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.

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.

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.

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.

Introduction

Aseptic loosening of the acetabular cup in total hip replacement (THR) remains a major problem. Current diagnostic imaging techniques are ineffective at detecting early loosening, especially for the acetabular component. The aim of this preliminary study was to assess the viability of using a vibration analysis technique to accurately detect acetabular component loosening.

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.

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.

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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.

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.

Introduction

The hip joint is subjected to cyclic loading during activities of daily living and this can induce micromotion at the bone-implant interfaces of uncemented implants. Osseointegration, which is essential for long term implant survival, will occur when micromotion at these interfaces is below 40µm and may occur up to 150µm [1].

Studies investigating the micromotion of press-fit acetabular cups only report micromotions in one direction. Standard methods also maintain a static cup position throughout testing; usually at the angle of maximum resultant force during gait. Current methods therefore do not take into account the effect of motion of the hip on micromotion of the cup, nor do they investigate all six degree of freedom (DoF) of motion.

The aim of this study was to assess press-fit cup micromotion in six DoF under physiological loading when the cup is held statically and moved in flexion-extension.

The extension facet angle (EFA) of the medial compartment of the knee has been implicated as a potential mechanical cause for anteromedial knee osteoarthritis.

We developed a novel sagittal plane flexion osteotomy of the medial tibiofemoral compartment. We then performed a cadaveric study to study the effect of the osteotomy on the intra-articular knee pressures under axial load mimicking the stance phase of gait. A Tekscan K400 pressure sensor was inserted submeniscally into the joint and 700N applied using an Instron machine. A topographical map of the pressure areas was then assessed pre- and post-osteotomy for the 10 cadaveric knees specimens.

We found that the intra-articular pressures are greatest in the anteromedial compartment in the native knee and after the osteotomy the area of highest pressure moves posterolaterally spread over a greater surface area.

We conclude that a flexion osteotomy of the medial compartment reduces intra-articular knee pressures concentrated anteromedially in full extension and may be beneficial in patients with an elevated EFA with anteromedial symptoms.

Summary Statement

The tensile properties of a number of synthetic fibre constructs and porcine MCLs were experimentally determined and compared to allow the selection of an appropriate synthetic collateral ligament model for use in a kinematic knee simulator.

Summary

The required torque leading to an abrasion of the passive layer in the stem-head interface positively correlates to the assembly force. In order to limit the risk of fretting and corrosion a strong hammer blow seems to be necessary.

Treatment of syndesmotic injuries is a subject of ongoing controversy. Locking plates have been shown to provide both angular and axial stability and therefore could potentially control both shear forces and resist widening of the syndesmosis. The aim of this study is to determine whether a two-hole locking plate has biomechanical advantages over conventional screw stabilisation of the syndesmosis in this pattern of injury. Six pairs of fresh-frozen human cadaver lower legs were prepared to simulate an unstable Maisonneuve fracture. The limbs were then mounted on a servo-hydraulic testing rig and axially loaded to a peak load of 800N for 12000 cycles. Each limb was compared with its pair; one receiving stabilisation of the syndesmosis with two 4.5mm quadricortical cortical screws, the other a two-hole locking plate with 3.2mm locking screws (Smith and Nephew). Each limb was then externally rotated until failure occurred. Failure was defined as fracture of bone or metalwork, syndesmotic widening or axial migration >2mm. Both constructs effectively stabilised the syndesmosis during the cyclical loading within 1mm of movement. However the locking plate group demonstrated superior resistance to torque compared to quadricortical screw fixation (40.6Nm vs 21.2Nm respectively, p value <0.03).

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

Objectives: To compare the biomechanical properties of lag screw insertion in a laboratory model. Two blades, the Synthes Dynamic Helical Hip Screw (DHHS) and Proximal Femoral Nail Antirotation (PFNA), and two screws, the Synthes Dynamic Hip Screw (DHS) and Stryker Gamma 3 lag screw, were compared.

Setting: Orthopaedic biomechanics laboratory.

Design: Insertion testing was carried out in high and low density polyurethane foam mounted and attached to a Zwick Roell Amsler Hydrowin.

Outcome Measures: The axial load and torque during insertion of the implants was measured.

Results: The force required to insert the DHHS and PFNA blades was greater than the DHS and Gamma 3 screws into both low and high density foam. The force required to insert the DHHS and PFNA blades into high density foam was greater than low density foam. The torque required to insert the DHHS and PFNA blades into high density foam was less than that to insert the DHS and Gamma 3 screws. The torque required to insert the DHS and Gamma 3 screws into low density foam was less than the DHHS and PFNA blades. The torque during insertion of the DHHS and PFNA blades seemed to be independent of foam density.

