The focus of this article has been upon process design. But the process design organization is in many instances contracted to provide to the owner a plan for executing the balance of the project, ...
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Drawing a portrait in profile, in front view, and three-quarter view. As well as how to draw the different features of the face.
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‘how to’ technique How to prepare a posterior crown using a
Issue 2 2013 Issue 2
plane-by-plane technique. by Dr Eddie Bass
Basic technique Basic ....... technique....... 1
Approximal surface preparation. >> Use a pear-shaped, rather than a tapering, diamond bur.
Lef t t : Use a long pear-shaped diamond bur for approximal surface reduction. Middle: It facilitates getting through a tight space without damaging the adjacent approximal surface. Right: Using a tapering diamond bur increases the risk of approximal surface damage.
Using a pear-shaped diamond bur for approximal surface reduction has many advantages over using a tapering diamond bur:
of an excess taper that can compromise resistance and retention form, especially on distal surfaces of molars.
• It can fit through a tight space whilst minimising the risk of damaging the approximal surface of the adjacent tooth.
• In the subsequent preparation the rounded end also facilitates the production of an ideal chamfer.
• The shape helps minimise the production >
An example bur that meets these requirements is shown on page 49.
Place a retraction cord before carrying out any further preparation.
retraction cord
t n e m u r t s n i c i t s a l p
s r e z e e w t
After clearing the approximal surface contacts place a retraction cord. TCA
Lef t t : A strand of Gingi-Pak retraction cord is placed in the gingival sulcus. Middle: It is seated in place with the end of a sharpened and polished plastic instrument. Right: Using the end of a pair of tweezers, previously dipped in trichloracaetic acid (TCA), the cord is moistened with the solution.
Source: www.dentaloutlook.com.au
The author prefers to use a strand of GingiPak (Belport). The strand is first placed into the gingival sulcus and pushed into position with the end of a sharpened and polished flat plastic instrument. Any moisture on the cord is then dried with a
small pellet of Gingi-Pak held in a pair of tweezers. A minuscule amount of trichloracetic acid (TCA) is applied at strategic locations using a pair of tweezers that had been previously dipped in a drop of TCA. Once the cord has been wetted it is pushed slightly further into the gingival sulcus with the end of the plastic instrument.
‘how to’ technique Tooth preparation for posterior crowns. (cont) 2
endodontic origin
Smooth surface preparation. >> Create a groove at the gingival level on the buccal and lingual surfaces. A small wheel diamond bur is used to prepare a groove along the buccal and lingual walls at the level of the gingival margin.
Either a small wheel diamond bur (lef t) or a small round diamond bur (righ t) is used to produce a groove on the buccal and lingual surfaces corresponding to the contour of the gingival margin.
Alternatively, a small round diamond bur can be used for the same purpose. Using either of these
>
burs results in a smooth groove following the buccal and lingual contours of the gingival margin. In so doing it establishes the cervical margin of the preparation in these locations. Example burs that meet these requirements are shown on the next pages.
Use a safe-ended diamond bur to prepare the buccal and lingual walls. After the grooves on the buccal and lingual surfaces have been prepared, a safe-ended, tapering diamond bur is used to complete the buccal and lingual reduction.
Lef t : A safe-ended tapering diamond bur is used for reducing the buccal and lingual walls. Middle: Reduction is carried to the appropriate depth and t aper. Right: The safe-ended tip sits in the previously established cervical groove which acts as a guide.
Holding the bur vertically will provide buccal and lingual walls with a convergence angle >
Round the corners of the preparation.
bur
A tapering diamond bur (lef t) is used to round the corners of the preparation (right). Source: www.dentaloutlook.com.au
The smooth surface preparation is completed using a tapering diamond bur. The corners are rounded and the cervical chamfer or shoulder extended in a smooth curve from the approximal to the buccal and lingual aspects of the preparation.
around 8 to 10 degrees which is ideal for retention. The presence of the previously established grooves provides a guide and helps keep the tapering bur on track and prevents any unwanted cutting into the sub-gingival region.
‘how to’ technique Tooth preparation for posterior crowns. (cont) 3
Occlusal surface preparation. >> Carry out a controlled reduction. To obtain a uniform and controlled reduction of the occlusal surface a suitable depth-cutting bur is used. Depth cuts are made across the surface in a bucco-lingual and >
mesio-distal direction. An oval-shaped diamond bur is then used to remove the remaining tooth structure to the depth of the cuts made by the end-cutting bur.
