Tooth enamel looks like a bundle of closely packed fibers. This is most likely a transversally isotropic material. Cracks with what orientation do you mean?

Studying your works since the 80s of last century, I honestly do not understand the perspective of your question.

I consider dental enamel as an outstanding example of ceramics with the highest possible fracture toughness. I thought you were of the same opinion.

When contacting hard particles, the tensile stresses do not occur in the enamel due to the relatively low shear strength of the interfaces between the fibers (rods).

Dr. Borovik:

Thank you for your rapid response.

Although I have studied tooth dentin and the dentin-enamel junction (DEJ) in the past, I have never studied tooth enamel per se, although I have just started to look at the enamel of the Giant Panda with Zengqian Liu at IMR in China. I guess that everything is relative, but I would most certainly never describe tooth enamel as "an outstanding example of ceramics with the highest possible fracture toughness". Indentation toughness measurements, which would be approximate of course, give values well below 1 MPa/m (Okazaki, et al., Shika Zairyo Kikai, May 1989), making enamel about half as tough as silicon. This is a material that is highly brittle "in my book", even compared to traditional ceramics such as alumina, silicon carbide and silicon nitride, which in their untoughened states still have (intrinsic) toughnesses around 3 MPa/m.

In teeth, if significant cracks reach the dentin, there is a good chance that the tooth will be lost. Your teeth are the oldest tissue in your body - they have to last 50-70 years or more, and so it is inevitable that nano- to micro-scale cracks must form in the enamel due to its minimal fracture toughness (I don't particularly care at this point at what orientation). Why then do they not reach the dentin, as fracture of the entire tooth is not a particularly regular occurrence? As I mentioned in my original question, the DEJ can stop many such cracks from penetrating into the dentin (see Imbeni et al., Nature Matls., March 2005), but are their mechanisms within the enamel, presumably hydration-related, which permit the healing of such cracks when they are small? That simply is my question.

ROR

Thanks for the detailed explanation.

I meant that the structure of the best structural ceramics ( T. Ishikawa, (SA-Tyrannohex)) is very similar to the structure of the tooth enamel. Mechanisms of fatigue, relaxation and healing of cracks in the ceramics are not clear so far as well as to the tooth enamel.

The first of all, I would like to note that it is philosophical question despite it sounds very simple. It seems to me that it is really interdisciplinary question laid between materials science/fracture mechanics and biology/health science. Regeneration of the living tissues is the biological basis for the healing of mechanical damages including cracks in some types of bones as the clinical practice has shown. I believe that this process is the driving force for the healing of cracks in materials science works. Concerning with teeth, I never heard that their hard basis - dentin exhibits an ability to regeneration. However, some dentists believe that tooth enamel is able to the regeneration, but this is the subject of a sharp discussion in the dental community. Besides, I would like to note that I mean the regeneration of tooth enamel in a tooth in "jaws", while the regeneration of teeth of some fishes and animals is the well-known fact.

This seems a very interesting question to answer. In a clinical situation, we clearly observe cracks in enamel. This also might answer this question, why cracks in enamel never become carious lesion? As you can see in the attached paper. One possible explanation could be the remineralization process, that is constantly happening in the oral cavity, in which calcium and phosphate ions from the saliva could be pumped into these cracks.

Thank you, Carina, that is what we have been thinking might be the case with tiny cracks that we observe to apparently heal in the enamel of Giant Pandas. I really appreciate your help.

ROR

This is an interesting discussion. Enamel cracks might mean that the enamel rods themselves have micro-fractures. Since the enamel-forming ameloblasts are no longer present on an erupted tooth an actual regenerative "repair" is not possible. However, calcification of the area can take place with the calcium and phosphate ions as mentioned by Carina Tanaka.

Your question has piqued my curiosity as to why the fractures or cracks, many times, seem to stay at the surface and not migrate down to the DEJ - or further through the tooth as in some cases. Since there is a "give" between the enamel rods when pressure is applied, does it mean that there is less give at the surface than closer to the DEJ? Why? I have not looked at slides of enamel rods in a while but now I'm wondering if the interstitial spacing between the enamel rods is different as the enamel rods get closer to the surface than at the DEJ? Is the interstitial space smaller allowing the enamel rods to physically touch each other and create the crack? If the space is smaller, how does this affect fluid movement from the pulp to the enamel surface? If the fluid movement is reduced from sugars or medications, would this cause the interstitial spacing to diminish and lead to cracks?

I have noticed over the years that people on anti-rejection drugs from organ transplants have more cracks in their enamel than is normal. Is the drug causing it in some way?

