Inflammation in Cancellous and Cortical Bone Healing

Sammanfattning: Fractures in humans most commonly occur near the joints, in the metaphyseal bone area mainly consisting of cancellous bone. Despite this, mainly cortical fractures, located in the diaphyseal bone area, have been studied in experimental models of bone healing. It is known from previous studies that the diaphyseal fracture is sensitive to anti-inflammatory treatment, while metaphyseal bone healing is more resistant. The aim of this thesis is to study the inflammatory response to bone trauma in cancellous and cortical bone. A flow cytometric method was established for the purpose of examining the cellular composition of the inflammatory process in models of bone healingIn paper I the cellular composition of metaphyseal bone healing was studied with flow cytometry. The proximal tibia was traumatized and then studied at day 1, 3, 5 and 10 afterwards and compared to healthy mice. The contralateral proximal tibia was also studied at the same time points to delineate the trauma site specific inflammation. A few changes could be noted that seemed specific to the trauma site in macrophage phenotype development. However, the cellular composition was similar at the trauma site and in the contralateral proximal tibia. This notion of a general skeletal response was confirmed with analysis of the humerus at day 5.In paper II a model of cortical bone healing apt for flow cytometry was developed and compared to cancellous bone healing. A furrow was milled along the femoral cortex and the healing bone tissue analyzed. The earliest time point that enough cells were present for flow cytometry was day 3. The cortical and cancellous model of bone healing was compared at day 3 and 5 to study how they evolve in comparison to each other. It was noted that they were similar in cellular composition at day 3, but had diverged at day 5. The cancellous model increased in neutrophilic granulocytes, whereas the cortical model increased in lymphocytes.In paper III the cancellous and cortical model were compared under experimental intervention of indomethacin. It is known that indomethacin leads to weakened biomechanical properties in cortical bone healing, but not in cancellous bone healing. The effect on cellular composition with indomethacin was studied with flow cytometry and the extracellular protein profile in the healing bone tissue with mass spectrometry. Unexpectedly, inflammatory monocytes were increased in the cortical model at day 3 with indomethacin, but otherwise the models were similar in cell composition at day 3 and 5. In mass spectrometry there was a large increase in detected proteins at day 3 in the indomethacin exposed cortical model, but otherwise the models were similar. This points to an early and model specific effect of indomethacin. The observed lack of indomethacin-induced effects in cancellous bone healing is in line with the previously noted lack of indomethacin-induced effects on bone weakening. The apparently increased inflammatory activity in the cortical model with indomethacin exposure at day 3 might indicate the healing process to be disturbed and not able to progress from the early proinflammatory state to a more anabolic, anti-inflammatory state.In paper IV the effect of macrophage depletion on healing of metaphyseal bone was studied. Clodronate was given for depletion at different time points prior to surgery and the pull-out force of a screw or tissue phenotyping of macrophages was performed a varying number of days after surgery. It was noted that metaphyseal bone healing was to a large extent inhibited by macrophage depletion up to two days after surgery, but not if depletion was done more than two days after surgery. Thus, macrophages seem to be most important during the first two days after trauma in cancellous bone healing. In summary this thesis provide insight to the natural development of bone healing. The findings emphasise that cancellous and cortical bone healing are different entities with differences in the inflammatory process leading to healing.