St6phane Lefebvre, DC*. Dwight Vallee, DC**. J David Cassidy, DC, MSc (Orth) ... was diagnosed and the child was treated with medication by his family doctor.
0008-3194/93/141-144/$2.00/©JCCA 1993
Unstable os odontoideum in young children St6phane Lefebvre, DC* Dwight Vallee, DC** J David Cassidy, DC, MSc (Orth), FCCS(C)*** Anne K Dzus, MD, FRCS(C)****
Os odontoideum is a rare anomaly of the odontoidprocess. When unstable, it can be a source ofneck pain that can result in serious neurological injury. In young children, the presence of a transverse lucency at the base of the dens represents the normal developing growth plate. However, os odontoideum should be considered in the differential diagnosis in a child with recurrent or persistent neckpain. A case ofan unstable os odontoideum in a two-year-old child is presented to illustrate this point. (JCCA 1993; 37(3):141-144)
L'os odontoide est une anomalie rare de l'apophyse odontoide. Lorsqu'elle est instable, elle cause parfois une douleur cervicale qui peut entramner des blessures neurologiques graves. Chez les enfants en bas age, la presence d'une lucency transverse a la base de la dent de l'axis represente une zone cartilagineuse normale d'accroissement des os longs. Toutefois, ilfaut tenir compte de l'os odontoide lors de la formulation du diagnostic differentiel d'un enfant qui souffre de douleurs cervicales recurrentes ou persistantes. Pour illustrer cette question, voici maintenent l'exemple d'un enfant age de deux ans dont l'os odontoide est instable. (JCCA 1993; 37(3):141-144)
KE YWORDS: os odontoideum, instability, neck pain.
MOTS CLS: Os odontoide, instabilite, douleur cervicale.
Introduction Os odontoideum is a rare anomaly of the odontoid process. Its prevalence is unknown, as many cases remain asymptomatic and hidden for life. It may be described as a wide transverse gap of fibrous tissue separating the main body of the dens from the centrum of the axis. Once believed to represent a defect of union of the ossification center of the odontoid to the body of axis, there is evidence suggesting that it results from an early fracture of the odontoid secondary to trauma. I Os odontoideum is not clinically significant unless it presents with atlanto-axial instability. This instability may progress and
cause severe neurologic deficit.2 Pain is the most common complaint and restricted neck mobility the most common physical finding.3 In adults, if routine radiographic views disclose a transverse lucency at the base of the dens further investigation for atlanto-axial instability is necessary. In children, the same transverse lucency represents the odontoid growth plate. Therefore, children complaining of recurrent or persistent neck pain should have flexion and extension views to rule out an unstable os odontoideum. The purpose of this case report is to illustrate the value of flexion and extension radiographs in children complaining of recurrent or persistent neck pain.
Case report * Clinical Resident, Canadian Memorial Chiropractic College, Toronto, Ontario. ** Private practice of Chiropractic, Saskatoon, Saskatchewan *** Research Associate, Department of Orthopaedics, Royal University Hospital, Saskatoon, Saskatchewan. *** Assistant Professor, Department of Orthopaedics, Royal University Hospital, Saskatoon, Saskatchewan. Reprint request to: Dr. J.D. Cassidy, Department of Orthopaedics, Royal University Hospital, University of Saskatchewan, Saskatoon, S7N OXO. © JCCA 1993.
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A two-and-a-half-year old boy was referred to the paediatric orthopaedic outpatient clinic at the Royal University Hospital for evaluation of recurrent neck pain. The first episode of neck pain had begun following a fall at a playground when he was one and a half years old. At that time, radiographs of his cervical spine had not disclosed any abnormality. (Figure 1) Muscle strain was diagnosed and the child was treated with medication by his family doctor. Six months later, the boy was still complaining of recurrent neck pain, but additional radiographs did not reveal any obvious change. Since he continued to have neck pain 11 months after the initial injury, he was referred for 141
Os odontoideum
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Figure 1 Anteroposterior and lateral radiographs of the cervical spine taken at the initial presentation show no evidence of abnormality.
