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Orthopaedic Proceedings
Vol. 105-B, Issue SUPP_16 | Pages 7 - 7
17 Nov 2023
Hayward S Gheduzzi S Keogh P Miles T
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Abstract

Objectives

Spinal stiffness and flexibility terms are typically evaluated from linear regression of experimental data and are then assembled into 36-element matrices. Summarising in vitro test results in this manner is quick, computationally cheap and has the distinct advantage of outputting simple characteristic values which make it easy to compare results. However, this method disregards many important experimental features such as stiffening effects, neutral and elastic zones magnitudes, extent of asymmetry and energy dissipation (hysteresis). Alternatives to the linear least squares method include polynomials, separation of the load-displacement behaviour into the neutral and elastic zones using various deterministic methods and variations on the double sigmoid and Boltzmann curve fits. While all these methods have their advantages, none provide a comprehensive and complete characterisation of the load-displacement behaviour of spine specimens. In 1991, Panjabi demonstrated that the flexion-extension and mediolateral bending behaviour of functional spinal units could be approximated using the viscoelastic model consisting of a nonlinear spring in series with a linear Kelvin element. Nowadays viscoelastic models are mainly used to describe creep and stress relaxation, rather than for cyclic loading. The aim of this study was to conclusively prove the viscoelastic nature of spinal behaviour subject to cyclic loading. Being able to describe the behaviour of spine specimens using springs and dampers would yield characterising coefficients with recognisable physical meaning, thus providing an advantage over existing techniques.

Methods

Six porcine isolated spinal disc specimens (ISDs) were tested under position and load control. Visual inspection of the load-displacement graphs from which the principal terms of the stiffness and flexibility matrices are derived suggest that the load-displacement behaviour could be idealised by a nonlinear spring system with damping. It was hypothesised that the contributions arising from non-linear spring-like behaviour and damping could be separated for each of the principal load-displacement graphs.


Orthopaedic Proceedings
Vol. 105-B, Issue SUPP_9 | Pages 41 - 41
17 Apr 2023
Hayward S Miles A Keogh P Gheduzzi S
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Lower back pain (LBP) is a global problem. Countless in vitro studies have attempted to understand LBP and inform treatment protocols such as disc replacement devices (DRDs). A common method of reporting results is applying a linear fit to load-displacement behaviour, reporting the gradient as the specimen stiffness in that axis. This is favoured for speed, simplicity and repeatability but neglects key aspects including stiffening and hysteresis. Other fits such as polynomials and double sigmoids better address these characteristics, but solution parameters lack physical representation. The aim of this study was to implement an automated method to fit spinal load-displacement behaviour using viscoelastic models.

Six porcine lumbar spinal motion segments were dissected to produce isolated disc specimens. These were potted in Wood's metal, ensuring the disc midplane remained horizontal, sprayed with 0.9% saline and wrapped in saline-soaked tissue and plastic wrap to prevent dehydration. Specimens were tested using the University of Bath spine simulator operating under position control with a 400N axial preload.

Specimens were approximated using representative viscoelastic elements. These models were constructed in MATLAB Simulink R2020b using the SimScape library. Solution coefficients were determined by minimizing the sum of squared errors cost function using a non-linear least squares optimization method.

The models matched experimental data well with a mean % difference in model and specimen enclosed area below 6% across all axes. This indicates the ability of the model to accurately represent energy dissipated. The final models demonstrated reduced RMSEs factors of 3.6, 1.1 and 9.5 smaller than the linear fits for anterior-posterior shear, mediolateral shear and axial rotation respectively.

These nonlinear viscoelastic models exhibit significantly increased qualities of fit to spinal load-displacement behaviour when compared to linear approximations. Furthermore, they have the advantage of solution parameters which are directly linked to physical elements: springs and dampers. The results from this study could be instrumental in improving the design of DRDs as a mechanism for treating LBP.


