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Experimental Analysis of Fabricated Synthetic Midthoracic Paediatric Spine as Compared to the Porcine Spine Based on Range of Motion (ROM)

Experimental Analysis of Fabricated Synthetic Midthoracic Paediatric Spine as Compared to the... Hindawi Applied Bionics and Biomechanics Volume 2021, Article ID 2799415, 10 pages https://doi.org/10.1155/2021/2799415 Research Article Experimental Analysis of Fabricated Synthetic Midthoracic Paediatric Spine as Compared to the Porcine Spine Based on Range of Motion (ROM) 1,2 1,3 1,3 Nor Amalina Muhayudin, Khairul Salleh Basaruddin , Ruslizam Daud, 4 4 Fiona McEvoy, and Tansey Faculty of Mechanical Engineering Technology, Universiti Malaysia Perlis, 02600, Pauh Putra Campus, Perlis, Malaysia Faculty of Electronic Engineering Technology, Universiti Malaysia Perlis, 02600 Perlis, Malaysia Sports Engineering Research Centre, Centre of Excellence (SERC), Universiti Malaysia Perlis, 02600 Perlis, Malaysia Mechanical Engineering Department, Institute of Technology Tallaght, Dublin, Dublin 24, Ireland Correspondence should be addressed to Khairul Salleh Basaruddin; khsalleh@unimap.edu.my Received 22 April 2021; Accepted 24 August 2021; Published 25 September 2021 Academic Editor: Raimondo Penta Copyright © 2021 Nor Amalina Muhayudin et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The present study is aimed at investigating the mechanical behaviour of fabricated synthetic midthoracic paediatric spine based on range of motion (ROM) as compared to porcine spine as the biological specimen. The main interest was to ensure that the fabricated synthetic model could mimic the biological specimen behaviour. The synthetic paediatric spine was designed as a 200% scaled-up model to fit into the Bionix Servohydraulic spine simulator. Biomechanical tests were conducted to measure the ROM and nonlinearity of sigmoidal curves at six degrees of freedom (DOF) with moments at ±4 Nm before the specimens failed. Results were compared with the porcine spine (biological specimen). The differences found between the lateral bending and axial rotation of synthetic paediatric spine as compared to the porcine spine were 18% and 3%, respectively, but was still within the range. Flexion extension of the synthetic spine is a bit stiff in comparison of porcine spine with 45% different. The ROM curves of the synthetic paediatric spine exhibited nonlinearities for all motions as the measurements of neutral zone (NZ) and elastic zone (EZ) stiffness were below “1.” Therefore, it showed that the proposed synthetic paediatric spine behaved similarly to the biological specimen, particularly on ROM. spine started with manipulation of the adult finite element 1. Introduction model to the paediatric model to incorporate anatomical dif- Human adult and animal cadaveric spines such as porcine, ferences between adult and paediatric spines. Although there sheep, baboon, and calf are commonly used in biomechanical were distinct differences between adult and paediatric spines investigation [1–5]. Over the years, the understanding of such as the morphology of their vertebra, the orientation of human spinal biomechanics was based on comprehensive facet joints was more horizontal in the paediatric spine and studies of human adult spine [1, 3, 5]. On the other hand, thus made it more mobile as compared to the adult spine information on paediatric spinal biomechanics was very lim- and ossification state of the vertebrae and size of nucleus pul- ited due to difficulties in obtaining paediatric human posus were larger in the paediatric intervertebral disc as com- cadavers. Although paediatric and adult spines were distinc- pared to adult disc. Paediatric spine is not miniature of adult spine; therefore, it cannot be treated as such. A few studies on tively different from each other in both anatomically and mechanically, studies on paediatric spines started by scaling paediatric biomechanical investigation still used human down from the adult size model to paediatric size model in adult and immature porcine spine as their specimens due to finite element analysis [6–10]. The studies on paediatric the limitations of paediatric specimens [11–13] 2 Applied Bionics and Biomechanics aimed at developing a working synthetic paediatric spine as The main challenge in developing the paediatric biome- chanical analyses was limited information of paediatric another alternative in paediatric spine biomechanical test- experimental data to enable a direct comparison with the ing. The key element is to ensure that the synthetic model performs similarly to the biological model. Therefore, the finite element model. Recently, a few studies were reported on the use of paediatric human cadavers to investigate the objective of this paper is to investigate the ROM of synthetic paediatric biomechanical response [3, 14]. Ouyang et al. midthoracic paediatric spine as compared to porcine spine. [15] investigated bending and tensile tests of paediatric cer- vical spines from neck to head of 2 years old to 12 years old. 2. Methods The study found that the distraction load for 6 years old to 12 years old was significantly higher as compared to 2 to 4 The first step in synthetic paediatric spine fabrication was years old. Another study to investigate the failure tolerance the development of physical paediatric spine model. Since was conducted by Lopez-Valdes et al. [3] by using human the actual physical size of paediatric model was relatively paediatric and adult thoracic spines. Similar findings were small to be tested with the MTS Bionix Servohydraulic spine found by Lopez-Valdes et al. whereby 7-year-old spines simulator, a scaling process was considered. To the author’s showed lower tolerance as compared to 15-year-old spines. knowledge, no data on physiological ROM of human paedi- The study suggested that the 15-year-old spine tolerance atric spine exists, particularly in the thoracic region, and was comparable to adult spines. On the other hand, Clarke thus, it is essential to generate an experimental protocol by et al. [14] used sheep spines to investigate the biomechanical using biological specimens before ROM of the synthetic differences between mature and immature spines by using spine was determined. newborn and 2-year-old specimens. The study focused on ROM and found that immature spines (newborn specimens) 2.1. Scaling of Paediatric Spine. Another important factor exhibited a significantly lower ROM as compared to mature that was considered while developing the synthetic paediat- spines (2 years old). In paediatric in vivo testing, these were ric spine was the actual size of human paediatric spine. the only studies found as a guide to compare the synthetic Assuming that size of the paediatric spine is 100%, the size paediatric spine with human paediatric spine. This limita- of adult spine is normally scaled up to 141%, and the size tion of paediatric human spines can be overcome by devel- of porcine spine is larger than an adult spine by an average oping a working synthetic paediatric spine. Hence, more difference of 50% [4, 27, 28]. Therefore, the porcine spine biomechanical testing in regards with paediatric spines can size is approximately 190% as compared to paediatric spine. be performed such as paediatric trauma during motor vehi- In this research, the synthetic paediatric spine was scaled up cle collision, paediatric sports-related injuries, and other to 200% of the paediatric spine size to fit the size of the MTS common recreational injuries that required paediatric speci- Bionix Servohydraulic system spine simulator which was in mens for further investigations. reference to the size of porcine spine that was used as the Development of a synthetic paediatric spine is essential cadaver control data as shown in Figure 1. to investigate the mechanical behaviour of paediatric cases A physical model of paediatric vertebra was purchased such as scoliosis. The common surgical procedure among from Sawbones (Inc., Vashon Island, USA) for all regions, paediatric cases is early onset scoliosis surgical treatment, and these vertebrae represent the anatomical dimension of and studies on the effect and accuracy of various paediatric a juvenile group (8 to 9 years old). Since this study focused spinal instrumentations of scoliosis normally were con- on T4-T8, five individual vertebrae were scanned in three- ducted only using FE studies, animal