“Oh No You Didn’t!” Why Patients Keep Breaking Restorations

John J. Maggio, DDS Clinical Assistant Professor Department of Restorative SUNY at Buffalo, School of Dental Medicine [email protected]

GUIDANCE AND INTERFERENCE

The prognosis of any restorative therapy is heavily dependent on a consideration of all of aspects of the patient’s function.

The ideal features only static posterior contacts in physiologic occlusion, with only anterior teeth functioning in protrusive and lateral protrusive movements.

That means posterior teeth only touch in maximum intercuspation, and only the anteriors touch when the mandible moves.

from Dawson, Functional Occlusion: From TMJ to Smile Design, 2006

Posterior teeth are designed to withstand compressive forces from the opposing teeth. Anterior teeth are designed to undergo shearing forces (or tension) from the opposing teeth in protrusive and lateral protrusive movements.

THE CONFORMATIVE APPROACH

“Conformative Occlusion” – placing a new restoration so that it does not alter the current occlusal contacts on all other teeth.

1

This is indicated when: - The patient has a stable maximum intercuspal position - The patient has no joint/muscular disease - The teeth are free of disease

“Conformative” occlusion is not the easiest approach, but it is the SAFEST approach for the teeth, periodontium, muscles and TMJ’s. (Davies et al., 2001)

SUMMARY - In the ideal occlusion posterior teeth contact only in maximum intercuspation. - In the ideal occlusion only anterior teeth contact in mandibular movements. - Posterior teeth are designed to withstand compressive forces. - Anterior teeth are designed to withstand tensile forces. - Occlusions that shift compressive forces to anterior teeth can cause chipping, wear and fremitus. - The conformative approach to restorative dentistry respects occlusal contacts of all teeth. - If a patient has a stable intercuspal occlusion, no pain and no TMJ disease, a non- ideal occlusion can be maintained. - Occlusal contacts should be observed BEFORE treating a tooth. - Articulating paper can identify static contacts and protrusive contacts. - Occlusion should also be observed visually. - Occlusion should be studied BEFORE anesthesia. - All mandibular movements a patient can perform should be considered when restoring a tooth.

MISSING & TIPPED MOLARS

The importance of a bilateral molar occlusion cannot be overstated. When molars are missing, the mandible advances to find a position of maximal contact of remaining (anterior) teeth. The anterior teeth are then subjected to compressive forces. The results of a lack of a bilateral molar occlusion are: - unhealthy stresses on the TMJ’s, - muscle strain, - excessive and inappropriate forces on anterior teeth (and even remaining premolars). Teeth in such a situation will BREAK, WEAR or FLARE.

Therefore, it is very important for us to encourage patients to retain all of their teeth. When this is not possible, missing teeth should be replaced, to minimize forces on the remaining teeth. For patients who will not replace their missing teeth, restorations on the remaining teeth will have a weaker prognosis.

2 THREE TYPES OF SUPERERUPTION

Craddock et al, 2007

Active Eruption involves loss of attachment, as seen in the presence of periodontal disease. The clinical increases in height.

With Passive Eruption no additional tooth structure is exposed. - most common in: • younger patients • women • maxilla • premolars

In Relative Wear, all teeth wear except the unopposed tooth. - most common in: • older patients • mandibular teeth

Active Eruption was the most frequent mode seen, followed by Periodontal Growth, with Relative Wear much less frequently observed.

So, we have a lot of information to rely on when planning with our patients... We know that: • Most unopposed teeth will supererupt. • Younger patients are very likely to have a supererupting tooth if they lose a tooth. • Maxillary teeth are more likely to supererupt than mandibular teeth. • Teeth on either side of a lost tooth are likely to tip toward the space. • Adjacent mandibular molars will tip the most. • Adjacent mandibular premolars and maxillary molars are most likely to rotate. • After ten years, or with periodontitis, teeth will actually erupt out of the tissue, exposing roots. • Opposing teeth with partial contact will most likely still supererupt (and to the same degree). • Opposing teeth with partial contact are more likely to tip. • Passive Eruption (Periodontal Growth) is common in young people, women, maxillary teeth, premolars and non-perio involved teeth. • Tipping and rotation of teeth adjacent to a space are more likely with a small 3 overbite. • Rotated, tipped and supererupted teeth cause occlusal disturbances and interferences.

