Old Tricks for New Dogs - Vertical Margin in Fixed Prosthodontics

The vertical margin in full veneer crown preparation can offer an esthetic, minimally invasive and biocompatible solution for teeth that are periodontally or structurally compromised.

For example, this patient, which was restored as a fixed-removable combination case:

The definition of the vertical margin

The finish line is the border between the tooth and the most apical point of the preparation (Castellini, 1994), but the finish line may be:

1)Linear Finish Line (sometimes known as "Horizontal Margin").

This is where there is a distinct finish line and the clinician defines with a burr the exact position where the restoration should finish on the tooth. The technician then fabricates the restoration to this margin. See Figure 5 (prepared margin) and Figure 6 (crowns on recall).

Linear finish lines may be simple (for example, a chamfer or shoulder) or complex (a shoulder with a bevel).

2) Area Finish Line (also known as “Vertical Margin”).

In this approach there is a finishing area rather than a line. The clinician defines an area where the crown margin may finish. The clinician and technician then collaborate to determine a finish line position that will give optimal esthetics, biomechanics, and periodontal health.

See Figure 7 (prepared tooth: observe no apparent margin) and Figure 8 (final outcome; full coverage monolithic zirconia crowns on both upper lateral incisors). The area finish line may be either feather-edged (where the axial wall meets the root at 180 degrees) or knife-edged (where the axial wall meets the root at less than 180 degrees).

However, in reality a combination of feather and knife edges often co-exist on the same prepared tooth, especially if the tooth is multi-rooted.

When is the use of the vertical margin advantageous?

Restoration of patients with advanced periodontal disease: When a patient has recession and an exposed root face it is often desirable to finish at the gingival margin to optimize esthetics.

However, preparing a root face for a linear margin obviously requires cutting into the root and may run the risk of devitalization of the tooth or a reduction of the biomechanical performance of the tooth risking fracture. The vertical margin requires no preparation of the root at all and is therefore more conservative in these cases.

Restoration of endodontically treated teeth: For the same reason as above the vertical margin does not compromise the horizontal ferrule of root treated teeth.

Provision of splinted crowns: Used to stabilize mobile teeth multiple units of cross-arch braced crowns can be difficult to prepare with parallel paths of insertion. The vertical margin makes this a simpler proposition since the technician has some degree of flexibility in margin positioning and can compensate for minor alignment errors.

Preparation of crowns with a previous biologic width invasion: The existing horizontal margin (if minimal depth) can be deleted during the preparation of a vertical margin. If the margin of the provisional crown is placed more coronally than the previous margin the tissue can re-establish a healthy biologic width without the need for crown lengthening surgery. This is often advantageous with challenging crown: root ratios.

What are the challenges of the vertical margin?

Esthetics: this margin design is unsuitable for porcelain-fused to metal crowns since a visible metal margin will result.

Stress-distribution: If not carefully managed this margin creates high stress distributions in comparison with other margin types during firing and when occlusally loaded (El Ebrashi et al, 1969). This may result in a margin which is weak in tension and therefore subject to distortion (Kuwata, 1989).

In view of this, the vertical margin is only suitable for porcelain fused to metal crowns with a metal collar (for non-esthetic zones, e.g. lower molars) or zirconia crowns. Some recent research has also seen success with lithium disilicate.

How does the technician define margin position?

Since there is no clearly defined linear finish line, the vertical margin presents a unique challenge: where should the crown margin begin?

Clearly, if the margin is too coronal it will be visible and present an esthetic challenge. Conversely, if the margin is placed too apically then there is a risk of impinging on the biologic width resulting in a periodontal compromise.

The solution is very simple. We will illustrate the approach with reference to this case shown in Figure 9.

After pouring the dies (or on the scan) prior to trimming the die the technician marks the position of the gingival margin on the preparation in a black pencil (Figure 10).

This represents the maximal coronal limit of the potential margin. The die is then trimmed in the conventional manner; the technician then marks the most apical extension (usually the base of the sulcus) of the impression in a red pencil.

The finish line can then be created anywhere in the zone between red and black pencil lines: clearly in zones where the esthetics are less of an issue (for example, the palatal) the margin is finished closer to the black line.

Where esthetics are more of an issue (for example, discolored substrates), or where creation of an ideal emergence contour is more challenging (for example, spaced teeth or peg laterals) you may choose to move the finish line more gingivally.

What is the preparation sequence for vertical margins?

The following illustrates an A-Z Stepwise sequence for the preparation of an anterior tooth:

The first phase is to create incisal depth grooves for reduction guidance: the workhorse burr for the entire procedure is the 863 016C burr (Figure 11).

