Evaluation of a more ventral starting point for thoracic pedicle screws: higher maximal insertional arc and more medial and safer screw angulation

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To demonstrate that a more ventral starting point for thoracic pedicle screw insertion, produced by aggressively removing the dorsal transverse process bone down to the superior articular facet (SAF), results in a larger margin for error and more medial screw angulation compared to the traditional dorsal starting point (DSP). The margin for error will be quantified by the maximal insertional arc (MIA).


The study population included 10 consecutive operative patients with adult idiopathic scoliosis who underwent primary surgery. All measurements were performed using 3D visualization software by an attending spine surgeon. The screw starting points were 2 mm lateral to the midline of the SAF in the mediolateral direction and in the center of the pedicle in the cephalocaudal direction. The DSP was on the dorsal cortex. The ventral starting point (VSP) was at the depth of the SAF. Measurements included distance to the pedicle isthmus, MIA, and screw trajectories.


Ten patients and 110 vertebral levels (T1–11) were measured. The patients’ average age was 41.4 years (range 18–64 years). The pedicle isthmus was largest at T1 (4.04 ± 1.09 mm), and smallest at T5 (1.05 ± 0.93 mm). The distance to the pedicle isthmus was 7.47 mm for the VSP and 11.92 mm for the DSP (p < 0.001). The MIA was 15.3° for the VSP and 10.1° for the DSP (p < 0.001). Screw angulation was 21.7° for the VSP and 16.8° for the DSP (p < 0.001).


A more ventral starting point for thoracic pedicle screws results in increased MIA and more medial screw angulation. The increased MIA represents an increased tolerance for error that should improve the safety of pedicle screw placement. More medial screw angulation allows improved triangulation of pedicle screws.

ABBREVIATIONS DSP = dorsal starting point; MIA = maximal insertional arc; SAF = superior articular facet; VSP = ventral starting point.

Article Information

Correspondence Lawrence G. Lenke: The Spine Hospital, NewYork-Presbyterian/Allen, New York, NY. ll2989@cumc.columbia.edu.

INCLUDE WHEN CITING Published online December 14, 2018; DOI: 10.3171/2018.8.SPINE18175.

J.D.L. and C.W. share first authorship of this work.

Disclosures Dr. Lenke receives research support from AOSpine, Scoliosis Research Society, EOS, and Setting Scoliosis Straight Foundation. Dr. Lenke is a paid consultant for and/or received royalties from Medtronic, DePuy, K2M, Quality Medical Pub. Dr. Lenke is on the editorial or governing board of Journal of Neurosurgery: Spine, Spine Deformity Journal, Spine, Scoliosis, Journal of Spinal Disorders & Techniques, www.iscoliosis.com, www.spineuniverse.com, Backtalk (Scoliosis Association), Global Spine Outreach, and Orthopaedic Research and Education Foundation. Dr. Lenke also reports having received reimbursement for airfare/hotel from Broadwater, the Seattle Science Foundation, Stryker Spine, and the Spinal Research Foundation; grant support from EOS; and philanthropic research funding from the Fox Family Foundation and Evans Family Donation; and serving as an expert witness in a patent infringement case for Fox Rothschild, LLC.

Dr. Lehman is on the editorial or governing board of Spine Deformity, The Spine Journal, AOSpine, Cervical Spine Research Society, North American Spine Society, and Scoliosis Research Society. Dr. Lehman is a paid presenter, consultant, or speaker for DePuy, a Johnson & Johnson Company; Medtronic; and Stryker. Dr. Lehman receives publishing royalties from Wolters Kluwer Health–Lippincott Williams & Wilkins.

© AANS, except where prohibited by US copyright law.



  • View in gallery

    Sawbones model demonstrating the area of dorsal transverse bone to be removed (circled in black with marking pen) to access the VSP. Figure is available in color online only.

  • View in gallery

    Intraoperative images demonstrating key steps of freehand pedicle screw insertion using the VSP technique. A: Exposure of dorsal bony landmarks to the tips of the transverse process. B: A large Leksell rongeur is placed with one jaw on the superior articular facet to set the depth of bone removal. C: Removal of dorsal transverse process bone to the depth of the superior articular facet. D: Creation of pilot hole with a matchstick burr. E: Cannulation of the pedicle with a thoracic gearshift probe. F: Insertion of pedicle screw. Figure is available in color online only.

  • View in gallery

    Determination of starting point on the left pedicle of the T8 vertebra. Left: The mediolateral position of the starting point is 2 mm lateral to the midline of the SAF. Right: The cephalocaudal position is in the center of the pedicle. The VSP is marked by blue/green crosshairs. The DSP is marked by the tip of the white arrow. Figure is available in color online only.

  • View in gallery

    MIA from VSP (blue) and DSP (pink). degs = degrees. Figure is available in color online only.

  • View in gallery

    Mean pedicle isthmus. The y-axis values are mean endosteal diameter measurements in millimeters. Figure is available in color online only.

  • View in gallery

    Distance from starting point to isthmus. The y-axis values are mean distance measurements in millimeters. SP = starting point. Figure is available in color online only.

  • View in gallery

    Maximal insertional arc. The y-axis values are mean MIA measurements in degrees. Figure is available in color online only.

  • View in gallery

    Screw angles. The y-axis values are mean angle measurements in degrees. Figure is available in color online only.



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