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Study Design: Evaluation using 3-dimensional screw trajectory software and computed tomographic scans., Objective: To investigate the anatomic feasibility of laminar screw placement in the upper thoracic spine compared with pedicle screw placement., Summary of Background Data: Although laminar screws have been suggested as an alternative to pedicle screws in the upper thoracic spine, previous anatomic feasibility studies have some limitations., Methods: Four types of screws were simulated from T1 to T6: unilaminar screw (US), superior bilaminar screw (SBS), inferior bilaminar screw (IBS), and pedicle screw (PS). Maximum allowable screw dimensions and the success rates of 4.5-mm screw placement were compared for each level. Laminar screw dimensions with more than 90% success rate at each level were determined for reference., Results: Computed tomographic scans of 132 patients were analyzed. Laminar screw diameters gradually increased from T1 (4.4-5.4 mm, for each type) to T6 (4.8-6.7 mm), whereas PS diameter steeply declined from T1 (5.9 mm) to T4 (3.4 mm) and then leveled off. At T1, PS had greater success rate of 4.5-mm screw placement than laminar screws (US > IBS > SBS); at T2, US had greater success rate than IBS, followed by PS and SBS; and at T3 to T6, laminar screws (US > IBS > SBS) had greater success rate than PS in all comparisons. Except for SBS at T1, laminar screw diameters with more than 90% success rates were between 3.5 and 5.0 mm., Conclusion: In view of their anatomic feasibility, laminar screws can be a viable alternative to PSs in the upper thoracic spine. Particularly at T3 to T6 where the pedicle width is inherently small, the success rates of laminar screw placement were significantly and consistently higher than those of PS placement. The comparable success rates of laminar screws using commercially available screw sizes further emphasize their potential clinical use.

Study Design: Numerical modeling and simulations of scoliotic spine instrumentation using monoaxial, uniaxial, polyaxial, and multiple-degrees-of-freedom (6DOF) postloading pedicle screws., Objective: To biomechanically analyze the general curve reduction effects and bone-screw force levels of monoaxial, uniaxial, polyaxial, and 6DOF pedicle screws for scoliotic spine instrumentation., Summary of Background Data: The ideal spinal fusion construct for treating scoliosis is still debatable. Studies on the effects of different types of implants were mainly based on postoperative radiograph measurements. Systematic studies are yet to be done on how bone-screw forces are correlated with screw types., Methods: Computer biomechanical models were built using 3-dimensional geometry and spine stiffness of 10 patients with adolescent idiopathic scoliosis having undergone spinal instrumentation. The surgical instrumentations were simulated each time, using a different type of screw. For each case and screw type, 15 screw placement variations were simulated to investigate their effects on bone-screw forces., Results: The maximum differences between different screw types were 6.4°, 1.1°, and 4.7°, respectively, for main thoracic Cobb angles, main thoracic apical vertebral rotation, and thoracic kyphosis (1.2°, 0.3°, and 0.3° on average). The average bone-screw forces were higher for monoaxial (229 N ± 140 N) than uniaxial (206 N ± 122 N), polyaxial (141 N ± 99 N), and 6DOF screws (103 N ± 42 N). Bone-screw forces with monoaxial screws were, respectively, 1.1, 2.5, and 25 times more sensitive to screw placement variation than uniaxial, polyaxial, and 6DOF screws., Conclusion: The bone-screw loads of different screws were significantly different. The descending order of bone-screw loads was monoaxial, uniaxial, polyaxial, and 6DOF screws. For patients with large and stiff spinal deformities or for patients with compromised bone quality, screws with more degrees of freedom offer better perspective to reduce bone-screw connection failure.

