Abstract
Experimental studies report that ligaments of the ankle joint are prestrained. The prestrain is an important aspect of modern biomechanical analysis, which can be included in the models by: applying symmetrical, arbitrary prestrains to the ligaments, assuming a strain-free location for the joint or by using experimental prestrain data. The aim of the study was to comparatively analyze these approaches. In total, 4 prestraining methods were considered. In order to do so, a symmetrical model of the ankle with six nonlinear cables and two sphere–sphere contact pairs was assumed. The model was solved in statics under moment loads up to 5 Nm. The obtained results showed that the arbitrary prestrains caused an unbalanced load for the model at rest, and in turn modified its rest location in an unpredictable way. Due to the imbalance, it was impossible to enforce the assumed prestrains and thus cartilage prestrain was required to stabilize the model. The prestraining had a significant effect on the angular displacements and the load state of the model. The findings suggest that the prestrain values are patient specific and arbitrary prestrains will not be valid for most models.
Highlights
A mechanical structure is in a state of prestrain if its elements remain strained when the system is at rest and no external loads are applied on it [1]
The model was solved under static conditions based on equilibrium equations [10], which were composed from forces/moments generated by the cables and the contact pairs, as well as an external moment acting on TCS
The model experienced a shift in the rest location due to the imbalance of internal loads generated by the prestrained ligaments at the neutral location
Summary
A mechanical structure is in a state of prestrain if its elements remain strained when the system is at rest and no external loads are applied on it [1] This phenomenon occurs in most human body joints and can be visually observed when a joint’s ligaments retract after excision from the body. The non-symmetrical distribution of ligaments in the body joints could prove to be difficult to model with the 2.0% prestrain approach. The second option is to assume prestrain values based on the available experimental results While this approach accounts for the asymmetrical ligament placement, due to the lack of complete data sets, the experimental data are often combined with arbitrary values, as in [9] for the elbow joint. For the sake of completeness, it is worth noting that the prestrain can be indirectly addressed by numerical estimation [15]
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