Development of a stable closed femoral fracture model in mice.

BACKGROUND: Mice have become of increasing interest as experimental model for fracture studies. Due to their small size, most studies use simple pins for fracture stabilization, although insufficient rigidity of fixation critically affects fracture healing. Herein, we studied whether longitudinal fracture compression by an intramedullary screw represents a standardized, stable osteosynthesis technique in mice, and whether it may accelerate fracture healing. MATERIALS AND METHODS: A micro-screw (MouseScrew) was constructed, allowing closed fracture stabilization without traumatizing surgery. Fracture stabilization was achieved by longitudinal compression, which was confirmed by biomechanical testing of osteotomized cadaver femora. Bone repair was analyzed histomorphometrically at 2 and 5 wk after surgery. RESULTS: Ex vivo analyses showed a significantly increased rotational and axial stiffness after screw stabilization (n = 8 each) compared with stabilization techniques using a conventional pin (n = 8 each) or a locking nail (n = 8 each). In the in vivo setting, 2 wk of screw stabilization (n = 8) demonstrated a significantly decreased fibrous tissue formation and an increased cartilage production compared with fractures stabilized by the locking nail (n = 8). After 5 wk callus consisted exclusively of bone in all animals studied without differences between the two stabilization techniques (n = 8 each). CONCLUSIONS: Because prolonged fibrous tissue formation indicates delayed fracture healing, we conclude that the increased stability of the fracture by the use of our newly developed MouseScrew accelerates initial bone repair. Further, this fracture model may represent an ideal tool to study bone repair in mice under conditions of stable fixation.