The atlantoaxial capsular ligaments and transverse ligament are the primary stabilizers of the atlantoaxial joint in the craniocervical junction: a finite element analysis

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OBJECTIVE

Prior studies have provided conflicting evidence regarding the contribution of key ligamentous structures to atlantoaxial (AA) joint stability. Many of these studies employed cadaveric techniques that are hampered by the inherent difficulties of testing isolated-injury scenarios. Analysis with validated finite element (FE) models can overcome some of these limitations. In a previous study, the authors completed an FE analysis of 5 subject-specific craniocervical junction (CCJ) models to investigate the biomechanics of the occipitoatlantal joint and identify the ligamentous structures essential for its stability. Here, the authors use these same CCJ FE models to investigate the biomechanics of the AA joint and to identify the ligamentous structures essential for its stability.

METHODS

Five validated CCJ FE models were used to simulate isolated- and combined ligamentous–injury scenarios of the transverse ligament (TL), tectorial membrane (TM), alar ligament (AL), occipitoatlantal capsular ligament, and AA capsular ligament (AACL). All models were tested with rotational moments (flexion-extension, axial rotation, and lateral bending) and anterior translational loads (C2 constrained with anterior load applied to the occiput) to simulate physiological loading and to assess changes in the atlantodental interval (ADI), a key radiographic indicator of instability.

RESULTS

Isolated AACL injury significantly increased range of motion (ROM) under rotational moment at the AA joint for flexion, lateral bending, and axial rotation, which increased by means of 28.0% ± 10.2%, 43.2% ± 15.4%, and 159.1% ± 35.1%, respectively (p ≤ 0.05 for all). TL removal simulated under translational loads resulted in a significant increase in displacement at the AA joint by 89.3% ± 36.6% (p < 0.001), increasing the ADI from 2.7 mm to 4.5 mm. An AACL injury combined with an injury to any other ligament resulted in significant increases in ROM at the AA joint, except when combined with injuries to both the TM and the ALs. Similarly, injury to the TL combined with injury to any other CCJ ligament resulted in a significant increase in displacement at the AA joint (significantly increasing ADI) under translational loads.

CONCLUSIONS

Using FE modeling techniques, the authors showed a significant reliance of isolated- and combined ligamentous–injury scenarios on the AACLs and TL to restrain motion at the AA joint. Isolated injuries to other structures alone, including the AL and TM, did not result in significant increases in either AA joint ROM or anterior displacement.

ABBREVIATIONS AA = atlantoaxial; AACL = AA capsular ligament; ADI = atlantodental interval; AL = alar ligament; CCJ = craniocervical junction; DOF = degrees of freedom; FE = finite element; OA = occipitoatlantal; OACL = OA capsular ligament; ROM = range of motion; TL = transverse ligament; TM = tectorial membrane.

Article Information

Correspondence Benjamin J. Ellis: Scientific Computing and Imaging Institute, University of Utah, Salt Lake City, UT. ben@sci.utah.edu.

INCLUDE WHEN CITING Published online June 14, 2019; DOI: 10.3171/2019.4.SPINE181488.

Disclosures Dr. Dailey reports that he is a consultant for Zimmer Biomet, receives clinical or research support for the current study from K2M, and has received an honorarium from AOSpine.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    A CCJ FE model of a 14-year-old girl (model 4). Anterior view of the discretized CCJ FE model with various letter points of the anatomy. A) OACL; B) AACL; C) anterior atlantooccipital membrane; D) anterior longitudinal ligament; E) PAOM (posterior atlantooccipital membrane); F) PAAM (posterior AA membrane); G) TM; H) TL; I) apical ligament; and J) alar ligament. Figure is available in color online only.

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    Percentage increase from normal in flexion (A), extension (B), axial rotation (C), and lateral bending (D). Asterisks indicate a significant difference (p < 0.05) from the normal, noninjured model. Figure is available in color online only.

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    Percentage increase in displacement from normal for an applied load of 15 lbf on C0 toward the anterior for isolated and combined injuries to the CCJ. Asterisks indicate a significant difference from the normal, noninjured model. Figure is available in color online only.

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    CCJ FE model of a 26-year-old woman. Normal (left side) and AACL injury (right side) ROM comparison in degrees for flexion (A), extension (B), axial rotation (C), and lateral bending (D). The colors represent relative change in ROM from fixed C2 (0°).

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    CCJ FE model of a 59-year-old woman. Sagittal cross-sectional views of a normal and injured CCJ in translation simulations. A: Normal CCJ under a translation load of 15 lbf. B: Injured CCJ with the TL removed under a translation load of 15 lbf. C: Enlarged view of the ADI (2.7 mm) of a normal CCJ. D: Enlarged view of the ADI (4.5 mm) of an injured CCJ with the TL removed. E: FE fringe plot of a normal CCJ under translation. F: FE fringe plot of an injured CCJ with the TL removed under translation. The colors represent relative change in translation from a fixed C2 (0 mm).

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