Effect of a New Annular Incision on Biomechanical Properties of the Intervertebral Disc.

Abstract

To compare the biomechanical properties of a novel annular incision technique, an oblique incision made approximately 60° to the spinal column, with the traditional transverse and longitudinal annular slit incision in an ex vivo sheep lumbar spine model. Sixteen sheep lumbar spines were used for the current ex vivo biomechanical comparative study. Functional spine unit (FSU) specimens composed of two vertebrae and one disc in the middle was cut from the whole lumbar spine. Annular slit incisions of 5 mm were made in different directions with a 15-blade knife at the intervertebral disc, following which partial discectomy was performed to produce the following groups: control with no incision, transverse slit, longitudinal slit and oblique slit groups. The specimens were then subjected to flexion-extension, lateral bending, axial rotation and compression tests. As expected, the control group showed the least range of motion (ROM) in the flexion-extension test. The oblique slit group showed a trend toward a smaller ROM than the transverse and longitudinal groups in 1, 2, 3 and 5 Nm flexion-extension tests; these differences were not statistically significant (P > 0.05). In addition, the transverse (5.80° ± 0.20°), longitudinal (5.77° ± 0.67°) and oblique (5.47° ± 0.43°) slit groups showed a significantly larger ROM than the control group (3.22° ± 0.28°) in 2 Nm lateral bending tests (P < 0.05). Compared with the transverse and longitudinal groups, the oblique group also showed a trend toward a smaller ROM in lateral bending tests (P > 0.05). Following increments in the axial torsion force, the ROM was greater in all four experimental groups than the ROM with 1 Nm axial torsion. Furthermore, a significantly smaller axial rotational ROM was found in the oblique than the transverse group for 1 and 5 Nm force (P < 0.05). With increase in the axial force to 5 Nm, the ROM in the oblique slit group (4.71° ± 0.52°) was significantly smaller than that in the transverse group (7.25° ± 0.46°, P < 0.05), but not significantly different from that of the longitudinal slit group (5.84° ± 0.23°, P > 0.05). Comparable ultimate loads to failure were found in the oblique, transverse and longitudinal groups; the highest ultimate load to failure being in the control group (P > 0.05). The novel oblique slit annular incision to the intervertebral disc showed a trend toward better biomechanical properties than the traditional transverse and longitudinal slit incisions.