Radiologic Evaluation of Developmental Anomalies of The Odontoid Process: A Cone-Beam-Compated Tomography Study

  

    
OO (n)
    
Sex
    
D1
    
D2
    
D3
    
EDI
    
AADI
    
PADI
    
LADI

      asym
    
BDI
    
AOJA
  
  

    
1
    
K
    
10.5
    
5.9
    
4.7
    
2.0
    
1.9
    
16.4
    
0.9
    
5.2
    
115
  
  

    
2
    
E
    
8.9
    
3.6
    
4.7
    
0.8
    
1.2
    
19.2
    
1.4
    
8.5
    
106
  
  

    
3
    
K
    
2.4
    
2.9
    
2.5
    
1.0
    
1.4
    
19.2
    
1.3
    
2.7
    
129
  
  

    
4
    
K
    
8.4
    
3.7
    
3.8
    
0.7
    
0.4
    
17.2
    
0.2
    
4.2
    
116
  
  

    
OO: Os Odontoideum. D1:    Horizontal Dimension. D2: Vertical Dimension. D3: Sagittal Dimension; EDI:    Extraossicle-Dental Interval; AADI: Anterior Atlantodental Interval; PADI:    Posterior Atlantodental Interval; LADI asym: Lateral Atlanto Dens Interval Asymmetry;    BDI: Basion-Dens Interval; AOJA: Atlanto-Occipital Joint Angle

      *Measurements are in millimeters 
  

Table 2: OOs size and radiographic measurements.

Discussion

The odontoid process has been observed to have many congenital and acquired anatomical variants. Congenital anomalies include various types of odontoid dysgenesis, such as os odontoideum, persistent os-terminale, odontoid aplasia or hypoplasia, dens bicornis, inclination of the odontoid process, malposition of the odontoid process, dublicated odontoid process, fused nonseperated odontoid process to the anterior arch of the atlas, and dolicho-odontoid [1]. Acquired anomalies of the odontoid may be traumatic, degenerative, inflammatory, or neoplastic. The etiology of this disease is not fully understood but is likely multifactorial.

The prevalence [13] of congenital cervical vertebra anomalies has been reported to range from 0.9% to 41.5%; a greater number of congenital cervical vertebra anomalies is likely based on false-positive findings on lateral cephalometric radiographs [15]. Alsufyani NA [13]. evaluated the incidental findings in the cervical vertebra and clivus on CBCT images and reported that 16.2% of the findings were congenital. In this study, CBCT images of fourteen patients (0.7%) showed developmental anomalies of the odontoid process.

During normal development of the C2 vertebra, the odontoid process itself is derived from two lateral primary ossification centers, and the apex is derived from an apical secondary ossification center. This secondary ossification center, referred to as the ossiculum terminale, does not appear until 3 to 6 years of age and does not complete fusion with the remainder of the odontoid until the beginning of puberty [1]. However, small, well-corticated ossicles in the vicinity of C1 and in the dens of C2 fail to fuse and form OTP. The presence of OTP should not be considered pathological until it persists beyond the age at which skeletal maturity is attained [3]. Because of this information, this study was conducted with patients older than 12 years.

An increased incidence of odontoid ossicles has been reported in patients suffering from Down syndrome. Viswanathan et al. [4] reported the case of a 10-year-old boy without any syndrome or other associated abnormalities who presented with multiple falling episodes and who was found to have undiagnosed OTP on CT scan. Constantoyannis et al. [5] presented the case of a 36-year-old woman with gradual onset of neck pain and neurological signs who was also found to have OTP close to the arc of the atlas. Alsufyani NA. [13] found 14 OTP among 7689 CBCT images, 1.9% of which were incidental findings. In this study, OTP was found in ten (0.5%) patients without any syndrome or other associated abnormalities.

Os odontoideum is an uncommon diagnosis. An oval-shaped, well-corticated bony ossicle is positioned cephalad to the body of the axis. However, the etiology/pathogenesis of this lesion remains controversial. However, there are two theories about the origin of os odontoideum: traumatic or congenital. [16] The most widely accepted etiology of OO is fracture of the dens, which is often unnoticed by the patient, followed by nonunion. Conversely, the incidence of this anomaly in identical twins and people with familial relations also weakens the prior injury argument. In addition, OO has also been described in association with many other syndromes, with Down syndrome being the most common syndrome. They may be present in the normal position of the odontoid process (orthotopic), displaced cranially (dystopic), or fused to the clivus.

