Intramedullary nails with two lag screws

Evaluation of fixation after plating of distal radius fractures - a validation study

We used five fresh-frozen cadavers with 10 upper limbs to evaluate by finite element analysis (FEA) the plate fixation of distal radius fractures. The distal radius of the cadavers was fractured using a comminution fracture model. Plate fixation was performed using Synthes VATCP. Compression tests were performed on these specimens and force displacement curves were obtained. FEA was performed using Mechanical Finder. The Keyak, Keller vertebra, Carter, and Matsuyama conversion equations without contact analysis, and the Matsuyama equations with contact analysis, were used for the boundary conditions. We found strong positive correlations with the Matsuyama conversion equations either with or without contact analysis. The validated FEA model will be used for preoperative simulation of actual fractures.

The effect of distance between holes on the structural stability of subchondral bone in microfracture surgery: a finite element model study

Background

Microfracture is a surgical technique that involves creating multiple holes of 3–4 mm depth in the subchondral bone to recruit stem cells in the bone marrow to the lesion, inducing fibrocartilage repair and knee cartilage regeneration. Recently, it has been reported that increasing the exposed area of the lower cartilaginous bone (drilling a lot of holes) increases the outflow of stem cells, which is expected to affect the physical properties of the subchondral bone when the exposed area is large. The purpose of this study was to analyse the effect of the distance between the holes in the microfracture procedure on the structural stability of the osteochondral bone using a finite element method.

Methods

In this study, lateral aspects of the femoral knee, which were removed during total knee arthroplasty were photographed using microtomography. The model was implemented using a solitary walks program, which is a three-dimensional simplified geometric representation based on the basic microtomography data. A microfracture model was created by drilling 4 mm-deep holes at 1, 1.5, 2, 2.5, 3, 4, and 5 mm intervals in a simplified three-dimensional (3D) geometric femoral model. The structural stability of these models was analysed with the ABAQUS program. We compared the finite element model (FEM) based on the microtomography image and the simplified geometric finite element model.

Results

Von Mises stress of the subchondral bone plate barely increased, even when the distance between holes was set to 1 mm. Altering the distance between the holes had little impact on the structural stability of the subchondral bone plate. Safety factors were all below 1.

Conclusions

Although we did not confirm an optimal distance between holes, this study does provide reference data and an epidemiological basis for determining the optimal distance between the holes used in the microfracture procedure.

Biomechanical investigation of a novel hybrid dorsal double plating for distal radius fractures by integrating topology optimization and finite element analysis

BACKGROUND

Currently available dorsal locking plates for the treatment of distal radius fractures are far less then volar locking plates, and there is limited evidence about biomechanical strength of dorsal plates. The aim of this study is to develop a novel hybrid dorsal double plating, which enhance biomechanical strength in the articular fixation region and achieve the minimally invasive surgical technique requirement of distal radius fracture treatment by combining weighted topology optimization and finite element (FE) analysis METHODS: A dorsal template bone plate design (based on dorsal double plating (DDP)) was constructed to perform weighted topology optimization and FE analysis under six fracture models with 50%, 30%, and 20% weighting of the joint subjected to axial, bending, and torsion moments, respectively. A novel hybrid dorsal double plating (HDDP) was generated using the union of six single dorsal plates to subtract the intersection of the original template dorsal model. A 100 N axial load with 1 Nm bending and torsion moments were applied at the end of the distal radius onto six fracture FE models to investigate the biomechanical differences between the DDP and HDDP approaches.

RESULTS

Results of weighted topology optimization showed that the profile of the HDDP presented a "Y" shape. Simulation results showed that the bone plate stress values for the distal radius fractures fixed with HDDP was much smaller than those with DDP regardless of the type of bone fractures and load conditions. The maximum bone stress value of the DDP approach was much higher than that of HDDP when the distal radius was a complete sagittal articular fracture and partial articular fracture involving lunate fossa. The corresponding maximum bone stress values for different loads might be higher than the ultimate strength of bone (150  MPa) and induced the risk of future bone fractures.

CONCLUSIONS

It is concluded that the novel HDDP demonstrated better resistance to functional loads, provided sufficient screw fixation at the articular surface, and can be placed on the dorsal site of the distal radius through the standard dorsal approach to minimize invasive surgeries and eliminate tendon irritations.

Stress distribution in delayed replanted teeth splinted with different orthodontic wires: a three-dimensional finite element analysis

AIM

The aim was to evaluate the biomechanical behavior of the supporting bony structures of replanted teeth and the periodontal ligament (PDL) of adjacent teeth when orthodontic wires with different mechanical properties are applied, with three-dimensional finite element analysis.

