ORTHODONTIC BRACKET

BRACKET D'ORTHODONTIE

English Abstract

ABSTRACT OF THE DISCLOSURE



An orthodontic bracket of the type intended for
cementing directly to a previously etched surface of the tooth
enamel is made by applying to the lingual face thereof that
contacts the tooth enamel surface a thin porous layer of
sintered metal powder which is arranged to have a porosity such
that it has keying characteristics close to that of the etched
enamel and thereby facilitates the cementing adhesion of the
bracket to the tooth. A particularly suitable metal powder
consists of particles of the same alloy as the bracket metal and
of size in the range about 10 to about 150 microns (-100 mesh),
preferably from about 10 to about 50 microns (-300 mesh) applied
in a thickness of about 0.05 to 0.2 mm, by means of a sintering
operation that fuses the particles to the lingual face and to
one another. The porous layer preferably is pre-filled with a
polymer material compatible with the bonding cement, such as the
sealant employed with the cement, so as to protect the layer
against mechanical damage caused by bending of the bracket by
the orthodontist to conform the bracket surface more closely to
the tooth surface to which it is to be applied.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An orthodontic bracket comprising a bracket body having
at the lingual face thereof a thin layer of metal powder for
reception of a bonding material for fixing the bracket to a
respective tooth.



2. An orthodontic bracket as claimed in claim 1, wherein
the said particles are adhered to the bracket and to one another
by sintering.



3. An orthodontic bracket as claimed in claim 2, wherein
the metal powder is of particle size about 10 to 150 microns.



4. An orthodontic bracket as claimed in claim 1, wherein
the metal powder is of particle size about 10 to 150 microns.



5. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the material of the metal powder layer is the
same as that of the bracket lingual face to which it is applied.



6. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the said layer of metal powder is of thickness
about 0.05 to 0.2 mm.



7. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the said layer of metal powder is carried by a
separate thin metal foil constituting part of the bracket and






fixed to the remainder of the bracket body.

8. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the said metal foil is of thickness about 0.1 to
0.25 mm, and the total thickness of the metal foil and the layer
of the metal powder is about 0.15 to 0.45 mm.



9. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the metal powder is of particle size about 30 to
50 microns.



10. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the particles are of size within a range of
about 50 microns from the smaller to the larger particles to
avoid dense packing thereof.



11. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the thin layer is filled with a
cement-compatible sealant material to protect it against
compaction during mechanical handling thereof.



12. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the material of the metal powder is the same as
that of the bracket lingual face to which it is applied, and
wherein the said layer of metal powder is of thickness about
0.05 to 0.2 mm.



13. An orthodontic bracket as claimed in any one of claims






1 to 3, wherein the material of the metal powder is the same as
that of the bracket lingual face to which it is applied, and
wherein the said layer of metal powder is carried by a separate
thin metal foil constituting part of the bracket and fixed to
the remainder of the bracket body.



14. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the material of the metal powder is the same as
that of the bracket lingual face to which it is applied, wherein
the said metal foil is of thickness about 0.1 to 0.25 mm, and
wherein the total thickness of the metal foil and the layer of
the metal powder is about 0.15 to 0.45 mm.



15. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the material of the metal powder is the same as
that of the bracket lingual face to which it is applied, and
wherein the metal powder is of particle size about 30 to 50
microns.



16. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the material of the metal powder is the same as
that of the bracket lingual face to which it is applied, and
wherein the particles are of size within a range of about 50
microns from the smaller to the larger particles to avoid dense
packing thereof.




17. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the material of the metal powder is the same as




11


that of the bracket lingual face to which it is applied, and
wherein the thin layer is filled with a cement-compatible
sealant material to protect it against compaction during
mechanical handling thereof.



18. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the said layer of metal powder is of thickness
about 0.05 to 0.2 mm, and wherein the said layer of the metal
powder is carried by a separate thin metal foil constituting
part of the bracket and fixed to the remainder of the bracket
body.



19. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the said layer of metal powder is of thickness
about 0.05 to 0.2 mm, and wherein the said metal foil is of
thickness about 0.1 to 0.25 mm, and wherein the total thickness
of the metal foil and the layer of the metal powder is about
0.15 to 0.45 mm.



20. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the said layer of metal powder is of thickness
about 0.05 to 0.2 mm, and wherein the metal powder is of
particle size about 30 to 50 microns.



21. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the said layer of metal powder is of thickness
about 0.05 to 0.2 mm, and wherein the particles are of size
within a range of about 50 microns from the smaller to the



12


larger particles to avoid dense packing thereof.



22. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the said layer of metal powder is of thickness
about 0.05 to 0.2 mm, and wherein the thin layer is filled with
a cement-compatible sealant material to protect it against
compaction during mechanical handling thereof.



23. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the said layer of metal powder is carried by a
separate thin metal foil constituting part of the bracket and
fixed to the remainder of the bracket body, and wherein the
metal powder is of particle size about 30 to 50 microns.



24. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the said layer of metal powder is carried by a
separate thin metal foil constituting part of the bracket and
fixed to the remainder of the bracket body, and wherein the
particles are of size within a range of about 50 microns from
the smaller to the larger particles to avoid dense packing
thereof.



25. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the said layer of metal powder is carried by a
separate thin metal foil constituting part of the bracket and
fixed to the remainder of the bracket body, and wherein the thin
layer is filled with a cement-compatible sealant material to
protect it against compaction during mechanical handling thereof.


13


26. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the said metal foil is of thickness about 0.1 to
0.25 mm, wherein the total thickness of the metal foil and the
layer of the metal powder is about 0.15 to 0.45 mm, and wherein
the metal powder is of particle size about 30 to 50 microns.



27. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the said metal foil is of thickness about 0.1 to
0.25 mm, wherein the total thickness of the metal foil and the
layer of the metal powder is about 0.15 to 0.45 mm, and wherein
the particles are of size within a range of about 50 microns
from the smaller to the larger particles to avoid dense packing
thereof.



28. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the said metal foil is of thickness about 0.1 to
0.25 mm, wherein the total thickness of the metal foil and the
layer of the metal powder is about 0.15 to 0.45 mm, and wherein
the thin layer is filled with a cement-compatible sealant
material to protect it against compaction during mechanical
handling thereof.



29. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the metal powder is of particle size about 30 to
50 microns, and wherein the particles are of size within a range
of about 50 microns from the smaller to the larger particles to
avoid dense packing thereof.




14



30. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the metal powder is of particle size about 30 to
50 microns, and wherein the thin layer is filled with a
cement-compatible sealant material to protect it against
compaction during mechanical handling thereof.



31. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the particles are of size within a range of
about 50 microns from the smaller to the larger particles to
avoid dense packing thereof, and wherein the thin layer is
filled with a cement-compatible sealant material to protect it
against compaction during mechanical handling thereof.



32. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the material of the metal powder is the same as
that of the bracket lingual face to which it is applied, wherein
the said layer of metal powder is of thickness about 0.05 to 0.2
mm, and wherein the said layer of metal powder is carried by a
separate thin metal foil constituting part of the bracket and
fixed to the remainder of the bracket body.



33. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the material of the metal powder is the same as
that of the bracket lingual face to which it is applied, wherein
the said layer of metal powder is of thickness about 0.05 to 0.2
mm, wherein the said metal foil is of thickness about 0.1 to
0.25 mm, and wherein the total thickness of the metal foil and
the layer of the metal powder is about 0.15 to 0.45 mm.






34. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the material of the metal powder is the same as
that of the bracket lingual face to which it is applied, wherein
the said layer of metal powder is of thickness about 0.05 to 0.2
mm, and wherein the metal powder is of particle size about 30 to
50 microns.



35. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the material of the metal powder is the same as
that of the bracket lingual face to which it is applied, wherein
the said layer of metal powder is of thickness about 0.05 to 0.2
mm, and wherein the particles are of size within a range of
about 50 microns from the smaller to the larger particles to
avoid dense packing thereof.



36. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the material of the metal powder is the same as
that of the bracket lingual face to which it is applied, wherein
the said layer of metal powder is of thickness about 0.05 to 0.2
mm, and wherein the thin layer is filled with a
cement-compatible sealant material to protect it against
compaction during mechanical handling thereof.



37. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the material of the metal powder is the same as
that of the bracket lingual face to which it is applied, wherein
the said layer of metal powder is of thickness about 0.05 to 0.2
mm, wherein the said layer of metal powder is carried by a



16



separate thin metal foil constituting part of the bracket and
fixed to the remainder of the bracket body, wherein the said
metal foil is of thickness about 0.1 to 0.25 mm, and wherein the
total thickness of the metal foil and the layer of the metal
powder is about 0.15 to 0.45 mm.



38. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the material of the metal powder is the same as
that of the bracket lingual face to which it is applied, wherein
the said layer of metal powder is of thickness about 0.05 to 0.2
mm, wherein the said layer of metal powder is carried by a
separate thin metal foil constituting part of the bracket and
fixed to the remainder of the bracket body, and wherein the
metal powder is of particle size about 30 to 50 microns.



39. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the material of the metal powder is the same as
that of the bracket lingual face to which it is applied, wherein
the said layer of metal powder is of thickness about 0.05 to 0.2
mm, wherein the said layer of metal powder is carried by a
separate thin metal foil constituting part of the bracket and
fixed to the remainder of the bracket body, and wherein the
particles are of size within a range of about 50 microns from
the smaller to the larger particles to avoid dense packing
thereof.




40. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the material of the metal powder is the same as




17



that of the bracket lingual face to which it is applied, wherein
the said layer of metal powder is of thickness about 0.05 to 0.2
mm, wherein the said layer of metal powder is carried by a
separate thin metal foil constituting part of the bracket and
fixed to the remainder of the bracket body, and wherein the thin
layer is filled with a cement-compatible sealant material to
protect it against compaction during mechanical handling thereof.



41. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the material of the metal powder is the same as
that of the bracket lingual face to which it is applied, wherein
the said layer of metal powder is of thickness about 0.05 to 0.2
mm, wherein the said layer of metal powder is carried by a
separate thin metal foil constituting part of the bracket and
fixed to the remainder of the bracket body, wherein the said
metal foil is of thickness about 0.1 to 0.25 mm, wherein the
total thickness of the metal foil and the layer of the metal
powder is about 0.15 to 0.45 mm, and wherein the metal powder is
of particle size about 30 to 50 microns.



42. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the material of the metal powder is the same as
that of the bracket lingual face to which it is applied, wherein
the said layer of metal powder is of thickness about 0.05 to 0.2
mm, wherein the said layer of metal powder is carried by a
separate thin metal foil constituting part of the bracket and
fixed to the remainder of the bracket body, wherein the said
metal foil is of thickness about 0.1 to 0.25 mm, wherein the



18


total thickness of the metal foil and the layer of the metal
powder is about 0.15 to 0.45 mm, and wherein the particles are
of size within a range of about 50 microns from the smaller to
the larger particles to avoid dense packing thereof.



43. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the material of the metal powder is the same as
that of the bracket lingual face to which it is applied, wherein
the said layer of metal powder is of thickness about 0.05 to 0.2
mm, wherein the said layer of metal powder is carried by a
separate thin metal foil constituting part of the bracket and
fixed to the remainder of the bracket body, wherein the said
metal foil is of thickness about 0.1 to 0.25 mm, wherein the
total thickness of the metal foil and the layer of the metal
powder is about 0.15 to 0.45 mm, and wherein the thin layer is
filled with a cement-compatible sealant material to protect it
against compaction during mechanical handling thereof.



44. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the material of the metal powder is the same as
that of the bracket lingual face to which it is applied, wherein
the said layer of metal powder is of thickness about 0.05 to 0.2
mm, wherein the said layer of metal powder is carried by a
separate thin metal foil constituting part of the bracket and
fixed to the remainder of the bracket body, wherein the said
metal foil is of thickness about 0.1 to 0.25 mm, wherein the
total thickness of the metal foil and the layer of the metal
powder is about 0.15 to 0.45 mm, wherein the metal powder is of




19


particle size about 30 to 50 microns, and wherein the thin layer
is filled with a cement-compatible sealant material to protect
it against compaction during mechanical handling thereof.



45. An orthodontic bracket as claimed in any one of claims
1 to 3, wherein the material of the metal powder is the same as
that of the bracket lingual face to which it is applied, wherein
the said layer of metal powder is of thickness about 0.05 to 0.2
mm, wherein the said layer of metal powder is carried by a
separate thin metal foil constituting part of the bracket and
fixed to the remainder of the bracket body, wherein the said
metal foil is of thickness about 0.1 to 0.25 mm, wherein the
total thickness of the metal foil and the layer of the metal
powder is about 0.15 to 0.45 mm, wherein the particles are of
size within a range of about 50 microns from the smaller to the
larger particles to avoid dense packing thereof, and wherein the
thin layer is filled with a cement-compatible sealant material
to protect it against compaction during mechanical handling
thereof.