spines, or postoperative dimension (3D) before being scaled to 200% in a selective studies that required years of observation [11, 16, 17]. There- laser sintering (SLS) machine to fabricate the prototype. fore, it will be beneficial to have a working synthetic paediat- ric spine that can be used in spinal instrumentations of 2.2. Fabrication of Synthetic Paediatric Spine. The materials biomechanical investigations or preplanning of complicated to fabricate the synthetic spine were divided into three main surgical treatment. The main advantage in using synthetic components, which were vertebra, intervertebral disc, and materials is that they can be tailored to a specific require- spinal ligaments. All materials used in the synthetic paediat- ment, and they offer constant material properties. On the ric spine were structurally and mechanically close to human hand, studies conducted by Suh et al., Du et al., and Oros- data. This was to ensure that the final product (synthetic zlány et al. [18–20] proved that thoracic was the most com- paediatric spine) could replicate human behaviour. Details mon affected region in spine with more than 50% cases of analysis to select the material for each component of spine amongst children. Therefore, the present study was focused are available in previous author’s publications, namely, for on the synthetic spine development of the thoracic region, vertebra [4], intervertebral disc [5], and spinal ligament particularly on T4-T8. In recent years, synthetic materials [6]. Table 1 summarises the properties of the selected mate- were commonly used as alternatives in biomechanical test- rials for each component of fabricated synthetic paediatric ing, especially in trabecular bone [21–24]. Bohl et al. started spine. to develop adult synthetic spine model of L3-L5 segments by The paediatric synthetic spine was fabricated as a single using a 3D printer, and they found that although there were functional spinal unit (FSU), consisting of two vertebrae, great differences in ROM data, the study claimed that the an intervertebral disc, and associated ligaments. The process model could mimic a specific ROM on standard ROM test- started with embedding the cortical around the trabecular ing applied to cadavers [25, 26]. The present study is mainly structure. Next, the disc was attached within the two Applied Bionics and Biomechanics 3 Scaling factor Paediatric spine Adult spine Synthetic paediatric spine Reference: Porcine spine 100 120 140 160 180 200 Specimen size (%) Figure 1: Size comparison among all spine models in this study. vertebrae by using the “spinous processes” natural structure 2.3.2. Experimental Setup. These experiments were con- as reference. The next process was to mould the spinal liga- ducted by using MTS Bionix Servohydraulic system spine ments within the posterior elements and attach the anterior simulator. Specimens were fixed at its natural position in longitudinal ligament (ALL) and posterior longitudinal liga- the spine simulator before testing, as shown in Figure 4. ment (PLL) within the vertebral body. Finally, the vertebral Twelve specimens were then tested without a preload to body was covered with sheet wax. All materials used in the avoid buckling in an alternating sequence of flexion/exten- synthetic paediatric spine were geometrically and mechani- sion, lateral bending right/left, and axial rotation right/left cally close to human data. The fabrication process flow is under pure moments. All specimens were tested at five summarised in Figure 2. cycles, and the first two cycles were considered as precycles. The applied moments and angular displacements were recorded for each cycle. The experiments were performed 2.3. Biomechanical Testing. To provide an indication of under ±7.5 Nm load with 1.7 deg/sec for all DOF, and the whether the materials selected to fabricate the synthetic pae- results used were at the fifth cycle. diatric spine could mimic human spine movement or not, a All specimens were tested to obtain flexion, extension, series of experiments were carried out for both porcine spine right and left lateral bending, and right and left axial rota- and synthetic paediatric spine. tion by using the MTS Bionix Servohydraulic spine simu- lator under similar experimental procedures developed 2.3.1. Specimen Preparation. Six porcine spines from 6 prior to the porcine spine test. To observe the perfor- months to 7 months old were provided from a local abattoir. mance of synthetic paediatric spine, three specimens were The breed was a cross Saddleback with Gloucester porcine. tested until failure by using a pure moment of ±1Nm Average weight of spines used was 82.34 kg (±4.9 kg). The (0.1 deg/sec) for all six DOF with an increment of full spines were freshly dissected into single FSUs (T4-T5, ±1 Nm. Results from these three specimens showed that T5-T6, T6-T7, and T7-T8) with three specimens for each the specimens failed at ±5 Nm, whereby the disc started FSU, as shown in Figure 3. All ligaments, disc, and vertebra to detach from the vertebrae. Therefore, an assumption were preserved, while muscle tissues were carefully removed was made that the valid ROM for the paediatric synthetic and frozen at -20 spine in this study was at ±4 Nm. The rest of specimens C. The specimens were thawed for 24 hours before testing. The specimens were potted to ensure were tested at ±4 Nm with 1 deg/sec rate. All specimens that the middle disc was aligned horizontally with the spine were tested to five cycles, whereby the first two cycles were simulator. The upper half of top vertebra and lower half of considered as the precycle and results were determined at bottom vertebra were embedded in a polyurethane liquid the fifth cycle. plastic (Smooth Cast 300). Four FSUs of synthetic paediatric spine from T4 to T8 2.3.3. Analysis of ROM. ROM was determined from the sig- were prepared based on the fabrication process mentioned moidal curve for each loading direction from the fifth cycle: flexion, extension, right and left lateral bending, and right in Section 2.2, with three specimens for each FSU. All twelve specimens were potted to ensure that the middle disc was and left axial rotation. The typical curve for each loading aligned horizontally with the spine simulator. Similar to por- direction is shown in Figure 5. The arrows indicated the cine specimens, the upper half of top vertebra and lower half loading and unloading direction. Key parameters in the of bottom vertebra were embedded in liquid resin (Smooth curve are total ROM, NZ ROM, NZ stiffness (S1), and EZ stiffness (S2). Cast 300). Specimen type 4 Applied Bionics and Biomechanics Table 1: Material properties of the components in the fabricated synthetic paediatric spine. Properties Material Spine component Compression Tensile modulus Stiffness modulus (MPa) (MPa) (N/mm) Expandable polyurethane foam (0.24 g/cm density) Trabecular bone 89.9 - - Urethane plastic (Smooth Cast 385) Cortical bone 3200 - - Polyurethane elastomer (Monothane) Annolous fibrosus 14.1 3.3 - Silicone (Lightweight) Nucleus pulposus 1.6 0.6 - EZ = 37:8 Silicone with fiber glass tape (Sorta Clear 40 in woven fibre 45 ) ALL & PLL - 22.5 NZ = 109:6 (a) (b) (c) (d) (f ) (e) Figure 2: The process flow to manufacture paediatric synthetic spine. (a) Cortical, (b) completed vertebra, (c) disc, (d) assembled ALL and PLL, (e) added posterior ligaments, and (f) completed FSU. Results collected from synthetic paediatric spine test specimen 2 was the most flexible porcine spine. Overall, all were plotted in the sigmoidal curve to observe if any nonlin- FSUs showed the same pattern, whereby it increased from earity existed in the ROM. The nonlinearities or sigmoidal flexion extension to lateral bending and axial rotation, patterns were essential to prove that the synthetic paediatric except for T4-T5. The wide interspecimen variability was spine exhibited a viscoelastic behaviour, which is normally expected as each FSU was from a different porcine spine found in the biological specimens. The nonlinearities of specimen. graphs were observed for all specimens, as it was the key As for ROM at moment of ±4 Nm, it increased from parameter to determine the performance of synthetic spine flexion extension to lateral bending and axial rotation, as compared to the biological specimens. The value of except for T5-T6. The differences between each FSU in S1/S2 was expected to be lower than 1.0 to prove that the each DOF were only around 20%, except for T5-T6. curve was a nonlinear curve, whereby the smaller the value, Data indicated that T5-T6 in axial rotation was the stiff- the stronger the sigmoidicity. est as compared to other FSUs and DOF. The ROM data for each FSU at ±4 Nm moment is summarised in Table 3. 