eMedicine Specialties > Orthopedic Surgery > Spine

Os Odontoideum

Author: Eeric Truumees, MD, Consulting Surgeon, Department of Orthopedic Surgery, William Beaumont HospitalOrthopaedic Director, Gehring Biomechanics LaboratoryAdjunct Faculty, Bio-Engineering Center, Wayne State University
Contributor Information and Disclosures

Updated: Sep 12, 2008

Introduction

In 1863, separation of the odontoid process from the body of the axis was first described in a postmortem specimen. In 1886, Giacomini coined the term os odontoideum as a condition in which the dens is separated from the axis body. This entity is clinically important because the mobile or insufficient dens renders the transverse atlantal ligament (TAL) ineffective at restraining atlantoaxial motion. Translation of the atlas on the axis may lead to impingement of the upper cervical cord or vertebral artery.

Os odontoideum is rare, but the exact prevalence and incidence are unknown. Many cases are either incidentally detected or are diagnosed when patients become symptomatic. To date, no large-scale screening studies have been performed.

The age at diagnosis varies significantly from the first to the sixth decades of life. With increased awareness, however, os odontoideum has been diagnosed in younger patients. While the etiology remains controversial, an increased frequency of os odontoideum has been reported in patients with Morquio syndrome,1,2 multiple epiphyseal dysplasia, and/or Down syndrome.

Etiology

Initially, os odontoideum was thought to represent a congenital failure of fusion of the dens to the remainder of the axis. As such, the condition is usually grouped with other craniocervical junction abnormalities, such as dental aplasia and hypoplasia.7,8,9,10,11,12,13

Today, it seems clear that failure of the secondary ossification center of the dens to fuse with the base of the odontoid represents a separate entity known as persistent ossiculum terminale.14  Differentiation between os odontoideum and persistent ossiculum terminale is clinically critical. The ossicle of ossiculum terminale is much smaller than that of os odontoideum. More important, that ossicle lies at the level of the atlantal ring above the transverse atlantal ligament. In this cranial location, ossiculum terminale, unlike os odontoideum, is not associated with significant instability.

Some authors speculate that os odontoideum represents a previous fracture of the odontoid synchondrosis before its closure at age 5-6 years.15,16,17  These authors describe os odontoideum in patients with previously normal cervical radiographs. For example, Schuler et al elegantly described the evolution of an os odontoideum following trauma in a child.18 In this model, os odontoideum develops gradually. Following a fracture of the odontoid synchondrosis, with growth, the alar ligaments carry the dens fragment away from the axis base. The cranial portion of the dens fragment continues to receive a blood supply from the apical arcade. The avascular caudad portion resorbs, leaving the characteristic rounded ossicle.

Authors who favor a congenital basis for os odontoideum point out that the craniovertebral junction is one of the most common sites for malformation. Included are clefts or aplasia of anterior and posterior arches of atlas.7,10,11,17  On the other hand, unlike most congenital malformations, os odontoideum tends to occur as an isolated entity without other regional anomalies.19 Garg et al reported a case of os odontoideum in a myelopathic 16-year-old patient with bipartite atlas. They concluded that coexistence of these conditions support the embryologic basis for os odontoideum. In this case, the dens had an “unusual bony projection” on its anterior surface.

Crockard and Stevens reviewed the embryologic and comparative anatomy data of clinical syndromes associated with craniocervical instability. They concluded that os odontoideum is the product of excessive movement at the time of ossification of the cartilaginous dens and is analogous to the unfused type II odontoid fracture. True hypoplasia of the odontoid peg, on the other hand, was found to be part of a wider segmentation defect associated with Klippel-Feil syndrome,20 occipitalized atlas, or basilar invagination and rarely was found to be associated with instability.21

Sankar and colleagues reviewed 519 consecutive patients with radiographic abnormalities in the occipitocervical region. Os odontoideum was confirmed in 16. Only 3 of those patients had a history of remote trauma. The authors concluded that this supported an embryologic basis for the condition.22

The size of the os odontoideum may vary, but it typically is smaller than the normal dens, particularly at its base. Perhaps there are 2 etiologic groups. Certainly, in patients with other congenital anomalies or odontoid malformations, an embryologic basis may be assumed.  Unfortunately, individual correlations do not “prove” the case one way or the other.  