Many questions come to my mind from you original question. Thank you for this!

dan

Dan:

Thank you for your fascinating reply.

With respect to the "self-healing" of the enamel, we had come to the conclusion that no actual regenerative repair could ever take place in enamel , but that nano- to near micro-scale cracks could "self-heal" by calcification. On the basis of what you and Carina are saying, this seems to be the case.

However, your second point is also intriguing concerning what the total function of the DEJ actually is. There are so many unsolved questions here, as you articulate.

In our work on the DEJ, we noticed that it was a region where cracks entering the dentin from the enamel could experience "shielding", i.e., locally reduced crack driving forces from the formation of bridging ligaments formed across the crack faces (Imbeni et al., Nature Matls. March 2005); this leads to the arrest of enamel cracks in the mantle dentin within 5 to 10 microns of the optical DEJ.

However, what also interested me was the fluted nature of the DEJ in certain animals, e.g., humans, in relation to the organization/orientation of the enamel rods as they impinge on the DEJ. In certain species, the enamel rods are disorganized whereas in humans they are well oriented roughly normal to the DEJ. From a mechanics perspective, a crack normal to a dissimilar materials interface would be most likely to penetrate the interest rather than be arrested or deflected (all other things being equal). By making the DEJ fluted in nature, enamel cracks parallel to the rods would now impinge on the DEJ at an angle other than 90 deg, making penetration of the DEJ into the dentin more difficult. By the same token, if the DEJ were planar, as it is in some species, a disordered array of enamel rods would also likely generate cracks that would not normally impinge on the DEJ at 90 deg, again lessening the possibility of penetrating the interface. I have never followed up these rambling thoughts but the various functions of the highly effective crack-stopping DEJ are fascinating to think about.

Thank you for your comments, you too Carina and Peter.

Cheers

ROR

Prof. Ritchie,

This is an interesting discussion. Enamel is one of the most fascinating materials in nature. It is highly inorganic with less than 1-2% organic material. The key to enamels toughness is its microstructure! The decussation of enamel rods that starts somewhere in the middle (between the cusp and the DEJ), creates an intricate network of rods that invoke a number of toughening mechanisms. Interestingly, we noted that these structures guides a crack that start from the tooth surface in towards the DEJ and in this process the toughness increases nearly 5X (from about 0.5 MPa.m^0.5 near the surface to almost 2.5 MPa.m^0.5 near the DEJ; Bajaj and Arola., Biomaterials 2009). So in that respect, enamel is as tough as some of the traditional ceramic materials. Some very interesting work was also done by Brian Lawn's group, where they demonstrated that tufts that are present near the DEJ, act as shock absorbers and dissipate energy (Chai et al., PNAS 2008). There are hundreds of such tufts in each tooth, which also protects the enamel from fracture.

As pointed out in the comment by Carina, there is evidence that remineralization can occur from saliva, which could partially close heal micro-cracks. Although if one was to measure the toughness of such healed cracks, I do not expect a jump in the stress intensity value. If indeed a crack can be healed with remineralization that would be extremely interesting.

Regards,

Devendra

Thank you, Devendra, for your insights. I really appreciate hearing your thoughts.

ROR

Thanks professor Ritchie for the start of this discussion!

Carina has mentioned the important function of the tooth enamel as the protection of dentin from an aggressive environment. Contact with saliva in a cavity on the enamel induces the forming of the phosphate layer, which protects dentin from aggressive saliva. However, it is not clear what happens with mechanical properties of this layer in comparison with the enamel? If the properties are the same we would say that the crack has been healed. The similar situation takes place on the surface of aluminum and its alloys on air.

I have treated many teeth with cracked tooth syndrome by full cusp coverage gold restorations. On the proximal surfaces the contact area has to be broken but the buccal and lingual or palatal surfaces the restoration is extended only 2 mm to 3 mm. I have never seen a cracked tooth healing by itself. I use direct wax technic to make the gold restorations and do the prep and fit on the same day.

I have written a book, "Colour Atlas of a New Concept of Enamel Caries" in 1987. This book is available on the net www.enamelcaries.com.

Demineralisation and remineralisation is a surface phenomenon and body of the lesion never remineralises. Some external minerals can reach the deeper layers mixed with the acids at low pH. As the pH rises the minerals precipitate and cannot enter the carious lesion. Bulk of the enamel caries occurs at the contact of two or more enamel surfaces where the acids can enter but the saliva or biofilm cannot enter. Acid attack is clearly ahead of bacterial front. Bulk of the enamel caries start at the edge of the plaque where the plaque is thin.

