Properties of PMMA end cap holders affect FE stiffness predictions of vertebral specimens.

Bone cement is often used, in experimental biomechanics, as a potting agent for vertebral bodies (VB). As a consequence, it is usually included in finite element (FE) models to improve accuracy in boundary condition settings. However, bone cement material properties are typically assigned to these models based on literature data obtained from specimens created under conditions which often differ from those employed for cement end caps. These discrepancies can result in solids with different material properties from those reported. Therefore, this study aimed to analyse the effect of assigning different mechanical properties to bone cement in FE vertebral models. A porcine C2 vertebral body was potted in bone cement end caps, µCT scanned, and tested in compression. DIC was performed on the anterior surface of the specimen to monitor the displacement. Specimen stiffness was calculated from the load-displacement output of the materials testing machine and from the machine load output and average displacement measured by DIC. Fifteen bone cement cylinders with dimensions similar to the cement end caps were produced and subjected to the same compression protocol as the vertebral specimen and average stiffness and Young moduli were estimated. Two geometrically identical vertebral body FE models were created from the µCT images, the only difference residing in the values assigned to bone cement material properties: in one model these were obtained from the literature and in the other from the cylindrical cement samples previously tested. The average Youngs modulus of the bone cement cylindrical specimens was 1177 ± 3 MPa, considerably lower than the values reported in the literature. With this value, the FE model predicted a vertebral specimen stiffness 3% lower than that measured experimentally, while when using the value most commonly reported in similar studies, specimen stiffness was overestimated by 150%.

Material property calibration is more important than element size and number of different materials on the finite element modelling of vertebral bodies: A Taguchi study.

Finite element (FE) modelling of a vertebral body (VB) is considered challenging due to the many parameters involved such as element size and type, and material properties. Previous studies have reported how these parameters affect the mechanical behaviour of a VB model; however, most studies just compared results without any specific statistical tool to quantify their influence. The Taguchi Method (TM) has been successfully used in manufacturing and biomechanics to evaluate process parameters and to determine optimum set-up conditions. This study aimed to evaluate the influence of the main finite element modelling parameters on the mechanical behaviour of a VB model using the Taguchi Method. A FE model was developed based on a C2 juvenile porcine vertebral body and three of the most commonly used modelling parameters were evaluated using TM in terms of the change in the predicted stiffness in comparison to experimental values: element size, number of different material properties for VB (based on grey-scale bins) and calibration factor for grey-scale to density to Young's Modulus equation. The influence of the combined factors was also assessed. The Taguchi analysis showed that the three factors are independent. The calibration factor is the main contributor, accounting for 97% of the predicted stiffness, with the value of 0.03 most closely aligning the numerical and experimental results. Element size accounted for 2% of the predicted stiffness, with 0.75 mm being the optimal, while the number of grey-scale bins influenced the results by less than 1%. Our findings indicate that the calibration factor is the main modelling parameter, with the element size and number of bins accounting for less than 3% of the predicted stiffness. Therefore, calibration of material properties should be done based on a large number of samples to ensure reliable results.

A Novel Modelling Methodology Which Predicts the Structural Behaviour of Vertebral Bodies under Axial Impact Loading: A Finite Element and DIC Study.