Orthopaedic Proceedings
Vol. 105-B, Issue SUPP_9 | Pages 42 - 42
17 Apr 2023
Hayward S Miles A Keogh P Gheduzzi S
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Lower back pain (LBP) is a worldwide clinical problem and a prominent area for research. Numerous in vitro biomechanical studies on spine specimens have been undertaken, attempting to understand spinal response to loading and possible factors contributing to LBP. However, despite employing similar testing protocols, there are challenges in replicating in vivo conditions and significant variations in published results. The aim of this study was to use the University of Bath (UoB) spine simulator to perform tests to highlight the major limitations associated with six degree of freedom (DOF) dynamic spine testing.

A steel helical spring was used as a validation model and was potted in Wood's metal. Six porcine lumbar spinal motion segments were harvested and dissected to produce isolated spinal disc specimens. These were potted in Wood's metal, ensuring the midplane of the disc remained horizontal and then sprayed with 0.9% saline and wrapped in saline-soaked tissue and plastic wrap to prevent dehydration. A 400N axial preload was used for spinal specimens. Specimens were tested under the stiffness and flexibility protocols.

Tests were performed using the UoB custom 6-axis spine simulator with coordinate axes. Tests comprised five cycles with data acquired at 100Hz. Stiffness and flexibility matrices were evaluated from the last three motion cycles using the linear least squares method.

According to theory, inverted flexibility matrices should equal stiffness matrices. In the case of the spring, the matrices matched analytical solutions and inverted flexibility matrices were equivalent to stiffness matrices. Matrices from the spinal tests demonstrated some symmetry, with similarities between inverted flexibility- and stiffness matrices, though these were unequal overall. Matrix element values were significantly affected by displacements assumed to occur at disc centre.

Spring tests proved that for linear, elastic specimens, the spine simulator functioned as expected. However, multiple factors limit the confidence in spine test results. Centre of rotation, displacement assumptions and rigid body transformations are known to impact the results from spinal testing, and these should be addressed going forward to improve the replication of in vivo conditions.


Orthopaedic Proceedings
Vol. 105-B, Issue SUPP_9 | Pages 43 - 43
17 Apr 2023
Hayward S Miles A Keogh P Gheduzzi S
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Injury of the intervertebral disc (IVD) can occur for many reasons including structural weakness due to disc degeneration. A common disc injury is herniation. A herniated nucleus can compress spinal nerves, causing pain, and nucleus depressurisation changes mechanical behaviour. Many studies have investigated in vitro IVD injuries including endplate fracture, incisions, and nucleotomy. There is, however, a lack of consensus on how the biomechanical behaviour of spinal motion segments is affected. The aim of this study was to induce defined changes to IVDs of spine specimens in vitro and apply 6 degree of freedom testing to evaluate the effect of these changes.

Six porcine lumbar spinal motion segments were harvested from organically farmed pigs. Posterior structures were removed to produce isolated spinal disc specimens. Specimens were potted in Wood's metal, ensuring the midplane of the IVD remained horizontal. After potting, specimens were sprayed with 0.9% saline, wrapped in saline-soaked tissue and plastic wrap to prevent dehydration. A 400N axial preload was equilibrated for 30 minutes before testing. Specimens were tested intact and after a partial nucleotomy removing ~0.34g of nuclear material with a curette through an annular incision.

Stiffness tests were performed using the University of Bath's custom 6-axis spine simulator with coordinate axes and displacement amplitudes. Tests comprised five cycles with data acquired at 100Hz. Stiffness matrices were evaluated from the last three motion cycles using the linear least squares method.

Stiffness matrices for intact and nucleotomy tests were compared. No significant differences in shear, axial or torsional stiffnesses were noted. Nucleotomy caused significantly higher stiffness in lateral bending and flexion-extension with increased linearity and the load-displacement behaviour in these axes displayed no neutral zone (NZ).

Induced changes were designed to replicate posterolaterally herniated discs. Unaffected shear, axial and torsional stiffnesses suggest the annulus is crucial in these axes. However, reduced ROM and NZ after nucleotomy suggests bending is most affected by herniation. Increased linearity and lack of defined NZ in these axes demonstrates herniation causes major changes to the viscoelastic behaviour of spine specimens in response to loading.


