Introduction: Computational modelling of the spine offers a particularly difficult challenge to analysts due to its complex structure and high level of functionality. Previous studies [Wijayathunga, 2008; Jones, 2007] have shown that finite element (FE) predictions of vertebral stiffness are highly sensitive to the applied boundary conditions and therefore validation requires careful matching between the experimental and simulated situation. The aim of this study was to develop and experimentally validate specimen specific FE models of porcine vertebrae in order to accurately predict the stiffness of single vertebra specimens.

METHOD: Nine single vertebra specimens were excised from the thoracolumbar region of two porcine spines. The specimens were mounted between two parallel PMMA housings and each specimen was imaged using a micro computed tomography (μCT) system (μCT80; Scanco Medical, Switzerland). In order to accurately match the experimental conditions, a radio-opaque marker was positioned on the specimen housing at the point of load application. The vertebrae were separated into two groups: a development set (set 1) consisting of three specimens and a validation set (set 2) of six specimens. Specimens from set 1 were used to establish the optimum method of conversion from image greyscale, to element material properties. The models in set 2 were used to assess the accuracy of the stiffness predictions for each model. The vertebrae were tested in a materials testing machine (AGS-10kNG; Shimadzu Corp., Japan) under axial compression and the stiffness for each specimen was calculated. The μCT data was imported into an image processing package (Scan IP, Simpleware, UK). The software enabled the images to be segmented and the vertebra, cement housings and position of load application to be identified. The segmented images were down-sampled to 1mm voxels, enabling a FE mesh to be generated (Scan FE; Simpleware, UK) based on direct voxel to element conversion. The Young’s modulus of each bone element was assigned, based on the greyscale of the corresponding image voxel. The PMMA housing plates were assigned homogeneous material properties (E = 2.45 GPa). Abaqus CAE 6.8 (Simula, Providence, Rhode Island, USA) was used for the processing and post-processing of all the models.

Results: The mean experimental stiffness was 4321 N/ mm (standard deviation = 415 N/mm). The optimum conversion factor was calculated for set 1, which yielded a root mean squared (RMS) percentage error of 7.5% when compared with the experimental stiffness. Using this optimised scale factor, FE models of specimens from set 2 were created. The predicted stiffnesses for set 2 were compared to the corresponding experimental values and yielded an RMS error of 10.1%.

Conclusion: The results indicate that specimen specific models can provide good agreement with the corresponding experimental specimen stiffness. In addition, the method employed in this study proved robust enough to be applied to vertebral tissue obtained from different animals of the same species. This method will now be developed to assess treatments for traumatic spinal injuries.

Purpose of the study: to demonstrate a mechanism of loosening of the Souter-Strathclyde Total Elbow Replacement (TER) using evidence from revision surgery.

Methods: nine Souter-Strathclyde humeral and ulnar components retrieved from revision surgery for aseptic loosening were examined macroscopically and then microscopically under low power magnification. The wear patterns were compared and photographed.

Results and conclusion: inspection of the retrieved cobalt chrome steel humeral components revealed no evidence of surface wear. However on examination of the polyethylene ulnar components six of the nine exhibited macroscopic wear taking the form of deep linear grooves on either the medial or lateral articulating surface. Microscopic examination revealed wear exhibited as complete disruption of the polyethylene machining lines on the medial and lateral articular surfaces, but almost complete preservation on the central gliding ridge. The findings are best explained in the context of normal elbow kinematics and congruence of the Souter-Strathclyde components. The normal elbow joint is not a simple hinge joint. In addition to flexion/extension, axial rotation and abduction/adduction motion patterns occur. However articulating surfaces of the Souter-Strathclyde components are highly congruent and thus resist the elbow’s normal translational and rotational movements. Our wear patterns are the result of humeral component rocking during flexion and extension as a result of this resistance. The central gliding ridge is preserved because the humeral component is not always in contact with it as it rocks out of its articulation in the coronal plane. Furthermore as the humeral component rocks, the sharp edge of its articulating surface makes contact with the articulating surface of the ulna causing abrasion and in the extreme circumstance the deep linear grooves observed. The biomechanics eventually lead to component loosening.

Introduction: It has been shown that acrylic bone cement is weakened by its porosity, which enhances the formation of micro-cracks that contribute to major crack propagation. It has also been observed, that mixing procedures play a significant role in determining the quality of bone cement produced. A high degree of porosity is found to exist in cement that is inadequately mixed.

