Anterior thoracolumbar instrumentation: Stiffness and load sharing characteristics of plate and rod systems

Study Design. An in vitro biomechanical study using a thoracolumbar corpectomy model to compare load sharing capabilities and stiffnesses of six different anterior instrumentation systems (three rod styles and three plate styles) for stabilizing the thoracic and lumbar spine. Objectives. To evaluate the axial load sharing capabilities of the instrumentation in a thoracolumbar corpectomy model and to measure the bending stiffness of the anterior instrumentation systems for the axes of flexion extension, lateral bending, and axial rotation with and without an anterior column graft in place. Summary of Background Data. Prior publications have analyzed biomechanical characteristics of many spinal instrumentation systems. These reports have compared anterior instrumentation systems with posterior instrumentation systems, in situ fusion techniques, intervertebral spacers, structural allograft and instrumentation, and combined anterior and posterior instrumentation. Other reports have published data on the biomechanical characteristics of typical anterior and posterior spinal instrumentation systems. However, there are no published reports that specifically compare the characteristics of anterior plate-style with anterior rod-style systems, or examining load sharing capabilities. Methods. Six constructs of each of six instrumentation systems were mounted on simulated vertebral bodies. A custom four-axis spine simulator was used to apply independent flexion-extension, lateral bending, and axial rotation moments as well as axial compressive loads. Axial load sharing was measured through a range of applied axial loads from 50 N to 500 N with rotational moments maintained at 0 Nm. The bending stiffness of each construct was calculated in response to +/-5.0 Nm moments about each axis of rotation with a 50 N compressive axial load with a full-length corpectomy graft in place, simulating reconstruction of the anterior column, and with no graft in place, simulating catastrophic graft failure. Statistical significance was determined using an analysis of variance and Fisher PLSD post hoc test with an alpha less than or equal to 0.05. Results. Load sharing results ranged from 63% to 89%. There was an inverse relationship between load sharing and stiffness. No correlation was found between load sharing and implant style (rod vs. plate). With the graft in place, stiffness result varied by instrumentation system rather than by plate/rod style. Without the graft, the stiffness of the constructs decreased approximately one-third in flexion-extension, two-thirds in lateral bending, and one-fifth in axial rotation, underlying the importance of the graft in overall construct stiffness. Conclusions. For both load sharing and stiffness, there is more influence from the design of the instrumentation system, than whether it is a plate or rod style system. The graft contributed to overall construct stiffness, particularly in lateral bending.