A biomechanical investigation of dual growing rods used for fusionless scoliosis correction

• Published in: Clinical biomechanics (Bristol) 2015-01, Vol.30 (1), p.33-39
• Main Authors: Quick, M.E, Grant, C.A, Adam, C.J, Askin, G.N, Labrom, R.D, Pearcy, M.J
• Format: Article
• Language: English
• Subjects: Animals; Biomechanical; Biomechanical Phenomena; Biomechanics; Constants; Disease Models, Animal; Equipment Design; Fusionless correction; Growing rod; Humans; In vitro; In Vitro Techniques; Internal Fixators; Intervertebral; Mathematical analysis; Movement; Orthopedic Procedures; Physical Medicine and Rehabilitation; Porcine; Range of motion; Reduction; Rods; Rotation; Scoliosis; Scoliosis - physiopathology; Scoliosis - surgery; Spine; Spine - physiology; Spine - surgery; Stiffness; Swine; Three dimensional; Weight-Bearing
• ISSN: 0268-0033; 1879-1271
• DOI: 10.1016/j.clinbiomech.2014.11.008

Abstract Background The use of dual growing rods is a fusionless surgical approach to the treatment of early onset scoliosis which aims to harness potential growth and correct spinal deformity. The purpose of this study was to compare the in-vitro biomechanical response of two different dual rod designs under axial rotation loading. Methods Six porcine spines were dissected into seven level thoracolumbar multi-segment units. Each specimen was mounted and tested in a biaxial Instron machine, undergoing nondestructive left and right axial rotation to peak moments of 4 Nm at a constant rotation rate of 8 deg. s−1 . A motion tracking system (Optotrak) measured 3D displacements of individual vertebrae. Each spine was tested in an un-instrumented state first and then with appropriately sized semi-constrained and ‘rigid’ growing rods in alternating sequence. The range of motion, neutral zone size and stiffness were calculated from the moment–rotation curves and intervertebral range of motion was calculated from Optotrak data. Findings Irrespective of test sequence, rigid rods showed a significant reduction of total rotation across all instrumented levels (with increased stiffness) whilst semi-constrained rods exhibited similar rotational behavior to the un-instrumented spines ( P < 0.05). An 11.1% and 8.0% increase in stiffness for left and right axial rotation respectively and 14.9% reduction in total range of motion were recorded with dual rigid rods compared with semi-constrained rods. Interpretation Based on these findings, the Semi-constrained growing rods were shown to not increase axial rotation stiffness compared with un-instrumented spines. This is thought to provide a more physiological environment for the growing spine compared to dual rigid rod constructs.