3. Results and Discussions 3.2. ROM of Synthetic Paediatric Spine. Variables of interest were the ROM and value of S1/S2, as summarised in Table 4. 3.1. ROM of Porcine Spine. The ROM data for each FSU at ±7.5 Nm moment is summarised in Table 2. The data In Table 4, axial rotation exhibited more linear curves as showed that for flexion extension, T5-T6 and T7-T8 were compared to other ROMs because the average value of stiffer than T4-T5 and T6-T7 by around 40%. In lateral S1/S2 was 0.7, which was closed to 1.0. The most nonlinear bending, all FSUs were in good agreement with less than curves were observed in lateral bending with an average value of 0.16. In flexion and extension curves, the upper 10% difference between each FSU. As for axial rotation, dif- ference between the biggest ROM (T6-T7) and lowest ROM FSUs (T4-T5 and T5-T6) showed more linear curves as (T4-T5) was around 30%. Interestingly, T4-T5 and T6-T7 compared to lower FSUs (T6-T7 and T7-T8). Although axial results came from the same specimens. In this research, rotation curves were inclined towards a linear curve, the Applied Bionics and Biomechanics 5 (a) (b) Figure 3: The porcine specimens were dissected into single FSUs, view in (a) frontal plane and (b) transverse plane. (a) (b) Figure 4: Experimental setup for (a) porcine spine and (b) synthetic paediatric spine in the MTS Bionix Servohydraulic spine simulator. EZ stiffness Angular (S2) displacement (degree) NZ stiffness (S1) Moment (Nm) NZ Total ROM ROM Figure 5: Typical sigmoidal curve of spine ROM. values of S1/S2 for all cases were still lower than 1.0, which The second variable was ROM of each FSU, whereby the suggested that all six DOF exhibited nonlinearity curves in pattern that emerged for ROM values showed distinct differ- their ROMs. ences between FSUs. The results can be divided into two 6 Applied Bionics and Biomechanics obtained by Wilke et al. [2] for all FSUs, as presented in Table 2: Porcine spine data of the ROM for each DOF of each specimen at moment of ±7.5 Nm. Figure 6. The average of all FSUs for the midthoracic region (T4- ROM ( ) FSU T8) is summarised in Figure 7 in all six DOF. The difference Flexion extension Lateral bending Axial rotation between lateral bending and axial rotation was less than 2%, 6:72 ± 3:61 10:59 ± 4:87 9:51 ± 5:23 T4-T5 while flexion/extension was stiffer by 36% [7]. The signifi- 3:08 ± 0:19 10:46 ± 4:00 10:82 ± 4:00 cant difference in flexion/extension might be potentially T5-T6 due to weight and size of the porcine tested. Although the T6-T7 6:69 ± 4:97 12:78 ± 8:58 16:51 ± 12:02 ROM in lateral bending and axial rotation from these FSUs 4:78 ± 1:29 12:03 ± 1:27 13:07 ± 6:95 T7-T8 were within the range provided by literature, the average was lower as compared to other FSUs. As suggested by Muhayu- din et al. and White and Panjabi, specimen weight played a Table 3: Porcine spine data of the ROM for each DOF of each significant effect on the anatomical spine dimension, which specimen at moment of ±4 Nm. may subsequently affect the ROM and in this research, whereby it significantly affected the flexion/extension [28, ROM ( ) 29]. Therefore, by considering the differences in flexion/ex- FSU Flexion extension Lateral bending Axial rotation tension between current study and literature, the results of T4-T5 1:97 ± 0:48 5:40 ± 1:00 7:15 ± 3:61 porcine spine from this study were used in the comparative analysis with synthetic paediatric spine. 2:80 ± 0:28 5:60 ± 0:96 4:92 ± 0:74 T5-T6 Since no data is available on ROM of human paediatric T6-T7 2:47 ± 0:10 5:63 ± 1:51 7:22 ± 2:01 spine, further analysis was required to compare the porcine 2:47 ± 0:42 6:45 ± 1:81 8:73 ± 5:80 T7-T8 ROM with human adult ROM from White and Panjabi under the same moment [29]. The ROM presented in Figure 8 was an average ROM measurement taken from a groups, which were the upper FSUs (T4-T5 and T5-T6) and single FSU, ranging from T4 to T8. As expected, both sets lower FSUs (T6-T7 and T7-T8) in all six DOF. The upper of porcine data were comparable, except for flexion/exten- FSUs and lower FSUs were within the same range for all sion. However, when compared to human adult ROM, the six DOF. The percentage differences within upper FSUs differences ranged from 60% to 90% for all six DOF. In lat- and lower FSUs were more than 50% for all six DOF. Flex- eral bending and axial rotation, the differences between por- ion/extension was the stiffest motion, followed by lateral cine and human adult ROMs were approximately around bending and axial rotation. Despite differences between 93% and 66%, respectively. In contrast, the flexion/extension FSUs, the pattern emerged similar to the pattern showed in from this study was comparable to human adult with 12% porcine spine, whereby the ROM increased from flexion/ex- difference, while Wilke et al. was 74% larger than that of a tension to axial rotation. human adult. The size of porcine spines was evidently larger than human adult spines, which subsequently resulted in a larger ROM. However, the relation between specimen size 4. Discussion and ROM was not linear as the material properties, specifi- The biomechanical analysis of a spine was normally carried cally the viscoelasticity of the soft tissues which was different for each specimen. This was considered when the compara- on a single FSU. Despite testing a small segment of the spine, it can exhibit the characteristics of the entire spine. Due to tive analysis was conducted between the synthetic paediatric limited information on the physiological ROM of paediatric and porcine spine. spine, the porcine spine was an essential substitute to guide The crucial element in testing synthetic paediatric spine in biomechanical testing. The physiological ROM included was to ensure that the plotted curve showed nonlinearities to replicate the viscoelastic behaviour of soft tissues in were flexion and extension, left and right lateral bending, and left and right axial rotation. The series of tests were con- human spine. Therefore, the first parameter was to deter- ducted on porcine spines from T4 to T8 on a single FSU mine the ratio of EZ stiffness (S1) to NZ stiffness (S2). In under ±7.5 Nm moments at all six DOF. The ROM was mea- Table 4, axial rotation exhibited more linear curves as com- sured for all six DOF at ±7.5 Nm moments and was directly pared to other ROMs because the average value of S1/S2 was compared with a previous study on porcine spines by Wilke 0.7, which was close to 1.0. The most nonlinear curves were et al. [2], since the average weight for tested porcine spines observed in lateral bending with an average value of 0.16. In was within the same range used in this research. flexion and extension curves, the upper FSUs (T4-T5 and In Figure 6, the flexion/extension of the porcine spines T5-T6) showed more linear curves as compared to lower presented was varied in all FSUs. The flexion/extension of FSUs (T6-T7 and T7-T8). Although axial rotations curve T4-T5 and T6-T7 was 20% different as compared to research were inclined towards linear curve, the values of S1/S2 for but was within the range. In contrast, T5-T6 and T7-T8 had all cases were still lower than 1.0, which suggested that all 50% average difference with that in literature and T7-T8 six DOF exhibited nonlinearity curves in their ROMs. flexion was the only DOF that was within the range. On The nonlinear behaviour in ROM was a result from both the other hand, the average of lateral bending and axial rota- soft and hard tissues. The lateral bending exhibited nonlin- tion was in good agreement and was within the range ear curves with a ratio closer to 0, suggesting that the Applied Bionics and Biomechanics 7 Table 4: Results of the ROM and S1/S2 for each motion of synthetic paediatric specimen at moment of ±4 Nm. Flexion extension Lateral bending Axial rotation FSU ° ° ° ROM ( ) S1/S2 ROM ( ) S1/S2 ROM ( ) S1/S2 0:87 ± 0:03 0:46 ± 0:28 0:18 ± 0:03 3:55 ± 0:81 0:76 ± 0:05 T4-T5 2:46 ± 0:25 T5-T6 0:82 ± 0:02 0:55 ± 0:09 2:89 ± 0:46 0:24 ± 0:05 3:30 ± 0:95 0:87 ± 0:04 1:72 ± 0:14 0:29 ± 0:16 7:00 ± 0:20:12 ± 0:04 9:20 ± 1:06 0:51 ± 0:25 T6-T7 T7-T8 1:92 ± 0:42 0:21 ± 0:06 6:56 ± 0:39 