Regardless of the underlying cause, sound treatment selection requires an understanding of the natural history of this process. We know that, in a subset of patients, the secondary ligamentous restraints become lax, resulting in instability. When the instability has been long-standing, it becomes multidirectional. Less clear are the percentages and rates of progression to instability. As no population studies are available, assumptions are made from case series data of nonoperatively managed patients.

Based on the position of the dens tip, 2 types of os odontoideum are described: orthotopic and dystopic. In the orthotopic type, the dens is in anatomic position. In dystopic os odontoideum, the dens tip is in any other position. Most commonly, the fragment is located near the foramen magnum, where it may fuse with the clivus. Alternatively, the os may be fixed to the anterior ring of the atlas.

Subluxation and instability are described in both types of os odontoideum.  Some authors feel that dystopic os odontoideum is more likely to be symptomatic.

Pathophysiology

A significant, but unknown, percentage of patients with os odontoideum are and remain asymptomatic. Os odontoideum often is incidentally detected after screening or after evaluating a patient following trauma. Given the frequency of asymptomatic os odontoideum, it is difficult to determine with certainty whether the os odontoideum is the true cause of symptoms when they do occur. Typical symptoms include the following:

  • Local mechanical neck pain
  • Torticollis and headache
  • Neurologic symptoms
  • Neurovascular symptoms

Neurologic symptoms may develop in patients with cervical instability.23,24  This neurologic involvement is often limited to one transitory episode of diffuse paresis following trauma. In others, a progressive myelopathy is noted.25  Weakness and ataxia usually predominate over sensory changes. Less frequently, atlantoaxial instability results in vertebral artery compression precipitating neurovascular symptoms. These vascular symptoms arise from cervical cord and brainstem ischemia and encompass a bewildering array of signs and symptoms. Early sequelae include ataxia, syncope, vertigo, and visual disturbances. Later, cerebellar and brainstem infarcts and seizures are seen.26,27  Sudden death is rare but can occur.

In patients suspected of having os odontoideum, a thorough physical examination is mandatory.  This assessment begins with a complete neck and cervical spine examination.  Evaluate for tenderness, range of motion (ROM), and associated anomalies.  A careful neurologic examination should include assessment of cerebellar and brainstem function, gait evaluation, and Romberg testing.  In patients with atlantoaxial instability, upper motor neuron findings are commonly reported, including spasticity, hyperreflexia, clonus, and proprioceptive loss.

Relevant Anatomy

Successful treatment of os odontoideum requires an understanding of the unique anatomic characteristics of the cervicocranium (occiput-C2). The bony elements here develop through enchondral ossification. The tip of the dens and its associated ligaments arise from the fourth occipital through the cervical-0 (C0) somites, which do not ossify until middle childhood.3,4

The base of the dens forms from the C0 and C1 sclerotomes as 2 paired structures that ossify just before birth. The C2 and C3 sclerotomes give rise to the body of the axis, which fuses with the dens at age 4 years (see Image 1).  In a study of human embryos at 8 weeks of gestation, O’Rahilly et al reported that no transverse segmentation formed within the odontoid process at any time. An embryologic anomaly characterized by a complete/partial segmentation of 2 rostral parts will result in bipartite dens rather than an os odontoideum.5

The atlantoaxial joint (C1-C2) consists of biconcave articulations with loose capsules and small contact areas. Stability is therefore conferred by associated ligaments, including the TAL, which is the primary restraint to flexion and extension. Other important restraints include the apical ligaments, the alar ligaments, the tectorial membrane, and the atlanto-occipital membranes

The vertebral arteries are intricately invested in the bony anatomy of the atlantoaxial segment.  They pass just inferior to the C1-C2 facet joint, then course laterally through the transverse foramen of C2. Just above the C1 lateral mass, they turn medially and meet to progress cephalad into the foramen magnum.6  Aberrancy of the vertebral artery course is not rare and may limit fixation options in some patients selected for operative management of os odontoideum. This deviant course may be unilateral or bilateral and is best shown on thin-cut 1-mm CT scan. If the vertebral artery is in the path of a C1-C2 transarticular screw, an alternative fixation strategy must be considered.

