Biomedical subjects

Ching-Chi Hsu

Publications and source records attributed to Ching-Chi Hsu.

4 recordsLinked to original sources

PubMed2006

Increasing bending strength of tibial locking screws: mechanical tests and finite element analyses.

BACKGROUND: Healing of tibial fractures treated by locked nailing is threatened by locking screw failure. However, the effects of the design factors of the screws on their mechanical strength have rarely been studied. METHOD: Three-point bending tests and finite element analyses were used to investigate the bending strength of five types of commercially available tibial locking screws and two types of specially designed screws. Yielding strength and fatigue life measured in bending tests were correlated to total strain energy and maximal tensile stress computed in finite element analyses. Parametric analysis and design optimization were done according to the Taguchi method. Validation studies to assess the stress rising effect of the threads on the fatigue strength were conducted in two types of new screws made of either stainless steel or titanium alloy. FINDINGS: The yielding strength of the screws was closely related to their total strain energy, and the logarithm of the fatigue life was closely related to the maximal tensile stress with correlation coefficients of -0.95 and -0.90, respectively. Parametric studies indicated that fatigue strength of the screws was affected mainly by inner diameter (contribution, 63.8%) and root radius (27.8%). The yielding strength was determined primarily by inner diameter (88.5%). Titanium screws had a longer fatigue life than stainless steel screws, especially in screws with larger root radii. INTERPRETATION: A screw's strength is closely related to its design factors. Finite element models, which can reliably reflect the mechanical strength of screws can save time and effort during screw design. Larger root radius can effectively improve the fatigue strength, especially for titanium screws as compared with stainless steel screws.

Biomechanical Phenomena↗

PubMed2006

Multiobjective optimization of tibial locking screw design using a genetic algorithm: Evaluation of mechanical performance.

Breakage or loosening of locking screws may impair fracture fixation or bone healing in locked nailing of tibial fractures. Bending strength and bone holding power, two important design objectives of locking screws, may conflict with each other. The present study used multiobjective optimization with a genetic algorithm to investigate the optimal designs with respect to these two objectives. Three-dimensional finite element models for analyzing bending strength and bone holding power of locking screws were created first. Through use of a Taguchi L25 orthogonal array, two objective functions were developed by least-squares regression analyses. Then, the trade-off solutions between the two objectives known as Pareto optima were explored by a weighted-sum aggregating approach under geometric constraints. The objective functions, reliably reflecting the finite element results, were valid for multiobjective studies. The Pareto fronts of the screws with 4.5-mm and 5.0-mm outer diameters were similar. The "knee" region of the Pareto front, characterized by the fact that a small improvement in either objective will cause a large deterioration in the other objective, might be the favored choice of optimal designs. The commercially available locking screws compared with the Pareto optima were found to be dominated designs and could be improved. In conclusion, the multiobjective optimization with a genetic algorithm was useful for optimization of locking screw design with many variables and conflicting objectives. Choosing an optimal design requires a thorough knowledge of the inherent problems. This method could reduce the time, cost, and labor associated with the screw development process.

Algorithms↗

PubMed2004

Increase of pullout strength of spinal pedicle screws with conical core: biomechanical tests and finite element analyses.

Screw loosening can threaten pedicle screw fixation of the spine. Conical screws can improve the bending strength, but studies of their pullout strength as compared with that of cylindrical screws have shown wide variation. In the present study, polyurethane foam with two different densities (0.32 and 0.16 gm/cm3) was used to compare the pullout strength and stripping torque among three kinds of pedicle screws with different degrees of core tapering. Three-dimensional finite element models were also developed to compare the structural performance of these screws and to predict their pullout strength. In the mechanical tests, pullout strength was consistently higher in the higher density foam and was closely related to screw insertion torque (r=0.87 and 0.81 for the high and low density foam, respectively) and stripping torque (r=0.92 and 0.78, respectively). Conical core screws with effective foam compaction had significantly higher pullout strength and insertion torque than cylindrical core screws (p<0.05). The results of finite element analyses were closely related to those of the mechanical tests in both situations with or without foam compaction. This study led to three conclusions: polyurethane foam bone yielded consistent experimental results; screws with a conical core could significantly increase pullout strength and insertion torque over cylindrical; and finite element models could reliably reflect the results of mechanical tests.

Bone Screws↗

PubMed2004

Mechanical tests and finite element models for bone holding power of tibial locking screws.

OBJECTIVE: To investigate the bone holding power of tibial locking screws. DESIGN: The bone holding power was assessed by mechanical testing and finite element analysis. BACKGROUND: Screw loosening might threaten fracture fixation and bone healing. METHODS: In mechanical tests, six types of different tibial locking screws were inserted into low-density polyurethane foam tubes, which simulated osteoporotic bone. The screws were pushed out of the foam bone by an axial load, and the maximal pushout load was recorded. In finite element analysis, three-dimensional finite element models with a nonlinear contact interface between the screws and the bones were created to simulate the mechanical testing. The total strain energy of the bone and total reaction force of the screws were recorded. The contribution of the design factors was analyzed by the Taguchi method. RESULTS: In the mechanical tests, foam bone was stripped by the screw threads without screw deformation. The testing results were closely related to those of finite element analysis. The Taguchi analysis showed that the descending order of contribution of the design factors was outer diameter, pitch, half angle, and inner diameter. Root radius and thread width had minimal effects. CONCLUSIONS: The bone holding power of the screws could be reliably assessed by finite element models, which could analyze the effects of all the design factors independently and were potentially applicable to screws with irregular thread patterns.

Analysis of Variance↗