Understanding the mechanical properties of natural teeth has been an important pursuit, informing the development of effective restorative materials for use in dental crowns. Ceramics, composites and metals are routinely used in innovative ways to replicate the durability, function, aesthetics and feel of teeth.
A challenge in the field of materials science has been to find permanent restorative materials that replicate the amazing shock-absorbing qualities of natural teeth; ensuring their ability to manage repeated pressure without breaking, or damaging other teeth.[i]
The scaly origins of teeth
Research[ii] now supports the theory that the teeth of all vertebrates share their origin in the scales of ancient fish, before they migrated into the mouth about 400 million years ago. When examining the fossilised remains of an extinct sawfish species, scientists found a complex but primitive form of tooth enamel – with a high resistance to mechanical stress.
Modern human teeth have evolved with a highly efficient and unique structure, enabling them to withstand a great deal of bite pressure and wear.[iii] Teeth that can properly grind food without fracturing help to ensure that adequate nutrition can be absorbed from food.[iv]
Complex, elastic microstructure
Flexural strength measures a material’s ability to withstand pressure from a static load before breaking. Elasticity, one of the most important features of natural teeth, refers to the material’s rigidity under pressure and when the pressure is removed.[v] Many of the shock-absorbing qualities of teeth are provided by the periodontal ligament and root structure. This foundation, combined with the elastic nature of enamel and dentine in the tooth crown is designed to withstand and exert hundreds of kilograms of vertical force.iv
Surface enamel is primarily composed of hydroxyapatite crystals,iv arranged in fish-scale, or key-hole patterns in configurations that allow stress to be distributed across the material, reducing impact stress.[vi] The 65-70% inorganic mineral content in dentineiv consists mainly of smaller hydroxyapatite crystals than those found in enamel. The remaining content, similar to bone, is comprised mainly of collagen, proteins and water, all of which make it highly effective in managing pressure.[vii]
The density of cementum covering the root, as well as the dentine inside is lower than the material in the natural tooth.iv This provides a soft landing, absorbing impact, and distributing stress across both the tooth material and the periodontal ligament to reduce the risk of damage or pain.
Restorative materials – the challenges
Shock-absorbing and impact properties, accounting for flexural strength, elasticity and strain, are extremely important in designing new dental and biomaterials that remain intact under strain or impact.[viii]
Metal, porcelain fused to metal, pressed ceramic, all-ceramic or porcelain crowns all have advantages and disadvantages. Despite being strong, porcelain options lack elasticity and can chip, although they can increasingly replicate the shade and translucency of natural teeth. Metal is durable, and less tooth reduction is required, however, the metallic appearance is not for every patient. Pressed ceramics can mimic the translucency of natural enamel, but lack elasticity, and can crack over time.[ix]
Zirconia crowns have very high impact resistance, but are very costly, risk wear on opposing teeth and look less natural than ceramics or composites.[x] When zirconia crowns do crack, or when treatment is needed to the tooth under the crown, their hardness can also make their removal very challenging.[xi]
Convenient composites
Composite materials continue to develop quickly to offer permanent restorations for patients at a lower cost and much reduced chair time than alternatives. Composites have additionally been shown to have the same flexural strength, elasticity, microhardness, microstructure and fractal dimension when used in direct applications as in printable CAD/CAM, offering patients a much quicker and cheaper alternative to precious metals and ceramics.[xii]
BRILLIANT Crios from COLTENE is a reinforced CAD/CAM composite benefitting from high flexural strength and low modulus of elasticity. Its similarities with dentine give it excellent shock-absorbing qualities, making BRILLIANT Crios perfect for implant-supported crowns, as well as conventional indications such as inlays, onlays crowns and veneers. Stress peaks are reduced during chewing, resulting in decreased risk of material fatigue.
Restorative options can increasingly recreate not only the function and aesthetics of natural teeth, but the natural feeling of teeth in the mouth. Increasingly available to patients are affordable materials that also emulate the incredible powers of natural teeth to manage stress, wear and impact without cracking under pressure.
