The term os odontoideum (OO) refers to an anatomic anomaly of the upper cervical spine which was first described by Giacomini in 1886. This condition is defined radiologically as an oval or round-shaped ossicle with smooth circumferential cortical margins representing a hypoplastic odontoid process (dens) that has no continuity with the C2 vertebral body. (See Figure). The ossicle is usually cranially migrated relative to the expected position of the odontoid tip and can adopt two anatomic types: orthotopic (ossicle located in the position of the normal odontoid) and dystopic (ossicle located near the occiput in the area of the foramen magnum).
Researchers have described variable radiological sizes regarding OO. However, most present half the size of a normal odontoid process, some are so small and cephalic that it may be hard to diagnose on plain X-rays or computed tomography (CT). The OO is often attached to the anterior arch of C1 through an intact transverse ligament.
One of the main risks of this anatomical entity is the association of anterior atlantoaxial subluxation. Posterior atlantoaxial subluxation is extremely rare. This atlantoaxial instability can lead to cervical spinal stenosis with resultant cervical myelopathy due to vascular compromise, bony compression, and/or stretching of the spinal cord.
The etiology/pathogenesis of this lesion remains controversial. However, there are two theories about the origin of os odontoideum: traumatic or congenital.
Most studies favor the hypothesis of the traumatic theory, considering it as an acquired pathology resulting from avascular necrosis caused by an odontoid fracture. The acquired theory purports that the os odontoideum forms after a traumatic event, with the preservation of the blood supply to the fractured odontoid tip; this can occur in either the prenatal or postnatal periods, and therefore may not be recalled by the patient. There have been case reports documenting the formation of an os odontoideum after trauma in patients with previously documented normal odontoid processes.
On the other hand, the congenital hypothesis considers it a segmental defect, which represented a failed fusion of odontoid and axis vertebral body, an incomplete migration of the axis centrum, failure of segmentation, and/or nontraumatic avascular necrosis. The congenital theory may explain os odontoideum in identical twins without a history of trauma or reports of familial expression.
This thought line is reinforced by Straus et al., who demonstrated trends in gene expression profiles between os odontoideum patients and normal subjects.
The congenital theory has support from certain associations with upper cervical anomalies such as hypoplastic posterior arch of C1, Klippel-Feil syndrome, trisomy 21 syndrome, Morquio syndrome, or neurofibromatosis.
Os odontoideum is a rare entity with an unknown estimated prevalence due to the usual asymptomatic course and the absence of large-scale screening studies.
Perdikakis et al. retrospectively reviewed the magnetic resonance (MR) of the odontoid process configuration in 133 patients, aged between 19 and 81 years old, finding OO in one case (0.7%).
Embryological pathophysiologic development:
Ossiculum terminale or the apical odontoid epiphysis: Odontoid apex is derived from the fourth occipital sclerotome
The odontoid and axis bodies: Develop from the first and second cervical sclerotomes
Neurocentral synchondrosis: The epiphyseal growth plate that separates the first and second cervical sclerotomes
Terminal apical arcade of blood vessels - The odontoid process depends on this arcade mostly for its blood supply and this anastomoses caudally with the deep penetrating branches arising from the posterior ascending arteries. The latter arise from the vertebral artery. Since the blood supply to the odontoid is precarious in this way, any vascular insufficiency of the terminal arcade can lead to ischemia and necrosis during embryologic development.
Clinical presentation may vary widely, ranging from asymptomatic incidental findings on imaging to neck discomfort and/or neurological deficits with permanent paralysis in severe cases.
Its initial clinical presentation is often nonspecific with neck pain, shoulder pain, torticollis or headache and upper extremity paresthesia like intermittent tingling and numbness in the neck and upper limbs. Other common complaints are lower limb weakness and gait difficulties. These symptoms are thought to result from static or dynamic compression or repeated minor trauma to the spinal cord.
Increased motion at the C1-C2 level can lead to vertebral artery occlusion, ischemia of the brainstem and posterior fossa structures, resulting in seizures, syncope, vertigo, visual disturbances and even sudden death after minor trauma.
During the physical examination, it is common to find neurological deficits or loss of motors milestones. Hypoesthesia, hyperreflexia (patellar tendon reflex), scapulohumeral (Shimizu) reflex, Tromner reflex, Hoffmann reflex, Babinski reflex, and clonus may result in positive in chronic spinal cord compression. Spastic or broad-based gait and decreased hand dexterity are signs of myelopathy. All those previous sings and symptoms are nonspecific, and usually, another diagnosis that causes myelopathy should be ruled out.
Early and accurate diagnosis is critical in mitigating morbidity and mortality. Minor trauma in undiagnosed cervical instability might end in catastrophic neurological results.
Routine anteroposterior and lateral cervical spine radiographs with an open-mouth odontoid are the first approach to this condition and can make the diagnosis.
