Expanded – threaded – driven – headed – tool-deformed – or locked-thr – Externally threaded fastener element – e.g. – bolt – screw – etc. – Thread or shank structure
Reexamination Certificate
2001-06-18
2002-12-17
Saether, Flemming (Department: 3627)
Expanded, threaded, driven, headed, tool-deformed, or locked-thr
Externally threaded fastener element, e.g., bolt, screw, etc.
Thread or shank structure
C411S016000, C411S438000
Reexamination Certificate
active
06494657
ABSTRACT:
FIELD OF THE INVENTION
The field of the invention is mechanical fasteners mainly for orthopedic use, for high vibration environments and for substrates subject to erosion.
BACKGROUND AND SUMMARY OF THE INVENTION
Screws are used to hold together various materials and generally depend upon wedging the threads and shaft of the screw into the substrate into which the screw is driven for a secure connection. However, this wedging action applies forces and resulting stresses on substrate that in certain circumstances speed substrate degradation at or adjacent to the screw and substrate interface and this in turn may lead to the loosening of the screw.
An example of this effect occurs in orthopedic screws that are driven into bones. These screws can damage the bone cells at and near the bone and screw interface, causing the bone to slowly recede from the screw and eventually causing the screw to loosen.
Another example occurs in wood where the wedging forces crushes the wood adjacent to the wood and screw interface. Also stone and concrete often crack due to the wedging forces of normal screws and bolts, especially where they are located near edges. In metal fabrication where the wedging action of a screw being driven into a metal substrate can alter the metallurgy of the substrate immediately adjacent to the screw and substrate interface. This metallurgical alteration can in certain circumstances speed corrosion and lead eventually to fastener failure.
In many cases there is a very limited elastic range in the substrate into which the screw is driven. Any recession of the substrate from the original substrate and screw interface will result in a dramatic reduction in the wedging force that the screw depends upon for purchase and hold, since the reaction forces exerted by the rigid substrate fall off rapidly with substrate recession.
One method presently used to increase the holding power of the screw is to attach a spring washer beneath the head of the screw that increases the function and wedging forces between the thread of the screw and the substrate into which the screw is driven. The difficulty with this approach is that the spring washer pulls the screw out of the lumen into which it is driven rather than pushing it in. If the screw does become loose for any reason, this pulling force may act to accelerate the further loosing of the screw. A further difficulty with this method is that it often exerts uneven pressure on different parts of thread of the screw. This uneven application of pressure can result in lower overall friction available to keep the screw secure in the substrate and also increase in some cases the damage to the substrate that in turn may cause further loosing.
What is needed is a screw system that does not depend principally upon the wedging forces to maintain its purchase and hold in the substrate.
What is also needed is a screw system that tends to push the screw into the substrate into which it is driven rather than pull it out.
What is also needed is a screw system that evenly distributes the frictional forces along the interface between the screw and the threat on which it slides.
What is also needed is a screw system that can maintain steady holding forces, even as the substrate recedes away from the screw.
U.S. patent application Ser. No. 09/541,508 by Unsworth and Waram, entitled “Self Adjusting Screw System”, which patent is incorporated herein by specific reference describes various means to meet the requirements set out above. The present invention describes a system that permits higher holding forces to be maintained and in some preferred embodiments does not require adhesives to hold the screw and coil of the system together for the application of torque and radial loading to the coil prior to insertion.
The present invention is a screw system that maintains purchase and hold in the substrate by maintaining a relatively low and constant force normal to the longitudinal axis of the screw even if and as the substrate recedes away from the screw and substrate interface. This force normal to the said longitudinal axis of the screw can be kept relatively constant and can be established in advance for various specific purposes to prevent unnecessary damage to the substrate into which it is inserted. Also the screw system expands as the substrate recedes maintaining intimate contact between the two. Additionally the screw system may apply a controlled and relatively constant force parallel to the longitudinal axis of the screw, pushing the screw into the hole into which it is driven and increasing the friction between the screw and the thread into which it driven thereby reducing the chance of the screw turning back out the hole.
A preferred embodiment of the invention is a system comprised of a screw and a coil or helicoil (the terms having the same meaning in this patent). Both the screw and the coil are inserted into the substrate. The helicoil is a coil formed usually from metal wire, but can be formed from any material that can have springiness imparted to it, including plastic and biodegradable plastic. The screw threads into the center of the coil, the turns of which, on the interior surface, describe the thread that meshes with the threads of the screw. The outside of the coil also forms a thread which can in turn be threaded into a thread tapped or cut into the interior walls of a hole in the substrate into which the screw and coil system are driven. In the event that the walls of the hole are not prepared with a thread, the coil may still be screwed into the hole and the coil itself will press or cut threads which will mesh with those threads (formed by the turns of the coil) on the outside of the coil.
This combination of screw and coil are well known to the art. U.S. Pat. No. 4,712,955 by Reece et al. describes a screw and helicoil system where the screw is larger than the helicoil and forces the helicoil out normal to the longitudinal axis of the coil and screw, utilizing ramp-like threads on the screw and receivers on the inside of the coil. This action creates very strong wedging forces that hold the screw and coil assembly in the hole of the substrate. This method while suitable for some purposes is not suitable where the substrate is likely to recede away from the original interface between the coil and the substrate. As explained above, even thought the wedging forces are very high, they are maintained over a small distance normal to the longitudinal axis of the coil and screw, and drop off dramatically when the substrate recedes away from the said interface.
The preferred embodiment of the invention includes a coil and screw system, but the coil, by various means tends to expand once it is inserted into a substrate independently, and not by being forced to do so by another element of the attachment system. These means are referred to herein as “expansion means”. While expansion is desirable, it must not be at the expense of the loosing of the screw from the substrate as the coil moves away from the screw in response to the coil following the recession of the substrate. The U.S. patent application Ser. No. 09/541,508 by Unsworth and Waram above referred to describes means for ensuring that the connection between the screw and coil is maintained under these conditions, and these means can be combined with the preferred embodiments herein described to effect the same purpose.
Unlike the wedging action of the conventional screw or conventional screw and coil combination, the coil will expand radially a relatively large distance following any recession of the substrate away from the original substrate coil interface and especially if superlastic shape memory alloy (SMA) material is used for the coil the forces exerted by the expanding coil on the interfacing substrate into which the screw and coil are driven will be relatively even, predictable and repeatable. The expanding screw and coil combination will also by various means described below maintain the purchase and hold of the screw and coil combination on the lumen of the substrate i
Unsworth John D.
Waram Thomas
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