For more on COLTENE, visit www.coltene.com/CRIOS,
email info.uk@coltene.com or call 0800 254 5115.
[i] Zhang YR, Du W, Zhou XD, Yu HY. Review of research on the mechanical properties of the human tooth. Int J Oral Sci. 2014 Jun;6(2):61-9. doi: 10.1038/ijos.2014.21. PMID: 24743065; PMCID: PMC5130056.
[ii] Leonard D. Ancient Sawfish Help to Illuminate Our Teeth’s Scaly Origins. Scientific American. Decemver 2022. Available at: https://www.scientificamerican.com/article/ancient-sawfish-help-to-illuminate-our-teeths-scaly-origins/ Accessed August 2024
[iii] Physiology, Tooth. StatPearls. March 2023. Available at: https://www.ncbi.nlm.nih.gov/books/NBK538475 Accessed August 2024
[iv] Peyron MA , Santé-Lhoutellier V , François O , Hennequin M . Oral declines and mastication deficiencies cause alteration of food bolus properties. Food Funct. 2018 Feb 21;9(2):1112-1122. doi: 10.1039/c7fo01628j. PMID: 29359227.
[v] Poonacha V, Poonacha S, Salagundi B, Rupesh PL, Raghavan R. In vitro comparison of flexural strength and elastic modulus of three provisional crown materials used in fixed prosthodontics. J Clin Exp Dent. 2013 Dec 1;5(5):e212-7. doi: 10.4317/jced.51136. PMID: 24455084; PMCID: PMC3892269.
[vi] Akasapu A, Hegde U, Murthy PS. Enamel Surface Morphology: An Ultrastructural Comparative Study of Anterior and Posterior Permanent Teeth. J Microsc Ultrastruct. 2018 Jul-Sep;6(3):160-164. doi: 10.4103/JMAU.JMAU_27_18. PMID: 30221142; PMCID: PMC6130241.
[vii] https://www.elsevier.com/resources/anatomy/tooth-mandibular-first-molar/micro-anatomy/dentin/15558
[viii] Niek de Jager, Tijmen J.A.G. Münker, Luis F. Guilardi, Victor J. Jansen, Yvon G.E. Sportel, Cornelis J. Kleverlaan, The relation between impact strength and flexural strength of dental materials, Journal of the Mechanical Behavior of Biomedical Materials, Volume 12 2021, 104658, ISSN 1751-6161, https://doi.org/10.1016/j.jmbbm.2021.104658.
[ix] Isgrò G, Rodi D, Sachs A, Hashimoto M. Modulus of Elasticity of Two Ceramic Materials and Stress-Inducing Mechanical Deformation following Fabrication Techniques and Adhesive Cementation Procedures of a Dental Ceramic. Int J Biomater. 2019 Nov 19;2019:4325845. doi: 10.1155/2019/4325845. PMID: 31827519; PMCID: PMC6885839.
[x] Soleimani F, Jalali H, Mostafavi AS, Zeighami S, Memarian M. Retention and Clinical Performance of Zirconia Crowns: A Comprehensive Review. Int J Dent. 2020 Oct 15;2020:8846534. doi: 10.1155/2020/8846534. PMID: 33123199; PMCID: PMC7584951.
[xi] Keeling FL, Taft RM, Haney SJ. Bur Choice When Removing Zirconia Restorations. J Prosthodont. 2023 Apr;32(4):347-352. doi: 10.1111/jopr.13564. Epub 2022 Jul 15. PMID: 35771711.
[xii] Grzebieluch W, Kowalewski P, Grygier D, Rutkowska-Gorczyca M, Kozakiewicz M, Jurczyszyn K. Printable and Machinable Dental Restorative Composites for CAD/CAM Application-Comparison of Mechanical Properties, Fractographic, Texture and Fractal Dimension Analysis. Materials (Basel). 2021 Aug 29;14(17):4919. doi: 10.3390/ma14174919. PMID: 34501009; PMCID: PMC8434230.
