Abstract
In vitro testing of spinal motion segments provides valuable information about the effects of surgical procedures on the biomechanics of the spine. Few studies, however have investigated the effect of varying laboratory testing environments on the outcome of these tests. This study aims to identify differences in mechanical properties induced by testing in one of three testing environments, and trends due to repeated testing over time.
27 sheep lumbar motion segments were tested in either,
-
air at 18°C while wrapped with gauze soaked in Phosphate Buffered Saline (PBS),
-
a PBS bath at 37°C, or
-
at 37°C and 100% humidity.
Specimens were cycled through +/−8Nm in axial rotation, lateral bending, and flexion/extension. Tests were repeated every hour for 6 hours. Torque and angle were recorded and each bending mode was repeated for 4 cycles, the last 3 of which were used in calculations. Stiffness (5–7Nm), neutral zone (NZ), NZ stiffness, Range of Motion (ROM) energy under the loading curve and hysteresis area were calculated and evaluated with ANOVA.
Post hoc comparisons found differences in stiffness, hysteresis area and energy of bending between room temperature and both heated conditions during flexion/extension. Differences were also noted between the room temperature and PBS bath conditions for stiffness and hysteresis area during lateral bending. One explanation of the results could be the thermo-sensitive properties of spinal ligaments and intervertebral fibrocartilages.
Repeated testing was a factor that affected the outcome of NZ, NZ stiffness, ROM and energy under the loading curve in all modes of torsion. If not accounted for during repeated tests this could lead to confounding results. Many of the traditionally reported variables (NZ, ROM) showed changes with repeated testing while hysteresis area remained relatively steady during repeated tests while identifying differences between testing groups. This variable may be useful in evaluating the condition of a motion segment with less time related effects.
Correspondence should be addressed to EORS Secretariat Mag. Gerlinde M. Jahn, c/o Vienna Medical Academy, Alserstrasse 4, 1090 Vienna, Austria. Fax: +43-1-4078274. Email: eors@medacad.org
