A Review Paper Occipitocervical Junction: Imaging, Pathology, Instrumentation Michael Benke, MD, Warren D. Yu, MD, Sean C. Peden, MD, and Joseph R. O’Brien, MD, MPH ANATOMY Abstract The occiput, a wide, flat bone at the posteroinferior aspect The occipitocervical junction (OCJ) is a highly special- of the skull, encases the cerebellum, the pons, and the ized area of the spine. Understanding the unique anato- medulla in the posterior fossa. The sphenoid bone also my, imaging, and craniometry of this area is paramount makes up part of the posterior fossa as it articulates with in recognizing and managing the potentially devastating the anterior border of the occiput. The clivus (Latin for effects that pathology has on it. Instrumentation tech- niques continue to evolve, the goal being to safely obtain “slope”) is the superior portion of the sphenoid bone that durable, rigid constructs that allow immediate stability, meets the portion of the occiput anterior to the foramen anatomical alignment, and osseous fusion. This article magnum (Figure 1). Through the foramen magnum, a large, reviews the pathologic conditions at the OCJ and the oval aperture in the occiput, the cranium communicates current instrumentation and fusion options available for with the vertebral canal. The foramen magnum is bound treatment. The general orthopedist needs to recognize anteriorly by the basion, posteriorly by the opisthion, and the pathology common in this region and appropriately laterally by the convex occipital condyles; it transmits the refer patients for treatment. medulla oblongata, the spinal accessory nerves (CN11), the vertebral arteries, the anterior and posterior spinal arteries, he occipitocervical junction (OCJ) is the unique set the tectorial membrane, and the alar ligaments. The hypo- of articulations among the occipital bone or occiput glossal nerve (CN12) courses through the hypoglossal canal (C0), the atlas (C1), and the axis (C2). This anatom- in the base of each occipital condyle.2 ical area is also known as the craniovertebral junc- The atlas (C1) is a ring-shaped vertebra composed of Ttion; in the literature, the 2 terms are used interchangeably. right and left lateral masses connected by anterior and pos- The OCJ lacks intervertebral disks and depends on strong terior arches. The atlas has no vertebral body. The C1 lateral capsular and ligamentous attachments for stability.1 This masses articulate superiorly with the occipital condyles and region affords a large degree of mobility, with about half of inferiorly with the C2 superior articular facets. The dens all cervical flexion-extension coming from C0–C1 and half (a.k.a. odontoid process) of C2 articulate with the posterior of all cervical rotation occurring at C1–C2.1 Recognition and surface of the anterior arch of C1 and is held in place by treatment of OCJ pathology require a thorough understand- strong ligamentous attachments to the atlas and the skull ing of osseous, ligamentous, and neurovascular anatomy. (Figure 2). Craniocervical instability can originate at the Understanding imaging and associated craniometry of the C0–C1 and C1–C2 joints.3 OCJ is paramount. General orthopedists needs to recognize The unique combination of motion and stability at the the pathology common in this region and appropriately refer OCJ is provided by its strong ligamentous structures.4-6 patients for treatment. Dr. Benke is Resident, Department of Orthopaedic Surgery, George Washington University, Washington, DC. Dr. Yu is Chief, Spine Section, Department of Orthopaedic Surgery, George Washington University Hospital, and Associate C Professor of Orthopaedic Surgery and Neurosurgery, George Washington University School of Medicine, Washington, DC. Dr. Peden is Resident, Department of Orthopaedic Surgery, O George Washington University. Dr. O’Brien is Associate Director of Spine Surgery, George Washington University Hospital, and Assistant Professor, George Washington University School of Medicine. Figure 1. Coned-down cross-section of skull shows important anatomical structures and their relationships in occipitocervical Address correspondence to: Sean C. Peden, MD, 2150 junction. Clivus (C) is posterior portion of sphenoid bone, with Pennsylvania Avenue NW, 7th Floor, Washington, DC 20037 (tel, smooth inferior slope. Occipital bone (occiput) makes up pos- 203-918-0409; e-mail, [email protected]). terior rim of foramen magnum. The anterior point of occiput— posterior margin of foramen magnum at its midpoint—is opis- Am J Orthop. 