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Krag Versus Caudad Trajectory Technique for Pedicle Screw Insertion in Osteoporotic Vertebrae: Biomechanical Comparison and Analysis

Study Design. To compare in detail the effects of pedicle screw insertion in osteoporotic vertebrae via Krag and Caudad trajectory techniques. Objective. To compare the biomechanical stability of 2 pedicle screw fixation techniques and to correlate the stability of the pedicle screw with quantitative computed tomography (QCT). Summary of Background Data. Pedicle screw fixation is commonly used to facilitate fusion and postoperative rehabilitation. Fixation failure and loosening in the metal-bone interface are frequent, with osteoporosis usually a major factor. Pedicle screw fixation in osteoporotic spines is of particular concern regarding implant failure. Few reports have addressed which fixation method provides better biomechanical strength and thus presents less risk of failure. Methods. Eleven cadaveric vertebrae were harvested and subjected to dual-energy x-ray absorptiometry and QCT to assess bone mineral density. Matched, polyaxial pedicle screws were inserted into the left and right pedicles of each vertebra. Screws were randomly assigned to the Caudad or Krag group by right or left side. They were inserted under 3-dimensional navigation system assistance. Cyclic loading tests were performed (maximum load 250 N, 3 Hz, up to 30,000 cycles) while recording load and displacement. Pullout tests were performed if the cyclic loading test was completed. Stiffness quotients were calculated. Results. Cycle-displacement curves showed more pedicle screw dislodgement in the Krag than the Caudad group (P < 0.01). Initially, stiffness was significantly higher in the Caudad group (P < 0.01), but the difference diminished thereafter. In the Caudad group, bone mineral density measured by QCT was significantly correlated with several biomechanical parameters. Conclusion. Pedicle screws inserted in osteoporotic lumbar vertebrae using the Caudad trajectory displayed significantly higher biomechanical strength than those inserted using the Krag trajectory, especially during early fixation. Stability of pedicle screw fixation using the Caudad trajectory technique can be estimated by QCT. Level of Evidence: A in vitro biomechanical cadver study