AN APPARATUS FOR TESTING THE PASSIVE FIT OF SCREW RETAINED STRUCTURES

APPAREIL D'ESSAI DE L'AJUSTEMENT PASSIF DE STRUCTURES RETENUES PAR VIS

English 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.

French Abstract

L'invention concerne un appareil indicateur de passivité qui s'utilise lors de l'assemblage de structures à l'aide d'éléments filetés et lorsqu'on cherche à obtenir un assemblage passif précis. Cet appareil comprend un moyen (9) qui permet d'indiquer la précision d'ajustement des structures à assembler et qui réagit à un changement de la résistance à la rotation de la vis de retenue (4) ou de tout autre élément rotatif fileté.

Claims

CLAIMS:
1. An apparatus for use with structures to be joined
by a retaining screw for testing the passive fit of the
structures to be joined, including a dental handpiece with
an appropriate screw driving attachment for tightening said
retaining screw to at least one of said structures and a
micro-motor for driving said dental handpiece, characterised
by appropriate circuitry for monitoring the rotation of the
micro-motor from which the change in torque delivered to the
retaining screw may be derived during tightening over a time
period and display means in the form of (a) a numerical
display for displaying data relating to the number of
rotations used to tighten the screw, or the rate of
tightening the screw, over a selected range of torques, or
(b) a visual display which is adapted to display a graphical
representation of the torque applied to the retaining screw
as it is tightened, over a selected range of torques, or
(c) indicating lights which illuminate at appropriate
thresholds, adapted to receive an output from said
monitoring means of said change in torque over said time
period, the change in torque over said time period being
indicative of the quality of fit of the structures to be
joined, for indicating the precision of fit between said
structures.
2. An apparatus according to claim 1 characterised in
that said monitoring means is adapted to measure change in
resistance to rotation during tightening of said retaining
screw.
3. An apparatus according to claim 1 characterised in
that said monitoring means includes electronic circuitry.
5

4. An apparatus according to claim 1 characterised in
that said monitoring means is adapted to measure change in
powder supply voltage or current for said micro-motor over
time.
5. An apparatus according to claim 1 characterised in
that it is adapted to operate over a torque range which is
commonly used for dental applications in which retaining
screws are used to connect crowns or bridges to teeth or
dental implants.
6. An apparatus according to claim 1 characterised in
that said dental handpiece for tightening said retaining
screw is a compressed air driven handpiece and said
monitoring means is adapted to measure a change over said
time period in air flow to said compressed air driven
handpiece.
7. An apparatus according to claim 1 characterised in
that said monitoring means are adapted to count a number of
rotations of the screw driving attachment over a specified
range of torque as the retaining screw is tightened to
indicate precision of fit between said structures.
8. An apparatus according to claim 1 characterised in
that said monitoring means are adapted to count a number of
rotations of the screw driving attachment over a specified
range of time as the retaining screw is tightened over a
specified range of torque to indicate precision of fit
between said structure and said retaining screw.
9. A method for determining a precision of fit
between dental structures to be joined by a threaded
component, said method comprising the steps of:
6

tightening said threaded component to said dental
structures;
measuring a torque applied to said threaded
component during said tightening during a period of time;
determining the precision of fit between said
dental structures based on a variation in a rate of change
in said torque during said period of time; and
indicating the precision of fit between said
dental structures in a visual display.
10. The method according to claim 9, wherein said
measuring and determining are performed with electronic
circuitry.
11. The method according to claim 9, further
comprising the step of:
counting a number of rotations of said threaded
component over a specified range of torque.
12. The method according to claim 9 wherein said
determining step comprises:
detecting a passive fit between said dental
structures when said rate of change in said torque is
substantially constant during said predetermined period of
time; and
detecting an imprecise fit between said dental
structures when said rate of change of said torque varies
during said predetermined period of time.
13. A method of determining a precision of fit between
dental structures to be joined by a threaded component, said
method comprising the steps of:
7

tightening said threaded component with a dental
handpiece supplied with a constant voltage to join said
dental structures;
measuring a torque applied to said threaded
component during a predetermined period of time during said
tightening;
determining a minimum threshold of torque which
indicates a precise fit between said dental structures;
visually indicating said precision of fit between
said dental structures based on said measured torque and
said minimum threshold.
14. The method of claim 13 wherein the step of
visually indicating further comprises indicating various
levels of an imprecise fit based on a difference between
said measured torque and said minimum threshold.
8

Description

WO 95/20146 PCTJI895100055
An apparatus for testing the passive fit of screws 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
passivel~~. 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 surprising that small discrepancies in the
mechanical fit of
the bridge superstructure to the dental implants may arise.
SUBSTITUTE S,~IEtT (RULE 26)

CA 02158854 2006-03-O1
29277-48
When positioned in the mouth a small discrepancy
may sometimes go unnoticed 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 loss of the implant.
When the implant supported bridge superstructure
fits against the implants precisely it is commonly said to
fit passively - 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.
According to one aspect the invention provides an
apparatus for use with structures to be joined by a
retaining screw for testing the passive fit of the
structures to be joined, including a dental handpiece with
an appropriate screw driving attachment for tightening said
retaining screw to at least one of said structures and a
micro-motor for driving said dental handpiece, characterised
by appropriate circuitry for monitoring the rotation of the
micro-motor from which the change in torque delivered to the
retaining screw may be derived during tightening over a time
period and display means in the form of (a) a numerical
display for displaying data relating to the number of
rotations used to tighten the screw, or the rate of
tightening the screw, over a selected range of torques, or
2

CA 02158854 2006-03-O1
29277-48
(b) a visual display which is adapted to display a graphical
representation of the torque applied to the retaining screw
as it is tightened, over a selected range of torques, or
(c) indicating lights which illuminate at appropriate
thresholds, adapted to receive an output from said
monitoring means of said change in torque over said time
period, the change in torque over said time period being
indicative of the quality of fit of the structures to be
joined, for indicating the precision of fit between said
structures.
According to another aspect the invention provides
a method for determining a precision of fit between dental
structures to be joined by a threaded component, said method
comprising the steps of: tightening said threaded component
to said dental structures; measuring a torque applied to
said threaded component during said tightening during a
period of time; determining the precision of fit between
said dental structures based on a variation in a rate of
change in said torque during said period of time; and
indicating the precision of fit between said dental
structures in a visual display.
According to another aspect the invention provides
a method of determining a precision of fit between dental
structures to be joined by a threaded component, said method
comprising the steps of: tightening said threaded component
with a dental handpiece supplied with a constant voltage to
join said dental structures; measuring a torque applied to
said threaded component during a predetermined period of
time during said tightening; determining a minimum threshold
of torque which indicates a precise fit between said dental
structures; visually indicating said precision of fit
between said dental structures based on said measured torque
and said minimum threshold.
2a

CA 02158854 2006-03-O1
29277-48
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
fits 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 slowly 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 passively, whereas a
relatively slow build up in tension with prolonged rotation
of the screw as the tension increases, indicates that the
superstructure is initially separated from the
2b

WO 95/20146
PCTI1B95/00055
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) ~. 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, a g a green light emitting diode. Below a
predetermined treshoid 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
3
SUBSTITUTE SHEET (RULE 26)

WO 95/20146 ~r PCT/IB95100055
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.
4
SUBSTITUTE SHEET (RULE 28)

Representative Drawing