Description

~219 ~75


ORTHODONTIC BRACKET
Field of the Invention
The present invention is concerned with improvements in
or relating to orthodontic brackets and especially to such brackets
of the type which in use are bonded to the teeth.
Review of the Prior Art
In the early days of orthodontic practice the brackets
were fastened to respective metal bands which were then placed
around the teeth, but this procedure is now rapidly being replaced
by systems in which the brackets are bonded by a cement directly
to the teeth surfaces. In one such known system the bracket
lingual face to which the cement is applied is provided by a thin
sheet of fine metal mesh supported by a thin metal foil attached
(e.g. welded) to the bracket body. The tooth surface enamel is
lS first etched to a depth of about 0.01 mm., so as to improve the
mechanical bonding of the cement thereto, and a thin layer or
un~illed cement is applied over the etched area. A relatively
thick layer of filled cement is applied to the lingual face of the
metal mesh and pressed firmly into the interstices thereof, so as
to maximise the mechanical bonding; the bracket is then pressed
firmly into place on the t~oth until the excess cement squeezes
from the edges, the surplus "flash" being removed after the cement
has hardened su~ficiently for the bracket to be held securely in
place.
The filling of the cement, usually with very finely
ground silica, is necessary to provide it with adequate anti-
abrasion resistance in the highly hostile environment of the


-- 1 --
I .

~Z~L99t~5

human mouth, unfilled cement being eroded relatively quickly. The
use of a surplus. of cement is necessary since the tooth surface
usually is not particularly smooth and the foil is too stiff to
be ab.le to deform enough to follow the tooth contours. It is
important to ensure that there are no voids or crevices between
the bracket and the tooth.surfaces in which food particles, bacteria,
etc., can lodge, since these could promote the formation of decay
and could in an extreme cas.e result in a cavity or cavities. A
good peripheral s.eal is also des.irable to avoid edges against
~hich.the patient ' 5 tongue could snag ~ith consequent discomfort.
It is of cours.e fun~amental that there be an excellent bond between
each tooth and the-bracket of sufficient strength to withstand the
relatiYely high forces that are applied by normal everyday activities,
extending o~er a period of up to about three years, while permitt-

ing the bond to be broken relatively easily at the conclusion ofthe procedure.
The use of the above-described fine metal mesh has proven
to be relatively satisfactory, the interwoven wires providing a
large number of re~entrant crevices, and the like into which the
~onding cement can penetrate to provide the desired mechanical
keying. Studies of ~racket failures show that predominantly (90~)
they result from breaking of the bond between the cement and the
bracket, and any improvement in such keying that can be obtained
is therefore highly desirable. The production of the brackets
involves the secure fastening of the mesh to the foil, and of the
foil to the bracket, without damaging. any of the components and
any improvements in cost, speed of manufacture, scrap rate




- 2 - a

34~

reduction, etc., that can be obtained is of course desirable.
There is moveover a constant endeavour to reduce the size of the
brackets, not only from the point of view of patient comfort
because of the reduced bulk and protrusion, but also to give the
orthodontist greater flexibility in the positioning of the
brackets on the teeth, so that the required tooth movements can
be obtained more readily and without the need to move the
position of the brackets on the teeth during the procedure.
Therefore any reduction in thickness of the bracket element that
is interposed between the bracket body and the tooth is highly
desirable. However, such miniaturization reduces the area of
mesh available for bonding to the tooth.
Definition of the Invention
It is therefore an object of the invention to provide
an orthodontic bracket having a new kind of lingual surface for
facilitating the bonding of the bracket to a tooth.
In accordance with the present invention there is
provided an orthodontic bracket comprising a bracket body having
on the lingual face thereof a thin porous layer of metal powder
for reception of a bonding material for fixing the bracket to a
respective tooth surface.
Preferably the said particles are adhered to the
bracket and to one another by sintering.
The material of the metal powder layer may be the same
as that of the bracket lingual face to which it is applied, and
the said layer may be of thickness about 0.05 to 0.2 mm.
The said layer of metal powder may be carried by a
separate thin metal foil constituting part of the bracket and




-- 3

9'~75
fixed to the re~ainder of the bracket body, and preferably the
said metal foil is of thickness about 0.15 to 0.45 mm.
The metal powder may be of particle size about 10 to
150 microns and preferably is of particle size about 30 to 50
microns.
The particles may be of size within a range of about 50
microns from the smaller to the larger particles to avoid dense
packing thereof.
The thin layer may be filled with a cement-compatible
sealant material to protect it against compaction during
mechanical handling thereof.
Description of the Drawings
Particular preferred embodiments of the invention will
now be described, by way of example, with reference to the
accompanying diagrammatic drawings, wherein:-

