Failure analysis of a locking compression plate with asymmetric holes and polyaxial screws

Predicting overloading plate failure using specimen-specific finite element models combined with implantable sensors

BACKGROUND

Mechanical failure of plate osteosyntheses, such as plate bending, still occur in patients. While finite element (FE) models can simulate the mechanical behavior of a bone-plate construct, they lack in vivo validation due to unknown loads. The advent of implantable sensors, which monitor fracture healing by measuring plate deformation, presents an opportunity to validate these FE models in vivo. However, there is currently no established link between the sensor signal and the predicted implant failure. The aim of this study was to bridge this gap by combining FE simulations with sensor data to predict experimentally obtained implant failure of bone-plate constructs.

METHODS

Seven cadaveric ovine tibia shaft fractures, fixed with locking plates, were tested for quasi-static failure, with implantable sensors monitoring plate bending deformation. These setups were mirrored in FE models, where virtual sensor signals, calibrated from a four-point bending test on the isolated sensor, were compared to experimental signals at the onset of plate bending.

RESULTS

There was a high correlation between the experimental and virtual sensor signals from the four-point bending test (R2 > 0.99). The construct-specific FE models, with the calibrated virtual sensor signals, demonstrated a strong correlation with experimental sensor signals at yield (concordance correlation coefficient = 0.89, standard error of estimate = 187.0, relative standard error = 11.9 %).

CONCLUSION

FE models accurately predicted sensor signals at plate bending onset, enabling retrospective in vivo validation without load data and supporting tailored rehabilitation to lower patient complication rates.

The effect of screw hole inserts for the unused screw holes on the strength of a plate

INTRODUCTION

The purpose of this study was to determine if the use of specially designed screw hole inserts in empty locking screw holes improves the strength and failure characteristics of locking plates.

METHODS

Forty-two 7-hole locking LC/DCP plates were mounted on cylindric UHMW Polyethylene blocks with a 1-cm gap between blocks, simulating a fracture with comminution and bone loss. 21 plates had a screw hole insert placed in the center hole (centered over the simulated fracture), while 21 of the plates remained empty in the center hole. The plate-block constructs were placed in a mechanical testing machine and subjected to loading conditions. The axial, bending, and torsional stiffness and displacements needed for the failure of each plate-block construct were calculated. The statistical analysis was performed using the Mann-Whitney U test for independent variables.

RESULTS

All plates were then loaded to failure. There were significant differences in the axial load to failure (p = 0.017), bending load to failure (p < 0.01), and bending displacements (p < 0.01) of the test groups favoring the screw hole insert group as higher mechanical strength.

DISCUSSION/CONCLUSION

In conclusion, the study demonstrates that the addition of the specially designed locking screw hole insert does significantly change the strength of the locking LC/DCP plates and might be suggested in the clinical application.

Impact of Strength Parameters and Material Structure of Bone Plates on Displacement of Bone Fragments in the Injured Area

The study is a metallographic analysis of commercial bone plates used for stabilizing long bones. The plates examined were delivered to the hospital in different years, and the course of treatment of patients with similar goniometric and anthropometric parameters varied dramatically. To determine the characteristics of displacement of bony fragments in the area of the simulated fracture and relate it to the strength parameters of the bone plate, experimental tests were carried out on composite femurs loaded according to the biomechanical loading model at known values of forces acting on the femoral head. In order to assess the influence of material parameters of the plate on the biomechanics of the bone-bone plate system, microstructural and strength tests were performed, i.e., three-point bending tests, chemical composition and hardness assessments, as well as evaluation of the state of internal stresses in the tested materials. The research conducted allowed us to develop guidelines for companies producing bone fixations and orthopedic surgeons who use bone plates to stabilize bones after mechanical trauma, allowing the plates to be tailored to individual patient characteristics.

