The present results have biomechanical implications in considering causes of instrumentation failure following single-intervertebral PS fixation of the lumbar spine. That is, even under simple compressive loads, combined compressive and bending stresses are generated simultaneously in the rods, with bending stress an order of magnitude greater than compressive stress. In clinical cases, many instances of rod damage or breakage have been reported in association with long fusions such as scoliosis surgery, osteotomy, and total en bloc spondylectomy [15, 16]. In such cases, the rods were speculated to have been damaged or broken as a result of the extremely high bending stress applied.
Moreover, the present results showed that the stresses applied to the left and right rods are unequal, with a large difference between values. In addition to individual differences in test specimens, such as the shape of the vertebral body and spinal alignment, other possible causes are an asymmetrical angle of screw insertion, and in particular the fact that positioning the central axis of the specimens in the center of the testing machine is difficult because of its asymmetrical shape. However, the results of this study suggest that stresses on the left and right rods were asymmetrical despite the simple axial compression applied to the spine, with greater stress on one side of the rod. Considering only the prevention of rod damage and breakage, it seems important to use a strong design that prevents damage and breakage even if a load is applied to only a single rod. For example, such a design could correspond to increasing the thickness of the rods or using cobalt-chrome to make the rods, to provide sufficient resistance to bending stress [17].
Regarding the data on the rate of decrease in intradiscal pressure in this study, considerable variation was seen between test specimens, but the maximum rate of decrease was only 40% after PS fixation. Consequently, stress shielding by PS fixation was found to be not very significant, and intervertebral disks, as anterior elements, are still subjected to heavy loads. As a result, in clinical practice, if the anterior stabilizing elements of the spine are deemed unstable in single-intervertebral PS fixation, proper stabilization of the anterior stabilizing elements of the spine through the use of bone grafts or interbody cages appears critical for preventing instrumentation failure.
Furthermore, the average compressive load applied to the PS instrument and the average rate of decrease in intradiscal pressure obtained from the pressure sensors in this study were nearly equal, at approximately 26%. Consequently, the decrease in intradiscal pressure due to PS fixation appears to be replaced by an increase in the compressive load on the PS instrument. In other words, this result also appeared to clarify how the compressive load applied to the spine is distributed to the anterior stabilizing element and posterior PS instrument.
To investigate how the compressive load applied to the spine is distributed to anterior elements (vertebral body and intervertebral disks) and posterior elements (facet joints and PS), we believe that the question must be approached from the perspectives of both rod strain and intradiscal pressure, as in the present study.
Notable limitations of the present research were: 1) the small number of specimens; and 2) the fact that only simple axial compression loading tests were conducted. In future studies, we would like to increase the number of specimens, prepare a variety of models, and conduct both bending tests and rotation tests.