Biomechanical characteristics of the porcine denticulate ligament in different vertebral levels of the cervical spine-preliminary results of an experimental study.

BACKGROUND: Few studies exist on the mechanical properties of denticulate ligaments and none report the variation in these properties at different levels of the spine. The aim of this study was to perform an experimental determination of load-extension and stress-strain characteristics of the denticulate ligament and to establish if their properties change at different vertebral levels of the cervical spine. METHOD: The study was carried out on a total of 98 porcine denticulate ligament samples dissected from seven fresh porcine cervical spinal cord specimens. All of the samples were subjected to an uniaxial tensile test at a speed of 2mm/min, during which the load-extension characteristics were registered. RESULTS: The analysis revealed a decrease of the failure force in the caudal orientation indicated by significant differences between the C1 (1.04±0.41N) and C7 (0.55±0.12N) vertebral levels (P=0.037). The average ultimate force that broke the denticulate ligaments was 0.88N. The mean value of Young׳s modulus was 2.06MPa with a minimum of 1.31MPa for C7 and maximum of 2.46MPa for C5. CONCLUSIONS: The values of the denticulate ligament failure force in samples from different cervical vertebrae levels differ significantly. The presented data should be taken into consideration during numerical modelling of the human cervical spinal cord.

Analysis of the influence of a metha-type metaphysical stem on biomechanical parameters.

The full postoperative loading of the limb is possible if patients are properly selected and qualified for hip arthroplasty and the requirements as to the proper position of the metaphysial stem are met. The lack of precision, and patient qualification which does not satisfy the fixed criteria may result in stem setting inconsistent with the assumptions. An analysis based on the finite element method (FEM) will enable one to find out how to plan the magnitude of operated joint loading on the basis of the position of the stem in the postoperative radiograph. By analyzing the distribution of bone tissue deformations one can identify the zones where the spongy bone is overloaded and determine the strain level in comparison with the one determined for a model of the bone with the stem in proper position. On the basis of the results obtained one can estimate the range of loads for the operated limb, which will not result in the loss of the stem's primary stability prior to obtaining secondary stability through osteointegration. Moreover, an analysis of the formation of bone structures around the stem showed that the incorrect setting of a Metha-type stem may lead to the initiation of loosening.

Biomechanics of distal femoral fracture fixed with an angular stable LISS plate.

Fractures of the distal end of the femur are infrequent and constitute less than 1% of all fractures. Only 3% to 6% of femoral fractures occur at the distal end. The two groups most at risk of the said fractures are young men and older women. The aim of treatment of fractures of the distal femur is to restore normal function of the knee joint. The authors asked themselves whether, following fixation of a 33-C2 fracture (according to the AO classification) with a LISS plate, a rehabilitation program can be undertaken immediately after surgery with the implementation of active movements in the knee joint of the operated limb. In order to answer this question, we created a digital model of a fractured femur fixed with the LISS method. The model was subjected to loads corresponding to the loads generated during active lifting of a limb extended in the knee joint and during flexing of a limb in the knee joint to the 90° angle. Interfragmentary movement (IFM) is one of the key parameters taken into account in the treatment of bone fractures. It allows classification of the treatment in terms of its quality both from the mechanical and histological points of view. We analyzed interfragmentary movement in all fracture gaps. The largest recorded displacement reached in our model was 243 µm, which, in the light of the literature data, should not interfere with bone consolidation, and thus implementation of active movement in the operated knee joint (keeping in mind the simplifications of the experimental method used) is possible in the early postoperative period.

Mechanical properties of cervical dura mater.

The aim of the study was to determine experimentally the stress as strain function as well as the orthotropy and heterogeneity of porcine dura mater of the cervical spinal cord. Material was divided into groups based on the place of collection, considering the dorsal side and ventral side, specifying the number of cervical vertebra, and the direction of tension of the sample - longitudinal or circumferential. Experimental studies were conducted with the MTS Synergie 100 testing machine. The tensile test was performed for each sample at a speed of 2 mm/min until the sample's break. There were determined the characteristics of stress as a function of strain in particular samples. Distribution maps of the stress and strain values at the characteristic points were then drawn (the beginning and the end of the linear range of the stress-strain characteristic and the point corresponding to the complete sample damage) for each set of samples, taking account of their collection place and direction of tension. The results confirmed the orthotropy of mechanical properties of dura mater. Stress and strain differed also in the value at the height of each vertebra and exhibited diversification on the ventral side compared to dorsal one.

Finite element modelling of the cervical spinal cord injury -- clinical assessment.

