Editorial: Biomechanical effects of interspinous process devices using a hybrid testing protocol

Arnold is a consultant for Medtronic Sofamor Danek, Stryker Spine, and Fzio Med; has direct stock ownership in Z-Plasty; and has had sponsored or reimbursed travel from AOSpine North America.T he use of lumbar interspinous process (ISP) devices for the treatment of symptomatic lumbar stenosis has dramatically increased in the past decade, 1,2 very likely because of the increased number of elderly patients in the population.Such patients tend to have several comorbidities, which increase the risk of lumbar laminectomy; thus, a less invasive surgical procedure was required to manage this common problem.Interspinous process devices are appealing because they can often be inserted with less risk than a standard laminectomy. 6n 2005, the FDA approved the use of ISP devices for the treatment of neurogenic claudication.Currently, several devices are on the market in Europe and the United States, and the most frequently used device is the X-Stop (Medtronic Inc.).The goal of an ISP device is to prevent extension and functionally increase the area of the spinal canal and neuroforamina at the level at which the device is placed.The procedure often takes less than 30 minutes, and minimal dissection of the posterior elements is required. 9,12 Knowledge of the biomechanics of these devices is useful to the practicing surgeon.Several in vitro and in vivo studies have assessed range of motion (ROM), intradiscal pressure (IDP), facet loading, and effects on adjacent segments. 4The study by Erbulut et al. 3 is a finite-element (FE) analysis in which the authors looked at several biomechanical effects of an X-Stop-like device on the implanted and adjacent lumbar segments.The authors used a hybrid testing model that applied the moment levels necessary to achieve displacements identical to those of the intact specimen. 10,11 While this approach may better represent the in vivo mechanics of motion, this particular test protocol has been studied infrequently. 7he authors found that the device tested would be predicted to decrease ROM, facet load, and IDP at the index level, although this was only tested in extension.Notably,