Comparison of pedicle screw loosening under uniaxial and multiaxial loading.

Damage evolution and modeling of sintered metals under multi-axial loading conditions

Sintered metals are characterized by the high porosity (⩾ 8%) and voids/micro-cracks in microns. Inelastic behavior of the materials is coupled with micro-crack propagation and coalescence of open voids. In the present work the damage evolution of the sintered iron under multi-axial monotonic loading conditions was investigated experimentally and computationally. The tests indicated that damage of the sintered iron initiated already at a stress level much lower than the macroscopic yield stress. The damage process can be divided into the stress-dominated elastic damage and the plastic damage described by the plastic strain. Based on the uniaxial tensile tests an elastic-plastic continuum damage model was developed which predicts both elastic damage and plastic damage in the sintered iron under general multi-axial monotonic loading conditions. Computational predictions agree with experiments with different multi-axial loading paths. A phenomenological continuum damage model for the sintered metal is developed based on the experimental observations to predict the inelastic behavior and damage process to failure under multi-axial loading conditions. The proposed damage model is experimentally verified under different loading conditions.

A prospective single center observational study. The aim of this study was to examine the potential role of sonication in the diagnosis of low-grade infections and its association with pedicle screw (PS) loosening, and to describe risk factors and radiological findings associated with spinal implant infection. Although PS loosening has mainly been attributed to mechanical overload, implant colonization and biofilm formation have recently been suggested. Culturing of sonication fluid implants is promising in the field of spine instrumentation infection, but little data are available. We prospectively included all patients who were subjected to implant removal. PS loosening was assessed with computed tomography (CT) scan. Different clinical and radiological parameters which could serve as indicators of implant infection were studied. Thirty-eight patients were included in the study and 11 of them (29%) had a positive sonication result. Patients with spinal implant infection were associated with screw loosening (P = 0.005). Particularly, those screws with a positive microbiological culture showed signs of screw loosening in the preoperative CT scan (P < 0.001). Our results also showed that radiological screw loosening at L1-L3 level, and loosened larger constructs were associated with screw microbial colonization. The most common isolated microorganisms were coagulase-negative staphylococci and Cutibacterium acnes. An implant-based multivariate analysis indicated that screw loosening, the absence of prophylactic cefazolin, ICU hospitalization, screw breakage, and L1-L3 spine level were independent risk factors for implant-associated infection. Our model exhibited a high predictive power with an area under the curve of 0.937. As clinical presentation of deep implant chronic infection is unspecific, consideration of these factors enables preoperative prediction and risk stratification of implant colonization, thus helping patient's management.Level of Evidence: 3.

Currently, the diagnosis of pedicle screw (PS) loosening is based on a subjectively assessed halo sign, that is, a radiolucent line around the implant wider than 1 mm in plain radiographs. We aimed at development and validation of a quantitative method to diagnose PS loosening on radiographs. Between 11/2004 and 1/2010 36 consecutive patients treated with thoraco-lumbar spine fusion with PS instrumentation without PS loosening were compared with 37 other patients who developed a clinically manifesting PS loosening. Three different angles were measured and compared regarding their capability to discriminate the loosened PS over the postoperative course. The inter-observer invariance was tested and a receiver operating characteristics curve analysis was performed. The angle measured between the PS axis and the cranial endplate was significantly different between the early and all later postoperative images. The Spearman correlation coefficient for the measurements of two observers at each postoperative time point ranged between 0.89 at 2 weeks to 0.94 at 2 months and 1 year postoperative. The angle change of 1.9° between immediate postoperative and 6-month postoperative was 75% sensitive and 89% specific for the identification of loosened screws (AUC = 0.82). The angle between the PS axis and the cranial endplate showed good ability to change in PS loosening. A change of this angle of at least 2° had a relatively high sensitivity and specificity to diagnose screw loosening.