Orthopaedic Proceedings
Vol. 103-B, Issue SUPP_16 | Pages 24 - 24
1 Dec 2021
Hayward S Miles T Keogh P Gheduzzi S
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Abstract

Introduction

Back pain affects 80% of the population at some stage in their life with significant costs to society. Mechanisms and causes of pain have been investigated by studying the behaviour of functional spinal units (FSUs) subjected to displacement- or load control protocols in 6 degrees of freedom (DOF). Load control allows specimens to move physiologically in response to applied loads whereas displacement control constrains motion to individual axes. The displacement control system of the Bath University six-axis spine simulator has been validated and the load control system is in the process of iterative development.

Objectives

The objective was to build a computational model of the spine simulator to develop a complete 6 DOF load control system to enable accurate specimen testing under load control.


Orthopaedic Proceedings
Vol. 103-B, Issue SUPP_2 | Pages 45 - 45
1 Mar 2021
Czerbak K Gheduzzi S Clift S
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Abstract

Objectives

A fibril reinforced multiphasic cartilage model was developed to improve the understanding of the depth-dependent cartilage internal structure and its through thickness biomechanical response. The heterogeneous model of cartilage was validated against full-field strain measurement obtained via Digital Image Correlation (DIC) during free swelling experiments.

Methods

Hemi-cylindrical cartilage cores of 5mm diameter were obtained from porcine femoral condyles and humeral heads. The full field behaviour of these samples was monitored using DIC during an osmotic free swelling experiment performed following a standardised protocol [1]. Computational models were created in FEBio (version 2.8, febio.org). The cartilage, submerged in saline solution was represented by a 1×1mm cube [2] with geometry and constrains set up to mimic the experimental conditions. Cartilage was modelled as a multiphasic material represented by one inhomogeneous layer with depth-dependent Young's modulus [3], zonally varied water content and zonally oriented collagen fibrils [4]. Experimental and predicted strain maps were compared to each other both qualitatively and quantitatively.


Orthopaedic Proceedings
Vol. 103-B, Issue SUPP_2 | Pages 47 - 47
1 Mar 2021
Hayward SJ Lammey GP Gheduzzi S Miles AW Keogh PS
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Abstract

Introduction

Altered mechanical loading is a contributing factor to low back pain, a condition affecting 80% of the population at some point in life. A plethora of in-vitro studies exist focusing on 6 degree of freedom (dof) testing of functional spinal units (FSU) to obtain a specimen stiffness matrix. Due to differences in the performance of test apparatus and in the technique used to manipulate raw data it is difficult to compare results from different groups.

Objectives

Our primary objective was to develop a standardised technique to benchmark the performance of testing apparatus; a secondary objective was to standardise the data manipulation technique.


Orthopaedic Proceedings
Vol. 101-B, Issue SUPP_2 | Pages 38 - 38
1 Jan 2019
Hernandez BA Gill HS Gheduzzi S
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Several specimen specific vertebral (VB) models have been proposed in the literature; these replicate the typical set-up of a vertebral body mounted in bone cement and subject to a compressive ramp. VB and cement geometries are obtained from micro-CT images, the cement is typically assigned properties obtained from the literature while VB properties are inferred from the Hounsfield units- where the conversion factor between grayscale data and Young's modulus is optimised using experimental load-displacement data. Typically this calibration is performed on VBs dissected from the same spines as the study group. This, alongside the use of non-specific cement properties, casts some doubts on the predictivity of the models thus obtained. The predictivity of specimen specific FE models was evaluated in this study.

VBs obtained from three porcine cervical segments (C2-C6) were stripped of all soft tissues, potted in bone cement and subject to a compressive loading ramp. A speckle pattern was applied to the anterior part of the specimen for DIC imaging. Specimen specific FE models were constructed from these specimens and a conversion factor between grayscale and material properties was optimised. Cement properties were assigned based on literature data. VBs from a further cervical spine (C2-C7) were subject to the same experimental protocol. In this case, the models generated from microCT images the material properties of bone were assigned based on the average conversion factor obtained previously. The predicted load-displacement behaviour thus obtained was compared to experimental data. Generally, poor agreement was found between overall load-displacement. The use of generic cement properties in the models was found to be partly responsible for this. When the load displacement behaviour of the VB was studied in isolation, good agreement within one standard deviation was found with 4 out of 6 models showing good correlation between simulation and DIC data.


