Background <p>Clinically, fracture healing is typically monitored though serial radiographs. Specifically, callus development (growth and mineralization) is an indicator of healing and associated with local mechanical strain. However, a sustainable relationship between mechanical conditions and the respective healing progress has not been shown so far.</p> Material and methods <p>One hundred sixty-six patients with extra-articular lower-limb fractures treated by osteosynthesis plates or intramedullary nails were included. Callus formation (visible area in X-ray relative to bone shaft) and quality (image intensity relative to the cortex) were measured by consecutive X-ray analyses as well as the modified Radiographic Union-Score for Tibia (mRUST) during follow-up. Corresponding load- and fixation-matched finite element analysis (FEA) modelling was developed for tibia or femur loading as well as plate or intramedullary nail fixation. Mechanical strains (medially, laterally, dorsally, anteriorly) were evaluated from FEA and compared to the progress in X-ray callus formation and quality to perform a correlation analysis between observed callus formation and simulated local mechanical strains.</p> Results <p>For femoral fractures, callus size was largest dorso-medially (1.41 ± 1.57 cm²/cm and 1.18 ± 1.11 cm²/cm at 180 ± 45&#xa0;days post-surgery) while largest callus formations were found laterally in tibial fractures (0.75 ± 0.49 cm²/cm at 365 ± 45&#xa0;days post-surgery). These locations of maximal callus size in femur and tibia matched the locations of extreme principal strains from FEA. In femur, callus density increased steadily and exceeded cortex density at 365 ± 45&#xa0;days post-surgery. For tibia, no such clear trend was observable. While initially showing a similar increase in callus bridging score mRUST, increase over 2&#xa0;years was 48% higher for the tibial fractures compared to femoral fractures. While principal strains correspond to increases in early callus formation in both femur and tibia (Kendall-Tau-b: <i>p</i> = 0.021 for volumetric strain at 90 ± 45&#xa0;days post-surgery), shear strains are consistently associated with less callus formation (Kendall-Tau-b: <i>p</i> = 0.048 for volumetric/shear strain associated with callus size*density at 365 ± 45&#xa0;days post-surgery).</p> Conclusions <p>Callus formation during bone healing may be associated with local mechanical strain in lower limb fractures within a clinically relevant cohort including different fracture locations and fixation types. Shear strain at the fracture site appeared to be associated with reduced quality callus formation, whereas principal strain was observed to correlate with increased early callus formation. The presented methodology may have potential as a predictor of healing processes and could help identify mechanically challenging fracture fixation settings.</p> Level of evidence <p>II.</p> Trial registration <p>Ethical approval was obtained from the local ethics board to this retrospective study design (EA4/099/24).</p>