ABSTRACT
Objective To study the vibration characteristics of a normal C2-7 cervical spine finite element model and vibration changes of a cervical facet joint with different degrees of impairment or that is resected. Methods The finite element model of a normal C2-7 cervical spine based on computed tomography (CT) scan images was established and validated. Next, the normal frequency and first ten modes of the normal cervical spine model were extracted. A facet joint was considered without or with joint constraints with a friction coefficient of 0.01, 0.1 and 0.2 to simulate a resected facet joint or a facet joint with mild, moderate, and severe damage, respectively. Thus, the effects of different types of damage to the cervical spine on their natural frequencies could be studied. Results The minimum natural frequency of the normal model occurred in the extension and lateral bending modes and it was approximately 12 Hz. A large displacement in the model occurred mainly in the atlas. The frequency of the model with the constrained joint was higher than that without joint constraints; however, the natural frequencies of the facet joints with different friction coefficients remained almost unchanged. Conclusions The study of the natural frequency, mode shape, and amplitude of the cervical spine provided the basis for further studying its dynamic characteristics, which is of tremendous significance in the nursing and treatment of cervical vertebrae. A vibration environment of 12 Hz should be avoided in daily activities and cervical treatment to prevent severe damage to the cervical spine.
ABSTRACT
Objective:To investigate the wear resistance of four low-shrinkage commercial composite resins and two traditional composite resins in vitro and discuss the relative influencing factors, and to illustrate the inner relationship of the wear resistance of resins and material composition.Methods:Four low-shrinkage commercial composite resins including CLEARFIL MAJESTY Posterior(CMP), Filtek LS(LS), Admira(AD), Kalore(KA) and two traditional composite resins including Filtek Z350XT (Z350), Solitaire2(S2) were chosen.The bar-shaped specimens were fabricated and mounted in a UMT-2 wear testing machine and abraded with the two-body media (distilled water) with a Si3N4 ball as antagonist.The maximum wear depth was determined after 14 400 cycles.The friction coefficient was determined during the test.The worn surfaces were examined with SEM.Results:CMP showed the lowest maximum wear depth and KA presented the highest maximum wear depth.The maximum wear depth ranked as follows: CMP
ABSTRACT
STATEMENT OF PROBLEM: Implant screw loosening remains a problem in implant prosthodontics. Some abutment screws with treated surfaces were introduced to prevent screw loosening and to increase preload. DLC(Diamond Like Carbon) film has similar properties on hardness, wear resistance, chemical stability, biocompatibility as real diamond materials. PURPOSE: The purpose of this study was to investigate the effect of lubricant layer on abutment screw and to discriminate more effective method between soft lubricant and hard lubricant to prevent screw loosening. MATERIAL AND METHOD: In this study, 1mum thickness DLC was used as protective, lubricating layer of titanium screws and 3 times removal torque was measured on the abutment screws to investigate the difference in 10 coated and 10 non-coated abutment screws. RESULTS: The results indicated that the implants with DLC coating group were not more resistant to the applied force in screw loosening. At 32Ncm, the 3 times removal torque in DLC group were 27.7+/-2.89, 25.85+/-2.35 and 26.2+/-2.57. The removal torque in no-coated abutment screws were 27.85+/-4.23, 27.35+/-2.81 and 27.9+/-2.31, respectively. CONCLUSION: The lubricant layer used in this study was Diamond Like Carbon(DLC) and it have a properties of hard and stable layer. The DLC coating layer was hard enough to prevent distortion of screws in the repeated unscrewing procedure in clinical situation. The reduced friction coefficient in hard DLC layer was not effective to prevent screw loosening.
Subject(s)
Carbon , Diamond , Friction , Hardness , Prosthodontics , Titanium , TorqueABSTRACT
The effects of the surface friction of a grasped object on the regulation of grip force during holding tasks using a precision grip were investigated. Using a force transducer-equipped grip apparatus, the grip force and load force acting on the object were measured continuously while surface materials (silk, wood, suede and sandpaper) and load weights (0.98N, 1.96N, 2.94N, 4.90N and 9.81N) were varied. From the recorded data, the average static grip force, slip force, safety margin force and static friction coefficient were evaluated.<BR>It was found that both the slip force and safety margin force increased as the slipperiness of the object surface increased. Significant interactions between surface type and weight were observed in the slip force and static friction coefficient. The interaction effect resulted from the fact that the frictional relationships with the fingers changed according to both weight and surface conditions. This was considered due to the viscoelastic nature of finger skin. An increase in the safety margin force with surface slipperiness was considered due to psychological reaction, probably fear of dropping the object. Unexpected changes in surface conditions caused a greater safety margin force than trials without a surface change, which might also have been associated with psychological reaction to uncertainty of the new surface condition. A relatively large inter-subject variation was found in the slip force and safety margin force relative to slippery surfaces.