Vertebral body and posterior element morphology: the normal spine in middle life.

Multiple morphologic parameters were studied in 50 normal adult vertebral columns ranging in age from 20 to 40 years at the time of death. Posterior element morphology is highly variable and largely unpredictable. Minimum pedicle diameters ranged from 1.8 mm at T6 to 6.4 mm at L5, and did not correlate with any more readily measured vertebral dimensions. Penetration of 5 mm screw threads through pedicle cortex can be expected to occur routinely in the lower thoracic spine, and in a lesser but significant number of lumbar vertebrae.

A new technique to improve cortical-evoked potentials in spinal cord monitoring. A ratio method of analysis.

Systemic effects such as anesthesia, hypotension, hypothermia, and hypoxia affect the cortical evoked responses. We propose, that by sequential stimulation of the median and posterior tibial nerves, and the construction of a ratio from the value of their amplitudes, the systemic effects can be eliminated and thus improve the reliability of the cortical evoked responses. Two groups of scoliosis patients who underwent spinal surgery with instrumentation were analyzed retrospectively. Both groups had spinal cord monitoring using peripheral nerve stimulation and cortical recordings of the somatosensory-evoked response (SER). In Group I, 50 patients were analyzed for changes in posterior tibial nerve response before and after distraction. Wide variability in the response suggested this method to be less reliable in predicting spinal cord conduction deficits. Thirty-eight patients in Group 2 were analyzed using both the median and posterior tibial nerve amplitudes. A ratio of the posterior tibial to median nerve wave amplitude was constructed, thus eliminating any systemic variables. A critical value, alerting the surgeons to possible decreases in spinal cord conduction, was calculated by subtracting one standard deviation from the mean of the postdistraction ratios of the posterior tibial to median nerves (1.20-.633 = .567).