Conclusions: The insertional properties of blades are significantly different to screws and this may have clinical importance.

Introduction: Vertebroplasty is increasingly used in the treatment of painful osteoporotic vertebral fractures, and involves transpedicular injection of bone cement into the fractured vertebral body. Effective infiltration of the vertebral body cancellous bone by the cement is determined by the cement viscosity, and by the permeability of the bone. However, it is unclear how permeability is influenced by regional variations in porosity and architecture of bone within the vertebral body. The aim of the present study was to investigate how permeability is influenced by porosity and architecture of cancellous bone mimics.

Methods: Cylindrical polyamide mimics of two types of cancellous bone structures were fabricated using selective laser sintering (SLS) techniques. Structure A had the rod-like vertical and horizontal trabeculae typical of the anterior vertebral body, while structure B had oblique trabeculae typical of the posterior-lateral vertebral body. Structure B had fewer trabeculae than A. Porosities of 80 and 90% were represented for both structures. Golden syrup, which has a viscosity similar to bone cement1, was injected into the mimics at a constant speed using a ram driven by a materials testing machine. Pressure drop measurements across the mimic, made using a differential pressure transducer, were obtained at five different injection speeds. Permeability of each mimic was calculated from these measurements2. Two more repeat permeability measurements were performed on each mimic.

Results: Repeat measurements were always within 12% of the mean value. For structure A the mean permeabilities were 1.26×10-7 and 1.82×10-7m2 for the 80 and the 90% porosity mimics respectively. The corresponding mean permeabilities for structure B were 1.92×10-7 and 2.86×10-7m2.

Discussion: These preliminary results indicate that higher permeabilities occur in structures with higher porosities, and with structures containing fewer trabeculae that are arranged obliquely. Since permeability is a determinant of cement infiltration, taking into account patient-specific bone architecture parameters may improve the safety and clinical outcome of vertebroplasty. Future experiments will clarify in more detail the architectural parameters that have greatest effect on permeability.

The growing interest in the development of spinal implants has led to an increasing need for biomechanical studies. Porcine spines are commonly used in such studies. Quantitative data of the normal porcine tho-racolumbar spine is lacking, yet these data are crucial to discussion of such studies. In this study we aim to provide such a database to highlight the differences between the porcine and human specimen with a view to help plan future studies contemplating their use.

6 adult (18-24 month old, 60-80 kilograms) male porcine spines were dissected of soft tissue. The lowest thoracic and all the lumbar vertebrae were studied (n=42). 15 anatomical parameters from each vertebra were measured by 2 independent observers using digital calipers (Draper PVC150D, accuracy ± 0.03mm). The mean, SD and SEM were calculated using Micro-soft Excel. Results were compared with available data on human vertebra (Zindrick et al 1987;Panjabi et al 1991,1992; Kumar et al 2000).

The inter class correlation coefficient for the observers was 0.997. The intra-observer agreement was statistically robust (0.994). The vertebral body height of the porcine vertebra was larger while both the upper and lower endplate depth and width were smaller than the human specimens. The pedicle width and depth was greater than the human specimen. The spinal canal length and depth of the porcine spine were smaller than humans indicating a narrow spinal canal. The spinous process length showed an increase from T16 to L1. This was in contrast to human spinous process. The results for the measured parameters and their comparison to human specimen will be presented.

Results from our study provides a database of anatomical measurements for the porcine vertebrae and highlights the differences with the human specimen. The data would help design future studies contemplating the use of pig spines. Biomechanical studies involving interbody cages, disc replacements and pedicle screw systems should take into account the differences and match implant size accordingly. It also provides valuable information for geometric and Finite Element Modelling of the porcine spine. Further, the results are useful in extrapolation of data from experiments which have used the porcine model.

The visco-elastic behaviour of cement, is a key feature of cement-implant performance in total hip arthroplasty.

The aim of this study was to describe the creep behaviour of the leading plain bone cements under standardised physiological in-vitro conditions.

Methods: Cements were mixed under vacuum conditions as per manufacturers instructions. Moulds were used to to produce beams of standard dimensions. These were stored in saline at 37oC for 21 days to ensure thorough polymerisation. Under the same conditions, the beams were tested for 72 hours in a 12-station quasi-static creep rig, using a four-point bending configuration. The rig applied a constant stress of 8MPa to each beam and the deflection was recorded at 8-minute intervals by a data-logging device. The porosity was measured in the mid-cross section of each beam sample using a digital image technique.