Currently mixing system allow for the preparation of the bone cement under the application of a vacuum in a closed, sealed chamber by means of a repeatable mixing action. These systems are perceived to be repeatable, reliable, and operator independent. The objective of this study is to evaluate the quality and consistency of acrylic bone cement prepared by scrub staff in an orthopaedic theatre using a commercially available third generation vacuum mixing syringe, in terms of the level of voids within the cement microsturcture.

Materials and Methods: The mixing devices were stored at 23°C ± 1°C for a minimum of 24 hours prior to mixing. The acrylic bone cement (Palacos R® with gentamicin, Biomet Merck, UK) was stored at 4°C ± 1°C for a minimum of 24 hours prior to mixing.

Bone cement was mixed using a commercially available third generation mixing device (vacuum = −550mmHg) at Musgrave Park Hospital, Belfast, Northern Ireland. The cement was mixed according to the device manufacturers’ instructions for use. Mixing was carried out during a joint replacement surgery by a number of experienced theatre scrub staff (n = 35). The cement remaining at the end of the procedure was allowed to cure within the delivery nozzle, made from linear low-density polyethylene (LLDPE) and having an internal diameter of 10mm. 205 nozzles were collected post-operatively and stored at 23°C ± 1°C prior to testing. The percentage porosities were determined by measuring the apparent densities based on Archimedes principle and, as a direct result; it was possible to calculate the mean percentage porosities.

Discussion: It can be observed that the majority of the theatre nurses, ie 46.8% prepared bone cement using the vacuum mixing system containing a porosity of between 2% to 4%. A cement porosity of this range would be the accepted optimum content for acrylic bone cement. However, 6.4% of the theatre nurses prepared cement demonstrating a porosity content ranging from 8–16%, which is highly unsatisfactory when you consider that the cement mixing system is perceived to be a consistent and reliable mixing device that is operator independent.

Figure 2 illustrates a bar chart representing the bone cement porosity as a function of which orthopaedic theatre the cement was prepared. There was no significance difference when comparing the quality of the cement mixed in terms of porosity with the different theatres. The mean porosity values of the cement mixed ranged between 2.5% and 5.2% depending on which theatre was used.

Conclusions: Bone cement mixed using the commercially available third generation device in theatre by 35 scrub staff was found to have a high degree of variability. Thus demonstrating that even an alleged reproducible mixing system is independent on mixing technique when used in a clinical situation by a number of users. Thus illustrating the system is not wholly user independent.

As a consequence of this investigation it is recommended that the key to ensuring high quality bone cement, with a good mechanical strength, that can be consistently prepared in theatre by scrub staff are two fold.

The orthopaedic staff must be aware of the significance of cement mixing and how it is affected by a number of factors including the type of mixing system, vacuum level applied, and mixing technique.

Introduction: Wear, and the resultant loosening and revision, of Total Hip Replacements (THRs) remains the limiting factor in the long term success of the prosthesis. Over 1 million Total Hip Replacements (THRs) are implanted each year, of which about 15% are revisions, most of which are a consequence of loosening of either femoral or acetabular components. This is frequently caused by either the mechanical (Wroblewski, 1986) or biological (Besong et al, 1997) response to the wear of ultra-high molecular weight polyethylene (UHMWPE) acetabular component.

In a previous study Bennett (2002, 2000) has demonstrated that the walking patterns of THR patients 5 years post operation directly correlated with the wear of the acetabular component, as measured radiographically. The present study considers THR patients 10 years post-operatively, ensuring more accurate wear measurements and more meaningful outcome measures.

Materials and Methods: Gait Analysis was performed on a number of THR patients following routine review using a Vicon 370 data capture system and a lower body marker set. This data was processed using Polygon software and joint angles were derived for the hip in the sagittal, coronal and transverse planes. A computer simulation was used to determine the path which each of 20 points on the prosthetic femoral head traces on the acetabulum during walking.

Results: It was found that patients exhibited different patterns of movement ranging from liner to multi-directional. Normal subjects have previously been found to exhibit multi-directional movement. Patients with mult-directional movement showed evidence of greater wear (Bennett et al., 2000).

Discussion and conclusion: Linear movement causes orientation hardening and wear resistance while multi-directional movement cause increased shear and greater wear rates. These differences in movement loci have a significant influence on UHMWPE wear rate and the long term survival of the implant.