The biomechanical effect of postoperative hypolordosis in instrumented lumbar fusion on instrumented and adjacent spinal segments

Study Design. Change in lumbar lordosis was measured in patients that had u ndergone posterolateral lumbar fusions using transpedicular instrumentation . The biomechanical effects of postoperative lumbar malalignment were measu red in cadaveric specimens. Objectives. To determine the extent of postoperative lumbar sagittal malali gnment caused by an intraoperative kneeling position with 90 degrees of hip and knee flexion, and to assess its effect on the mechanical loading of th e instrumented and adjacent segments. Summary of Background Data. The importance of maintaining the baseline lumb ar lordosis after surgery has been stressed in the literature. However, the re are few objective data to evaluate whether postoperative hypolordosis in the instrumented segments can increase the likelihood of junctional breakd own. Methods. Segmental lordosis was measured on preoperative standing, intraope rative prone, and postoperative standing radiographs. In human cadaveric sp ines, a lordosis toss of up to 8 degrees was created across L4-S1 using cal ibrated transpedicular devices. Specimens were tested in extension and unde r axial loading in the upright posture. Results. In patients who underwent L4-S1 fusions, the lordosis within the f usion decreased by 10 degrees intraoperatively and after surgery. Postopera tive lordosis in the proximal (L2-L3 and L3-L4) segments increased by 2 deg rees each, as compared with the preoperative measures. Hypolordosis in the instrumented segments increased the load across the posterior transpedicula r devices, the posterior shear force, and the lamina strain at the adjacent level. Conclusions. Hypolordosis in the instrumented segments caused increased loa ding of the posterior column of the adjacent segments. These biomechanical effects may explain the degenerative changes at the junctional level that h ave been observed as long-term consequences of lumbar fusion.