Catherine A. Dobson

Dynamic stochastic simulation of cancellous bone resorption.

A stochastic simulation of cancellous bone resorption was developed and applied to a simple two-dimensional lattice structure representing the vertebral body. The simulation is based upon the concept of a basic multicellular unit (BMU) where net resorption (-deltaB.BMU) is considered at bone/marrow surfaces. The cancellous bone structure is defined as a binary matrix with the size of the pixels corresponding to a square element of approximately 20 microm dimension. The simulation considers both the probability that any surface pixel will be activated into a BMU and, if activated, the length of the resorption cavity. The relationship between relative stiffness and density for the simulation was predicted by finite element analysis. The stochastic simulation was iterated eight times with the mechanical properties assessed after each stage. Perforation of a single trabeculae was first observed at step 2, the structure completely lacking connectivity and mechanical integrity by step 8. The slope of the stiffness-porosity graph was greater than unity for the first five steps, but thereafter approached zero because the structure had lost connectivity and effectively collapsed. The eight-step simulation was repeated five times and demonstrated that, although the stiffness/density relationships were similar at the extremes of density, the dependence of stiffness upon density varied. This clearly demonstrates the stochastic nature of the simulation upon cancellous bone structure, and is probably indicative of a significant dependence of mechanical integrity upon perforation effects.

Tibial impacts and muscle activation during walking, jogging and running when performed overground, and on motorised and non-motorised treadmills

PURPOSE To examine tibial acceleration and muscle activation during overground (OG), motorised treadmill (MT) and non-motorised treadmill conditions (NMT) when walking, jogging and running at matched velocities. METHODS An accelerometer recorded acceleration at the mid-tibia and surface EMG electrodes recorded rectus femoris (RF), semitendinosus (ST), tibialis anterior (TA) and soleus (SL) muscle activation during OG, MT and NMT locomotion whilst walking, jogging and running. RESULTS The NMT produced large reductions in tibial acceleration when compared with OG and MT conditions across walking, jogging and running conditions. RF EMG was small-moderately higher in the NMT condition when compared with the OG and MT conditions across walking, jogging and running conditions. ST EMG showed large and very large increases in the NMT when compared to OG and MT conditions during walking whilst SL EMG found large increases on the NMT when compared to OG and MT conditions during running. The NMT condition generated very large increases in step frequency when compared to OG and MT conditions during walking, with large and very large decreases during jogging and very large decreases during running. CONCLUSIONS The NMT generates large reductions in tibial acceleration, moderate to very large increases in muscular activation and large to very large decreases in cycle time when compared to OG and MT locomotion. Whilst this may decrease the osteogenic potential of NMT locomotion, there may be uses for NMTs during rehabilitation for lower limb injuries.

Algorithms for accurate rapid prototyping replication of cancellous bone voxel maps

A set of algorithms has been developed and evaluated for 3D and 21/2D rapid prototyping replication of 3D reconstructions of cancellous bone samples. The algorithms replicate a voxel map without any loss of fidelity, so as to increase the validity of the comparison of mechanical tests on the 3D reconstructed models with those predicted by finite element analyses. The evaluation is both in terms of algorithmic complexity and the resultant data set size. The former determines the feasibility of the conversion process, whereas the latter the potential success of the manufacturing process. The algorithms and their implementation in PC software is presented.

Finite Element Analysis of Simulations of Cancellous Bone Resorption

A stochastic simulation of the resorption of cancellous bone has been developed and integrated with a finite element model to predict the resultant change in structural properties of bone as bone density decreases. The resorption represents the net imbalance of osteoclast and osteoblast activity that occurs in osteoporosis. A simple lattice structure of trabecular bone is considered, with an examination of the lattice geometry and discretization indicating that just five trabeculae need to be modelled. The results from the analysis show how the mechanical properties of the cancellous bone degrade with osteoporosis and demonstrate how the method can be used to predict the relationships between stiffness and density or porosity.

Three Dimensional Stereolithography Models of Cancellous Bone Structures From μCT Data: Testing and Validation of Finite Element Results

Abstract Stereolithography (STL) models of complex cancellous bone structures have been produced from three-dimensional micro-computed tomography data sets of human cancellous bone histological samples from four skeletal sites. The STL models have been mechanically tested and the derived stiffness compared with that predicted by finite element analysis. The results show a strong correlation (R2 = 0.941) between the predicted and calculated stiffnesses of the structures and show promise for the use of STL as an additional technique to complement the use of finite element models, for the assessment of the mechanical properties of complex cancellous bone structures.

The effect of boundary constraints on finite element modelling of the human pelvis

The use of finite element analysis (FEA) to investigate the biomechanics of anatomical systems critically relies on the specification of physiologically representative boundary conditions. The biomechanics of the pelvis has been the specific focus of a number of FEA studies previously, but it is also a key aspect in other investigations of, for example, the hip joint or new design of hip prostheses. In those studies, the pelvis has been modelled in a number of ways with a variety of boundary conditions, ranging from a model of the whole pelvic girdle including soft tissue attachments to a model of an isolated hemi-pelvis. The current study constructed a series of FEA models of the same human pelvis to investigate the sensitivity of the predicted stress distributions to the type of boundary conditions applied, in particular to represent the sacro-iliac joint and pubic symphysis. Varying the method of modelling the sacro-iliac joint did not produce significant variations in the stress distribution, however changes to the modelling of the pubic symphysis were observed to have a greater effect on the results. Over-constraint of the symphysis prevented the bending of the pelvis about the greater sciatic notch, and underestimated high stresses within the ilium. However, permitting medio-lateral translation to mimic widening of the pelvis addressed this problem. These findings underline the importance of applying the appropriate boundary conditions to FEA models, and provide guidance on suitable methods of constraining the pelvis when, for example, scan data has not captured the full pelvic girdle. The results also suggest a valid method for performing hemi-pelvic modelling of cadaveric or archaeological remains which are either damaged or incomplete.

