Richard M. Hall

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.

A dynamic investigation of the burst fracture process using a combined experimental and finite element approach

Spinal burst fractures account for about 15% of spinal injuries and, because of their predominance in the younger population, there are large associated social and healthcare costs. Although several experimental studies have investigated the burst fracture process, little work has been undertaken using computational methods. The aim of this study was to develop a finite element model of the fracture process and, in combination with experimental data, gain a better understanding of the fracture event and mechanism of injury. Experimental tests were undertaken to simulate the burst fracture process in a bovine spine model. After impact, each specimen was dissected and the severity of fracture assessed. Two of the specimens tested at the highest impact rate were also dynamically filmed during the impact. A finite element model, based on CT data of an experimental specimen, was constructed and appropriate high strain rate material properties assigned to each component. Dynamic validation was undertaken by comparison with high-speed video data of an experimental impact. The model was used to determine the mechanism of fracture and the postfracture impact of the bony fragment onto the spinal cord. The dissection of the experimental specimens showed burst fractures of increasing severity with increasing impact energy. The finite element model demonstrated that a high tensile strain region was generated in the posterior of the vertebral body due to the interaction of the articular processes. The region of highest strain corresponded well with the experimental specimens. A second simulation was used to analyse the fragment projection into the spinal canal following fracture. The results showed that the posterior longitudinal ligament became stretched and at higher energies the spinal cord and the dura mater were compressed by the fragment. These structures deformed to a maximum level before forcing the fragment back towards the vertebral body. The final position of the fragment did not therefore represent the maximum dynamic canal occlusion.

A biomechanical investigation of vertebroplasty in osteoporotic compression fractures and in prophylactic vertebral reinforcement.

Study Design. Cadaveric single vertebrae were used to evaluate vertebroplasty as a prophylactic treatment and as an intervention for vertebral compression fractures. Objective. To investigate the biomechanical characteristics of prophylactic reinforcement and postfracture augmentation of cadaveric vertebrae. Summary of Background Data. Percutaneous vertebroplasty is a treatment option for osteoporotic vertebral compression fractures. Short-term results are promising, but longer-term studies have suggested a possible accelerated failure rate in the adjacent vertebral body. Limited research has been conducted into the effects of prophylactic vertebroplasty in osteoporotic vertebrae. This study aims to elucidate the biomechanical differences between the 2 treatment groups. Methods. Human vertebrae were assigned to 2 scenarios: Scenario 1 simulated a wedge fracture followed by cement augmentation; Scenario 2 involved prophylactic augmentation using vertebroplasty. Micro-CT imaging was performed to assess the bone mineral density, vertebral dimensions, fracture pattern, and cement volume. All augmented specimens were then compressed under an eccentric flexion load to failure. Results. Product of bone mineral density and endplate surface area gave a good prediction of failure strength when compared with actual failure strength of specimens in Scenario 1. Augmented vertebral bodies showed an average cement fill of 23.9% ± 8.07%. There was a significant postvertebroplasty increase in failure strength by a factor of 1.72 and 1.38 in Scenarios 1 and 2, respectively. There was a significant reduction in stiffness following augmentation for Scenario 1 (t = 3.5, P = 0.005). Stiffness of the vertebral body in Scenario 2 was significantly greater than observed in Scenario 1 (t = 4.4, P = 0.0002). Conclusion. Results suggest that augmentation of the vertebrae postfracture significantly increases failure load, while stiffness is not restored. Prophylactic augmentation was seen to increase failure strength in comparison to the predicted failure load. Stiffness appears to be maintained suggesting that prophylactic vertebroplasty maintains stiffness better than vertebroplasty postfracture.

Measurement of canal occlusion during the thoracolumbar burst fracture process

Post-injury CT scans are often used following burst fracture trauma as an indication for decompressive surgery. Literature suggests, however, that there is little correlation between the observed fragment position and the level of neurological injury or recovery. Several studies have aimed to establish the processes that occur during the fracture using indirect methods such as pressure measurements and pre/post impact CT scans. The purpose of this study was to develop a direct method of measuring spinal canal occlusion during a simulated burst fracture by using a high-speed video technique. The fractures were produced by dropping a mass from a measured height onto three-vertebra bovine specimens in a custom-built rig. The specimens were constrained to deform only in the impact direction such that pure compression fractures were generated. The spinal cord was removed prior to testing and the video system set up to film the inside of the spinal canal during the impact. A second camera was used to film the outside of the specimen to observe possible buckling during impact. The video images were analysed to determine how the cross-sectional area of the spinal canal changed during the event. The images clearly showed a fragment of bone being projected from the vertebral body into the spinal canal and recoiling to the final resting position. To validate the results, CT scans were taken pre- and post-impact and the percentage canal occlusion was calculated. There was good agreement between the final canal occlusion measured from the video images and the CT scans.

