The long bones grow by the process of endochondral ossification, which occurs at the growth plate. This process is regulated by biological factors and mechanical factors. The biological factors which contribute to endochondral ossification process are genes, hormones, nutrients etc. The mechanical factor is the load acting on the bone. The major forces on the bone are due to joint contact load and muscle forces, which induce stresses in the bone. Carter and Wong proposed in a theory that cyclic or intermittent octahedral shear stress promotes the bone growth and cyclic or intermittent hydrostatic compressive stress inhibits the bone growth. Previously this theory has been used to predict the morphological development of long bones, but with studies using simplified femur and growth plate models. Furthermore, the Carter and Wong theory has a limitation that it does not intrinsically incorporate the resulting growth direction.In the first study, the importance of a subject-specific growth plate over a simplified growth plate has been studied, and growth has been simulated using two different growth direction models: Femoral neck shaft deformation direction and minimum shear stress direction. This study favors the minimum shear stress growth direction model, as it is less sensitive to applied boundary condition than the femoral neck shaft deformation direction model.The second study aims to understand how different muscle groups affect the bone growth tendency. Subject-specific femur and growth plate models of able-bodied children were used. The muscle forces and associated hip contact force from specific muscle groups were applied, and neck shaft angle and femoral anteversion growth tendencies were predicted. This study indicated a tendency for reduction of neck shaft angle and femoral anteversion. Hip abductor muscle forces contribute most, and hip adductor muscle forces least, to bone growth rate.Accurate prediction of bone growth tendency and knowledge of the influence of different muscle groups on bone growth tendency may help in better treatment planning for children at risk of developing bone deformity problems.

Longitudinal growth of long bone occurs at growth plates by a process called endochondral ossification. Endochondral ossification is affected by both biological and mechanical factors. This thesis focuses on the mechanical modulation of femoral bone growth occurring at the proximal growth plate, using mechanobiological theories reported in the literature. Finite element analysis was used to simulate bone growth.

The first study analyzed the effect of subject-specific growth plate geometry over simplified growth plate geometry in numerical prediction of bone growth tendency. Subject-specific femur finite element model was constructed from magnetic resonance images of one able- bodied child. Gait kinematics and kinetics were acquired from motion analysis and analyzed further in musculoskeletal modelling to determine muscle and joint contact forces. These were used to determine loading on the femur in finite element analysis. The growth rate was computed based on a mechanobiological theory proposed by Carter and Wong, and a growth model in the principal stress direction was introduced. Our findings support the use of subject- specific geometry and of the principal stress growth direction in prediction of bone growth.

The second study aimed to illustrate how different muscle groups’ activation during gait affects proximal femoral growth tendency in able-bodied children. Subject-specific femur models were used. Gait kinematics and kinetics were acquired for 3 able-bodied children, and muscle and joint contact forces were determined, similar to the first study. The contribution of different muscle groups to hip contact force was also determined. Finite element analysis was performed to compute the specific growth rate and growth direction due to individual muscle groups. The simulated growth model indicated that gait loading tends to reduce neck shaft angle and femoral anteversion during growth. The muscle groups that contributes most and least to growth rate were hip abductors and hip adductors, respectively. All muscle groups’ activation tended to reduce the neck shaft and femoral anteversion angles, except hip extensors and adductors which showed a tendency to increase the femoral anteversion.

The third study’s aim was to understand the influence of different physical activities on proximal femoral growth tendency. Hip contact force orientation was varied to represent reported forces from a number of physical activities. The findings of this study showed that all studied physical activities tend to reduce the neck shaft angle and anteversion, which corresponds to the femur’s natural course during normal growth.

The aim of the fourth study was to study the hypothesis that loading in the absence of physical activity, i.e. static loading, can have an adverse effect on bone growth. A subject-specific model was used and growth plate was modeled as a poroelastic material in finite element analysis. Prendergast’s indicators for bone growth was used to analyse the bone growth behavior. The results showed that tendency of bone growth rate decreases over a long duration of static loading. The study also showed that static sitting is less detrimental than static standing for predicted cartilage-to-bone differentiation likelihood, due to the lower magnitude of hip contact force.

The prediction of growth using finite element analysis on experimental gait data and person- specific femur geometry, based on mechanobiological theories of bone growth, offers a biomechanical foundation for better understanding and prediction of bone growth-related deformity problems in growing children. It can ultimately help in treatment planning or physical activity guidelines in children at risk at developing a femur or hip deformity.