0:10 ± 0:05 11:00 ± 1:40 0:63 ± 0:15 16 16 T4-T5 T5-T6 T6-T7 T7-T8 T4-T5 T5-T6 T6-T7 T7-T8 T4-T5 T5-T6 T6-T7 T7-T8 T4-T5 T5-T6 T6-T7 T7-T8 –4 –4 Lateral bending left Lateral bending right Flexion Extension Current study Current study Wilke et al. Wilke et al. (a) (b) 15 16 T4-T5 T5-T6 T6-T7 T7-T8 T4-T5 T5-T6 T6-T7 T7-T8 Axial rotation left Axial rotation right Flexion Axial rotation Lateral bending extension Current study Current study Wilke et al. Wilke et al. (c) (d) Figure 6: ROM of porcine spine between this research and Wilke et al. [2] at ±7.5 Nm moments in (a) flexion extension, (b) lateral bending, (c) axial rotation, and (d) overall ROM. materials selected as synthetic intervertebral disc replicated the comparative analysis. The assumptions were that the the human behaviour. The lateral bending movement was relation between specimen size and ROM was not linear, and that the synthetic paediatric spine was fabricated by dependent on the nonlinearity of intervertebral disc as the facet joints made less contact with each other. As for the using synthetic material, which did not necessarily exhibit axial rotation and flexion/extension, the nonlinearity curves all the behaviour of biological soft tissues. Theoretically, measured were closer to 1, which suggested that the ROM the synthetic materials used in the synthetic paediatric spine tended to be more linear. These movements involved facet were supposed to allow wider movement as compared to the contacts that tended to be stiffer due to stiffness of the bone. human adult spine. Therefore, the ROM of synthetic paedi- This was potentially caused by assuming that the facet joints atric spine was expected to be within the same range as por- had the same material properties as spinal ligaments. cine spine, because the porcine ROM was larger than the The ROM of porcine was compared to synthetic paediat- human adult ROM (Figure 8). ric spine at ±4 Nm moments. As the size of synthetic paedi- Generally, porcine spine ROM was more flexible as com- atric spine (200%) and porcine spine (190%) was 10% pared to synthetic paediatric spine in all six DOF, with a different, the average ROM was expected to differ, but was larger difference in flexion/extension by 45%. The flexion still within the same range. Two assumptions were made in and extension was expected to be lower in the synthetic Displacement (degree) Displacement (degree) Displacement (degree) Displacement (degree) 8 Applied Bionics and Biomechanics Flexion extension Lateral bending Axial rotation Human adult spine-White & Panjabi (1990) Porcine spine-Wilke et al. (2011) Porcine spine-current study Figure 7: Comparison between porcine spine from current study and Wilke et al. [2] with human adult spine from White and Panjabi [29]. Flexion extension Lateral bending Axial rotation Synthetic paediatric spine Porcine spine Figure 8: Synthetic paediatric spine versus porcine spine at moment of ±4 Nm. paediatric spine as compared to the porcine spine because analysis between the synthetic paediatric spine and porcine the experimental data of all synthetic paediatric spines were spine, the synthetic paediatric spine developed in this relatively lower as compared to other ROMs. The difference research mimicked the behaviour of biological specimen. found for synthetic paediatric spine and porcine spine for Future works will consider using finite element analysis of paediatric spine to investigate the correct loading required lateral bending was 18% while it was only 3% in axial rotation. in the biomechanical testing to obtain paediatric ROM. The average ROM in lateral bending and axial rotation The limitations in this study are there is no data of pae- of synthetic paediatric spine were within the acceptable diatric ROM to enable a direct comparison and the synthetic range with porcine spine while flexion/extension differed paediatric spine has to be scaled up to 200% from the actual by 45%. One of the research limitations is the simplified size to fit the fixation holder in the spine simulator. shape of intervertebral disc for the synthetic paediatric spine, Although it was a scaled-up model, the morphology of the which potentially caused a significant difference in flexio- paediatric vertebra was still maintained. n/extension. However, synthetic paediatric spine ROM exhibited a nonlinear curve for all six DOF, suggesting that 5. Conclusion the ROM measured was acceptable because the synthetic paediatric spine demonstrated a viscoelasticity behaviour In the present study, fabricated synthetic paediatric spine in that existed in human soft tissues. From the comparative FSU unit was tested with a MTS Bionix Servohydraulic spine ROM (degree) ROM (degree) Applied Bionics and Biomechanics 9 [5] L. Wang, Y. Wang, L. Shi et al., “Can the sheep model fully rep- simulator to obtain the ROM in flexion, extension, lateral resent the human model for the functional evaluation of cervi- bending, and axial rotation. Overall, the ROM curves of syn- cal interbody fusion cages?,” Biomechanics and Modeling in thetic paediatric spine exhibited nonlinearities as all mea- Mechanobiology, vol. 18, no. 3, pp. 607–616, 2019. surements of NZ and EZ stiffness were below than 1. The [6] D. D. Jebaseelan, C. Jebaraj, N. Yoganandan, and ROM was then compared with the porcine spine for com- S. Rajasekaran, “Validation efforts and flexibilities of an parative analysis by using a comparable size model at eight-year-old human juvenile lumbar spine using a three- ±4 Nm moments. The porcine spine ROM was more flexible dimensional finite element model,” Medical & Biological Engi- than synthetic paediatric spine in all DOF, with a difference neering & Computing, vol. 48, no. 12, pp. 1223–1231, 2010. of 45% in flexion/extension, while the lateral bending and [7] S. Kumaresan, N. Yoganandan, F. A. Pintar, and D. J. Maiman, axial rotation of synthetic paediatric spine were in good “Finite element modeling of the cervical spine: role of interver- agreement with the porcine spine, with differences of 18% tebral disc under axial and eccentric loads,” Medical Engineer- and 3%, respectively. The difference in flexion/extension ing & Physics, vol. 21, no. 10, pp. 689–700, 1999. was potentially due to the simplified design of synthetic [8] S. Kumaresan, N. Yoganandan, F. A. Pintar, D. J. Maiman, and intervertebral disc, as it did not reflect the unique shape S. Kuppa, “Biomechanical study of pediatric human cervical within the vertebral body for each individual FSU. The spine: a finite element approach,” Journal of Biomechanical results presented in this research showed that the fabricated Engineering, vol. 122, no. 1, pp. 60–71, 2000. synthetic paediatric spine had nonlinearity characteristic in [9] L. Dong, G. Li, H. Mao, S. Marek, and K. H. Yang, “Develop- all DOF. The synthetic paediatric spine ROM was acceptable ment and validation of a 10-year-old child ligamentous cervi- as compared to the porcine spine at ±4 Nm moments, specif- cal spine finite element model,” Annals of Biomedical ically in lateral bending and axial rotation. Therefore, the Engineering, vol. 41, no. 12, pp. 2538–2552, 2013. fabricated synthetic paediatric spine particularly in thoracic [10] R. Phuntsok, M. D. Mazur, B. J. Ellis, V. M. Ravindra, and D. L. region in this study can mimic the biological ROM. It could Brockmeyer, “Development and initial evaluation of a finite ele- potentially be used as replacement of paediatric spine for ment model of the pediatric craniocervical junction,” Journal of biomechanical research that is related to spine deformity. Neurosurgery. Pediatrics, vol. 17, no. 4, pp. 497–503, 2016. [11] K. Shiba, H. Taneichi, T. Namikawa, S. Inami, D. Takeuchi, and Y. Nohara, “Osseointegration improves bone–implant Data Availability interface of pedicle screws in the growing spine: a biomechan- ical and histological study using an in vivo immature porcine The data used to support the findings of this study are avail- model,” European Spine Journal, vol. 26, no. 11, pp. 2754– able from the corresponding author upon request. 