Biomechanical comparison of an interspinous device and a rigid stabilization on lumbar adjacent segment range of motion.

PURPOSE OF THE STUDY Decompression surgery with or without fusion is the gold standard treatment of lumbar spinal stenosis, but adjacent segment degeneration has been reported as a long-term complication after fusion. This led to the development of dynamic implants like the interspinous devices. They are supposed to limit extension and expand the spinal canal at the symptomatic level, but with reduced effect on the range of motion of the adjacent segments. The aim of the present study is the evaluation of the biomechanical effects on the range of motion (ROM) of adjacent lumbar segments after decompression and instrumentation with an interspinous device compared to a rigid posterior stabilization device. MATERIALS AND METHODS Eight fresh frozen human cadaver lumbar spines (L2-L5) were tested in a spinal testing device with a moment of 7.5 Nm in flexion/extension, lateral bending and rotation with and without a preload. The preload was applied as a follower load of 400N along the curvature of the spine. The range of motion (ROM) of the adjacent segments L2/L3 and L4/L5 was measured with the intact segment L3/L4, after decompression, consisting of resection of the interspinous ligament, flavectomy and bilateral medial facetecomy, and insertion of the Coflex® (Paradigm Spine, Wurmlingen) and after instrumentation with Click X® (Synthes, Umkirch) as well. RESULTS The interspinous and the rigid device caused a significant increase of ROM at both adjacent segments during all directions of motion and under follower load, without significant difference between these devices. The ROM of L2/L3 tends to increase more than the ROM of L4/L5 after instrumentation without statistical significance. DISCUSSION The "dynamic" Coflex device caused a significant increase of ROM at both adjacent lumbar segments comparable to the increase of ROM after instrumentation with the rigid Click X device. Other in vitro studies observed comparable biomechanical effects on the adjacent segments after fusion, but biomechanical spacer studies concentrated on the "noncompressible" X-Stop® and could not demonstrate a significant adjacent segment effect of this device. CONCLUSIONS The hypothesis, that an interspinous device would reduce the stress on adjacent segments compared to a rigid posterior stabilization device, could not be demonstrated with this biomechanical in vitro study. Therefore, the protection of adjacent segments after instrumentation with dynamic devices is still not completely achieved.