Tomorrow Tooth Journal 20-03-2017;1:1-29
The development of new materials and equipment, as well as a better comprehension of soft tissue biology, has given birth to a new revolutionary approach in prosthetic dentistry:
the shoulderless concept
The preparation of the abutment using horizontal finish lines (shoulder and chamfer) has been widely used in the last three or more decades and acknowledged by the academic world as the gold standard.
“Horizontalists” authors claim for the following alleged advantages:
1 - Avoiding overhangs
2 - Avoiding horizontal and vertical over-contouring
3 - Respecting of the biologic width
4 - Finish line capturing and determination on the die
5 - Better lab-clinician workflow
The same “horizontalists” authors have alleged preparations with vertical finish lines as having
the following alleged drawbacks:
1 - Unavoidable overhangs
2 - Unavoidable over-contouring
3 - Uncontrolled invasion of the gingival sulcus and biologic width
4 - Difficulty of capturing and determining a finishing line on the die
5 - More difficult lab-clinician workflow
Thus on the basis of prevailing thought, the academic world has unanimously considered the vertical preparation an incorrect approach.
Over the years, a minority of professionals dealing with “verticalism” have developed two main approaches and schools of thought:
- the shoulderless approach
- the edgeless approach
The shoulderless approach, known also as bevel, has been used extensively in the Gold Era, thanks to the possibility to finish metal margins at a minimal thickness. Shoulderless design has been always unanimously acknowledged as the most conservative approach towards dental structure and the less prone to marginal gap.
The edgeless approach, known also as gingitage, was born with the work of Vick Pollard and Rex Ingraham. A variant of this approach was proposed by Morton Amsterdam and later by the team of Di Febo and Carnevale (Mascarella’s school) and recently by Ignazio Loi (BOPT).
The edgeless approach, distinct from shoulderless approach, aims to place finish lines subgingivally and to seal the preparation coronally to the finish lines on the prepared portion of the tooth with the indirect restoration, according to the need of the technician and the clinician.
This approach, useful in periodontally compromised teeth, was not well suited for periododontally sound teeth, often resulting in irreversible damage to the periodontal attachment. Moreover this approach would be truly aggressive in the pericervical area and would result in a rough prepped surface on the uncrowned and exposed area.
The suggested burs and protocol of Mascarella's school and BOPT are not easy to be handled without an high level of magnification (microscope) and require exceptional dexterity in preparing teeth. Following Mascarella or BOPT approach clinician and technicians have therefore to face the following problems:
- Problematic and inevitable undercuts
- Excessive taper of the prepared tooth
- Inevitable damage of connective attachment
- Profuse bleeding
- Six weeks of waiting/healing period prior to impression
- Unpredictable soft tissue regeneration
- Time consuming trial and error on temporaries during the healing period
- Absence of a neat, clear-cut, visible finishing area and finish lines
It has been a common and frustrating experience among the colleagues starting to practice the Mascarella’s approach.
The three aims of this preset paper are:
A- Demystify the “hypothesis” of periodontal damage from vertical finish lines
B- Show the biomechanical advantages of the vertical finish lines
C- Introduce a completely new approach in designing and managing the vertical finish lines
Demystify the “hypothesis” of periodontal damage from vertical preparation
This new technique is periodontally and biologically friendly. With the aid of the special burs (see part C of the paper) it is virtually impossible to violate the Biological Width (BW).
BW is defined by the sulcus, the junctional epithelium (JE) and the connective tissue attachment (CT). The only structure that has a biologic reaction after invasion is the connective attachment. We may highlight that the JE is not important, as it may simply be juxtaposition of epithelial structures on a surface, that in a “healthy environment” is represented by
- the enamel
- the emidesmosomes, connecting epithelial cells to a surface as long as this surface is hard, smooth and clean. This surface may be either enamel, cement, dentin, composite or zirconia.
Only two conditions are mandatory:
- the surface has to be smooth, hard, clean
- the epithelium has to be pressed and supported from that surface on behalf of a healthy connective tissue
Rotary curettage performed inside the sulcus area or in the JE zone is actually an advantage because it produces a smooth, non lacerated, sulcular wall surface compared to other techniques like electrosurgery or retraction chords. This wound heals by re-epithelization of exposed connective tissue from contiguous oral epithelium. The formation of a new lamina propria and junctional epithelium and a return to the original quality and distribution of microvascularization of the tissue, occur rapidly and predictably.
The sooner the final restoration (crown) is delivered the better will be GUIDED the regeneration of the new pattern of epithelium. This represents the core of the biological approach borrowed from the post-extractive immediate loading of implants philosophy: the cervical anatomy provided from our technician will GUIDE the regeneration of the soft tissues.
Violation of the CT attachment (and in particular of the root cementum into whom the connective tissue fibers insert) induces an inflammatory reaction leading to the production of inflammatory molecules (proteases, cytokines, prostaglandins, and host enzymes), that will activate osteoclasts to induce bone resorption and risk of soft tissue recession. This is to be avoided.
In the technique proposed over these years by other authors (Di Febo, Loi, etc.) the introduction of the bur in the sulcus was very dependent by the operator’s skills and feeling.
With the technique proposed by the authors the violation of BW is virtually impossible because the non-working tip of the special bur is calibrated in order not to touch the first millimeter of the root where the connective tissue fibers insert into the cementum. Moreover the possibility to use a smaller tip in comparison to conventional vertical preparation techniques allows a rotary curettage involving only the epithelial component of the sulcus with minor or no bleeding and faster healing.
