This child has pseudosubluxation of the cervical spine, which is considered a normal radiographic finding in a child this age.
Catell et al claim that anterior displacement of the second cervical vertebra on the third during neck flexion could not be measured accurately because there are no constant reference points in the young cervical spine. Differences in configuration, size, and degree of ossification at different ages rendered direct measurement of anterior displacement unreliable. This phenomenon of anterior displacement of C2 on C3 is a combination of forward shift and flexion of the second cervical vertebra on the third. Anteroposterior movement of the second cervical vertebra on the third during flexion and extension of the spine is one of the components of pseudosubluxation. Almost half of the children under 8 in their study had measured AP movement between the 2nd and 3rd cervical vertebra of 3 mm or more. Of this subgroup, 60% had observed anterior displacement of the second on the third cervical vertebra in flexion. The reason for this is 4 fold in children under 8 years old: hypermobility, unique vertebral configuration, incomplete ossification, and the presence of epiphyses-singly or in combination. This is a normal variation in children; it is a pseudosubluxation. This is supported by the fact that the spinolaminar point on C2 is within 1.5 mm of spinolaminar line.
Swischuk, LE. Anterior displacement of C2 in children: Physiologic or pathologic. Radiology 1977.; 122:759-763
PMID:841068 (Link to Abstract)
Cattell HS, Filtzer DL. Pseudosubluxation and other normal variations in thecervical spine in children. A study of one hundred and sixty children. J Bone Joint Surg Am. 1965 Oct;47(7):1295-309.
PMID:5837630 (Link to Abstract)