Three-dimensional Finite Element Research Of Effect Of Stress-shielding After Rigid Internal Fixation On Fractures Of The Mandible.

OBJECTIVE:To analysis the stress distribution and the effect of stress-shielding of a variety of rigid internal fixations (RIF) on fractures of symphysis, fractures of mental foramen and fractures of mandibular angle under six kinds of occlusion. And research the relevant factors resulting in the effect of stress-shielding. That will provide a reference for clinical treatment of mandibular fractures.METHODS:The three-dimensional finite element models of mandibular fractures, which were created through ANSYS software, were adopted to simulate mandibular fractures. The stress distribution and the stress-shielding effect, of which a variety of rigid internal fixations on fractures of symphysis, fractures of mental foramen and fractures of mandibular angle, which were under six kinds of occlusion—occlusion of anterior teeth, intercuspal occlusion, injured side premolar, injured side molar, uninjured side premolar, uninjured side molar, at the initial, middle and later stages of fracture healing, were assessed through methods of finite element.RESULTS:1. The stress-shielding effect after rigid internal fixations on fractures of symphysis:(1)The effect of stress-shielding after rigid internal fixation on fractures of symphysis will always occur. With the fracture healing, the rates of stress-shielding gradually decrease, and the rates are ordered from larger to little as the initial period, the middle period and the later period of fracture healing. (2)At the same period of fracture healing, the rate of stress-shielding of double titanium miniplates under the same occlusion is higher than that of single titanium miniplate, as well as the rate of double stainless steel miniplates is higher than that of single stainless steel miniplate. (3)Under the same occlusion, from high to low, the order of rate of stress-shielding is single titanium miniplate plus single reconstruction plate, double stainless steel miniplates, single reconstruction plate, double titanium miniplates, double biomechanical miniplates, single stainless steel miniplate, single titanium miniplate respectively.2. The stress-shielding effect after rigid internal fixations on fractures of mental foramen:(1)The effect of stress-shielding after rigid internal fixation on fractures of symphysis will always occur. With the fracture healing, the rates of stress-shielding gradually decrease, and the rates are ordered from larger to little as the initial period, the middle period and the later period of fracture healing. (2)At the same period of fracture healing, the rate of stress-shielding of double titanium miniplates under the same occlusion is higher than that of single titanium miniplate, as well as the rate of double stainless steel miniplates is higher than that of single stainless steel miniplate. (3)Under the same occlusion, from high to low, the order of rate of stress-shielding is single titanium miniplate plus single reconstruction plate, double stainless steel miniplates, double titanium miniplates, single reconstruction plate, single stainless steel miniplate, single titanium miniplate, double biomechanical miniplates respectively.3. The stress-shielding effect after rigid internal fixations on fractures of mandibular angle:(1)The effect of stress-shielding after rigid internal fixation on fractures of symphysis will always occur. With the fracture healing, the rates of stress-shielding gradually decrease, and the rates are ordered from larger to little as the initial period, the middle period and the later period of fracture healing. (2)At the same period of fracture healing, the rate of stress-shielding of double titanium miniplates under the same occlusion is higher than that of single titanium miniplate, as well as the rate of double stainless steel miniplates is higher than that of single stainless steel miniplate. (3)Under the same occlusion, from high to low, the order of rate of stress-shielding is single titanium miniplate plus single reconstruction plate, double stainless steel miniplates, double titanium miniplates, single reconstruction plate, single stainless steel miniplate, single titanium miniplate, double biomechanical miniplates respectively.CONCLUSIONS:1. The effect of stress-shielding after rigid internal fixation on fractures of mandible will always occur. With the fracture healing, the rates of stress-shielding gradually decrease, and the rates are ordered from higher to lower as the initial period, the middle period and the later period of fracture healing. 2. Material properties have an effect on rates of stress-shielding. The larger the modulus of elasticity (E) is, the higher the rate of stress-shielding is, and the more obvious the effect of stress-shielding is.3. At the same period of fracture healing, under the same occlusion, the larger the number of plates is, the higher the rate of stress-shielding is, and the more obvious the effect of stress-shielding is.4. The value of bite force has hardly any proven effect on rates of stress-shielding, while the overbite force may more likely cause refracture after plates are removed.5. The effect of stress-shielding has both the positive effects in fracture healing and the negative effects in deferring or even harming normal fracture healing. The ideal RIF should offer solid and reliable sustaining force and resist harmful stress outside, bringing forth axis press stress for acceleration of bone healing at the initial period of fracture healing. While at the later period, the ideal RIF should minimize the impact of stress-shielding effect.