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Poroviscoelastic models have been widely employed to the modeling of hydrated biological tissues, since they allow to investigate the biomechanical responses associated with the interstitial fluid flow. Such problems present strong physical couplings arising from material and geometrical nonlinearities. In this regard, the present study investigates the numerical performance of five biphasic solution algorithms within the context of soft biological tissues: monolithic, drained, undrained, fixed-strain, and fixed-stress scheme. To this end, two classical tests were studied within a finite element framework: confined and unconfined compression tests. Since these tests behave differently in terms of biphasic coupling, the sensitivity of different permeability values and time increments to the algorithms' performance were assessed. The results highlight that iterative techniques perform well over the monolithic one for weak coupling cases but suffer from lack of convergence when the coupling strength increases. This means that, in contrast to what is recommended in the vast majority of geomechanics papers, the iteratively-coupled schemes are not always well-suited methods for problems related to soft biological tissues mechanics. The monolithic scheme thus emerges as the most reliable choice to solve biphasic problems in a biomechanics context, specifically when high coupling strength problems and small time increments take place.

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This paper proposes a new testing method based on the toggle effect under transverse loads (cranial-caudal) to investigate the loosening potential of pedicular screw designs. A three-step in vitro testing procedure was developed to mimic the loosening mechanism of pedicular screws. Firstly, the pedicular screw of a certain design is inserted into a bone substitute model specifically designed for the test. Secondly, a controlled cyclic cranial-caudal loading is applied transversally to the longitudinal axis of the screw for three ascendent load levels (staircase) by a pre-determined number of load cycles. Lastly, each pedicular screw is adjusted and submitted to axial pull-out quasi-static testing. The results are used to calculate a loosening index that, together with statistical analysis, indicates the potential for loosening of the specific design evaluated. The proposed testing method effectively provides a simulated environment to evaluate the loosening potential of pedicular screw designs. The proposed loosening index calculation may be used to compare different pedicular screw designs. The proposed methodology was verified as a valuable tool to investigate the influence of the cranial-caudal loads on pedicular screw behavior. It offers a new alternative for use in pre-clinical studies on the loosening potential of pedicular screw designs.