Ching-Kong Chao

Increasing Bending Strength and Pullout Strength in Conical Pedicle Screws: Biomechanical Tests and Finite Element Analyses

Study Design Comparative in vitro biomechanical study and finite element analysis. Objectives To investigate the bending strength and pullout strength of conical pedicle screws, as compared with conventional cylindrical screws. Summary of Background Data Transpedicle screw fixation, the gold standard of spinal fixation, is threatened by screw failure. Conical screws can resist screw breakage and loosening. However, biomechanical studies of bending strength have been lacking, and the results of pullout studies have varied widely. Methods Ten types of pedicle screws with different patterns of core tapering and core diameter were specially manufactured with good control of all other design factors. The stiffness, yielding strength, and fatigue life of the pedicle screws were assessed by cantilever bending tests using high-molecular-weight polyethylene. The pullout strength was assessed by pullout tests using polyurethane foam. Concurrently, 3-dimensional finite element models simulating these mechanical tests were created, and the results were correlated to those of the mechanical tests. Results In bending tests, conical screws had substantially higher stiffness, yielding strength, and fatigue life than cylindrical screws (P<0.01), especially when there was no step at the thread-shank junction. In pullout tests, pullout strength was higher in screws with a conical core and smaller core diameter and also in situations with higher foam density (P<0.01). In finite element analysis, the maximal deflection and maximal tensile stress were closely related to yielding strength (r=−0.91) and fatigue life (r=−0.95), respectively, in the bending analyses. The total reaction force was closely related to the pullout strength in pullout analyses (r=0.84 and 0.91 for different foam densities). Conclusions Conical screws effectively increased the bending strength and pullout strength simultaneously. The finite element analyses reliably predicted the results of the mechanical tests.

Notch sensitivity of titanium causing contradictory effects on locked nails and screws

The purpose of this biomechanical study was to compare the mechanical properties of specially designed locked nails and screws with the same structures and made from either stainless steel or titanium alloy. The structural factors investigated included inner diameter and root radius for locking screws and outer diameter and nail hole size for locked nails. The mechanical properties investigated included yield load, cyclic bending stiffness, and fatigue life. Finite element models were used to simulate the mechanical tests and compute the stress concentration factors. Increasing the root radius and the inner diameter could effectively increase the fatigue life of the locking screws. The fatigue life of titanium screws was higher (by 1.4- to >6-fold) than that of stainless steel screws, especially when the inner diameter was increased. In contrast, the fatigue life of titanium locked nails was lower (by about 1/4 to 1/3) than that of their stainless steel counterparts. Finite element models could closely predict the results of the biomechanical tests with a Pearson correlation coefficient that ranged from -0.58 to -0.84 for screws and was -0.98 for nails. The stress concentration factors ranged from 1 to 1.97 for screws and from 3.89 to 4.99 for nails. The present study suggested that with larger root radius and inner diameter, titanium locking screws could provide much higher fatigue life than stainless steel counterparts. However, titanium locked nails might lose their advantages of superior mechanical strength because of high notch sensitivity, and this limitation should be taken seriously during the design process.

Parametric study on the interface pullout strength of the vertebral body replacement cage using FEM-based Taguchi methods

Improper design of vertebral body cages may seriously affect the interface strength and cause the lose of fixation for a vertebral body replacement. This research used a FEM-based Taguchi method to investigate the effects of various factors to find the robust design of the body cage. Three-dimensional finite element models with a nonlinear contact analysis have been developed to simulate the pullout strength of the body cage. Then, the Taguchi robust design method was used to evaluate the spike design. In a situation without bone fusion, the spike row, the spike oblique, and the spike height were especially important factors. The optimum combination has been found to be the pyramidal spike type, a spike height of 2mm, a spike diameter of 2.2mm, an oblique geometry, 11 rows per 28 mm, and an inner diameter of 10mm. In a situation with bone fusion, the spike row, the spike height, and the inner diameter were the most significant factors. Here, the optimum combination has been found to be the conical spike type, a spike height of 2mm, a spike diameter of 2.2mm, an oblique geometry, 11 rows per 28 mm, and an inner diameter of 20mm. The finite element analyses could be used to predict the interface stiffness of the body cages. The FEM-based Taguchi methods have effectively decreased the time and effort required for evaluating the design variables of implants and have fairly assessed the contribution of each design variable.