Lateral radiograph of the cervical spine demonstrates widening of the anterior atlantoaxial distance and disruption of the spinolaminar junction line posteriorly. A well-corticated rounded ossicle located just dorsal and slightly cranial to the anterior arch of C1 presents in this projection without prevertebral soft tissue widening. Cervical spine flexion-extension radiographs help to determine the degree of atlantoaxial instability.
Some authors also perform bedside lateral radiographs during preoperative skull traction as they consider them to be very important for atlantoaxial joint instability and subluxation, and could help determine the surgical method used.
Computed tomography images confirm the corticated margin of this ossicle. It shows a shortened odontoid process and a smooth ossicle of bone. Reconstruction of CT images may show the congenital incomplete union of the C1 posterior arch. A hypertrophied anterior arch of C1 often suggests underlying chronic instability.
CT scan is also used to assess anatomical landmarks, as well as length and direction of the pedicles and vascular anatomy (VA), especially for the presence or absence of an anomalous VA.
Magnetic resonance imaging (MRI) demonstrates fluid within the widened anterior atlantoaxial space. It is possible to find a focal narrowing of the spinal canal with the posteriorly tilted odontoid process abutting the ventral cervical spinal cord, which may contain some areas of increased T2-weighted signal within the central gray matter, most consistent with myelomalacia.
There are some measurements taken at the lateral cervical radiographs or at the sagittal CT reconstruction that can help us to define cervical instability or surgical approach (See Figure).
Treatment options vary from no therapy and control to surgical treatment depending on symptoms and atlantoaxial instability, assessed by radiological images.
For asymptomatic patients, there is an ongoing debate between conservative therapies with imaging follow-up versus prophylactic spinal fusion. In cases without symptoms and no evidence of C1–C2 instability, regular clinical and radiographic follow-up, with avoidance of all contact sports is recommended.
Meanwhile, asymptomatic patients with radiographic evidence of atlantoaxial instability need to consider surgery, although there is a controversy regarding whether C1–C2 fusion can significantly influence neck rotation and daily activities of patients. Although the procedure could induce the loss of normal neck rotation by as much as 50%, the remaining joints could compensate for most of the function of C1–C2. The benefits of avoiding neurological compromise secondary to cord compression outweigh the loss of neck rotation associated with this procedure.
Surgery is usually indicated in patients who had one of the following conditions: neurological involvement (even if this is transient), radiological signs of instability (more than 5 mm of translation in flexion and extension X-rays), progressive instability, persistent neck pain associated with atlantoaxial instability.
The principles of treatment are to prevent sudden death from neurological compromise, improve the neurological status, stabilize the cervical spine, and improve the quality of life.
The treatment strategies for this condition can be resume in four steps:
Surgical options include atlantoaxial fusion, occiput-C2 fusion, and occiput-C3 fusion. The decision depends on spinal cord compression location, area for arthrodesis, and bone quality. All of them can be complemented with an additional transoral decompression in cases of irreducible subluxation of C1–C2. However, this procedure often has accompanying complications such as wound infection, cerebrospinal fluid leakage, neurological injury, and hardware loosening.
Occipito-cervical fusion is indicated in occipito-cervical instability or failed attempt of atlantoaxial fusion, also suggested in cases of poor bone quality due to the increased risk of screw pull-out.
Summary of treatment options:
OO is one of the causes of atlantoaxial instability and dislocation, the differential diagnosis that can produce this instability should be ruled out, such as:
The prognosis of this condition may vary according to symptoms and radiological parameters previously mentioned. The definition of C1-C2 instability is greater than 3 mm of anterior and posterior displacement of the atlas on the axis in adults and more than 4 to 5 mm in children. Greater displacement predicts the poorest outcomes. The degree of atlantoaxial instability is less sensitive than the absolute diameter of the spinal canal in predicting poor outcomes. On the other hand, a posterior atlanto-dens interval (PADI) less than 12 mm is a predictor for the development of paralysis in all causes of C1-C2 instability.
Complications of this entity rely on the severity of cord compression, instability, and unexpected events since minor trauma can lead to catastrophic neurological damage, including sudden death. This risk is due to the potential risk of C1-C2 instability secondary to abnormal development of the dens. Proper detection of radiological signs of instability is imperative to avoid complications related to spinal cord compression.
Upon arriving at the diagnosis of symptomatic os odontoideum, surgical treatment should be a consideration. Conservative treatment is an option only when the patient is asymptomatic, and there is no evidence of atlantoaxial instability. The patients should be aware of the possible catastrophic consequences of this pathology. Regular clinical and radiographic follow-up is necessary, and clinicians should strongly advise the avoidance of all contact sports.
If the patient points out any symptoms relative to this condition, or there is radiological evidence of C1-C2 instability, surgery is the most appropriate treatment.
