Practice Essentials

Atlantoaxial instability (AAI), also known as atlantoaxial subluxation, is radiologically identified increased laxity between the body of the first cervical vertebra (atlas) and the odontoid process of the second cervical vertebra (axis) (see the image below).^{[1, 2, 3, 4]} The subluxation can be anterior, posterior, or lateral, and symptoms occur as a result of cervical cord impingement.

Medial-sagittal cross-sectional view of the atlas (C1) and the odontoid process of the axis (C2).

Potential disability due to instability of the unique atlantoaxial joint results in controversies regarding the management of acute trauma and also the screening evaluation of particular at-risk individuals. The purposes of this article are to define AAI; to describe the relatively rare symptomatic lesions with significant morbidity and mortality; and finally to discuss the rationale for and against screening and restricting the activities of at-risk individuals.

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Epidemiology

United States data

Although traumatic lesions involving the atlantoaxial region are relatively rare, certain disease states and conditions present a higher theoretic risk of instability.

Surveys indicate 10-25% of patients with trisomy 21 have AAI.^{[5, 6]} Two thirds of these cases are due to laxity of transverse ligament, whereas one third are due to abnormal odontoid development. Although this association has been depicted on radiographs, the clinical incidence of serious cervical spine injury is not increased in this population compared with other populations.

About 25% of patients with rheumatoid arthritis have atlantoaxial instability, which is thought to be due to chronic inflammation.^[7] Congenital skeletal dysplasias may cause resultant odontoid hypoplasia. Marfan syndrome may involve ligamentous laxity, and acute inflammatory processes can affect the retropharyngeal, neck, or pharyngeal spaces.

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Etiology

Symptomatic AAI is due to acute trauma, usually cervical hyperflexion, hyperextension, or a direct axial load on the head or cervical spine. Although this type of injury can occur in any athlete, certain conditions predispose an individual to AAI dysfunction and disability. These include congenital odontoid anomalies such as odontoid aplasia, odontoid hypoplasia, and a separate odontoid process known as os odontoideum which is due to nonunion of an early childhood fracture and can lead to an unstable lesion or ligamentous laxity. Acute or chronic inflammatory processes can also lead to increased risk of AAI.

Next: Epidemiology

Functional Anatomy

The articulation of the odontoid process of C2 (axis) with the anterior arch of C1 (atlas) allows for 50% of cervical lateral rotation. The transverse and alar ligaments maintain joint integrity and limit posterior motion of the odontoid process relative to the C1 anterior arch. Abnormal posterior translation (or subluxation) can cause cervical cord impingement with the potential for significant neurologic compromise and even death.

Next: Epidemiology

Sport-Specific Biomechanics

During extremes of cervical flexion or extension, competent transverse and alar ligaments limit posterior translation of the odontoid process. Incompetent ligaments or a damaged odontoid process can allow for significant translation and potential damage in cases of cervical hyperflexion or hyperextension in which axial compression is delivered to the head and cervical spine. Given the potentially serious sequelae of significant atlantoaxial dysfunction, patients with defined instability are restricted from participating in contact sports and in sports requiring significant cervical flexion or extension.^{[8, 9]}

Clinical Presentation

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Media Gallery

Medial-sagittal cross-sectional view of the atlas (C1) and the odontoid process of the axis (C2).

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Author

Chris G Koutures, MD, FAAP Consulting Staff, Department of Pediatrics, Children's Hospital of Orange County and St Joseph Hospital; Team Physician, California State University, Fullerton; Team Physician, USA Volleyball

Chris G Koutures, MD, FAAP is a member of the following medical societies: American Academy of Pediatrics, American College of Sports Medicine, American Medical Association, American Medical Society for Sports Medicine, California Medical Association

Disclosure: Received royalty from SLACK Publishing for independent contractor.

Specialty Editor Board

Francisco Talavera, PharmD, PhD Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Received salary from Medscape for employment. for: Medscape.

Russell D White, MD Clinical Professor of Medicine, Clinical Professor of Orthopedic Surgery, Department of Community and Family Medicine, University of Missouri-Kansas City School of Medicine, Truman Medical Center-Lakewood

Russell D White, MD is a member of the following medical societies: Alpha Omega Alpha, American Academy of Family Physicians, American Association of Clinical Endocrinology, American College of Sports Medicine, American Diabetes Association, American Medical Society for Sports Medicine

Disclosure: Nothing to disclose.

Chief Editor

Craig C Young, MD Professor, Departments of Orthopedic Surgery and Community and Family Medicine, Medical Director of Sports Medicine, Medical College of Wisconsin

Craig C Young, MD is a member of the following medical societies: American Academy of Family Physicians, American College of Sports Medicine, American Medical Society for Sports Medicine, Phi Beta Kappa

Disclosure: Nothing to disclose.

Additional Contributors

David T Bernhardt, MD Director of Adolescent and Sports Medicine Fellowship, Associate Professor, Department of Pediatrics/Ortho and Rehab, Division of Sports Medicine, University of Wisconsin School of Medicine and Public Health

David T Bernhardt, MD is a member of the following medical societies: American Academy of Pediatrics, American College of Sports Medicine, American Medical Society for Sports Medicine

Disclosure: Nothing to disclose.

Acknowledgements

John Munyak, MD Associate Program Director, Director of Sports Medicine Education, Department of Emergency Medicine, Lincoln Medical and Mental Health Center

Disclosure: Nothing to disclose.

Chantal D Simpson, MD Staff Physician, Department of Emergency Medicine, Lincoln Hospital of New Jersey

Chantal D Simpson, MD is a member of the following medical societies: American College of Emergency Physicians and American College of Obstetricians and Gynecologists

Disclosure: Nothing to disclose.