EIGURES 1 and 2 are respectively back and frontperspective views of an orthodontic bracket of the invention,




- 3~ -

~'~19 ~75


FIGURE 3 is a top plan view of another orthodontic
bracket embodying the invention, and
FIGURE 4 is a photograph of a microscopic view of a
small portion of the lingual surface of the brackets of Figures 1
and 2 to a magnification of 300 times, in order to show the nature
of the surface obtained by the application of the invention.
Description of the Preferred Embodiments
-
The bracket illustrated in Figures 1 and 2 is that
descri~ed and claimed in Patent Serial NoO 4,248,588 issued to
Herbert G. Hanson. Briefly, the bracket consists of a body 10 of
stainless steel having therein a mesial-distal extending slot 12
that receives an arch wire 14. The labial side of the slot is
opened and closed as required, respectively for the insertion and
retention of the arch wire, by respective movem~nts of a generally
V-shaped spring member 16 that embraces the brac~et body and has
an end that in the slot-closed position is urged by its inherent
spring action to protrude into the slot 12 for engagement with the
arch wire therein. The bracket has many other features that adapt
it for its special task, but the exact structure of the bracket is
not crucial to a complete disclosure of the present invention, and
further description thereo~ is believed not be necessary. One of
the advantages of the Hanson bracket is the possibility of making
it of very small dimensions and a current series has an occlusal-
gingival height of 2.49 mm (0.098 in), a mesial-distal width of 2.50
mm (0.1 in) and a lingual-labial thickness of 1.52 mm (0.060 in).
The bracket body 10 has fastened to the lingual face thereof by
accurate laser welding a thin piece 17 of a thin stainless steel

~9 ~ 5

metal foil of 0.15 mm (0.006 in) thickness and about 3 mm by 4 mm
(0.12 to 0.16 in) dimensions with the longer edges extending
mesially-distally.
Prior to its attachment to the bracket body this foil
piece has applied to the lingual face thereof by a sintering
operation one or more layers 18 of stainless steel metal particles
in the size range of about 37 to 44 microns, the layer or layers
18 being of thickness 0.05 mm (0.002 in), so that the total thick-
ness of the foil plus metal layer/s is 0.20 mm (0.008 in). The
particles are of the same material as the foil, and the sintering
operation is such that they are not only fused to the foil but
they are fused to one another to form a resultant integral porous
layer or porous layers with excellent mechanical keying properties
when the cement is applied to the lingual face of the layer or
outermost layer for attachment of the bracket to a tooth. It is
preferred to use the same metal for the foil 16 and the metal
powder layer 18 since ready adhesion by the sintering operation is
thereby assured, but different metals can of course be used
A typical relatively simple test of the adhesion
capability of a particular cement/metal powder layer combination
involves supporting a newly extracted tooth in some suitable
manner, e.g. by burying ik in a cement in a holder with the labial
face exposed, cementing a bracket to the exposed tooth face and,
after the cement has set, applying a progressively increasing
tensile force to the bracket until it separates from the tooth. A
minimum acceptable figure for the applied force is about 6 kg.
Tests performed with brackets of the invention, employing the

7~
cement sold by 3M Corporation under the trade mark "CONCISE",
showd that the brackets did not separate when a force of about
6.7 kg was applied, failure occurring at the laser weld.
The embodiment of Figure 2 is a bracket of the type
which is attached to an arch wire (not shown) by means of one or
more tie wires ~also not shown). In this embodiment the metal
powder layer 18 is applied directly to the lingual face of the
body 10 instead of to a separate foil. When an intermediate
foil is used this preferably will be of thickness from about
0.11 mm to 1.25 mm, (0.004 to 0.010 in); the thickness of the
metal powder layer preferably is from about 0.5 mm to 0.2 mm,
(0.002 to 0.008 in) so that the thickness of the resultant
sandwich is from about 0.15 mm to 0.45 mm, (0.006 to 0.018 in).
The minimum satisfactory thickness of a prior art foil/metal
mesh sandwich is about 0.2 mm (0.008 in).
~ It is believed that the excellent bonding obtained with
the brackets of the invention is due to the fact that the
sintered metal porous layer or layers corresponds at least
approximately in its keying characteristics to the etched enamel
tooth surface, so that the cement employed is able to provide a
satisfactory bond both to the tooth surface and the bracket
surface.
The size of the metal powder particles should be within
the range 10 to 150 microns, but preferably are within the range
30 to 50 microns. It is preferred that the particles be of size
such that the difference between the smaller and the larger
particles is not more than about S0 microns, this uniformity in
size ensuring that they will not paclc too densely, so as to


