Downloaded from http://journals.lww.com/jorthotrauma by BhDMf5ePHKbH4TTImqenVA+lpWIIBvonhQl60EtgtdlLYrLzSPu+hUapVK5dvms8 on 12/15/2020 ORIGINAL ARTICLE Posterior Inferior Comminution Signiﬁcantly Inﬂuences Torque to Failure in Vertically Oriented Femoral Neck Fractures: A Biomechanical Study David J. Wright, MD, 1 Christopher N. Bui, MD, 1 Hansel E. Ihn, MD, 2 Michelle H. McGarry, MS, 3 Thay Q. Lee, PhD, 3 and John A. Scolaro, MD 1 Objectives: To evaluate axial fracture obliquity and posterior inferior comminution in vertically oriented femoral neck fractures (FNFs) in the physiologically young patient. A biomechanical investigation was designed to evaluate the impact of these fracture elements on torque to failure using cannulated screw (CS) and sliding hip screw ﬁxation. Methods: Four Pauwels III FNF models were established in synthetic femurs: (1) vertically oriented in the coronal plane (COR), (2) coronal plane with axial obliquity (AX), (3) coronal plane with posterior inferior comminution (CCOM), and (4) coronal plane with axial obliquity and posterior inferior comminution (ACOM). In each group (n = 10), speci- mens were ﬁxed using either 3 CSs or a sliding hip screw with supple- mental antirotation screw (SHS). Quasistatic cyclic ramp-loading to failure was performed using a custom testing jig combining axial pre- loading and torsional ramp-loading. The primary outcome was torque to failure, deﬁned as angular displacement $5 degrees. Results: In the CS group, torque to failure was 40.2 6 2.6 Nm, 35.0 6 1.4 Nm, 29.8 6 1.5 Nm, and 31.8 6 2.2 Nm for the COR, AX, CCOM, and ACOM fracture groups, respectively (P , 0.05). In the SHS group, torque to failure was 28.6 6 1.3 Nm, 24.2 6 1.4 Nm, 21.4 6 1.2 Nm, and 21.0 6 0.9 Nm for the COR, AX, CCOM, and ACOM fracture groups, respectively (P , 0.05). In both constructs, groups with posterior inferior comminution demonstrated signiﬁcantly lower torque to failure compared to the COR group (P , 0.05). The CS construct demonstrated higher torque to failure in all groups when compared to the SHS construct (P , 0.01). Conclusions: Posterior inferior comminution signiﬁcantly affects torque to failure in vertically oriented FNFs. Three peripherally placed CSs may resist combined axial and torsional loading better than a sliding hip screw construct. Key Words: vertical, Pauwels III, femoral neck fracture, biome- chanical, comminution, cannulated screws, sliding hip screw, poste- rior, torsion (J Orthop Trauma 2020;34:644–649) INTRODUCTION Vertically oriented femoral neck fractures (FNFs) are commonly the result of high energy trauma in physiologically young patients. Surgical ﬁxation is the primary treatment for these injuries. However, discussion continues to surround timing of ﬁxation, reduction, and optimal ﬁxation con- struct. 1,2 Dissimilar to more horizontally oriented FNFs com- monly seen in geriatric patients, where compressive forces predominate, vertically oriented fractures result in marked shear forces across the applied ﬁxation constuct. 3 Multiple biomechanical investigations have evaluated various implants and ﬁxation constructs in simulated, verti- cally oriented fracture models. In most studies, a single coronal plane osteotomy has been used. 4–12 Collinge et al evaluated a series of computed tomography scans in young patients with displaced FNFs and noted that although the fractures had a vertical orientation in the coronal plane, an obliquity on axial reformats also existed. In addition, commi- nution was present in the majority of fractures, most com- monly located in the posterior inferior quadrant of the neck. 13 Based on these clinical observations, we aimed to create a vertically oriented FNF model that accurately accounted for the axial obliquity and posterior inferior comminution seen in these injuries. We then used this model to evaluate the effect of these fracture elements on torque to failure in the 2 most commonly used ﬁxation methods for these fractures [cannu- lated screws (CSs) and sliding hip screw]. 1,2,13 Consistent with prior studies examining the effects of posterior commi- nution in these fractures, 14 we hypothesized that, regardless of ﬁxation method, torque to failure would decrease when a standard single coronal-plane fracture model was compared to models that included axial obliquity and posterior inferior comminution. MATERIALS AND METHODS Fracture Model A custom cutting jig was used to create 4 groups of morphologically unique, transcervical, vertically oriented Accepted for publication May 20, 2020. From the 1 University of California Irvine Department of Orthopedic Surgery, Orange, CA; 2 Department of Orthopedic Surgery, University of Southern California, Los Angeles, CA; and 3 Orthopaedic Biomechanics Laboratory, Congress Medical Foundation, Pasadena, CA. Supported by a grant from the Foundation for Orthopedic Trauma. Implants were provided by DePuy Synthes, West Chester, PA. The authors report no conﬂict of interest. Presented as a poster at the Annual Meeting of the Orthopaedic Trauma Association, October 11–14, 2017, Vancouver, BC. Reprints: John A. Scolaro, MD, UC Irvine Department of Orthopedic Surgery, 101 the City Dr. S., Pavillion III, Building 29A, Orange, CA 92868 (e-mail: jscolaro@uci.edu). Copyright © 2020 Wolters Kluwer Health, Inc. All rights reserved. DOI: 10.1097/BOT.0000000000001846 644 | www.jorthotrauma.com J Orthop Trauma  Volume 34, Number 12, December 2020 Copyright © 2020 Wolters Kluwer Health, Inc. Unauthorized reproduction of this article is prohibited.