Odontoid process

Muscles located at the anterior surface of the cervical spine are  

Rectus capitis anterior—a small muscle running from the anterior 

Longus colli which originates from the side of the upper thoracic vertebral bodies and inserts into the side of the upper cervical vertebral bodies. It contributes to flexion of the head. 

Longus capitis which originates from the transverse processes of the lower cervical bodies and inserts into the basilar part of the occipital bone. It is also a flexor of the head. 

The last two muscles do not reach the midline. This is why only the medial margin of the long cervical muscles has to be tetracted in transoral approach. 

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C2 develops from four ossification centres at birth, one for each neural arch, one for the body and one for the odontoid process. The odontoid process itself forms from two separate ossification centres that fuse in the midline by the 7 fetal month, but occasionally can persist as a vertical lucent cleft in the odontoid (Ogden 1984). A secondary ossification centre appears at the tip of the odontoid process between 3 and 6 years of age and fuses by 12 years. This can be mistaken for an avulsion fracture (Fig. 7.57). The body of C2 fuses with the odontoid process by 2-6 years leaving a fusion line or remnant of the cartilaginous synchondrosis until 11 years of age and may be confused with a fracture either on radiographs or CT (Fig. 7.58). Pseudofractures of the odontoid process can be produced by the Mach effect by overlapping of the teeth, the posterior arch of Cl, occiput or soft tissues such as the tongue (Fig. 7.59). The neural arches of C2 fuse posteriorly by 2-3 years of age and the body fuses with the neural arches by 3-6 years. Additional ossification centres may be found at the tips of the spinous... 

Fig. 7. 56. Unilateral offset of the lateral masses of Cl and C2 (arrows) due to rotation. The lateral mass to odontoid process distances are unequal...

Fig. 7. 57. a,b Sagittal and coronal CT reconstructions showing two foci of calcification within the ossification centre for the tip of the odontoid process in a 6-year-old. c Unfused, well developed ossification centre for the tip of the odontoid process in a 7-year-old... 

Fig. 7.58. Sagittal CT reconstruction. Almost fused synchondrosis between the body of C2 and the odontoid process simulating a fracture in an 11-year-old...

Fig. 7.59. Pseudofracture of the odontoid process produced by the Mach effect from overlapping of the posterior arch of Cl and the occiput...

Type 4 AARF with posterior displacement of the atlas and occurs with a deficiency of odontoid process. This is the least common type. 

The atlantoaxial articulation is specially adapted for (nearly) pure rotation. In addition to the inferior articular facets of the atlas and the superior articular facets of the axis, movement other than rotation is limited by the anteriorly located odontoid process (dens) of the axis. The odontoid process is held close to the anterior arch of the atlas by the transverse ligament of the atlas, which allows only a slight amount of flexion of the atlas on the axis. 

The distance from the posterior aspect of the anterior arch of the atlas to the anterior aspect of the odontoid process is less than 3 mm in adults. A larger distance bespeaks atlantoaxial dislocation. The antero-posterior diameter of the spinal canal in the atlanto-axial region is 20-26 mm in men and 19-25 mm in women. According to Apuzzo et al. (1978), patients develop clinical symptoms when the anteroposterior diameter is less than 14 mm at C1-C2 level. 

In cases of basilar impression, CT of the cranium and upper cervical spine can image the invaginated odontoid process and other bony anomalies. A typical CT finding is a complete dense ring of bone on the lower cut of the posterior fossa at which level in normal cases there is still brain tissue (Boles et al. 1977). Moreover, such accompanying anomalies as syrinx, Chiari malformation, hydrocephalus are also detectable (di Chiro and Schellinger 1976, Nakagawa et al. 1977, Lee et al. 1978, Oberson and Azam 1978, Carella et al. 1981, Isu et al. 1983). 

Fig. 8. The resection plane of the odontoid proposed by us. In the case of a high upward and backward projecting odontoid process we advise starting the resection of the odontoid 4-5 mm deeper than its neck, in the central part of the body of the...

The odontoid process is also removed with the high speed drill and diamond burr. The latter prevents soft tissue (ligamentous) damage and is used in the deeper parts of the bony structures. 

Menezes et al. (1980) gained remarkable experience in children with abnormalities of the craniocervical junction. They emphasized that in children, at the resection of the odontoid it is important to leave the transverse portion of the cruciate ligament complex intact as well as the cruciate notch on the axis. This provides better post-operative stability and may allow for future bone formation, since the periosteum is intact. They were able to demonstrate reformation of the odontoid process after such surgery. 

A. D., a 30-year-old man fell from a ladder and lost consciousness for a short while in July, 1984. A few hours later he felt cervical pain. Investigating the cause of the persistent neck pain it became evident that his odontoid process had broken and tilted to the right and the axis was displaced backward. Attempts at closed reduction failed and he was placed in an external fixation device. He was admitted to our Institute in August,... 

P. S. Z. A 27-year-old man who complained of painful stiffness of his neck for 3 months. Routine cervical X-ray showed some loss of bony structure in the C2 body he was placed in a cervical collar for partial fixation. His complaints increased. 2 months later repeat X-ray showed total loss of the C2 body (Fig. 24). He was admitted to our Institute in 1980. Neurological examination revealed tactile hypaesthesia in the area of the left C2, and hyperreflexia all over the extremities X-ray tomograms showed severe destruction of the body of C2 and of the odontoid process. The... 

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