Ching-Chi Hsu

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.

Shape optimization for the subsidence resistance of an interbody device using simulation-based genetic algorithms and experimental validation.

Subsidence of interbody devices into the vertebral body might result in serious clinical problems, especially when the devices are not well designed and analyzed. Recently, some novel designs were proposed to reduce the risk of subsidence, but those designs are based on the researchers experience. The purpose of this study was to discover the interbody device design with excellent subsidence resistance by changing the devices shape. The three‐dimensional nonlinear finite element models, which consisted of the interbody device and vertebral body, were created first. Then, the simulation‐based genetic algorithm, which combined the finite element model and the searching algorithm, was developed by using ANSYS® Parametric Design Language. Finally, the numerical results were carefully validated with the use of biomechanical tests. The optimum shape design obtained in this study looks like a flower with many petals and it has excellent subsidence resistance when compared with the other designs provided by the past studies. The results of the present study could help surgeons to understand the subsidence resistance of interbody devices in terms of their shapes and has directly provided the design rationales to engineers.

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.

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.

BIOMECHANICAL INVESTIGATION OF PEDICLE SCREW-BASED POSTERIOR STABILIZATION SYSTEMS FOR THE TREATMENT OF LUMBAR DEGENERATIVE DISC DISEASE USING FINITE ELEMENT ANALYSES

Fusion has been the gold standard treatment for treating lumbar degenerative disc disease. Many clinical studies have demonstrated that adjacent segment degeneration was observed in patients over time. Various instrumentations of pedicle screw-based stabilization systems have been investigated using numerical approaches. However, numerical models developed in the past were simplified to reduce computational time. The aim of this study was to evaluate and to compare the biomechanical performance of rigid, semi-rigid, and dynamic posterior instrumentations using a more realistic numerical model. Three-dimensional nonlinear finite element models of the T11-S1 multilevel spine with various posterior instrumentations were developed. The intersegmental rotation, the maximum disc stress, and the maximum implant stress were calculated. The results indicated that the rigid instrumentation resulted in greater fixation stability but also a greater risk of adjacent segment degeneration and implant failure. The biomechanical performance of the dynamic instrumentation was closer to that of the intact spine model compared with the rigid and semi-rigid instrumentations. The results of this study could help surgeons understand the biomechanical characteristics of different posterior instrumentations for the treatment of lumbar degenerative disc diseases.

Biomechanical analyses of static and dynamic fixation techniques of retrograde interlocking femoral nailing using nonlinear finite element methods.

Femoral shaft fractures can be treated using retrograde interlocking nailing systems; however, fracture nonunion still occurs. Dynamic fixation techniques, which remove either the proximal or distal locking screws, have been used to solve the problem of nonunion. In addition, a surgical rule for dynamic fixation techniques has been defined based on past clinical reports. However, the biomechanical performance of the retrograde interlocking nailing systems with either the traditional static fixation technique or the dynamic fixation techniques has not been investigated by using nonlinear numerical modeling. Three-dimensional nonlinear finite element models were developed, and the implant strength, fixation stability, and contact area of the fracture surfaces were evaluated. Three types of femoral shaft fractures (a proximal femoral shaft fracture, a middle femoral shaft fracture, and a distal femoral shaft fracture) fixed by three fixation techniques (insertion of all the locking screws, removal of the proximal locking screws, or removal of the distal locking screws) were analyzed. The results showed that the static fixation technique resulted in sufficient fixation stability and that the dynamic fixation techniques decreased the failure risk of the implant and produced a larger contact area of the fracture surfaces. The outcomes of the current study could assist orthopedic surgeons in comprehending the biomechanical performances of both static and dynamic fixation techniques. In addition, the surgeons could also select a fixation technique based on the specific patient situation using the numerical outcomes of this study.