Lef t : A depth-cutting bur is used as a guide for the amount of tooth structure needing to be removed from the occlusal surface. Right: An oval-shaped bur is used to complete the occlusal reduction.
Complete final smoothing and finishing. >> Lef t: An endcutting tungsten carbide bur with a bevelled or rounded edge can be used for smoothing the shoulder or chamfer of the preparation.
A slow-speed, end-cutting tungsten carbide bur, that has had its corners bevelled or rounded, can be used for any final smoothing or finishing of the shoulder or chamfer. It is used at around
40,000 rpm in a friction grip, red-band slowspeed handpiece. At this speed it gives very good tactile feedback as to the presence of any irregularity or roughness of the shoulder or chamfer.
Example case.....
A pear-shaped bur was used to prepare the approximal surfaces and establish the appropriate taper.
1
Aust. distributor Horico burs - Henry Schein Halas. Source: www.dentaloutlook.com.au
The diamond bur FG 239 012 (Horico) was used because it had the appropriate length and taper.
2
Before any further preparation was carried out a retraction cord was placed in the gingival sulcus.
3
‘how to’ technique Tooth preparation for posterior crowns. (cont)
There are numerous methods of achieving gingival retraction. In this case a cord prepared from a pellet of Gingi-Pak (Belport) was used.
4
After placement, the Gingi-Pak cord became wet with crevicular fluid and minor gingi val bleeding. This was dried with another pellet of Gingi-Pak .
7
The wheel diamond bur was used to cut a groove at the level of the gingival margin on the buccal and lingual surfaces.
10
The pellet was held on the tip of a probe and teased out into a long strand.
5
The strand was wound in the fingers to form a cord. Just one string was enough for effec tive retraction. No local anaesthetic was required.
6
The tip of a pair of tweezers was dipped into a drop of trichloracetic acid. Minuscule amounts of the solution were deposited at strategic locations around the cord.
The cord was pushed further into the sulcus with a plastic instrument. A K ome t 825 016 wheel diamond bur (above) was selected for the next step (see footnote*).
A safe-ended tapering diamond bur (K ome t 857314014) was used to reduce the buccal and lingual surfaces. The smooth tip fitted into the cervical groove on the buccal and lingual surfaces.
After the buccal and lingual reduction (illustrated) the line angles were rounded and irregularities in the gingival shoulder corrected with a K omet 856016 tapering diamond bur.
8
11
*Left: A K omet 801314014 round diamond bur can also be used to prepare a groove at the level of the gingival margin.
9
12
Aust. distributor Komet burs - Henry Schein Halas.
‘how to’ technique Tooth preparation for posterior crowns. (cont)
To obtain a con trolled reduction of the occlusal surface a depth-cutting bur was used. A K omet 8804009 Bur (above) was selected (see footnote *).
13
Grooves were prepared to the depth of the tip of the end-
14 cutting bur.
An oval-shaped diamond bur (K omet 379314023) was used to finish the occlusal reduction to the level of the depth cuts. This provided 1.0 1.5 mm of clearance.
A tungsten carbide, end-cutting bur (K omet H207010-014) was selected for the final smoothing and finishing of the cervical shoulder. The e dge of the bur had been slightly bevelled.
Appearance of finished preparation after the strand of Gingi-Pak had been removed prior to impression taking.
The gingival tissues two weeks later had completely recovered and were typical of those seen when the retraction technique employing trichloracetic acid is used.
16
19
17
20
At the completion the occlusal surface had grooves running bucco-lingually and grooves running mesio-distally.
15
The end-cutting bur was used at 40,000 rpm in a friction grip red-band handpiece. (If the interproximal space is small, a 010 or 012 diameter bur may be needed).
18
In the case shown a full gold crown was placed. The margins finished slightly subgingivally. The technique described can be used, with the appropriate modifica tions, for other types of crowns.
21
* Left: A Premier D.C. Dep t h-Cu t diamond bur (DC1.0 or DC1.5) can also be used in this step. These burs work better if the flange is removed as shown. Aust. distributors Komet burs - Henry Schein Halas.
