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The Traditional Mechanically Retained Restoration Paradigm: Subgingival Margins are Normal and a Necessary Byproduct of Restorative Needs

Four images showing periodontal inflammation caused by subgingival margin placement.

Introduction

In traditional mechanically retained dentistry such as full crowns, subgingival margin placement is considered to be normal, necessary, and a byproduct of traditional restorative techniques. The over 37 million crowns placed every year by dentists in the United States [1,2,3] show that it is by far the most popular indirect procedure, regardless of the material used, and is viewed as ideal for most cases, predictable and relatively easy to perform. However, familiarity must not be confused with simplicity [4,5]. Partial coverage supragingival adhesive techniques are often considered to have a more unpredictable outcome, to be more difficult to perform and less successful in general. It is human nature to think that what we do every day and know well is better than something unfamiliar.

Yes, tooth-colored restorative materials are currently extremely popular, but often these new materials are being used in conjunction with traditional mechanically retained techniques, and some computer-aided designed and manufactured (CAD-CAM) restorations. This often leads to subgingival margin placement. When all ceramic full crowns, inlays, onlays, and veneers are performed using principles and techniques extrapolated from traditional mechanically retained principles such as unnecessary subgingival margin placement, there is a risk of increased failure, postoperative sensitivity, pulp injury, non-esthetic results, and a defective margin. All these outcomes will result in an unhealthy periodontium. This incorrect use of modern restorative and adhesive materials ultimately produces no net benefit to the patient or the dentist (Figures 1.1-1.4).

Figure 1.1 Veneer and crown with deep subgingival margins, and poor periodontal health.

Figure 1.2 Recently completed computer-aided design and fabricated IPS e-max® (Ivoclar) crowns. The patient had severe sensitivity, and the gums were inflamed and bleeding. Some of the margins were deep subgingivally, especially interproximally.

Figure 1.3 Improperly aggressive preparation for a zirconia crown. Both lateral incisors required root canal therapy, owing to damage to the pulp.

Figure 1.4 Bonded onlays with deep subgingival margins. The extreme difficulty of bonded cementation with bleeding subgingival margins led to catastrophic failure.

The negative consequences of periodontal disease are well understood, as they affect not only the longevity of the teeth but also the overall health of our patients. The negative consequences of subgingival deposits of tartar in the ultimate periodontal health of our patients are also understood [6,7]. Overhangs and open margins (in fact, any defective margin, when it is subgingival) have the same effect as permanent tartar on the periodontal health of our patients (Figures 1.5-1.8) [8,9,10,11]. Taking the above statement into account, supragingival restoration margin placement would be ideal, and most clinicians, if asked, would say that this would be their first choice [12]. This prompts the question why are so many crowns, onlays, veneers, and direct class II and many class III restorations being placed with a subgingival margin? The reality is that habit instills the idea that placing margins subgingivally is a "normal" and necessary adverse effect. Subgingival margin placement is so common that it goes undetected and unnoticed. When using traditional mechanically retained restorative principles, it is believed that subgingival margin placement has important advantages, which outweigh the negative consequences. Subgingival margin placement should no longer be considered normal or necessary. The continued use of mechanically retained traditional restorative dentistry principles has many disadvantages, and the solution is supragingival minimally invasive adhesive dentistry, which is discussed in subsequent chapters. First, it is important to understand how unnecessary subgingival margin placement happens, and the consequences of such placement.

Figure 1.5 Periodontal tartar in proximity to defective crown margins, with similar unhealthy effects.

Figure 1.6 This patient was happy and unaware of unhealthy gingiva.

Figure 1.7 Inflamed gums, and poor margin.

Figure 1.8 X-ray showing severe periodontal damage likely caused by poor fitting subgingival restorations.

Subgingival Restorative Margin Placement

Subgingival margin placement is a byproduct of traditional restorative techniques. The majority of these types of restoration include subgingival margin placement, and it often goes unnoticed. There are four primary reasons why the margin is placed subgingivally, and a fifth which is based on misinformation:

1. Mechanical retention and lack of trust in adhesion
2. Traditional restorative techniques, boxes, and cervical clearance
3. Subgingival margin placement for esthetic purposes
4. Actual subgingival caries or existing restoration
5. Belief that the tooth will be better off and less likely to have recurrent caries if the margin is placed below the level of the gingiva.

Mechanical Retention and Lack of Trust in Adhesion

Historically, mechanical retention with traditional direct and indirect restorations, amalgams, composites and crowns was necessary [13,14,15,16,17]. For over 100 years, dentistry has relied on mechanical retention to hold restorations in place: axial walls with certain minimum heights, usually 3-4 mm, offsets, boxes, shoulders, and additional mechanical features. Restorations were retained in the tooth primarily by friction before the benefits of adhesion became available. Unfortunately, many dentists still do not fully trust adhesives to support and retain their restorations, and thus they continue to prepare teeth using traditional mechanical features. Even when the caries is supragingival, the need for axial wall and mechanical retention forces a degree of ­preparation that will invade the gingival sulcus (Figure 1.9). This is most common after the removal of mesial and distal caries and old restorations that leave the cavosurface margin already close to or at gum level. A common scenario is when a patient has a short clinical crown, without enough supragingival tooth remaining for a long retentive axial wall. Extension for retention will ­inevitably lead to a subgingival margin placement (Fig­ure 1.10). An adverse effect of axial reduction is the increase in the amount of tooth reduction. A full crown requires approximately 70% of the clinical crown to be drilled off (Figure 1.11); partial coverage restorations require a fraction of this amount [18,19]. The damage to the pulp by the additional unnecessary drilling (considering that bonded restoration does not require axial reduction) ranges from increased postoperative pain to irreversible pulpitis and pulpal necrosis, correlating with remaining dentin thickness [20,21,22,23,24]. Trust in adhesion and proper supragingival minimally invasive restorative techniques renders mechanical retention unnecessary.

Figure 1.9 Before and after finished adhesively retained onlay preparations. This is most commonly followed by axial reduction for mechanical retention with deep subgingival margins as a byproduct.

Figure 1.10 Flat tooth showing the inevitable need for subgingival axial walls if resistance and retention are the goal.

Figure 1.11 The need for mechanical reduction will create subgingival margins; a fully adhesive restoration without axial reduction will stay supragingival.

Traditional Restorative Techniques, Boxes, and Cervical Clearance

Traditionally, in both direct and indirect restorative dentistry, there has been a need to create clearance with the adjacent tooth facially, lingually, and gingivally, to place a matrix band, take an impression, confirm restoration seal, and so on. Facial and lingual clearance is of concern because of excessive tooth removal, but traditional gingival clearance techniques are the source of subgingival margin placement (Figure 1.12). To gain gingival clearance from the adjacent tooth, dentists have been taught to drop the gingival floor of the box for direct restorations, and the gingival interproximal margin for indirect restorations (Figure 1.13). This technique is extremely counterproductive for any adhesive dentistry procedure, as it leads to subgingival margin placement and causes the loss of the enamel margin, with negative consequences in both cases (Figure 1.14a,b). Techniques such as the "cervical margin separation" can replace the need to drop the margin and go subgingival, as subsequent chapters illustrate.

Figure 1.12 A traditional geometric box (courtesy of Dr Boris Keselbrener).

Figure 1.13 Gingival clearance created by dropping box technique (courtesy of Dr Boris Keselbrener).

Figure 1.14...