Bilateral separation of the neural arch was found in five per cent. of 200 skeletons examined.
Spondylolisthesis was present as an associated lesion in nearly fifty per cent. of the subjects having bilateral separation of the neural arch. The existence of this defect in the arch cannot generally be demonstrated, roentgenologically, in the living subject. When, however, spondylolisthesis is present and there is a considerable space between the two parts of the divided arch, the defect may be visualized. Its existence is suggested when the anteroposterior diameter of the vertebra involved in spondylolisthesis is greater than that of the other lumbar vertebrae, as seen in the lateral roentgenogram.
Bilateral separation of the arch results in a marked instability of the lumbar spine.
Increasing knowledge, gained from roentgenographic examination and operative findings, indicates that this defect is the fundamental etiological factor in a rather large group of cases of spondylolisthesis.