2762, 2017. [12] T. F. Fekete, F. S. Kleinstück, A. F. Mannion, Z. S. Kendik, and Conflicts of Interest D. J. Jeszenszky, “Prospective study of the effect of pedicle screw placement on development of the immature vertebra The authors do not have any conflict of interest that may in an in vivo porcine model,” European Spine Journal, affect the outcomes of this study. vol. 20, no. 11, pp. 1892–1898, 2011. [13] W. F. Lavelle, M. Moldavsky, Y. Cai, N. R. Ordway, and B. S. Bucklen, “An initial biomechanical investigation of fusionless Acknowledgments anterior tether constructs for controlled scoliosis correction,” The Spine Journal, vol. 16, no. 3, pp. 408–413, 2016. This study was financially supported by the Ministry of Higher Education, Malaysia, under Fundamental Research [14] E. C. Clarke, R. C. Appleyard, and L. E. Bilston, “Immature sheep spines are more flexible than mature Spines,” Spine, Grant Scheme (FRGS/1/2020/TK0/UNIMAP/02/20). vol. 32, no. 26, pp. 2970–2979, 2007. [15] J. Ouyang, Q. Zhu, W. Zhao, Y. Xu, W. Chen, and S. Zhong, References “Biomechanical assessment of the pediatric cervical spine under bending and tensile loading,” Spine, vol. 30, no. 24, [1] I. Busscher, A. J. van der Veen, J. H. van Dieën, I. Kingma, G. J. pp. E716–E723, 2005. Verkerke, and A. G. Veldhuizen, “In vitro biomechanical char- [16] Z. Kamal, G. Rouhi, N. Arjmand, and S. Adeeb, “A stability- acteristics of the Spine,” Spine, vol. 35, no. 2, pp. E35–E42, based model of a growing spine with adolescent idiopathic sco- liosis: a combination of musculoskeletal and finite element [2] H.-J. Wilke, J. Geppert, and A. Kienle, “Biomechanical in vitro approaches,” Medical Engineering & Physics, vol. 64, pp. 46– evaluation of the complete porcine spine in comparison with 55, 2019. data of the human spine,” European Spine Journal, vol. 20, [17] H.-K. Wong, J. H. P. Hui, U. Rajan, and H.-P. Chia, “Idio- no. 11, pp. 1859–1868, 2011. pathic Scoliosis in Singapore Schoolchildren,” Spine, vol. 30, [3] F. J. Lopez-Valdes, S. Lau, P. Riley, J. Lamp, and R. Kent, “The no. 10, pp. 1188–1196, 2005. biomechanics of the pediatric and adult human thoracic [18] S. W. Suh, H. N. Modi, J. H. Yang, and J. Y. Hong, “Idiopathic spine,” Annals of Advances in Automotive Medicine, vol. 55, scoliosis in Korean schoolchildren: a prospective screening pp. 193–206, 2011. study of over 1 million children,” European Spine Journal, [4] S.-R. Sheng, X.-Y. Wang, H. Z. Xu, G. Q. Zhu, and Y. F. Zhou, vol. 20, no. 7, pp. 1087–1094, 2011. “Anatomy of large animal spines and its comparison to the human spine: a systematic review,” European Spine Journal, [19] Q. du, X. Zhou, S. Negrini et al., “Scoliosis epidemiology is not vol. 19, no. 1, pp. 46–56, 2010. similar all over the world: a study from a scoliosis school 10 Applied Bionics and Biomechanics screening on Chongming Island (China),” BMC Musculoskele- tal Disorders, vol. 17, no. 1, 2016. [20] Á. Oroszlány, P. Nagy, and J. G. 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Veldhuizen, “Comparative anatomical dimensions of the com- plete human and porcine spine,” European Spine Journal, vol. 19, no. 7, pp. 1104–1114, 2010. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Applied Bionics and Biomechanics Hindawi Publishing Corporation

Experimental Analysis of Fabricated Synthetic Midthoracic Paediatric Spine as Compared to the Porcine Spine Based on Range of Motion (ROM)

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Copyright © 2021 Nor Amalina Muhayudin et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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Hindawi Applied Bionics and Biomechanics Volume 2021, Article ID 2799415, 10 pages https://doi.org/10.1155/2021/2799415 Research Article Experimental Analysis of Fabricated Synthetic Midthoracic Paediatric Spine as Compared to the Porcine Spine Based on Range of Motion (ROM) 1,2 1,3 1,3 Nor Amalina Muhayudin, Khairul Salleh Basaruddin , Ruslizam Daud, 4 4 Fiona McEvoy, and Tansey Faculty of Mechanical Engineering Technology, Universiti Malaysia Perlis, 02600, Pauh Putra Campus, Perlis, Malaysia Faculty of Electronic Engineering Technology, Universiti Malaysia Perlis, 02600 Perlis, Malaysia Sports Engineering Research Centre, Centre of Excellence (SERC), Universiti Malaysia Perlis, 02600 Perlis, Malaysia Mechanical Engineering Department, Institute of Technology Tallaght, Dublin, Dublin 24, Ireland Correspondence should be addressed to Khairul Salleh Basaruddin; khsalleh@unimap.edu.my Received 22 April 2021; Accepted 24 August 2021; Published 25 September 2021 Academic Editor: Raimondo Penta Copyright © 2021 Nor Amalina Muhayudin et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The present study is aimed at investigating the mechanical behaviour of fabricated synthetic midthoracic paediatric spine based on range of motion (ROM) as compared to porcine spine as the biological specimen. The main interest was to ensure that the fabricated synthetic model could mimic the biological specimen behaviour. The synthetic paediatric spine was designed as a 200% scaled-up model to fit into the Bionix Servohydraulic spine simulator. Biomechanical tests were conducted to measure the ROM and nonlinearity of sigmoidal curves at six degrees of freedom (DOF) with moments at ±4 Nm before the specimens failed. Results were compared with the porcine spine (biological specimen). The differences found between the lateral bending and axial rotation of synthetic paediatric spine as compared to the porcine spine were 18% and 3%, respectively, but was still within the range. Flexion extension of the synthetic spine is a bit stiff in comparison of porcine spine with 45% different. The ROM curves of the synthetic paediatric spine exhibited nonlinearities for all motions as the measurements of neutral zone (NZ) and elastic zone (EZ) stiffness were below “1.” Therefore, it showed that the proposed synthetic paediatric spine behaved similarly to the biological specimen, particularly on ROM. spine started with manipulation of the adult finite element 1. Introduction model to the paediatric model to incorporate anatomical dif- Human adult and animal cadaveric spines such as porcine, ferences between adult and paediatric spines. Although there sheep, baboon, and calf are commonly used in biomechanical were distinct differences between adult and paediatric spines investigation [1–5]. Over the years, the understanding of such as the morphology of their vertebra, the orientation of human spinal biomechanics was based on comprehensive facet joints was more horizontal in the paediatric spine and studies of human adult spine [1, 3, 5]. On the other hand, thus made it more mobile as compared to the adult spine information on paediatric spinal biomechanics was very lim- and ossification state of the vertebrae and size of nucleus pul- ited due to difficulties in obtaining paediatric human posus were larger in the paediatric intervertebral disc as com- cadavers. Although paediatric and adult spines were distinc- pared to adult disc. Paediatric spine is not miniature of adult spine; therefore, it cannot be treated as such. A few studies on tively different from each other in both anatomically and mechanically, studies on paediatric spines started by scaling paediatric biomechanical investigation still used human down from the adult size model to paediatric size model in adult and immature porcine spine as their specimens due to finite element analysis [6–10]. The studies on paediatric the limitations of paediatric specimens [11–13] 2 Applied Bionics and Biomechanics aimed at developing a working synthetic paediatric spine as The main challenge in developing the paediatric biome- chanical analyses was limited information of paediatric another alternative in paediatric spine biomechanical test- experimental data to enable a direct comparison with the ing. The key element is to ensure that the synthetic model performs similarly to the biological model. Therefore, the finite element model. Recently, a few studies were reported on the use of paediatric human cadavers to investigate the objective of this paper is to investigate the ROM of synthetic paediatric biomechanical response [3, 14]. Ouyang et al. midthoracic paediatric spine as compared to porcine spine. [15] investigated bending and tensile tests of paediatric cer- vical spines from neck to head of 2 years old to 12 years old. 2. Methods The study found that the distraction load for 6 years old to 12 years old was significantly higher as compared to 2 to 4 The first step in synthetic paediatric spine fabrication was years old. Another study to investigate the failure tolerance the development of physical paediatric spine model. Since was conducted by Lopez-Valdes et al. [3] by using human the actual physical size of paediatric model was relatively paediatric and