International Journal of Research in Orthopaedics | May-June 2023 | Vol 9 | Issue 3 Page 1 International Journal of Research in Orthopaedics Sait A et al. Int J Res Orthop. 2023 May;9(3):xxx-xxx http://www.ijoro.org Original Research Article Biomechanical analysis of the effect of ‘intermediate screws’ in short segment posterior fixation of unstable burst fractures of thoracolumbar spine in calf spine model Azad Sait 1 , Monosha Priyadarshini 1 , N. Arunai Nambi Raj 1 *, Kenny Samuel David 2 INTRODUCTION Unstable burst fractures are commonly seen in young individuals in the thoracolumbar region due to high energy trauma. 1,2 Approximately fifty percent of the affected population are estimated to have neurological deficits. 3-5 The affected individual is usually poly-traumatized, and the associated injuries warrant a quick surgical stabilization of the spine to aid early rehabilitation. Short segment instrumentation was preferred in young individuals because of the preservation of motion levels. Posterior short segment spanning fixation (SSSF) depends on the integrity of the anterior column and is not reliable in an unstable burst situation. 6,7 Although circumferential fixation provides reliable and stable short segment constructs, it adds to the further morbidity of an already poly traumatized individual. 8 Various augmentation techniques to SSSF have been described in the literature such as percutaneous balloon vertebroplasty, use of laminar hooks, and insertion of screws into the fractured level commonly termed as ‘intermediate screws’. 9-11 The short segment posterior fixation with intermediate screws (SSPI) was performed through the posterior approach familiar to most spine surgeons, and the associated morbidity was less. Our study aims to evaluate the biomechanical advantages of intermediate screws in ABSTRACT Background: Efforts in preserving motion levels in unstable thoracolumbar burst fractures steered to short segment fixation. However, short segment spanning fixation in clinical scenario reported high failure rates. Augmentation of spanning fixation by inserting intermediate screws into the fracture level is proposed to enhance stability. An experimental comparative study was performed to assess the biomechanical role of the ‘intermediate screws.’ Methods: Five calf spine specimens were freshly prepared to record the biomechanical characteristics, range of motion (ROM), and stiffness. CT scan confirmed an unstable burst fracture in each specimen. Each specimen was instrumented with short-segment posterior fixation with an intermediate screw. The same test protocols were repeated with and without intermediate screws. Results: Intermediate screws contribute to 20.2%, 16.5%, 14.5% and 23% decrease in ROM and 15.4%, 25.6%, 48.3%, and 160.2% increase in construct stiffness. Conclusions: Intermediate screws significantly increase the construct stiffness and decrease the ROM. Keywords: Bending moment, Pedicle screw, Load, Displacement curve, Universal testing device, Posterior instrumentation 1 School of Advanced Sciences, Vellore Institute of Technology, Vellore, Tamil Nadu, India 2 Spinal Disorders Surgery Unit, Department of Orthopaedics, Christian Medical College Vellore, Tamil Nadu, India Received: 22 March 2023 Revised: 04 April 2023 Accepted: 07 April 2023 *Correspondence: Dr. N. Arunai Nambi Raj, E-mail: narunainambiraj@vit.ac.in Copyright: © the author(s), publisher and licensee Medip Academy. This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. DOI: https://dx.doi.org/10.18203/issn.2455-4510.IntJResOrthop20231050