Orthodontic bracket with integral ball hook and method of making

A machined orthodontic bracket has a hook integrally formed with a gingival tiewing of the bracket. The hook includes a square shank, and a head with a cylindrical surface. The hook is machined by an end mill or formed cutter, and is achieved in part by changing the orientation of the bracket midway through the machining operation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a bracket that is attached to a tooth for 
orthodontic treatment, and particularly concerns a bracket having a ball 
hook that is integrally formed with a tiewing of the bracket. 
2. Description of the Related Art 
Orthodontic treatment involves movement of malpositioned teeth to desired 
positions for correct occlusion and improved aesthetics. During treatment, 
small slotted bodies known as brackets are affixed to the teeth and an 
archwire is secured in the slot of each bracket. The archwire serves as a 
track to guide movement of the teeth toward desired positions. 
In some instances, a small hook is affixed to certain brackets for movement 
of the teeth in particular directions. One end of an elongated elastic 
member is connected to the hook, and the other end is connected to a hook 
mounted elsewhere such as on another bracket, on an archwire, or on 
another orthodontic appliance in the oral cavity. The elastic member is 
under sufficient tension when in place to provide a resilient force that 
urges the teeth in desired directions. 
U.S. Pat. No. 5,125,831, assigned to the assignee of the present invention, 
describes an orthodontic bracket having four tiewings and a hook that is 
integrally connected to one of the tiewings. (Tiewings are tiny wings on 
the bracket that function to grasp an O-ring or a ligature wire that 
connects the archwire to the bracket.) The hook has opposed notches to 
provide connection to an elastic member extending in a direction away from 
either notch. The bracket described in U.S. Pat. No. 5,125,831 is similar 
to a bracket presently manufactured in a machining operation and sold by 
the assignee of the present invention under the trademark "UNI-TWIN". 
The hook of the bracket shown in U.S. Pat. No. 5,125,831 has a generally 
flat configuration when viewed toward the mesial side of the bracket 
(i.e., toward the side of the bracket facing midline of the arch) or the 
distal side of the bracket (i.e., toward the side of the bracket facing 
away from the midline of the arch). However, other brackets having hooks 
termed ball hooks are also known; such hooks include a shank with an 
enlarged, somewhat spherical head, and the shank is elongated and has a 
generally circular configuration in transverse cross section. Optionally, 
the shank of such ball hooks can be bent by the orthodontist in a desired 
direction to decrease the likelihood that the elastic member does not 
unintentionally detach from the hook during treatment. Ball hooks are 
preferred by some orthodontists over flat hooks because the enlarged ball 
head provides better retention of the elastic member in some instances. 
As can be appreciated, the strength of the ball hook, and especially of the 
shank, is a matter of significant importance. For example, if the shank 
breaks from the tiewing as the shank is bent to shift the hook to a 
certain orientation, the entire bracket must be detached from the tooth 
and replaced with a new assembly. Such a procedure is time consuming and a 
nuisance to both the orthodontist and the patient. 
In some instances, the shanks of ball hooks are connected to a tiewing of 
the bracket by a brazing operation. However, brackets having a hook brazed 
to a tiewing are not entirely satisfactory, since the brazing is 
relatively expensive, and careful alignment of the small parts during 
brazing is needed. Also, errors during brazing operation may not be 
readily apparent, but may result in a weak joint between the shank of the 
hook and the tiewing such that the shank detaches from the bracket when 
bent. 
In other instances, ball hooks are made integrally with the brackets when 
the brackets (including the hooks) are manufactured in a casting process 
or in a sintering operation. Cast and sintered brackets having integral 
ball hooks are generally less expensive to manufacture than brackets 
having brazed hooks, but there is a possibility that the shank of the hook 
may have insufficient strength as a result of the casting or sintering 
operation, due in part to the relatively small transverse cross-sectional 
areas of the shank. For example, when brackets are made by a metal 
injection molding operation, the metal powder may not flow and/or pack in 
a satisfactory manner in tiny cavities of the die that correspond to the 
shank of the resultant bracket hook, with the result that the strength of 
the shank is diminished. 
SUMMARY OF THE INVENTION 
The present invention concerns an orthodontic bracket that comprises a 
base, a body extending from the base and a tiewing extending from the 
body. The bracket includes a hook connected to the tiewing. The hook 
includes an elongated shank and a head. The head has a mesiodistal width 
that is greater than the mesiodistal width of the shank, and a 
labiolingual depth that is greater than the labiolingual depth of the 
shank. The hook and the tiewing are integrally formed during a machining 
operation. 
