The authors present the anatomic and experimental basis of an original technique for screwing at the first sacral level employed in lumbosacral fusion. The anatomic studies were based on specimens from the anatomy museum, frozen sections of the sacrum and CT examinations with three-dimensional reconstruction and assessment of the density of the different structures of S1 in Hounsfield units (HU). The findings were that the ala and lateral portions of S1 contain yellow marrow forming what amounts to a fatty sphere bounded by the cortical bone of the sacroiliac joint, the linea terminalis and the spongy bone of the pedicles and of the body of S1. The experimental study was made by avulsion of sacral screws (system of Cotrel Dubousset), each of 7mm diameter. No screw perforated the sacral cortex. Three directions were tested. The insertion of a screw through the pedicle and body of S1 is advised, with the point of insertion below and lateral to the articular process of S1 and an oblique course forward and inward at an angle of 10 degrees to the sagittal plane. This internal obliquity is limited by the posterior prominence of the iliac ala.
The authors present a retrospective study based on a homogeneous series of 34 patients with burst fractures of the thoracolumbar junction, fixed using Cotrel-Dubousset instrumentation. These patients underwent instrumentation using a short construct of hooks and screws gripping the two vertebrae above the lesion (2HS) and screws and hooks gripping the first vertebra below the lesion (1SH). This construct was therefore called "2HS-1SH". In order to evaluate just the material resistance after getting up, only the patients who were upright on the 4th day without a body cast and with no secondary anterior osteosynthesis were included in this study. Four patients showed some neurological symptoms on admission but recovery was so quick that they could be included in this study. Mean follow-up was 4 years 1 month (range 3 years 1 month and 6 years 2 months). Vertebral and regional kyphosis angles were measured preoperative, postoperatively and at the final follow-up. Functional recovery and complications were analyzed. Mean vertebral kyphosis was 21.2 degrees preoperatively, 3.8 degrees postoperatively and 5.3 degrees at the final follow-up. Regional kyphosis angles were respectively 19.2, 0.2, and 2.7 degrees. We had two cases of deep suppuration, one early and the other late. None of the patients required analgesics for more than 6 months after the operation. Patients returned to work after 5 months on average. The authors concluded that fixation by screw-and-hook constructs is an effective way to stabilize thoracolumbar junction burst fractures.
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