Figure 2 The flexion and extension radiographs taken two years later show 5 to 6 mm of anterior displacement of C1 on C2. Note the anterior displacement of the os odontoideum with the anterior arch of atlas during flexion. (arrow)
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Fiure 3 The tomogram shows that the odontoid is not united to the body of C2. (arrow)
orthopaedic evaluation. The boy was otherwise healthy except for occasional asthma attacks. He was the product of a full-term pregnancy and uncomplicated delivery. He had achieved the normal developmental milestones. Examination revealed a normally developed boy for his age. Motion of the cervical spine was normal, except for extension, which he was reluctant to perform. The upper extremities were neurologically intact. Palpation of the cervical spine revealed no tenderness. Radiographs of the cervical spine, including flexion and extension views, revealed 5 to 6 mm of displacement of C1 on C2. (Figure 2) Tomograms added evidence of a fibrous defect at the base of the odontoid. (Figure 3) A magnetic resona,nce image obtained to assess the state of the spinal cord was unremarkable. Surgical fusion of C1 to C2 was scheduled to relieve the persisting pain and to prevent neurological compromise. One year after surgery, the patient was completely pain-free, except after extreme physical exertion. On examination, range of motion of the cervical spine was full and painless. He remained neurologically intact. Radiographs showed a solid fusion of Cl and C2. (Figure 4)
Discussion In young children, the radiographic appearance of an os odontoideum and a normally developing odontoid process are indistinguishable. In fact, the growth plate of a normally developing axis and the bone defect characteristic of an os odontoideum both appear lucent on plain film radiographs. This radiographic lucency persists until the growth plate starts to close, some time during the third year of life. By age four the growth plate has completely closed in about 50% of children, and the remaining close by age six.4 Therefore, prior to age six, it is not possible to diagnose a clinically significant os odontoideum without flexion and extension radiographs. Usually atlanto-axial instability is assessed by measuring the atlanto-dental interspace. This method is invalid in cases of os odontoideum. This remains true even when flexion and extension radiographs are used. The reason is that the transverse ligament fixes the free ossicle of the os odontoideum to the anterior arch of the atlas, thus maintaining the normal atlantodental interspace throughout flexion and extension. In order to determine instability in cases of os odontoideum, movement must be observed between the centrum of the axis and the os odontoideum in the sagittal plane on plane radiographs. There is the potential for neurological compromise when the
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Os odontoideum
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Figure 4 The anteropostenor and lateral radiographs taken after surgery show a solid posterior fusion of Cl and C2.~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
space available for the spinal cord is encroached upon. During sagittal flexion, the space is represented by the distance between the posterior arch of the atlas and the posterior border of the centrum of the axis. However, during extension, it is the distance between the posterior aspect of the os odontoideum and spinous process that defines this space.56 Anatomical relationships at the Cl-C2 vertebral levels are well described by Steel's law of thirds. This law suggests a third of the vertebral canal is occupied by the spinal cord, another third occupied by the odontoid, and the last third represents a safe zone. In cases where instability exists, the risk of neurological compromise increases as the safe zone is diminished. Most commonly, patients with unstable os odontoideum present with a long history of neck pain, decreased cervical range of motion, torticollis and headache. Symptoms of myelopathy (spinal cord compression) and cervical and brain stem ischemia (vertebral artery compression) are rarely present on the initial presentation, but constitute potential complications of unstable os odontoideum.7 If the instability is allowed to persist, the patient risks permanent neurological impairment.6 Therefore, the treatment usually consists of surgical fusion.
firmed by flexion and extension radiographs of the upper cervical spine.
Acknowledgements We thank the Chiropractors' Association of Saskatchewan, the Chiropractic Foundation for Spinal Research, and the Canadian Memorial Chiropractic College for financial assistance in preparing this manuscript, and the Department of Medical Photography at the Royal University Hospital for assistance with photography.
References
Conclusions Unstable os odontoideum is a rare anomaly of the axis. It should be suspected as a possible cause of persistent neck pain in children with decreased neck mobility. The diagnosis is con-
1 Fielding JW, Griffin PP. Os odontoideum: an acquired lesion. J Bone Jt Surg 1974; 56A:187-190. 2 Wadie NH. Myelopathy complicating congenital atlanto-axial dislocation: a study of 28 cases. Brain 1967; 90:449-72. 3 Fielding JW, Hensinger RN, Arbor A, Hawkins RJ. Os odontoideum. J Bone Jt Surg 1980; 62A:376-383. 4 Fielding JW. Selected observations on the cervical spine in the child. Curr Pract Orthop Surg 1973; 5:31-55. 5 Hensinger RN. Osseous anomalies of the craniovertebral junction. Spine 1986; 11:323-333. 6 Lovel WW, Winter RB. Pediatric Orthopedics. 2nd ed. Philadelphia: JB Lippincott Company, 1986:715-738. 7 Shirasaki N, Okado K, Oka S, Hosono N, Yonenobu K, Ono K. Os odontoideum with posterior atlantoaxial instability. Spine 1991; 16:706-715.
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