The present invention also concerns a method for making an orthodontic 
bracket having a hook, and comprises the step of shaping round stock in a 
lathe to form surfaces generally corresponding to labial sides of an 
occlusal and a gingival tiewing of the bracket as well as to form surfaces 
generally corresponding to labial and lingual sides of a hook of the 
bracket. The method also includes the step of cutting the stock along 
spaced apart reference lines to form surfaces generally corresponding to 
mesial and distal sides of the occlusal and gingival tiewings of the 
bracket and also to remove portions of the stock located on mesial and 
distal sides of the hook. The method further includes the step of moving a 
cutting tool along a path generally parallel to the labial surface of the 
bracket to form surfaces generally corresponding to an occlusal edge of 
the occlusal tiewing, a gingival edge of the hook and a pair of notches of 
the hook. The method also includes the step of moving a cutting tool along 
a path about an axis generally perpendicular to the occlusal plane of the 
bracket to form surfaces generally corresponding to labial and lingual 
surfaces of a head of the hook. 
The present invention is particularly advantageous in that the bracket is 
relatively inexpensive to manufacture, and can be made in automated 
fashion with little human attention. Yet, the hook of the bracket exhibits 
satisfactory strength in use. The integral, machined nature of the hook of 
the present invention avoids problems such as porosity or joint 
dysfunction as may be observed in connection with cast or sintered 
brackets, or in connection with brackets wherein a hook is brazed to a 
tiewing of the bracket. Moreover, machining of the bracket avoids the 
costs associated with making molds or dies such as are used to make 
brackets by a casting or sintering operation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
An orthodontic bracket constructed in accordance with the present invention 
is broadly designated by the numeral 10 in FIGS. 1-4. The bracket 10 
includes a base 12 having a foil mesh bonding surface for directly bonding 
the bracket 10 to the surface of a tooth. The base 12 has a concave, 
compound contour that matches the convex shape of the tooth surface, and 
is somewhat similar to bonding bases sold under the trademark "DYNA-BOND 
II" (3M Unitek Corporation), except that the bonding base is smaller. 
The bracket 10 includes a body 14 that extends from the base 12 in a 
buccolabial direction (i.e., in a direction toward the patient's cheeks or 
lips). A mesial (i.e., toward the middle of the dental arch) rotation arm 
16 and a distal (i.e., in a direction away from the middle of the dental 
arch) rotation arm 18 are integrally connected to the body 14 directly 
adjacent the base 12. 
A gingival tiewing 20 extends from the body 14 in a gingival direction 
(i.e., toward the gums). An occlusal tiewing 22 extends from the body 14 
in a occlusal direction (i.e., toward the outermost end of the teeth). An 
archwire slot 24 is located between the tiewings 20, 22 and has a 
rectangular configuration in transverse cross-section in order to matingly 
receive an archwire having a similar rectangular cross-sectional 
configuration. 
A hook 26 is integrally connected to the gingival edge of the gingival 
tiewing 20. The hook 26 includes a shank 28 having a longitudinal axis 
that extends in an occlusal-gingival direction parallel to the mesial and 
distal sides of the tiewings 22, 24. The outermost end of the hook 26 
includes a head 30 that is connected to the gingival end of the shank 28. 
The shank 28 has a generally rectangular cross-sectional configuration in 
views transverse to its longitudinal axis, and in the embodiment 
illustrated has a mesiodistal width of 0.016 inch (0.4 mm) and a 
labiolingual depth of 0,013 inch (0.3 mm), although other dimensions may 
also be used. The occlusal end of the shank 28 includes three fillets 32 
(on mesial, distal and labial sides) that smoothly blend in curved fashion 
the transition between the gingival end of the gingival tiewing 20 and the 
reduced dimension of the central portion of the shank 28. The shank 28 
also includes four fillets 34 (on mesial, distal, labial and lingual 
sides) that smoothly blend in curved fashion the transition between the 
central portion of the shank 28 and the occlusal side of the head 30. 
The head 30 includes a cylindrical surface 36 that extends around mesial, 
labial, distal and lingual sides of the head 30 in smoothly curved 
fashion. An outermost end 38 of the head 30 has the shape of a partial 
cylinder that smoothly blends with edges of the cylindrical surface 36. 
The rectangular cross-sectional shape of the shank 28 is an advantage when 
the hook 26 is adjusted in use, as the rectangular shape facilitates 
bending of the shank 28 in one of four particular directions, and hinders 
bending in other directions. More specifically, the shank 20 can be 
readily bent in a labial, lingual, mesial or distal direction so that the 
head 30 is moved labially, lingually, mesially, or distally respectively, 
as such directions correspond to the four flat sides of the shank 28. 
However, bending of the shank 28 in other directions is hindered due to 
the orientation of the rectangular cross-sectional shape of the shank 28. 
Such construction is an advantage over hooks having round shanks, since it 
is more likely that the head 30 of the present invention will be oriented 
in the desired direction after bending even in instances where the force 
exerted by the pliers or other bending tool is not exactly aligned in the 
proper direction. 
