title: On the importance of aging to the crack growth resistance of human enamel.
abstract: With improvements in oral health and an overall increase in quality of life, the percentage of fully or largely dentate seniors is increasing. Understanding the effects of aging on the mechanical properties of teeth is essential to the maintenance of lifelong oral health. In this investigation the effects of aging on the fracture toughness of human enamel were evaluated from incremental crack growth experiments performed on tissue of donor teeth representing "young" (17 ⩽ age ⩽ 25) and "old" (age ⩾ 55) age groups. Results showed that the old enamel exhibited significantly lower resistance to fracture than that of the young tissue in two orthogonal directions of crack growth. For crack growth transverse to the enamel rods, the fracture toughness of the old enamel (0.37 ± 0.15 MPa m(0.5)) was nearly 70% lower than that of tissue from the young teeth (1.23 ± 0.20 MPa m(0.5)). Based on results from a mechanistic analysis of crack growth, the reduction in fracture resistance is attributed to a decrease in the degree of extrinsic toughening. The practice of restorative dentistry should account for these changes in tooth tissues in the treatment of senior patients.
  
  
  CONCLUSIONS
  The mechanical behavior of enamel has been studied for over 3 decades. Due to the limited volume of tissue available for evaluation, past work has been largely based on indentation methods. In this investigation we have evaluated the resistance to fracture of human enamel using a conventional fracture mechanics approach and incremental crack growth. We compared the fracture resistance of cuspal enamel obtained from the teeth of representative "young" and "old" donor groups. Our results show that there is a substantial reduction in the resistance to fracture with age, that it is anisotropic, and that the degradation is more severe than that which occurs to dentin. As such, we feel this work is a significant contribution to the field.
authors.affiliation: Department of Material Science and Engineering, University of Washington, Seattle, WA, USA; Department of Mechanical Engineering, University of Maryland Baltimore County, Baltimore, MD, USA.
authors.forename: Mobin
authors.initials: M
authors.lastname: Yahyazadehfar
authors.affiliation: Department of Mechanics, Shanghai University, Shanghai 200444, PR China.
authors.forename: Dongsheng
authors.initials: D
authors.lastname: Zhang
authors.affiliation: Department of Material Science and Engineering, University of Washington, Seattle, WA, USA; Department of Restorative Dentistry, School of Dentistry, University of Washington, Seattle, WA, USA; Department of Oral Health Sciences, School of Dentistry, University of Washington, Seattle, WA, USA. Electronic address: darola@uw.edu.
authors.forename: Dwayne
authors.initials: D
authors.lastname: Arola
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000375:Aging / Q000502:physiology*; D003743:Dental Enamel / Q000473:pathology*; D005260:Female; D020342:Finite Element Analysis; D006801:Humans; D008297:Male; D008875:Middle Aged; D014082:Tooth Fractures / Q000473:pathology*; D055815:Young Adult
keywords: Aging; Crack growth; Enamel; Fracture; Tooth; Toughness