More on Os Odontoideum

Overview: Os Odontoideum
Workup: Os Odontoideum
Treatment: Os Odontoideum
Follow-up: Os Odontoideum
Multimedia: Os Odontoideum
References
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References

  1. Lipson SJ. Dysplasia of the odontoid process in Morquio''s syndrome causing quadriparesis. J Bone Joint Surg Am. Apr 1977;59(3):340-4. [Medline].

  2. Stevens JM, Kendall BE, Crockard HA, Ransford A. The odontoid process in Morquio-Brailsford''s disease. The effects of occipitocervical fusion. J Bone Joint Surg Br. Sep 1991;73(5):851-8. [Medline].

  3. McRae D. The significance of abnormalities of the cervical spine. AJR. 1960;84:3-25.

  4. Shaffrey CI, Chenelle AG, Avel MF. Anatomy and physiology of congenital spine lesions. In: Benzel EC, ed. Spine Surgery: Techniques, Complication Avoidance and Management. Churchill Livingstone;1999:265.

  5. O'Rahilly R, Müller F, Meyer DB. The human vertebral column at the end of the embryonic period proper. 2. The occipitocervical region. J Anat. Jan 1983;136(Pt 1):181-95. [Medline].

  6. Yukata K, Katoh S, Sairyo K, Matsui Y, Hamada Y, Yasui N. Os odontoideum in achondroplasia: a case report. J Pediatr Orthop B. Mar 2008;17(2):103-5. [Medline].

  7. Bassett F, Goldner JL. Aplasia of the odontoid process. J Bone Joint Surg. 1968;50A:833-4.

  8. Chang H, Park JB, Kim KW, Choi WS. Retro-dental reactive lesions related to development of myelopathy in patients with atlantoaxial instability secondary to Os odontoideum. Spine. Nov 1 2000;25(21):2777-83. [Medline].

  9. Fielding JW, Griffin PP. Os odontoideum: an acquired lesion. J Bone Joint Surg Am. Jan 1974;56(1):187-90. [Medline].

  10. Gwinn J, Smith J. Acquired and congenital absence of the odontoid process. AJR. 1962;88:424-31.

  11. Hawkins RJ, Fielding JW, Thompson WJ. Os odontoideum: congenital or acquired. A case report. J Bone Joint Surg Am. Apr 1976;58(3):413-4. [Medline].

  12. Morgan MK, Onofrio BM, Bender CE. Familial os odontoideum. Case report. J Neurosurg. Apr 1989;70(4):636-9. [Medline].

  13. Stillwell WT, Fielding JW. Acquired os odontoideum. A case report. Clin Orthop. Sep 1978;(135):71-3. [Medline].

  14. Wollin D. The os odontoideum. Separate odontoid process. J Bone Joint Surg. 1963;45-A:1459-1471.

  15. Hukuda S, Ota H, Okabe N, Tazima K. Traumatic atlantoaxial dislocation causing os odontoideum in infants. Spine. May-Jun 1980;5(3):207-10. [Medline].

  16. Kuhns LR, Loder RT, Farley FA, Hensinger RN. Nuchal cord changes in children with os odontoideum: evidence for associated trauma. J Pediatr Orthop. Nov-Dec 1998;18(6):815-9. [Medline].

