Hertizan Contact Stress-Strain Response of Enamel

Hertizan Contact Stress-Strain Response of Enamel

Carlos AO Fernandes, Marinee Cabrera, Alisa Leesavan, Nelson RFA Silva, Van P Thompson.

Objectives: This study evaluated the macroscopic stress strain response of enamel to Hertzian contact with variation in indenter diameter and loading rate.

Methods: Third human molars were cut into halves from mesial to distal obtaining specimens at both the buccal and lingual sites.  All samples were then embedded in acrylic resin (PMMA) and polished to expose flat enamel surfaces and lightly etched (5 sec, 10% phosphoric acid). Samples were triangular wave loaded at 0.5, 10 or 500 N/sec over a range of loads. Indenters were WC spheres (r=1.59 or 3.18 mm). After loading, the specimens were observed under a stereomicroscope and the indent measurements were recorded for a/r (contact radius/indenter radius) determination. Contact stress and contact strain were calculated. ANOVA and group comparisons were determined.

Results: At loads of 150 N and above with the small indenter ring cracks and radial cracks were observed.  The radial cracks were seen to originate from the DEJ when specimens were sectioned. With the larger indenter no cracking was observed.  Contact modulus for loads and rates employed are below:

  

	Contact Modulus  (Gpa) (r=3.18 mm)	Contact Modulus (Gpa) (r=1.59 mm)	Contact Modulus (GPa) (r=1.59)
Load (N) / Rate	10 N/sec (n=15/)	0.5 N/sec (n=8/)	500 N/sec (n=8/)
50	26.7*
150	16.4*	12.3#	16.20#
200		13.5	13.7
250		10.7^	16.25^

*, #, ^ Significant difference (p<0.05)

Conclusion:  The Hertzian contact modulus of enamel is strongly influenced by indenter load and radius.  Small indenters create damage lowering contact modulus.  Contact modulus is load rate dependent