Biomechanical effect of different interspinous devices on lumbar spinal range of motion under preload conditions

Interspinous devices are used as an alternative to the current gold standard treatment, decompressive surgery with or without fusion, for lumbar spinal stenosis. They are supposed to limit extension and expand the spinal canal and foramen at the symptomatic level, but still allow lateral bending and axial rotation in the motion segment. The aim of the present study is the biomechanical evaluation of the change in the range of motion of the affected and adjacent segments following implantation of different interspinous devices under load in all directions of motion. 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 (follower load of 400 N). The ROM was measured after implantation of Aperius(A (R)) (Kyphon, Mannheim), In-Space(A (R)) (Synthes, Umkirch), X-Stop(A (R)) (Tikom, Furth) and Coflex(A (R)) (Paradigm Spine, Wurmlingen) into the segment L3/L4. All interspinous devices caused a significant reduction in extension of the instrumented segment without significantly affecting the other directions of motion. The flexion was reduced by all implants only when the follower load was applied. All devices caused a higher ROM of the whole spine during lateral bending and rotation. The actual evaluated interspinous devices led to a significant reduction in ROM during flexion-extension, but to a significant increase in ROM for the whole specimen (L2-L5) during lateral bending and rotation, which could increase the risk of adjacent segment degeneration.