'7~;
leave a large number of voids into which the cement can
penetrate for mechanical keying thereto. It will be understood
by those skilled in the art that when dealing with particles of
this size there is no abrupt cut-off in size and although the
difference between the statistically smaller and larger
particles is 50 microns there will in fact be present a much
wider range of sizes, but of numbers too small to be
significant. It will also be appreciated that the range of
particle size for a particular metal powder will be chosen
predominantly to ensure a satisfactory bond of the cement to the
layer 18; adequate bonding of the metal particles to one
another will usually be less critical in such choice since the
required strength is more easily obtained and, as indicated
above, the majority of failures have previously occurred in the
cement/foil interface and not the bracket/foil junction.
The particle size employed is readily determined by the
size of mesh through which the powdered material will pass.
Thus, a 100 mesh sieve will pass particles of size less than 149
microns but retain any larger, while a 300 mesh sieve will pass
particles less than 50 micron size; the preferred material is
that which will pass through the 300 mesh sieve.
It is also well known to those skilled in the art that
the shapes of the particles are affected substantially by the
conditions under which they are prepared. The usual method of
preparation is to finely divide the molten metal and then to
cool it, and the shapes are affected by the cooling procedure;
thus, it is known that gas cooling tends to result in particles
of regular spherical shape, while liquid cooling tends to result




,~ .

S
in particles of irregular shape.
In other embodiments it may be preferred to apply the
metal powder layer 18 in more than one stage. For example, a
first sub-layer can be applied directly to the foil or bracket
lingual face can be of smaller size particles and/or of a wide
range to achieve denser packing, the layer 18 being completed by
the application of one or more other sub-layers of particle size
and distribution preferred to provide the desired mechanical
characteristics for adequate keying of the cement thereto.
A very satisfactory procedure for fastening the
particles of the metal powder layer to the remainder of the
bracket, and to one another, is by sintering at about three
quarters of the melting temperature. As a specific example,
when the material of the foil 16 and the layer 18 is 17/4
stainless steel with a melting point of about 1480C (2700F)
the sintering will be carried out at about 1100C to 1250C
(2012F to 2282F) for about one half hour in a vacuum or
hydrogen atmosphere. The coated foil may subsequently be heat
treated to restore its hardness to its former value, if the
material used will permit such treatment.
It is found to be advantageous to apply to the sintered
metal powder layer a coating of a cement-compatible sealant that
will completely fill the pores of the layer. This is found to
protect the layer against compaction and consequent loss of
porosity during mechanical manipulation in applying a coated
foil to the bracket body, and also as the orthodontist applies
the bracket to a tooth. A suitable material is the filler-free
sealant used to coat the etched portion of a tooth prior to the



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application of the bracket, such as the unfilled material sold
by 3M Company under the trade mark ~licence~, or that sold by
~eliance Orthodontic Products of Itasco, Illinois under the
trade mark "Phase II~.
About one half of the brackets must be manipulated in
this manner. It has been found that even if this bending
manipulation is of such severity that cracks appear in the
polymer these appear to be repaired by usual application of
sealant by the orthodontist just prior to the application of the
beacket to the teeth, so that the orthodontist is given much
more freedom of operation in this regard. The unfilled sealant
material is used since it is of course inherently compatible
with the paste material of the system, but any other compatible
material can be employed.
The porous layer is found to be inherently somewhat
easily crushed by the type of mechanical handling and
deformation required to conform the foil to the tooth shape.
Visual inspection of such a crushed portion shows the surface to
have a burnished, compacted appearance contrasting clearly with
the characteristic porous appearance of the remainder of the
surface, and the crushed portion no longer exhibits the required
excellent keying characteristics it possessed prior to the
handling and deformation. A bracket of the invention with the
porous layer pre-filled as described is found to be capable of
manipulation and deformation without this loss of keying
characteristic, and is preferred by the orthodontist because of
the ease of operation that it provides.




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