Os odontoideum is an uncommon diagnosis. However, early recognition and proper identification of radiological and clinical signs of instability may help to avoid catastrophic complications secondary to spinal cord compression. Once the physician has confirmation of the diagnosis, patients and their families require education from the surgeon and orthopedic nurses about this condition and its possible outcomes.
Os odontoideum requires an interprofessional team approach, including physicians, surgeons, specialists, and specialty-trained nurses, all collaborating across disciplines to achieve optimal patient results. [Level V]
|||Perrini P,Montemurro N,Iannelli A, The contribution of Carlo Giacomini (1840-1898): the limbus Giacomini and beyond. Neurosurgery. 2013 Mar; [PubMed PMID: 23208067]|
|||Spierings EL,Braakman R, The management of os odontoideum. Analysis of 37 cases. The Journal of bone and joint surgery. British volume. 1982; [PubMed PMID: 7096415]|
|||Matsui H,Imada K,Tsuji H, Radiographic classification of Os odontoideum and its clinical significance. Spine. 1997 Aug 1; [PubMed PMID: 9259779]|
|||Klassov Y,Benkovich V,Kramer MM, Post-traumatic os odontoideum - case presentation and literature review. Trauma case reports. 2018 Dec; [PubMed PMID: 30533483]|
|||Arvin B,Fournier-Gosselin MP,Fehlings MG, Os odontoideum: etiology and surgical management. Neurosurgery. 2010 Mar; [PubMed PMID: 20173524]|
|||Robson KA, Os odontoideum: rare cervical lesion. The western journal of emergency medicine. 2011 Nov; [PubMed PMID: 22224150]|
|||Pluemvitayaporn T,Kunakornsawat S,Piyaskulkaew C,Pruttikul P,Pongpinyopap W, Chronic posterior atlantoaxial subluxation associated with os odontoideum: a rare condition. A case report and literature review. Spinal cord series and cases. 2018; [PubMed PMID: 30588336]|
|||Nguyen JC,Pollock AN, Os odontoideum. Pediatric emergency care. 2015 Mar; [PubMed PMID: 25738246]|
|||Tang X,Tan M,Yi P,Yang F,Hao Q, Atlantoaxial dislocation and os odontoideum in two identical twins: perspectives on etiology. European spine journal : official publication of the European Spine Society, the European Spinal Deformity Society, and the European Section of the Cervical Spine Research Society. 2018 Jul; [PubMed PMID: 28508240]|
|||Fielding JW,Griffin PP, Os odontoideum: an acquired lesion. The Journal of bone and joint surgery. American volume. 1974 Jan; [PubMed PMID: 4812163]|
|||Schuler TC,Kurz L,Thompson DE,Zemenick G,Hensinger RN,Herkowitz HN, Natural history of os odontoideum. Journal of pediatric orthopedics. 1991 Mar-Apr; [PubMed PMID: 2010525]|
|||Verska JM,Anderson PA, Os odontoideum. A case report of one identical twin. Spine. 1997 Mar 15; [PubMed PMID: 9089945]|
|||Wang S,Wang C, Acquired os odontoideum: a case report and literature review. Child's nervous system : ChNS : official journal of the International Society for Pediatric Neurosurgery. 2012 Feb; [PubMed PMID: 22134416]|
|||Zygourakis CC,Cahill KS,Proctor MR, Delayed development of os odontoideum after traumatic cervical injury: support for a vascular etiology. Journal of neurosurgery. Pediatrics. 2011 Feb; [PubMed PMID: 21284467]|
|||Wada E,Matsuoka T,Kawai H, Os odontoideum as a consequence of a posttraumatic displaced ossiculum terminale. A case report. The Journal of bone and joint surgery. American volume. 2009 Jul; [PubMed PMID: 19571099]|
|||Currarino G, Segmentation defect in the midodontoid process and its possible relationship to the congenital type of os odontoideum. Pediatric radiology. 2002 Jan; [PubMed PMID: 11819061]|
|||FLEMMING C,HODSON CJ, Os odontoideum; a congenital abnormality of the axis; case report. The Journal of bone and joint surgery. British volume. 1955 Nov; [PubMed PMID: 13271492]|
|||Ellies DL,Krumlauf R, Bone formation: The nuclear matrix reloaded. Cell. 2006 Jun 2; [PubMed PMID: 16751095]|
|||McHugh BJ,Grant RA,Zupon AB,DiLuna ML, Congenital os odontoideum arising from the secondary ossification center without prior fracture. Journal of neurosurgery. Spine. 2012 Dec; [PubMed PMID: 23039156]|
