A methodology for the generation and non-destructive characterisation of transverse fractures in long bones.

Long bone fractures are common and although treatments are highly effective in most cases, it is challenging to achieve successful repair for groups such as open and periprosthetic fractures. Previous biomechanical studies of fracture repair, including computer and experimental models, have simplified the fracture with a flat geometry or a gap, and there is a need for a more accurate fracture representation to mimic the situation in-vivo. The aims of this study were to develop a methodology for generating repeatable transverse fractures in long bones in-vitro and to characterise the fracture surface using non-invasive computer tomography (CT) methods. Ten porcine femora were fractured in a custom-built rig under high-rate loading conditions to generate consistent transverse fractures (angle to femoral axis < 30 degrees). The bones were imaged using high resolution peripheral quantitative CT (HR-pQCT). A method was developed to extract the roughness and form profiles of the fracture surface from the image data using custom code and Guassian filters. The method was tested and validated using artificially generated waveforms. The results revealed that the smoothing algorithm used in the script was robust but the optimum kernel size has to be considered.

Experimental and computational approach investigating burst fracture augmentation using PMMA and calcium phosphate cements.

The aim of the study was to use a computational and experimental approach to evaluate, compare and predict the ability of calcium phosphate (CaP) and poly (methyl methacrylate) (PMMA) augmentation cements to restore mechanical stability to traumatically fractured vertebrae, following a vertebroplasty procedure. Traumatic fractures (n = 17) were generated in a series of porcine vertebrae using a drop-weight method. The fractured vertebrae were imaged using µCT and tested under axial compression. Twelve of the fractured vertebrae were randomly selected to undergo a vertebroplasty procedure using either a PMMA (n = 6) or a CaP cement variation (n = 6). The specimens were imaged using µCT and re-tested. Finite element models of the fractured and augmented vertebrae were generated from the µCT data and used to compare the effect of fracture void fill with augmented specimen stiffness. Significant increases (p < 0.05) in failure load were found for both of the augmented specimen groups compared to the fractured group. The experimental and computational results indicated that neither the CaP cement nor PMMA cement could completely restore the vertebral mechanical behavior to the intact level. The effectiveness of the procedure appeared to be more influenced by the volume of fracture filled rather than by the mechanical properties of the cement itself.

Development of calcium phosphate cement for the augmentation of traumatically fractured porcine specimens using vertebroplasty.

The study aim was to develop and apply an experimental technique to determine the biomechanical effect of polymethylmethacrylate (PMMA) and calcium phosphate (CaP) cement on the stiffness and strength of augmented vertebrae following traumatic fracture. Twelve burst type fractures were generated in porcine three-vertebra segments. The specimens were randomly split into two groups (n=6), imaged using microCT and tested under axial loading. The two groups of fractured specimens underwent a vertebroplasty procedure, one group was augmented with CaP cement designed and developed at Queen's University Belfast. The other group was augmented with PMMA cement (WHW Plastics, Hull, UK). The specimens were imaged and re-tested . An intact single vertebra specimen group (n=12) was also imaged and tested under axial loading. A significant decrease (p<0.01) was found between the stiffness of the fractured and intact groups, demonstrating that the fractures generated were sufficiently severe, to adversely affect mechanical behaviour. Significant increase (p<0.01) in failure load was found for the specimen group augmented with the PMMA cement compared to the pre-augmentation group, conversely, no significant increase (p<0.01) was found in the failure load of the specimens augmented with CaP cement, this is attributed to the significantly (p<0.05) lower volume of CaP cement that was successfully injected into the fracture, compared to the PMMA cement. The effect of the percentage of cement fracture fill, cement modulus on the specimen stiffness and ultimate failure load could be investigated further by using the methods developed within this study to test a more injectable CaP cement.

Development of a bovine collagen-apatitic calcium phosphate cement for potential fracture treatment through vertebroplasty.

The aim of this study was to examine the potential of incorporating bovine fibres as a means of reinforcing a typically brittle apatite calcium phosphate cement for vertebroplasty. Type I collagen derived from bovine Achilles tendon was ground cryogenically to produce an average fibre length of 0.96±0.55 mm and manually mixed into the powder phase of an apatite-based cement at 1, 3 or 5 wt.%. Fibre addition of up to 5 wt.% had a significant effect (P ≤ 0.001) on the fracture toughness, which was increased by 172%. Adding ≤ 1 wt.% bovine collagen fibres did not compromise the compressive properties significantly, however, a decrease of 39-53% was demonstrated at ≥ 3wt.% fibre loading. Adding bovine collagen to the calcium phosphate cement reduced the initial and final setting times to satisfy the clinical requirements stated for vertebroplasty. The cement viscosity increased in a linear manner (R²=0.975) with increased loading of collagen fibres, such that the injectability was found to be reduced by 83% at 5 wt.% collagen loading. This study suggests for the first time the potential application of a collagen-reinforced calcium phosphate cement as a viable option in the treatment of vertebral fractures, however, issues surrounding efficacious cement delivery need to be addressed.

Spinal cord-fragment interactions following burst fracture: an in vitro model.

OBJECT: The purpose of the study was to develop an in vitro model of the bone fragment and spinal cord interactions that occur during a burst fracture and further the understanding of how the velocity of the bone fragment and the status of the posterior longitudinal ligament (PLL) affect the deformation of the cord. METHODS: An in vitro model was developed such that high-speed video and pressure measurements recorded the impact of a simulated bone fragment on sections of explanted bovine spinal cord. The model simulated the PLL and the posterior elements. The status of the PLL had a significant effect on both the maximum occlusion of the spinal cord and the time for occlusion to occur. Raising the fragment velocity led to an overall increase in the spinal cord deformation. Interestingly the dura mater appeared to have little or no effect on the extent of occlusion. CONCLUSIONS: These findings may indicate the importance of the dura's interaction with the cerebrospinal fluid in protecting the cord during this type of impact.

A dynamic study of thoracolumbar burst fractures.

BACKGROUND: The degree of canal stenosis following a thoracolumbar burst fracture is sometimes used as an indication for decompressive surgery. This study was performed to test the hypothesis that the final resting positions of the bone fragments seen on computed tomography imaging are not representative of the dynamic canal occlusion and associated neurological damage that occurs during the fracture event. METHODS: A drop-weight method was used to create burst fractures in bovine spinal segments devoid of a spinal cord. During impact, dynamic measurements were made with use of transducers to measure pressure in a synthetic spinal cord material, and a high-speed video camera filmed the inside of the spinal canal. A corresponding finite element model was created to determine the effect of the spinal cord on the dynamics of the bone fragment. RESULTS: The high-speed video clearly showed the fragments of bone being projected from the vertebral body into the spinal canal before being recoiled, by the action of the posterior longitudinal ligament and intervertebral disc attachments, to their final resting position. The pressure measurements in the synthetic spinal cord showed a peak in canal pressure during impact. There was poor concordance between the extent of postimpact occlusion of the canal as seen on the computed tomography images and the maximum amount of occlusion that occurred at the moment of impact. The finite element model showed that the presence of the cord would reduce the maximum dynamic level of canal occlusion at high fragment velocities. The cord would also provide an additional mechanism by which the fragment would be recoiled back toward the vertebral body. CONCLUSIONS: A burst fracture is a dynamic event, with the maximum canal occlusion and maximum cord compression occurring at the moment of impact. These transient occurrences are poorly related to the final level of occlusion as demonstrated on computed tomography scans.