Figure 1. Illustration of the C2 vertebra showing the locations of the pedicle and pars interarticularis. (A) Superior view. Note the illustration of the right showing the hemitubular pedicle after removal of the superior facet. (B) Lateral (left) and inferior (right) views. (Reprinted with permission from Ebraheim NA, Fow J, Xu R, Yeasting RA. The location of the pedicle and pars interarticularis in the axis.Spine.2001;26:E34-E37.)

Figure 2. A 32-year-old woman with mildly asymmetric traumatic spondylolisthesis, Effendi I. (A) Radiograph shows a 2- to 3-mm wide fracture through the partes interarticulares. There is mild anterior translation of the C2 vertebral body relative to C3 with a normal disk space. Also note the anterior translation of the posterior arch of C1 relative to C2. (B and C) Sequential axial images through the fracture show involvement of the pars interarticularis. (D) Right, (E) mid-, and (F) left sagittal images show slightly asymmetric fractures through the partes interarticulares. Note the anterior translation of the C2 vertebral body without widening of the disk. The facet joints at C2-C3 are normal.

Figure 3. A 16-year-old adolescent with asymmetric traumatic spondylolisthesis, Effendi I. (A) Lateral radiograph reveals a minimally displaced fracture through the pars interarticularis (arrows). The fracture is partially obscured by the superimposed inferior articular facet. (B) Axial CT image shows a pars interarticularis fracture on the right and a fracture involving the posterior aspect of the vertebral body and the transverse process on the left. Asymmetric fractures are frequently encountered. (C) Right sagittal image shows an oblique fracture through the pars interarticularis. (D) Fracture through the anterior aspect of the left pars interarticularis. The fracture extends into the vertebral body and transverse process on other images, not shown.

Figure 4. A 21-year-old woman with asymmetric atypical traumatic spondylolisthesis, Effendi II. (A) Radiograph shows a posteriorly located fracture involving the lamina superiorly and extending to the inferior articular facet. Fracture on the opposite side involves a small fragment of the posterior vertebral body. The C2 vertebral body and the dens are tilted in flexion. Prevertebral soft-tissue swelling is present. (B and C) Axial images illustrate the right pars interarticularis fracture. The left fracture extends from the vertebral body, to the left pars, and then to left transverse process. (D) Right sagittal image shows the right pars fracture extending to the margin of the facet joint. (E) Midsagittal image illustrates flexion of the C2 vertebral body and dens relative to C3. The posteroinferior vertebral body fragment is characteristic of the "atypical" pattern.

Figure 5. A 61-year-old man with traumatic spondylolisthesis characterized by anterior translation and extension. Radiograph shows marked anterior translation of C2 vertebral body relative to C3. Note fracture lines through the anterior portion of the partes interarticulares extending into the posteroinferior aspects of the vertebral body bilaterally.

Figure 6. A male patient with traumatic spondylolisthesis and bilateral facet joint dislocation, Effendi III. The partes interarticulares are fractured with anterior translation and flexion of the vertebral body. The facet joints at C2-C3 are dislocated bilaterally. The anteroinferior corner of the C2 vertebral body has been avulsed. Note severe prevertebral soft-tissue swelling with resultant compromise of the pharyngeal airway. (Image courtesy of the John H. Harris, Jr., MD, DSc, collection.)

Figure 7. A 6-year-old with traumatic spondylolisthesis and bilateral facet joint subluxation, Effendi III. (A) Conventional radiograph obtained after cervical halo placement reveals severe subluxation of the C2-C3 facets bilaterally. Hairline fracture through the pars interarticularis (arrow) is noted. (B and C) Two axial images show the minimally displaced fracture through the partes interarticulares. (D) Right sagittal image shows perched C2-C3 facets (arrow) and hairline pars fracture. (E) Midsagittal image reveals kyphotic angulation of C2 relative to C3. (F) Left sagittal image shows severe facet subluxation (arrow) and hairline pars fracture.

Figure 8. A 47-year-old man with multiple cervical spine fractures, including bilateral lamina fractures of C2, Effendi II. (A) Postoperative plain film focusing on C2 shows bilateral overlapping fractures of the lamina (arrow). (B) Axial image through the neural arch of C2 shows minimally displaced fractures at the right spinolaminar junction, and in the midportion of the left lamina. Minimally displaced laminar fractures are classified as Effendi I.

Figure 9. A 49-year-old woman with symmetric traumatic spondylolisthesis, Effendi I, with associated hyperextension teardrop fracture. (A) Conventional radiograph exhibits prevertebral soft-tissue swelling and a large triangular fragment avulsed from the anteroinferior corner of the C2 vertebral body (white arrow). Fractures of the partes interarticulares are difficult to appreciate. The most apparent manifestation is disruption of Harris' ring posteriorly (black arrows). (B) Axial CT image shows mildly displaced fractures through the partes interarticulares. (C) Right sagittal image shows extension of the fracture from the pars interarticularis into the inferolateral portion of the vertebral body. (D) Midsagittal image illustrates the triangular hyperextension teardrop fracture. (E) Left sagittal image shows a fracture of the leftpars interarticularis that is nearly a mirror image of the opposite side. The mild displacement with normal C2-C3 disk space indicates an Effendi I injury.