We can tell our patients: • tooth loss will have a lot of consequences, even if it’s “just one back tooth”. • lost teeth are best restored as soon as possible. • nutrition will be best with a fixed or an implant. • TMD is likely following tooth loss, and will not necessarily improve by replacing the teeth.

COMBINATION SYNDROME

Combination Syndrome is commonly seen in patients who only have mandibular anteriors left. Severe anterior resorption of the maxilla results. Treatment with Complete Upper Denture and Mandibular RPD are not always successful, as the maxilla continues to be resorbed. The author suggests implants to retain the alveolar bone and end the destructive process. (Tosltunov, 2007)

MISSING MOLAR SUMMARY - Lack of a bilateral molar occlusion can result in advancement of the mandible to maximum contact of anterior teeth. - Lack of a bilateral molar occlusion will cause wear, chipping and/or flaring of anterior teeth. - Restorations placed on remaining teeth have a weaker prognosis than with a complete dentition. - Loss of posterior teeth can lead to TMD. - Even single posterior tooth loss can cause major disturbances in the occlusal plane and excursions. - The ultimate disastrous result of loss of molars is Combination Syndrome.

Fremitis is not always seen in these cases, because periodontal ligament proprioceptors will affect a change in the occlusion to achieve maximum intercuspation and spread forces among all remaining teeth. This change can bring a deviation from the midline in protrusion.

Some patients (even with a full compliment of teeth) initially close in a centric position and then skid forward to a preferred anterior position.

The causes of this unconscious adaptation could include: - a posterior prematurity in centric, - a diseased tooth (endo or perio lesion), - a diseased TMJ or TMJ’s, including disc disease, - muscular disease, - lack of a posterior position that provides maximal intercuspation. 4 Most people (even with full dentitions) deviate from the midline when protruding. This is an adaptation to achieve maximum contact and to avoid overloading one tooth. Teeth wear accordingly.

Posterior interferences in protrusive and lateral protrusive excursions can: - interfere with proper anterior function, and - overload posterior teeth in function.

Anterior guidance protects posterior teeth, and reduces stress on anterior teeth. In anterior guidance, posterior teeth disclude and the muscles of mastication shut down.

THE NEUTRAL ZONE (from Dawson, Functional Occlusion: From TMJ to Smile Design, 2006)

Teeth are positioned in the neutral zone, that area where the forces from the tongue are equal to the forces from the lips or cheeks.

Teeth exist in their current position because the musculature wants them to be there.

The teeth are positioned where the forces from facial muscles equal the lingual muscles.

• The buccinator muscle is essentially one continuous band from left to right. • The neutral zone can explain why orthodontic cases, even those involving orthognathic surgery, often fail or relapse. • The neutral zone can also explain restorative failures, especially when a restoration extends beyond the neutral zone. • Crowded dentitions are the result of teeth that are larger than the neutral zone can tolerate. The musculature can tolerate an odd tooth out of place better than it could tolerate an arch form that violates it.

5 • The concept of the neutral zone also explains why inappropriately increasing the vertical dimension of occlusion can cause lingual tipping of anterior teeth, as the suddenly stretched buccinator seeks to return to its original length. • Unless the muscular cause of the position of a tooth is corrected, treatment will fail. • Attention to the neutral zone is a necessary prerequisite to restoring dentitions.

Patients with small mouth openings and narrow smiles will have more powerful buccinator muscles. They will be most difficult to treat. Success will depend on respecting the neutral zone, or a successful effort to change it.