Cervical spine injuries (CSIs) arising from collisions are uncommon in contact sports, such as rugby union, but their consequences can be devastating. Several FE modelling approaches are available in the literature, but a fully calibrated and validated FE modelling framework for cervical spines under compressive dynamic-impact loading is still lacking and material properties are not adequately calibrated for such events. This study aimed to develop and validate a methodology for specimen-specific FE modelling of vertebral bodies under impact loading. Thirty-five (n = 35) individual vertebral bodies (VBs) were dissected from porcine spine segments, potted in bone cement and µCT scanned. A speckle pattern was applied to the anterior faces of the bones to allow digital image correlation (DIC), which monitored the surface displacements. Twenty-seven (n = 27) VBs were quasi-statically compressively tested to a load up to 10 kN from the cranial side. Specimen-specific FE models were developed for fourteen (n = 14) of the samples in this group. The material properties were optimised based on the experimental load-displacement data using a specimen-specific factor (kGSstatic) to calibrate a density to Young's modulus relationship. The average calibration factor arising from this group was calculated (K¯GSstatic) and applied to a control group of thirteen (n = 13) samples. The resulting VB stiffnesses was compared to experimental findings. The final eight (n = 8) VBs were subjected to an impact load applied via a falling mass of 7.4kg at a velocity of 3.1ms-1. Surface displacements and strains were acquired from the anterior VB surface via DIC, and the impact load was monitored with two load cells. Specimen-specific FE models were created for this dynamic group and material properties were assigned again based on the density-Young's modulus relationship previously validated for static experiments, supplemented with an additional factor (KGSdynamic). The optimised conversion factor for quasi-static loading, K¯GSstatic, had an average of 0.033. Using this factor, the validation models presented an average numerical stiffness value 3.72% greater than the experimental one. From the dynamic loading experiments, the value for KGSdynamic was found to be 0.14, 4.2 times greater than K¯GSstatic. The average numerical stiffness was 2.3% greater than in the experiments. Almost all models presented similar stiffness variations and regions of maximum displacement to those observed via DIC. The developed FE modelling methodology allowed the creation of models which predicted both static and dynamic behaviour of VBs. Deformation patterns on the VB surfaces were acquired from the FE models and compared to DIC data, achieving high agreement. This methodology is now validated to be fully applied to create whole cervical spine models to simulate axial impact scenarios replicating rugby collision events.

Musculoskeletal modelling of the human cervical spine for the investigation of injury mechanisms during axial impacts.

Head collisions in sport can result in catastrophic injuries to the cervical spine. Musculoskeletal modelling can help analyse the relationship between motion, external forces and internal loads that lead to injury. However, impact specific musculoskeletal models are lacking as current viscoelastic values used to describe cervical spine joint dynamics have been obtained from unrepresentative quasi-static or static experiments. The aim of this study was to develop and validate a cervical spine musculoskeletal model for use in axial impacts. Cervical spine specimens (C2-C6) were tested under measured sub-catastrophic loads and the resulting 3D motion of the vertebrae was measured. Specimen specific musculoskeletal models were then created and used to estimate the axial and shear viscoelastic (stiffness and damping) properties of the joints through an optimisation algorithm that minimised tracking errors between measured and simulated kinematics. A five-fold cross validation and a Monte Carlo sensitivity analysis were conducted to assess the performance of the newly estimated parameters. The impact-specific parameters were integrated in a population specific musculoskeletal model and used to assess cervical spine loads measured from Rugby union impacts compared to available models. Results of the optimisation showed a larger increase of axial joint stiffness compared to axial damping and shear viscoelastic parameters for all models. The sensitivity analysis revealed that lower values of axial stiffness and shear damping reduced the models performance considerably compared to other degrees of freedom. The impact-specific parameters integrated in the population specific model estimated more appropriate joint displacements for axial head impacts compared to available models and are therefore more suited for injury mechanism analysis.

The equivalence of multi-axis spine systems: Recommended stiffness limits using a standardized testing protocol.

The complexity of multi-axis spine testing often makes it challenging to compare results from different studies. The aim of this work was to develop and implement a standardized testing protocol across three six-axis spine systems, compare them, and provide stiffness and phase angle limits against which other test systems can be compared. Standardized synthetic lumbar specimens (n=5), comprising three springs embedded in polymer at each end, were tested on each system using pure moments in flexion-extension, lateral bending, and axial rotation. Tests were performed using sine and triangle waves with an amplitude of 8Nm, a frequency of 0.1Hz, and with axial preloads of 0 and 500N. The stiffness, phase angle, and R2 value of the moment against rotation in the principal axis were calculated at the center of each specimen. The tracking error was adopted asa measure of each test system to minimize non-principal loads, defined as the root mean squared difference between actual and target loads. All three test systems demonstrated similar stiffnesses, with small (<14%) but significant differences in 4 of 12 tests. More variability was observed in the phase angle between the principal axis moment and rotation, with significant differences in 10 of 12 tests. Stiffness and phase angle limits were calculated based on the 95% confidence intervals from all three systems. These recommendations can be used with the standard specimen and testing protocol by other research institutions to ensure equivalence of different spine systems, increasing the ability to compare in vitro spine studies.

