Lumbar sagittal contour after posterior interbody fusion: Threaded devices alone versus vertical cages plus posterior instrumentation

Study Design. An observational radiographic study examining lumbar sagittal contour of patients undergoing posterior interbody arthrodesis. Objectives. To compare operative alterations of lumbar sagittal contour after posterior interbody fusion using threaded interbody devices alone versus vertical cages combined with posterior compression instrumentation. Summary of Background Data. Technique-related alterations of lumbar sagittal contour during interbody arthrodesis have received little attention in the spinal literature. Methods. Standing lumbar radiographs were measured for preoperative and postoperative segmental lordosis at levels undergoing posterior interbody arthrodesis using either stand-alone side-by-side threaded devices or vertical cages combined with posterior transpedicular compression instrumentation. Sagittal plane segmental correction (or loss of correction) was calculated and statistically compared. Results. The radiographs of 30 patients (34 spinal segments) undergoing lumbar or lumbosacral arthrodesis were compared. Seventeen patients (18 segments) had undergone interbody fusion using threaded cages, whereas 13 patients (16 segments) underwent fusion using vertically oriented mesh cages combined with posterior compression instrumentation. Preoperative segmental lordosis averaged 8° for both groups. For patients undergoing fusion with threaded cages, there was a mean lordotic loss of 3°/segment. For patients undergoing fusion with vertically oriented mesh cages combined with posterior compression instrumentation, there was a mean lordotic gain of 5°/segment. This difference in segmental sagittal plane contour was highly significant (P = 0.00). Conclusion. Threaded fusion devices placed under interbody distraction with the endplates parallel fail to preserve or reestablish segmental lordosis. Vertical cages, however, when combined with posterior compression instrumentation, not only maintain segmental lordosis, but also can correct sagittal plane deformity.