Altering the neutral zone can be accomplished by several means: - orthodontics - elimination of noxious habits - myofunctional therapy - reduction of tongue size - surgical lengthening of the buccinators - vestibuloplasty

Aside from changing the neutral zone, the arch form must be observed and obeyed in the course of restorative therapy.

An overcontoured facial can cause the buccinator muscle to put pressure on the veneer. This could result in: 1. DEBONDING AND LOSS OF THE VENEER 2. VENEER FRACTURE, AND LOSS OF ONE OR MORE FRAGMENTS 3. LINGUAL MOVEMENT OF THE TOOTH, WITH WEAR OF THE OPPOSING TOOTH.

6 PHONETICS & ENVELOPES

Length of anterior teeth can be determined by observing the patient’s envelope of function.

When patients are asked to make fricative (“F” or “V”) sounds, the incisal edges of the maxillary incisors should contact the Vermillion border of the lower lip.

To see if a maxillary incisal edge can be lengthened, non-bonded composite can be temporarily placed. The new length can be tested with fricative sounds.

Length may not be the issue: a thickness of composite could be added to see if the new facial contour can be tolerated phonetically.

When patients are asked to make sibilant sounds (“S” or “SH” or “CH”), maxillary and mandibular teeth should be just shy of contacting each other, and the sound should be crisp (not slurred) with no air leakage.

Does the patient have a vertical envelope or a horizontal envelope? How do I know? Have the patient count the 60’s while you watch from the side.

7 Patients with horizontal envelopes are harder to treat, as they will have a greater tendency to stress the incisal edges of their teeth, potentially fracturing or dislodging restorations.

NEUTRAL ZONE SUMMARY • The Neutral Zone is the area where a tooth receives equal pressure from the buccinator and the tongue. • An extreme example of the power of the musculature is the severely constricted palate with tongue thrust and splaying of anterior teeth. • The Neutral Zone concept helps explain why teeth are crowded, why some orthodontic therapy fails, and why some cosmetic treatment fails. • Orthodontic and restorative treatment must take the Neutral Zone into consideration. • Length of anterior teeth is dictated by the patient’s envelope of function, as demonstrated by fricative and sibilant speech patterns.

Oral habits can be detrimental to cosmetic or esthetic treatment. This can include: - thumb or finger sucking, - lip sucking, - tongue thrusting, - biting, chewing or sucking on foreign objects. Careful diagnosis and habit cessation are critical to the prevention of failures.

All teeth involved in restorative therapy should be checked for mobility, fremitis and hyperfunction.

The periodontal and pulpal health of involved teeth should also be evaluated before treatment begins.

Bruxism can cause treatment failure. Dentofacial type can give an indication of risk.

Patients with short, square faces tend to have a higher incidence of parafunctional habits.

Patients with active bruxing or clenching habits should wear occlusal splints after treatment.

Any patient who has significant anterior treatment should be followed by occlusal splints.

Patients should be educated on the importance of wearing the appliance(s).

8 CROWN RETENTION

Conclusion: Repreparing the cervical half of an overtapered crown preparation increases resistance to crown dislodgement.

CROWN PREPARATION SUMMARY

Studies show that: • Tall preparations are more resistant to dislodging forces than short preparations. • Overtapered crown preparations are less able to withstand dislodging forces. • The most effective way to improve the resistance of an overtapered crown preparation is parallel repreparing of the cervical 1.5mm. • Proximal grooves, occlusal isthmuses and occlusal planes can contribute to increased resistance. • Proximal grooves are more effective than boxes. • M & D grooves are more effective than B & L. • Proximal grooves can add resistance to short crown preparations.

POST FAILURES

WHAT IS THE PURPOSE OF A POST?

It has never been shown that endo teeth are more brittle than vital teeth. - One study showed resistance to fracture is the same for vital and non-vital teeth. - BUT, endo teeth seem to have a lower moisture content.1

Posts do NOT reinforce roots. Posts WEAKEN ROOTS, by removing dentin.

But...

