Experimental Evaluations of Intervertebral Disc Mechanics Following Posterolateral Fusion are Dependent on Testing Protocol

IntroductionNumerous studies have sought to quantify the biomechanical effects of posterior spinal fusion and disc arthroplasty; however, the results from these studies are conflicting. Additionally, the variety of techniques utilized during biomechanical testing complicates comparing results of instrumentation studies. Two distinct methods of experimental loading protocols have emerged to test the efficacy of spinal instrumentation. The load control testing method utilizes a known load to induce specimen rotation with the displacement as the experimental output, while displacement control testing methods induce movement of the specimen to a target rotation with the reaction moment being the output. A number of studies have tested spinal fusion and disc arthroplasty devices using load control testing methods and have reported a mixture of results. While all load control studies have consistently reported decreased total spine mobility due to loss of rotation at the fused segment, several studies have demonstrated increased intersegmental rotations and intervertebral disc mechanics at segments adjacent to the fusion, including a finite element study performed by Tang et al. that reported increases in nucleus pulposus pressures of 66%, intersegmental rotations of 30%, and annulus fibrosus stresses of 200% in the segment cranial to a fusion tested under load control. Other studies have shown no change in spinal kinematic or mechanical parameters in adjacent segments under the same loading protocol. The major assumption of the load control testing protocol is that fusion patients will be content to live with a limited amount of total spine mobility; however, this may not be feasible in order to perform the required activities of daily living. Thus, an argument has been put forth that the same amount of total rotation should be applied to implanted specimens as healthy specimens in biomechanical tests that intend to measure the efficacy of fusion and arthroplasty devices. This technique is also known as the hybrid method. The intent of this study was to analyze the difference in intradiscal pressure and annulus fibrosus stress measurements in fused spinal specimens tested under load control and the hybrid method using a finite element model.