Orthopaedic Proceedings
Vol. 101-B, Issue SUPP_2 | Pages 18 - 18
1 Jan 2019
Boyd S Silvestros P Hernandez BA Cazzola D Preatoni E Gill HS Gheduzzi S
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Digital image correlation (DIC) is rapidly increasing in popularity in biomechanical studies of the musculoskeletal system. DIC allows the re-construction of full field displacement and/or strain maps of the surface of an object. DIC systems typically consist of two cameras focussing on the same region of interest. This constrains the angle between the cameras to be relatively narrow when studying specimens characterised by complex geometrical features, giving rise to concerns on the accuracy of the out of plane estimates of movement.

The aim of this research was to compare the movement profiles of bony segments measured by DIC and by an optoelectronic motion capture system.

Five porcine cervical spine segments (C2-C6) were obtained from the local butcher. These were stripped of all anterior soft tissues while the posterior structures were left intact. A speckle pattern was applied to the anterior aspect of the specimens, while custom made infrared clusters were rigidly attached to the 3 middle vertebral bodies (C3-C5). The specimens were mounted in a custom made impact rig which fully constrained C6 but allowed C2 to translate in the axial direction of the segment. Images were acquired at 4kHz, both for the DIC (Photron Europe Ltd, UK) and motion capture cameras (Qualisys Oqus 400, Sweden). The in-plane and out of plane displacements of each of the VBs were plotted as a function of time and the similarity between the curves thus obtained was analysed using the SPM1D technique which allowed a comparison to be made in terms of t-statistics. No statistical differences were found between the two techniques in all axis of movement, however the out of plane movements were characterised by higher variance which is attributed to the uncertainty arising from the near parallel positioning of the cameras in the experimental set-up.


Orthopaedic Proceedings
Vol. 101-B, Issue SUPP_2 | Pages 5 - 5
1 Jan 2019
Silvestros P Boyd S Hernandez BA Gheduzzi S Gill HS Preatoni E Cazzola D
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Head collisions in sport can result in catastrophic cervical spine injuries. Musculo-skeletal (MSK) modelling can help analyse the relationship between players' motion, external loading and internal stresses that lead to injury. However, the literature lacks sport specific MSK models. In automotive research the intervertebral disc behaviour has been represented as viscoelastic elements (“bushing”), whose stiffness and damping parameters are often estimated under quasi-static conditions and may lack validity in dynamic impacts. The aim of this study was to develop a validated cervical spine model for axial impacts for future use in the analysis of head-first rugby collisions.

A drop test rig was used to replicate a sub-catastrophic axial head impact. A load of 80 N from 0.5 m was applied to the cranial aspect of a C2-C6 porcine spinal specimen mounted in the neutral position. The 3D motion of C3-C5 vertebras (4 kHz) and the cranial axial load (1 MHz) were measured via motion capture (Qualysis, Sweden) and a uniaxial load cell (RDP Electronics Ltd, UK). Specimen specific models were created in NMSBuilder and OpenSim after the vertebrae geometries were obtained from the segmentation of micro-CT images of the specimens. The compressive viscoelastic properties of four vertebral joints (C2-C3 through to C5-C6) were optimised via a Genetic Algorithm (MATLAB v2016b, The Mathworks Inc) to minimise tracking errors.

The optimisation converged to a solution of 140–49000 kN/m and 2000–8000 Ns/m for stiffness and damping respectively (RMSE=5.1 mm). Simulated joint displacements ranged between 0.09 – 1.75 mm compared to experimental 0.1 – 0.8 mm.

Optimal bushing parameters were higher than previously reported values measured through quasi-static testing. Higher stiffness and damping values could be explained by the higher-dynamics nature of the event analysed related to a different part of the non-linear intervertebral disc load-displacement curve.


Orthopaedic Proceedings
Vol. 100-B, Issue SUPP_3 | Pages 10 - 10
1 Apr 2018
Gosiewski J Gheduzzi S Gill R
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Introduction

The THR is the second most successful and cost-effective surgical procedure of all time. Data shows that hip cup failure is a significant problem. The aim of this study is to improve methods of cemented cup fixation through validation experiments and FEA.