The application of physiological loading using a dynamic, multi-axis spine simulator.

In-vitro testing protocols used for spine studies should replicate the in-vivo load environment as closely as possible. Unconstrained moments are regularly employed to test spinal specimens in-vitro, but applying such loads dynamically using an active six-axis testing system remains a challenge. The aim of this study was to assess the capability of a custom-developed spine simulator to apply dynamic unconstrained moments with an axial preload. Flexion-extension, lateral bending, and axial rotation were applied to an L5/L6 porcine specimen at 0.1 and 0.3Hz. Non-principal moments and shear forces were minimized using load control. A 500N axial load was applied prior to tests, and held stationary during testing to assess the effect of rotational motion on axial load. Non-principal loads were minimized to within the load cell noise-floor at 0.1Hz, and within two-times the load-cell noise-floor in all but two cases at 0.3Hz. The adoption of position control in axial compression-extension resulted in axial loads with qualitative similarities to in-vivo data. This study successfully applied dynamic, unconstrained moments with a physiological preload using a six-axis control system. Future studies will investigate the application of dynamic load vectors, multi-segment specimens, and assess the effect of injury and degeneration.

The influence of tibial component malalignment on bone strain in revision total knee replacement.

Revision total knee replacement is a challenging surgical procedure typically associated with significant loss of bone stock in the proximal tibia. To increase the fixation stability, extended stems are frequently used for the tibial component in revision surgery. The design of the tibial stem influences the load transfer from tibial component to the surrounding bone and is cited as a possible cause for the clinically reported pain in the location of the stem-end. This study aimed to analyse the strain distribution of a fully cemented revision tibial component with a validated finite element model. The model was developed from a scanned composite tibia (Sawbones), with an implanted, fully cemented stemmed tibial component aligned to the mechanical axis of the tibia. Loading was applied to the tibial component with mediolateral compartment load distributions of 60:40 and 80:20. Three strain gauged composite tibias with implanted tibial components of the same design using the same loading distribution were tested to obtain experimental strains at five locations in the proximal tibia. The finite element model developed was validated against strain measurements obtained in the experimental study. The strains displayed similar patterns (R(2) = 0.988) and magnitudes with those predicted from the finite element model. The displacement of the stem-end from the natural mechanical axis in the finite element model demonstrated increased strains in the stem-end region with a close proximity of the distal stem with the cortical bone. The simulation of a mediolateral compartment load of 80:20 developed peak cortical strain values on the posterior-medial side beneath the stem. This may possibly be related to the clinically reported pain at the stem-end. Furthermore, stem positioning in close proximity or contact with the posterior cortical bone is a contributory factor for an increase in distal strain.

Dynamic, six-axis stiffness matrix characteristics of the intact intervertebral disc and a disc replacement.

Thorough pre-testing is critical in assessing the likely in vivo performance of spinal devices prior to clinical use. However, there is a lack of data available concerning the dynamic testing of lumbar (porcine model) total disc replacements in all six axes under preload conditions. The aim of this study was to provide new data comparing porcine lumbar spinal specimen stiffness between the intact state and after the implantation of an unconstrained total disc replacement, in 6 degrees of freedom. The dynamic, stiffness matrix testing of six porcine lumbar isolated disc specimens was completed using triangle waves at a test frequency of 0.1 Hz. An axial preload of 500 N was applied during all testing. Specimens were tested both in the intact condition and after the implantation of the total disc replacement. Sixteen key stiffness terms were identified for the comparison of the intact and total disc replacement specimens, comprising the 6 principal stiffness terms and 10 key off-axis stiffness terms. The total disc replacement specimens were significantly different to the intact specimens in 12 of these key terms including all six principal stiffness terms. The implantation of the total disc replacement resulted in a mean reduction in the principal stiffness terms of 100%, 91%, and 98% in lateral bending, flexion-extension, and axial rotation, respectively. The novel findings of this study have demonstrated that the unconstrained, low-friction total disc replacement does not replicate the stiffness of the intact specimens. It is likely that other low-friction total disc replacements would produce similar results due to stiffness being actively minimised as part of the design of low-friction devices, without the introduction of stiffening elements or mechanisms to more accurately replicate the mechanical properties of the natural intervertebral disc. This study has demonstrated, for the first time, a method for the quantitative comparative mechanical function testing of total disc replacements and provides baseline data for the development of future devices.