The cements tested were Palacos R, CMW1 and Smartset GHV and Surgical Simplex P.

All data were analysed using ANOVA with Bonfer-roni post-hoc test (SPSS).

Results: Palacos R exhibited the highest mean deflection at 72 hours (0.86+/- 0.21mm) followed by Surgical Simplex P (0.85 +/- 0.18mm), CMW1 (0.72 +/- 0.09mm) and Smartset GHV (0.60 +/- 0.16mm). The difference between the two DePuy cements and Palacos R (p=0.03) and Surgical Simplex P (p=0.04) were statistically sig-nificant. None of the beams failed during the test. The creep behaviour correlated with the cross-sectional porosity measurements.

Conclusions: This study has shown that there are sig-nificant differences in the creep bahaviour of the leading medium and high viscosity bone cements. In particular Palacos R and Surgical Simplex P demonstrate ‘High’ creep and the DePuy cements ‘Low’ creep. Creep appears sensitive to subtle changes in the composition of the material. This may be reflected in the clinical behaviour of different bone cements and stresses the importance of the time-dependent properties of PMMA.

Introduction: Joint replacement procedures such as revision impaction grafting and spinal fusion interbody operations are stretching allograft bone stocks to their limits. The need for synthetic alternatives that offer a structural and biological matrix for graft incorporation are paramount for future bone regeneration procedures. Synthetic bone graft alternatives that offer biocompatibility to the host bone (i.e. a biological response) such as hydroxyapatite/tricalcium phosphate (HA/TCP), in addition to possessing an interconnected porosity network have been shown to have a strong influence on the osteoinductive potential of these materials. The current method allows the production of calcium phosphate ceramic components (CPC) that possess an interconnected open porous network in the required size range for osteoid growth and revascularisation.

Materials and Methods: The method can be described as the reticulated foam technique, whereby two grades of calcium phosphate powder are blended together to form a HA/TCP ceramic slip. The slip is then ball milled for 24hrs with zirconia milling media. This slip is used to impregnate polyurethane (PU) foam via a mechanical plunging procedure. The impregnated foam is then held above the slip bath in order for the slip to flow and coat the struts of the foam. The impregnated foam is then dried on tissue paper and treated with high velocity compressed air to avoid the formation of any closed cells. Samples are dried at 120°C for 15hrs. The PU foams are graded as 30 and 45ppi (pores per inch). The slip viscosity ranges from 6000 – 8000 cps (measured with a Brookfield Viscometer, spindle no. 5 and at 10rpm). Samples are sintered slowly until 600°C to ensure PU burnout is complete. Sintering continues up to 1280°C to ensure densification. Image analysis was performed using optical microscopy, digital photography and SEM analysis. Mechanical testing was performed by 3 point bending using an 1122 Instron.

Results: Macroporosity in the samples varied from 40 – 70%. Typical pore sizes far exceeded 300μm (the pore size acknowledged as that needed for osteogenesis). Approx. 79% of all pores were between 150 – 450μm in area equivalent diameter. Typical strut thicknesses ranging from 100 – 500μm were also reported, as was a strut thickness-pore size-mechanical strength relationship. One hundred and twenty samples possessed a breaking stress with a 95% confidence level of 0.30MPa±0.01MPa. The low strengths reported are due to the formation of blow-out holes at triple point junctions on the interconnected struts.

Conclusions: Major requirements for replacement bone materials have been met including a wide range of interconnected porosity from 50 – 1000μm. Bioactivity combined with an excellent porosity size range suggests excellent possibility of osteogenesis. In addition, this fabrication procedure offers consistency and reliability. Future work will focus on improving the strength of these open porous calcium phosphate ceramics.

Introduction: Immediate postoperative stability of cementless hip stems is one of the key factors for the long-term success of total hip replacement. The ability to discriminate between stable and unstable stems in the laboratory constitutes a desirable tool for the industry, as it would allow the identification of unsuitable stem designs prior to clinical trials. The use of composite femora for stability investigations is wide spread [1,2] even though their use in this application is yet to be validated. This study is aimed at establishing whether Sawbones composite femora are suitable for the assessment of migration and micromotion of a cementless hip stem. The stability of two SL Plus stems (Precision Implants, CH) implanted into Sawbone was compared to that of two SL Plus stems implanted into cadaveric femora. Ethical approval was obtained for the harvest and use of cadaveric material.