MATERIALS AND METHODS

Based on tomographic and microtomographic data, a three-dimensional model of the anterior maxilla with the corresponding teeth (tooth 13-tooth 23) was generated to simulate avulsion and replantation of the tooth 21. The teeth were splinted with orthodontic wire (Ø 0.8 mm) and composite resin. The elastic modulus of the three orthodontic wires used, that is, steel wire (FA), titanium-molybdenum wire (FTM), and nitinol wire (FN) were 200 GPa, 84 GPa, and 52 GPa, respectively. An oblique load (100 N) was applied at an angle of 45° on the incisal edge of the replanted tooth and was analyzed using Ansys Workbench software. The maximum (σmax) and minimum (σmin) principal stresses generated in the PDL, cortical and alveolar bones, and the modified von Mises (σvM) values for the orthodontic wires were obtained.

RESULTS

With regard to the cortical bone and PDL, the highest σmin and σmax values for FTM, FN, and FA were checked. With regard to the alveolar bone, σmax and σmin values were highest for FA, followed by FTM and FN. The σvM values of the orthodontic wires followed the order of rigidity of the alloys, that is, FA > FTM > FN.

CONCLUSION

The biomechanical behavior of the analyzed structures with regard to all the three patterns of flexibility was similar.

Design optimisation and experimental evaluation of dorsal double plating fixation for distal radius fracture

This study determines the relative effects of changes in osteoporosis condition, plate/screw design factors (plate angle/length/width/thickness and screw diameter) and fixation methods (screw number and screw length) on the biomechanical response of dorsal double plating (DDP) fixation at a distal radius fracture to determine the optimal design and evaluate its biomechanical strength using the dynamic fatigue test. Eighteen CAD and finite element (FE) models corresponding to a Taguchi L18 array were constructed to perform numerical simulations to simulate the mechanical responses of a DDP fixed in a simply distal radius fracture bone. The Taguchi method was employed to determine the significance of each design factor in controlling bone/plate/screw stress and distal fragment displacement under axial (100 N), bending (1 N m) and torsion (1 N m) loads. Simulation results indicated that the order rank to determine the mechanical response was the plate thickness, plate width, screw diameter, and number of screws. Dorsal intermediate (L) plate with 60 mm length, 1.8 mm thickness, 6.0 mm width and 2.8 mm diameter, 20 mm length dual-thread locking screw can be found for optimisation. The DDP, including an L plate with 0°, 30° and 60° angles and a straight I plate, were made with Ti6Al4V to fix onto the sawbones with three corresponding radius fractures to perform the dynamic testing. The specimens were oscillated with loads between 10 N and 150 N at 5 Hz for 20,000 cycles. The average stiffness in 20,000 test cycles was 425.7 N/mm, 461.1 N/mm and 532.1N/mm for the 0°, 30° and 60° constructs, respectively. No difference in stiffness was found in the same angled constructs throughout the 20,000 cycles of testing (p > 0.05). Lack of gross construct failures during cyclic testing and reasonable stiffness corroborated that our new constructs tested to date seem stable enough to support restricted post-operative loads.

Comparative study of two materials for dynamic hip screw during fall and gait loading: titanium alloy and stainless steel

BACKGROUND

Internal fixation with dynamic hip screw is a choice of treatment for hip fractures to stabilize a femoral fracture. Choosing the proper implant and its material has a great effect on the healing process and failure prevention. The purpose of this analysis was to assess biomechanical behavior of dynamic hip screw with two different materials implanted in the femur during fall and gait.

METHODS

A 3D finite element model of an intact femur and a 3D implant within the same femur were developed. A finite element analysis was carried out to establish the effect of load conditions and implant material properties on biomechanical behavior of the dynamic hip screw after internal fixation. Two load configurations are chosen: one simulating the stance phase of the normal gait cycle, and the other replicating a low-energy fall. The implanted femur was investigated with two different materials for the dynamic hip screw: stainless steel and titanium alloy.

RESULTS

During stance, more stress is placed on the implanted femur compared with the intact femur. During a fall, the implanted femur is in a greater state of stress, which mostly occurs inside the dynamic hip screw. Titanium alloy decreases stress levels by an average of 40% compared with stainless steel. However, deformation is slightly reduced with a stainless steel dynamic hip screw during both load cases.

