Internal loads in the human tibia during gait
Introduction
Internal loads acting in vivo in long bones under daily activities are of special interest in fracture healing research. In vitro tests of fixation devices mounted on bone fragments are often used to analyze the primary as well as the long term stability of the bone–implant complex (BIC) (Ahmad et al., 2007, Stoffel et al., 2007). To decide whether the in vitro determined load to failure of the tested BIC is critical, the internal loads have to be known.
Telemetrizised implants were successfully used to determine the internal loads acting inside the implant during fracture healing based on the deformations of the fixation device (Schneider et al., 2001, Seide et al., 2004). Often, the fracture site could carry some of the load, which leads to smaller deformations in this device and therefore to an underestimation of the internal loads. The effect of this load sharing mechanism is amplified in the late healing stage due to the newly formed bone in the healing region. Due to this underestimation, which is difficult to quantify, measured loads are always patient specific and therefore not suitable for describing the internal loads in general.
Beside these in vivo measurements, inverse dynamic models were used to calculate the musculoskeletal loads during daily activities (Brand et al., 1994, Glitsch and Baumann, 1997, Heller et al., 2001, Stansfield et al., 2003, Taylor et al., 2004). Often, the musculoskeletal loads are expressed by the calculated muscle forces and joint reaction loads, which were used to apply physiological-like loading conditions on Finite-Element (FE) models (Duda et al., 1998, Duda et al., 2001, Duda et al., 2002, Gomez-Benito et al., 2007). For performing in vitro tests and discussing the results thereof, there is a need to know the internal loads, expressed as internal forces and moments. However, there is no information about these forces and moments along the human tibia during gait in the literature.
The development of a numerical musculoskeletal model is very complex and time consuming. Therefore we modified a muskuloskeletal model of the human lower extremities, which was developed and provided under public domain by the AnyBody Research Group (repository 6, www.anybody.aau.dk ). The aim of our study was to determine the internal forces and moments along the longitudinal axis of the human tibia during gait.
Section snippets
Methods
The musculoskeletal model used consists of seven rigid bodies representing the bones as well as the surrounding tissues of the lower human extremities, i.e. pelvis, thigh, shank and foot of both legs (Fig. 1). The hip joints were modeled as spherical joints with three rotational degrees of freedom (DoF). Both knee joints were simulated as hinge joints with one rotational DoF whereas the ankle joints had two rotational DoF according to a cardan joint. There were 35 Hill type muscles (Winters,
Results
The calculated resultant hip contact force as well as the axial force on the tibial plateau showed loading peaks at the beginning ( ) and the end ( ) of the stance phase (Fig. 2). The peak values were 4.4 times bodyweight (BW) for the hip contact force and 3.3 BW for the knee contact force. The highest internal force in the tibia always acted in the longitudinal direction ( ) during gait with the extreme value at the end of the stance phase (Fig. 3 and Supplementary data
Discussion
In this study we successfully modified an existing musculoskeletal model to determine the internal forces and moments in the human tibia during gait. These internal loads were strongly dependent on the time point of the gait cycle as well as the location along the tibial axis. The highest internal loads occurred in the late stance phase. At this time point of the gait cycle, the centre of pressure was located in the medio-ventral position of the foot and the GRF was in the medial, dorsal and
Conclusions
This study provides for the first time all internal forces and moments along the human tibial axis during gait. The results of this study could be used to define physiological loading scenarios for in vitro testing of osteosynthesis implants with respect to the location of the fracture along the tibial axis. Therefore, the mechanical behaviour of current implant designs could be optimized and implant failures in vivo might be avoided.
Acknowledgements
This work was supported by Stryker Osteosynthesis. We would like to thank Dr. Blakytny for his comments on this manuscript.
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