A neurogenetic approach to a multiobjective design optimization of spinal pedicle screws.

A pedicle screw fixation has been widely used to treat spinal diseases. Clinical reports have shown that the weakest part of the spinal fixator is the pedicle screw. However, previous studies have only focused on either screw breakage or screw loosening. There have been no studies that have addressed the multiobjective design optimization of the pedicle screws. The multiobjective optimization methodology was applied and it consisted of finite element method, Taguchi method, artificial neural networks, and genetic algorithms. Three-dimensional finite element models for both the bending strength and the pullout strength of the pedicle screw were first developed and arranged on an L(25) orthogonal array. Then, artificial neural networks were used to create two objective functions. Finally, the optimum solutions of the pedicle screws were obtained by genetic algorithms. The results showed that the optimum designs had higher bending and pullout strengths compared with commercially available screws. The optimum designs of pedicle screw revealed excellent biomechanical performances. The neurogenetic approach has effectively decreased the time and effort required for searching for the optimal designs of pedicle screws and has directly provided the selection information to surgeons.

Biomechanical comparisons of different posterior instrumentation constructs after two-level ALIF: a finite element study.

Anterior lumbar interbody fusion (ALIF) with cylindrical cages and supplemental posterior fixation has been widely used for internal disc derangement. However, most researchers have focused on single-level ALIF. Therefore, the biomechanical performance of various fixation constructs after two-level ALIF is not well characterized. This research used three-dimensional finite element models (FEM) with a nonlinear contact analysis to evaluate the initial biomechanical behavior of five types of fixation devices after two-level ALIF (L3/L4, L4/L5) under six loading conditions. These fixation constructs included a three-level pedicle screw and rod, a two-level translaminar facet screw, a two-level transfacet pedicle screw, a bisegmental pedicle screw and rod, and a bisegmental pedicle screw and rod with cross-linking. The FEMs developed in this study demonstrate that, compared to the other four types of posterior fixation constructs analyzed, the three-level pedicle screw and rod provide the best biomechanical stability. Both two-level facet screw fixation constructs showed unfavorable loading in lateral bending. For the construct of the three-level pedicle screw and rod, the middle-segment pedicle screw should not be omitted even though a cross-link is used. The two-level ALIF models with cages and posterior fixation constructs that we developed can be used to evaluate the initial biomechanical performance of a wide variety of posterior fixation devices prior to surgery.

Comparison of multiple linear regression and artificial neural network in developing the objective functions of the orthopaedic screws

Optimizing the orthopaedic screws can greatly improve their biomechanical performances. However, a methodical design optimization approach requires a long time to search the best design. Thus, the surrogate objective functions of the orthopaedic screws should be accurately developed. To our knowledge, there is no study to evaluate the strengths and limitations of the surrogate methods in developing the objective functions of the orthopaedic screws. Three-dimensional finite element models for both the tibial locking screws and the spinal pedicle screws were constructed and analyzed. Then, the learning data were prepared according to the arrangement of the Taguchi orthogonal array, and the verification data were selected with use of a randomized selection. Finally, the surrogate objective functions were developed by using either the multiple linear regression or the artificial neural network. The applicability and accuracy of those surrogate methods were evaluated and discussed. The multiple linear regression method could successfully construct the objective function of the tibial locking screws, but it failed to develop the objective function of the spinal pedicle screws. The artificial neural network method showed a greater capacity of prediction in developing the objective functions for the tibial locking screws and the spinal pedicle screws than the multiple linear regression method. The artificial neural network method may be a useful option for developing the objective functions of the orthopaedic screws with a greater structural complexity. The surrogate objective functions of the orthopaedic screws could effectively decrease the time and effort required for the design optimization process.

Comparison study of the pullout strength of conventional spinal pedicle screws and a novel design in full and backed-out insertions using mechanical tests.

