Fig. 1

schematic representation of longitudinal sections of bone segments implanted with variable fixation. In grey the bone plate, in green the forming bone callus and in blue the axial loading on the construct. Panel a - At the beginning of the treatment, in the inflammatory phase, the rigidity of the fixation and the displacements at cis and trans cortices are equivalent to those provided by standard locking screws [16]. These conditions are known to promote the formation of bone callus mainly at the trans-cortex. Panel b - VFLS sleeve degrades through superficial hydrolysis following the profile characteristic of resorbable polymers of the same family [17,18,19,20], namely progressive and spaced over time molecular weight loss, loss of mechanical properties and loss of mass. As the sleeve starts losing mechanical properties in the callus formation phase, the entire fixation progressively becomes more flexible. The displacements at cis and trans cortices progressively change, with a marked increase of interfragmentary displacements at the cis-cortex [16]. These changes aim at progressively straining the entire fracture gap in the “window of opportunity”, namely a strain range promoting the formation of bone callus