Atlantoaxial instability

In anatomy, the transverse ligament of the atlas is a ligament which arches across the ring of the atlas (the topmost cervical vertebra, which directly supports the skull), and keeps the odontoid process in contact with the atlas. Anatomy It is concave in front, convex behind, broader and thicker in the middle than at the ends, and firmly attached on either side to a small tubercle on the medial surface of the lateral mass of the atlas.As it crosses the odontoid process, a small fasciculus (crus superius) is prolonged upward, and another (crus inferius) downward, from the superficial or posterior fibers of the ligament.The former is attached to the basilar part of the occipital bone, in close relation with the membrana tectoria; the latter is fixed to the posterior surface of the body of the axis; hence, the whole ligament is named the cruciate ligament of the atlas.The transverse ligament divides the ring of the atlas into two unequal parts: of these, the posterior and larger serves for the transmission of the medulla spinalis and its membranes and the accessory nerves; the anterior and smaller contains the odontoid process.The neck of the odontoid process is constricted where it is embraced posteriorly by the transverse ligament, so that this ligament suffices to retain the odontoid process in position after all the other ligaments have been divided. [Source: Wikipedia ]

May Be Caused by
 Common
Atlanto-occipital fusion
Incompetent transverse atlantoaxial ligament
Juvenile chronic arthritis
Odontoid fracture
Rheumatoid arthritis
Torn transverse spinal ligaments
Trauma
 Uncommon
Aarskog syndrome
Absent anterior arch of atlas
Absent odontoid process
Achondroplasia
Ankylosing spondylitis
Behçet disease
C2-C3 block vertebra
Calcium pyrophosphate dihydrate crystal deposition disease
Campomelic dysplasia
Cervical adenitis
Chondrodysplasia punctata
CREST syndrome
Diastrophic dysplasia
Dyggve-Melchior-Clausen dysplasia
Erosion of odontoid
Fracture
Gout
Grisel syndrome
Hemophilia
Hypochondrogenesis
Hypoplastic odontoid process
Klippel-Feil syndrome
Laxity of transverse spinal ligaments
Marfan syndrome
Mastoiditis
Metaphyseal chondrodysplasia Jansen type
Metaphyseal chondrodysplasia McKusick type
Metatropic dysplasia
Morquio syndrome
Mucolipidosis type 3
Mucopolysaccharidosis
Multiple epiphyseal dysplasia
Nasopharyngeal infection
Neurofibromatosis type 1
Nontraumatic atlantoaxial subluxation
Odontoid anomaly
Opsismodysplasia
Os odontoideum
Parotitis
Patterson syndrome
Pseudoachondroplasia
Psoriatic arthritis
Reiter syndrome
Reiter/reactive arthritis
Retropharyngeal infection
Retropharyngeal or nasopharyngeal inflammation
Scleroderma
Spondyloepimetaphyseal dysplasia short limb-hand abnormal calcification type
Spondyloepimetaphyseal dysplasia Strudwick type
Spondyloepiphyseal dysplasia
Spondyloepiphyseal dysplasia congenita
Spondyloepiphyseal dysplasia tarda
Spondylometaphyseal dysplasia
Storage disease
Systemic lupus erythematosus
Tooth abscess
Trisomy 21
Tuberculosis
Turner syndrome
Winchester syndrome