1. Field of the Invention
The present invention relates generally to the field of restorative dentistry and, more particularly, to a prosthodontic torque limiting wrench provided with a torque limiting load spring for tightening dental implants.
2. Description of the Prior Art
Years of research has shown that living hard and soft tissues not only accept certain types of non-biological material, such as titanium, but also they actually incorporate it into the tissue itself. As a result, modern prostheses are being utilized in almost every facet of medical and dental science.
Prosthodontic restorative systems and techniques are well known in the prior art. For partially or fully edentulous patients, a dental implant fixture is implanted in a cylindrical bore made in a patient's jaw bone after the gum tissue has been displaced.
In the past few years, the use of dental implants has risen dramatically. Such implants typically consist of a titanium screw threaded into the patient's jaw bone and left alone for a number of months while the bone grows around it. On the top side, the screw (also called an anchor) contains a female threaded hole into which an adaptor (also known as an abutment) and a prosthetic attachment (such as a tooth or combination of teeth made, for example, of a composite material) are fastened. The fastening takes place after the implant has been sufficiently encapsulated in bone--a process that usually takes 3 to 6 months.
The torquing levels associated with the installation of the anchor, abutment and attachment units are usually different. Because each of these units is physically very small and made of highly malleable material, torque accuracy is critical. Over-torquing can easily damage the threads while under-torquing will cause the units to become unscrewed and loose in the mouth.
Torque-limiting devices, i.e., special-purpose prosthodontic wrenches for use with implants, used in the past are no longer sufficient given the increased emphasis on dental hygiene in recent years. Sterilization with torque-limiting prosthodontic wrenches is now routinely performed after use on any one patient to prevent infection of the next-to-use patient by communicable diseases, such as hepatitis or human immunodeficiency virus (HIV).
Precise and complete tightening of prosthodontic components in connection with dental restorations becomes difficult with torque-limiting wrenches whose precision has been degraded by the wear, tear and other such adverse effects of the sterilization process. For example, the most common and effective sterilization technique by dentists is the process of autoclaving. Repeated sterilization cycles of torque wrenches in an autoclave subjects such precision instruments to increased wear, including fretting and galling of metal parts. Some prior art instruments in particular are provided with complicated mechanical clutches that use lubricants. These instruments were found to be too heat sensitive to be adequately sterilized in an autoclave and as a result quickly became defunct in the current market.
The challenge to develop a torque-limiting mechanism that would provide long life and lasting calibration without the use of heat sensitive lubricants has lead to the development of an instrument by Implant Innovations, Inc. (I.I.I.) that operates to control torque with a strain-type system rather than by friction. This device is disclosed in an article by Dan Rogers entitled "Engineering a Device for Installing Dental Implants", Mechanical Engineering, April 1992, pp. 72-73. This I.I.I. torque driver is described as allowing the practitioner (dentist) to avoid the risks of over- or under-tightened screws and arguably offers the benefits of complete and adequate sterilization without degradation of precision. However, there remain disadvantages.
First, the strain-type system with the I.I.I. device referred to above includes two roller-coaster-type cams that face each other to transform a rotational force generated by turning the screw into the linear direction so that it can be applied to a strain gauge. The overall construction of this torque driver is, therefore, extremely complex, bulky, and expensive to manufacture,
Secondly, because the I.I.I. torque driver device should be sterilized (e.g., in an autoclave) after its use on a patient, there remains the obvious risk that a practitioner may forget to sterilize the device before using it on the next patient.