Fracture Patterns From Three-Point Bending of Sheep Femora

Fracture patterns of long bones broken under various loading conditions have been well documented [1,8]. Of particular interest in the current study was the wedge or “butterfly” type fracture that occurs as a result of bending forces on a bone. Butterfly fractures generally consist of a characteristic “Y” shape across the long axis [1,7]. While no studies were found that examine the mechanism of such a fracture, it is generally accepted and widely published that the bottom of the Y fracture is found on the tensile side and the top split portion occurs on the compressive side of the neutral axis (Figure 1). This phenomenon is explained using basic solid mechanics principles. Since the tensile strength of bone is 133 MPa compared to a compressive strength of 193 MPa [6], bending initiates a transverse fracture on the tensile side. However, across the neutral axis, compressive forces dominate and tensile failure is thought not to continue. Since the shear strength of bone is 51.6 MPa [10], less than half the compressive strength, the bone appears to fail along the planes of maximum shear stress at 45° to the transverse split [3].