Methods

Five Sawbones composite pelves with cemented UHMWPE cups were tested. Each pelvis was instrumented with triaxial strain gauges at four locations of predicted high strain. Each sample (n = 5) was bolted at the sacroiliac joint in a uniaxial testing machine. A load of 500 N was applied in the direction of the peak force during normal walking, for five repetitions. The directional surface strains were used to evaluate the equivalent strain. Specimen specific finite element models were developed based on CT scan data using ScanIP. Each mesh consisted of an average of 2.5 million linear tetrahedral elements and was solved in ANSYS.


Orthopaedic Proceedings
Vol. 100-B, Issue SUPP_4 | Pages 33 - 33
1 Apr 2018
Hernandez BA Blackburn J Cazzola D Holsgrove TP Gill H Gheduzzi S
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Cervical spine fractures are frequent in impact sports, such as rugby union. The consequences of these fractures can be devastating as they can lead to paraplegia, tetraplegia and death. Many studies have been conducted to understand the injury mechanisms but the relationship between player cervical spine posture and fracture pattern is still unclear. The aim of this study was to evaluate the influence of player cervical spine posture on fracture pattern due to an impact load. Nineteen porcine cervical spines (C2 to C6) were dissected, potted in PMMA bone cement and mounted in a custom made rig. They were impacted with a mean load of 6 kN. Eight specimens were tested in an axial position, five in flexion and six in lateral bending. All specimens were micro-CT imaged (Nikon XT225 ST Scanner, Nikon Metrology, UK) before and after the tests, and the images were used to assess the fracture patterns. The injuries were classified according to Allen-Ferguson classification system by three independent observers. The preliminary results showed that the main fracture modalities were consistent with those seen clinically. The main fractures for the axial orientation were observed in C5-C6 level with fractures on the articular process and endplates. These findings support the concept that the fracture patterns are related to the spine position and give an insight for improvements on sports rules in order to reduce the risk of injury.


Orthopaedic Proceedings
Vol. 100-B, Issue SUPP_6 | Pages 17 - 17
1 Apr 2018
Pascual SR Gheduzzi S Miles A Keogh P
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Back pain is a significant socio-economic problem affecting around 80% of the population at some point during their lives. Chronic back pain leads to millions of days of work absence per year, posing a burden to health services around the world. In order to assess surgical interventions, such as disc replacements and spinal instrumentations, to treat chronic back pain it is important to understand the biomechanics of the spine and the intervertebral disc (IVD). A wide range of testing protocols, machines and parameters are employed to characterise the IVD, making it difficult to compare data across laboratories.

The aim of this study was to compare the two most commonly used testing protocols in the literature: the stiffness and the flexibility protocols, and determine if they produce the same data when testing porcine specimens in six degrees of freedom under the same testing conditions. In theory, the stiffness and the flexibility protocols should produce equivalent data, however, no detailed comparison study is available in the literature for the IVD, which is a very complex composite structure.

Tests were performed using the unique six axis simulator at the University of Bath on twelve porcine lumbar functional spinal unit (FSU) specimens at 0.1 Hz under 400 N preload. The specimens were divided in two groups of six and each group was tested using one of the two testing protocols. To ensure the same conditions were used, tests were firstly carried out using the stiffness protocol, and the equivalent loading amplitudes were then applied using the flexibility protocol.

The results from the two protocols were analysed to produce load-displacement graphs and stiffness matrices. The load-displacement graphs of the translational axes show that the stiffness protocol produces less spread between specimens than the flexibility protocol. However, for the rotational axes there is a large variability between specimens in both protocols. Additionally, a comparison was made between the six main diagonal terms of the stiffness matrices using the Mann-Whitney test, since the data was not normally distributed. No statistically significant difference was found between the stiffness terms produced by each protocol. However, overall the stiffness protocol generally produced larger stiffnesses and less variation between specimens.

This study has shown that when testing porcine FSU specimens at 0.1 Hz and 400 N preload, there is no statistically significant difference between the main diagonal stiffness terms produced by the stiffness and the flexibility protocols. This is an important result, because it means that at this specific testing condition, using the same testing parameters and environment, both the stiffness and flexibility methods can be used to characterise the behaviour of the spine, and the results can be compared across the two protocols. Future work should investigate if the same findings occur at other testing conditions.


