1. Field of the Invention
The present invention relates to medical implants, and in particular, to threaded devices used for anchoring medical implants.
2. Description of the Related Art
To be effective, medical implants, such as intramedullary fixation rods, must be securely positioned and remain stable once installed. For example, once a long bone fracture has been reduced and an intramedullary fixation rod has been installed, axial, rotational and angular stability of the rod must be maintained. If not, the benefits otherwise afforded by the use of the fixation rod will not be fully realized. To date, several different means have been used to anchor fixation rods to provide the requisite stabilities.
One type of anchoring device uses a pin installed transversely within the bone. For example, see U.S. Pat. Nos. 3,763,855 and 4,212,294. A transverse hole is drilled through the cortex of the bone. A pin having a transverse threaded aperture within its shaft is installed and the aperture is aligned with the medullary canal of the bone. As an intramedullary fixation rod is installed, it is threaded through the pin aperture. The threaded engagement of the pin and rod provides axial stability for the rod. A disadvantage of this type of anchoring device is the risk of additional trauma imposed upon the patient by the transverse hole in the bone required for the pin.
Another type of anchoring device also requires transverse holes for the installation of pins or screws. Some types of intramedullary fixation rods are fabricated with longitudinal and transverse holes or slots. When the fixation rod is installed, transverse holes are drilled through the cortex of the bone in alignment with the holes o slots in the rod. Pins, screws or bolts are then installed within these transverse holes and through the holes or slots of the rod. The holes in the fixation rod may be threaded to receive screws or bolts, or sometimes brackets or plates are used on the outside of the bone in conjunction with the screws or bolts. For example, see U.S. Pat. 4,135,507 and 3,709,218. As with the apertured transverse pin described above, a disadvantage of this type of anchoring device is the risk of additional trauma imposed upon the patient.
These types of anchoring devices have the further disadvantage of making the surgery for their installation and removal more complex and time-consuming. With so many pins, screws, plates, etc. to install, the surgeon and patient both must spend that much more time in surgery. And for the surgeon in particular, the extra operational steps needed for installing all that hardware can be quite time consuming.
Still another type of anchoring device for a bone fixation rod does not use transverse pins, screws or bolts. Instead, the distal end of the fixation rod is equipped with radially expanding projections. For example, see U.S. Pat. Nos. 3,678,925 and 3,716,051. Once the rod is installed, the projections are caused to radially expand within the shaft (e.g. medullary canal) into which the rod has been installed. The expanding projections thereby provide a tight friction fit for the rod within the bone.
This type of anchoring device can have two disadvantages. First, the fixation rod is more complicated with the radially expandable projections and the mechanical coupling necessary to remotely activate those projections. Second, the expanding projections can cause the rod to become incarcerated within the bone, making extraction difficult.
Yet another type of anchoring device for an intramedullary fixation rod consists of a form of lag bolt. A hollow fixation rod is used with this device. For example, see U.S. Pat. Nos. 3,530,854 and 3,990,438. After the rod has been installed, the bolt is passed longitudinally through the hollow core of the rod. The threaded portion of the screw protrudes from the distal end of the rod and is threaded into the bone until the head of the lag screw engages some form of blocking structure within the fixation rod.
This type of anchoring device can have two disadvantages. First, installation of the lag screw requires an additional hole within the bone at the distal end of the fixation rod. This can introduce risks of undesirable stress and trauma within the bone. Second, since the fixation rod must be hollow, and the lag screw must be sufficiently large to be effective in grabbing into the bone when threaded therein, the fixation rod diameter must be relatively large. This will limit the use of such a rod to only large bones.
Therefore, it can be seen that an alternative anchoring or locking mechanism for bone fixation rods o other types of medical implants is desirable. In particular, it would be desirable to have such an alternative device which requires no difficult or time-consuming drilling of additional holes in the bone or installation of extra hardware, nor any complex mechanical features such as radially expanding projections which can cause undesirable incarceration of the device.