1 Helfer AR, et al: Determination of the moisture content of vital and pulpless teeth. Oral Surg Oral Med Oral Pathol 34:661, 1972. 9 A posted canal is stronger than an empty canal.2 - It is critical to utilize all of the prepared post space with the post. Empty space apical to the post can cause the post to become loose. It can also cause the root to fracture.

So... The purpose of a post is to RETAIN THE CORE.

The root retains the post. The post retains the core. The core retains the crown.3

WHEN DO WE NEED A POST? → When we need to retain the core.

In general, ANTERIOR teeth with minimal access preparations and intact marginal ridges do NOT need crowns.

Anterior teeth that are not crowned should NOT receive posts.4

In general, endo-treated posterior teeth should be crowned. EXCEPTIONS might be lower premolars and first molars with: - Conservative access cavities, - Intact marginal ridges, AND - Absence of excessive occlusal forces.5

Maxillary premolars with 2 or 3 surface amalgams have a high incidence of fracture. - Endo-treated upper premolars with previous sizable restorations should be crowned.

Even if we have a conservative access, we need to consider a post in cases of: - big access or big instrumentation - horizontal cracks on the crown - heavy function6

WHAT IS THE PURPOSE OF A POST? - To retain a core

So… WHEN DO WE NEED A POST? → When we need to retain the core.

2 Hunter et al, 1989 3 Henry & Bower, 1977 4 Sorensen & Martinoff showed no improvement in prognosis with posts present. 1984 5 Rosenstiel, Contemporary Fixed , 4th Ed, 2006 6 Christensen, 2004 10 Posts are not needed in molars if the crown prep (in natural tooth) is 3 to 4 mm in length.7 - If one is missing, we might consider a post.8

Another consideration… A post could interfere with our ability to retreat the endo.

HOW MUCH TOOTH DO WE NEED?

ANSWER: 2 mm all the way around the tooth. (FERRULE)

If you have 2mm of natural tooth all the way around, the actual type and design of the post does not matter as much.9 10 11

A better option might be forced orthodontic extrusion.

Tooth on left represents no surgery and no ferrule. Tooth on right represents surgery to obtain ferrule. Center tooth represents ortho extrusion to obtain ferrule.

- While this also alters the root length and changes the crown-to-root ratio, the crown is NOT lengthened, so the crown-to-root ratio is not as unfavorable as a crown- lengthened tooth.12

WHAT POST QUALITIES ARE BEST?

LENGTH OF THE POST

It is best to leave 5mm of gutta percha. - to avoid disturbing the apical seal - curves and lateral canals occur in the apical 5mm. 20 The absolute minimum is 3mm. 20

7 Kane, JJ et al, 1990 8 Rosenstiel, Contemporary Fixed Prosthodontics, 4th Ed, 2006 9 Assif et al, 1989 10 Milot & Stein, 1992 11 Rosenstiel, Contemporary Fixed Prosthodontics, 4th Ed, 2006 12 Rosenstiel, Contemporary Fixed Prosthodontics, 4th Ed, 2006 11 RETENTION - LONG posts are more retentive than SHORT posts. 20 13 14 15

FRACTURE RESISTANCE - SHORT posts are more likely to cause a root to fracture. - LONG posts resist fracture better because stress concentration is lowered.16

So, HOW long is long enough? - Retention seems adequate with ⅔ root length.17 - Resistance to deflection is greatest at ¾ the root length.18 19

But, can we always get posts that are that long? - Often, 5mm gutta percha cannot be retained with a post ¾ the length of the root (or even ⅔ length of the root).20 - The solution might be make the post as long as possible, while still leaving 3mm of Gutta Percha.

