Modeling attachment and compressive loading of locking and non-locking plate fixation: a finite element investigation of a supracondylar femur fracture model.

This study developed a finite element (FE) model of simulated locking plate fixation to examine the strain response following supracondylar femoral plate attachment and under compressive loading. An implicit FE model of a synthetic femur with a distal fracture gap stabilized with a lateral plate was evaluated following attachment and 500 N loading, considering locking and non-locking proximal screws configurations. Screw pre-tension values of 60 N for both distal and proximal non-locking screws yielded good agreement with plate experimental strain data in attached (unloaded) and loaded conditions. The results highlight the importance of pre-tensioning in modeling plate attachment using non-locking screws.

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

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's 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's 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.

Rotating hinge knee causes lower bone-implant interface stress compared to constrained condylar knee replacement.

PURPOSE: To compare the stresses at bone-arthroplasty interface of constrained and semi-constrained knee prostheses, using the finite element (FE) method as a predictor of the survivorship of the implants. METHODS: Three-dimensional FE models of the knee implanted with rotating hinge (RHK) and legacy constrained condylar (LCCK) prostheses were generated to study the loads and stresses for two situations: medial- and lateral collateral ligament deficiencies in full extension. RESULTS: On average, the shear stress developed at bone-implant interface dropped from 16.9 to 13.7 MPa (18.9%), and the interface von Mises stress lowered from 37.6 to 30.2 MPa (19.6%) in RHK compared to those in LCCK prostheses. RHK design also resulted in a more uniform stress distribution at the interfaces in both femur and tibia. The average polyethylene liner stress dropped from 9.6 to 2.6 MPa (a 72.7% decrease) in RHK design when compared to that in LCCK design. CONCLUSION: The more uniform interface stress suggests fewer density changes at the periprosthetic regions due to bone remodelling. Moreover, the lower polyethylene stresses are likely to reduce wear and damage. These findings reveal that the RHK design may have more favorable mechanical features compared to LCCK design in full extension boundary conditions, implying a potentially better survivorship. However, the findings should be interpreted cautiously as other configurations were not investigated.

The impact of surgical manipulation on lower lateral cartilage stiffness.

BACKGROUND: Cephalic trimming of the alar (or lower lateral) cartilage may cause weakening leading to external nasal valve collapse. Numerous methods have been proposed to combat this weakening in order to maintain lateral crural stiffness. The purpose of this study was to quantify the effect of mucosal stripping, cephalic trimming, cephalic turn-in flap, and lateral crural strut grafting on lateral crural stiffness. METHODS: In situ cyclic compressive loading was performed on eight lateral crura in 4 fresh frozen cadaveric specimens. Testing was performed on the unaltered degloved cartilage (intact) and following each of the following interventions: mucosal stripping, cephalic turn-in flap, cephalic trimming, and lateral crural strut grafting. Linear regression of the generated force-displacement curves was used to calculate stiffness. Each intervention was compared to the intact cartilage. RESULTS: Alar cartilage of all of the specimens demonstrated a linear response to compressive loading. Intact cartilage had a mean stiffness of 3.53 N/mm. Mucosal stripping and cephalic turn-in flaps yielded similar stiffness values to intact cartilage. Cephalic trimming reduced stiffness in all cases by a mean of 1.09 N/mm (p = 0.003). Lateral crural strut grafting significantly increased stiffness by a mean of 3.67 N/mm (p = 0.0001). CONCLUSIONS: Cephalic trimming leads to decreased lateral crural stiffness in cadaveric specimens. Cephalic turn-in flaps restore pre-trimmed stiffness, and lateral crural strut grafting increases overall stiffness of the cartilage. These findings should be considered in patients undergoing rhinoplasty, particularly if there are concerns regarding potential external valve collapse.

Biomechanical analysis using FEA and experiments of a standard plate method versus three cable methods for fixing acetabular fractures with simultaneous THA.

