Surgery – Instruments – Orthopedic instrumentation
Reexamination Certificate
2000-04-18
2001-06-26
Philogene, Pedro (Department: 3732)
Surgery
Instruments
Orthopedic instrumentation
C606S075000
Reexamination Certificate
active
06251112
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed to a closure for an open-ended medical implant that receives a rod or related rod-like structure in an open end of the implant and then in turn the rod captured within the end of the implant by the closure. This application is also directed to tools utilized in installing and removing such a closure.
A wide range of medical implants are utilized within the human body. With respect to the spine these implants include bone screws that are implanted in vertebrae and various types of connectors and the like that are used to interconnect with bone screws and each other to form an overall support matrix or spinal implant system. Many of these implants, especially bone screws, but also hooks and connectors, are open-ended having a yoke with a pair of upstanding arms that receive rods or other structures in a channel formed between the arms. Typically, each yoke receives an elongate rod, but other elements of the system having cylindrical shaped nipples or the like may also be received in the yokes. Because such bone screws and the like are open-ended, some closure must be used in order to capture the rod or other structure received within the open end of the implant.
To insure that the closure remains on the bone screw or other implant and that the rod or other structure being held in the implant does not slip either axially or rotationally, in many systems a substantial amount of torque must be applied in installing the closure to both maintain the closure in position and urge the closure against the rod to lock the rod in position relative to the bone screw or other implant. The force required to torque the closure presents either a destabilizing force to the overall completed implant in that it may spread the arms or alternatively, if the closure has been made in such a way to counteract the destabilizing force of the torque, the closure and overall implant have become too large to be used effectively within the system or the closure becomes very complex and difficult to manufacture and/or use.
In particular, the prior art has developed plug closures which are simply threaded and are screwed into threads on the inside of the arms in the open end of the implant to hold the rod or the like in place. High torque exerted on the plug tends to spread the arms during installation or the arms of the implant may later spread due to forces acting on the implant system, thereby loosening the closure and possibly allowing the rod to slip or the plug to work loose of the bone screw. In addition, the rods are normally curved or non-linear at the location where they are placed in the implant. That is, typically, the spine is not straight and rods used for supporting the spine are normally bent to follow the curvature of the spine. Consequently, the rod often has a substantial amount of curvature where the closure plugs engage the rod. Because of this curvature, the rod often is somewhat arcuate underneath the plug even after the plug is fully installed. This presents substantial problems, if the rod is later subjected to forces in use in the body, such as where the patient encounters a fall or automobile accident, that cause the rod to bow somewhat against the desired curvature. This then spaces the rod from engagement with the plug or at least reduces the frictional engagement of the plug with the rod to a level that allows the rod to easily rotate or move axially and/or the plug to become loose and even work free of the bone screw over time.
Other closure caps are not threadably received as a plug by the implant, but rather have a complex system of interlocking parts with the implant in order to bridge the gap between the arms. Such caps normally slide into place and then are locked by tightening a set screw or the like. Such closures are very difficult to manufacture and are relatively very expensive. Closures are often also difficult to install, because the parts are small and the working space provided to the surgeon during surgery is very limited. Further, the curvature of the rod may make it very difficult to slide the cap into place so that it may then interlock with mating elements in the bone screw.
Yet another type of closure has been a closure that provides a ring that goes entirely about the outside of the arms of the implant. This ring may be threaded and received on mating threads on the outsides of the arms of the implant. The ring concept works well in preventing the arms of the implant from spreading due to forces. However, this type of system is subject to problems. One of the problems is that like the above noted system, when the ring secures a curved rod, the rod may later flex so as to release pressure and reduce friction between the closure and rod, so that the rod becomes loose within the implant and can slip by rotation or axial movement therein. Such systems are usually not very effective in locking the rods in place, as the plug normally engages part of the ring and not the rod.
Furthermore, the outside circumferential ring substantially increases the thickness of the implant along the axis of the rod. This presents a substantial problem, since there is often very limited space along the rod for all of the various connectors, bone screws and the like that are necessary for the overall implant system and large closures take up too much space. Additionally, the large caps that are necessary for closing this type of implant leave very little room for the surgeons to work and block access of bender devices to the rod that are needed to shape the rod in some types of implant surgery which require that the curvature of the rod to be modified after connection to the bone screws. The large head of such caps is also contrary to the general desire to reduce size, weight and overall volume of such implants so as to reduce their impact on the patient.
It is noted that other problems with the prior art include that, while some types of caps prevent outward movement of the arms, such allow the tops of the arms to bend radially inward toward each other so the cap becomes loose. Further, where the cap is threaded and goes over the outside of the arms, it is often very difficult to line up the threads properly between opposite arms and the cap because of the tight working space, so the cap does not sit correctly and does not tighten correctly against the rod.
SUMMARY OF THE INVENTION
A closure cap for an open-ended implant wherein the implant has a pair of upstanding arms located on either side of a rod receiving channel and wherein the arms are externally or outwardly threaded to threadably receive the closure. The closure is an incomplete round cap having opposed and equally sized segments missing so that the cap does not form a complete circle. When positioned to act as a closure, this makes the closure cap comparatively thin in profile relative to a cap of the same radius that is complete. The cap includes an upper plate joined to opposed flanges. The flanges are radially inwardly or interiorly threaded and positioned to allow receipt of the flanges around the threaded arms of the implant.
The closure includes a central cylindrical member that depends from the plate and is positioned between but in spaced relation to the flanges. The cylindrical member is sized and shaped to maintain the spacing of the arms when installed on the implant by keeping the arms from bending inwardly.
The plate and the cylindrical member also include a central top to bottom bore which receives a set screw. In use the set screw engages and is urged under torque against a rod within the channel so as to fix in position and lock the rod against movement relative to the implant.
A set of tools is also provided within the invention. A cap installation tool saddles on the closure cap and extends downwardly on either side of the closure cap so as to complete the threads of the cap where the segments are missing, especially forming a circumferential or 360 degree threaded structure with the flanges of the closure cap. In particular, the inside of the
McMahon John C.
Philogene Pedro
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