Study Design: An anatomic study evaluating the feasibility of posterior screw fixation on the lateral mass of the atlas and comparing the pullout force of C1 posterior lateral mass screws with that of occipital and C2 pedicle screws., Objectives: To study the feasibility, anatomy, and biomechanics of posterior screw fixation within the lateral mass of the atlas., Summary of Background Data: Occipitocervical fusion is a common choice for atlantoaxial dislocations. After that the movement of occipitocervical region will be restricted. If screws could be placed in the lateral mass of the atlas, 1) the movement of C0-C1 junction will be preserved; 2) through the screw the dislocated atlas could be pulled backward; and 3) the fractured lateral mass can be fixed using the posterior lateral mass screw directly., Methods: Thirty atlas specimens of native adults were used to measure pertinent clinical data. Six fresh upper cervical specimens were used to observe the association of the C2 nerve and the lateral mass of the atlas. The pullout force of the C1 lateral mass was measured and compared with that of screws placed into the occiput and C2 pedicle, respectively., Results: The distances from the midline to the midpoint of the C1 lateral mass, to the inner wall of the transverse foramen, and to the inner edge of the pedicle were 17.6 +/- 1.2 mm, 23.0 +/- 1.7 mm, and 14.2 +/-1.1 mm, respectively. The width and height of the C1 lateral mass were 11.6 +/- 1.4 mm and 12.7 +/- 1.0 mm, respectively. The distance between the inferior midpoint of the C1 lateral mass in the transverse plane of the midposterior arch of the atlas to the edge of the inferior atlas joint line was 4.1 +/- 0.7 mm. The width of the posterior arch at the point of vertebral artery overpass was 4.7 +/- 1.0 mm. The observation on the fresh specimens showed that the C2 nerve passes inferolateral to the C1-C2 joint and could be pulled sideways. The screw pullout force within the lateral mass of the atlas was 1818.16 +/- 422.67 N. This is significantly less than that of screws within the occiput, and there was no difference with that of a C2 pedicle screw., Conclusion: The width and height of the atlas lateral mass were larger than that of the C2 pedicle, and there was enough space to insert a 3.5-mm diameter screw in the atlas lateral mass over the C2 nerve. The pullout force of the screw on the lateral mass of the atlas was the same as that of the C2 pedicle screw. It is possible toinsert a 3.5-mm screw in the lateral mass of the atlas. The direction of the screw should be about 20 degrees anterosuperior in the vertical plane and 15 degrees inward in the horizontal plane. The suitable length of the screw should be approximately 22 mm inside the lateral mass.

Study Design: Clinically relevant aspects of pedicle screws were subjected to electrical resistance testing., Objectives: To catalog commonly used pedicle screws in terms of electrical resistance, and to determine whether polyaxial-type pedicle screws have the potential to create a high-resistance circuit during stimulus-evoked electromyographic testing., Summary of Background Data: Although stimulus-evoked electromyography is commonly used to confirm the accuracy of pedicle screw placement, no studies have documented the electrical resistance of commonly used pedicle screws., Methods: Resistance measurements were obtained from eight pedicle screw varieties (5 screws of each type) across the screw shank and between the shank and regions of the screw that would be clinically accessible to stimulus-evoked electromyographic testing with a screw implanted in a pedicle. To determine measurement variability, resistance was measured three times at each site and with the crown of the polyaxial-type screw in three random positions., Results: Resistance across the screw shank ranged from 0 to 36.4 ohms, whereas resistance across the length of the monoaxial-type screws ranged from 0.1 to 31.8 ohms. Resistance between the hexagonal port and shank of polyaxial-type screws ranged from 0 to 25 ohms. In contrast, resistance between the mobile crown and shank of polyaxial-type screws varied widely, ranging from 0.1 ohms to an open circuit (no electrical conduction). Polyaxial-type screws demonstrated an open circuit in 28 of 75 measurements (37%) and a high-resistance circuit (exceeding 1000 ohms) in 5 of 75 measurements (7%)., Conclusions: Polyaxial-type pedicle screws have the potential for high electrical resistance between the mobile crown and shank, and therefore may fail to demonstrate an electromyographic response during stimulus-evoked electromyographic testing in the setting of a pedicle breech. To avoid false-negative stimulus-evoked electromyographic testing, the cathode stimulator probe should be applied to the hexagonal port or directly to the screw shank, and not to the mobile crown.