STAGES OF HEALING

Inflammation

The stage of inflammation begins at the time of injury when bleeding at the fracture site from the bone and surrounding soft tissue causes hematoma formation. The organization of the fracture hematoma is one of the earliest stages of fracture repair. The damaged cells release inflammatory mediators that cause local edema and attract inflammatory cells. Inflammatory mediators promote local angiogenesis, and as the process progresses fibroblasts and chondrocytes are recruited that begin to form callus. The type of healing that occurs, and the rate at which it does, is dependent on intrinsic characteristics of the bone based on location and on extrinsic factors such as the method of treatment.

Repair—Formation of Soft Callus

The fracture hematoma is replaced with granulation tissue containing many of the cells responsible for healing at the fracture site. The development of soft callus involves the early formation of external bridging callus as well as the late formation of medullary callus. Soft callus formation is characterized by vigorous mitotic and metabolic activity and may at times be mistaken for a low-grade connective tissue malignancy.

Repetitive micromovement at the fracture site is an important mechanical stimulus for the formation of soft callus. The soft callus is composed of fibrous tissue, cartilage, and woven bone and provides mechanical scaffolding for the formation of hard bony callus, which stabilizes and eventually bridges the fracture gap. Despite the intense angiogenesis that accompanies soft callus formation, oxygen tension at the fracture site is low and pH is acidic. The callus formed at the periphery is harder and that formed in the central region is composed of more cartilage and fibrous tissue. The intense cellularity of the soft callus far outstrips the increased oxygen supply stimulated by angiogenesis. If the vascular supply is interrupted early in fracture healing, the regenerative response is impaired, preventing normal fracture repair.

Repair—Formation of Hard Callus

The transition from soft callus to hard callus occurs 3 to 4 weeks into the fracture healing process with the appearance of calcified cartilage, and it continues until the bone ends are firmly united. The process of hard callus formation mimics similar events in the normal growth plate. Calcified cartilage provides a scaffold for osteoblasts and for the subsequent deposition and mineralization of bone matrix. Primitive fiber bone at the fracture site is eventually transformed into normal lamellar bone in both medullary and external bridging callus. Blood supply and oxygen tension at the fracture site continue to increase during this stage.