AN APPARATUS FOR TESTING THE PASSIVE FIT OF SCREW RETAINED STRUCTURES

Abstract: A passivity indicating apparatus for use where structures are to be joined by threaded components and a passive, precise, fit is sought, comprising means (9) for indicating the precision of fit of structures that are to be joined which means is responsive to a change in the resistance to rotation of the retention screw (4) or other threaded rotary component.

Description: An apparatus for testing the passive fit of screw retained structures There are many situations where a screw retained structure must fit against another structure to which it is to be connected with a high degree of accuracy and passivity - that is as the retaining screws are tightened, there should be no stress or strain induced in either structure. This invention relates to an apparatus for checking that screw retained structures fit passively. It is clear that there are numerous situations where structures must be precisely and passively joined by screws, bolts, or other threaded components, over a wide range of accuracy and torque; this invention has a particular application in dentistry, as described below, but it is understood that the governing principles could equally be applied in any other class of engineering application. Where teeth have been lost, as a result of, for example, dental disease, it may become necessary to place dental implants. Dental implants are usually made of titanium or other bio-compatible materials, and form a strong inflexible union with bone. This type of union has been termed "osseointegration". It is now understood that osseointegration will endure for many years in favorable circumstances. However this union may rapidly break down if the implant is overloaded by unfavorable forces. It is common practice to use several implants to support a dental bridge. It is also common practice to use retaining screws to connect such bridges to the implants. Bridges at present are fabricated by casting wax or plastic patterns in dental alloys, or by using computer aided design and computer aided milling technology. Most stages of construction are carried out on models prepared from impressions of the patient ' s mouth. There is thus a great potential for error in the fabrication of the bridge framework, and so it is not suφrising that small discrepancies in the mechanical fit of the bridge superstructure to the dental implants may arise. When positioned in the mouth a small discrepancy may sometimes go un-noticed with the result that when the bridge is screwed into place in spite of a small error, flexure of the implant, surrounding bone, bridge superstructure, retaining screw, and any other prosthetic components may permit the bridge to be seated in spite of the imperfect fit. As a result of this flexure there may then be a considerable build up of stress or strain in the system. This later manifests itself.as failure of one of the components, or failure of osseointegration and ultimately the ioss of the implant. When the implant supported bridge superstructure fits against the implants precisely it is commonly said to fit passiveiy - that is when the screws are tightened no stress or strain is introduced. Under these circumstances, tightening the screws that retain the bridge superstructure merely compresses the framework against the head of the implant. Figure 1 illustrates the rapid increase in torque as the retaining screws are tightened when the screw head contacts the bridge superstructure, and the superstructure fits the implants passively. Where the implant retained bridge superstructure fite the implants poorly, tightening the retaining screws results in a relatively slower build up of tension in the screw as the various components of the system are flexed. Figure 2 demonstrates how the tension in (or torque applied to) the retaining screw increases siowly as the various components of the system are flexed, and then rapidly increases as the superstructure is compressed against the implant when the superstructure finally contacts the implant. The present invention is a device which monitors the build up of tension in the retaining screw as it is tightened. A rapid build up in tension, with minimal rotation of the retaining screw, indicates that a bridge superstructure fits the implants passiveiy. whereas a relatively slow build up in tension with proionged rotation of the screw as the tension increases, indicates that the superstructure is initially separated from the implant. A specific embodiment of this invention wiii now be described with reference to Figures 3 and 4. With reference to Figure 3, a conventional, or dedicated, dental handpiece 1 is powered by a miniature electric motor (micro-motor) 2. An appropriate screw-driving attachment 3 is loaded in the handpiece to permit its use for tightening a retention screw 4. With reference to Figure 4, power to the micro-motor 2 is regulated by the power supply 5 such that voltage is maintained at a constant level, and current may be monitored by appropriate circuitry. The handpiece and micro-motor are calibrated such that current in the power supply to the micro-motor is correlated with the torque delivered by the handpiece as it tightens the retaining screw. In this embodiment of the invention an analogue to digital converter 7 samples the current to the micro-motor after current to voltage conversion and appropriate amplification 6. The digital output is then processed by a microcomputer 8 which then displays a graphic representation of a curve of current (torque) against time on a visual display 9; the shape of the curve may then be qualitatively assessed. Further processing will also return a numerical result, such as rate of change in torque. Alternatively circuitry monitors the rate of change in current as the retaining screw is tightened. A rapidly increasing current, indicative of a passively fitting superstructure is indicated by an indicator light, e g a green light emitting diode. Below a predetermined treshold other indicators illuminate, thus demonstrating various ieveis of inadequacy of fit. Similarly, a numerical display may be provided. The instrument may be calibrated by using the apparatus first upon standardised structures along with the desired retention screw and will also permit comparison between the fit of the bridge when seated on the model or in the patient's mouth. An alternative embodiment of this invention may employ components such as optical encoders or Hall effect sensors, or other types of transducers, in conjunction with appropriate circuitry, or may employ circuitry which monitors fluctuations induced in the supply to the micro-motor by the motor, which permits the rotation of the micro¬ motor to be monitored. The number of rotations, or parts of rotations of the motor, which correlates with the rotation of the ecrew driving head, can thus be counted as the torque delivered to the retaining screw increases; as torque builds up over a specified range, the number of rotations of the micro-motor counted over the specified range is inversely related to the accuracy of the fit of the structures. The result of the test may be displayed numerically, or may be indicated by indicating lights which illuminate at appropriate tresholds. The instrument may be calibrated by using the apparatus first upon standardised bridge frameworks and models with the appropriate retention screws, and will also permit comparison between the bridge when seated on the model or in the patient's mouth. In both these embodiments, signal processing and other processing functions may be carried out with the aid of a micro computer and associated software. This facilitates the calibration process and permits comparison of the digital data derived from the analogue to digital converter from measurements carried out on a patient, with measurements carried out on a standardised bridge framework, or the patient's bridge framework when seated on the model upon which it was fabricated. This will also facilitate compensation for play or friction in the system which may result in increasing current in the motor, or rotation of an element of the apparatus, prior to actual rotation of the retaining screw.

