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Effects of hypofunction and hydrocortisone on the mechanical strength of the periodontal ligament in the mesial root of the rat mandibular first molar

, : Effects of hypofunction and hydrocortisone on the mechanical strength of the periodontal ligament in the mesial root of the rat mandibular first molar. Tsurumi University Dental Journal 12(1): 19-34

An experimental system to evaluate quantitively the mechanical properties of the periodontal ligament has been developed. The technique involves measurement of the ultimate load required to separate transverse sections, approximately 440 .mu.m thick, of the mesial root of the rat mandibular first molar from its surrounding alveolar bone. This ultimate load was divided by the surface area of the root (thickness of the section .times. circumference of the tooth) and the quotient was regarded as the mechanical strength/unit area of the periodontal ligament at this site. Using this technique, the effects of hypofunction, and of hydrocortisone on the mechanical strength of the periodontal ligament were examined in the present studies. In those teeth rendered hypofunctional by removal of their antagonists, it was found that the average mechanical strength per unit area of the periodontal ligament was reduced to 19% of the control value. The average value obtained from those experimental animals was 23 g/mm2 after a period of 10 days whereas average value for the control animals was 121 g/mm2. Following daily injections of 10 mg of hydrocortisone/kg body weight for 10 days, the mechanical strength of the periodontal ligament increased to 157% (190 g/mm2 on average) of the control value. However in those animals rendered hypofunctional and given hydrocortisone, the mechanical strength of the periodontal ligament was reduced. Radiographic observations showed that the cross sectional area of the periodontal ligament decreased significantly in the hypofunctional teeth relative to the control animals indicating apposition of bone had occurred at the alveolar wall during the experimental period. In both groups of rats with and without antagonistic teeth, hydrocortisone did not show any significant effects on the area of the periodontal ligament. It is suggested that changes in the mechanical strength of the periodontal ligament would be caused by changes in the composition and structure of the periodontal collagen fibres and/or changes in the Sharpey fiber connection between the fibres and the surfaces of the alveolar bone and of cementum. It is assumed that method employed here would be useful to assess the mechanical properties of the periodontal ligament under other various experimental conditions and therefore enable the examination of the degrees of injury and of repairs in periodontal diseases.


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