Orthopaedic Proceedings
Vol. 99-B, Issue SUPP_9 | Pages 87 - 87
1 May 2017
Mahmoodi P Gheduzzi S Gill H
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Background

Understanding vertebral fracture is important in order to reduce fracture risk. Previous studies have used FE to investigate mechanical behaviour, typically using a linear material response. This study aimed to establish a novel model that could represent the plastic behaviour leading to fracture.

Method

Porcine vertebrae were mCT scanned and they were loaded to failure in a material test machine (Instron 5965). The specimens were then rescanned. From the first scan, specimen specific FE models were created (ScanIP, Simpleware, UK). Mesh convergence was studied and tetrahedral elements with an approximate element size of 0.7 were used for computational simulations. The relationship between greyscale values (GS) and Young's modulus (E) was optimised to match the experimental load displacement data using Ansys. Further, a plastic material response was modelled.


Orthopaedic Proceedings
Vol. 99-B, Issue SUPP_8 | Pages 28 - 28
1 Apr 2017
Rastetter B Wright S Gheduzzi S Miles A Clift S
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Background

Finite element (FE) models are frequently used in biomechanics to predict the behaviour of new implant designs. To increase the stability after severe bone loss tibial components with long stems are used in revision total knee replacements (TKR). A clinically reported complication after revision surgery is the occurrence of pain in the stem-end region. The aim of this analysis was the development of a validated FE-model of a fully cemented implant and to evaluate the effect of different tibial stem orientations.

Methods

A scanned 4th generation synthetic left tibia (Sawbones) was used to develop the FE-model with a virtually implanted fully cemented tibial component. The 500 N load was applied with medial:lateral compartment distributions of 60:40 and 80:20. Different stem positons were simulated by modifying the resection surface angle posterior to the tibias shaft axis. The results were compared with an experimental study which used strain gauges on Sawbones tibias with an implanted tibial TKR component. The locations of the experimental strain gauges were modelled in the FE study.


Orthopaedic Proceedings
Vol. 99-B, Issue SUPP_5 | Pages 104 - 104
1 Mar 2017
Pascual SR Keogh P Miles A Gheduzzi S
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Chronic back pain is the leading cause of disability worldwide, affecting millions of people. The source of pain is usually the intervertebral disc (IVD), thus there has been a growing interest in developing new improved implants such as disc replacements to treat the condition. However, to ensure the artificial devices being designed replicate the intact disc, the biomechanical behaviour of the IVD must be well understood (Adams and Dolan, 2005). The two most widely used testing procedures in the spinal industry to characterise the behaviour of the disc are the flexibility and the stiffness protocols (Stokes et al, 2002 and Panjabi et al, 1976).

For elastic specimens, the results produced by the flexibility and the stiffness protocols should in theory be identical. However, this does not hold true for inelastic specimens, such as the IVD. For this reason, the custom developed Spine Simulator (Holsgrove et al, 2014) at the University of Bath has been used to compare, in six degrees of freedom, the extent of the difference produced by these two testing protocols.

A biomechanical model of the IVD was tested, which consisted of two cylindrical nylon blocks attached together with a layer of nitrile rubber, representing respectively the vertebral bodies (VB) and the IVD. Two steel pins were inserted into the VB, spanning the thickness of the disc, to ensure the stiffness raise either side of the neutral zone was replicated by the model. Tests were performed at a frequency of 0.1 Hz using triangular wave cycles. The specimen was firstly subjected to the stiffness protocol, characterised by displacements of ±0.5 mm in anterior-posterior and lateral shear, ±0.35 mm in axial compression and ±1.5 deg in all rotational axes. The resulting loads were applied to the specimen when subjected to the flexibility protocol. In addition, the effect of a preload was studied by testing specimens with an axial compressive load of 250 N.

The stiffness matrix was calculated for each test and the main diagonal terms produced for the two protocols were compared using the Mann-Whitney test. Overall, results showed that there was a significant difference in the stiffness terms produced by the two protocols when tests were performed with (p ≤ 0.016) and without (p = 0.004) a preload. The only exception was found in the flexion-extension axis when the test was performed with a preload (p = 0.337). Additionally, differences were also recorded when comparing the shape and linearity of the load-displacement hysteresis curve (Figure 1) and the area enclosed by the curve.

