Abstract:

Objective　To evaluate the intervertebral disc heights of porcine lumbar spines implanted with 3 flexible /dynamic pedicle screw fixations under axial compressive force at 8 locations, and compare it with conventional pedicle screw fixation.   Methods  Eight fresh porcine spine specimens (L2~L5) were collected. 3 flexible /dynamic and 1 conventional pedicle screw fixations were applied to L3-L4 segment following resection of the posterior structures. Each specimen was subjected to flexion-compression, extension-　compression, lateral-bending-compression, lateral-flexion-compression and lateral-extension-compression with a compressive force of 400N on the top of the specimen at 8 locations. 3D motion was measured using the Optotrak, and the L3-L4 anterior disc height was calculated. The specimen conditions included the intact and instrumentation with 5.5 mm rod, 3mm rod and sliding rod, and with ball-socket joint pedicle screws.   Results  Under the flexion-compression, lateral-bending-compression and lateral-flexion compression, there were no significant differences in the disc height change between the intact and each fixation condition（P＞0.05）. In extension-compression, the disc height change with the 3.0 mm rod was significantly different from the intact or the fixations with 5.5mm rod（P＜0.05)，while no significant difference among other groups were detected; In lateral-extension-compression, there was significant difference in the disc height change between each fixation and intact status (P<0.05), and between fixations with 3.0mm rod and the ball-socket joint screw fixation (P<0.05).　Conclusions　Flexible/dynamic fixations enhanced the compressive stability of lumbar spine when compared to the intact status. The fixation with sliding rods achieved compressive stability close to the conventional pedicle screw fixation. The compressive stability with ball-socked joint screw fixation was comparable to the conventional fixation in extension-compression and lateral- extension-compression, but was poor in flexion-compression or lateral-flexion- compression. Fixation with thinner rods was not strong enough in extension-compression or lateral-extension-compression.

Key words: Pedicle screw fixation, Flexible, Dynamic, Compression, Biomechanics