Methods: Stability was assessed in terms of micromotion and migration. Micromotion was defined as the recoverable movement of the implant relative to the bone under cyclic loading. Migration was defined as the non-recoverable movement of the implant with respect to the surrounding bone. Movement of the implant with respect to the surrounding bone was monitored at two locations on the lateral side of the stem by means of two custom made transducers based on the concept described by Berzins et al [3]. Each femur was tested in two different sinusoidal loading configurations: single leg stance (SLS-11° of adduction and 7° of flexion) [4] loaded up to 400N and stair climbing (SC-11° of adduction and 32° of flexion) loaded up to 300N. The effect of the abductor muscles was included in the model [5]. Each test consisted of 200 loading cycles applied at 50 Hz. The captured data was post-processed by a MATLAB routine and converted into translations and rotations of the stem with respect to the bone.

Results: The proximal part of the implant was subject to the highest amplitudes of micromotion in both loading configurations independent of the host. During SLS the largest micromotion was measured in the direction of the axis of the femur, this amplitude was in the order of 20 μm for the stems implanted in sawbones and varied between 13 and 39 μm for the stems implanted in cadaveric femora. The migration of the implants was minimal both in SLS and SC for both hosts with values measured in the sawbones model nearly on order of magnitude smaller than the cadaveric. In the case of SLS the prevalent movement consisted of a translation along the axis of the bone, while during SC the rotations became prevalent.

Discussion: This study has demonstrated that Sawbones provide an effective model to establish micromotion with oscillation patterns and orders of magnitiude similar to cadaveric bone. However the migration is much more dependent on the quality of fit and the internal geometry of the femur and therefore more caution should be placed on interpreting migration data from Sawbones models.

Introduction: Calcium phosphate based ceramics with a porous configuration are attraction for use as synthetic bone grafts as the porous network allows tissue ingrowth, which further enhances the implant-tissue attachment. The degree of interconnectivity and the nominal pore size are the critical factors that determine the success of the implants. It is generally accepted that a minimum pore size of 100 μm is necessary for the porous implant materials to function well and a pore size greater than 200 μm is an essential requirement for osteo-conduction. However, research has suggested that the degree of interconnectivity is more critical than the pore size. In this study, porous Hydroxyapatite/Tricalcium phosphate (HA/TCP) bioceramics with interconnected porosity and controlled pore sizes were fabricated by a novel technique involving vacuum impregnation of reticulated polymeric foams with ceramic slip. HA/TCP samples with a range of pore sizes and functionally gradient materials (FGM) with porosity gradients were made.

Materials and Methods: Two grades of calcium phosphate powder, TCP 118 and TCP 130, were used. Varying the blend ratios could change the ratios of HA and TCP in the sintered samples. The foams used comprised polyurethane (PU) which had one of three different porosities 20, 30 and 45 pores per inch (ppi). In order to make a FGM with porosity gradients mimicking the bimodal structure of cortical and cancellous bone, two different foams were either joined together by sewing or pressfitting together. The foams were substantially impregnated with slip by vacuum impregnation. The impregnated foams were removed from the vacuum chamber and dried on tissue for at least 24 hours then sintered at temperatures of up to 1280°C.

Results and Discussion: Using a slip with the appropriate viscosity, porous HA/TCP bioceramics having interconnecting pores and a range of pore sizes can be produced successfully. By joining different ppi foams together, it is possible to develop functional gradient materials in which the porosity varies through the thickness of the samples. No weakness could be seen at the interface between the two different structures. This demonstrated that porous HA/TCP with two or more different levels of porosity could be produced in a single block. Image analysis shows the porosity measured for the three different foams was similar. The area equivalent diameters of the pore structure are 197–254 μm with 20ppi foam, 143–183 μm with 30ppi foam and 105–127 μm with 45ppi foam. The compressive strengths of the HA/TCP samples are in the range of 30–170 MPa and the apparent densities were 2.34–2.76 g/cm3. The technique developed for fabricating porous bioceramics can be extended to produce a range of bone substitute materials with properties tailored to specific clinical applications.

