Dentistry – Prosthodontics – Holding or positioning denture in mouth
Reexamination Certificate
2000-12-07
2002-03-19
Lucchesi, Nicholas D. (Department: 3732)
Dentistry
Prosthodontics
Holding or positioning denture in mouth
C433S072000, C073S761000
Reexamination Certificate
active
06358051
ABSTRACT:
BACKGROUND
This invention was made with government support under grant No. 1R43DE13454 awarded by National Institutes of Health Office of Extramural Programs SBIR/STTR Grant Programs. The government has certain rights in the invention.
This invention relates to a method and apparatus for ensuring that small screws used to hold together dental implant components are tightened to the correct initial stress level, or “preload.” According to the National Institute of Health, among the factors involved in the design of a dental implant are the forces produced during implant loading, the dynamic nature of loading, and the mechanical and structure properties of the prosthesis in stress transfer to tissues. Unfortunately, accurate data on such parameters are incomplete.
National Institutes of Health Consensus Development Conference Statement on Dental Implants.
Jun. 13-15, 1988.
During the early 1970's the dental profession was very hesitant to use dental implants or fixtures surgically implanted into a patient's jawbone as a treatment option to replace missing teeth. However, success with implants in the past 30 years has replaced this skepticism. This is due to the efforts of P-I Brånemark and co-workers in Sweden who introduced the concept of osseointegration in humans. When the principles of osseointegration are followed, the anchorage of a non-biological titanium implant unit to living bone will occur, with approximately 95% and 85% implant survival rates for the lower and upper jaws, respectively. See, for example in U.S. Pat. Nos. 4,824,372, 4,872,839 and 4,934,935 to Jorneus et al., Brajnovic and Edwards, respectively.
One of most critical aspects in the replacement of missing teeth using dental implants is the ability of small screws positioned within the implant complex to hold the various implant parts together during loading and stress transfer. As any screw in the implant system is tightened, the initial stress level developed within the screw becomes critical to the maintenance of the joint stability between the parts the screw is clamping together. Owing to the high strain level that the assembled joint experiences in everyday life, this initial stress level called the preload is of paramount importance. Insufficient tightening of a screw in the implant system can result in the screw becoming loose rather quickly, and over time this looseness can lead to fracture of the screw and potentially failure of the implant reconstruction. This is particularly critical for screws that secure spacers or abutments to the implant or fixture.
The stability of the screw joint is considered a function of the preload stress achieved in the screw when applying the preload tightening torque to clamp the implant components together. The optimum preload torque is influenced by the geometry of the screw, the contact relationships between the screw and its bore, between the screw and its threads, and between the bearing surfaces of the components clamped together by the screw, friction, and the properties of the materials used. One example is the joint formed between the bearing surface of the implant and the bearing surface of the spacer or abutment. Another example is the joint formed between a prosthesis and an abutment, also held together by a small screw in the implant system.
When the screw joint experiences instability, the screw will either loosen or fracture. Screw joint failure occurs in two stages. The first stage consists of external functional loading applied to the screw joint that gradually leads to the effective erosion of the preload in the screw joint. Any transverse or axial external force that causes a small amount of slippage between the threads releases some of the stress, and therefore, some of the preload is lost. The greater the preload applied to a screw joint up to a maximum equal to the proportional limit), the greater the resistance to loosening and the more stable the joint. As long as the frictional forces between the threads remain large, a greater external force will be required to cause loosening.
Once the critical load exceeds the screw joint preload, it becomes unstable. The external load rapidly erodes the remaining preload and results in vibration and micromovement that leads to the screw backing out. Once this second stage has been reached, the screw joint ceases to perform the function for which it was intended and has failed.
Optimizing the preload of a screw used in a dental implant system is critical for implant screw joint stability. As was stated earlier, implant screw loosening and fractures are quite common. The fact that on average complications with implant screw will occur in one out of every four implants surgically placed is significant. The need for optimum preload in screw tightening at the initial stages of implant component assembly and completion of the final implant restoration cannot be left to chance. An instrument that scientifically records the preload established in these implant screws following tightening and prior to any external load applications is essential to implant performance and the quality of life of the patient who receives implants as part of their dental rehabilitations.
It has been reported by Patterson and Johns that to achieve the maximum preload possible in component screws for dental implants, it is necessary to apply the appropriate tightening torque to each screw. Torque tightening devices for implant screws arc discussed, for example, in U.S. Pat. No. 6,109,150 and 5,626,474. However, most screw torque-tightening devices lack accuracy because of a number of variables beyond the control of these conventional instruments. This means that the maximum stress developed in an implant screw tightened by conventional torque-tightening devices may be less than 70% of the yield strength of the screw itself and therefore well below the maximum possible preload for a stable joint. If the screw is loaded to the appropriate preload level one can be confident that the screw will not fail during the life of a patient when “normal” external loads are applied.
Ultrasound instrumentation has been used to measure the preload established in large bolts and screws in industrial applications. Thus far, however, it has not been applied to small screws the size of those used in implant systems. In industrial applications for large bolts and screws, the most common ultrasonic instruments for control of screw tension are called “pulse-echo” or “transit time” instruments. Bickford has described the use of this method with large bolts. A drop of fluid is placed on the head of the bolt to reduce the acoustic impedance between the transducer and bolt head. An acoustic transducer of some sort is placed against the bolt head. The instrument is then zeroed for this particular bolt because each bolt will have a slightly different acoustic length even if their physical lengths are the same. The zero load is recorded before tightening. Next, the bolt is tightened. If the transducer can remain in place during tightening, it will show the buildup of stretch or tension in the bolt during tightening. If it must be removed, it is repositioned on the bolt again after tightening to show the stress level achieved. If at some future time one wishes to measure the tension present within the bolt, the original data can be input to the instrument computer unit and after placing the transducer on the top of the bolt, the instrument will record the existing tension and the zero stress conditions.
In principle, the electronic instrument delivers a voltage pulse to the transducer, which emits a brief burst of ultrasound (typically five to seven or more cycles). This burst passes down through the bolt, echoes off the far end, and returns to the transducer. The electronic instrument measures very precisely the time delay required for the burst of sound to make its round trip in the bolt. As the bolt is tightened, the amount of time required for the ultrasound to make its round trip increased for two reasons: 1) the bolt stretches as it is tightened,
Gr. Maev Roman
Lang Brien R.
Ptchelintsev Andrei
Lucchesi Nicholas D.
Welsh & Katz Ltd.
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