Initially the portion of the burr closest to the shank, which is a uniform 1.6 mm in diameter, is used at high speed with water spray to create depth grooves at the full diameter of the burr (see Figs. 12, 13 and 14). Caution should be taken not to use the tapered aspect of the burr since this will create non-uniform depth guidance.

Next, the preparation is separated from the adjacent teeth using an 862 010C burr at medium speed with a water spray (Figure 15): focus should be on the adjacent tooth rather than the tooth being prepared in order to avoid iatrogenic damage.

I prefer this burr over a “needle” type design since the flame is much less likely to lose diamond particles and blunt over the needle. The final separation should be slightly over-tapered in terms of Total Occlusal Convergence (TOC) for two reasons: firstly iatrogenic damage to the adjacent teeth is less likely, and secondly it should be noted that in the subsequent “gingitage” (Ingraham et al, 1981) stages the TOC tends to reduce as the tooth is prepared. The slight initial over-taper compensates for this and avoids undercuts (Figure 16).

The incisal edge is then reduced to the full depth orientation grooves with a wheel diamond, with my choice being a 909 040 1.5-mm at high speed with water spray (Figure 17). The wheel is preferred over a cylinder diamond since it offers more control and is less likely to create divots on the incisal edge.

The operator should then revert back to the 863 016C burr using the aspect of the burr closest to the shank to reduce the incisal aspect of the facial surface. Be aware that the facial is reduced in two distinct planes to ensure the correct thickness of ceramic is available for optimal esthetics (Figure 19).

The same burr is then held in the long axis of the tooth to reduce the facial. At this point the burr should only contact the height of contour, which will be in the mid third of the tooth (Figure 20).

As reduction continues at high speed with water spray the contact point of the burr gradually moves apically so that finally the burr will be in contact with the tooth at the level of the gingival margin (Figure 21).

The burr is then worked interproximally and palatally in much the same manner until axial reduction is complete. The aim is for 10-20 degrees of taper with a minimum cingulum height of 3 mm (Goodacre et al, 2001).

At this point, the progression is toward the “gingitage” phase. The aim is to remove undercut and create an uninterrupted linear progression from root surface to the coronal aspect of the prep; further to remove sulcus epithelium but not connective tissue, to remove chronically inflamed tissue and establish a blood clot which will ultimately recreate an ideal emergence profile.

An 863 012C burr is employed at medium speed with a water spray. The burr is initially advanced at an angle into the sulcus (Figure 22) and as the undercut is removed, slowly inclined into the long axis of the prep (Figure 23); this ensures the tip of the burr is not cutting and reduces the risk of gouging of the root surface.

Prior to penetrating the sulcus with the burr it is prudent to assess sulcus depth with a periodontal probe to avoid inadvertent damage to the connective tissue.

The preparation is then smoothed with fine burrs and discs to create a glassy smooth finish to allow accurate fit of the restoration and soft tissue resolution: initially an 863 012F is employed at low speed with water spray within the sulcus (Figure 24) and then over the reduction planes of the preparation (Figure 25).

The palatal surface is then smoothed with a 379 023 5mm fine football diamond at very low speed (Figure 26). Finally the entire prep is polished with fine discs to create a flowing prep with no sharp line angles (Figure 27): the final outcome being seen in Figure 28.

Figures 29-31 show the excellent esthetic and soft tissue outcomes that can be achieved in compromised teeth with vertical margins in combination with zirconia crowns.

Jason Smithson, BDS (Lond), DipRestDentRCS (Eng), is a member of Spear Resident Faculty.

References

Castellini D. La preparazione dei pilastri per corone in metal ceramic. Bologna: Martina 1994: 207-208

El Ebrashi MK, Craig RG, Peyton FA. Experimental stress analysis of dental restorations. Part III. The concept of geometry of proximal margins. J Prosthet Dent 1969; 22: 333-345

Kuwata M. Atlante a colori sulla tecnologia delle riconstruzioni ceramo-metaliche. Utet 1989; 2: 235-250

Ingraham R, Sachat P, Hansing Fj. Rotary Gingival Curettage-a Technique For Tooth Preparation And Management Of The Gingival Sulcus For Impression Taking. Int J Perio Rest Dent. 1981 ; 1(4):8-33

Goodacre Cj, Campagni Wv And Aquilino Sa. Tooth Preparations For Complete Crowns: An Art Form Based On Scientific Principles. J Prosthet Dent 2001; 85:363-76