The most common presenting symptom associated with OO was neck pain, followed by weakness and paresthesia of the upper and lower limbs, with the upper limbs more commonly involved. Gait disturbance, neck stiffness, and headaches were uncommon presenting complaints, as was torticollis. A mixture of upper and lower motor neuron signs was present in a proportion of these patients. Minor trauma in these patients might cause sudden death [8].

Reports of OO in the dental literature are limited to a few studies [13,18]. Alsufyani NA. [13] reported only one case of OO (0.01%) as an incidental finding of the cervical spine and clivus in a retrospective CBCT study. In this study, OO was observed in four (0.2%) patients, and all the patients were in the orthotopic position. However, we do not know whether these patients have any neurologic symptoms due to OO.

The presence of OTP and OO may be clinically difficult to distinguish from a type I odontoid fracture that occurs at the tip of the dens or a type II odontoid fracture that occurs at the base of the dens. [19] Acute fractures show irregular, lucent margins on imaging, whereas OTP and OO patients present with smooth, corticated margins. Chronic type I fractures of the odontoid may exhibit nonunion and display corticated margins on imaging. In addition, ossicles must be differentiated from “joint mice” (those with loose intra-articular bodies from those with degenerative joint diseases). [13] Additionally, OTP must be differentiated from OO. Constantoyannis et al. [5] noted the difficulty of distinguishing between OTP and other odontoid anomalies, such as OO, and concluded that surgical intervention is necessary regardless of the final diagnosis if a patient is presenting with neurological signs.

One of the limitations of this study is that we could not find any other developmental or congenital anatomical variants of the odontoid process except OTP or OO among the 1950 CBCT images, and we cannot discuss their prevalence or any other features.

Atlantoaxial Instability (AAI) with normal vertebral morphology is usually evaluated by measuring the Atlas-Dens/odontoid Interval (ADI). The ADI of the atlantoaxial joint, including the anterior Atlantodental Interval (AADI) and the Lateral Atlantodental Interval (LADI), has been studied extensively using conventional radiographs and is widely used in the evaluation of AAI.

The AADI is known as the predental space. Chen et al. [20] reported that the AADI was <3.0 mm in most males (97.9%) and <2.5 mm in most females (97.7%). Wang et al. [10] reported 24 patients with OO and used 5 mm to diagnose AAI. In this study, a distance greater than 3 mm was considered to indicate AAI because of the studied adult population, and we could not detect any AAI in OTP or OO patients. The maximum AADI was 1.9 mm, which was considered the normal interspace.

Chen et al. [20] reported that the AADI was 1.83 ± 0.46 mm (0.9–3.4 mm) in males and 1.63 ± 0.43 mm (0.5–3.2 mm) in females, which indicated that the AADI was significantly greater in males than in females. In this study, the mean AADI was found to be 1.2 mm in males and 1.1 mm in females.

The LADI is routinely evaluated for the spread of lateral masses in C1 and for LADI asymmetry. Asymmetry of the LADI in patients with a history of recent trauma is indicative of rotatory subluxation of the atlas on the axis. Persistent LADI asymmetry not corrected by rotation should be used as the basic radiological criterion for the diagnosis of atlantoaxial rotatory fixation. Ajmal and O’Rourke [21] studied 13 neck injury patients with an LADI asymmetry =2.0 mm and suggested that LADI asymmetry might be a sign of significant cervical trauma. In this study, all the CBCT scans were performed with the patient in a normal head position, aiming to diminish the influence of head rotation on the results. We identified only an OTP patient with LADS asymmetry equal to 2 mm.

PADI measurements are important if they are less than 14 mm, which is associated with an increased risk of neurological injury and is an indication for surgery. [8] In this study, the smallest PADI was 14.5 mm, which was obtained for an OTP patient.

One of the measurements used in this study was the Basion-Dens Interval (BDI). At the beginning of this study, we hypothesized that the BDI would increase among OTP and OO patients because of the additional ossicles at the tip of the dens. However, the maximum BDI measurement found in an OO patient was 8.5 mm, which was within the normal limit.

Adult patients with AAI associated with OTP [4,5] and OO [10,11] have been described in the literature. An increased incidence of OTP has been reported in patients suffering from Down syndrome, and its presence may contribute to the AAI, which is often associated with this condition. In this study, the patients did not suffer from acute trauma or any syndrome, but the limitation of this retrospective study is that we cannot determine the clinical findings of these patients.

