Purpose: Little is known of roles for relaxin in connective tissues outside reproductive system. The present study explores how relaxin modulates the physical properties of periodontal ligament(PDL) in rats. Materials&Methods: Three-month-old rats were treated for 1 or 3 days with either human relaxin(8 ėg/kg/hr) plus a bolus injection(0.5mg,1.43mg/kg) or control vehicle(sodium acetate) via implanted Alzet osmotic minipumps(5 in each group). A maxillary molar segment and 2-mm-thick mandibular section anterior to 1st molar were cut and embedded to form testing blocks using orthodontic self-curing resin. After 2nd and 3rd molar crowns were removed, bucco-lingual wiggle range of 1st molars was measured using a differential variable reluctance transducer(DVRT) under a given horizontally-oriented force. The mandibular blocks containing incisor's root with its alveolar socket was tested by applying a compressive load on root to push it down until failure(>20% stress reduction) using MTS. The yield and failure loads(Kg), energy(Kg-mm), elongations(mm) and strains(%) to failure, and stiffness(modulus, Kg/mm2) were measured. All samples were fresh and kept moist with a 0.9% saline drip during testing. Results: Compared to vehicle-treated controls, wiggle range of 1st molar increased up to 170%(p=0.026) and 130% for 1- and 3-day relaxin-treated rats respectively. All values from push-out tests were lower in relaxin-treated than control animals. These differences were more obvious in 1- than 3-day treatment groups. The energy and enlongation to failure loads were significantly(or marginally) lower(p=0.046 or p=0.067) in relaxin-treated rats whose PDL also showed decreased yield stress and stiffness, especially for 3-day treatment. A significantly negative associations was found between wiggle range of 1st molars and PDL stiffness of incisors(r=-0.54,p=0.025). Conclusion: Human relaxin plays an important roles in modulating the physical properties of the PDL thus altering tooth looseness. The relaxin may reduce strength and stiffness of the PDL and increase tooth mobility. Supported by BAS Medical, Inc

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