|||White IK,Mansfield KJ,Fulkerson DH, Sequential imaging demonstrating os odontoideum formation after a fracture through the apical odontoid epiphysis: case report and review of the literature. Child's nervous system : ChNS : official journal of the International Society for Pediatric Neurosurgery. 2013 Nov; [PubMed PMID: 23649960]|
|||Morgan MK,Onofrio BM,Bender CE, Familial os odontoideum. Case report. Journal of neurosurgery. 1989 Apr; [PubMed PMID: 2926504]|
|||Hadley MN,Walters BC,Grabb PA,Oyesiku NM,Przybylski GJ,Resnick DK,Ryken TC, Os odontoideum. Neurosurgery. 2002 Mar; [PubMed PMID: 12431299]|
|||Straus D,Xu S,Traynelis VC, Os odontoideum in identical twins: Comparative gene expression analysis. Surgical neurology international. 2014; [PubMed PMID: 24818044]|
|||Perdikakis E,Skoulikaris N, The odontoid process: various configuration types in MR examinations. European spine journal : official publication of the European Spine Society, the European Spinal Deformity Society, and the European Section of the Cervical Spine Research Society. 2014 May; [PubMed PMID: 24366357]|
|||Raj A,Srivastava SK,Marathe N,Bhosale S,Purohit S, Dystopic Os Odontoideum Causing Cervical Myelopathy: A Rare Case Report and Review of Literature. Asian journal of neurosurgery. 2020 Jan-Mar; [PubMed PMID: 32181211]|
|||Shirasaki N,Okada K,Oka S,Hosono N,Yonenobu K,Ono K, Os odontoideum with posterior atlantoaxial instability. Spine. 1991 Jul; [PubMed PMID: 1925742]|
|||Watanabe M,Toyama Y,Fujimura Y, Atlantoaxial instability in os odontoideum with myelopathy. Spine. 1996 Jun 15; [PubMed PMID: 8792520]|
|||Klimo P Jr,Kan P,Rao G,Apfelbaum R,Brockmeyer D, Os odontoideum: presentation, diagnosis, and treatment in a series of 78 patients. Journal of neurosurgery. Spine. 2008 Oct; [PubMed PMID: 18939918]|
|||Dai L,Yuan W,Ni B,Jia L, Os odontoideum: etiology, diagnosis, and management. Surgical neurology. 2000 Feb; [PubMed PMID: 10713186]|
|||Sasaki H,Itoh T,Takei H,Hayashi M, Os odontoideum with cerebellar infarction: a case report. Spine. 2000 May 1; [PubMed PMID: 10788864]|
|||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. 2000 Nov 1; [PubMed PMID: 11064523]|
|||Wu X,Wood KB,Gao Y,Li S,Wang J,Ge T,Zhao B,Shao Z,Yang S,Yang C, Surgical strategies for the treatment of os odontoideum with atlantoaxial dislocation. Journal of neurosurgery. Spine. 2018 Feb; [PubMed PMID: 29148933]|
|||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. 1989 Oct; [PubMed PMID: 2781009]|
|||Boden SD,Dodge LD,Bohlman HH,Rechtine GR, Rheumatoid arthritis of the cervical spine. A long-term analysis with predictors of paralysis and recovery. The Journal of bone and joint surgery. American volume. 1993 Sep; [PubMed PMID: 8408150]|
|||Stulik J,Vyskocil T,Sebesta P,Kryl J, Atlantoaxial fixation using the polyaxial screw-rod system. European spine journal : official publication of the European Spine Society, the European Spinal Deformity Society, and the European Section of the Cervical Spine Research Society. 2007 Apr; [PubMed PMID: 17051397]|
|||Fielding JW,Hensinger RN,Hawkins RJ, Os Odontoideum. The Journal of bone and joint surgery. American volume. 1980 Apr; [PubMed PMID: 7364809]|
|||Qureshi MA,Afzal W,Malik AS,Ullah JS,Aebi M, Os-odontoideum leading to atlanto-axial instability--report of surgery in four cases. JPMA. The Journal of the Pakistan Medical Association. 2008 Nov; [PubMed PMID: 19024140]|
|||Salunke P,Behari S,Kirankumar MV,Sharma MS,Jaiswal AK,Jain VK, Pediatric congenital atlantoaxial dislocation: differences between the irreducible and reducible varieties. Journal of neurosurgery. 2006 Feb; [PubMed PMID: 16506499]|
|||Bourdillon P,Perrin G,Lucas F,Debarge R,Barrey C, C1-C2 stabilization by Harms arthrodesis: indications, technique, complications and outcomes in a prospective 26-case series. Orthopaedics [PubMed PMID: 24629457]|
|||Stock GH,Vaccaro AR,Brown AK,Anderson PA, Contemporary posterior occipital fixation. Instructional course lectures. 2007; [PubMed PMID: 17472317]|
|||Sánchez-Aparcero B,Alemán I,Botella MC, Fracture of the odontoid process in a male individual from the medieval necropolis of Maro (Málaga, Spain). International journal of paleopathology. 2018 Sep; [PubMed PMID: 30126702]|