FIELD OF INVENTION The invention relates to securing objects using wires, and in particular to securing broken or cut bones in the human body using wires as well as tightening ligature wires in orthodontics.
BACKGROUND OF THE INVENTION The use of wires to secure objects by twisting the ends of the wires dates back many centuries, but so does the uncertainty of this process. FIG. 1 shows an object broken or cut into two parts 1A and 1B and being secured by wires 2 twisted to form joints 3 using pliers 4 or a similar tool. Joint 3 is reliable when it consists of multiple twists of wire. Quite frequently the wire breaks closer to the object, leaving an insufficient number of turns as shown by 3A. In such cases the joint frequently opens when stressed, as shown by 3B. This is particularly undesirable in surgery such as sternotomy where the sternum is held together by such twisted wires after the surgery. Any joint failure as shown by 3B requires a new surgery. Similar use of wires and similar problems is found in orthodontics and many other areas of medicine. While this problem is critical in surgery it is common to many non-medical fields such as tightening re-enforcement bars in concrete work, using wires as hose clamps, twisting wires to form electrical connections and many other applications. The most common solution to this problem in prior art is to replace the wire twisting with a crimped ferrule 13. This solution, while reliable, adds cost, complexity and sometimes is not feasible because of limited access to the area.
SUMMARY OF THE INVENTION The invention enables the break-off point of twisted wires to predictably occur at the edge of the tightening tool. This is achieved by slightly weakening the wires at this point, using the pressure of the tightening tool and the shape of the jaw to introduce a controlled weak point. Such a uniform break-off point allows secure and uniform twisted wire joints.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a perspective view of the prior art.
FIG. 2 shows a perspective view of the uniform break-off in wires twisted according to the invention.
FIG. 3 shows a tool that can be used to practice the invention.
FIG. 4 shows an alternate form of a tool used to practice the invention.
FIG. 5 shows an enlarged view of the wires breaking off.
DESCRIPTION OF THE PREFERRED EMBODIMENT It was found that the break-off point of the twisted wires can occur at the edge of the pliers in a completely predictable manner if the wire at this point will be slightly weaker than at the rest of the twisted section. FIG. 2 shows the appearance of objects 1A and 1B held together by twisted wires 2 having the twisted section 3 break-off at a uniform point. Clearely 1A and 1B can also be separate objects. It was also found that the simplest way to introduce such a controlled weakening is to squeeze the twisted wires at a high pressure, significantly higher than the pressure created when regular pliers or similar tools are used. This allows varying the length of the twisted section simply by changing the point of gripping the wires with the tool. The pressure depends on the wire diameter; for stainless steel wires commonly used in sternotomy (0.7-1 mm diameter) the pressure of the jaws of the tool should be in the range of 100-300 Kg. In order to easily generate the required high pressure, a high leverage system or toggle system is used as shown in the tools of FIG. 3 and FIG. 4. In order to have a uniform pressure, not dependent on operator, it is desired to build into the tool some form of snap action which locks the tool arms 9A and 9B by latch 11 when the correct pressure is reached. Such locking pliers are well known in surgery and in general use, often referred to a “vise grip pliers”.
Referring now to FIGS. 3 and 4, tool 4 comprises of hardened steel jaws 5 pivoting on hardened pivot 6 and actuated by arms 9A and 9B via compound leverage generated by pivots 7 and 8. All pivots should be hardened because of the significant loads. A design as shown in FIG. 3 and FIG. 4 can easily have a mechanical advantage of 30:1 thus no more than 10 Kg of force are needed at handles 10 to created 300 kg of pressure at tip of jaws 5. FIG. 3 shows a tool shape similar to currently used surgical pliers while FIG. 4 shows a different style of handles 10 to make the tool 4 acquire the shape of a screwdriver when handles 10 are latched together by latch 11. This shape facilitates twisting of the wires. In both FIG. 3 and FIG. 4 the latch 11 is released by sideways pressure on the handles. Another advantage of the latch is that the piece of wire that beaks off stays locked in the tool and can not accidentally drop into an undesired spot. Such a tool can also be made as a disposable tool, where only jaws 5 are made of high quality materials, such as type 440 stainless steel, while the rest of the tool is made of bent sheet metal or molded filled plastic.
FIG. 5 shows a close up view of the controlled break-off. The twisted part of wires 2 breaks at the edge 5A of jaws 5 into section 3A which secures objects 1A and 1B, and a section 3B which remains locked in jaws 5. Because of the high pressure of jaws 5 section 3B has a slightly reduced cross section of wire 2, causing the wires to break exactly at edge 5A of jaw 5. Both the pressure and shape of edge 5A are important and have to be determined experimentally for each wire size. The edge 5A of jaw 5 has to be rounded with a very small radius, typically 0.05 mm to 0.2 mm. The hardness of jaws 5 in the areas coming in contact with the wire 2 has to be at least 60 Rc because of the very high local pressure. The ends of jaws 5 coming in contact with the wire can be flat or slightly concave. The concave jaws tend to center the wire in the jaw. Because the pressure and shape of the tip of the jaws are quite critical and depend on the exact wire size, it is best to have a separate non-adjustable tool for each size of wire. Handles 10 are squeezed together until the tool locks at the correct setting. The snap action of the tool gives clear tactile feedback to the user.
While the preferred embodiment controls the break point by compressing the wire the invention should be understood to cover any method of controlling the break point of the wire.