It is critical, in case of sub-gingival finish lines, that the crown restoration be designed with an emergence profile that will support the gingival tissues and cause them to produce a tight gingival cuff that will act as a tight gasket around the crown’s cervical area. A tight cervical cuff will protect against food impaction in the gingival sulcus as well as prevent plaque and tartar from accumulating inside the loose gingival tissues on the untreated surface of the tooth. The emergence profile advocated here is considered to be over-contouring by conventional standards. The concept presented here is borrowed from the anatomy of the emergence profile of enamel on natural teeth and its relationship to the gingival tissues in the early dentition.
In periodontally healthy teeth the use of a non cutting tip allows not only to do a better controlled invasion of the sulcus , but to accomplish two critical elements:
- a more conservative preparation of the tooth
- the maintenance of a non-prepared area of the Tooth (shoulderless approach). The convergence of the root with the neighboring teeth keeps the same inter-root distance and allows to maintain and get better inter-proximal tissues.
showing the biomechanical advantages of the vertical finish lines
The biomechanical perspective in the tooth abutment design is intimately correlated with the concept of ferrule. The word “ferrule” has been widely misunderstood over the years and not properly addressed. In common usage the word “ferrule” is referring to the residual dental structure in two dimensions when it really refers to the “ferrule effect.”
Ferrule is a corruption of Latin “viriola” (small bracelet) under the influence of “ferrum” (iron).
Ferrule is any of a number of objects used for fastening, joining or reinforcement. They are often narrow circular metallic rings. Thus, the ferrule in prothetic dentistry is the PROSTHETIC CROWN, which is going to fasten, like a bracelet, the residual dental structure.
As in mechanical engineering, the ferrule in prosthetic dentistry should have designed and built according to these two following features:
- being built in a material with modulus of elasticity larger than that of dental structure
- being designed not at the expense of residual dental structure
First then, the crown should be in material with high modulus of elasticity, as metals or zirconia. Material like composite or similar are not suitable for this goal.
Second, being not at the expense of the residual dental structure, means that we should not reduce residual dental structure, i.e. the dentin, especially the pericervical dentin (PCD). To distinguish common usage of ferrule from the remaining tooth structure, Clark and Khademi refer to this remaining tooth structure as 3DF three-dimensional-ferrule.
Enamel to some extent is not a structural material. It’s main functions are to seal the denting and protect the tooth from wear. These two functions are in some way “replaceable” by prothetic material. Evidence that enamel is not necessary for long-term tooth survival is given by the many cases of recession with exposed root structure, often with little or no functional impairment.
The primary problem with all “horizontal” finish line designs is that the ferrule effect is created at the expense of the residual dental structure: the 3DF. This is further aggravated by more apically placed finish lines in a futile attempt to improve retention, as dentin thickness decreases. This is very detrimental, especially in endodontically treated teeth.
The shoulder version of the horizontal finish line is the worst scenario, because not only it reduces the thickness of the 3DF but it creates a stress-concentration point at the shoulder itself undermining the ferrule effect it was attempting to provide.
As a first approximation to give the clinician a sense of things, the shear strength of a brittle cylinder is primarily dependent on the cross section of the cylinder (A = π r2). Reduction (1 mm) in a radius of a cylinder from 4.5 mm to 3.5 mm will result in a surface area reduction of 40%. Shoulder preparations require a minimum of 1 mm in tooth reduction at the cervical level.
Assume for a moment a shoulder preparation for a porcelain jacket crown on a 10-mm diameter root of a maxillary central incisor with a 1-mm diameter pulp. A shoulder preparation of 1 mm in depth will result in a 40% decrease in surface area (bulk) of the tooth at the finish line, compared to a 20% reduction in bulk that can result from a 0.5 mm finish line depth. Increasing the finish line depth from 0.5 mm to 1 mm will result in 50% reduction in shear strength of the prepared tooth.
Based on this example, shoulder preparations should be abandoned in favor of a shallow finish line preparation to preserve tooth bulk and maximize tooth resistance to cervical fractures.
Failure in bending (snap-off) is most likely to occur as the shoulder acts as a stress concentration point.
Super imposition of different preparation designs
- chamfer of 0.4 mm above finish line
- chamfer of 0.8 mm above finish line
The only way we have to prep the “ferrule” without affecting the 3DF is to avoid any “horizontal” invasion (like shoulder or chamfer), but to create a shoulderless preparation.
For the ferrule to be effective, the residual dental structure should be well engineered by the dentists. The clinician should prepare the abutment maximizing the 3DF taking care of the following parameters about the remaining dental structure:
- spatial position
introducing a new approach in designing and managing vertical finish lines
Integral to these preparation protocols is a round-ended tapered diamond bur with a non-cutting end.
This bur is well known in the endodontic field as Batt-Bur.
This bur has the following advantages:
- Taper of about 2 degrees facilitates an optimal taper of the abutment
- Coronal diameter of 1.2 mm and apical diameter of only 0.7 mm facilitate a very conservative approach.
- Non-cutting end of 1 mm reduces or avoids the damage to the connective attachment.
- Non-cutting end of 1 mm allows-facilitates a tooth-guided preparation procedure.
- Facilitates a machined preparation. What you drill is what you get.
- Non-cutting end allows a bloodless gingitage, creating space for the immediate impression and an easy provisional relining.
- Non-cutting end allows to work in the presence of cord or teflon tape, without any tearing or impingement.
- Bur design allows even the non-expert clinician to avoid any undercuts into the prepped abutment.
- Non-cutting end allows to be used as a periodontal probe.
- Design differs from flame burs used in the approach of Mascarella’s school and BOPT, allowing a pure geometrical frustum and true vertical finish lines.
- Encourages immediate impression and provisionalization, in contrast to the other approaches.
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