Acetabular fractures potentially account for up to half of all pelvic fractures, while pelvic fractures potentially account for over one-tenth of all human bone fractures. This is the first biomechanical study to assess acetabular fracture fixation using plates versus cables in the presence of a total hip arthroplasty, as done for the elderly. In Phase 1, finite element (FE) models compared a standard plate method versus 3 cable methods for repairing an acetabular fracture (type: anterior column plus posterior hemi-transverse) subjected to a physiological-type compressive load of 2207N representing 3 x body weight for a 75kg person during walking. FE stress maps were compared to choose the most mechanically stable cable method, i.e. lowest peak bone stress. In Phase 2, mechanical tests were then done in artificial hemipelvises to compare the standard plate method versus the optimal cable method selected from Phase 1. FE analysis results showed peak bone stresses of 255MPa (Plate method), 205MPa (Mears cable method), 250MPa (Kang cable method), and 181MPa (Mouhsine cable method). Mechanical tests then showed that the Plate method versus the Mouhsine cable method selected from Phase 1 had higher stiffness (662versus 385N/mm, p=0.001), strength (3210versus 2060N, p=0.009), and failure energy (8.8versus 6.2J, p=0.002), whilst they were statistically equivalent for interfragmentary sliding (p≥0.179) and interfragmentary gapping (p≥0.08). The Plate method had superior mechanical properties, but the Mouhsine cable method may be a reasonable alternative if osteoporosis prevents good screw thread interdigitation during plating.

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

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, 0.48mm posterior position, and 0.23mm 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.

Long-term response of femoral density to hip implant and bone fracture plate: Computational study using a mechano-biochemical model.

Although bone fracture plates can provide appropriate stability at the fracture site and lead to early patient mobilization, they significantly change the loading pattern in the bone after union (Stress shielding). This phenomenon results in a bone density decrease, which may cause premature failure of the implant. This paper presents the first study that quantifies the long-term response of femoral density to hip implantation and plating (lateral and anterior plating) using a mechano-biochemical model which considers the coupling effect between mechanical loading and biochemical affinities as stimuli for bone remodeling. The results showed that the regions directly beneath the plate experienced severe bone loss (i.e. up to ∼ -70%). However, some level of bone formation was observed in the vicinity of the most proximal and distal screw holes in both lateral and anterior plated femurs (i.e. up to ∼ +110%). The bone under the plate was divided into six zones. With respect to bone remodeling response, the findings revealed that anterior plating was not superior to lateral plating since the maximum and average bone losses among the zones in the anterior plated femur (i.e. -36% and -24%, respectively) were approximately the same as their corresponding values in the lateral plated femur (i.e. -38% and -24%, respectively).

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

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's 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 > 0.05). Stress shielding was significantly higher in the standard-stem implant (63% change from intact femur, p < 0.001) than in short-stem implants (29-39% change, p < 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 properties of a structurally optimized carbon-fibre/epoxy intramedullary nail for femoral shaft fracture fixation.

Intramedullary nails are the golden treatment option for diaphyseal fractures. However, their high stiffness can shield the surrounding bone from the natural physiologic load resulting in subsequent bone loss. Their stiff structure can also delay union by reducing compressive loads at the fracture site, thereby inhibiting secondary bone healing. Composite intramedullary nails have recently been introduced to address these drawbacks. The purpose of this study is to evaluate the mechanical properties of a previously developed composite IM nail made of carbon-fibre/epoxy whose structure was optimized based on fracture healing requirements using the selective stress shielding approach. Following manufacturing, the cross-section of the composite nail was examined under an optical microscope to find the porosity of the structure. Mechanical properties of the proposed composite intramedullary nail were determined using standard tension, compression, bending, and torsion tests. The failed specimens were then examined to obtain the modes of failure. The material showed high strength in tension (403.9±7.8MPa), compression (316.9±10.9MPa), bending (405.3±8.1MPa), and torsion (328.5±7.3MPa). Comparing the flexural modulus (41.1±0.9GPa) with the compressive modulus (10.0±0.2GPa) yielded that the material was significantly more flexible in compression than in bending. This customized flexibility along with the high torsional stiffness of the nail (70.7±2.0Nm(2)) has made it ideal as a fracture fixation device since this unique structure can stabilize the fracture while allowing for compression of fracture ends. Negligible moisture absorption (~0.5%) and low porosity of the laminate structure (< 3%) are other advantages of the proposed structure. The findings suggested that the carbon-fibre/epoxy intramedullary nail is flexible axially while being relatively rigid in bending and torsion and is strong enough in all types of physiologic loading, making it a potential candidate for use as an alternative to the conventional titanium-alloy intramedullary nails.

