[Atlanto-axial dislocation--instability index and indications for surgery (author's transl)].

Thirty cases of atlanto-axial dislocation were investigated and the results are reported. The may be divided into 2 groups; (a) 15 cases of abnormalities of the odontoid process, i. e. 7 cases of os odontoideum, 7 cases of fractured odontoid, 1 cases of deformed odontoid, and (b) 15 cases with no abnormalities of the odontoid process. Neurological signs were noted in 14 cases which may be divided into the following three types, i.e. acute, recurrent, and chronic-progressive. The spinal canal diameter at the atlanto-axial level was measured in the position of flexion and extension using cervical tomography. The degree of motility thus obtained was expressed in terms of "Instability Index" (I. I.) devised in our clinic. In the group with neurological signs, the acute type showed an I. I. of 39 percent which was the highest value, and followed by 33 percent in the recurrent type. In the chronic-progressive type, the I. I. value was as low as 3.75 percent with a mean maximal diameter of 11.0 mm. Operation was performed in 16 cases. In 12 cases, posterior fusion was performed from the occipital bone to C3 or in some cases C4. In 3 cases, posterior decompression and fusion from the occipital bone to C3 were conducted. In one case, anterior decompression and fusion was performed by the transoral approach. The outcome was generally satisfactory. As a new method of posterior fusion, we have used "single bone-flap" method since 1972, that could fix the occipital bone and the cervical spines more tightly. In conclusion, the operation is indicated in the following cases; (1) when neurological signs are evident, (2) even in the abcence of neurological signs, (a) when the I. I. is high (over 20 percent), (b) even when the I. I. is low, when the maximal diameter of the canal is under 14.0 mm.