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Condylus Tertius
Jeffrey S. Ross, MD
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KEY FACTS

  • Terminology

    • Top Differential Diagnoses

      • Clinical Issues

        TERMINOLOGY

        • Synonyms

          • 3rd occipital condyle
        • Definitions

          • Condylus tertius
            • Bony process in median line at front rim of foramen magnum, articulated with tip of dens or anterior C1 arch
            • Uncommon variation = 0.5-5% incidence
            • 1st described by J. F. Meckel in 1815
            • Medial residue of hypochordal arch of proatlas (4th occipital sclerotome)
            • Typically single, but supranumerary ossicles may be present
            • Forms joint or pseudojoint with clivus, odontoid process, or anterior arch of C1
          • Basilar process (processus basilaris)
            • Bony variant in similar region as condylus tertius but with slightly different imaging features and different embryology
            • Also known as mamillary or papillary processes
            • Uncommon = 4% incidence
            • Unilateral or bilateral, paramedian, sphere-shaped bony projections off of anterior inferior margin of foramen magnum
            • Lateral residue of hypochordal arch of proatlas
              • Variable persistence of lateral residue gives broad mix of morphology (unilateral, bilateral, small, large)

        IMAGING

        • General Features

          • Imaging Recommendations

            DIFFERENTIAL DIAGNOSIS

              PATHOLOGY

              • General Features

                • Staging, Grading, & Classification

                  CLINICAL ISSUES

                  • Presentation

                    Selected References

                    1. McKay SD et al: Review of cervical spine anomalies in genetic syndromes. Spine (Phila Pa 1976). 37(5):E269-77, 2012
                    2. Muhleman M et al: The proatlas: a comprehensive review with clinical implications. Childs Nerv Syst. 28(3):349-56, 2012
                    3. Klimo P Jr et al: Incidental os odontoideum: current management strategies. Neurosurg Focus. 31(6):E10, 2011
                    4. Goel A et al: Unusual bone formation in the anterior rim of foramen magnum: cause, effect and treatment. Eur Spine J. 19 Suppl 2:S162-4, 2010
                    5. Menezes AH et al: Remnants of occipital vertebrae: proatlas segmentation abnormalities. Neurosurgery. 64(5):945-53; discussion 954, 2009
                    6. Hanihara T et al: Frequency variations of discrete cranial traits in major human populations. III. Hyperostotic variations. J Anat. 199(Pt 3):251-72, 2001
                    7. Prescher A et al: Anatomic and radiologic appearance of several variants of the craniocervical junction. Skull Base Surg. 6(2):83-94, 1996
                    8. Smoker WR: Craniovertebral junction: normal anatomy, craniometry, and congenital anomalies. Radiographics. 14(2):255-77, 1994
                    Related Anatomy
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                    Related Differential Diagnoses
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                    References
                    Tables

                    Tables

                    KEY FACTS

                    • Terminology

                      • Top Differential Diagnoses

                        • Clinical Issues

                          TERMINOLOGY

                          • Synonyms

                            • 3rd occipital condyle
                          • Definitions

                            • Condylus tertius
                              • Bony process in median line at front rim of foramen magnum, articulated with tip of dens or anterior C1 arch
                              • Uncommon variation = 0.5-5% incidence
                              • 1st described by J. F. Meckel in 1815
                              • Medial residue of hypochordal arch of proatlas (4th occipital sclerotome)
                              • Typically single, but supranumerary ossicles may be present
                              • Forms joint or pseudojoint with clivus, odontoid process, or anterior arch of C1
                            • Basilar process (processus basilaris)
                              • Bony variant in similar region as condylus tertius but with slightly different imaging features and different embryology
                              • Also known as mamillary or papillary processes
                              • Uncommon = 4% incidence
                              • Unilateral or bilateral, paramedian, sphere-shaped bony projections off of anterior inferior margin of foramen magnum
                              • Lateral residue of hypochordal arch of proatlas
                                • Variable persistence of lateral residue gives broad mix of morphology (unilateral, bilateral, small, large)

                          IMAGING

                          • General Features

                            • Imaging Recommendations

                              DIFFERENTIAL DIAGNOSIS

                                PATHOLOGY

                                • General Features

                                  • Staging, Grading, & Classification

                                    CLINICAL ISSUES

                                    • Presentation

                                      Selected References

                                      1. McKay SD et al: Review of cervical spine anomalies in genetic syndromes. Spine (Phila Pa 1976). 37(5):E269-77, 2012
                                      2. Muhleman M et al: The proatlas: a comprehensive review with clinical implications. Childs Nerv Syst. 28(3):349-56, 2012
                                      3. Klimo P Jr et al: Incidental os odontoideum: current management strategies. Neurosurg Focus. 31(6):E10, 2011
                                      4. Goel A et al: Unusual bone formation in the anterior rim of foramen magnum: cause, effect and treatment. Eur Spine J. 19 Suppl 2:S162-4, 2010
                                      5. Menezes AH et al: Remnants of occipital vertebrae: proatlas segmentation abnormalities. Neurosurgery. 64(5):945-53; discussion 954, 2009
                                      6. Hanihara T et al: Frequency variations of discrete cranial traits in major human populations. III. Hyperostotic variations. J Anat. 199(Pt 3):251-72, 2001
                                      7. Prescher A et al: Anatomic and radiologic appearance of several variants of the craniocervical junction. Skull Base Surg. 6(2):83-94, 1996
                                      8. Smoker WR: Craniovertebral junction: normal anatomy, craniometry, and congenital anomalies. Radiographics. 14(2):255-77, 1994