adult thoracic spines. Similar findings were small to be tested with the MTS Bionix Servohydraulic spine found by Lopez-Valdes et al. whereby 7-year-old spines simulator, a scaling process was considered. To the author’s showed lower tolerance as compared to 15-year-old spines. knowledge, no data on physiological ROM of human paedi- The study suggested that the 15-year-old spine tolerance atric spine exists, particularly in the thoracic region, and was comparable to adult spines. On the other hand, Clarke thus, it is essential to generate an experimental protocol by et al. [14] used sheep spines to investigate the biomechanical using biological specimens before ROM of the synthetic differences between mature and immature spines by using spine was determined. newborn and 2-year-old specimens. The study focused on ROM and found that immature spines (newborn specimens) 2.1. Scaling of Paediatric Spine. Another important factor exhibited a significantly lower ROM as compared to mature that was considered while developing the synthetic paediat- spines (2 years old). In paediatric in vivo testing, these were ric spine was the actual size of human paediatric spine. the only studies found as a guide to compare the synthetic Assuming that size of the paediatric spine is 100%, the size paediatric spine with human paediatric spine. This limita- of adult spine is normally scaled up to 141%, and the size tion of paediatric human spines can be overcome by devel- of porcine spine is larger than an adult spine by an average oping a working synthetic paediatric spine. Hence, more difference of 50% [4, 27, 28]. Therefore, the porcine spine biomechanical testing in regards with paediatric spines can size is approximately 190% as compared to paediatric spine. be performed such as paediatric trauma during motor vehi- In this research, the synthetic paediatric spine was scaled up cle collision, paediatric sports-related injuries, and other to 200% of the paediatric spine size to fit the size of the MTS common recreational injuries that required paediatric speci- Bionix Servohydraulic system spine simulator which was in mens for further investigations. reference to the size of porcine spine that was used as the Development of a synthetic paediatric spine is essential cadaver control data as shown in Figure 1. to investigate the mechanical behaviour of paediatric cases A physical model of paediatric vertebra was purchased such as scoliosis. The common surgical procedure among from Sawbones (Inc., Vashon Island, USA) for all regions, paediatric cases is early onset scoliosis surgical treatment, and these vertebrae represent the anatomical dimension of and studies on the effect and accuracy of various paediatric a juvenile group (8 to 9 years old). Since this study focused spinal instrumentations of scoliosis normally were con- on T4-T8, five individual vertebrae were scanned in three- ducted only using FE studies, animal spines, or postoperative dimension (3D) before being scaled to 200% in a selective studies that required years of observation [11, 16, 17]. There- laser sintering (SLS) machine to fabricate the prototype. fore, it will be beneficial to have a working synthetic paediat- ric spine that can be used in spinal instrumentations of 2.2. Fabrication of Synthetic Paediatric Spine. The materials biomechanical investigations or preplanning of complicated to fabricate the synthetic spine were divided into three main surgical treatment. The main advantage in using synthetic components, which were vertebra, intervertebral disc, and materials is that they can be tailored to a specific require- spinal ligaments. All materials used in the synthetic paediat- ment, and they offer constant material properties. On the ric spine were structurally and mechanically close to human hand, studies conducted by Suh et al., Du et al., and Oros- data. This was to ensure that the final product (synthetic zlány et al. [18–20] proved that thoracic was the most com- paediatric spine) could replicate human behaviour. Details mon affected region in spine with more than 50% cases of analysis to select the material for each component of spine amongst children. Therefore, the present study was focused are available in previous author’s publications, namely, for on the synthetic spine development of the thoracic region, vertebra [4], intervertebral disc [5], and spinal ligament particularly on T4-T8. In recent years, synthetic materials [6]. Table 1 summarises the properties of the selected mate- were commonly used as alternatives in biomechanical test- rials for each component of fabricated synthetic paediatric ing, especially in trabecular bone [21–24]. Bohl et al. started spine. to develop adult synthetic spine model of L3-L5 segments by The paediatric synthetic spine was fabricated as a single using a 3D printer, and they found that although there were functional spinal unit (FSU), consisting of two vertebrae, great differences in ROM data, the study claimed that the an intervertebral disc, and associated ligaments. The process model could mimic a specific ROM on standard ROM test- started with embedding the cortical around the trabecular ing applied to cadavers [25, 26]. The present study is mainly structure. Next, the disc was attached within the two Applied Bionics and Biomechanics 3 Scaling factor Paediatric spine Adult spine Synthetic paediatric spine Reference: Porcine spine 100 120 140 160 180 200 Specimen size (%) Figure 1: Size comparison among all spine models in this study. vertebrae by using the “spinous processes” natural structure 2.3.2. Experimental Setup. These experiments were con- as reference. The next process was to mould the spinal liga- ducted by using MTS Bionix Servohydraulic system spine ments within the posterior elements and attach the anterior simulator. Specimens were fixed at its natural position in longitudinal ligament (ALL) and posterior longitudinal liga- the spine simulator before testing, as shown in Figure 4. ment (PLL) within the vertebral body. Finally, the vertebral Twelve specimens were then tested without a preload to body was covered with sheet wax. All materials used in the avoid buckling in an alternating sequence of flexion/exten- synthetic paediatric spine were geometrically and mechani- sion, lateral bending right/left, and axial rotation right/left cally close to human data. The fabrication process flow is under pure moments. All specimens were tested at five summarised in Figure 2. cycles, and the first two cycles were considered as precycles. The applied moments and angular displacements were recorded for each cycle. The experiments were performed 2.3. Biomechanical Testing. To provide an indication of under ±7.5 Nm load with 1.7 deg/sec for all DOF, and the whether the materials selected to fabricate the synthetic pae- results used were at the fifth cycle. diatric spine could mimic human spine movement or not, a All specimens were tested to obtain flexion, extension, series of experiments were carried out for both porcine spine right and left lateral bending, and right and left axial rota- and synthetic paediatric spine. tion by using the MTS Bionix Servohydraulic spine simu- lator under similar experimental procedures developed 2.3.1. Specimen Preparation. Six porcine spines from 6 prior to the porcine spine test. To observe the perfor- months to 7 months old were provided from a local abattoir. mance of synthetic paediatric spine, three specimens were The breed was a cross Saddleback with Gloucester porcine. tested until failure by using a pure moment of ±1Nm Average weight of spines used was 82.34 kg (±4.9 kg). The (0.1 deg/sec) for all six DOF with an increment of full spines were freshly dissected into single FSUs (T4-T5, ±1 Nm. Results from these three specimens showed that T5-T6, T6-T7, and T7-T8) with three specimens for each the specimens failed at ±5 Nm, whereby the disc started FSU, as shown in Figure 3. All ligaments, disc, and vertebra to detach from the vertebrae. Therefore, an assumption were preserved, while muscle tissues were carefully removed was made that the valid ROM for the paediatric synthetic and frozen at -20 spine in this study was at ±4 Nm. The rest of specimens C. The specimens were thawed for 24 hours before testing. The specimens were potted to ensure were tested at ±4 Nm with 1 deg/sec rate. All specimens that the middle disc was aligned horizontally with the spine were tested to five cycles, whereby the first