  17. Brecknell JE, Malham GM. Os odontoideum: report of three cases. J Clin Neurosci. Mar 2008;15(3):295-301. [Medline].

  18. Schuler TC, Kurz L, Thompson DE, et al. Natural history of os odontoideum. J Pediatr Orthop. Mar-Apr 1991;11(2):222-5. [Medline].

  19. Garg A, Gaikward S, Gupta V, et al. Bipartite Atlas With Os Odontoideum: Case Report. Spine. January 2004 15;29(2):E35-E38.

  20. Mintken PE, Metrick L, Flynn TW. Upper cervical ligament testing in a patient with os odontoideum presenting with headaches. J Orthop Sports Phys Ther. Aug 2008;38(8):465-75. [Medline].

  21. Crockard HA, Stevens JM. Craniovertebral junction anomalies in inherited disorders: part of the syndrome or caused by the disorder?. Eur J Pediatr. Jul 1995;154(7):504-12. [Medline].

  22. Sankar W, Wills B, Dormans J, Drummond D. Os Odontoideum Revisited: The Case for a Multifactorial Etiology. Spine. 2006;31:979-984.

  23. Dyck P. Os odontoideum in children: neurological manifestations and surgical management. Neurosurgery. Mar-Apr 1978;2(2):93-9. [Medline].

  24. Rowland L, Shapiro J, Jacobson H. Neurological syndromes associated with congenital absence of the odontoid process. Arch Neurol Psych. 1958;80:286-291.

  25. Watanabe M, Toyama Y, Fujimura Y. Atlantoaxial instability in os odontoideum with myelopathy. Spine. Jun 15 1996;21(12):1435-9. [Medline].

  26. Bhatnagar M, Sponseller PD, Carroll C IV, Tolo VT. Pediatric atlantoaxial instability presenting as cerebral and cerebellar infarcts. J Pediatr Orthop. Jan-Feb 1991;11(1):103-7. [Medline].

  27. Sasaki H, Itoh T, Takei H, Hayashi M. Os odontoideum with cerebellar infarction: a case report. Spine. May 1 2000;25(9):1178-81. [Medline].

  28. Minderhoud JM, Braakman R, Penning L. Os odontoideum, clinical, radiological and therapeutic aspects. J Neurol Sci. May-Jun 1969;8(3):521-44. [Medline].

  29. Satte A, Ech-Cherif El Kettani N, El Quessar A, El Hassani MR, Chakir N, Boukhrissi N, et al. [Os odontoideum: clinical and radiological aspects]. Rev Neurol (Paris). Feb 2008;164(2):177-80. [Medline].

  30. Surov A, Hess S, Spielmann RP, Kunze C. A delayed diagnosis of an unstable os odontoideum. Eur J Med Res. Mar 31 2008;13(3):136-8. [Medline].

  31. Holt RG, Helms CA, Munk PL, Gillespy T 3rd. Hypertrophy of C-1 anterior arch: useful sign to distinguish os odontoideum from acute dens fracture. Radiology. Oct 1989;173(1):207-9. [Medline].

  32. Hadley MN. Os Odontoideum. Neurosurgery. March 2002;50(3) Supplement:S148-S155.

  33. Aksoy FG, Gomori JM. Symptomatic cervical synovial cyst associated with an os odontoideum diagnosed by magnetic resonance imaging: case report and review of the literature. Spine. May 15 2000;25(10):1300-2. [Medline].

  34. Hughes TB Jr, Richman JD, Rothfus WE. Diagnosis of Os odontoideum using kinematic magnetic resonance imaging. A case report. Spine. Apr 1 1999;24(7):715-8. [Medline].

  35. shellock FG. Functional Assessment of the Joints Using Kinematic Magnetic Resonance Imaging. Seminars in Musculoskeletal Radiology. Dynamic and Functional Musculoskeletal Imaging. 2003;7(4):249-276.

  36. Hosono N, Yonenobu K, Ebara S, Ono K. Cineradiographic motion analysis of atlantoaxial instability in os odontoideum. Spine. Oct 1991;16(10 Suppl):S480-2. [Medline].