An Effective Approach for Optimization of a Composite Intramedullary Nail for Treating Femoral Shaft Fractures.

The high stiffness of conventional intramedullary (IM) nails may result in stress shielding and subsequent bone loss following healing in long bone fractures. It can also delay union by reducing compressive loads at the fracture site, thereby inhibiting secondary bone healing. This paper introduces a new approach for the optimization of a fiber-reinforced composite nail made of carbon fiber (CF)/epoxy based on a combination of the classical laminate theory, beam theory, finite-element (FE) method, and bone remodeling model using irreversible thermodynamics. The optimization began by altering the composite stacking sequence and thickness to minimize axial stiffness, while maximizing torsional stiffness for a given range of bending stiffnesses. The selected candidates for the seven intervals of bending stiffness were then examined in an experimentally validated FE model to evaluate their mechanical performance in transverse and oblique femoral shaft fractures. It was found that the composite nail having an axial stiffness of 3.70 MN and bending and torsional stiffnesses of 70.3 and 70.9 N⋅m², respectively, showed an overall superiority compared to the other configurations. It increased compression at the fracture site by 344.9 N (31%) on average, while maintaining fracture stability through an average increase of only 0.6 mm (49%) in fracture shear movement in transverse and oblique fractures when compared to a conventional titanium-alloy nail. The long-term results obtained from the bone remodeling model suggest that the proposed composite IM nail reduces bone loss in the femoral shaft from 7.9% to 3.5% when compared to a conventional titanium-alloy nail. This study proposes a number of practical guidelines for the design of composite IM nails.

QCT-based failure analysis of proximal femurs under various loading orientations.

In this paper, the variations of the failure strength and pattern of human proximal femur with loading orientation were analysed using a novel quantitative computed tomography (QCT)-based linear finite element (FE) method. The QCT images of 4 fresh-frozen femurs were directly converted into voxel-based finite element models for the analyses of the failure loads and patterns. A new geometrical reference system was used for the alignment of the mechanical loads on the femoral head. A new method was used for recognition and assortment of the high-risk elements using a strain energy-based measure. The FE results were validated with the experimental results of the same specimens and the results of similar case studies reported in the literature. The validated models were used for the computational investigation of the failure loads and patterns under 15 different loading conditions. A consistent variation of the failure loads and patterns was found for the 60 different analysed cases. Finally, it was shown that the proposed procedure can be used as a reliable tool for the failure analysis of proximal femurs, e.g. identification of the relevant loading directions for specific failure patterns, or determination of the loading conditions under which the proximal femurs are failure-prone.

Investigating stress shielding spanned by biomimetic polymer-composite vs. metallic hip stem: A computational study using mechano-biochemical model.

Periprosthetic bone loss in response to total hip arthroplasty is a serious complication compromising patient's life quality as it may cause the premature failure of the implant. Stress shielding as a result of an uneven load sharing between the hip implant and the bone is a key factor leading to bone density decrease. A number of composite hip implants have been designed so far to improve load sharing characteristics. However, they have rarely been investigated from the bone remodeling point of view to predict a long-term response. This is the first study that employed a mechano-biochemical model, which considers the coupling effect between mechanical loading and bone biochemistry, to investigate bone remodeling after composite hip implantation. In this study, periprosthetic bone remodeling in the presence of Carbon fiber polyamide 12 (CF/PA12), CoCrMo and Ti alloy implants was predicted and compared. Our findings revealed that the most significant periprosthetic bone loss in response to metallic implants occurs in Gruen zone 7 (-43% with CoCrMo; -35% with Ti) and 6 (-40% with CoCrMo; -29% with Ti), while zone 4 has the lowest bone density decrease with all three implants (-9%). Also, the results showed that in terms of bone remodeling, the composite hip implant is more advantageous over the metallic ones as it provides a more uniform density change across the bone and induces less stress shielding which consequently results in a lower post-operative bone loss (-9% with CF/PA12 implant compared to -27% and -21% with CoCrMo and Ti alloy implants, respectively).

