| Summary: | 碩士 === 國立臺灣科技大學 === 機械工程系 === 100 === Interlocking intramedullary nail had been widely used in proximal femoral fractures, but it still threatened by lag screw failures, including breakage and loosening. It is necessary to perform the second surgery when implant failure occurred. The second surgery costs additional time and money, and it also causes the additional injury to the patients. The purpose of this study is to improve the lag screw design, and to find an optimal design which can maintain bending and pullout strength simultaneously.
This study used Taguchi L25 orthogonal array to define the design space of lag screws, and used finite element analysis (FEA) to solve the bending and pullout models. The results of FEA were used to construct artificial neural network (ANN). We chose the best ANN model to perform multi-objective genetic algorithms (GA), and determined the optimal design of lag screw. Finally, we chose five different lag screw designs (L5 screw, L16 screw, L23 screw, commercial screw, and optimal screw) to perform biomechanical tests, and prove the optimal design which can maintain bending and pullout strength simultaneously.
In this study, we defined the optimal design from GA results, the six factors were initial position of conical angle 29 mm, inner diameter 3.3 mm, proximal root radius 0.22 mm, pitch 3.3 mm, proximal half angle 5°, thread width 0.2 mm. From the FEA results, the maximum tensile stress of bending models descended as commercial screw, L23 screw, L16 screw, optimal screw, and L5 screw (455.36-1312.10 MPa); and total reaction force of pullout models descended as optimal screw, commercial screw, L16 screw, L23 screw, and L5 screw (23.61-38.14 N). Further, from biomechanical tests, the yielding load descended as L5 screw, optimal screw, L16 screw, L23 screw, and commercial screw (454.08-605.01 N); the fatigue life descended as optimal screw, L5 screw, L16 screw, L23 screw, and commercial screw (80339-1000000 cycles); the pullout strength descended as optimal screw, commercial screw, L16 screw, L23 screw, and L5 screw (964.31-1635.36 N) and L16 screw, optimal screw, commercial screw, L23 screw, and L5 screw (299.09-661.12 N) in 0.32 and 0.16 g/cm3 Sawbone densities. The correlation coefficient between the total strain energy of bending models and the yielding load of bending tests was -1.00; and between the maximum tensile stress of bending models and logarithmic fatigue life of fatigue tests was -0.91; and between the total reaction force of pullout models and pullout strength of pullout tests were 0.99 and 0.93 in 0.32 and 0.16 g/cm3 Sawbone densities .
In this study, the influential factors were initial position of conical angle and inner diameter in bending cases; inner diameter, proximal root radius and pitch in pullout cases. From the results of ANN, GA and biomechanical tests, it showed the optimal lag screw could maintain 95％ of maximal bending and pullout strength simultaneously.
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