Recently, new pedicle screw designs have been developed. However, these designs’ performances are still unclear, especially when backed out after insertion. The objective of this study was to investigate the performances of different screw designs when backed out from full insertion. Seven conventional designs of the pedicle screw and one novel design were inserted into polyurethane foam (0.32 g/cm3). All screws were first fully inserted (43 mm) and were backed out 360°. Axial pullout tests were performed and the reaction force was measured. The results showed that the conical screw of type 1 with a small inner diameter provided the highest pullout strength in both full insertion and backed-out insertion (2401.85 and 2169.82 N, respectively). However, this screw’s pullout strength significantly decreased (9.7%) when backed out from full insertion. There was no significant difference between the conical screw of type 1 with a small inner diameter and double duo core screw (p > 0.01) in backed-out insertion. The cylindrical screw with a small diameter, dual inner core screw and double dual core screw also provided good results in both full insertion (2115.44, 2182.99 and 2226.93 N, respectively) and backed-out conditions (2065.80, 2014.28 and 1941.29 N, respectively). The increased pullout strength of the conical design could be due to the effect of bone compaction. However, the screw exhibited less consistent pullout strength when backed out when compared with the other designs. The conical screw should be inserted to the precise position without turning back, especially in osteoporosis patients. The dual inner core screw and double dual core screw could provide greater stability in both conditions. Care should be taken when using both the cylindrical screw with a small thread depth and the dual outer core screw.

Pushout strength of tibial locking screws: Development of finite element models

Abstract This study investigated the bone holding power of tibial locking screws by mechanical testing and finite element analysis. In mechanical tests, five types of commercially available tibial locking screws: Howmedica, Osteo AG, Richards type I, Richards type II, and Synthes were inserted in a cylinder of polyurethane foam bone. Axial load was applied to the screw tip to push the screws out of the foam bone by a material testing machine. The pushout strength of each screw was recorded. In finite element analysis, three‐dimensional finite element models with nonlinear contact interface between the screws and the bones were created to simulate the mechanical testing. The results showed that the order of the pushout strength of the locking screws from high to low was Osteo AG, Richards type II, Richards type I, Howmedica, and Synthes. The results of mechanical tests were highly correlated to the results of finite element analysis. Finite element models with low elastic modulus of bone and no frictional force between the screws and bones can better simulate the situations of mechanical testing. The finite element models built in this study may help the manufacturers evaluate new designs of locking screws before manufacture and assist surgeons to select suitable devices for their patients.

Multiobjective optimization design of spinal pedicle screws using neural networks and genetic algorithm: mathematical models and mechanical validation.

Short-segment instrumentation for spine fractures is threatened by relatively high failure rates. Failure of the spinal pedicle screws including breakage and loosening may jeopardize the fixation integrity and lead to treatment failure. Two important design objectives, bending strength and pullout strength, may conflict with each other and warrant a multiobjective optimization study. In the present study using the three-dimensional finite element (FE) analytical results based on an L25 orthogonal array, bending and pullout objective functions were developed by an artificial neural network (ANN) algorithm, and the trade-off solutions known as Pareto optima were explored by a genetic algorithm (GA). The results showed that the knee solutions of the Pareto fronts with both high bending and pullout strength ranged from 92% to 94% of their maxima, respectively. In mechanical validation, the results of mathematical analyses were closely related to those of experimental tests with a correlation coefficient of −0.91 for bending and 0.93 for pullout (P < 0.01 for both). The optimal design had significantly higher fatigue life (P < 0.01) and comparable pullout strength as compared with commercial screws. Multiobjective optimization study of spinal pedicle screws using the hybrid of ANN and GA could achieve an ideal with high bending and pullout performances simultaneously.

Biomechanical evaluation of bending strength of spinal pedicle screws, including cylindrical, conical, dual core and double dual core designs using numerical simulations and mechanical tests.

Pedicle screws are used for treating several types of spinal injuries. Although several commercial versions are presently available, they are mostly either fully cylindrical or fully conical. In this study, the bending strengths of seven types of commercial pedicle screws and a newly designed double dual core screw were evaluated by finite element analyses and biomechanical tests. All the screws had an outer diameter of 7 mm, and the biomechanical test consisted of a cantilever bending test in which a vertical point load was applied using a level arm of 45 mm. The boundary and loading conditions of the biomechanical tests were applied to the model used for the finite element analyses. The results showed that only the conical screws with fixed outer diameter and the new double dual core screw could withstand 1,000,000 cycles of a 50-500 N cyclic load. The new screw, however, exhibited lower stiffness than the conical screw, indicating that it could afford patients more flexible movements. Moreover, the new screw produced a level of stability comparable to that of the conical screw, and it was also significantly stronger than the other screws. The finite element analysis further revealed that the point of maximum tensile stress in the screw model was comparable to the point at which fracture occurred during the fatigue test.