Figure 10. A 38-year-old man with traumatic spondylolisthesis, Effendi IIA, with wide distraction of the C2-C3 disk. (A) Conventional lateral radiograph shows fractures through both partes interarticulares. The C2-C3 intervertebral disk space is 3 times normal width. One of the two C2-C3 facet joints is also widened and subluxed. A fragment of the left lateral mass, which could be mistaken for a dislocated inferior articular facet, projects through the widened C2-C3 disk space. (B) Axial CT shows fractures through the partes interarticulares. (C) Right sagittal image shows the typical fracture line of traumatic spondylolisthesis. (D) Midsagittal image emphasizes the severe widening of the C2-C3 intervertebral disk. (E) Left sagittal image shows a mirror image fracture line, but also illustrates facet joint widening and subluxation at C2-C3.

Figure 11. A 38-year-old woman with atypical traumatic spondylolisthesis, Effendi II. (A) Conventional lateral radiograph shows oblique fracture through both partes interarticulares. On one side, the fracture involves a large fragment of the posterior vertebral body that impinges upon the spinal canal (arrow). The remainder of the C2 vertebral body has translated anteriorly. (B) Axial CT shows a displaced fragment on the left side of the C2 vertebral body impinging upon the spinal canal, the defining feature of the “atypical’ pattern.

Figure 12. A 57-year-old woman with asymmetric atypical traumatic spondylolisthesis, Effendi I. (A) Fracture through the right pars interarticularis is shown on this axial image. (B) More caudally, the fracture on the right extends into the posteroinferior vertebral body with minimal posterior displacement of the fracture fragment. A minimally displaced fracture is present in the left lamina. (C and D) Two sequential sagittal images show the fracture through the pars interarticularis extending into the posterior vertebral body. The lack of displacement of the body fragment makes the atypical fracture pattern less clinically significant. (E) Left sagittal image shows a fracture line through the left lamina extending to the pars inferiorly.

Figure 13. An 89-year-old woman with Type III C2 vertebral body fracture illustrating the "fat C2" sign. (A) Lateral conventional radiograph shows a comminuted fracture of the C2 vertebral body with posterior displacement and fragmentation of the posterior cortex (arrows). Note the appearance of widening between the anterior and posterior cortical lines of C2 -- the "fat C2" sign. (B) Midsagittal CT image shows posterior displacement of the C2 vertebral body. (C) Axial image through the upper C2 vertebral body shows a comminuted fracture involving the body and superior articular facets. (D) Axial CT image through the inferior vertebral body shows coronally oriented fracture with posterior displacement of the posterior wall of the C2 body.

Figure 14. A 29-year-old man with hyperextension teardrop fracture of C2. (A) Lateral conventional radiograph shows a triangular fragment at the anteroinferior corner of the C2 vertebral body. There is marked prevertebral soft-tissue swelling. (B) Sagittal image clearly reveals the triangular fracture fragment. (C and D) Two sequential axial images illustrate the ease with which even large, substantially displaced hyperextension teardrop fractures may be overlooked in the axial plane.

Figure 15. A 36-year old woman with Type II C2 body fracture. (A and B) Axial images through the upper and midportions of the C2 vertebral body exhibit an obliquely oriented fracture involving the right C1-C2 joint and extending across the C2 body inferiorly. (C) Coronal image shows the oblique orientation of the fracture and involvement of the right superior articular facet.

Figure 16. A 74-year-old woman with Type IV C2 body fracture. (A) Lateral conventional radiograph shows fat C2 sign. Note the 6-mm posterior displacement of the posterior cortex of C2 vertebral body relative to the posterior cortex of the dens. Also note the absence of prevertebral soft-tissue swelling. (B) Coronal image shows an obliquely oriented, comminuted fracture extending from the left superior articular facet of C2 to the right side of the inferior endplate. (C, D, and E) Axial CT images through the (C) superior, (D) middle, and (E) inferior portions of the C2 vertebral body show the obliquely oriented, comminuted fracture.

Figure 17. A 59-year old man with minimally displaced left lateral mass fracture of C2. (A and B) Two sequential axial CT images show a hairline fracture through the left superior articular facet.

Figure 18. An 18-year-old man with a depressed left lateral mass fracture. (A and B) Two sequential axial images reveal a comminuted fracture of the right lateral mass. A large fracture fragment is displaced anteriorly. (C) Coronal image shows depression of the right lateral mass. (D) Sagittal image shows a centrally depressed fracture fragment. Note abundant prevertebral soft-tissue gas.