Fixation and Stability of Hardware and Fractures

The proper function of orthopedic hardware requires that a constant configuration is maintained between bone and hardware when external forces are applied. This constant configuration is referred to as fixation. Stability is the degree of fixation of a bone-hardware construct. An unstable construct allows motion to occur at bone-hardware or bone-bone interfaces which can result in delayed fracture healing or failure of a joint replacement. The choice and positioning of orthopedic hardware as well as the condition of the underlying bone are major determinants of the stability of a fixation.

The stability of a fixation is relative to the type and magnitude of the applied forces. A fixation may be stable to one type of force but not to another. Forces or stresses can be categorized into nonrotational and rotational types along the three spatial axes at a fracture/hardware implant site. To illustrate these forces, consider a transverse fracture of the femoral diaphysis. Nonrotational forces consist of axial forces (compressive/loading forces and tensile/distraction forces) directed along the diaphysis or perpendicular to the fracture plane. Shear forces act in the plane of the fracture perpendicular to the diaphysis producing horizontal displacement of the distal femur. Rotation of the femur distal to the fracture can occur about the long axis of the femur or along two axes perpendicular to the diaphysis at the fracture site. Rotation of the distal femur about the diaphyseal axis is usually referred to simply as rotation. A pure axial rotation produces no angulation at the fracture site. Rotation about the two other axes which lie in the horizontal plane produces angulation of the fracture or bending of hardware at the fracture site.

For several types of orthopedic hardware implants, bone grafting can be employed to enhance fixation. When reading x-rays of hardware implants, it is important to look for the presence of bone grafts. Bone grafts can be of autologous or heterologous (allograft) origin and can consist of a bone segment, bone fragments, or morcellized bone. The most recent innovations include unique materials incorporating bone growth factors that increase the speed and degree of graft incorporation to native bone.

In any orthopedic procedure where bone growth or remodeling is an important component of healing, such as with fracture repairs, osteotomies, arthrodeses, and bone grafting, there must be an adequate blood supply to the involved boney structures. In addition, adequate soft tissue support must be present for proper healing, especially with complex fractures.