Fracture repair is influenced by the mechanical environment, particularly when cyclic loads are applied across the fracture
site. However, the specific mechanical loading parameters that accelerate fracture healing are unknown. Intact bone adaptation
studies show enhanced bone formation with pauses inserted between loading cycles. We hypothesized pause-inserted noninvasive
external loading to mouse tibial fractures would lead to accelerated healing. Eighty mice underwent tibial osteotomies with
intramedullary stabilization and were divided into four loading protocol groups: (1) repetitive loading (100 cycles, 1 Hz);
(2) pause/time-equivalent (10 cycles, 0.1 Hz); (3) pause/cycle-equivalent (100 cycles, 0.1 Hz); and (4) no load control. Loading
was applied daily for 2 weeks. Healing was assessed using histology, biomechanical bending tests, and microcomputed tomography.
The pause-inserted, cycle-equivalent group had a greater percentage of osteoid present in the callus cross-sectional area
compared with no-load controls, indicating more advanced early healing. The pause-inserted, cycle-equivalent group had a failure
moment and stiffness that were 37% and 31% higher than the controls, respectively. All three loaded groups had smaller overall
mineralized callus volumes than the control group, also indicating more advanced healing. At an early stage of fracture healing,
pause-inserted loading led to more histologically advanced healing.
One or more of the authors have received funding from the Orthopaedic Trauma Association (MJG), the Orthopaedic Research and
Education Foundation (MJG), and NIH Musculoskeletal Core Center P30AR046121 (TMW, MCHvdM).
Each author certifies that his or her institution has approved the animal protocol for this investigation and that all investigations
were conducted in conformity with ethical principles of research.