- The post should be at least the same size as the height of the crown.21

- Others say ½ the length of the root in bone is enough.22

- MOLAR posts should not exceed 7mm in length.23 - DISTAL CANAL OF LOWER MOLARS - PALATAL CANAL OF UPPER MOLARS

- If placing a second post in a MOLAR, try to avoid: - the MB canal on upper molars (root curvature and concavities), - the ML canal on lower molars. (not uniform)24

- Try to avoid: - the DISTAL surfaces of the MESIAL canals in mandibular molars (thin walls). - the M & D surfaces of the BUCCAL canals in maxillary molars. (easy to strip) 32 - Many authors recommend removing the Gutta Percha with HEAT, and not rotary instruments. Rotary instruments can be used to clean the walls, and to prepare a post space. This not only preserves dentin, but also helps avoid perforations.

13 Standlee et al, 1978 14 Kurer et al, 1977 15 Cooney et al, 1986 16 Standlee and Caputo, 1992 17 Hunter et al, 1989 18 Johnson & Sokumura, 1978 19 Leary et al, 1989 20 Zillich, 1984 21 Henry & Bower, 1977 22 Stern & Hirshfeld, 1973 23 Abou-Rass et al, 1982 24 Tilk et al, 1979 12 PERFORATIONS - Do not use a gates glidden larger than #3 in a molar. (Don’t use a gates glidden #4 in a molar.) - After endo, some furcation areas are already less than 1mm in thickness. - Length is more important than width. - 7mm post in distal of mandibular molars and palatal canal of maxillary molars.25

PERFORATIONS produce a distinct, round radiolucency with borders.

FRACTURED ROOTS produce a diffuse radiolucency with an oval “tear-drop” shape.

WIDTH OF THE POST

- Post diameter should not be greater than one-third the root diameter. Root concavities are not always easy to see on radiograph.26

- A wide post generates more stress when it is SHORT.

- Post length is more important than width in generating stresses. Medium to wide posts should be avoided if the post is short (causes too much stress.)27

- A thicker post actually stresses a tooth more, since it is sitting in a thinner tooth. Since length is more important than width, going for a wider post is not worth the risk of fracture.28

TEXTURE OF THE POST - Serrations, roughening, & grooves increase retention.29 30 31 - Serrated posts are 30-40% more retentive than smooth posts.32

SHAPE OF THE POST Categories of posts: 1. parallel 2. tapered 3. threaded (screw)

25 Kuttler et al, 2004 26 Tilk et al, 1979 27 Hunter et al, 1989 28 Rosenstiel, Contemporary Fixed Prosthodontics, 4th Ed, 2006 29 Colley et al 1968 30 Ruemping et al, 1979 31 Wood, 1983 32 Henry & Bower, 1977 13 RETENTION - The most retentive post is the THREADED (screw) POST.33 34 35 - The least retentive post is a TAPERED post. A PARALLEL post is more retentive than a tapered post.41 46 47

RESISTANCE TO FRACTURE

- SCREW posts are the most likely to fracture a root. - In that study of teeth extracted for vertical root fractures, 67% of those teeth had SCREW posts.36

- TAPERED posts are more likely to fracture teeth than PARALLEL posts. - TAPERED posts have a wedging effect and show the highest shoulder stress concentrations.37

- Smooth-sided parallel posts generate the greatest apical stresses.50 - Serrated parallel posts distribute forces on both sides of the post.50 - Serrated parallel posts place the root under less stress than other posts.50

But, this all depends on having: - adequate length - vertical adaptation (using all of the post space we created) - a round canal: - max central - max 1st molar - mand 2nd premolar - db canals of max molars

33 Kurer et al, 1977 34 Standlee et al, 1978 35 Ruemping et al, 1979 36 Fuss et al, 2001 37 Standlee et al, 1972 14 SUMMARY The ideal post is: - at least as long as the crown. - not wider than 1/3 root diameter. - serrated, rough, or grooved. - parallel.

WHAT POST TYPES ARE BEST?