On optimization of a composite bone plate using the selective stress shielding approach.

Bone fracture plates are used to stabilize fractures while allowing for adequate compressive force on the fracture ends. Yet the high stiffness of conventional bone plates significantly reduces compression at the fracture site, and can lead to subsequent bone loss upon healing. Fibre-reinforced composite bone plates have been introduced to address this drawback. However, no studies have optimized their configurations to fulfill the requirements of proper healing. In the present study, classical laminate theory and the finite element method were employed for optimization of a composite bone plate. A hybrid composite made of carbon fibre/epoxy with a flax/epoxy core, which was introduced previously, was optimized by varying the laminate stacking sequence and the contribution of each material, in order to minimize the axial stiffness and maximize the torsional stiffness for a given range of bending stiffness. The initial 14×4(14) possible configurations were reduced to 13 after applying various design criteria. A comprehensive finite element model, validated against a previous experimental study, was used to evaluate the mechanical performance of each composite configuration in terms of its fracture stability, load sharing, and strength in transverse and oblique Vancouver B1 fracture configurations at immediately post-operative, post-operative, and healed bone stages. It was found that a carbon fibre/epoxy plate with an axial stiffness of 4.6 MN, and bending and torsional stiffness of 13 and 14 N·m(2), respectively, showed an overall superiority compared with other laminate configurations. It increased the compressive force at the fracture site up to 14% when compared to a conventional metallic plate, and maintained fracture stability by ensuring the fracture fragments' relative motions were comparable to those found during metallic plate fixation. The healed stage results revealed that implantation of the titanium plate caused a 40.3% reduction in bone stiffness, while the composite plate lowered the stiffness by 32.9% as compared to the intact femur. This study proposed a number of guidelines for the design of composite bone plates. The findings suggest that a composite bone plate could be customized to allow for moderate compressive force on the fracture ends, while remaining relatively rigid in bending and torsion and strong enough to withstand external loads when a fracture gap is present. The results indicate that the proposed composite bone plate could be a potential candidate for bone fracture plate applications.

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

BACKGROUND: Intramedullary 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. METHODS: In 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. RESULTS: Post-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. INTERPRETATION: Our 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.

Effect of patellar thickness on knee flexion in total knee arthroplasty: a biomechanical and experimental study.

A biomechanical computer-based model was developed to simulate the influence of patellar thickness on passive knee flexion after arthroplasty. Using the computer model of a single-radius, PCL-sacrificing knee prosthesis, a range of patella-implant composite thicknesses was simulated. The biomechanical model was then replicated using two cadaveric knees. A patellar-thickness range of 15 mm was applied to each of the knees. Knee flexion was found to decrease exponentially with increased patellar thickness in both the biomechanical and experimental studies. Importantly, this flexion loss followed an exponential pattern with higher patellar thicknesses in both studies. In order to avoid adverse biomechanical and functional consequences, it is recommended to restore patellar thickness to that of the native knee during total knee arthroplasty.

Displacement of the hip center of rotation after arthroplasty of Crowe III and IV dysplasia: a radiological and biomechanical study.

To study the direction and biomechanical consequences of hip center of rotation (HCOR) migration in Crowe type III and VI hips after total hip arthroplasty, post-operative radiographs and CT scans of several unilaterally affected hips were evaluated. Using a three-dimensional model of the human hip, the HCOR was moved in all directions, and joint reaction force (JRF) and abductor muscle force (AMF) were calculated for single-leg stance configuration. Comparing to the normal side, HCOR had displaced medially and inferiorly by an average of 23.4% and 20.8%, respectively, of the normal femoral head diameter. Significant decreases in JRF (13%) and AMF (46.13%) were observed in a presumptive case with that amount of displacement. Isolated inferior displacement had a small, increasing effect on these forces. In Crowe type III and IV hips, the HCOR migrates inferiorly and medially after THA, resulting in a decrease in JRF, AMF, and abductor muscle contraction force.