Finite element analysis and optimization studies on tibia implant of SS 316L steel and Ti6Al4V alloy

Tibial fractures account for approximately 15% of all fractures, typically resulting from high-energy trauma. A critical surgical approach to treat these fractures involves the fixation of the tibia using a plate with minimally invasive osteosynthesis. The selection and fixation of the implant plate are vital for stabilizing the fracture. This selection is highly dependent on the plate's stability, which is influenced by factors like the stresses generated in the plate due to the load on the bone, as well as the plate's length, thickness, and number of screw holes. Minimizing these stresses is essential to reduce the risk of implant failure, ensuring optimal stress distribution and promoting faster, more effective bone healing. In the present work, the finite element and statistical approach was used to optimize the geometrical parameters of the implant plate made of SS 316L steel and Ti6Al4V alloy. A 3D finite element model was developed for analyzing the stresses and deformation, and implant plates were manufactured to validate the results with the help of an experiment conducted on the universal testing machine. A strong correlation was observed between the experimental and predicted results, with an average error of 8.6% and 8.55% for SS316L and Ti6Al4V alloy, respectively. Further, using the signal-to-noise ratio for the minimum stress condition was applied to identify the optimum parameters of the plate. Finally, regression models were developed to predict the stresses generated in SS316L and Ti6Al4V alloy plates with different input conditions. The statistical model helps us to develop the relation between different geometrical parameters of the Tibia implant plate. As determined by the present work, the parameter most influencing is implant plate length. This outcome will be used to select the implant for a specific patient, resulting in a reduction in implant failure post-surgery.

Biomechanical optimization of the magnesium alloy bionic cannulated screw for stabilizing femoral neck fractures: a finite element analysis

Purposes

The magnesium alloy bionic cannulated screw (MABCS) was designed in a previous study promoting cortical-cancellous biphasic healing of femoral neck fractures. The main purpose was to analyze the bore diameters that satisfy the torsion standards and further analyze the optimal pore and implantation direction for stabilizing femoral neck fractures.

Methods

The MABCS design with bionic holes with a screw diameter of less than 20% met the torsion standard for metal screws. The MABCS was utilized to repair the femoral neck fracture via Abaqus 6.14 software, which simulated the various stages of fracture healing to identify the optimal biomechanical environment for bionic hole size (5%, 10%, 15%, and 20%) and implantation direction (0°, 45°, 90°, and 135°).

Results

The stress distribution of the MABCS fracture fixation model is significantly improved with an implantation orientation of 90°. The MABCS with a bionic hole and a screw diameter of 10% provides optimal stress distribution compared with the bionic cannulated screw with diameters of 5%, 15%, and 20%. In addition, the cannulated screw fixation model with a 10% bionic hole size has optimal bone stress distribution and better internal fixation than the MABCS fixation models with 5%, 15%, and 20% screw diameters.

Conclusion

In summary, the MABCS with 10% screw diameter bionic holes has favorable biomechanical characteristics for stabilizing femoral neck fractures. This study provides a biomechanical foundation for further optimization of the bionic cannulated screw.

Effect of screw angulation and multiple insertions on load-to-failure of polyaxial locking system

PURPOSE

Polyaxial locking plates rely on the alignment between the thread-to-thread connections of the screw head and the plate hole. These implants have provided substantial support for surgeons. In particular, extended screw positioning have proven to be beneficial in the fixation of challenging fractures. This study aimed to investigate the mechanical properties of ChM 5.0 ChLP polyaxial screws inserted in off-axis trajectories, including multiple insertions and to correlate these parameters with the screw head and the plate hole thread-to-thread engagement.

METHODS

Polyaxial locking screws were inserted into the plates at various angles (0°,10°,15°, -15° off-axis). Multiple time inserted screws were placed firstly at 15°, then 0° and finally -15° off-axis in the same plate hole. A microCT scan of the plate-hole and screw-head interface was conducted before destructive tests. Representative screws from each group were also examined by Scanning Electron Microscope.

RESULTS

The standard insertion at 0° sustained the greatest maximum bending strength without relocation in the screw hole. Screws inserted at 10° and 15° (one time) showed a significant reduction in load-to-failure of up to 36% and 55%, (p = 0.001) (p = 0.001) respectively. Screws inserted at -15° after a maximum of three multiple insertions with angle shift, showed a total reduction in force of up to 70% (p = 0.001). A microCT analysis of thread engagement showed significant correlations. However, the results obtained for multiple insertions were highly variable.

CONCLUSIONS

ChM 5.0 ChLP polyaxial locking system has valuable properties that foster fracture fixation, providing various surgical options. Nevertheless, the freedom of off-axis placement and multiple insertions of the screws comes at the price of reduced force. When possible surgeons should minimize the angles of insertions.