Measurement of table elevation of the Jackson spine table for studies involving the reverse Trendelenburg position.

To JNA Readership: Before conducting physiologic studies using the Jackson spine table (Orthopedic Systems, Inc, Union City, CA), we measured the angle of table rise produced by reverse Trendelenburg (RT) positioning using a hand-made goniometer during ‘‘dry runs.’’ We intuitively believed that 15-degree positioning, previously described in another study, would result in a reasonable change in head elevation that would not interfere with an ‘‘in vivo’’ surgical procedure. We were surprised to find that the elevation of the head of the table relative to the foot was enormous and not compatible with performing prone lumbar spine surgery. The Jackson table is a rigid 1-piece table. Using simple trigonometry, one can calculate the elevation of the head (in cm) above the foot of the table which is represented by sine angle (y) x length of table (in cm) (Fig. 1). Therefore, the Jackson table length (hypotenuse) of 213.4 cm and a 15-degree table angle (sin y=0.259) produces a head elevation (opposite side) of 55.3 cm (21.8 inches) above the foot. Two clinical studies, one with patients and another with volunteers, have described measurements of intraocular pressure in the prone position taken in 15-degree and 10-degree RT, respectively. In the first study, the table type was not specified and the elevation is therefore difficult to estimate. In the second study using the Jackson table, a 10-degree RT position would result in a 37.1-cm (14.6 inches) elevation, a height which would likely pose significant surgical difficulties during a prone lumbar spine procedure. In future studies, we recommend that the elevation from the parallel be specified as a vertical measure of the difference in elevation between head and foot. This measure, given a rigid nonflexing table, can be obtained by subtracting the difference of the height of the foot of the table above the floor from the height of the head of the table above the floor (Fig. 1). A height of 15 cm (6 inches) from the parallel results in a head elevation that does not interfere with standard operative and anesthetic technique, and corresponds to an angle of 4 degrees. The methods are mathematically equivalent and thus neither method accounts for differences in patient height, but using a 4-degree angle measurement (sin y=0.07) with an error of 1 degree would result in a 25% change in vertical height from 14.9 to 18.6 cm. This compares to an error of 6.7% with a 1-cm error in height measurement. Using angle designations between tables is also less accurate because tables with a shorter length (hypotenuse) would result in a lower head elevation (opposite side) using the same angle. (Fig. 1).