A Review of a New Clinical Entity of 'Central Atlantoaxial Instability': Expanding Horizons of Craniovertebral Junction Surgery

Abstract

The author discusses the novel form of central or axial atlantoaxial instability and analyses its clinical significance. High degree of clinical and radiological understanding of the region is mandatory to diagnose and then treat such atlantoaxial instability. Evaluation of alignment of facets of atlas and axis and observations on direct manipulation of facets of atlas and axis forms the basis of diagnosis. The treatment of clinical entities like basilar invagination, Chiari formation, syringomyelia and myelopathy related to cervical spinal degeneration, spinal deformities, ossified posterior longitudinal ligament, and Hirayama disease can be influenced by the understanding of central or axial atlantoaxial instability.

Keywords: Atlantoaxial instability; Basilar invagination; Chiari formation; Syringomyelia.

Fig. 1.

Images of a 50-year-old female patient. (A) Computed tomography (CT) scan with the head in neutral position showing abnormal increase in the atlantodental interval. There is assimilation of atlas and C2–3 fusion. Posterior foramen magnum decompression has been done. (B) CT scan with sagittal cut passing through the facets showing facet of atlas dislocated anterior to facet of axis – type 1 atlantoaxial facetal dislocation. (C) T2-weighted magnetic resonance imaging showing neural indentation by the odontoid process. (D) Postoperative CT scan showing reduction in atlantoaxial dislocation and craniovertebral junction realignment. (E) Postoperative CT scan showing the implant. The facets are now in alignment.

Fig. 2.

Images of a 19-year-old male patient. (A) T1-weighted magnetic resonance imaging (MRI) showing severe basilar invagination. (B) T2-weighted MRI. (C) Computed tomography (CT) scan showing no atlantodental interval abnormality. (D) CT scan showing type 2 atlantoaxial facetal instability. The facet of atlas is dislocated posterior to the facet of axis. (E) Postoperative CT scan. (F) Postoperative CT scan showing the implant.

Fig. 3.

Images of a 33-year-old male patient. (A) T2-weighted magnetic resonance imaging (MRI) showing basilar invagination and Chiari formation. (B) Computed tomography (CT) scan showing basilar invagination. There is no abnormality in atlantodental interval. (C) The facets of atlas and axis are in alignment. Clinical and radiological features suggest type 3 atlantoaxial facetal instability. (D) Postoperative CT scan. (E) Postoperative CT scan showing the implant.

Fig. 4.

Images of a 35-year-old male patient. (A) T2-weighted magnetic resonance imaging (MRI) showing basilar invagination, Chiari formation and syringomyelia. (B) Computed tomography (CT) scan showing basilar invagination. (C) CT scan with the cut passing through the facets. The facets of atlas and axis are in alignment. (D) Postoperative CT scan. (E) Postoperative CT scan showing the implant in the lateral masses of atlas and axis. (F) Postoperative MRI showing reduction in the size of syrinx.

Fig. 5.

Images of a 52-year-old male patient. (A) T2-weighted magnetic resonance imaging (MRI) showing multisegmental spinal degeneration with evidences of cord compression and cord signal alteration. There is no neural compression in the region of craniovertebral junction. (B) MRI cut through the facets of atlas and axis showing type 2 atlantoaxial facetal instability. (C) Postoperative X-ray showing multisegmental fixation that includes atlantoaxial joint fixation.

Fig. 6.

Images of a 53-year-old female patient. (A) T2-weighted magnetic resonance imaging (MRI) showing multilevel cervical cord compression by ossification of the posterior longitudinal ligament (OPLL). (B) Computed tomography (CT) scan showing OPLL. (C) CT scan image passing through the facets showing no facetal instability. (D) Plain X-ray showing multisegmental spinal fixation that includes atlantoaxial joint.