Dentistry – Prosthodontics – Holding or positioning denture in mouth
Reexamination Certificate
2000-04-17
2002-09-10
Manahan, Todd E. (Department: 3732)
Dentistry
Prosthodontics
Holding or positioning denture in mouth
C433S174000
Reexamination Certificate
active
06447295
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to dental prosthetics, and more particularly to retaining screws to secure dental components, such as an abutment and/or prosthesis to a dental implant or fixture.
2. Description of the Related Art
In restorative dental implantology, an artificial root or “fixture” is surgically implanted into the jawbone of a patient. The implant is typically formed of titanium or a commercially pure titanium alloy. Titanium, and its biocompatible alloys, referred to herein simply as titanium, is the material of choice for surgical implants because it offers a combination of nearly ideal characteristics—high strength, light weight, bio-compatibility and essentially total resistance to corrosion in contact with tissues and bones.
After implanting and following an initial healing period of a few months, new bone growth is observed to occur around the titanium fixture supporting it securely in place. This process is known as osseointegration. In a secondary procedure, an abutment of a specific desired size and shape is then placed over the fixture and secured thereto by a bolt threaded into a cavity in the fixture. Typically, a titanium bolt is used, called the abutment retaining screw. The prosthesis is the third component of the system and may be fabricated of cast gold alloy and porcelain. However, since machined parts have greater accuracy than cast parts, the prosthesis is commonly cast to a machined component which is fastened to a threaded cavity in the abutment by a retaining screw.
Alternative approaches include attaching the prosthesis directly to the fixture without any intervening abutment. This method uses only one titanium retaining screw. A further variation includes cementing the restoration to the implant and/or abutment in a similar manner as is done in conventional fixed bridges on natural teeth. Typically, in this case, a tapered abutment without threads, often referred to as a cementable abutment, is fastened to the fixture with a titanium abutment retaining screw. This method also utilizes only one screw in the system.
Thus, implant dentistry relies upon one or more screws to fasten together component stacks. These components typically include fixtures, abutments and artificial teeth though they can include other components, for example, impression copings. Retaining screws have been used in dental prostheses for more than two decades and the problems inherent with screw securement have been studied in depth.
Typically, a special torque wrench is used to torque the abutment screw to the desired load or torque. However, the torque wrench measures total torque resistance only. As the abutment screw is turned, resistance increases indicating rising torque values on a dial gauge. But only part of this resistance comes about by the tension forces which are created as the abutment screw stretches under an applied load. The resistance forces which produce the clamping force are equal in magnitude and opposite in direction to the stretching force (tensile load). The important parameter of interest, however, is the resulting clamping load, which is produced when the screw is tightened at various torque values. As, noted above, this is a small component of the overall torque applied to a titanium screw, due to the large friction forces involved.
Thus, it is desirable to provide a high preload to the abutment screw so that the dental restoration can withstand the applied loads, and hence prevent joint failure and also to prevent screw loosening. Screw loosening generally occurs due to a “ratcheting” effect as significant loads, for example, during mastication, are repeatedly applied to the dental restoration, and causes the screw threads to turn or “back off” in steps. As the screw loosens, the preload decreases and this renders the implant-abutment joint further susceptible to failure.
The high loads encountered by the abutment screw can also cause the contacting surfaces of the components to open slightly on one side of the implant system by bending of the screw. This can create small gaps between the opposed surfaces of the abutment and the implant. Undesirably, oral fluids may gain access to the interior of the implant system through the gaps, thus risking infection. Movement of the implant components may also cause the screws to loosen or fail as they are repeatedly stretched and bent.
U.S. Pat. No. 5,482,463 to Wilson, et al. describes a screw joint for dental implant component stacks which has greater resistance to loosening, and therefore greater resistance to bending and/or breaking. This is accomplished by the use of spring washers and has the effect of increasing the preload and of increasing resistance to slippage of the entire joint. While the use of springs, as described by Wilson, et al. provides a partial solution to the long-standing problem of loosening of screws in dental implants, the fundamental problem of low preloading remains largely unsolved.
U.S. Pat. No. 5,711,669 to Hurson, incorporated by reference herein, teaches a system for improving the tightening efficiency of a titanium abutment screw using a malleable material, such as gold or silver, that is both biocompatible and has a low coefficient of friction. However, such coatings are expensive and, in some cases, can reduce the friction to such an extent that the screws can be easily over-torqued by the dental practitioner applying only minimal torque. Moreover, the malleability of the coating can render the threads of the titanium screw susceptible to galling.
SUMMARY OF THE INVENTION
Accordingly, it is an object and advantage of the prevent invention to overcome some or all of the above limitations by providing an amorphous “hard carbon” coated retaining screw for a dental prosthetic implant system. Though there are a wide variety of commercially available “hard carbon” coatings and some of the properties of hard carbon as a surface coating are known in the art, the present invention provides certain novel and unique benefits and advantages over the prior art in the field of screw retained prosthetic constructions particularly in the field of oral restorations, and more particularly in the field of dental screws for retaining abutments and/or dental restorations as related to the stacking and assembly of dental components.
The hard carbon coating provides a low friction surface finish which advantageously results in improved preloading of the screw, and hence a high clamping force between the components of the dental prosthetic implant system. The coating can comprise diamond-like carbon (DLC), amorphous diamond, crystalline diamond, or a combination thereof. The dental screw can include abutment retaining screws and prosthesis retaining screws. Other advantages provided by the hard carbon coating include high mechanical surface hardness, biocompatibilty, corrosion resistance, chemical inertness and low cost.
In accordance with one embodiment of the present invention, a retaining screw for fastening a dental component to an implant is provided. The implant has osseointegrated in a jawbone. The retaining screw generally comprises a head and a shank. The screw head comprises a seating surface sized and configured to engage a seating surface of the dental component. The screw head further comprises a cavity adapted to receive a tool for tightening the screw. The shank is in mechanical communication with the head and comprises threads adapted to threadably engage a threaded socket of the implant. A coating of hard carbon is applied to the seating surface of the screw head and to at least a portion of the shank. This reduces friction during tightening of the retaining screw and improves the preloading of the screw.
In accordance with another embodiment of the present invention, a dental implant system for supporting a prosthesis is provided. The dental implant system generally comprises a retaining screw, a dental implant and an abutment. The retaining screw generally comprises a head and a shank. The screw head comp
Kennard Don
Kumar Ajay
Knobbe Martens Olson & Bear LLP
Manahan Todd #E.
Nobel Biocare AB
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