Horizontal Cracks in Teeth

Horizontal cracks in teeth, also referred to as oblique cracks, are a common pathology, especially in teeth with large restorations. These cracks extend horizontally and vertically around a restoration, posing a risk to cusps that may fracture as a result of the crack. In more severe cases, horizontal cracks may propagate under a restoration near the pulp. When treated early, horizontal cracks can be treated conservatively, minimizing loss of healthy tooth structure. Advanced adhesive techniques, like those used in biomimetic restorative dentistry, not only restore teeth to their natural function but also prevent future cracks from forming.

A horizontal crack is clearly visible in this tooth, previously restored with a large amalgam restoration. Case by Dr. Davey Alleman, DMD.

What are horizontal cracks in teeth?

Any crack that extends beyond the dentin-enamel junction (DEJ) and into dentin poses a risk to the tooth and should be removed where possible. Horizontal cracks are identified by their movement across the tooth, either through cusps or directly across the pulp. These are also referred to as oblique cracks, not to be confused with craze lines, and should all be treated proactively to prevent loss of healthy tooth structure. 

Non-critical tooth structure is removed to aid in crack treatment. This crack can now been seen spanning the width of the tooth across the pulp. The modern treatment protocols of biomimetic restorative dentistry prevent cracks like these from forming under restorations. Case by Dr. Davey Alleman, DMD.

What causes horizontal cracks in teeth?

Teeth are able to withstand impressive occlusal forces thanks to the tough enamel exterior and resilient dentin interior. Cracks into enamel are one of a tooth’s natural defense mechanisms, but when these cracks extend into dentin, they put the tooth at risk. Horizontal cracks form most commonly in the following conditions:

Cracks that extend beyond the DEJ: teeth without a history of pathologies or treatment can crack over time. Patients’ bite forces vary, so stronger forces over time will weaken dentin molecules, gradually pulling them apart. Like any material, repeated force at a high enough strength will initiate a crack. 

Even composite restorations can put a tooth at risk of cracking if it is not adequately bonded.

Under this large restoration, horizontal cracks are visible around the pulp. Case by Dr. Davey Alleman, DMD.

Teeth with restorations: the occlusal surface of a tooth is supported by sub-occlusal oblique transverse ridges, also called “Rainey Ridges”, named after researcher J Tim Rainey (Rainey JT.  A sub-occlusal oblique transverse ridge: identification of a previously unreported tooth structure: the Rainey Ridge. J Clin Pediatr Dent. 1996;  21:9-13). These webs of enamel support the tooth during the forces of occlusion, so when they are removed like in a MOD or large Class II restoration, the tooth will bend and flex more than it is designed to. This puts the tooth at greater risk of crack initiation, making horizontal cracks, as well as vertical cracks, more common.

Thin cusps: large restorations that result in thin cusps increase the likelihood of horizontal cracks. Deep undercuts interfere with tooth hydration, cutting dentinal tubules that carry pulpal fluid to the dentin throughout the tooth. As these tubules are cut in thin cusps, this dentin loses its hydration making it more brittle (Kishen A, Vedantam S. Hydromechanics in dentine: Role of dentinal tubules and hydrostatic pressure on mechanical stress-strain distribution. Dent Materials. 2007; 23: 1296-1306) and, therefore, more susceptible to crack initiation under occlusal forces. 

Once the amalgam is removed, cracks in the thin cusps are visible and need to be removed for a long-lasting, asymptomatic restoration. Case by Dr. Davey Alleman, DMD.

Preventing horizontal cracks in teeth

Prevention is the best medicine, so teeth should be assessed for cracks during twice-annual dental exams. While preventing cracks in virgin teeth may not be possible, early diagnosis will allow for the crack to be treated as conservatively as possible.

Preventing horizontal cracks in teeth with previous restorations or teeth requiring restorative treatment is possible thanks to biomimetic restorative dentistry. The high bond strengths of biomimetic dentistry support a restored tooth at the same strength as a natural tooth is bonded to itself, meaning the tooth bends and flexes like a natural tooth, even after being restored. This natural tooth flexure means the tooth is not at a greater risk of crack initiation.

After the crack in this tooth was removed, it was restored using the biomimetic restorative dentistry protocols in the Six Lessons Approach in this case by Dr. Davey Alleman, DMD. The first photo shows the tooth after immediate dentin sealing and resin coating. The second photo shows the final restoration.

Achieving these bonds is highly technique sensitive, but offers superior results, compared to retention amalgam and even most composite restorations. Retention forms in restorations do nothing to support the remaining tooth structure, and composite restorations that do not account for bonding to various dental substrates and c-factor likely debond within minutes of light curing. This means they too do not support the tooth’s remaining structure and essentially function like amalgams.

To account for these variables, Dr. David Alleman, DDS created a systematic set of protocols for treating and restoring cracked teeth. He called The Six Lessons Approach to Biomimetic Restorative Dentistry, and doctors around the world are using this set of protocols to achieve predictable, long-lasting restorations, even in cracked teeth. 

Diagnosing horizontal cracks

Horizontal cracks in teeth can be diagnosed based on patient symptoms and treatment history.

Patient symptoms: frequent or intermittent pain or sensitivity indicates the tooth may have a crack. According to Martin Brännström’s Hydrodynamic Theory of Pain (Brännström M. The hydrodynamic theory of dentinal pain: sensation in preparations, caries, and the dentinal crack syndrome. J of Endodontics. 1986; 12:453-457), pain in the tooth is expressed when pulpal fluid dynamically moves in an opening in the dentinal tubules. Horizontal cracks may propagate slowly, which means dentinal tubules will remineralize, stopping the pain temporarily, but as the crack grows, the pain will return until the tubules are sealed. 

Once the old amalgam restoration is removed, cracks are visible underneath. The thin cusp and deep box likely led to the crack formation. Case by Dr. Davey Alleman, DMD.

1-2-3-4 crack risk assessment: Dr. David Alleman created a risk assessment as part of his Six Lessons Approach, to determine if a tooth was at risk of crack initiation based on its treatment history. These risk factors include restoration width, cusp width and box depth. Learn more about the 1-2-3-4 risk assessment in our Free Crack Treatment Webinar.

Treatment recommendations for horizontal cracks

Any crack that extends beyond the DEJ should be considered a pathology and treated just like caries — the earlier the better. Removing cracks while they are small conserves healthy and essential tooth structure, promoting a longer life for that tooth.

Cracks should be removed completely in the peripheral seal zone to protect the pulp. The Six Lessons Approach includes stress reduction protocols for severely cracked teeth and complete restorative steps to achieve biomimetic bond strengths that have been shown to last 20 years and counting.

View the full case showing treatment of horizontal cracks in teeth by Dr. Davey Alleman, DMD.

Dr. David Alleman was determined to find a better way to treat cracks, after having seen otherwise healthy teeth cut down for a crown based on traditional recommendations. Drawing on research and guidance from engineers, he began removing cracks from teeth in the early 2000s and the results were immediate. Once the crack was gone, the symptoms were gone. This led to restorations that remained symptom free, bonded and vital while providing alternatives to traditional crowns. Learn more about crack treatment protocols and the peer-reviewed research behind them in upcoming Alleman Center biomimetic dentistry training programs.

Learn more about cracks, the third most common dental pathology, in Dr. David Alleman’s Six Lessons Approach podcast.