Background and Purpose: There is a high incidence of arthritis in the hand, but joint replacement technology in the wrist and other small joints is still in its infancy compared with the larger joints. The wrist is the most complex small joint and so there is a need for fundamental research into the way in which it works. At present there is no generally agreed upon satisfactory explanation for the complex movement patterns of the carpal bones. The purpose of the work was to test a new hypothesis on wrist kinematics. The basis of the hypothesis was that the bones of the wrist move in such a manner as to maximise total contact area in the joint, thereby minimising contact stress. Such a strategy would minimise the bone mass requirements, thereby minimising the biological “cost” of creating and maintaining the joint. This agrees with the minimum energy principle, which governs many natural processes.

Methods: A computer model was created to test the hypothesis. A cadaveric wrist was dissected and 3D faceted models of the carpal bones were created using laser digitisation. The model contained a program to evaluate the closeness of packing of the carpal bones and an optimisation algorithm [1] to maximise this quantity by adjusting the positions of the bones. The evaluation program computed the contact area and level of intersection between nine pairs of interacting bones. Rotation in the radial-ulnar deviation plane was applied in 1.0° increments to four rigidly connected bones defining the overall posture of the wrist, and an optimisation algorithm was used to maximise the contact area by adjusting the positions and orientations of the remaining bones.

Results: The results of the work are encouraging because certain known characteristics of carpal behaviour were clearly predicted by the model. The results for the scaphoid in particular were similar to the characteristic movements of this bone in both radial and ulnar deviation. During 20° of unlar deviation, the bone demonstrated 14.3° of extension, which is near to the 20.4° reported by an experimental study [2]. In 10° of radial deviation, the bone underwent 6.4° of flexion, which again is close to the 8.1° experimental result.

Conclusion: Although the computer model predicted certain aspects of carpal behaviour, the initial hypothesis was not conclusively proved. This is due in part to the computational complexity of the task. Despite some simplifying assumptions, there were still a large number of degrees of freedom, and it is almost certain that the optimisation process was afflicted with local minima problems. If the technical hurdles can be overcome and the hypothesis is proved correct, then we will gain a new explanation of the laws governing the kinematics of the wrist joint, which are not fully understood at present. This will provide invaluable information for surgical applications, where a thorough understanding of normal kinematics is essential for the treatment of joint injury and instability.

The static properties of bone cements have been widely reported in the literature (Lewis, 1997, Khun, 2000, Armstrong 2002). Commercial bone cements are expected to perform above the minimum values in static tests specified by ISO 5833: 2002. It has been suggested that the viscoelastic properties of bone cement, such as creep and stress relaxation, might bear more relevance to the in-vivo behaviour of the cement-implant construct (Lee 2002). This study aimed to compare numerous properties of Simplex P, Simplex Antibiotic and Simplex Tobramycin and identify those properties most sensitive to subtle changes in cement composition. The three cements were chosen on the basis that they are characterised by the same liquid and powder compositions, the only difference being represented by the type and amount of added antibiotics. In Simplex Antibiotic the additives are 0.5g Erythromycin and 3 million I.U. Colistin, while in Antibiotic Simplex with Tobramycin the only additive is 0.5g of Tobramycin. The static properties of the cements were assessed following protocols described in ISO 5833: 2002, while the viscoelastic properties of the cement were measured with in-house developed apparatus in quasi-static conditions. Creep and stress relaxation tests were performed in four point bending configuration. Porosity was measured on the mid cross section of the creep samples using a digital image technique. All cements exhibited properties compatible with the ISO standard, but in plain Simplex the ISO minimum for bending and compressive strength was within the variation of the batches tested. Bending strength measurements were the least sensitive to differences in the cements. Plain Simplex displayed lower bending and compressive strength but higher bending modulus than the antibiotic laden options. The bending modulus could only discriminate between Simplex P and Simplex Antibiotic (p=0.02). Differences in the compressive strength of the three cements were significant, with the plain option being the weakest. Stress relaxation only discriminated between plain and Tobramycin loaded cement (p=0.028), while creep was more sensitive to differences and allowed distinction between plain and antibiotic loaded bone cements. The creep behaviour correlated with the cross sectional porosity measurements. This study demonstrated that the static tests specified by the current international standard are not as sensitive to subtle changes in the composition of the material as the time temperature dependent parameters characteristic of creep and stress relaxation. The authors advocate the evaluation of time and temperature dependent characteristics as a complement to the current standard.

Aseptic loosening remains a long-term problem in total hip replacement. This phenomenon is prevalent even if modern cementing techniques seem to have reduced its incidence. Osteolysis has been deemed as a disease of access to fixation interfaces (1), either the stem- or bone-cement interface in hip replacement. This can be attributed in part to the quality of the cement in the proximity of the stem. It has been noted that due to thermal effects, polymerisation of bone cement starts at the bone-cement interface and gradually moves inwards towards the stem.