The aim of the study was to evaluate the efficiency of Finite Element Method (FEM) modelling of the clinical cases of traumatic cervical spinal cord injury (SCI). The study population consisted of 28 patients suffering from traumatic cervical spine injury with (study group) and without (control) neurological deficits. A numerical simulation of the trauma event was performed, based on validated 3D FEM model. All the results obtained underwent statistical analysis. Statistically significant differences between both groups were found in severity of bony and neural structure damage as well as in stress and strain ratios. The highest values of tensile stress and deformation were noted in the sagittal (Y) axis. The maximum stress and strain were found in anterior spinothalamic, lateral spinothalamic and dorsal columns. It was also found that stress and strain in each segment and axis of the spinal cord model were positively correlated with the severity of the cervical spine injury (R-Spearman 0.39 to 0.64) and neurological symptoms of SCI (R-Spearman: 0.43 to 0.82). It is possible to create a clinical numerical model of the SCI with the use of FEM. The correlations between the mechanical force and neurological deficits show tendencies which require further studies based on an improved model and a greater number of patients.

Numerical model of the human cervical spinal cord--the development and validation.

The influence of mechanical load on the extent of nervous tissue damage in the spinal cord at the time of trauma is presently incontestable. Although numerical modelling cannot fully replace physical testing, it seems to be the perfect complement to experiments in terms of the analysis of such a complex phenomenon as traumatic spinal cord injury. Previous numerical models of the human cervical spinal cord have been limited by several factors: two-dimensional modelling, spinal cord geometry simplification and incomplete reflection of specific anatomical and biomechanical relations of the objects being modelled. The objective of this study was to develop and validate an accurate and universal numerical Finite Element Method (FEM) model of the human cervical spinal cord. Our survey focuses mainly on geometric, constraint and material aspects. Experimental validation was carried out based on a controlled compression of the porcine spinal cord specimens. Each stage of compression was simulated using the FEM model of the compressed segment. Our 3D numerical simulation results compared with experimental results show a good agreement. It is possible to use the developed numerical model of the human cervical spinal cord in the biomechanical analysis of the spinal cord injury phenomenon. However, further clinical evaluation is clearly justified.

Impact of some external factors on the values of mechanical parameters determined in tests on bone tissue.

The knowledge of the mechanical characteristics of bone structures is a prerequisite for theoretical, numerical, and experimental analyses describing the functioning of bone, which is a living organ. The description of the mechanical properties of bone tissue, such as conventional Young's modulus or strength, will enable the assessment of the degree of tissue degeneration through a comparison of the material properties of the examined bones to the properties determined for physiologically normal bones. However, the mechanical parameters published by different research centres often differ from each other by up to several hundred percent. These discrepancies arise primarily from the differences in the research methodology applied, and thus from many additional factors having a direct impact on the values of the mechanical parameters obtained in experimental tests. Therefore, in order to standardize and improve the interpretation of the results of measurements, we should develop universal criteria for the measuring conditions and quantify the impact of the factors being related to sample and measurements on the values obtained. In this paper, the authors present the dependence of some factors, i.e. the site and direction of sample excision as well as the rate and type of loading, on the values of the mechanical parameters. Those values were determined in experimental tests and the additional correlation coefficients proposed enabled an easier comparison of the results obtained with the values presented in the literature.

The biomechanical analysis of the traumatic cervical spinal cord injury using finite element approach.

According to up-to-date knowledge only mathematical modelling of the spinal cord injury (SCI) may provide real insight into a spatial location of the fields of the spinal cord mechanical strain generated by the injury. The purpose of our research was to correlate the results of Finite Element Analysis of SCI with the patient's neurological state and the injured spinal cord MR imaging. The 3D Finite Element Model of the cervical spinal cord and vertebral canal of a 21-year-old male patient was created. The moment of the injury was reconstructed by a simulation of the displacement of nonelastic structure to the light of vertebral canal. A detailed spatial analysis of the stress, strain and dislocation distribution was performed. The most injured region was the superficial zone of the white matter, the anterior part and central region of the grey matter, which was in good agreement with patient's neurological staus. An individualized Finite Element Model of traumatic SCI constructed by us enabled the evaluation of the influence of mechanical strain on a neurological condition of a patient. Further research will consist in validation of the results of endurance analyses based on a enlarged group of patients.

Computerized tomography evaluation of cortical bone properties in the tibia.

BACKGROUND: Tibia shaft fractures are among the most common fractures. However, the incidence of tibia fractures does not increase in the elderly. Osteoporotic bone resorption is possibly compensated for by periosteal apposition, which improves the mechanical properties of the bone. The purpose of this study was to identify a correlation between geometric indices of transverse bone scans and radiologic bone density based in CT studies. MATERIAL AND METHODS: 12 human cadaver tibiae were CT examined. Radiological density in Hounsfield units was evaluated in transverse bone scans taken at the junction of the distal and middle third of the tibial shaft. The average and maximum radiological density and area of cortical bone were evaluated. RESULTS: Average radiological bone density decreased with age. This was not accompanied by a corresponding increase in cortical bone area. CONCLUSION: The study did not provide direct confirmation that age-related osteoporotic bone resorption is compensated for by periosteal apposition.