Sensitivity to model geometry in finite element analyses of reconstructed skeletal structures: Experience with a juvenile pelvis

Biomechanical analysis of juvenile pelvic growth can be used in the evaluation of medical devices and investigation of hip joint disorders. This requires access to scan data of healthy juveniles, which are not always freely available. This article analyses the application of a geometric morphometric technique, which facilitates the reconstruction of the articulated juvenile pelvis from cadaveric remains, in biomechanical modelling. The sensitivity of variation in reconstructed morphologies upon predicted stress/strain distributions is of particular interest. A series of finite element analyses of a 9-year-old hemi-pelvis were performed to examine differences in predicted strain distributions between a reconstructed model and the originally fully articulated specimen. Only minor differences in the minimum principal strain distributions were observed between two varying hemi-pelvic morphologies and that of the original articulation. A Wilcoxon rank-sum test determined there was no statistical significance between the nodal strains recorded at 60 locations throughout the hemi-pelvic structures. This example suggests that finite element models created by this geometric morphometric reconstruction technique can be used with confidence, and as observed with this hemi-pelvis model, even a visual morphological difference does not significantly affect the predicted results. The validated use of this geometric morphometric reconstruction technique in biomechanical modelling reduces the dependency on clinical scan data.

New insights into the biomechanics of Legg-Calvé-Perthes’ disease

Objectives Legg–Calvé–Perthes’ disease (LCP) is an idiopathic osteonecrosis of the femoral head that is most common in children between four and eight years old. The factors that lead to the onset of LCP are still unclear; however, it is believed that interruption of the blood supply to the developing epiphysis is an important factor in the development of the condition. Methods Finite element analysis modelling of the blood supply to the juvenile epiphysis was investigated to understand under which circumstances the blood vessels supplying the femoral epiphysis could become obstructed. The identification of these conditions is likely to be important in understanding the biomechanics of LCP. Results The results support the hypothesis that vascular obstruction to the epiphysis may arise when there is delayed ossification and when articular cartilage has reduced stiffness under compression. Conclusion The findings support the theory of vascular occlusion as being important in the pathophysiology of Perthes disease. Cite this article: M. Pinheiro, C. A. Dobson, D. Perry, M. J. Fagan. New insights into the biomechanics of Legg-Calvé-Perthes’ disease: The Role of Epiphyseal Skeletal Immaturity in Vascular Obstruction. Bone Joint Res 2018;7:148–156. DOI: 10.1302/2046-3758.72.BJR-2017-0191.R1.

Histomorphometric algorithms for the direct derivation of morphological indices of simulations of strain-adaptation in cancellous bone

A set of algorithms has been developed and applied, in order to quantify the morphology of structures produced by our work on computer simulation of cancellous bone remodelling. The set of algorithms is organised in three themes, based on the set of metrics they compute. The first one deals with the direct derivation of basic metrics such as the trabecular thickness, separation and number. The second deals with the area/volume distribution of bone and marrow, while the third one considers surface roughness and perimeter/area. The algorithms were tested on sample sections of cancellous bone and applied to a simulation of strain adaptation incorporating complex multi-axial loading. The metrics have shown to be independent of structural anisotropy, in contrast to some established methods that assume certain morphologies.

Relationships between walking speed, T-score and age with gait parameters in older post-menopausal women with low bone mineral density

BACKGROUND The gait patterns of women with low bone mineral density (BMD) or osteoporosis have not been thoroughly explored, and when examined, often studied in relation to falls and kyphosis. RESEARCH QUESTION The aim of this study was to investigate the relationships between gait parameters and comfortable, self-selected walking speed and BMD in older post-menopausal women with a broad range of T-scores (healthy to osteoporotic). METHODS 3D kinematic and kinetic data were collected from forty-five women mean (SD) age 67.3 (1.4) years during level walking at their preferred speed. Multiple regression analyses explored the explained variance attributable to speed, femoral neck T-score, and age. RESULTS The mean (SD) walking speed 1.40 (0.19) m·s-1 explained the variance in most temporal-spatial, kinematic and joint powers (R2 = 12-68%, P ≤ 0.01). T-score accounted for (R2 = 23%, P ≤ 0.001) of the shared explained variance in stride width. It also increased the explanatory power for knee flexion (R2 = 7%, P ≤ 0.05) and knee range of motion (R2 = 12%, P ≤ 0.01). Power absorption by the knee flexors in terminal swing (K4) was the only power burst resulting in significant slope coefficients for all predictor variables (R2 = 52 and 54%) (P ≤ 0.001) and (R2 = 68%, P ≤ 0.05). SIGNIFICANCE Speed alone explained most of the variance in the gait parameters, while speed and T-score combined increased the explanatory power of the regression models for some of the knee joint variables. Our findings demonstrated that older post-menopausal women, with a broad range of T-scores, are able to walk at comfortably fast speeds, generating gait patterns similar to those of younger women. The results also suggest that strengthening the hip abductor, knee extensor and flexor muscle groups may benefit the gait patterns of older postmenopausal women with low BMD.