Cellular composition of the initial fracture hematoma compared to a muscle hematoma: a study in sheep.

Bone fracture leads to a cycle of inflammation, cellular migration, and proliferation to restore tissue integrity. Immune cells at the site of injury are involved especially in the early phase of the healing process, but little is known about the cells present in the initial fracture hematoma. The hypothesis of this study was that the cellular composition in a fracture hematoma differs from that found in a muscle hematoma and that these divergences get more pronounced over time. By using a reproducible osteotomy model and muscle trauma in sheep the distributions of the immune cell subpopulations were evaluated 1 and 4 h after surgery. The cell amount within the first 4 h increased in both hematoma. The number of dead cells was higher in the muscle hematoma. One hour postoperatively the initial fracture hematoma revealed a lower granulocyte percentage compared to the muscle hematoma. The ratio of T helper to cytotoxic T cells was higher in the fracture hematoma compared to the muscle hematoma at both investigated time points. B cell percentage increased in the fracture but not in the muscle hematoma from 1 to 4 h. This is the first study that compares the immune cell subpopulations of a fracture and muscle hematoma. (c) 2009 Orthopaedic Research Society.