Pedicle Screw Plowing in Adolescent Idiopathic Scoliosis

How Common Is It and Is It a Problem?

Walter Klyce, MD; Krishna V. Suresh, BS; Amit Jain, MD; Stefan Parent, MD, PhD; Suken A. Shah, MD; Patrick J. Cahill, MD; Stephen George, MD; David Clements, MD; Salil Upasani, MD; Burt Yaszay, MD; Firoz Miyanji, MD; Michael P. Kelly, MD; Baron S. Lonner, MD; Paul D. Sponseller, MD, MBA

Disclosures

Spine. 2022;47(12):873-878. 

In This Article

Abstract and Introduction

Abstract

Study Design: Multicenter retrospective review.

Objective: To calculate overall incidence of pedicle screw "plowing" in adolescent idiopathic scoliosis (AIS) patients who underwent posterior spinal fusion (PSF). To identify risk factors for pedicle screw plowing and associated postoperative outcomes, including loss of correction and revision rate.

Summary of Background Data: Curve correction of AIS generates perpendicular stresses that can cause pedicle screws to lose alignment and "plow" through pedicles craniocaudally.

Methods: We reviewed records of 1057 patients who underwent PSF for AIS from 2002 to 2015. Preoperative and first postoperative erect radiographs were evaluated by two observers to determine (1) presence of plowing and (2) subsequent loss of correction (LOC). Plowing was defined as more than 25° sagittal angulation compared with pedicle axis or entry of the most dorsal part of the screw outside the pedicle projection. LOC was defined as postoperative change in focal angulation of an instrumented spinal level, when in consensus of both reviewers. Bivariate analyses were performed (alpha = 0.05).

Results: Nineteen thousand five hundred sixty nine screws were assessed across our cohort of 1057 patients. Both observers agreed that 48 patients (4.5%) demonstrated plowing of more than or equal to one pedicle screw. For 72 screws (0.4%), both observers noted plowing, most commonly through the cranial cortex of the pedicle (65/72 screws) and at the lowest instrument vertebra (LIV) (17/72 screws). Factors associated with plowing included larger curves (P = 0.02); lower mean pedicle screw density (P = 0.0003); skeletal immaturity as measured by open triradiate cartilage (P = 0.04); and younger chronological age at time of surgery (P = 0.04). LOC occurred in 13 patients, most commonly at LIV (P < 0.0001). Revision rate for loss of screw fixation was higher in the plowing group (P = 0.003).

Conclusion: Pedicle screw plowing occurred in 4.5% of AIS patients, especially in those skeletally immature and with decreased implant density. Plowing commonly occurred in the cranial direction and was associated with LOC, particularly at the LIV.

Level of Evidence: 3

Introduction

Adolescent idiopathic scoliosis (AIS) is a three-dimensional deformity of the spine and is often treated by posterior spinal fusion (PSF).[1–4] Segmental fixation with dual-rod pedicle screw constructs is a safe and reliable method of fixation for patients with AIS, enabling a large magnitude of correction.[5,6] However, vertebral derotation, compression, and distraction maneuvers during surgical correction place stress directed perpendicular relative to the pedicle. Rod manipulation exerts craniocaudal and medio-lateral forces on the walls of the pedicle, which may cause the shaft of the screw to break through the cortical boundaries of the pedicle and/or vertebral body, subsequently causing loss of screw fixation at that spinal level.

This phenomenon, known as pedicle screw "plowing," has been described in previous biomechanical studies, which demonstrated that repeated craniocaudal stress forces in cadaveric bone can result in pedicle screw movement in bone, loss of screw purchase, and decreased axial pullout resistance.[7–11] However, to our knowledge, pedicle screw plowing has not been studied in vivo. Specifically, the incidence of pedicle screw plowing, its associated risk factors, and the need for subsequent revision surgery has not been previously studied in patients with AIS.

Quantifying incidence and establishing risk factors for pedicle screw plowing in the AIS population can focus awareness during surgery to minimize its frequency. We hypothesized that pedicle screw plowing would be associated with preoperative major curve size, skeletal maturity, and have characteristic direction and location in the curve.

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