Fracture-induced pain-like behaviours in a femoral fracture mouse model.

This study is the first comprehensive characterisation of the pain phenotype after fracture using both evoked and naturalistic behaviours in adult male and ovariectomised female mice. It also shows that an anti-nerve growth factor (NGF) therapy could be considered to reduce pain after fracture surgery. INTRODUCTION: Bone fractures are common due to the ageing population and very painful even after healing. The phenotype of this pain is still poorly understood. We aimed to characterise it in a femoral fracture model in mice. METHODS: We employed both adult male, and female ovariectomised (OVX) mice to mimic osteoporotic fractures. Mice underwent a unilateral femoral fracture maintained by an external fixator or a sham surgery. Pain behaviours, including mechanical and thermal sensitivity, weight bearing and LABORAS, were measured from baseline to 6 weeks after fracture. The effect on pain of an antibody against nerve growth factor (anti-NGF) was assessed. Changes in nerve density at the fracture callus were analysed by immunohistochemistry. RESULTS: Following surgery, all groups exhibited high levels of invoked nociception. Mechanical and thermal hyperalgesia were observed from 1 week after surgery, with nociceptive sensitization in the fracture group maintained for the 6 weeks, whereas it resolved in the sham group after 3 weeks. OVX induced reduction in pain thresholds, which was maintained after fracture. The frequency of naturalistic behaviours did not change between groups. Anti-NGF administered before and weekly after surgery alleviated fracture-induced mechanical nociception. The density of nerve fibres in the fracture callus was similar in all groups 6 weeks after surgery. CONCLUSIONS: Fractures in rodent models are highly painful in both sexes. This pain-like phenotype is prolonged and should be routinely considered in fracture healing studies as it can affect the study outcome. The anti-NGF alleviates fracture-induced mechanical pain.

Osteoarthritis-related nociceptive behaviour following mechanical joint loading correlates with cartilage damage.

OBJECTIVE: In osteoarthritis (OA), the pain-structure relationship remains complex and poorly understood. Here, we used the mechanical joint loading (MJL) model of OA to investigate both knee pathology and nociceptive behaviour. DESIGN: MJL was used to induce OA in the right knees of 12-week-old male C57BL/6 mice (40 cycles, 9N, 3x/week for 2 weeks). Mechanical sensitivity thresholds and weight-bearing ratios were measured before loading and at weeks one, three and six post-loading. At these time points, separate groups of loaded and non-loaded mice (n = 12/group) were sacrificed, joints collected, and fur corticosterone levels measured. µCT analyses of subchondral bone integrity was performed before joint sections were prepared for nerve quantification, cartilage or synovium grading (scoring system from 0 to 6). RESULTS: Loaded mice showed increased mechanical hypersensitivity paired with altered weight-bearing. Initial ipsilateral cartilage lesions 1-week post-loading (1.8 ± 0.4) had worsened at weeks three (3.0 ± 0.6, CI = -1.8-0.6) and six (2.8 ± 0.4, CI = -1.6-0.4). This increase in lesion severity correlated with mechanical hypersensitivity development (correlation; 0.729, P = 0.0071). Loaded mice displayed increased synovitis (3.6 ± 0.5) compared to non-loaded mice (1.5 ± 0.5, CI = -2.2-0.3) 1-week post-loading which returned to normal by weeks three and six. Similarly, corticosterone levels were only increased at week one post-loading (0.21 ± 0.04 ng/mg) compared to non-loaded controls (0.14 ± 0.01 ng/mg, CI = -1.8-0.1). Subchondral bone integrity and nerve volume remained unchanged. CONCLUSIONS: Our data indicates that although the loading induces an initial stress reaction and local inflammation, these processes are not directly responsible for the nociceptive phenotype observed. Instead, MJL-induced allodynia is mainly associated with OA-like progression of cartilage lesions.