gms | German Medical Science

Objectives: Formation of fracture callus during secondary bone healing is promoted by mechanical stimulation, i.e. the interfragmentary motion between the fractured bone fragments. In clinical settings, the extent of mechanical stimulation depends on (1) the stiffness of the fixation construct and (2) the degree of physiological loading. The goal of this preclinical study is to investigate the impact of the amount of postoperative activity - represented by the number of stimulation cycles applied in a day - on the formation of fracture repair tissue.

Methods: Eight Swiss White Alpine sheep received an experimental defect in the right tibia. The defectincorporated two partial osteotomies directed perpendicularly to each other, thus creating a floating bone fragment in the shape of a wedge (adapted from Hente and Perren [1]. The defect was then instrumented with an active external fixator that moves the bone fragment, thereby inducing mechanical stimulation to the fracture gap. The movements of the fixator were regulated by our in-house developed controller that enables the programming and autonomous execution of different stimulation protocols. For each animal, the stimulation was applied in ten batches evenly distributed over the day from 9 am and 9 pm. No stimulation was applied overnight. All animals were subdivided into four groups that differed in the number of cycles per batch – i.e. 1, 10, 100, 1,000 – resulting in the total number of administered stimuli per day being 10, 100, 1,000, and 10,000 respectively. At the end of the five-week study period, we evaluated the callus volume that was measured on the high-resolution computer tomography scan.

Results and conclusion: The surgical intervention was successful in all animals. Formation of fracture callus has been observed in all animals and ranged from 0.5 cm³ to 6.74 cm³. The largest callus volume (6.74 cm³ and 5.18 cm³) was observed for the two animals that received 10,000 loading cycles in a day, whereas the average of the animals pooled from the three other groups was equal to 1.57±1.09 cm³.

Our preliminary findings show that a high number of loading cycles can promote callus formation substantially. In previous experiments conducted with the tilting-wedge model (Hente and Perren [1]), continuous application of 10,000 loading cycles (8.6 sec rest between stimuli) inhibited fracture healing. Our data suggests that with more physiological temporal distribution - with overnight resting and resting between batches – even a high number of cycles can effectively promote healing. Further experimental work is currently ongoing to complete the study groups. An in-depth understanding of the effect of postoperative activity on fracture healing will help in evidence-based refining of the rehabilitation protocols for fracture patients.