An investigation into axial impacts of the cervical spine using digital image correlation.

BACKGROUND CONTEXT: High-energy impacts are commonly encountered during sports such as rugby union. Although catastrophic injuries resulting from such impacts are rare, the consequences can be devastating for all those involved. A greater level of understanding of cervical spine injury mechanisms is required, with the ultimate aim of minimizing such injuries. PURPOSE: The present study aimed to provide a greater understanding of cervical spine injury mechanisms, by subjecting porcine spinal specimens to impact conditions based on those measured in vivo. The impacts were investigated using high-speed digital image correlation (DIC), a method not previously adopted for spinal impact research. STUDY DESIGN: This was an in vitro biomechanical study. METHODS: Eight porcine specimens were impacted using a custom-made rig. The cranial and caudal axial loads were measured at 1 MHz. Video data were captured with two cameras at 4 kHz, providing measurements of the three-dimensional deformation and surface strain field of the specimens using DIC. RESULTS: The injuries induced on the specimens were similar to those observed clinically. The mean±standard deviation peak caudal load was 6.0±2.1 kN, which occurred 5.6±1.1 ms after impact. Damage observable with the video data occurred in six specimens, 5.4±1.1 ms after impact, and the peak surface strain at fracture initiation was 4.6±0.5%. CONCLUSIONS: This study has provided an unprecedented insight into the injury mechanisms of the cervical spine during impact loading. The posture represents a key factor in injury initiation, with lordosis of the spine increasing the likelihood of injury.

The dynamic, six-axis stiffness matrix testing of porcine spinal specimens.

BACKGROUND CONTEXT: Complex testing protocols are required to fully understand the biomechanics of the spine. There remains limited data concerning the mechanical properties of spinal specimens under dynamic loading conditions in six axes. PURPOSE: To provide new data on the mechanical properties of functional spinal unit (FSU) and isolated disc (ISD) spinal specimens in 6 df. STUDY DESIGN: Dynamic, six-axis stiffness matrix testing of porcine lumbar spinal specimens. METHODS: The stiffness matrix testing of lumbar porcine FSU (n=6) and ISD (n=6) specimens was completed in a custom six-axis spine simulator using triangle wave cycles at a frequency of 0.1 Hz. Specimens were first tested without an axial preload, then with an axial preload of 500 N, with equilibration times of both 30 and 60 minutes. RESULTS: The stiffness matrices were not symmetrical about the principal stiffness terms. The facets increased all the principal stiffness terms with the exception of axial compression-extension. Significant differences were detected in 15 stiffness terms because of the application of an axial preload in the ISD specimens, including an increase in all principal stiffness terms. There were limited differences in stiffness because of equilibration time of 30 and 60 minutes. CONCLUSIONS: The assumption of stiffness matrix symmetry used in many previous studies is not valid. The biomechanical testing of spinal specimens should be completed in 6 df, at physiologic loading rates, and incorporate the application of an axial preload. The present study has provided new data on the mechanical properties of spinal specimens and demonstrates that the dynamic stiffness matrix method provides a means to more fully understand the natural spine and quantitatively assess spinal instrumentation.

Pectoralis major tendon repair: a biomechanical study of suture button versus transosseous suture techniques.