BackgroundIn general, the diagnosis of pedicle screw (PS) loosening is evaluated qualitatively based on the presence of a radiolucent area around the implant wider than 1 mm on plain radiographs and computed tomography (CT). Digital tomosynthesis is a novel imaging technology that can acquire reconstructed tomographic images of patients in different postures with relatively low radiation. In this study, PS loosening is evaluated quantitatively by measuring the PS displacement angle in the vertebrae using digital tomosynthesis.MethodsWe evaluated 41 patients who underwent posterior spinal fusion surgery using PS. The 72 pedicle screws at the cranial end of the fused segments were evaluated. The patients were divided in two groups, one with PS loosening (7 patients, 12 screws) and the other without PS loosening (34 patients, 60 screws), based on conventional CT findings. All patients underwent tomosynthesis in two different postures during a single CT session.ResultsThe displacement angles of the PS in patients in a lying position and in a standing position were measured using selected slices of the same cross-sectional view from digital tomosynthesis. The displacement angle was significantly greater in the PS loosening group (5.7°) than in the group without PS loosening (0.6°) (p<0.01). Based on the ROC analysis, the optimal cut-off value of the PS displacement angle for identification of loosened screws was 1.7° with a sensitivity of 100% and specificity of 93% (AUC = 0.98).ConclusionsThis new method using digital tomosynthesis has the potential to aid diagnosis of PS loosening quantitatively and more accurately than conventional evaluations.

NiTi shape memory alloys (SMAs) are widely used in astronautics and bio-medical fields due to their shape memory property, superelasticity and excellent biological compatibility. In such applications, the NiTi SMA devices are often subjected to a kind of strain- or stress-controlled cyclic loading, leading to the fatigue failure of NiTi SMAs is a key issue that should be investigated. Meanwhile, with the ambient temperature higher than austenite transformation finish temperature (Af), the NiTi SMAs can transform into martensite phase during loading process and recover their austenite phase during unloading, the unique martensite transformation and its reverse of NiTi SMAs indicates their fatigue failure characters much different from ordinary metals. Thus, besides cyclic-loading experiments, the micro-structure observations on the morphology of residual martensite on fracture surfaces is also an important component of investigating the damage mechanisms of NiTi SMAs, which should be taken into considered as well. This paper aims at the micro-structure observations on the fractured NiTi micro-tubes after both uniaxial and multiaxial stress-controlled fatigue loadings, in which the morphology of residual martensite affected by different stress levels and loading paths are compared and analyzed. The results show that the bigger-sized residual martensite phase is inclined to appear in the loading cases with higher stress level or multiaxial paths, corresponding to the higher residue deformations in the fatigue-loading tests. Meanwhile, in multiaxial loadings, due to the effect of gradually increased shear stress along the radial direction, it leads to the size of residual martensite near the outer surface of the NiTi micro-tubes much bigger than that near the inner surface, whereas in uniaxial loadings, the distribution of martensite phase are nearly uniform on the cross section of NiTi specimens. It can be also found that the locations near the internal defects tend to produce bigger sized martensite phase as well, which indicates the stress-concentration effect induced therein. Moreover, the morphology of mutually hindered martensite phase in different directions can be also observed in the fractured specimens, it implies the propagation of the micro-cracks which caused by the transformation induced plasticity between the interface of austenite and martensite will be further prevented, leading to the relatively slower crack propagation rate of NiTi SMAs than ordinary metals. The findings can provide experimental support to investigating the damage mechanisms of NiTi SMAs in microscopic field, and it is also the foundation of establishing the reasonable damage evolution model and life prediction model of NiTi SMAs, describing their distinctive fatigue characters in both uniaxial and multiaxial loading conditions.

Hounsfield unit of screw trajectory as a predictor of pedicle screw loosening after single level lumbar interbody fusion

Multiaxial physiological loading generates complex fluid flow and strain patterns in the osteocyte lacunar-canalicular system.

This was a retrospective observational study of a cohort of consecutive patients. To compare the influence of (1) manual and (2) computer-assisted (CA) rod bending techniques on pedicle screw pull-out resulting in loosening during rod reduction and final tightening. Our recent study showed screw pull-out during rod reduction might be a risk factor for loosening of percutaneous pedicle screws (PPSs) postoperatively, resulting in worse postoperative back pain. We retrospectively analyzed data from 53 consecutive patients who underwent minimally invasive lumbar or thoracic spinal stabilization using intraoperative computed tomography image (CT)-guided navigation with conventional manual rod bending or CA rod bending and were followed up for 1 year. Screw pull-out length was measured on axial CT images obtained immediately after screw insertion and postoperatively. Loosening of screws and clinical outcome were evaluated radiographically, clinically, and by CT 1 year after surgery. The frequencies of screw pull-out and screw loosening between the 2 rod bending techniques were compared. Lumbar pain-related factors for both groups of patients were determined preoperatively and 1 year after surgery. Overall, 360 pedicle screws were placed in the manual group and 124 pedicle screws were placed in the CA group. There was no significant difference in the mean age, sex, bone mineral density, mean stabilized length, or smoking habits of patients between the groups. The diameters, lengths, and trajectory angle (axial and sagittal) of the screws placed were not significantly different between the groups. Screw pull-out rate/length and loosening in the CA group was significantly lower than that in the manual group. Postoperative low back pain improved significantly in the CA group compared with that in the manual group. CA bending is useful to avoid PPS pull-out during rod reduction and screw loosening postoperatively. CA bending is useful to avoid PPS pull-out during rod reduction and screw loosening postoperatively. This result might have been a factor leading to reduced postoperative back pain in the CA group, however, that future studies are need to investigate this association.