  37. Hensinger RN. Congenital anomalies of the cervical spine. In: Herkowitz HN, Garfin SR, Balderston RA, et al, eds. The Spine. Lippincott-Raven;1999:242-243.

  38. Hensinger RN. Osseous anomalies of the craniovertebral junction. Spine. May 1986;11(4):323-33. [Medline].

  39. Hensinger RN, Fielding JW, Hawkins RJ. Congenital anomalies of the odontoid process. Orthop Clin North Am. Oct 1978;9(4):901-12. [Medline].

  40. An H, ed. Congenital anomalies of the cervical spine. In: Principles and Techniques of Spine Surgery. 1st ed. Lippincott Williams & Wilkins;1997:165.

  41. Callahan RA, Lockwood R, Green B. Modified Brooks fusion for an os odontoideum associated with an incomplete posterior arch of the atlas. A case report. Spine. Jan-Feb 1983;8(1):107-8. [Medline].

  42. Brockmeyer DL, York JE, Apfelbaum RI. Anatomical suitability of C1-2 transarticular screw placement in pediatric patients. J Neurosurg. Jan 2000;92(1 Suppl):7-11. [Medline].

  43. Jun BY. Complete reduction of retro-odontoid soft tissue mass in os odontoideum following the posterior C1-C2 tranarticular screw fixation. Spine. Sep 15 1999;24(18):1961-4. [Medline].

  44. Prabhu VC, France JC, Voelker JL, Zoarski GH. Vertebral artery pseudoaneurysm complicating posterior C1-2 transarticular screw fixation: case report. Surg Neurol. Jan 2001;55(1):29-33; discussion 33-4. [Medline].

  45. Reilly CW, Choit RL. Transarticular screws in the management of C1-C2 instability in children. J Pediatr Orthop. Sep-Oct 2006;26(5):582-8. [Medline].

  46. Motosuneya T, Hirabayashi S, Yamada H, Kobayashi Y, Sekiya S, Sakai H. Posterior atlantoaxial subluxation due to os odontoideum combined with cervical spondylotic myelopathy: a case report. Eur Spine J. Sep 2008;17 Suppl 2:S275-9. [Medline].

  47. Wang J, Vokshoor A, Kim S, et al. Pediatric atlantoaxial instability: management with screw fixation. Pediatr Neurosurg. Feb 1999;30(2):70-8. [Medline].

  48. Takakuwa T, Hiroi S, Hasegawa H, et al. Os odontoideum with vertebral artery occlusion. Spine. Feb 15 1994;19(4):460-2. [Medline].

  49. Stulík J, Vyskocil T, Sebesta P, Kryl J. [Harms technique of C1-C2 fixation with polyaxial screws and rods]. Acta Chir Orthop Traumatol Cech. 2005;72(1):22-7. [Medline].

  50. Lamas E, Estevez J, Castillo R, Esparza J. Os odontoideum removed by a transoral approach. Surg Neurol. May 1977;7(5):312-4. [Medline].

  51. Laus M, Pignatti G, Malaguti MC, et al. Anterior extraoral surgery to the upper cervical spine. Spine. Jul 15 1996;21(14):1687-93. [Medline].

  52. Chang H, Park JB, Kim KW. Synovial cyst of the transverse ligament of the atlas in a patient with os odontoideum and atlantoaxial instability. Spine. Mar 15 2000;25(6):741-4. [Medline].

  53. Park J, Huhn SL, Lee M, et al. A Definitive Surgical Option without a Halo or Occipital Fusion. Neurosurgery. August 2005;57(2):437.

  54. Juhl M, Seerup KK. Os odontoideum. A cause of atlanto-axial instability. Acta Orthop Scand. Feb 1983;54(1):113-8. [Medline].

  55. Dai L, Yuan W, Ni B, Jia L. Os odontoideum: etiology, diagnosis, and management. Surg Neurol. Feb 2000;53(2):106-8; discussion 108-9. [Medline].