two cycles were simulator. The upper half of top vertebra and lower half of considered as the precycle and results were determined at bottom vertebra were embedded in a polyurethane liquid the fifth cycle. plastic (Smooth Cast 300). Four FSUs of synthetic paediatric spine from T4 to T8 2.3.3. Analysis of ROM. ROM was determined from the sig- were prepared based on the fabrication process mentioned moidal curve for each loading direction from the fifth cycle: flexion, extension, right and left lateral bending, and right in Section 2.2, with three specimens for each FSU. All twelve specimens were potted to ensure that the middle disc was and left axial rotation. The typical curve for each loading aligned horizontally with the spine simulator. Similar to por- direction is shown in Figure 5. The arrows indicated the cine specimens, the upper half of top vertebra and lower half loading and unloading direction. Key parameters in the of bottom vertebra were embedded in liquid resin (Smooth curve are total ROM, NZ ROM, NZ stiffness (S1), and EZ stiffness (S2). Cast 300). Specimen type 4 Applied Bionics and Biomechanics Table 1: Material properties of the components in the fabricated synthetic paediatric spine. Properties Material Spine component Compression Tensile modulus Stiffness modulus (MPa) (MPa) (N/mm) Expandable polyurethane foam (0.24 g/cm density) Trabecular bone 89.9 - - Urethane plastic (Smooth Cast 385) Cortical bone 3200 - - Polyurethane elastomer (Monothane) Annolous fibrosus 14.1 3.3 - Silicone (Lightweight) Nucleus pulposus 1.6 0.6 - EZ = 37:8 Silicone with fiber glass tape (Sorta Clear 40 in woven fibre 45 ) ALL & PLL - 22.5 NZ = 109:6 (a) (b) (c) (d) (f ) (e) Figure 2: The process flow to manufacture paediatric synthetic spine. (a) Cortical, (b) completed vertebra, (c) disc, (d) assembled ALL and PLL, (e) added posterior ligaments, and (f) completed FSU. Results collected from synthetic paediatric spine test specimen 2 was the most flexible porcine spine. Overall, all were plotted in the sigmoidal curve to observe if any nonlin- FSUs showed the same pattern, whereby it increased from earity existed in the ROM. The nonlinearities or sigmoidal flexion extension to lateral bending and axial rotation, patterns were essential to prove that the synthetic paediatric except for T4-T5. The wide interspecimen variability was spine exhibited a viscoelastic behaviour, which is normally expected as each FSU was from a different porcine spine found in the biological specimens. The nonlinearities of specimen. graphs were observed for all specimens, as it was the key As for ROM at moment of ±4 Nm, it increased from parameter to determine the performance of synthetic spine flexion extension to lateral bending and axial rotation, as compared to the biological specimens. The value of except for T5-T6. The differences between each FSU in S1/S2 was expected to be lower than 1.0 to prove that the each DOF were only around 20%, except for T5-T6. curve was a nonlinear curve, whereby the smaller the value, Data indicated that T5-T6 in axial rotation was the stiff- the stronger the sigmoidicity. est as compared to other FSUs and DOF. The ROM data for each FSU at ±4 Nm moment is summarised in Table 3. 3. Results and Discussions 3.2. ROM of Synthetic Paediatric Spine. Variables of interest were the ROM and value of S1/S2, as summarised in Table 4. 3.1. ROM of Porcine Spine. The ROM data for each FSU at ±7.5 Nm moment is summarised in Table 2. The data In Table 4, axial rotation exhibited more linear curves as showed that for flexion extension, T5-T6 and T7-T8 were compared to other ROMs because the average value of stiffer than T4-T5 and T6-T7 by around 40%. In lateral S1/S2 was 0.7, which was closed to 1.0. The most nonlinear bending, all FSUs were in good agreement with less than curves were observed in lateral bending with an average value of 0.16. In flexion and extension curves, the upper 10% difference between each FSU. As for axial rotation, dif- ference between the biggest ROM (T6-T7) and lowest ROM FSUs (T4-T5 and T5-T6) showed more linear curves as (T4-T5) was around 30%. Interestingly, T4-T5 and T6-T7 compared to lower FSUs (T6-T7 and T7-T8). Although axial results came from the same specimens. In this research, rotation curves were inclined towards a linear curve, the Applied Bionics and Biomechanics 5 (a) (b) Figure 3: The porcine specimens were dissected into single FSUs, view in (a) frontal plane and (b) transverse plane. (a) (b) Figure 4: Experimental setup for (a) porcine spine and (b) synthetic paediatric spine in the MTS Bionix Servohydraulic spine simulator. EZ stiffness Angular (S2) displacement (degree) NZ stiffness (S1) Moment (Nm) NZ Total ROM ROM Figure 5: Typical sigmoidal curve of spine ROM. values of S1/S2 for all cases were still lower than 1.0, which The second variable was ROM of each FSU, whereby the suggested that all six DOF exhibited nonlinearity curves in pattern that emerged for ROM values showed distinct differ- their ROMs. ences between FSUs. The results can be divided into two 6 Applied Bionics and Biomechanics obtained by Wilke et al. [2] for all FSUs, as presented in Table 2: Porcine spine data of the ROM for each DOF of each specimen at moment of ±7.5 Nm. Figure 6. The average of all FSUs for the midthoracic region (T4- ROM ( ) FSU T8) is summarised in Figure 7 in all six DOF. The difference Flexion extension Lateral bending Axial rotation between lateral bending and axial rotation was less than 2%, 6:72 ± 3:61 10:59 ± 4:87 9:51 ± 5:23 T4-T5 while flexion/extension was stiffer by 36% [7]. The signifi- 3:08 ± 0:19 10:46 ± 4:00 10:82 ± 4:00 cant difference in flexion/extension might be potentially T5-T6 due to weight and size of the porcine tested. Although the T6-T7 6:69 ± 4:97 12:78 ± 8:58 16:51 ± 12:02 ROM in lateral bending and axial rotation from these FSUs 4:78 ± 1:29 12:03 ± 1:27 13:07 ± 6:95 T7-T8 were within the range provided by literature, the average was lower as compared to other FSUs. As suggested by Muhayu- din et al. and White and Panjabi, specimen weight played a Table 3: Porcine spine data of the ROM for each DOF of each significant effect on the anatomical spine dimension, which specimen at moment of ±4 Nm. may subsequently affect the ROM and in this research, whereby it significantly affected the flexion/extension [28, ROM ( ) 29]. Therefore, by considering the differences in flexion/ex- FSU Flexion extension Lateral bending Axial rotation tension between current study and literature, the results of T4-T5 1:97 ± 0:48 5:40 ± 1:00 7:15 ± 3:61 porcine spine from this study were used in the comparative analysis with synthetic paediatric spine. 2:80 ± 0:28 5:60 ± 0:96 4:92 ± 0:74 T5-T6 Since no data is available on ROM of human paediatric T6-T7 2:47 ± 0:10 5:63 ± 1:51 7:22 ± 2:01 spine, further analysis was required to compare the porcine 2:47 ± 0:42 6:45 ± 1:81 8:73 ± 5:80 T7-T8 ROM with human adult ROM from White and Panjabi under the same moment [29]. The ROM presented in Figure 8 was an average ROM measurement taken from a groups, which were the upper FSUs (T4-T5 and T5-T6) and single FSU, ranging from T4 to T8. As expected, both sets lower FSUs (T6-T7 and T7-T8) in all six DOF. The upper of porcine data were comparable, except for flexion/exten- FSUs and lower FSUs were within the same range for all sion. However, when compared to human adult ROM, the six DOF. The percentage differences within upper FSUs differences ranged from 60% to 90% for all six DOF. In lat- and lower FSUs were more than 50% for all six DOF. Flex- eral bending and axial rotation, the differences between por- ion/extension was the stiffest motion, followed by lateral cine and human adult ROMs were approximately around bending and axial rotation. Despite differences between 93% and 66%, respectively. In contrast, the flexion/extension FSUs, the pattern emerged similar to the pattern showed in from this study was comparable to human adult with 12% porcine spine, whereby the ROM increased from flexion/ex- difference, while Wilke et al. was 74% larger than that of a tension to axial rotation. human adult. The size of porcine spines was evidently larger than human adult spines, which subsequently resulted in a larger ROM. However, the relation between specimen size 4. Discussion and ROM was not linear as the material properties, specifi- The biomechanical analysis of a spine was normally carried cally the viscoelasticity of the soft tissues