The tensile performance of coated woven fabrics under multiaxial loads is examined in the present study. Two groups of experiments were conducted to investigate the influences of the configuration of the fabric specimen and the loading speed on the multiaxial tensile properties of the fabrics. The configuration of the specimen for the multiaxial tensile tests is identified as gear‐shape with large arm widths. A loading speed of lower than 20 mm/min is suggested to obtain the tensile properties of the coated woven fabrics under multiaxial loads. The tensile performances of coated woven fabrics under uni‐, bi‐, and multiaxial loads were compared. We found that the tensile performances under bi‐ and multiaxial loads are much better than those under uniaxial loads. Therefore, for the application of the coated woven fabrics in lightweight structures, biaxial or multiaxial loading conditions will be necessary. Experiments on the specimens with an initial crack in the center under multiaxial loads show that, by comparison with other loading directions, the tensile properties in warp direction of the coated woven fabrics play an important role in the failure performance and crack propagation under multiaxial loads. To eliminate the dependence on the mechanical properties in warp direction, the balance of the two principle directions of coated woven fabrics should be improved. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

A new fatigue life prediction method using the energy‐based approach under uniaxial and multiaxial random loadings is proposed. The uniqueness of the proposed model is based on a time‐derivative damage accumulation unlike classical cycle‐based damage accumulation models. Thus, damage under arbitrary random loading can be directly obtained using time‐domain integration without cycle counting. First, a brief review of existing models is given focusing on their applicability to uniaxial/multiaxial, constant/random, and high cycle fatigue/low cycle fatigue loading regimes. Next, formulation of time‐derivative damage model is discussed in detail under uniaxial random loadings. Then, an equivalent energy concept for general multiaxial loading conditions is used to convert the random multiaxial loading to an equivalent random uniaxial loading, where the time‐derivative damage model can be used. Finally, the proposed model is validated with extensive experimental data from open literature and in‐house testing under various constant and random spectrum loadings.

The dislocation-based fatigue deformation mechanism of a RAFM steel under multi-axial loadings

A numerical investigation on multiaxial fatigue assessment of Nitinol peripheral endovascular devices with emphasis on load non-proportionality effects

In this study, we demonstrate that the strength of a short‐fiber reinforced polymer as measured in uniaxial tension is not a good indicator for its strength under multiaxial loading conditions. To illustrate this fact, the influence of physical aging on the strength of a 20 wt% short‐fiber reinforced polycarbonate is studied for a uniaxial and biaxial loading condition. Results demonstrate that aging strongly reduces the strength in multiaxial loading, whereas, it increases in uniaxial loading. To rationalize these observations, a micromechanical analysis of the local stress state is performed using three‐dimensional (3D) representative volume elements (RVE) in combination with a constitutive model that adequately describes the intrinsic deformation response of the matrix. RVE simulations demonstrate that biaxial loading results in higher local hydrostatic stresses and triaxiality than the uniaxial loadcase for the composite system considered, and aging results in a shift towards higher values. The high triaxiality, the magnitude of the hydrostatic stress, and the shift in hydrostatic stress upon aging, presents a clear rationalization why physical aging induces a strength reduction in biaxial loading and not in uniaxial loading. These results highlight that care should be taken when predicting strength under complex loading conditions with only uniaxial data available.

This paper contains an extension of the uniaxial state variable constitutive model of Ramaswamy et al. (1988) to the case of multiaxial loading. The correlation between uniaxial and multiaxial loading conditions is achieved through the assumptions of material isotropy and conservation of inelastic volume. The multiaxial extension is based only on the material parameters evaluated from uniaxial loading. The research is accompanied by a multiaxial experimental program to evaluate the response of Rene’ 80 at 871°C and 982° C. Experiments in the program include torsion, proportional axial and torsion, and nonproportional loading. It was shown experimentally that there is no extra hardening from the multiaxial loading than results from uniaxial loading. Further, it is shown that the multiaxial model is successful in predicting the experimental results using only the parameters determined from the uniaxial experiments.