Femoral component heating was first proposed as a method to reduce the curing time of bone cement (2). This practice was later found to reduce the porosity at the stem-cement interface (3) and also to improve the interface shear strength (4). This study aimed to investigate the effect of femoral stem heating on two bone cements (Simplex P (Stryker) and Palacos R (Biomet Merck)) over a range of mantle thicknessess.

The model femora used for this study were maintained at a constant temperature of 37C while the stem temperature varied between 21, 37 and 44C. The femoral moulds were formed from dental plaster with a similar thermal conductivity to bone. Mould sizes were created to generate cement mantles of 2, 5 and 7.5mm thickness.

In the 2mm Simplex P cement mantles there was very little porosity evident. It was concentrated in the proximity of the stem when the component was kept at 21C and disappeared as the stem was heated to higher temperatures. Minimal porosity could be identified in the thicker mantles with no apparent differences between temperatures. There were no temperature trends evident from within this cement group. Palacos R cement has been reported to have a higher porosity than Simplex in a number of studies (5, 6). With the 2mm Pala-cos mantles, the increased stem temperatures reduced the porosity at the stem-cement interface. There was however no obvious difference between the 37 and 44C temperatures, where porosity seemed to be evident in the midsection of the mantle. This trend was also identified in the thicker cement mantles. The porosity did not extend out to the cement-bone interface under any conditions.

This study analyses the changes in porosity across the mantle of the cement as the temperature of the stem component is increased. The initial results confirm that the porosity at the stem cement mantle is decreased but indicate that the porosity within the body of the cement is increased as the temperature of the stem is increased.

Aim: To compare the biomechanical properties of paired flexible steel and titanium nails in simulated transverse fractures of synthetic composite bones.

Methods: Steel and titanium nails (3mm diameter) were individually used in pairs of divergent configuration to study torsion, cantilever bending (anteroposterior and lateral), and axial loading properties of adolescent synthetic composite tibiae model (10mm diameter). Properties of the intact bone, simulated fresh fracture with nails and simulated healing fracture with nails were studied. Instron 4303 universal testing machine was used to study axial loading. Applying fibreglass layers around the fracture with epoxy resin simulated fracture healing with callus formation.

Results: Steel and titanium nails maintained good alignment of fracture fragments. Both the nails demonstrated very poor stability of fresh fractures in torsion loading. Steel nail/bone construct was 57% stronger than Titanium nail/bone construct under similar testing conditions during fracture healing (p< 0.05) but still < 50% stiffness of intact bone. In bending tests, both types of nails showed < 10% of the stiffness of intact bone in fresh fractures (p< 0.05). Mediolateral stiffness was better than anteroposterior stiffness. In fracture healing, the bending stiffness of both types of nail/bone constructs was > 50% that of intact bone. Axial stiffness of both nails was more than bending or torsion stiffness implying that fracture fragments play a significant role in the stability of the fracture.

Although both types of nail/bone constructs demonstrated similar stiffness results in fresh and healing fractures, steel nails performance was statistically better than Titanium nails in all loading tests (p< 0.05).

Conclusion: Fractures fixed with either type of flexible nails should be supplemented with splints or plaster for a short duration until callus formation. Flexible nails should be used with caution in comminuted fractures, over weight patients since they may not provide adequate stability or allow early mobilization. Additional research with cadaver bones may provide further insight into the performance of the flexible nails.

Introduction and Aims: Compression hip screws are considered to be the gold standard for treatment of trochanteric proximal femoral fractures. Two implant designs exist; the ‘keyed’ and ‘non-keyed’ barrel profiles. Many biomechanical studies have been published on the performance of sliding hip screws, but most have used only static testing, and none to our knowledge have sought to compare the two-barrel profiles. This study aimed to compare the sliding characteristics of keyed and non-keyed systems in both static and dynamic loading conditions.

Method: Tests were performed on the implants using a multi axis servo-hydraulic testing machine. The machine possessed both linear and torsional actuators, such that hip flexion/extension could be simulated during testing. Load to initiate sliding in both implants was measured in a variety of testing conditions; screw engagement in barrel (20–38mm), angle of hip flexion (0–40 degrees), perpendicular loading force (50–190N), and cycle frequency (0–1 Hz).