PURPOSE: Pectoralis major tendon avulsion injury benefits from surgical repair. The technique used and speed of rehabilitation in this demanding population remains subject to debate. We performed a biomechanical study comparing suture button (Pec Button™, Arthrex, Naples, FL) with a transosseous suture technique (FibreWire, Arthrex, Naples, FL). METHODS: Freshly slaughtered porcine humeri were prepared to model a single transosseous suture or suture button repair. A static, tensile load to failure experiment and a cyclic, tensile load experiment to model standard (10,000 cycles) and accelerated rehabilitation (20,000 cycles) philosophies were tested. The mode of failure, yield and ultimate failure load, extension (clinical failure >10 mm) and the resistance to cyclic loading was measured. RESULTS: The mode of failure was suture fracture in all the static load experiments with 10/11 occurring as the suture passed through the button and 7/11 as the suture passed through the bone tunnels. There was a significant difference in yield load, favouring transosseous suture [p = 0.009, suture button (SB) 673.0 N (647.2-691.7 N), transosseous suture (TOS) 855.0 N (750.0-891.4 N)] and median extension, favouring suture button [p = 0.009, SB 8.8 mm (5.0-12.4 mm), TOS 15.2 mm (13.2-17.1 mm)]. 2/3 transosseous suture and 0/3 suture buttons failed before completing 20,000 cycles. The difference in mean number of cycles completed was non-significant. The difference in mean extension was 5.1 mm (SB 6.7 mm, TOS 11.7 mm). CONCLUSIONS: Both techniques show advantages. The difference in extension is likely to be more clinically relevant than load tolerated at failure, which is well above physiological levels. The findings do not support an accelerated rehabilitation model.

The development of a dynamic, six-axis spine simulator.

BACKGROUND CONTEXT: Although a great deal of research has been completed to characterize the stiffness of spinal specimens, there remains a limited understanding of the spine in 6 df and there is a lack of data from dynamic testing in six axes. PURPOSE: This study details the development and validation of a dynamic six-axis spine simulator. STUDY DESIGN: Biomechanical study. METHODS: A synthetic spinal specimen was used for the purpose of tuning the simulator, completing positional accuracy tests, and measuring frequency response under physiological conditions. The spine simulator was used to complete stiffness matrix tests of an L3-L4 lumbar porcine functional spinal unit. Five testing frequencies were used, ranging from quasistatic (0.00575 Hz) to dynamic (0.5 Hz). Tests were performed without an axial preload and with an axial preload of 500 N. RESULTS: The validation tests demonstrated that the simulator is capable of producing accurate positioning under loading at frequencies up to 0.5 Hz using both sine and triangle waveforms. The porcine stiffness matrix tests demonstrated that the stiffness matrix is not symmetrical about the principal stiffness diagonal. It was also shown that while an increase in test frequency generally increased the principal stiffness terms, axial preload had a much greater effect. CONCLUSIONS: The spine simulator is capable of characterizing the dynamic biomechanics of the spine in six axes and provides a means to better understand the complex behavior of the spine under physiological conditions.

A biomechanical evaluation of hinged total knee replacement prostheses.

The number of total knee replacements being performed worldwide is undergoing an unprecedented increase. Hinged total knee replacements, used in complex salvage and revision procedures, currently account for a small but growing proportion of prostheses implanted. Modern hinged prostheses share the same basic configuration, allowing flexion-extension and tibial rotation. One aspect on which designs differ is the anteroposterior location of the hinge. A more posterior hinge is designed to increase the patellar tendon moment arm, reducing the quadriceps force required for a given activity and benefiting the patient. Five commonly used total knee replacements were evaluated in terms of quadriceps force and patellar tendon moment arm using a laboratory-based rig. Significant differences were identified between the five prostheses in quadriceps force and patellar tendon moment arm. Analysis of the correlation between these two parameters indicates that while patellar tendon moment arm influences quadriceps force, it is not the only factor. Also important is the lever function of the patella, and it is suggested here that the non-physiological nature of the prosthetic patellofemoral geometry may result in unnatural joint function. Thus, a thorough understanding of the resulting kinematic function of hinged total knee replacements is becoming increasingly important in complex revision total knee replacement to meet rising patient expectations and functional demands.

Ultrasound mimics the effect of mechanical loading on bone formation in vivo on rat ulnae.

