orthopedic fastener
2008-08-07

When bones are fractured, it is often necessary to relationally fix them with pins, which pins may be either plain or threaded. In practice the bone pieces are suitably and properly arranged, and then a hole is made in the pieces and a pin or screw is inserted into this hole, fastening the pieces together. Subsequently, as required, additional holes are made, and additional fasteners are inserted, to preclude relative movement between the bone pieces. It is necessary for the fractured surfaces to be in close, well aligned contact, to facilitate healing. 
It will be apparent that the fasteners must be of the proper length; too short will result in weak fastening, and too long will result in a fastener protruding from the bone into the surrounding tissue. It will also be appreciated that a number of fasteners may be required. Since bones are of varying dimensions, these fasteners will be of various lengths. The surgeon therefore finds it necessary to measure the depth of most of these holes. All of these measurements are done during the procedure, in the operating room, with the patient under anesthetic, which conditions call for spending as little time as necessary. Since time is critical, and precision is also critical, the measurement of these holes must be quick, accurate and convenient.
To use the orthopedic fastener, a small incision or hole is made in the tissue and a hole is predrilled into the bone of sufficient diameter to accommodate the orthopedic fastener. Referring to FIG. 6, the orthopedic fastener 300 is inserted into the predrilled hole 303 in bone 301. The orthopedic fastener 300 is pressed in such that barbs 110 bite into soft tissue 302 to hold tissue 302 in dose proximity to the surface of bone 301. The pin 200 is then pulled proximally by the surgeon (in the direction of arrow x) thereby urging flared section 206 through aperture 204 and between legs 115. The flared section 206 engages the inner surface of legs 115 and forces legs 115 radially outward so that barbs 116 bite into the side walls of predrilled hole 503 to hold the rivet firmly in place by increased friction. When the pulling force exerted by the surgeon on the proximal portion 203 of pin 200 exceeds the tensile strength of the pin, the pin 200 breaks at the notch 205 leaving a surface approximately flush with surface 102 of the rivet. The proximal portion 203 of the pin may then be discarded leaving the remainder of the orthopedic fastener in place to hold down soft tissue 302 for a time sufficient to promote healing and attachment of the soft tissue 302 to bone 301.

A satisfactory method was developed for evaluating the holding characteristics of fasteners in bone. Using this method in over 100 tests, the ultimate pull-out forces and shear stresses were determined for two sizes of sheet-metal type of screws with various interference fits, for a commercial orthopedic self-tapping screw, and for two sizes of machine screws in tapped bone, each at five sections of equine metacarpus. The ultimate pull-out force was maximum at the midlength of the bone, and minimum at the distal end. In general, the failure mechanisms were bone-thread shear for low pull-out forces, bone splitting at intermediate pull-out forces, and bone fragmentation at high pull-out forces. The failure mechanisms of the bone indicate that orthopedic fasteners should possibly not be designed for maximum holding force.
was formerly Engineering Research Associate, College of Engineering, Texas A & M University.