Results: Results showed a tendency towards greater sliding in the non-keyed system, although these were significant only for the screw engagement testing (p< 0.001). However, load to initiate sliding in both implants was significantly higher in dynamic as compared to static testing (p< 0.001), and increased as torsional frequency increased. The non-keyed system did not demonstrate any tendency for screw rotation within the barrel during dynamic testing.

Conclusion: The non-keyed compression hip screw system does show a trend towards improved sliding characteristics, and does not display the tendency for screw rotation within the barrel under loading, often quoted as a misgiving of this implant. Also, since forces to initiate sliding are significantly higher when these implants are loaded dynamically (which mimics more closely the in vivo performance), future biomechanical studies should include dynamic testing for any hip fracture implant.

Introduction: It is well known that the integrity of the bone cement interface is crucial for the long-term survival of a primary total hip arthroplasty (THR). Revision THR with impaction bone grafting has recently offered a solution to gross bone loss due to osteolysis. As graft becomes incorporated, clearly the bone graft/ cement interface is as crucial as the equivalent interface in primary THR. Our aim was to examine factors that inßuence this interface. Method: The study was designed to mimic clinical practice. Fresh femoral heads were harvested from primary THR. These were morcelised into large and small particles. These were characterised. The bone was impacted into a purpose built jig with measured force. Cement was pressurised onto the dried surface of the impacted bone after measured mixing times. Cement pressurisation was measured. The cement/graft specimen was extracted and transected with a band saw. Cement penetration was measured with digital image analysis. Results: Large fragment size was 29 mm², and small was 7.1mm². Light impaction was 2.2 Atm. Medium and heavy were 2.6 Atm. and 3.2 Atm. respectively. Cement penetration was inversely proportional to impaction force. Cement mixing time also signiþcantly affected cement penetration. Particle size had no effect. Conclusion: Allograft should be adequately but not excessively impacted, to allow good cement incursion. Cement should be introduced and pressurised perhaps as early as two minutes. Fragment size does not affect cement penetration.

Aims: Optimisation of femoral stem load transfer potentially encourages new bone growth. The effect of increasing the taper angle of a highly polished double tapered stem on stability and hoop strain is investigated.

Methods: An in-vitro model femur was instrumented at 3 levels with 10 strain gauges, proximally (channel 1–4), mid taper level (channel 5–8) and distally (channel 9–10). Under controlled conditions surgical Simplex P cement (Stryker Howmedica Osteonics) was prepared and introduced into the canal of the model femur. The Exeter (Stryker Howmedica Osteonics) stem with centraliser was inserted using a standardised technique. The cementation process was repeated 5 times for each stem. The mantle and stem were subjected to cyclic loading at 1 Hz to 0.5kN using an Instron 8511 servohydraulic materials testing machine. The subsidence of the stem and hoop strains generated were recorded. Each experiment was repeated 6 times.

Results: The average subsidence of all the stems was 0.2mm with a standard deviation varying between 0.1 and 0.2. All the stems showed similar patterns of loading, with no significant difference.

Conclusions: The results suggest that within a purely cemented environment the taper angle of the stem used is irrelevant with regard to the hoop strain and the stability of the construct. The authors therefore suggest that size of stem does not matter.

The wrist is arguably the most complex joint in the body and is essential for optimal hand function. The joint may be represented as two roughly orthogonal hinge axes, providing flexion-extension and radial-ulnar deviation. The location and orientation of these axes with respect to the underlying anatomy is essential for the design of successful joint prostheses. A population study was performed in order to obtain the parameters of this two-hinge joint.

Data for 108 normal right wrists was gathered using a Fastrak electrogoniometer with sensors fixed to the distal medial radial styloid and the distal third metacarpal head. Data was recorded as a series of three-dimensional coordinates covering the entire locus of movement.

The two-hinge geometry of the joint was represented mathematically with nine parameters describing the configuration of the axes and two angles controlling rotation about these axes. The configuration giving the closest kinematic match to the experimental data was determined using two nested optimisation processes. During the inner optimisation process, the third metacarpal head was brought as close as possible to each of the experimental points in turn by adjusting the two positioning angles. The sum of distances from each experimental point to the point of closest approach gave the “cost” of the current configuration. The outer optimisation process repeated the inner process iteratively, minimising the cost by adjusting the nine configuration parameters.

The double optimisation method was found to offer an innovative solution to the problem of analysing kinematic data from a population study. The mean joint configuration showed the axis of radial-ulnar deviation to be 1.9 mm (sd = 12.5 mm), distal to the flexion-extension axis, with axes almost orthogonal to one another. This data together with the radii of the rotations is invaluable in determining the optimal articulation geometries for wrist joint replacement prostheses.