which was different for each specimen. This was considered when the compara- on a single FSU. Despite testing a small segment of the spine, it can exhibit the characteristics of the entire spine. Due to tive analysis was conducted between the synthetic paediatric limited information on the physiological ROM of paediatric and porcine spine. spine, the porcine spine was an essential substitute to guide The crucial element in testing synthetic paediatric spine in biomechanical testing. The physiological ROM included was to ensure that the plotted curve showed nonlinearities to replicate the viscoelastic behaviour of soft tissues in were flexion and extension, left and right lateral bending, and left and right axial rotation. The series of tests were con- human spine. Therefore, the first parameter was to deter- ducted on porcine spines from T4 to T8 on a single FSU mine the ratio of EZ stiffness (S1) to NZ stiffness (S2). In under ±7.5 Nm moments at all six DOF. The ROM was mea- Table 4, axial rotation exhibited more linear curves as com- sured for all six DOF at ±7.5 Nm moments and was directly pared to other ROMs because the average value of S1/S2 was compared with a previous study on porcine spines by Wilke 0.7, which was close to 1.0. The most nonlinear curves were et al. [2], since the average weight for tested porcine spines observed in lateral bending with an average value of 0.16. In was within the same range used in this research. flexion and extension curves, the upper FSUs (T4-T5 and In Figure 6, the flexion/extension of the porcine spines T5-T6) showed more linear curves as compared to lower presented was varied in all FSUs. The flexion/extension of FSUs (T6-T7 and T7-T8). Although axial rotations curve T4-T5 and T6-T7 was 20% different as compared to research were inclined towards linear curve, the values of S1/S2 for but was within the range. In contrast, T5-T6 and T7-T8 had all cases were still lower than 1.0, which suggested that all 50% average difference with that in literature and T7-T8 six DOF exhibited nonlinearity curves in their ROMs. flexion was the only DOF that was within the range. On The nonlinear behaviour in ROM was a result from both the other hand, the average of lateral bending and axial rota- soft and hard tissues. The lateral bending exhibited nonlin- tion was in good agreement and was within the range ear curves with a ratio closer to 0, suggesting that the Applied Bionics and Biomechanics 7 Table 4: Results of the ROM and S1/S2 for each motion of synthetic paediatric specimen at moment of ±4 Nm. Flexion extension Lateral bending Axial rotation FSU ° ° ° ROM ( ) S1/S2 ROM ( ) S1/S2 ROM ( ) S1/S2 0:87 ± 0:03 0:46 ± 0:28 0:18 ± 0:03 3:55 ± 0:81 0:76 ± 0:05 T4-T5 2:46 ± 0:25 T5-T6 0:82 ± 0:02 0:55 ± 0:09 2:89 ± 0:46 0:24 ± 0:05 3:30 ± 0:95 0:87 ± 0:04 1:72 ± 0:14 0:29 ± 0:16 7:00 ± 0:20:12 ± 0:04 9:20 ± 1:06 0:51 ± 0:25 T6-T7 T7-T8 1:92 ± 0:42 0:21 ± 0:06 6:56 ± 0:39 0:10 ± 0:05 11:00 ± 1:40 0:63 ± 0:15 16 16 T4-T5 T5-T6 T6-T7 T7-T8 T4-T5 T5-T6 T6-T7 T7-T8 T4-T5 T5-T6 T6-T7 T7-T8 T4-T5 T5-T6 T6-T7 T7-T8 –4 –4 Lateral bending left Lateral bending right Flexion Extension Current study Current study Wilke et al. Wilke et al. (a) (b) 15 16 T4-T5 T5-T6 T6-T7 T7-T8 T4-T5 T5-T6 T6-T7 T7-T8 Axial rotation left Axial rotation right Flexion Axial rotation Lateral bending extension Current study Current study Wilke et al. Wilke et al. (c) (d) Figure 6: ROM of porcine spine between this research and Wilke et al. [2] at ±7.5 Nm moments in (a) flexion extension, (b) lateral bending, (c) axial rotation, and (d) overall ROM. materials selected as synthetic intervertebral disc replicated the comparative analysis. The assumptions were that the the human behaviour. The lateral bending movement was relation between specimen size and ROM was not linear, and that the synthetic paediatric spine was fabricated by dependent on the nonlinearity of intervertebral disc as the facet joints made less contact with each other. As for the using synthetic material, which did not necessarily exhibit axial rotation and flexion/extension, the nonlinearity curves all the behaviour of biological soft tissues. Theoretically, measured were closer to 1, which suggested that the ROM the synthetic materials used in the synthetic paediatric spine tended to be more linear. These movements involved facet were supposed to allow wider movement as compared to the contacts that tended to be stiffer due to stiffness of the bone. human adult spine. Therefore, the ROM of synthetic paedi- This was potentially caused by assuming that the facet joints atric spine was expected to be within the same range as por- had the same material properties as spinal ligaments. cine spine, because the porcine ROM was larger than the The ROM of porcine was compared to synthetic paediat- human adult ROM (Figure 8). ric spine at ±4 Nm moments. As the size of synthetic paedi- Generally, porcine spine ROM was more flexible as com- atric spine (200%) and porcine spine (190%) was 10% pared to synthetic paediatric spine in all six DOF, with a different, the average ROM was expected to differ, but was larger difference in flexion/extension by 45%. The flexion still within the same range. Two assumptions were made in and extension was expected to be lower in the synthetic Displacement (degree) Displacement (degree) Displacement (degree) Displacement (degree) 8 Applied Bionics and Biomechanics Flexion extension Lateral bending Axial rotation Human adult spine-White & Panjabi (1990) Porcine spine-Wilke et al. (2011) Porcine spine-current study Figure 7: Comparison between porcine spine from current study and Wilke et al. [2] with human adult spine from White and Panjabi [29]. Flexion extension Lateral bending Axial rotation Synthetic paediatric spine Porcine spine Figure 8: Synthetic paediatric spine versus porcine spine at moment of ±4 Nm. paediatric spine as compared to the porcine spine because analysis between the synthetic paediatric spine and porcine the experimental data of all synthetic paediatric spines were spine, the synthetic paediatric spine developed in this relatively lower as compared to other ROMs. The difference research mimicked the behaviour of biological specimen. found for synthetic paediatric spine and porcine spine for Future works will consider using finite element analysis of paediatric spine to investigate the correct loading required lateral bending was 18% while it was only 3% in axial rotation. in the biomechanical testing to obtain paediatric ROM. The average ROM in lateral bending and axial rotation The limitations in this study are there is no data of pae- of synthetic paediatric spine were within the acceptable diatric ROM to enable a direct comparison and the synthetic range with porcine spine while flexion/extension differed paediatric spine has to be scaled up to 200% from the actual by 45%. One of the research limitations is the simplified size to fit the fixation holder in the spine simulator. shape of intervertebral disc for the synthetic paediatric spine, Although it was a scaled-up model, the morphology of the which potentially caused a significant difference in flexio- paediatric vertebra was still maintained. n/extension. However, synthetic paediatric spine ROM exhibited a nonlinear curve for all six DOF, suggesting that 5. Conclusion the ROM measured was acceptable because the synthetic paediatric spine demonstrated a viscoelasticity behaviour In the present study, fabricated synthetic paediatric spine in that existed in human soft tissues. From the comparative FSU unit was tested with a MTS Bionix Servohydraulic spine ROM (degree) ROM (degree) Applied Bionics and Biomechanics 9 [5] L. Wang, Y. Wang, L. Shi et al., “Can the sheep model fully rep- simulator to obtain the ROM in flexion, extension, lateral resent the human model for the functional evaluation of cervi- bending, and axial rotation. Overall, the ROM curves of syn- cal interbody fusion cages?,” Biomechanics and Modeling in thetic paediatric spine exhibited nonlinearities as all mea- Mechanobiology, vol. 18, no. 3, pp. 607–616, 2019. surements of NZ and EZ stiffness were below than 1. The [6] D. D. Jebaseelan, C. Jebaraj, N. Yoganandan, and ROM was then compared with the porcine spine for com- S. 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Journal

Applied Bionics and BiomechanicsHindawi Publishing Corporation

Published: Sep 25, 2021

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