Development of specimen-specific finite element models of human vertebrae for the analysis of vertebroplasty:

Abstract The aim of this study was to determine the accuracy of specimen-specific finite element models of untreated and cement-augmented vertebrae by direct comparison with experimental results. Eleven single cadaveric vertebrae were imaged using micro computed tomography (mCT) and tested to failure in axial compression in the laboratory. Four of the specimens were first augmented with PMMA cement to simulate a prophylactic vertebroplasty. Specimen-specific finite element models were then generated using semi-automated methods. An initial set of three untreated models was used to determine the optimum conversion factors from the image data to the bone material properties. Using these factors, the predicted stiffness and strength were determined for the remaining specimens (four untreated, four augmented). The model predictions were compared with the corresponding experimental data. Good agreement was found with the non-augmented specimens in terms of stiffness (root-mean-square (r.m.s.) error 12.9 per cent) and strength (r.m.s. error 14.4 per cent). With the augmented specimens, the models consistently overestimated both stiffness and strength (r.m.s. errors 65 and 68 per cent). The results indicate that this method has the potential to provide accurate predictions of vertebral behaviour prior to augmentation. However, modelling the augmented bone with bulk material properties is inadequate, and more detailed modelling of the cement region is required to capture the bone—cement interactions if the models are to be used to predict the behaviour following vertebroplasty.

The Importance of Fluid-Structure Interaction in Spinal Trauma Models

While recent studies have demonstrated the importance of the initial mechanical insult in the severity of spinal cord injury, there is a lack of information on the detailed cord-column interaction during such events. In vitro models have demonstrated the protective properties of the cerebrospinal fluid, but visualization of the impact is difficult. In this study a computational model was developed in order to clarify the role of the cerebrospinal fluid and provide a more detailed picture of the cord-column interaction. The study was validated against a parallel in vitro study on bovine tissue. Previous assumptions about complete subdural collapse before any cord deformation were found to be incorrect. Both the presence of the dura mater and the cerebrospinal fluid led to a reduction in the longitudinal strains within the cord. The division of the spinal cord into white and grey matter perturbed the bone fragment trajectory only marginally. In conclusion, the cerebrospinal fluid had a significant effect on the deformation pattern of the cord during impact and should be included in future models. The type of material models used for the spinal cord and the dura mater were found to be important to the stress and strain values within the components, but less important to the fragment trajectory.

Preliminary biomechanical evaluation of prophylactic vertebral reinforcement adjacent to vertebroplasty under cyclic loading

BACKGROUND CONTEXT Percutaneous vertebroplasty has become a favored treatment option for reducing pain in osteoporotic patients with vertebral compression fractures (VCFs). Short-term results are promising, although longer-term complications may arise from accelerated failure of the adjacent vertebral body. PURPOSE To provide a preliminary biomechanical assessment of prophylactic vertebral reinforcement adjacent to vertebroplasty using a three-vertebra cadaveric segment under dynamic loads that represent increasing activity demands. In addition, the effects of reducing the elastic modulus of the cement used in the intact vertebrae were also assessed. STUDY DESIGN/SETTING Three-vertebra cadaveric segments were used to evaluate vertebroplasty with adjacent vertebral reinforcement as an intervention for VCFs. METHODS Nine human three-vertebra segments (T12-L2) were prepared and a compression fracture was generated in the superior vertebrae. Vertebroplasty was performed on the fractured T12 vertebra. Subsequently, the adjacent intact L1 vertebra was prophylactically augmented with cement of differing elastic moduli (100-12.5% modulus of the base cement value). After subfailure quasi-static compression tests before and after augmentation, these specimens were subjected to an incrementally increasing dynamic load profile in proportion to patient body weight (BW) to assess the fatigue properties of the construct. Quantitative computed tomography assessments were conducted at several stages in the experimental process to evaluate the vertebral condition and quantify the gross dimensions of the segment. RESULTS No significant difference in construct stiffness was found pre- or postaugmentation (t=1.4, p=.19). Displacement plots recorded during dynamic loading showed little evidence of fracture under normal physiological loads or moderate activity (1-2.5x BW). A third of the specimens continued to endure increasing load demands and were confirmed to have no fracture after testing. In six specimens, however, greater loads induced 11 fractures: 7 in the augmented vertebra (2xT12, 5xL5) and 4 in the adjacent L2 vertebra. A strong correlation was observed between the subsidence in the segmental unit and the incidence of fracture after testing (r(Spearmans)=-0.88, p=.002). Altering the modulus of cement in the intact vertebra had no effect on level of segmental compromise. CONCLUSIONS These preliminary findings suggest that under normal physiological loads associated with moderate physical activity, prophylactic augmentation adjacent to vertebroplasty showed little evidence of inducing fractures, although loads representing more strenuous activities may generate adjacent and peri-augmentation compromise. Reducing the elastic modulus of the cement in the adjacent intact vertebrae appeared to have no significant effect on the incidence or location of the induced fracture or the overall height loss of the vertebral segment.