Moreover, the fillets 32, 34 are an advantage when efforts are made to bend 
the hook 26, because the location of the bend is thus likely to occur in 
the central portion of the shank 28 between the fillets 32, 34 where the 
cross-sectional dimensions of the shank 28 are the smallest. As such, the 
resulting hook 26 when bent is more likely to have sufficient space in the 
"notches" of the hook 26 presented between the head 30 and the tiewing 20 
to properly receive an elastic member and to thereafter retain the elastic 
member on the hook 26 during treatment. 
The cylindrical outer surface 36 of the head 30 is an advantage from the 
standpoint of patient comfort. If, for example, the head 30 contacts the 
patient's cheeks, the cylindrical surface 36 is likely to touch the cheeks 
along a line corresponding to its outermost section. By way of comparison, 
a ball hook having a spherical head would be more likely to make point 
contact with the patient's cheeks, thereby increasing the chances that the 
patient will experience discomfort. 
In making the bracket 10, a process somewhat like the machining process 
described in U.S. Pat. No. 2,713,720 (incorporated by reference herein) is 
employed during the initial stages, wherein a number of brackets are 
formed around the circumference of a ring that is cut from round bar 
stock. First, a length of solid round bar stock, made of 17.4 PH stainless 
steel, is mounted on a lathe. As the stock rotates, a boring cutter having 
a round nose is moved along a path to cut and shape the lingual side of 
the gingival tiewing 20, the lingual sides of the shank 28, and portions 
of the stock corresponding to areas lingually of the head 30 including the 
lingual half of the end 38. Next, a turning tool is moved along the 
outside to shape and cut portions of the stock corresponding to areas 
labially of the head 30 (including the labial half of the end 38), the 
labial side of the shank 28, and the labial sides of the tiewings 20, 22. 
Next, a head spindle having a motor driven gripper is moved toward the 
stock to grip the labial side of the brackets 10 on the round stock in 
areas corresponding to gingival tiewings 20. A ring having a number of the 
partially formed brackets 10 is then cut from the bar stock. Subsequently, 
a cutter is used to shape areas corresponding to the lingual sides of the 
occlusal tiewing 22 as the ring is rotated by the spindle. 
Next, the ring is mounted in a CNC (Computer Numerical Control) milling 
machine by grasping inside walls of the ring (corresponding to occlusal 
and gingival sides of the bracket bodies 14) by a collet. In the steps 
that follow, the ring is indexed in rotary fashion after each step (or 
group of steps, as appropriate) so that each step is repeated for each of 
the brackets 10 that is to be made along the various circumferential 
locations on the ring. 
A rotary cutter, similar to a miniature circular saw, is used to cut the 
ring along pairs of spaced apart reference lines to form mesial and distal 
edges of the occlusal tiewing 22 and the gingival tiewing 20, as well as 
to remove portions of the ring located along mesial and distal sides of 
the hook 26. Optionally, a drill is used to drill holes in the rotation 
wings 16, 18. Next, a drill is deployed to form an identification dot 40 
(FIGS. 1 and 2). If desired, a rotary cutting tool is activated to cut a 
scribe line if desired (not shown in the drawings). A rotary cutting tool 
is then activated to saw the archwire slot 24 at a particular orientation 
to provide proper torque and angulation for the bracket 10. 
Next, the ring is turned about 90 degrees relative to the milling machine 
such that the shank 28 extends upwardly. Subsequently, a cutting tool is 
activated to rotate about a reference axis that is perpendicular to the 
occlusal plane (i.e., a reference plane located flatly between the 
patient's upper and lower dental arches when the bracket 10 is mounted on 
the patient's tooth). While rotating, the cutting tool also moves along a 
circular path about the aforementioned reference axis perpendicular to the 
occlusal plane and around the hook 26 such that the smooth cylindrical 
surface 36 on the head 30 is thereby formed. As illustrated in FIG. 2, the 
sides of the surface 36 are perpendicular to the occlusal plane and are 
oriented at an acute angle relative to the occlusogingivally extending 
sides of the tiewings 20, 22 in order to decrease the likelihood that an 
elastic member may slip off of the hook 26 during treatment. 
Next, a cutting tool, rotating about an axis corresponding to a 
labiolingual axis of the bracket 10, is used to cut the occlusal edge of 
the occlusal tiewing 22 (including the rounded corners shown in FIGS. 2) 
as well as to cut along portions corresponding to the end surface 38 of 
the head 30, and mesial and distal sides of the shank 28 so that mesial 
and distal notches in the hook 26 are formed. Next, the brackets 10 are 
cut from the ring into individual parts, tumbled and polished, and then 
heat treated. The mesh base 12 is then brazed to the lingual side of the 
body 14.