A rat muscle contusion model created by a modified drop-mass method using ultrasound

Aims Muscle contusion healing remains challenging because incomplete regeneration can lead to excessive scar formation, which may impair functional recovery. Therefore, reproducible animal models are important tools for investigating the healing process and evaluating potential therapeutic strategies. This study aimed to establish a reproducible muscle contusion model using a modified drop-mass method with ultrasound. Furthermore, the feasibility of using this model for molecular screening of muscle contusion healing was investigated. Methods A total of 70 male Wistar Hannover rats were initially enrolled in this study. Two rats were excluded because of tibial fractures after injury induction; therefore, 68 rats were included in the final assessments. In this model, the placement of the rat tibialis anterior muscle and the impactor tip was confirmed by ultrasound prior to dropping the weight. To evaluate the reproducibility of the injury, ultrasound measurements of muscle size, histological evaluation of muscle tissue structure, gene expression analysis of inflammation-related factors and macrophage markers, and muscle strength assessment were performed. Moreover, based on a comprehensive genetic analysis of samples collected from this model, molecules involved in the healing process of muscle contusions were screened. Results Abnormal ultrasound images, including hypoechoic areas, were observed in all the injured muscles. A peak increase in the maximum muscle cross-sectional area was observed 24 hours after injury, which subsequently decreased. Histological analysis revealed fibrous tissue at seven days post-injury, becoming more prominent at day 14. Muscle strength was reduced at three and seven days after the injury. Inflammation-related genes were markedly upregulated on the day of injury, while the expression of macrophage marker genes increased three days post-injury. Ribonucleic acid sequencing revealed gene clusters, including tissue kallikrein ( KLK1 ), with higher expression on day 7 than on day 3 post-injury. High expression of KLK1 was confirmed in the injured muscle tissue of model animals on day 7 after injury using quantitative polymerase chain reaction and immunohistochemical staining. Conclusion A reproducible model of skeletal muscle contusion can be established using a modified drop-mass method guided by ultrasound. This model demonstrated potential utility for exploring the molecules involved in the healing process of muscle contusions. Cite this article: Bone Joint Res 2026;15(7):767–774.