  56. Taggard DA, Menezes AH, Ryken TC. Treatment of Down syndrome-associated craniovertebral junction abnormalities. J Neurosurg. Oct 2000;93(2 Suppl):205-13. [Medline].

  57. Spierings EL, Braakman R. The management of os odontoideum. Analysis of 37 cases. J Bone Joint Surg Br. 1982;64(4):422-8. [Medline].

  58. Smith MD, Phillips WA, Hensinger RN. Fusion of the upper cervical spine in children and adolescents. An analysis of 17 patients. Spine. Jul 1991;16(7):695-701. [Medline].

  59. Fielding JW, Hensinger RN, Hawkins RJ. Os Odontoideum. J Bone Joint Surg Am. Apr 1980;62(3):376-83. [Medline].

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Further Reading

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Keywords

os odontoideum, atlanto-axial instability, atlantoaxial instability, cervical instability, atlanto-axial joint, atlantoaxial joint, atlas bone, axis bone

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Contributor Information and Disclosures

Author

Eeric Truumees, MD, Consulting Surgeon, Department of Orthopedic Surgery, William Beaumont HospitalOrthopaedic Director, Gehring Biomechanics LaboratoryAdjunct Faculty, Bio-Engineering Center, Wayne State University
Eeric Truumees, MD is a member of the following medical societies: Alpha Omega Alpha, American Academy of Orthopaedic Surgeons, American Orthopaedic Association, Cervical Spine Research Society, Michigan State Medical Society, Mid-America Orthopaedic Association, and North American Spine Society
Disclosure: Stryker Spine Consulting fee Consulting; DePuy Spine Consulting fee Consulting; Stryker Spine Royalty Other

Medical Editor

Lee H Riley III, MD, Chief, Division of Orthopedic Spine Surgery, Assistant Professor, Departments of Orthopedic Surgery and Neurosurgery, Johns Hopkins University
Disclosure: Nothing to disclose.

Pharmacy Editor

Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine
Disclosure: eMedicine Salary Employment

Managing Editor

William O Shaffer, MD, Professor, Vice-Chairman and Residency Program Director, Department of Orthopedic Surgery, University of Kentucky at Lexington
William O Shaffer, MD is a member of the following medical societies: American Academy of Orthopaedic Surgeons, American Orthopaedic Association, International Society for the Study of the Lumbar Spine, Kentucky Medical Association, Kentucky Orthopaedic Society, North American Spine Society, Southern Medical Association, and Southern Orthopaedic Association
Disclosure: DePuySpine 1997-2007 (not presently) Royalty Consulting; DePuySpine 2002-2007 (closed) Grant/research funds SacroPelvic Instrumentation Biomechanical Study; DePuyBiologics 2005-2008 (closed) Grant/research funds Healos study just closed; No present Industry grants or funds. None None

CME Editor

Dinesh Patel, MD, FACS, Associate Clinical Professor of Orthopedic Surgery, Harvard Medical School; Chief of Arthroscopic Surgery, Department of Orthopedic Surgery, Massachusetts General Hospital
Dinesh Patel, MD, FACS is a member of the following medical societies: American Academy of Orthopaedic Surgeons, American Association of Physicians of Indian Origin, American College of International Physicians, and American College of Surgeons
Disclosure: Nothing to disclose.

Chief Editor

Mary Ann E Keenan, MD, Professor, Vice Chair for Graduate Medical Education, Department of Orthopedic Surgery, University of Pennsylvania School of Medicine; Chief of Neuro-Orthopedics Program, Department of Orthopedic Surgery, Hospital of the University of Pennsylvania
Mary Ann E Keenan, MD is a member of the following medical societies: Alpha Omega Alpha, American Academy of Orthopaedic Surgeons, American Orthopaedic Association, American Orthopaedic Foot and Ankle Society, American Society for Surgery of the Hand, and Orthopaedic Rehabilitation Association
Disclosure: Nothing to disclose.

 
 
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