Laminar hook instrumentation in the cervical spine. An experimental study on the relation of hooks to the spinal cord

Several anterior and posterior methods are today available for stabilizatio n of the cervical spine. Factors such as level and degree of instability, m ethod of decompression, bone quality, length of fixation and safety factors influence the choice of method for a particular patient. The use of lamina r hooks in the cervical spine has been restricted by fear of cord compressi on with the potential of tetraplegia. The aim of the present study was to a ssess the safety and determine the anatomical relation between hooks insert ed in the cervical spinal canal and the dura and spinal cord. Thirteen cada vers from seven women and six men with no evidence of cervical spine disord er were included. The mean age was 81.3 years (range 65-101 years). The cer vical spine was instrumented with cervical Compact Cotrel Dubousset hooks a nd rods. The effect of the hook on the dura was studied by myelography in n ine cadavers. The deformation of the dural sac was quantified by measuremen t of the maximal width of the indentation of the contrast column at each le vel. A CT myelography scan was obtained in three cadavers. The ratio betwee n the distance of maximal hook intrusion into the spinal canal and the cana l diameter in the direction of the hook was calculated. The relation betwee n inserted hooks and the spinal cord and dura was documented in a fresh cad aver studied with CT myelography. A hemilaminectomy was performed at all le vels in three cadavers with direct visual inspection and photography of the hook sites before and after excision of the dura. A dural deformation of 2 mm or less, as observed by myelography, was found at four out of 77 (5%) h ook sites. The deformation was caused by a supralaminar hook at C3, C6 and C7 and by an infralaminar hook at C6. The mean hook intrusion in the spinal canal, as observed on CT, was 27% (range 8-43) of the canal diameter. On v isual inspection, 14 out of 18 hooks were in contact with the dura. After r emoval of the dura, two out of the 18 hooks in the same cadaver were in con tact with the spinal cord. However, no deformation of the cord was observed . To our knowledge this is the first study systematically documenting the r elation between hooks and the spinal cord in cadavers. In 95% of the hooks no deformation of the dural sac was observed and there was no evidence of s pinal cord deformation. From an anatomical point of view, laminar hook inst rumentation can be considered a safe procedure. The study shows, however, t hat hooks inserted in the cervical spine have a close anatomical relationsh ip with the neuraxis, and at stenotic levels the use of other techniques is therefore recommended.