In vitro bone cell response to tensile mechanical solicitations: is there an optimal protocol?

Bone remodelling is strongly linked to external mechanical signals. Such stimuli are widely used in vitro for bone tissue engineering by applying mechanical solicitations to cell cultures so as to trigger specific cell responses. However, the literature highlights considerable variability in devices and protocols. We reviewed here the major biological, mechanical and technical parameters implemented for in vitro tensile loading applications. The objective was to identify which values were used most, and whether there was an optimal protocol to obtain a functional tissue-engineering construct. First, we showed that a shift occurred from fundamental comprehension of bone formation, to its application in rebuilt tissues and clinical fields. Despite the lack of standardised protocols, consensual conditions relevant for in vitro bone development, in particular cell differentiation, could be highlighted. Culture processes were guided by physiological considerations, although out-of-range conditions were sometimes used without implying negative results for the development of rebuilt tissue. Consensus can be found on several parameters, such as strain frequency (1 Hz) or the use of rest periods, but other points have not yet been fully established, especially synergies with other solicitations. We believe the present work will be useful to develop new tissue-engineering processes based on stretching.