Preoperative Estimation of Screw Fixation Strength in Vertebral Bodies

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Abstract
Bone mineral density and bone cross-sectional area of human cadaveric vertebral bodies were investigated radiologically and histologically, respectively. After ventral instrumentation with ventral derotation spondylodesis screws, axial pullout force was measured and compared with radiologic and histologic data. To elucidate how well ventral derotation spondylodesis screw fixation strength can be estimated before surgery by specified applications of dual-energy x-ray absorptiometry, quantitative computed tomography, T2*-relaxation time in magnetic resonance imaging, and histomorphometry. It is postulated that bone quality plays a crucial role in initial strength of the instrumented spine. Bone quality is even more important in anterior fixation because of the prevalence of spongy bone in the vertebral body. Bone mineral density of human cadaveric lumbar-vertebral bodies was assessed by dual-energy x-ray absorptiometry and quantitative computed tomography (cancellous and cortical bone separately). Cancellous bone was also characterized by T2*-relaxation time, measured by magnetic resonance imaging and histomorphometric study. Vertebral bodies were instrumented ventrally with VDS screws, and screw axial pullout force was measured and correlated with each of the nonmechanical measures. Patients with manifest osteoporosis, osteomalacia and tumors were excluded. For statistical analysis, the Mann-Whitney rank sum test was used with a significance value of P < 0.05. The highest correlation with pullout force was for density of cancellous bone determined by quantitative computed tomography (r = 0.72; P < 0.001), immediately followed that determined by dual-energy x-ray absorptiometry (r = 0.70; P < 0.001). Results of measurement of T2*-relaxation time and those of histomorphometric study correlated moderately (r = 0.55; r = 0.50), whereas cortical bone density determined by quantitative computed tomography showed negligible correlation (r = 0.2). The absorptiometric techniques, quantitative computed tomographic scan of cancellous bone and dual-energy x-ray absorptiometric study, provide more accurate readings for preoperative estimation of initial VDS screw fixation strength than do the other methods studied.

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