Interesting question... Enamel needs dentine supports !?! right ...so :

Prof. Ralph Steinmann suggested in his life's works focusing on "the dentinal fluid movement " the dentine mineral's depletion as a metabolic related disease ( like in chronic inflammatory status & several well-know NCD's or drug's side-effects ). Caries would start by pulp pressure - dentinal fluid impairment (or reverse flow) followed by collagen mineral loss, increased susceptibility to acid attack & bacterial aggression ... the cavitated lesion is the end-resulting consequence of an "internal" long-lasting mineral's balance shift .

The occurring decreased dentine resistance , even if the outer enamel is not cavitated, could explain the tendency for cracks and fissure propagation in deeper areas.

I agree that gold onlays may be the best restoring option, nevertheless in many cracked tooth syndrome that I have treated , by placing a thick Base of ZOP -cement, ( which restore stiffness and reduce deformation ) simply covered by a thin composite layer, I observed often long - lasting recover, absence of symptoms, preserved vitality and radio-logical remineralisation at the dentin cement interface.

I'm firmly convinced that It's possible that some cracked tooth syndromes, and the majority of deep decay lesions can be "healed" by supporting the natural remineralization potential, targeting on material's choices and procedures intended to preserve/ maintain the physiological water & ions exchanges along the whole pulp-dentin organ and at the defect's interfaces .

This is why MTA & others inorganic cap materials works best ... exactly following the same concepts, despite lack of mechanical performances.

Regards

Nicola

Nicola:

Thanks for your thoughts on this issue. We have come to somewhat similar conclusions and have a paper coming out in Acta Biomaterialia shortly.

I appreciate your insight.

Cheers

ROR

Prof. Ritchie,

I'm very pleased and sincerely honored about your answer, thanks !

Looking forward to read the article you mention, I've been looking closer to some interesting aspects you describe extensively in several articles :

1. The tooth's structures high degree of complexity

2. Gradient interfaces .. to join dissimilar materials, as in the tooth.

> > enamel + dentin , 2 parts that perform distinctly different functions

3. Chemical factors, inorganic ions, bio-molecules, and the hydration level, >> influencing the tooth's physical properties and functionality .

Generally underestimated topics ... not sufficiently taught in dental schools

I'm impressed about the deep knowledge of dentistry matters and the interdisciplinary approach You and your lab's team are committed to.

Which I believe is very necessary , deserves to be implemented and widely spread among dental communities professionals.

Adhesive-based procedures on dentin are still disregarding the importance of bounded water and dentinal fluid, seems unable to deal with leakage issues ( MMP's ), are often hydrolyzed, generally cytotoxic ... etc. etc.

That said I have 2 conceptual questions:

1. What do you think about the contemporary dental care concepts, in relation to adhesive restorative procedures, the 4 decades lasting pursuit in seeking to replace 2 missing structures by a single polymer-based material ?

2. Why is a Bio-mimetic inorganic-organic sandwich architecture, delivering a real Bio-active effective ions ( Zn-Cu-Mg-P, etc) remineralisation , supporting the dentinal affected interface, hydration's aspects and improving mechanical performances , is generally still not considered as a valid option to follow ?

Thanks and best regards

Nicola

I

Hi Rob, I just noticed your question. I had a conversation with Mike Swain last year on almost this exact topic, and his conclusion was an emphatic yes. Enamel can heal in the correct conditions, including healing caries, but we rarely give it the opportunity. As for his evidence, I don't know it offhand, so I would suggest to reach out to him directly. Cheers, Jay

Dear professor Ritchie,

There are two different mechanisms :

1. The demineralisation/remineralisation process through a screw dislocation mechanism (Voegel JC, Frank RM. Calcif Tissue Res. 1977 Dec 14;24(1):19-27.

Stages in the dissolution of human enamel crystals in dental caries.)

2. The resistance to cracks (Gao et al. Materials become insensitive to flaws at nanoscale: Lessons from nature PNAS  May 13, 2003  vol. 100  no. 10  5597–5600)

We are currently working on the piezo electric properties of tooth enamel. We keep you informed.

best regards

étienne

Thank you Etienne - I will follow those up. ROR

Keep me informed

étienne

Very interesting discussion. Thanks a lot to all! It reminds me my talks with metallurgical engineers who needed in ideas on processing of some refractory alloys. I would not say that they followed our suggestions (in Materials Science), but the technology was ellabotated. May be it is one of directions of a progress.