The Influence of Posterior Instrumentation on Adjacent and Transfixed Facet Joints in Patients With Thoracolumbar Spinal Injuries: A Morphological in Vivo Study Using Computerized Tomography Osteoabsorptiometry

Abstract

Subchondral mineralization of adjacent and transfixed facet joints was analyzed in patients with thoracolumbar spinal injuries, both before posterior instrumentation and after removal of the spinal implant. To examine the influence of posterior instrumentation on content and distribution of subchondral mineralization as a correlate of the long-term load acting on the adjacent and transfixed zygapophysial joints. Posterior stabilization plays an important role in the treatment of spinal injuries and is a standard technique for the treatment of thoracolumbar spinal fractures. Studies have shown that stress and motion in the adjacent segments are altered in the presence of instrumentation. Twenty-three patients with thoracolumbar spinal injuries had computerized tomography (CT) during the course of routine posttraumatic diagnostics and subsequently received bisegmental posterior fixation with an internal fixator. Second CT were obtained after removal of the fixation device, which was performed on an average of 9.4 months after the trauma. Patients were divided into 2 groups with follow-up CT taken within either less than 3 months (group A: average 7.3 days, 15 patients) or 6 and more months (group B: average 17 months, 8 patients) after the internal fixator had been removed. Quantitative and qualitative CT osteoabsorptiometry were used to assess changes in subchondral mineralization, reflecting the altered load acting on the adjacent and bridged zygapophysial joints. There was a significant difference between preoperative and postoperative calcium values (P < 0.001) for the whole patient group. Mineralization decrease was significantly more often found than increase (P < 0.001). A separate analysis of the 2 groups of patients revealed significant differences between group A and B (P < 0.001). In group A, a mineralization decrease was found in 61.3% and an increase in 11.0% of the facet joints, while in group B, a mineralization decrease was shown in 21.9% and an increase in 41.0%. No significant differences between adjacent and transfixed facets were found except in group B, in which the suprajacent joints showed a significantly higher mineralization increase than the transfixed joints (P = 0.030). Decrease in subchondral mineralization indicates reduced load acting on the examined zygapophysial joints. This finding in patients with early follow-up CT seems to be caused by reduced activity in most of the patients until removal of the spinal implant. In patients with longer intervals between removal of the fixator and second CT, higher loads acting on the adjacent and bridged joints are shown morphologically. Whether or not these changes lead to spondylarthritis has to be studied in a long-term follow-up.