CONCLUSIONS

After internal fixation, dynamic hip screw generates greater stresses within the implanted femur compared with the intact femur under the same loading conditions. A titanium alloy implant appears to undergo less stress from a low-energy fall compared with stainless steel and can be considered the preferred implant material. The critical parts of the dynamic hip screw are the forth distal screw and the plate.

Finite element analysis of intramedullary devices: The effect of the gap between the implant and the bone

This paper examines the interaction interface between the implant and the bone for an intramedullary femoral nailing system using a finite element (FE) model and specifically considers the hypothesis that the local geometry at the interface is significant to the resulting localized contact stress between the medial and lateral aspect of nail and endosteum. Contact mechanics algorithms are used in the FE modelling technique that can be developed to deal with any form of intramedullary device for which contact at the bone—implant interface is important.

Global stiffness data from the FE model are compared with available data from an experiment carried out on a construct of the bone and the device that uses intramedullary femoral nails. Acceptable agreement is obtained.

The results demonstrate that the mechanical interface between the implant and the bone is significantly affected by the gap geometry and magnitude. In particular, larger gaps lead to greater concentrations of stress on the medial side, while the distribution of stress is more uniform at the lateral contacts. Furthermore, the results show that the gap can have a marked effect on the stresses that occur on the fracture plane.

Biomechanical evaluation of the modified double-plating fixation for the distal radius fracture

BACKGROUND

Distal radius fracture is among the most common type of skeletal injuries. To conquer the surgical and biomechanical complications of the most-frequent used double-plating operation for this fracture, modified double-plating technique was proposed in this study. The aim of this study was to investigate the biomechanical interactions of double-plating, modified double-plating and traditional single plating fixations coupled with various load conditions using nonlinear finite element analysis.

METHODS

A three-dimensional finite element distal radius fracture model with three fixation methods (double-plating, modified double-plating and single) was generated based on computer tomography data. After model verification and validation, frictional (contact) elements were used to simulate the interface condition between the fixation plates and the bony surface. The rigidity, stress values and displacements at the radius end were observed under axial, bending and torsion load conditions.

FINDINGS

The simulated results showed that the modified double-plating model demonstrated the highest rigidity and the least displacement among the three techniques in bending, but not in axial compression (similar results across the three) and torsion (modified double-plating technique possessed lowest rigidity). The maximum von Mises stress for bone was lower in modified double-plating model as well. These results indicated that modified double-plating technique demonstrated a better structural strength against bending with the least potential of fracture fragments and screw loosening.

INTERPRETATION

Although a lower torsional rigidity, modified double-plating technique was a better choice in distal radius fracture fixation since the bending force, which has the potential to separate the fracture ends, is more detrimental in hindering fracture healing.

Measurement of local strains induced into the femur by trochanteric Gamma nail implants with one or two distal screws

This study aims to evaluate experimentally the behaviour after fracture consolidation of two intramedullary Gamma nail implants, having one (G1) or two (G2) distal screws, respectively. Nails have been implanted into standardized synthetic femora, instrumented with strain gauges. Strains measurements, supported by finite element numerical modelling, showed that the G2 implant, although ensuring higher flexional and torsional stiffness, can lead to localized contacts that occur between the tip of the nail and the femoral endosteum. This might be one of the reasons of the complications associated with pain in the mid-portion of the thigh after implantation which has been reported in several clinical studies when Gamma nails with two distal screws are used.

Buttressing angle of the double-plating fixation of a distal radius fracture: a finite element study

Treatment of a distal radius fracture should consider principles including stable fixation and early motion. The aim of this study was to investigate the biomechanical interactions of plate-fixation angles in the internal double-plating method coupled with various load conditions using non-linear finite element analysis (FEA). A 3D finite element distal radius fracture model with three separation angles (50, 70, and 90 degrees ) between the buttressed L- and straight plates was generated based on computed tomography data. After model verification and validation, frictional (contact) elements were used to simulate the interface condition between the fixation plates and the bony surface. The stress/strain distributions and displacements at the radius end were observed under axial, bending, and torsion load conditions. The simulated results indicated that the bending and torsion increased the stress values more than the axial load. The radius and straight plate stress values decreased significantly with increasing fixation angles for all load conditions. However, the L-plate stress values increased slightly under the bending buckling effect. The displacements at the radius end and strains at the fracture healing interface decreased with increasing fixation angles for axial and torsion conditions but displayed a slight difference for the bending condition. The findings using FEA provide quantitative evidence to identify that much larger plate fixation angles could provide better mechanical strength to establish favorable stress-transmission and prevent distal fragment dislocation.