Biomechanical assessment of composite versus metallic intramedullary nailing system in femoral shaft fractures: A finite element study

AbstractBackgroundIntramedullary nails are the primary choice for treating long bone fractures. However, complications following nail surgery including non-union, delayed union, and fracture of the bone or the implant still exist. Reducing nail stiffness while still maintaining sufficient stability seems to be the ideal solution to overcome the abovementioned complications.MethodsIn this study, a new hybrid concept for nails made of carbon fibers/flax/epoxy was developed in order to reduce stress shielding. The mechanical performance of this new implant in terms of fracture stability and load sharing was assessed using a comprehensive non-linear FE model. This model considers several mechanical factors in nine fracture configurations at immediately post-operative, and in the healed bone stages.ResultsPost-operative results showed that the hybrid composite nail increases the average normal force at the fracture site by 319.23N (P<0.05), and the mean stress in the vicinity of fracture by 2.11MPa (P<0.05) at 45% gait cycle, while only 0.33mm and 0.39mm (P<0.05) increases in the fracture opening and the fragments' shear movement were observed. The healed bone results revealed that implantation of the titanium nail caused 20.2% reduction in bone stiffness, while the composite nail lowered the stiffness by 11.8% as compared to an intact femur.InterpretationOur results suggest that the composite nail can provide a preferred mechanical environment for healing, particularly in transverse shaft fractures. This may help bioengineers better understand the biomechanics of fracture healing, and aid in the design of effective implants

Biomechanical Response under Stress-Controlled Tension-Tension Fatigue of a Novel Carbon Fiber/Epoxy Intramedullary Nail for Femur Fractures

© 2020 IPEM Metallic intramedullary nails are the “gold standard” implant for repairing femur shaft fractures. However, their rigidity may eliminate axial micromotion at the fracture (causing delayed healing) and they may carry too much load relative to the femur (causing “stress shielding”). Consequently, some researchers have proposed fiber-reinforced composite nails, but only one evaluated cyclic fatigue performance. Therefore, this study assessed the cyclic fatigue response of a carbon fiber/epoxy nail with a novel ply stacking sequence of [02/-45/452/-45/0/-45/452/-452/452/-45/902] previously developed by the present authors. Nails were cyclically loaded in tension-tension at 5 Hz with a stress ratio of R=0.1 from 30% - 85% of the material\u27s ultimate tensile strength (UTS). Thermographic stress analysis, rather than conventional fatigue testing, was used to obtain high cycle fatigue strength (HCFS), below which the nail can be cyclically loaded indefinitely without damage. Also, the mechanical test machine\u27s built-in load cell and an extensometer were used to create stress-strain curves, from which the change in static EO and dynamic E* moduli were obtained. Results showed that HCFS was 70.3% of UTS (or about 283 MPa), while EO and E* remained at 42 GPa without any dRegradation during testing. The current nail shows potential for clinical use

Elevated Microdamage Spatially Correlates with Stress in Metastatic Vertebrae

Metastasis of cancer to the spine impacts bone quality. This study aims to characterize vertebral microdamage secondary to metastatic disease considering the pattern of damage and its relationship to stress and strain under load. Osteolytic and mixed osteolytic/osteoblastic vertebral metastases were produced in athymic rats via HeLa cervical or canine Ace-1 prostate cancer cell inoculation, respectively. After 21 days, excised motion segments (T12-L2) were µCT scanned, stained with BaSO4 and re-imaged. T13-L2 motion segments were loaded in axial compression to induce microdamage, re-stained and re-imaged. L1 (loaded) and T12 (unloaded) vertebrae were fixed, sample blocks cut, polished and BSE imaged. µFE models were generated of all L1 vertebrae with displacement boundary conditions applied based on the loaded µCT images. µCT stereological analysis, BSE analysis and µFE derived von Mises stress and principal strains were quantitatively compared (ANOVA), spatial correlations determined and patterns of microdamage assessed qualitatively. BaSO4 identified microdamage was found to be spatially correlated with regions of high stress in µFEA. Load-induced microdamage was shown to be elevated in the presence of osteolytic and mixed metastatic disease, with diffuse, crossed hatched areas of microdamage present in addition to linear microdamage and microfractures in metastatic tissue, suggesting diminished bone quality

The biomechanical effect of anteversion and modular neck offset on stress shielding for short-stem versus conventional long-stem hip implants

© 2015 IPEM. Short-stem hip implants are increasingly common since they preserve host bone stock and presumably reduce stress shielding by improving load distribution in the proximal femur. Stress shielding may lead to decreased bone density, implant loosening, and fracture. However, few biomechanical studies have examined short-stem hip implants. The purpose of this study was to compare short-stem vs. standard length stemmed implants for stress shielding effects due to anteversion-retroversion, anterior-posterior position, and modular neck offset. Twelve artificial femurs were implanted with either a short-stem modular-neck implant or a conventional length monolithic implant in 0° or 15° of anteversion. Three modular neck options were tested in the short-stem implants. Three control femurs remained intact. Femurs were mounted in adduction and subjected to axial loading. Strain gauge values were collected to validate a Finite Element (FE) model, which was used to simulate the full range of physiologically possible anteversion and anterior-posterior combinations (n = 25 combinations per implant). Calcar stress was compared between implants and across each implant\u27s range of anteversion using one and two-way ANOVA. Stress shielding was defined as the overall change in stress compared to an intact femur.The FE model compared well with experimental strains (intact: slope = 0.898, R = 0.943; short-stem: slope = 0.731, R = 0.948; standard-stem: slope = 0.743, R = 0.859); correction factors were used to adjust slopes to unity. No implant anteversion showed significant reduction in stress shielding (α = 0.05, p \u3e 0.05). Stress shielding was significantly higher in the standard-stem implant (63% change from intact femur, p \u3c 0.001) than in short-stem implants (29-39% change, p \u3c 0.001).Short-stem implants reduce stress shielding compared to standard length stemmed implants, while implant anteversion and anterior-posterior position had no effect. Therefore, short-stem implants have a greater likelihood of maintaining calcar bone strength in the long term

Biomechanical optimization of the angle and position for surgical implantation of a straight short stem hip implant

© 2016 IPEM Conservative hip implants preserve healthy bone for revision surgeries and improve physiological loading; however, they have little supporting biomechanical data with respect to their 3D orientation during implantation. This study endeavored to determine the optimal 3D orientation of a straight short stem hip implant within the proximal femur that would yield a stress distribution most similar to an intact femur. Synthetic femurs were implanted with a stem in one of seven maximum angles or positions and axially loaded, with resultant strain values used to validate a finite element model. Design of experiments was used to analyze the range of potential implant orientations under three gait cycle loading conditions. A global optimal orientation of 9.14 ° valgus, 2.49 ° anteversion, 0.48 mm posterior position, and 0.23 mm inferior position was found to yield stress distributions most similar to the intact femur across the gait cycle range. In general, it was determined that the valgus orientation was optimal throughout the gait cycle, consistently exhibiting a stress distribution more similar to that of the intact femur. Minimal levels of anterior/posterior and inferior positioning were seen to be beneficial in achieving more physiological stresses in specific regions of interest within the proximal femur, while the anteverted orientation was only beneficial in loading under flexion. Overall, orthopaedic surgeons should aim to implant straight short stem hip implants in valgus up to 10 °, with an otherwise neutral position and version, unless some degree of deviation would be beneficial for a patient-specific reason. This work has implications for the best surgical placement of straight short stem hip implants to yield maximal biomechanical stability