Reducing variance in a mouse defect healing model: Real-time Finite Element Analysis allows homogenization of tissue scale strains

Abstract

Mechanical loading allows both investigation into the mechano-regulation of fracture healing as well as interventions to improve fracture-healing outcomes such as delayed healing or non-unions. However, loading is seldom individualised or even targeted to an effective mechanical stimulus level within the bone tissue. In this study, we use micro-finite element analysis to demonstrate the result of using a constant loading assumption for all mouse femurs in a given group. We then contrast this with the application of an adaptive loading approach, denoted real time Finite Element (rtFE) adaptation, in which micro-computed tomography images provide the basis for micro-FE based simulations and the resulting strains are manipulated and targeted to a reference distribution. Using this approach, we demonstrate that individualised femoral loading leads to a better-specified strain distribution and lower variance in tissue mechanical stimulus across all mice, both longitudinally and cross-sectionally, while making sure that no overloading is occurring leading to refracture of the femur bones.

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

“…As the study applied the same load to all animals irrespective of healing progression, differences in the induced strains might have caused the high variability within groups. We addressed this by using our recently developed real-time micro-FE based loading approach 16 , which takes into account the individual strain distribution within the callus for determining individual loading settings at weekly post-operative intervals. So far, no study using a complete defect model assessed the effect of local cyclic mechanical loading (e.g.…”

“…From week 4 to week 7, individualized cyclic loading (8-16N, 10Hz, 3000cycles; 3x/week; controls -0N) was applied via the external loading fixator (Suppl. Fig 1B) based on computed strain distribution in the callus using animal-specific real-time micro-finite element (RT-microFE) analysis (for detailed description of methods see 16 . Briefly, after weekly micro-CT measurements of each animal, the images were pre-processed using threshold-binning to create a high-resolution multi-density FE mesh, which was then solved on a supercomputer within the same anaesthetic session for each mouse.…”

“…We aim to overcome current limitations by combining state-of-the-art time-lapsed in vivo imaging with a novel precisely controlled femur defect loading system in mice. By applying time-lapsed in vivo micro-CT and animal-specific real-time micro-finite element analysis (micro-FEA), the developed femur defect loading model allows scaling of loading settings based on strain distribution in the callus 16 , thereby considering individual differences in healing speed. To follow the healing progression in each animal, a recently developed longitudinal in vivo micro-CT based approach was applied, which was previously shown to capture the different healing phases and to discriminate physiological and impaired healing patterns 17 , without significant radiation-associated effects on callus properties 18 .…”

Development of a micro-CT mouse holder for a femur defect mouse model

“…The purpose is to hold a mouse, with the right femur fixated with a RISystem external fixator (RISystem, Landquart, Switzerland). This allows repeated measurements of the femur defect and prevents substantial movement between scans, facilitating image registration and longitudinal comparisons 2 . The holder is designed for use with a Scanco viva40 micro-CT scanner (Scanco, Brüttisellen, Switzerland) but with small modifications can be used with other in vivo scanners.…”

Application of subject-specific adaptive mechanical loading for bone healing in a mouse tail vertebral defect

“…Depending on the complexity and resolution deemed suitable, it is possible to do this immediately after imaging to limit unnecessary or additional handling and anesthesia in mice. This concept is concurrently presented in a mouse femoral defect model 19,20 , and in combination introduce the concept of real-time finite element (rtFE) analysis to describe this approach. Since mechanical loading is effective only within a certain strain window 21 , maintaining the applied loading within this window is critical.…”