It is well known that the integrity of the bone cement interface is crucial for the long-term survival of a primary total hip arthroplasty (THR). Revision THR with impaction bone grafting has recently offered a solution to gross bone loss due to osteolysis. As graft becomes incorporated, clearly the bone graft/cement interface is as crucial as the equivalent interface in primary THR.

The aim of this study was to examine factors that influence this interface. The effects of bone particle size, cement mixing time, and impaction force were examined.

The study was designed to mimic clinical practice. Fresh femoral heads were harvested from primary THR. These were morcelised into large and small particles. The bone was impacted into a purpose built jig with measured force. Cement was pressurised onto the dried surface of the impacted bone after measured mixing times. Cement pressurisation was measured. The cement/graft specimen was extracted and transected with a band saw. Cement penetration was measured with digital image analysis.

Large fragment size was 29 mm², and small was 7. 1mm². Light impaction was 2. 2 Atm. Medium and heavy were 2. 6 Atm. and 3. 2 Atm. respectively. Cement penetration was inversely proportional to impaction force. Cement mixing time also significantly affected cement penetration. Particle size had no effect.

Allograft should be adequately but not excessively impacted, to allow good cement incursion. Cement should be introduced and pressurised perhaps as early as two minutes. Fragment size does not affect cement penetration.

Aseptic loosening of the acetabular component is the main reason for revision hip arthroplasty surgery with loosening rates reported at 25% at 12–15years. The optimum method of acetabular preparation and cementation technique has not been fully evaluated. Clinical follow-up studies suggest multiple keyholes improve survival rates.

Keyholes increase penetration of cement and torsional resistance of the bone-cement interface. Some studies support the traditional three 1/2 inch keyholes other studies have shown multiple smaller holes improve stability. The optimum size and number of holes to provide the strongest fixation has yet to be determined.

Using an established 54mm diameter acetabular model, mahogany, three sizes of keyhole were tested-3x10mm holes, 12x5mm, and 48x2.5mm- the surface area of the defects created were the same in each group. The model acetabulum was filled with cement and a metal central bar inserted through which torque could be applied using an Instron machine. Six specimens from each group were tested. Three 10mm holes produced a significantly stronger resistance to torque when compared to 2.5mm (p≤0.017) and 5mm holes (p≤0.001). There was no significant difference between 2.5mm and 5mm holes (p≤0.139). Each addition of a further 10mm hole significantly increased the torque strength until the model was destroyed at six holes.

In laboratory testing larger key holes provide a stronger cement-acetabular interface.

We studied the effects of nine techniques of bone surface preparation on cement penetration and shear strength at the cement-bone interface in a standard model of bovine cancellous bone. In unprepared bone the mean penetration was 0.2 mm and the mean shear strength of the interface was 1.9 MPa, less than that of the underlying bone. Brushing with surface irrigation gave mean penetrations of 0.6 to 1.4 mm and mean shear strengths of 1.5 to 9.9 MPa. In 50% of specimens the interface was weaker than the underlying bone. The use of pressurised lavage resulted in mean penetrations of 4.8 to 7.9 mm and mean shear strengths of 26.5 to 36.1 MPa, which were greater than those of the cancellous bone in all specimens. Pressurised lavage was equally effective alone or in combination with brushing, and its efficacy was not altered by using pulsed or continuous jets, or by changing the temperature of the solution from 21 degrees C to 37 degrees C.

We report the results of a 4- to 17-year clinical and radiological follow-up of 264 Charnley first-generation stems in comparison with those of 402 second- and subsequent-generation stems. The incidence of fracture was 4.1% in first-generation stems and 0.5% in second- and subsequent-generation stems. The incidence of stem loosening requiring or likely to require revision was 3.1% in first-generation and 11.4% in second-generation stems. We believe that the increased loosening rate in second- and subsequent-generation stems is due to their larger cross-sectional area, which produces an increase in flexural stiffness.

In comparison with monofilament wire, multifilament cable was found to be a more suitable material for fixation of the greater trochanter. It is versatile, easy to work with and has superior mechanical properties. The "trochanter cable-grip system" was developed to exploit the use of multifilament cable as a means of reattaching the greater trochanter and experimental studies have yielded excellent results. This system was subjected to clinical trials for over four years in 321 hips and, at its present state of development, the incidence of detachment has been reduced to 1.5 per cent and that of cable breakage to 3.1 per cent.