The forces exerted by the soft and hard tissues of the thigh represent a system in equilibrium. This complete balance of loads must be considered when the system components are examined independently. However, in many biomechanical analyses of the thigh, the femur is studied independently. To attain a thorough understanding of the load situation in the femur and its influencing factors a three-dimensional model was developed. Considering all thigh muscles, the weight and the contact forces at the hip, patella-femoral and knee joints the internal loads of the bone were calculated. The load variations were analyzed as a function of the gait phase, the muscle attachment locations and the muscle force magnitudes. Further, the effect of muscle wrapping around the femur was considered. In addition to the muscles, the influence of the forces exerted by ligaments and compartment pressures were nvestigated. Femoral deformations were computed using beam theory. Using the model it is demonstrated that the internal loads of the femur decrease as a result of muscle activity from proximal to distal at the hip and from distal to proximal at the knee. The load reduction can be up to 50% of the internal forces at the hip, depending on gait phase. Maximal forces are found between 40% and 60% of the stance phase, whereas maximal torsional moments occur shortly after heel strike. The bending moments are not, as commonly assumed, oriented in one direction. Depending on the chosen set of muscle and joint forces the maximal loads range from 2.1 to 5.2BW, moments range from 0.2 to 0.4BWm. The differences between these force data sets can be of a factor of 1.5 to 2. If the muscle attachment locations are modified, the internal loads vary by as much as 1.5BW and 0.5BWm (mm. vasti, mm. gastrocnemi). Small influences are reported for the muscle wrapping approach (< 0.1BW and < 0.005 BWm) and if ligament forces are considered (< 0.5BW and < 0.08BWm). The former may be caused by inaccuracies in the determination of the bony contour and the later by an underestimation of the forces measured in in vitro experiments. An approximation of the influence of compartment pressure was also performed. The results indicate that the pressure differences which exist in the three thigh compartments produce small shear forces. With the method used, the extreme role that the muscles play in balancing the loads within the femur is demonstrated. The results suggest to consider muscle forces in any experimental or analytical investigation concerning femoral loads. For computational techniques, such as finite element analysis, simply including the forces associated with the mm. abductores may not produce accurate results. The findings of this thesis suggest, at least muscles attaching within the investigated region should be incorporated, if e.g. micro-motion or implant-interfaces are analyzed. Simple cantilever bending has been used to represent a worst case loading scenario for the fatigue testing of hip endoprosthesis. However, based on the findings of this study, appropriate testing procedures should be developed to apply physiologic loads within the region they occur. For example, the load state present in the hip should not be used for the purposes of testing implants used in the diaphyseal or distal portion of the bone. Implants used in these regions must account for the forces exerted by the mm. adductores, mm. vasti and mm. gastrocnemi. Dissertationen: Duda, G. N. (1996): Influence of Muscle Forces on the Internal Loads in the Femur during Gait. Dissertation, Technische Universität Hamburg-Harburg, Shaker-Verlag, Aachen.
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