PREOPERATIVE ESTIMATION OF SCREW FIXATION STRENGTH IN VERTEBRAL BODIES

Study Design. Bone mineral density and bone cross-sectional area of hu man cadaveric vertebral bodies were investigated radiologically and hi stologically, respectively. After ventral instrumentation with ventral derotation spondylodesis screws, axial pullout force was measured and compared with radiologic and histologic data. Objectives. To elucidat e how well ventral derotation spondylodesis screw fixation strength ca n be estimated before surgery by specified applications of dual-energy x-ray absorptiometry, quantitative computed tomography, T-2-relaxati on time in magnetic resonance imaging, and histomorphometry. Summary o f Background Data. It is postulated that bone quality plays a crucial role in initial strength of the instrumented spine. Bone quality is ev en more important in anterior fixation because of the prevalence of sp ongy bone in the vertebral body. Methods. Bone mineral density of huma n cadaveric lumbar-vertebral bodies was assessed by dual-energy x-ray absorptiometry and quantitative computed tomography (cancellous and co rtical bone separately). Cancellous bone was also characterized by T-2 -relaxation time, measured by magnetic resonance imaging and histomor phometric study. Vertebral bodies were instrumented ventrally with VDS screws, and screw axial pullout force was measured and correlated wit h each of the nonmechanical measures. Patients with manifest osteoporo sis, osteomalacia and tumors were excluded. For statistical analysis, the Mann-Whitney rank sum test was used with a significance value of P < 0.05. Results. The highest correlation with pullout force was for d ensity of cancellous bone determined by quantitative computed tomograp hy (r = 0.72; P < 0.001), immediately followed that determined by dual -energy x-ray absorptiometry (r = 0.70; (P < 0.001). Results of measur ement of T-2-relaxation time and those of histomorphometric study cor related moderately (r = 0.55; r = 0.50), whereas cortical bone density determined by quantitative computed tomography showed negligible. Con clusions. The absorptiometric techniques, quantitative computed tomogr aphic scan of cancellous bone and dual-energy x-ray absorptiometric st udy, provide more accurate readings for preoperative estimation of ini tial VDS screw fixation strength than do the other methods studied.