While the effect of ultrasound as an extreme example of low-magnitude high-frequency stimulation has been explored in the response of bone to injury, little is known about its effect on normal bone. This experiment was designed to test the hypothesis that ultrasound exerts a similar influence on bone as mechanical stimulation at a physiological level. Three groups of female Wistar rats were anaesthetised (6 per group). In one group, the left ulna was loaded cyclically in vivo 40 times, repeated on a further 5 occasions on alternate days. In a second group, transcutaneous low-intensity pulsed ultrasound stimulation was applied to the left ulnae for the same duration as the period of loading. In a third group, loading and ultrasound stimulation were applied concurrently. The right ulna served as non-loaded control in each animal. At the end of the experiment after 14 days, both ulnae were removed. Induced bone formation was assessed by measuring the proportion of medial periosteal bone surface with double label (dLS/BS, %) and by calculation of mineral apposition rate (MAR) from the inter-label distance. All three treatments induced a significant periosteal response, increasing dLS/BS values from <10% in control limbs to >80% in treated limbs. Increases in MAR of experimental ulnae versus contralateral control ulnae were 2.9 (+/-0.9), 8.6 (+/-2.4) and 8.7 microm (+/-3.2) for the ultrasound only, ultrasound and load, and load only groups, respectively. The effects of loading plus ultrasound were not significantly different from ultrasound alone. These data suggest that ultrasound is able to induce changes in bone that share at least some features with mechanical loading.

Intradiscal pressure changes with dynamic pedicle screw systems.

STUDY DESIGN: In vitro study using porcine spines instrumented with pedicle screw and rod fixation. OBJECTIVES: To determine the intradiscal pressure (IDP) changes with the use of dynamic and rigid pedicle screw systems in simulated spinal fusion. SUMMARY OF BACKGROUND DATA: The intervertebral discs are prone to injury under conditions of altered IDP. The effects of instrumentation with dynamic pedicle screw systems on IDP have not been clearly delineated. METHODS: A 2-level posterior instrumentation was applied to fresh porcine spinal segments (n=16). Dynamic and rigid pedicle screw constructs along with uninstrumented (n=6) spinal segments as controls were tested. The spinal segments were subjected to 24,000 cycles of flexion compression loading at 5 Hz. IDP within the instrumented (L2-L3 and L3-L4) and adjacent (L1-L2 and L4-L5) discs were measured using a pressure transducer needle. Results were recorded at 6000 cycle intervals. RESULTS: Instrumentation increased IDP. Within the instrumented levels, the greatest increase in IDP was found at the L2-L3 disc. Here, after 24,000 loading cycles, IDP for spines instrumented with mobile screws was 6.8 times higher than that of uninstrumented spines whereas for rigid screws the factor was 9.1. For the L3-L4 cases, the presence of instrumentation increased IDP by factors of 1.7 and 2.7 for mobile and rigid screws, respectively. In the uninstrumented levels, IDP at L1-L2 and L4-L5 was lower with mobile screws. These were statistically significant at for L1-L2 (24,000 cycles, P=0.008) and L4-L5 level (12,000, 18,000, and 24,000 cycles, P<0.04 in all cases). CONCLUSIONS: Of the 2 types, mobile screws produced the least increase in IDP. This feature might be beneficial for the fusion process while at the same time prevent secondary pathology such as premature disc degeneration and facet joint pathology due to excessive disc pressures.

Ultrasound transmission loss across transverse and oblique bone fractures: an in vitro study.

An axial transmission technique has been used to investigate the changes in the first arrival time and signal amplitude of 200 kHz ultrasonic waves travelling across different fracture geometries. Results taken from intact bovine femora were compared with those produced when a transverse and an oblique fracture were simulated. The arrival time and signal amplitude displayed a different variation with receiver position for the two geometries and a given fracture gap width. A comparison between this work and a previous study suggested that the marrow does not play a significant role in the mechanisms causing the change in arrival time and signal amplitude. Numerical modelling revealed that an oblique fracture caused a reduction in the extra time delay of the propagating wave compared with the transverse case, but a decrease in the corresponding signal amplitude, i.e., greater signal loss. The angle of the oblique fracture was investigated and results suggested that decreasing the fracture angle relative to the wave propagation direction caused a decrease in the extra time delay and an increase in signal loss. These findings are important for determining the sensitivity of systems for monitoring fracture healing using ultrasound arrival time and signal amplitude. (E-mail: spd@inorbit.com).

Locking plates increase the strength of dynamic hip screws.

