Objective:  When subject to a constant stress, the periodontal ligament exhibits a relaxation of strain over-time known as creep. This behavior in ligaments and tendons is called viscoelasticity.  To evaluate the viscoelastic properties of the periodontal ligament (PDL) during intrusive and extrusive loading, upon application of cyclic static loads. Strain elongation or creep studies were performed in uniaxial tensional experiments on segments of pig mandibles containing a tooth, alveolar bone and PDL. The effect of formaldehyde fixation on the mechanical properties of the PDL was also evaluated.  Methods: Creep was measured using a LVDT contact probe with cyclic loads of 0.03, 0.06, and 0.11 N. Each specimen yielded 5 compliance-time curves. Creep, displacement and recovery after each cyclic load were analyzed by 3-way ANOVAs.  Results: There was a significant difference between loading cycles. The displacements at 1000s were higher (p < 0.05) in the first loading cycle when compared to the other cycles. Within the first cycle, extrusive loading produced a higher displacement (p<0.05) than intrusive loading. However, no difference between extrusive vs. intrusive load was found in the other cycles.  Formaldehyde-fixed specimens presented a significantly lower (p<0.05) initial displacement when compared to the unfixed samples.

		Displacement (μm)		% Creep
Load	Cycles	Intrusive	Extrusive	Intrusive	Extrusive
0.11 N	1	111±25a	202±42a	7±4a	5±3a
0.11 N	2	77±26b	105±36b	8±3a	8±3b
0.11 N	3	79±36b	85±19b	8±3a	8±3a
0.11 N	Fixation	3± 2c	--	8±3a	--

Values are means ± SD (n=4).  Groups identified by different letters are significantly different (p<0.05).

Conclusion: These experimental results suggest that the stress-strain curve may be a function of load direction and that cross-linking agents altered the physiological behavior of the PDL.  Supported, in part, by DE014911 and DE015306 from the NIDCR.