FIELD

This invention relates to the field of bone implants for placement within intramedullary canals, specifically a mechanism for fixation.

BACKGROUND

Bone implants, or nails, act to augment the structure of a bone, correcting a fracture or other defect. While nails are considered superior to plates, rotation of the nails within the bone, specifically the intramedullary canal—the inside of a bone—can be a problem.

This problem was initially solved by the installation of screws through the bone, passing through the nail within the bone. But the installation of screws caused further trauma to the patient, while increasing the complexity of the surgery.

The solution was internal tangs that extend into the bone after placement of the nail.

While this methodology avoids the addition of screws, the prior art tang extension mechanisms cannot adapt to the internal structure of the bone.

What is needed is a mechanism for extension of tangs within the bone, the mechanism adapting to the internal structure of the bone.

SUMMARY

The bone fixation tangs with self-balancing extension compensate for the internal cavity shape of the intramedullary canal, thus avoiding the issues of over-extension and under-extension of tangs.

Over-extension, or pushing the tangs too deeply in the inner wall of the bone, is unnecessarily traumatic to the patient and may weaken the bone.

Under-extension, or stopping extension before the tangs have locked into the interior surface of the bone, results in reduced mechanical strength, thus compromising the ability of the nail to provide support to the bone.

The bone fixation tangs with self-balancing extension address this issue by allowing the tangs to balance their extension.

In the prior art, both sets of tangs are forced outward symmetrically, or in parallel—their extensions matching in magnitude.

This symmetric extension is generally achieved by the tangs having threaded hubs, the hubs translating the rotational movement of a central screw into linear movement.

The central screw is fixed, and the threading of the hubs matches each other and the central screw.

Thus, the central screw rotation causes equal and matching linear movement of the hubs, resulting in equal and matching linear movement of the tangs, and thus equal extension. This methodology is acceptable for a cylindrical intramedullary canal of consistent shape. But in practice, such a perfect interior surface is rare, and thus symmetric extension is an imperfect solution.

In contrast to symmetric extension, the tangs of the bone fixation tangs with self-balancing extension need not extend symmetrically, but can extend asymmetrically. This is achieved by allowing linear motion of the central screw, or tang actuation shaft, and using zero-lead threads on one tang hub, thus fixing the tang hub to the tang actuation shaft linearly/translationally, but still permitting free rotation.

In the disclosed embodiment, the first tang set uses a threaded hub with threads having a non-zero lead, lead being the amount of axial or linear advance of during one complete rotation. Stated differently, rotation of the hub causes linear motion of the hub with respect to the tang actuation shaft.

The second tang set uses a threaded hub with threads having a zero lead. Stated differently, rotation of the hub along its associated threads does not cause linear motion—the rotation of the screw does not cause the hub to move with along the tang actuation shaft.

Thus, the tang actuation shaft can rotate with respect to the second tang hub, and the actuation shaft and tang hub maintain the same linear position.

Correspondingly, if the actuation shaft moves up or down, it will carry the second tang set with it.

This combination of a first tang set that moves with respect to the actuation shaft, and the second tang set that does not, results in a balancing effect.

If during extension, the first tang set contacts the interior surface of the bone, the first tang set stops extending. Continued rotation of the actuation shaft carries the second tang set toward the first tang set, continuing its extension until the second set of tangs is placed.

Conversely, if during extension the second set of tangs contacts the interior surface of the bone, the second tang set stops extending.

Continued rotation of the actuation shaft causes no upward/downward movement of the actuation shaft, but carries the first tang set toward the second tang set, continuing its extension until the first set of tangs is placed.

In this manner, tang extension is balanced even in tapering or expanding bone cavities.