Cross-Shear Implementation in Sliding-Distance-Coupled Finite Element Analysis of Wear in Metal-on-Polyethylene Total Joint Arthroplasty: Intervertebral Total Disc Replacement as an Illustrative Application

Computational simulations of wear of orthopaedic total joint replacement implants have proven to valuably complement laboratory physical simulators, for pre-clinical estimation of abrasive/adhesive wear propensity. This class of numerical formulations has primarily involved implementation of the Archard/Lancaster relationship, with local wear computed as the product of (finite element) contact stress, sliding speed, and a bearing-couple-dependent wear factor. The present study introduces an augmentation, whereby the influence of interface cross-shearing motion transverse to the prevailing molecular orientation of the polyethylene articular surface is taken into account in assigning the instantaneous local wear factor. The formulation augment is implemented within a widely utilized commercial finite element software environment (ABAQUS). Using a contemporary metal-on-polyethylene total disc replacement (ProDisc-L) as an illustrative implant, physically validated computational results are presented to document the role of cross-shearing effects in alternative laboratory consensus testing protocols. Going forward, this formulation permits systematically accounting for cross-shear effects in parametric computational wear studies of metal-on-polyethylene joint replacements, heretofore a substantial limitation of such analyses.

Evaluation of the mechanical and architectural properties of glenoid bone

Successful glenoid fixation in shoulder arthroplasty is partly dependent on the properties of the underlying bone. Therefore, mapping of the glenoid surface and locating the bone with the highest quality, in terms of mechanical properties and morphology, is a key requirement in ensuring effective fixation. To this end, an investigation was undertaken to study the relationship between indentation behavior and the quality of the glenoid bone. Nineteen embalmed glenoids were obtained from human cadavers (mean age at death, 82 years). Each specimen was tested using a cylindrical indentor at 11 predetermined points to investigate load-displacement behavior. Microcomputed tomography analysis was performed to ascertain the bone volume (BV)/total volume (TV) fraction of the trabecular bone and the subchondral thickness. Statistical analysis showed that both strength and modulus varied with indentation position. Significant relationships were found between either strength or modulus and BV/TV or subchondral thickness, although the explained variance was relatively low.

Stem fixation in the Charnley low-friction arthroplasty in young patients using an intramedullary bone block

We report a prospective study of the use of intramedullary bone blocks to improve the fixation of a matt-finish femoral stem in Charnley low-friction arthroplasties. There were 379 patients (441 hips), but at a minimum follow-up of ten years there were 258 arthroplasties in 221 patients including some which had been revised. The mean age at surgery was 41 years (17 to 51) and the mean follow-up was 13.4 years (1 to 20 including the early revisions). Nine stems (3.5%) had been revised for aseptic loosening, but there were no stem fractures. Survivorship of stems was 99.2% at ten years and 94.35% at 15 and 20 years. We found that the patients gender, the position of the stem and the experience of the surgeon all influenced the outcome. Our findings suggest that using our method of stem fixation, follow-up of over 11 years was needed to reveal the effects of endosteal cavitation of the femur, and of over 13 years to assess any divergence between the clinical and the radiological outcomes of stem fixation.