European spine journal : official publication of the European Spine Society, the European Spinal Deformity Society, and the European Section of the Cervical Spine Research Society
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Accelerated degenerative changes at intervertebral levels adjacent to a spinal fusion, the so-called adjacent segment degeneration (ASD), have been reported in many clinical studies. Even though the pathogenesis of ASD is still widely unknown, biomechanical in vitro approaches have often been used to investigate the impact of spinal instrumentation on the adjacent segments. The goal of this review is (1) to summarize the results of these studies with respect to the applied protocol and loads and (2) to discuss if the assumptions made for the different protocols match the patients' postoperative situation. ⋯ The three test protocols mainly differ in the assumption on the postoperative motion behavior of the patients, which is the main reason for the conflicting findings. However, the protocols have never been validated using in vivo kinematic data. In a parallel review on in vivo kinematics by Malakoutian et al., it was found that the assumption that the patients move exactly the same after fusion implemented with the stiffness- and hybrid protocol does not match the patients' behavior. They showed that the motion of the whole lumbar spine rather tends to decrease in most studies, which could be predicted by the flexibility protocol. However, when the flexibility protocol is used with the "gold standard" pure moment, the difference in the kinematic changes between the cranial and caudal adjacent segment cannot be reproduced, putting the validity of current in vitro protocols into question.