Individualized cyclic mechanical loading improves callus properties during the remodelling phase of fracture healing in mice as assessed from time-lapsed in vivo imaging

Abstract

Fracture healing is regulated by mechanical loading. Understanding the underlying mechanisms during the different healing phases is required for targeted mechanical intervention therapies. Here, the influence of individualized cyclic mechanical loading on the remodelling phase of fracture healing was assessed in a non-critical-sized mouse femur defect model. After bridging of the defect, a loading group (n = 10) received individualized cyclic mechanical loading (8–16 N, 10 Hz, 5 min, 3 × /week) based on computed strain distribution in the mineralized callus using animal-specific real-time micro-finite element analysis with 2D/3D visualizations and strain histograms. Controls (n = 10) received 0 N treatment at the same post-operative time-points. By registration of consecutive scans, structural and dynamic callus morphometric parameters were followed in three callus sub-volumes and the adjacent cortex showing that the remodelling phase of fracture healing is highly responsive to cyclic mechanical loading with changes in dynamic parameters leading to significantly larger formation of mineralized callus and higher degree of mineralization. Loading-mediated maintenance of callus remodelling was associated with distinct effects on Wnt-signalling-associated molecular targets Sclerostin and RANKL in callus sub-regions and the adjacent cortex (n = 1/group). Given these distinct local protein expression patterns induced by cyclic mechanical loading during callus remodelling, the femur defect loading model with individualized load application seems suitable to further understand the local spatio-temporal mechano-molecular regulation of the different fracture healing phases.

Tissue-Level Regeneration and Remodeling Dynamics are Driven by Mechanical Stimuli in the Microenvironment in a Post-Bridging Loaded Femur Defect Healing Model in Mice

“… (A) Femur defect loading was performed by an electromagnetic actuator, a specially designed holder, and an external fixator ( Wehrle et al, 2021 ). (B) Three mask regions at week 0 of the femur defect regions.…”

“…In addition, we have not performed a traditional intergroup statistical analysis as this study aims to move toward the analysis of mechano-regulatory relationships as a continuum and not limited by simple threshold-based analyses (within the limitation of the imaging modality). Such conventional loading experiment outcomes, highlighting bone volume changes and the impact of loading, are well described by Wehrle et al (2019a ); Wehrle et al (2021 ). A final limitation that is worth noting is the lack of inclusion of the effect of the microstructure on mechanical stimuli.…”

“…Another limitation is the description of the boundary conditions. Here, we made use of a simple superposition of a uniaxial load and a bending load derived from geometries and loading parameters of the external fixator, while the fixator–bone arrangement is under a dynamic load and hence, the mechanical response is dynamic, the range of mechanical stimuli reflecting that within the literature ( Duda et al, 1998 ; Meakin et al, 2014 ; Razi et al, 2015 ; Liu et al, 2018 ; Wehrle et al, 2021 ). We, therefore, consider this static analysis sufficient for this study.…”

“…Details regarding animal experiments and mechanical-loading protocol are mentioned by Wehrle et al (2021 ) and Paul et al (2021 ). All animal procedures were approved by the relevant authorities (license number: 36/2014, Kantonales Veterinäramt Zürich, Zurich, Switzerland).…”

Effects of Exercise or Mechanical Stimulation on Bone Development and Bone Repair

“…Mechanical loading enhanced the effectiveness of BMP2 in promoting bone regeneration [ 69 ]. Using time lapse in vivo imaging, this research indicated that cyclic mechanical loading significantly increased the volume of the mineralized callus of the defective bone during the remodeling phase, which is associated with the regulation of Wnt signaling [ 70 ].…”