Os Odontoideum - Causes, Development, Symptoms, Diagnosis, and Treatment

Introduction:

The cervical vertebra consists of seven bones placed one over the other. The first bone right below the skull is called the atlas, and the second bone is called the axis. The axis bone has a projection that articulates with the atlas bone, which is called the odontoid process or the dens. The neck of the odontoid process is constricted and joins with the body of the axis bone.

Os odontoideum is a rare entity presenting with abnormal changes in the odontoid process, which is not connected to the axis bone.

What Is Os Odontoideum?

Os odontoideum (OO) is a condition that is characterized by a structural abnormality in the upper cervical spine (neck bones). Radiologically, it is presented as a smooth-edged calcified mass (ossicle) identified as a malformed odontoid process without connection with the axis bone (C2 vertebra). The ossicle can take up two positions,

• Orthotopic: Same position as the normal odontoid.

• Dystopic: Migrated towards the skull base.

An anatomist and a neuroscientist named Giacomini described this condition initially in 1886.

What Are the Causes of Os Odontoideum?

The reason for os odontoideum can be,

1) Trauma:

Injury to the neck bones can lead to odontoid process fracture and separation from the body of the axis bone. Though isolated, the small piece of bone may retain the blood supply and remain vital. This process can occur before or after the birth.

2) Congenital:

The congenital theory supports the fact of incomplete formation or fusion of the odontoid process with the axis body. The existence of this theory is because os odontoideum is often associated with genetic disorders like Klippel-Feil syndrome, neurofibromatosis, or Down syndrome.

How Does Os Odontoideum Develop?

The following are some of the hypotheses postulated for the development of os odontoideum,

• The odontoid process of the axis bone is formed from different embryonic tissue.

• The growth plates separate the tissues of origin of the odontoid and axis bodies.

• A band of cartilage may exist between the odontoid and the axis in kids below four years, and that gradually disappears as the child grows up.

• Vascular insufficiency from the terminal branches of the main arteries can cause oxygen deprivation and necrosis.

What Are the Symptoms of Os Odontoideum?

• Some patients may remain asymptomatic.

• Neck and shoulder pain, neck deformity, numbness in the neck and arm, and sometimes weakness in the legs are associated with os odontoideum. These symptoms are due to the mild compression of the spinal cord.

• Atlantoaxial instability - increased movement between the C1 and C2 vertebrae.

• Ischemia (reduced oxygen supply) results in visual disturbances, dizziness, seizures, and sudden death.

• Neurological problems like polyneuropathy, motor disturbances, and abnormal walking style are also found.

How Is Os Odontoideum Diagnosed?

Initially, the patients are physically assessed to check for neurological functions and motor abilities. Radiographic images are the gold standard in diagnosing os odontoideum.

1) X-rays:

• An anteroposterior (front to back) and lateral (side) view radiographs of the cervical vertebra with an open mouth will greatly help in the diagnosis.

• In the side view image, there will be a distance between the atlas and the axis bone and disruption of a distinct junction.

• A well-rounded ossicle (calcified mass) is visible close to the base of the skull.

• The instability and subluxation between the c1 and c2 bones can be assessed with cervical spine flexion-extension radiographs (taking an X-ray by bending and extending the neck).

• Bedside radiographs are used to decide the surgical procedure.

2) Computed Tomography (CT):

• Computed tomography (CT) scan images conform to the presence of ossicles.

• There is a shortening in the odontoid process.

• The chronic subluxation can be substantiated by the visibly increased thickening in the anterior arch of the atlas.

• CT scan is also used to evaluate normal anatomical structures like blood vessels (especially vertebral artery), nerves, and bony landmarks.

3) Magnetic Resonance Imaging (MRI):

• Magnetic resonance imaging (MRI) can illustrate the presence of fluid in the cervical bone spaces.

• Shrinkages in the spinal canal can be found.

4) Radiographic Measurements:

The following are some measurements assessed on the radiograph to confirm the cervical instability or surgical approach.

• Atlanto-dental Interval (ADI): It measures the horizontal distance between the odontoid process and the anterior arch of the atlas.

• Inference: In adults, more than 3.5 mm is regarded as unstable, and about 10 mm and more will require surgery. In the pediatric population, up to 5 mm of the atlanto-dental interval is considered normal.

• Space-available Cord (SAC) or Posterior Atlanto-dens Interval (PADI): It measures the distance between the odontoid process and the posterior arch of the atlas.

• Inference: In adults, around 14 mm and less will express an increased risk of neurological issues and will require surgery.

How Is Os Odontoideum Treated?

The treatment modality depends on the symptoms and the severity of the atlantoaxial instability, as evaluated in the radiographs. The main aim of treatment is to control the symptoms and enhance the patient's quality of life.

• Asymptomatic patients will need only observation and follow-up with radiographs.

• Some pioneers suggest a precautionary spinal fusion even before the symptoms and signs are evident.

• Symptomatic patients with neck pain, neurological issues, and progressive atlantoaxial instability will be considered for surgery.

What Are the Surgical Options for Os Odontoideum?

Surgery includes the fusion of cervical vertebrae. The choice of surgery depends on the location of the spinal cord compression and the quality of the bone.

• The most common is the merging of the atlas and the axis bone to stabilize them.

• Others involve the fusion of the occiput (the back of the skull) to the C2 or C3 vertebra when there is subluxation between the occiput and the cervical vertebra.

• Surgery can often present with complications like surgical site infection, cerebrospinal fluid (CSF) leakage, nerve injury, and separation of the fused bones.

Conclusion:

Early and precise diagnosis and prompt treatment planning are crucial in alleviating further medical issues and death. Any delay can result in irreversible neurological damage. Conservative management is considered only when the patient has no symptoms. If the patient is symptomatic or with the findings of atlantoaxial instability, definitive surgery will serve the purpose.