CUSTOM CAST POST ADVANTAGES: - fit to the canal - good for oval canals

DISADVANTAGES: - time-consuming (extra visit) - costly - requires removal of tooth structure (undercuts, bulk)

- As much coronal dentin should be retained as is possible, to minimize stress which can fracture the root.38 The amount of remaining tooth structure (crown and root) is the single most important factor in success and prevention of root fractures.39Cast posts require removal of coronal and radicular tooth structure. - If PARALLEL posts are more retentive and produce less stress, cast posts should be parallel. We can accomplish this using parallel post drills, instead of Gates Glidden burs. - When Cast Posts fail, they often fail catastrophically (root fracture).

The Split Shank threaded post (“FLEXIPOST”) - This redesigned threaded post is split 2/3 down the shaft. On cementation, the split section flexes, to lessen stress on the root. - Because the coronal 1/3 is NOT split, there is a lot of stress at the neck of the root. This causes a WEDGING EFFECT.40 - When they fail, the root fractures. - Standlee & Caputo deduced that this is no better than a SCREW POST with respect to the potential for catastrophic failure.

PREFABRICATED POST: STAINLESS STEEL ADVANTAGES: - easy to use - strong - inexpensive

38 Henry & Bower, 1977 39 Rosenstiel, Contemporary Fixed Prosthodontics, 4th Ed. 40 Standlee & Caputo, 1992 15 DISADVANTAGES: - can fracture the root (rigid) - nickel allergy - not esthetic - can corrode41

PREFABRICATED POST: TITANIUM ADVANTAGES: - not allergenic - does not corrode

DISADVANTAGES: - not as strong as steel - can fracture root (rigid) - not esthetic54

PREFABRICATED POST: CARBON FIBER ADVANTAGES: - can bond easily - flexible - can be removed

DISADVANTAGES: - not strong - not esthetic42

PREFABRICATED POST: GLASS FIBER (silica) ADVANTAGES: - can bond easily - flexible - can be removed - esthetic43 - high tensile strength - elasticity similar to dentin - flex under load44 DISADVANTAGES: - not strong - Bonding reduces the wedging effect of the canal in the root. - Can use a shorter, thinner post. This conservation of tooth structure lowers the chances of root fractures.45 - Bonding might mean that the shape (parallel vs. tapered) might be less important for retention.46

PREFABRICATED POST: CERAMIC (zirconia) ADVANTAGES: - esthetic - very strong / rigid

41 Christensen, 2004 42 Christensen, 1996 43 Christensen, 2004 44 Bateman et al, 2003 45 Pontius & Hutter, 2002 46 Qualtrough et al, 2003 16 DISADVANTAGES: - expensive - difficult to bond - difficult to remove - can fracture roots47 - When Zirconium Oxide posts are bonded with a resin cement, the bond between ceramic and resin is weak. This would diminish the retention of the post.48

- Bonded metal paraposts had better retention than ceramic posts, regardless of how they were cemented. Silanated ceramic posts cemented with resin had greater retention than those posts cemeted with glass ionomer. Sandblasting the posts had variable success.49

SUMMARY - CAST POSTS remove a lot of tooth structure. - STAINLESS STEEL POSTS can corrode. - TITANIUM POSTS are weaker than steel. - CERAMIC POSTS do not bond well. ALL of the above can fracture roots.

- CARBON FIBER POSTS bond well, but are black. - GLASS FIBER POSTS bond well. - QUARTZ FIBER POSTS are stronger than glass. NONE of the above render a tooth unrestorable after failure.

SUMMARY: - A post is indicated only when it is needed to retain a core. - A ferrule of at least 2mm is critical to prevent loss of post or fracture of the tooth. - For retention and fracture resistance, posts should be long, parallel and rough/serrated. - While metal posts are stronger, fiber posts (carbon, silica or quartz) generally do not cause catastrophic fractures that cannot be retreated. - Bonding a post gives the best retention and resistance to fracture. - Composite resin cores render an endodontically tooth more resistant to fracture. When resin cores fail, the tooth can usually still be restored.

47 Christensen, 2004 48 Hedlund et al, 2003 49 Purton et al, 2003 17