This preliminary study has provided important information regarding the differences in the data produced by the flexibility and the stiffness protocols, it is therefore impractical to compare results produced using these two methods. To ensure that in the future results can be compared across laboratories, there is a need for a standardised testing procedure in the spinal industry.

For figures/tables, please contact authors directly.


Orthopaedic Proceedings
Vol. 98-B, Issue SUPP_16 | Pages 26 - 26
1 Oct 2016
Czerbak K Clift S Gheduzzi S
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Osteoarthritis is one of the most common musculoskeletal diseases. It involves degeneration and loss of articular cartilage, leading to a painful bone on bone articulation during movement. Numerical FEA models exist to predict the mechanical behaviour of degenerated cartilage. One of the limitations of these models arises from the poor validation that can be attained with traditional experimental data. This typically relies on comparison with global mechanical quantities such as total tissue strain, which mask the individual contributions originating from the different layers. In order to improve on this, an experimental method was developed to visualise the through-thickness behaviour of articular cartilage.

Four experiments were performed on hemi-cylindrical cartilage plugs, harvested from a porcine femoral head, and immersed in a fluid solution. An Indian ink speckle pattern was applied to the flat surface of each hemi-cylinder. The specimens were equilibrated in 2.5M NaCl solution, transferred to a custom designed testing rig, and a reference image of the tissue cross-section was taken.

The solution concentration was then decreased to 0.15M and, predictably, the tissue thickness changed. Images of the tissue cross section were taken every 60s for the duration of the experiment (3600s). All images were analysed using a DIC algorithm (Ncorr open-source 2D digital image correlation matlab program), and documented the strain changes through the tissue thickness as a function of time. The measured total strain in the tissue was consistent with that reported by Lai et al. (1991). However the present technique allows to quantify the strain contribution from any of the tissue layers or sublayer. This poses a significant advantage over traditional methods as resulting information can further the understanding of the factors contributing to the mechanical behaviour of the tissue and provides an ideal platform for validating more and more refined models of tissue behaviour.


Orthopaedic Proceedings
Vol. 98-B, Issue SUPP_9 | Pages 147 - 147
1 May 2016
Rastetter B Wright S Gheduzzi S Miles A Clift S
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There has been an unprecedented increase in total knee replacement in recent years. The UK national joint registry recorded over 80,000 total knee replacements per year with a generally successful outcome. Improvements in modern knee replacement designs and surgical techniques has resulted in more and more young and active patients having knee replacements. Their more active lifestyles and increased life expectancy is also leading to a rise in revision knee surgery. The most common reason for revision knee replacement is for loosening as a result of wear and/or bone resorption. Revision knee tibial components typically use long stems to increase the stability in the presence of the proximal bone loss associated with implant removal and loosening. The stem design has been cited as a possible cause of the clinically reported pain at the stem end region. The aim of this study was to experimentally validate a finite element (FE) model and the analysis different load conditions and stem orientations in a stemmed tibial component. CT-scans of a composite tibia (Sawbones) were utilized to form a multi-body solid consisting of cortical bone and cancellous bone with an intramedullary canal. A fully cemented tibial component (Stryker) was virtually implanted in the composite tibia with the stem-end centred in the cancellous bone. The tibial compartment loads were distributed with a 60:40 (Medial: Lateral) and 80:20 ratio to simulate a normal and varus type knee. Several stem-end positions were developed with the modification of the tibias proximal resection angle. An experimental study using strain gauges applied to the same composite tibia was used to compare the results with the FE-model. The model was validated with the strain gauged experimental test specimens demonstrating a similar pattern and magnitude of predicted strains. The simulation of different stem-end orientations revealed an increase in strain to the posterior cortex below the stem-end with the stem in direct contact to the posterior cortical bone. A tibial stem fully surrounded by cancellous bone demonstrated a small increase to the proximal strains. The simulation of a varus aligned knee with a 80:20 (Medial: Lateral) load distribution shifted strain overall to the medial side and revealed a large increase of strain to the posterior-medial in the proximity of the stem-end. The intensification of the load on one side of the tibial plateau, associated with a varus aligned knee, developed the largest increase in strain beneath the stem-end region and is possibly a factor in the reported pain after surgery. The stem in close proximity to the posterior cortical bone is also a possible contributing factor to pain due to the increase of strain in the vicinity of the stem-end.


