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In Vitro Biomechanical Validation of a Self-Adaptive Ratchet Growing Rod Construct for Fusionless Scoliosis Correction

Chen, Zong-Xing MS; Kaliya-Perumal, Arun-Kumar MS∗,†; Niu, Chi-Chien MD; Wang, Jaw-Lin PhD; Lai, Po-Liang MD, PhD

doi: 10.1097/BRS.0000000000003119

Study Design. In vitro biomechanical evaluation of a novel self-adaptive unidirectional ratchet growing rod (RGR) system.

Objective. The aim of this study was to propose and biomechanically validate a novel RGR construct in vitro using porcine thoracic spines and calculate the tensile force required to elongate the RGR with springs, without springs, and with soft tissue encapsulation (induced in vivo in rabbits).

Summary of Background Data. Literature lacks clear consensus regarding the implant of choice for early-onset scoliosis. Multiple systems are currently available, and each has its own advantages and disadvantages. Therefore, studying novel designs that can credibly accommodate growth and curb deformity progression is of principle importance.

Methods. In vitro biomechanical motion tests were done using six porcine thoracic spines with pedicle screws at T3 and T8. A pure moment of ±5 Nm was loaded in lateral bending (LB) and flexion-extension. Range of motion (ROM) and neutral zone (NZ) of each specimen was determined after connecting the free movable growing rods (FGRs), RGRs, and standard rods (SRs). Tensile tests were done to measure the force required to elongate the RGR with springs, without springs, and with soft tissue encapsulation (induced in vivo in rabbits).

Results. Global ROM, implanted T3-T8 ROM, and the NZ of specimens with FGRs and RGRs were significantly higher than that with SRs. The RGRs favored unidirectional elongation in both LB and flexion. The tensile forces required for elongating the RGR without springs, with springs, and with soft tissue capsulation (by a scaled unit of 3 mm) were 3 ± 1.3 N, 10.5 ± 0.4 N, and 48.4 ± 14.4 N, respectively.

Conclusion. The RGR could stabilize and favor unidirectional elongation of the implanted spinal column when appropriate forces were present. There was no device failure as far as we have studied and it is anticipated that, with further safety and feasibility assessment, RGRs could be adapted for clinical use.

Level of Evidence: N/A

A self-adaptive ratchet growing rod was designed and in vitro biomechanical tests were conducted using porcine spines. We noted that the ratchet growing rods favored unidirectional elongation when appropriate forces were present. Therefore, it is perceived that, if normal growth provides adequate forces, ratchet growing rods can self-elongate and lengthening procedures can be avoided.

Department of Orthopaedic Surgery, Bone and Joint Research Center, Chang Gung Memorial Hospital and University College of Medicine, Taoyuan, Taiwan

Department of Orthopaedic Surgery, Melmaruvathur Adhiparasakthi Institute of Medical Sciences and Research, Melmaruvathur, Tamil Nadu, India

Institute of Biomedical Engineering, College of Medicine and College of Engineering, National Taiwan University, Taipei, Taiwan.

Address correspondence and reprint requests to Dr. Po-Liang Lai, MD, PhD, Department of Orthopedic Surgery, Spine Division, Chang Gung Memorial Hospital at Linkou, No. 5, Fuxing St., Guishan Dist., Taoyuan 33305, Taiwan; E-mail:

Received 25 February, 2019

Revised 6 April, 2019

Accepted 8 May, 2019

ZXC and AKKP contributed equally.

Research grants from the Ministry of Science and Technology, Taiwan (MOST 106-2221-E-182A-003-MY3), Chang Gung Memorial Hospital, Taoyuan, Taiwan (CRRPG3E0142) and AO Spine Research Grant for East Asia (AOSEA[R] 2016-07) funds were received in support of this work.

The legal regulatory status of the device(s)/drug(s) that is/are the subject of this manuscript is not applicable in my country.

No relevant financial activities outside the submitted work.

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