2011;40(10):E205-E215. Copyright Quadrant thion (O). Courtesy of Gray’s Anatomy of the Human Body. 20th HealthCom Inc. 2011. All rights reserved. US ed. Public domain. www.amjorthopedics.com October 2011 E205 Occipitocervical Junction B A C D E F Inferior body C3 Figure 2. Sketch of odontoid peg and its major ligaments attaching to occiput (C0) and atlas (C1). Posterior elements have been removed. The tectorial membrane, a continuation of the posterior longitudinal ligament, has been removed to better expose these ligaments. Figure 4. Midline sagittal computed tomography of normal anatomy. Hard palate is commonly not well visualized on neck CT. Wackenheim clivus baseline (A), posterior axial line or pos- terior vertebral line (B), and angle that they form, clivus–canal angle (a), are shown. McRae line (C), Chamberlain line (D), and McGregor line (E) are also shown. Lines are drawn to demon- strate Redlund-Johnell criterion. Distance from inferior body of axis (C2) to McGregor line (F) should be less than 29 mm in women or less than 34 mm in men. apical ligament (connects dens to basion). The posterior stabilizing structures are the ligamentum nuchae, the inter- spinous ligament, the posterior atlanto-occipital membrane, the ligamentum flavum, and the cervical musculature.4-6 OCCIPITOCERVICAL CRANIOMETRY Pathologic conditions at the OCJ may be evaluated with conventional radiography, computed tomography (CT), Figure 3. Cross-sectional view of occipitocervical junction (OCJ). and magnetic resonance imaging (MRI). Anatomical land- The following parameters are often used when interpreting sagit- marks, parameters, and relationships that were originally tal computed tomography scan or lateral radiograph for signs of described on conventional radiography,7-9 and that form the pathology at OCJ. Dens should remain below Wackenheim line, basis for occipitocervical craniometry, have been extrapo- which is drawn as tangent to posterior slope of clivus. McRae line lated for use with CT and MRI (Table). connects tip of clivus to opisthion, and protrusion of dens above it is abnormal. Chamberlain and McGregor lines use hard palate The Wackenheim clivus baseline is a tangent along the as reference point for 2 additional lines, and protrusion of dens superior surface of the clivus10 (Figure 3). Extension of the 3 mm above Chamberlain line and 4.5 mm above McGregor line tip of the dens above or posterior to this line is abnormal is abnormal. Atlantodental interval (ADI; sometimes anterior ADI and indicates basilar invagination. The clivus–canal angle is [AADI]), is usually less than 3 mm in adults and 5 mm in children. formed by the Wackenheim clivus baseline and the posterior At this spinal level, minimum space available for cord (SAC) to avoid compression is 14 mm. vertebral body line. The normal range is 180° in extension to 150° in flexion. Ventral spinal cord compression may occur Anteriorly, the OCJ is stabilized by 5 ligaments—the ante- when the angle is less than 150°.11 rior atlanto-occipital membrane (continuation of anterior The McRae line runs from the basion to the opisthion longitudinal ligament), the tectorial membrane (continua- (protrusion of tip of dens above line is abnormal12,13), the tion of posterior longitudinal ligament), the transverse liga- Chamberlain line from the hard palate to the opisthion (pro- ment (secures dens to anterior arch of C1), the alar liga- trusion of dens more than 3 mm above line is abnormal8), ments (connect dens to occipital condyles and are the main and the McGregor line from the hard palate to the most restraints to axial rotation and lateral bending4,5), and the caudal aspect of the occiput on midsagittal images (protru- E206 The American Journal of Orthopedics® www.amjorthopedics.com M. Benke et al Table. Radiographic Parameters of Occipitocervical Junction Eponym Parameters Pathology Wackenheim clivus baseline Tangent drawn along superior surface of Dens should be below line clivus Clivus–canal angle Angle formed by Wackenheim line and Normal range is 180° in extension to 150° posterior vertebral body line in flexion; angle of <150° is abnormal McRae line From basion to opisthion Protrusion of dens above line is abnormal Chamberlain line From hard palate to opisthion Protrusion of dens >3 mm above line is abnormal McGregor line From hard palate to most caudal point on Protrusion of dens >4.5 mm above line is midline occipital curve abnormal Ranawat criterion Distance from center of pedicle of C2 to <15 mm in men or <13 mm in women is transverse axis of C1 abnormal Redlund-Johnell criterion Distance from inferior C2 body to <34 mm in men or <29 mm in women is McGregor line abnormal Welcher basal angle Tangent to clivus as it intersects tangent to Normal range is 125°-143°; platybasia sphenoid bone occurs when basal angle is >143° Basion–dens interval From tip of basion to tip of dens >12 mm is abnormal Basion–atlanto interval From tip of basion to posterior axial line of >12 mm is abnormal C2 Powers ratio Ratio of line from basion to posterior arch >1.0 is abnormal of C1 (BP) to line from opisthion to anterior arch of C1 (OA) Anterior atlantodental interval From anterior arch of C1 to dens >3 mm in adults or >4.5-5 mm in children is abnormal Posterior atlantodental interval From dens to posterior arch of C1 <14 mm is abnormal Atlanto-occipital joint axis angle Angle formed by lines drawn parallel to Normal range is 124°-127° atlanto-occipital joints on coronal images sion of dens more than 4.5 mm above line is abnormal9).