The AOJA is formed at the intersection of tangents drawn parallel to the atlanto-occipital joints. It is normal for the peak to be between 124 and 127°. In condylar hypoplasia, the joint space is flattened, and the angle increases. In this study, in one of the OTP patients, the AOJA angle was greater than 127°, but the length of the odontoid process was 16.9 mm, which was within the normal range.

Early and accurate diagnosis is critical for mitigating morbidity and mortality due to these anomalies. Minor trauma in patients with undiagnosed cervical instability might result in catastrophic neurological results [16]. Imaging has an important role in the management of developmental anomalies of the odontoid process, from diagnosis to therapy. A multimodal imaging approach, including plain radiography, CT and MRI, may be used to identify and provide the detailed anatomy of these anomalies, changes in the surrounding structure, atlanto-axial instability, spinal cord compression, associated craniocervical junction malformation and favorable anatomy to guide surgical therapy [10]. Routine anteroposterior and lateral cervical spine radiographs with an open-mouth odontoid are the first approach to this condition and can aid in diagnosis. Computed tomography images confirmed the presence of a corticated margin in this ossicle. It shows a shortened odontoid process and a smooth ossicle of bone [16,17]. Several authors also perform bedside lateral radiographs during preoperative skull traction because they consider them to be very important for atlantoaxial joint instability and subluxation and could help determine the surgical method used. [22] Multidetector CT with isotropic resolution and multiplanar reformations has enabled better visualization of complex bony abnormalities. It also helps with craniometric measurements that cannot be accounted for by plain radiographs [23].

Cone-beam images are distinguished from CT images in part by their imaging geometry. The X-ray beam in a cone-beam unit diverges as a cone to the patient rather than being collimated into a fan beam, as in a CT machine. This allows the region of interest to be imaged with one cycle of the X-ray source, or less. It is possible to extract axial, sagittal or coronal images, as well as planar or curved reconstructions on the same window. Three-dimensional (3-D) images of bone or air/soft tissue interfaces can also be generated [24]. In this study, CBCT imaging allowed us to visualize cervical vertebra and developmental anomalies of the odonoid process in three planes in one window.

Although the region of immediate diagnostic interest may be limited to a specific site, it is important to evaluate all the structures imaged. Disease is often an incidental finding that is typically located in part of the image and is unrelated to the reason for which the image was obtained. Accordingly, it is critical to systematically evaluate the entire image or imaged volume. With the expanded use of CBCT equipment in the dental profession, identification, interpretation, and management of incidental findings are necessary, as some findings require a higher level of expertise and additional advanced imaging modalities. [24] In regard to incidental findings in the cervical spine, there is scant information in the dental literature, and the pressure on OMRs is great because of the complex anatomy, complicated pathophysiology, and lack of soft tissue depiction via CBCT [13]. Early recognition and proper identification of radiological and clinical signs of AAI may help to avoid catastrophic complications secondary to spinal cord compression. Once the physician has confirmed the diagnosis, patients and their families require education from surgeons and orthopedic nurses about this condition and its possible outcomes [8]. Surgery is usually indicated for patients who have one of the following conditions: neurological involvement, radiological signs of instability, progressive instability, or persistent neck pain associated with atlantoaxial instability [9]. Conservative treatment is an option only when the patient is asymptomatic and when there is no evidence of atlantoaxial instability. Patients should be aware of the possible catastrophic consequences of this pathology [16].

Conclusion

In conclusion, within the limitations of this study, developmental anomalies of the odontoid process, such as OT and OO, are rare anomalies of the odontoid process that may cause AAI. These lesions may appear incidentally on large-FOV CBCT images. Dentomaxillofacial radiologists should be able to identify these anomalies, especially for AAI, via CBCT and note them in their reports. Awareness of these findings will be quite helpful in assessing the risk of complications and in informing patients about their condition. Awareness can also help clinicians when it is necessary to refer patients to an expert physician.

Author Statements

Funding

Authors declare that there is no source of funding or financial interest.

Conflict of Interest

Authors declare that there is no conflict of interest.

Authors Contributions

AZZ: Data collection, Data analysis, Manuscript writing/editing. Concept and design of study or acquisition of data or analysis and interpretation of data;

APS: Data analysis, Manuscript writing/editing. Drafting the article or revising it critically for important intellectual content and Final approval of the version to be published.

KC: Manuscript writing, Taking ethic permision Drafting the article or revising it critically for important intellectual content and Final approval of the version to be published.

Ethics: This study was reviewed and approved by the Institutional Review Board of the University (OMU KAEK 2022/499). All procedures followed were in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national) and with the Helsinki Declaration of 1975, as revised in 2008 (5).

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