Orthopaedic Proceedings
Vol. 98-B, Issue SUPP_10 | Pages 133 - 133
1 May 2016
Wright S Gheduzzi S Miles A
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Introduction

Traditional applied loading of the knee joint in experimental testing of RTKR components is usually confined to replicating the tibiofemoral joint alone. The second joint in the knee, the patellofemoral joint, can experience forces of up to 9.7 times body weight during normal daily living activities (Schindler and Scott 2011). It follows that with such high forces being transferred, particularly in high flexion situations such as stair climbing, it may be important to also represent the patellofemoral joint in all knee component testing.

This research aimed to assess the inclusion of the patellofemoral joint during in vitro testing of RTKR components by comparing tibial strain distribution in two experimental rigs. The first rig included the traditional tibiofemoral joint loading design. The second rig incorporated a combination of both joints to more accurately replicate physiological loading. Five implanted tibia specimens were tested on both rigs following the application of strain gauge rosettes to provide cortical strain data through the bone as an indication of the load transfer pattern. This investigation aimed to highlight the importance of the applied loading technique for pre-clinical testing and research of knee replacement components to guide future design and improve patient outcomes.

Methods

Five composite tibias (4th Generation Sawbones) were prepared with strain gauge rosettes (HBM), correctly aligned and potted using guides for repeatability across specimens. The tibias were then implanted with Stryker Triathlon components according to surgical protocol.

The first experimental rig was developed to replicate traditional knee loading conditions through the tibiofemoral joint in isolation. The second experimental rig produced an innovative method of replicating a combination of the tibiofemoral and patellofemoral joint loading scenarios. Both rigs were used to assess the load distribution through the tibia using the same tibia specimens and test parameters for comparison integrity (Figure 1). The cortical strains were recorded under an equivalent 500 N cyclical load applied at 10° of flexion by a hydraulic test machine.


Orthopaedic Proceedings
Vol. 93-B, Issue SUPP_IV | Pages 485 - 485
1 Nov 2011
Pollintine P Harrison S Patel A Tilley D Miles A Gheduzzi S
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Introduction: Vertebroplasty is used to treat painful osteoporotic vertebral fractures, and involves transpedicular injection of bone cement into the fractured vertebral body. During injection, the fluid cement begins to “harden” to a solid, enabling it to support mechanical load. But the mechanical efficacy of vertebroplasty can be improved by using cements which disperse evenly throughout the vertebral body during injection (1). We hypothesise that a better cement dispersion is obtained with cements that have a slower viscosity increase during hardening. We test this using a numerical model.

Methods: A computer model mimicking the plate- and rod-like morphologies of cancellous bone was loaded into a commercial fluid dynamics package (CFX). During injection, viscosity increased linearly with time to simulate the hardening behaviour of the cement (2). The rate of viscosity increase was altered to mimic the hardening behaviour of 5 different cements, with the rates of increase chosen to encompass the hardening behaviour of commercial vertebroplasty cements (1). Simulations were run for 13 seconds, with cement injection at 1.5 mm/s. Cement dispersion was quantified by the proportion of marrow replaced by cement during injection. Injection pressure was also recorded.

Results: Injection pressure increased with time (p< 0.001), and maximum pressure correlated with the rate of viscosity increase (r2=0.7). The proportion of marrow replaced at the end of the experiment was inversely proportional to the rate of viscosity increase (r2=0.85). Cements with a rapidly increasing viscosity do not fully infiltrate regions of bone with plate-like morphologies, leading to a poorer cement dispersion.

Conclusion: Cements with slower hardening characteristics are dispersed more evenly throughout cancellous bone. Such cements may provide safer and more effective vertebroplasty procedures.

Conflicts of Interest: None

Source of Funding: Bupa Foundation