Morphological study of the spinal canal content for subarachnoid endoscopy

This study was designed to examine the morphology of the spinal dural sac and contents, using magnetic resonance imaging in order to define the inner geometrical dimensions that confine the manoeuvre of an endoscope inserted in the lumbar region and along the thoracic and cervical spine.

The morphology of the spine has been studied since the development of myelography. However, most studies have measured the diameters of the spinal cord only, not the size of the subarachnoid space. In addition, the few studies available on the subarachnoid space have focused on the cervical spine, leaving a near-complete dearth of data on the subarachnoid space dimensions along the thoracic spine.

Based on MRI images of the spine from 42 patients, the dimensions of the spinal cord, dural sac, and subarachnoid space were measured at mid-vertebral and inter-vertebral disc levels.

It was found that at each selected transverse level, the subarachnoid space tends to be symmetrical on the right and left sides of the cord, and measures 2.5 mm on average. However, the posterior and anterior segments, measured on the mid-sagittal plane, are generally asymmetrical and vary widely in size, ranging from 1 to 5 mm. These measurements match those found in previous studies, where these are available. The coefficient of variance for the dimensions of the subarachnoid space is as high as 42.4%, while that for the dimensions of the spinal cord is 10-15%.

The findings presented here expand our knowledge of the spinal canal's morphology, and show that an endoscope designed to travel within the subarachnoid space must be smaller than 2.5 mm in diameter.