Influence of Age on Fracture Healing in Young and Middle-Aged Mice in a Proximal Femur Fracture Model

Osteoporotic hip fractures are a considerable cause of pain and disability particularly among the elderly. Osteoporosis causes loss of bone stability, which in turn leads to an increased risk of fractures especially in metaphyseal bone. Moreover, the body's capacity for healing is diminished, resulting in prolonged recovery times following these fractures. The underlying mechanisms are not fully elucidated yet. Therefore, the aim of the current study was to determine the influence of age on a murine proximal femur fracture model in young and middle-aged mice and whether the model could be suitable to study the molecular mechanisms of age-related delayed fracture healing. A proximal femur fracture was performed in 12- and 52-week-old male C57BL/6 J mice. Multiplex cytokine analysis was conducted at 6 h and 24 h after fracture. µCT analysis, histomorphometry and immunostainings were performed at 14 and 21 days. Furthermore, spatial transcriptomics was performed at 14 days after fracture. Our data revealed an osteopenic phenotype in intact tibiae in middle-aged animals. Moreover, fracture callus size, cartilage formation, expression of late chondrogenic factors and osteoblast and osteoclast numbers were significantly decreased in the fracture callus of middle-aged mice. This resulted in less bone formation. Our data further suggested increased presence of p21⁺ senescent cells and a higher expression of senescence-associated secretory phenotype (SASP, e.g.: IL-6, CCL-5 and CCL-2) in older mice, whereas CXCL12 expression was reduced in these animals. In conclusion, we could confirm a diminished healing capacity of metaphyseal hip fractures in mice that were 52 weeks old. This was accompanied by differences in gene and protein expression including SASP-factors.