Claims: CLAIMS 1. A passivity indicating apparatus for use in any situation where structures are to be joined by threaded components such as screws or bolts, and a passive, precise, fit is indicated. 2. An apparatus according to claim 1 where change in the resistance to rotation of the retention screw or other threaded component is used to indicate the precision of fit of structures that are to be joined. 3. An apparatus according to any of the preceding claims, where change in the torque over a specified time, for the torque delivered by the motor driving the handpiece which is tightening screws or other threaded components, is measured by electronic circuitry to gain an impression of the quality of fit structures that are to be connected together. 4. An apparatus according to any of the preceding claims, where change in the power supply voltage or current over time for the motor driving the handpiece which is tightening screws or other threaded components, is measured, and the data used to gain an impression of the quality of fit of structures that are to be connected together. 5. An apparatus as described in any of the preceding claims, which operates over the range of torque which is commonly used for dental applications, in which retaining screws connect crowns or bridges to teeth or dental implants. 6. An apparatus as described in any of the preceding claims, where change in air flow over time for a compressed air driven handpiece which is tightening screws or other threaded components, is measured to gain an impression of the quality of fit of structures that are to be connected together. 7. An apparatus as described in any of the preceding claims, where the rotations of the screw driving attachment are counted over a specified range of torque as the retention screw or other threaded component is tightened. 8. An apparatus as described in any of the preceding claims, where the rotations of the screw driving attachment are counted over a specified range of time as the retention screw or other threaded component is tightened over a specific range of torques. 9. A test apparatus used for purposes described in any of the previous claims, with a numerical display that displays data relating to the number of rotations used to tighten a retaining screw, or the rate of tightening of a retaining screw, over a selected range of torques. 10. A test apparatus used for purposes described in any of the previous claims, with a visual display that displays a graphical representation of the torque applied to the retention screw as it is tightened, over a selected range of torques. 11. An apparatus as described in any of the preceding claims intended for use in circumstances other than dentistry, which operates with tools of appropriate dimension, over the range of torques encountered in the intended application.

Inventors: DAWOOD ANDREW JOSEPH STANLEY, DAWOOD, ANDREW, JOSEPH, STANLEY
Applicants: DAWOOD ANDREW JOSEPH STANLEY, DAWOOD, ANDREW, JOSEPH, STANLEY
Application year: 1995
Main classification: G01L5
IPC codes: A61C1, A61C8, A61C19, B25B23, G01L5