P25. Analysis of cage stability and interbody pressure distribution in compression of cortical bone trajectory fixation

BACKGROUND CONTEXT The compressive stiffness in posterior lumbar interbody fusion (PLIF) secure grafts. The cortical bone trajectory (CBT) fixation is effective for the initial fixation. However, it has been reported that the bone fusion rate of CBT fixation is lower than traditional pedicle screw fixation. One of these factors is less stability of anterior column in CBT fixation, which leads to the cage displacement. PURPOSE In this study, to elucidate the instability of anterior column in CBT fixation, we investigated the experimental initial compressive loading and distribution in CBT fixation. Furthermore, to clarify the cage displacement in CBT fixation, we retrospectively studied the relationship between the initial cage position and displacement in CBT fixation. STUDY DESIGN/SETTING Comparative study using lumbar vertebrae of pigs and human model bones, and retrospective study to clarify cage stability and interbody pressure distribution in compression. METHODS We inserted screws for lumbar vertebrae of pigs and human model bones by traditional trajectory (n=3) and CBT (n=4). Then we placed the pressure measurement film (PRESCALE; Fujifilm, Tokyo, Japan) into the intervertebral space. We analyzed the initial compression loading and distribution. A total of 67 patients (34 men, 33 women, mean age 66.7 years) who underwent CBT fixation using single polyetheretherketone cage in our institution were retrospectively investigated. A total of 114 lumbar cages were analyzed for place in axial plane of intervertebral space using computed tomography (CT) images. Then, we evaluated the range of displacement within 12 months after surgery. RESULTS In the experiment using pig lumbar vertebrae, the average pressure of the anterior column of intervertebral space was significantly lower (p=0.0044) in CBT (0.60±0.01MPa) than traditional trajectory (0.90±0.07MPa), and the compression loading was also significantly lower (p=0.0016) in CBT (12±5N) than traditional trajectory (73±9N). The similar result was observed in the human model bones. In the retrospective study of cage position, cage displacement was observed more frequently in the anterior location (14 cages, 16.3%) than the posterior location (2 cages, 7.1%). However, the cage position was not a significant risk factor for displacement (OR 2.528, 95% CI 0.578-11.75, p=0.227). CONCLUSIONS These findings suggest that the compression power of the anterior column in compression of CBT fixation is insufficient to grasp the cage followed by the cage instability and displacement. FDA DEVICE/DRUG STATUS Unavailable from authors at time of publication.