INTRODUCTION: Failure of a dynamic hip screw (DHS) fixation leads to decreased mobility of the patient and frequently to a decrease in general health. The most common mode of failure of a DHS is cut out of the lag screw from the femoral head. The second most common mode of failure is lift-off of the plate from the femur. The aim of this laboratory-based experimental study was to determine whether a DHS secured to an osteoporotic femur with a locking screw plate would provide a stronger construct than the standard DHS plate. METHOD: The standard DHS design was compared to a DHS with fixed angle locking screws holding the DHS plate to the femur. Standard dynamic compression plates (DCP) and locking compression plates (LCP) were attached to synthetic, osteoporotic bone. A load was applied to replicate the forces occurring following the fixation of unstable, intertrochanteric hip fractures. A bracket on the proximal end of the plate replicated the lag screw in the femoral head. The constructs were cyclically loaded by a screw-driven material-testing machine and the number of cycles before failure occurred was determined. RESULTS: The mean number of cycles to failure for the locking plate construct was 2.6 times greater than for the standard screw construct (285 versus 108 cycles, respectively p=0.016). CONCLUSION: A dynamic hip screw with fixed angle locking screws would reduce the risk of DHS failure. A locking screw DHS would be particularly useful in patients with osteoporotic bone, and in patients with less stable fracture configurations.

In vitro pressurization of the acetabular cement mantle: the effect of a flange.

A model was developed to assess the effect of an acetabular flange on pressure within different zones of the cement mantle during insertion. Two prosthetic designs were assessed in 3 different sizes. Flanged components produced significantly higher mean pressures than unflanged ones (P < .01). The effect of a flange was more pronounced at the rim than at the pole. Delayed insertion resulted in a further significant rise in mean pressure (P < .01), but this did not compensate for the lack of a flange. This experimental model supports the use of a flange to increase pressure within the cement mantle on component insertion. The beneficial effect is more marked in the area that is most likely to show deficiency in the cement-bone interface on postoperative radiographs.

Mechanical characterisation of three percutaneous vertebroplasty biomaterials.

Percutanous vertebroplasty (PVP) is gaining popularity for the treatment of vertebral compression fractures. The need of obtaining low viscosity materials for injection through small bore needles and the necessity of visualising the fluid flow during injection have led users to the formulation of a number of ad-hoc recipes aimed at adapting PMMA cements for this use. Industry, on its part, has addressed these requirements by developing specific products for this application. This study aimed at providing a direct comparison of a wide range of mechanical properties between three commercially available biomaterials developed for PVP: two PMMA based materials, Osteopal V (Merck Biomaterial GMBH, Dermstedt, D) and Verterbroplastic (DePuy Acromed, Inc, MA, USA), and a Bis-GMA composite, Cortoss (Orthovita, PA, USA). Cortoss consistently exhibited higher values for compressive strength, bending modulus and shear strength to both Osteopal V and Vertebroplastic. The creep behaviour of Cortoss was also different from that of the two PMMA cements.PVP can take advantage from the development of new injectable biomaterials in response to the problems associated with the use of PMMA in a highly vascularised area such as the vertebral body. In addition careful modulation of the mechanical properties of the material has the potential to further improve the outcome of PVP, possibly reducing the risk of adjacent level fractures associated with the procedure.

An in-vitro investigation of the CPS-Plus femoral stem: influence of the proximal centraliser on cement pressurisation during stem insertion.

The CPS-Plus stem is designed for use with a proximal and distal stem centraliser. This in-vitro study examined the cement pressurisation achieved during insertion of the CPS-Plus femoral stem into a model femur. Cement pressures were measured at proximal, mid and distal stem levels. Pressures were recorded during insertion of the CPS-Plus stem with both proximal and distal centralisers and compared with those achieved when only a distal centraliser was used with digital occlusion of the proximal femur. The CPS-Plus with a proximal centraliser generated significantly greater cement pressures than the CPS-Plus without a proximal centraliser at proximal and mid-stem regions. The use of a proximal stem centraliser may improve the cement-bone interdigitation and shear strength at the cement-bone interface, particularly in the region of the proximal femur.