2°, O-EAa: 103.8°, and AADI: 11 mm. B: Cervical extension image showing the decrease of nPAS to 13.4 mm in spite of the increase of O-C2a to 17.7°. nPAS decreases due to the reduction of AAS (AADI: 0 mm) and O-EAa also decreases to 97.5°, reflecting the reduction of AAS. Figure is available in color online only.

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