According to the state of the art intramedullary load supports, e.g. intramedullary nails, locked intramedullary nails and so on are implanted in the marrow space by means of hammer strokes or through axial pressure onto the nail end. The forces generated by such implantation methods are directly transferred to the bone through frictional and normal forces. The magnitude of the generated forces depends on many factors, including the diameter of the marrow space relative to the diameter of the nail as well as on the location of the entrance of the nail relative to the longitudinal axis of the marrow space.
The marrow space may be intraoperatively bored by means of a reamer as far as its diameter is 1-2 mm greater than the diameter of the nail. This reaming procedure reduces the forces to be applied during implantation of the nail, but has deleterious effects on endosteal blood flow.
A reduction of the forces generated during implantation or explantation of the nail by using an entry location which is in line with the center of the medullary space and which allows a nail insertion parallel to the longitudinal axis of the bone is problematic. This problem arises in case of intramedullary nailing of most of the tubular bones since the longitudinal axis of these tubular bones extends either through a bone joint (knee joint, shoulder joint, elbow joint) or through critical soft tissue like attachments of tendons or blood vessels (hip joint). A curved entry channel from the surface of the bone to the intramedullary space allows to pass articulate surfaces or critical soft tissue. In order to match the intramedullary space as well as the entry channel the respective intramedullary nails are slightly angled in the region of the nail end (Herzog's curvature in case of tibia nails, curved nail end of humerus nails, double curvature in case of femur nails and so on).
During implantation the nail must be elastically deformed due to the entry channel being eccentric to the axis of the marrow space until the curvature of the nail end which corresponds to the entry channel immerges in the bone. The force being necessary to elastically deform the intramedullary load support so far that it may compensate the deviation of the shape of the marrow channel entirely acts on the bone. Load peaks arise at the nail insertion site as well as in the region where the nail tip reaches the inner wall of the bone.
USPTO Backgrounds
USPTO Backgrounds