Postoperative Fluid Accumulation is Associated With Underestimation of AKI Severity in Lung Transplant Recipients.

BACKGROUND: Post-lung transplantation (LTx) fluid accumulation can lead to dilution of serum creatinine (SCr). We hypothesized that fluid accumulation might impact the diagnosis, staging, and outcome of posttransplant acute kidney injury (AKI). METHODS: In this retrospective study, we analyzed data from 131 adult LTx patients at a single German lung center between 2005 and 2018. We assessed the occurrence of AKI within 7 days posttransplant, both before and after SCr-adjustment for fluid balance (FB), and investigated its impact on all-cause mortality. Transient and persistent AKIs were defined as return to baseline kidney function or continuation of AKI beyond 72 h of onset, respectively. RESULTS: AKI was diagnosed in 58.8% of patients according to crude SCr values. When considering FB-adjusted SCr values, AKI severity was underestimated in 20.6% of patients, that is, AKI was detected in an additional 6.9% of patients and led to AKI upstaging in 23.4% of cases. Patients initially underestimated but detected with AKI only after FB adjustment had higher mortality compared to those who did not meet AKI criteria (hazard ratio [HR] 2.98; 95% confidence interval [CI] 1.06, 8.36; p = 0.038). Persistent AKI was associated with higher mortality than transient AKI, regardless of using crude or adjusted SCr values (p < 0.05). Persistent AKI emerged as an independent risk factor for mortality (HR 2.35; 95% CI 1.29, 4.30; p = 0.005). CONCLUSION: Adjusting for FB and evaluating renal recovery patterns post-AKI may enhance the sensitivity of AKI detection. This approach could help identify patients with poor prognosis and potentially improve outcomes in lung transplant recipients. TRIAL REGISTRATION: ClinicalTrials.gov identifier: NCT03039959, NCT03046277.

Load distribution between cephalic screws in a dual lag screw trochanteric nail.

BACKGROUND: It has been observed clinically that the Z-effect is a potential cause of failure of an intramedullary nail with two cephalic screws. It describes the migration behavior of the cephalic screws in the femoral head. The primary objective was to examine different cephalic screw configurations and test the load distribution between them as a function of their relative placement and their relative movement in the nail. It has been hypothesized that different cephalic screw positions may have an influence on the stress in the implant and bone and therefore on implant failures, such as the Z-effect. METHODS: To quantify the load distribution of a dual cephalic screw intramedullary femoral nail (Citieffe, Calderara di Reno, BO, Italy), a finite element model of the femur, focusing on the loading of the cephalic screws, was prepared. Four different screw lengths (90-105 mm) were examined. The investigation considered the stresses and strains in the bone and implant as well as the relative movement of the screws. RESULTS: If the inferior cephalic screw had a shorter length, then the superior one and the femoral nail had to bear higher loads. In that case, the "equivalent von Mises stress" increased up to 10 % at the superior cephalic screw and up to 5 % at the femoral nail. The analysis of the relative movement showed that sliding of the inferior cephalic screw occurred in the nail. The total movement ranged from 0.47 to 0.73 mm for the different screw configurations. CONCLUSIONS: The stresses were distributed more equally between the two cephalic screws in the bone and the implant if a longer inferior screw was used. The stresses in the bone and implant were reduced with a longer inferior cephalic screw. Therefore, a configuration using a longer inferior cephalic screw is preferable for trochanteric fracture fixation with a dual cephalic screw intramedullary device.

A new approach to prevent contralateral hip fracture: Evaluation of the effectiveness of a fracture preventing implant.

BACKGROUND: Among the millions of people suffering from a hip fracture each year, 20% may sustain a contralateral hip fracture within 5 years with an associated mortality risk increase reaching 64% in the 5 following years. In this context, we performed a biomechanical study to assess the performance of a hip fracture preventing implant. METHODS: The implant consists of two interlocking peek rods unified with surgical cement. Numerical and biomechanical tests were performed to simulate single stance load or lateral fall. Seven pairs of femurs were selected from elderly subjects suffering from osteoporosis or osteopenia, and tested ex-vivo after implantation of the device on one side. FINDINGS: The best position for the implant was identified by numerical simulations. The loadings until failure showed that the insertion of the implant increased significantly (P<0.05) both fracture load (+18%) and energy to fracture (+32%) of the implanted femurs in comparison with the intraindividual controls. The instrumented femur resisted the implementation of the non-instrumented femur fracture load for 30 cycles and kept its performance at the end of the cyclic loading. INTERPRETATION: Implantation of the fracture preventing device improved both fracture load and energy to fracture when compared with intraindividual controls. This is consistent with previous biomechanical side-impact testing on pairs of femur using the same methodology. Implant insertion seems to be relevant to support multiple falls and thus, to prevent a second hip fracture in elderly patients.

Individual density-elasticity relationships improve accuracy of subject-specific finite element models of human femurs.

In a previous study on subject-specific finite-element-models, we found that appropriate density-elasticity relationships to compute the mechanical behavior of femurs seem to be subject-specific. The purpose of this study was to test the hypothesis that the predictive error of a cohort of subject-specific finite element-models is lower with subject-specific density-elasticity relationships than with a cohort-specific density-elasticity relationship. Finite-element-analysis and inverse optimization based on response surface methodology were employed to test the hypothesis. Subject-specific FE-models of 17 human femurs and corresponding experimental data from biomechanical tests were taken from a previous study. A power function for the relation between radiological bone density and elastic modulus was set up with the optimization variables a and b: E(MPa)=aρqCT(b)(gK2HPO4/cm(3)). The goal of the optimization was to minimize the root-mean-square error in percent (RMSE%) between computational and experimental results. A Wilcoxon test (p=0.05) was performed on all absolute relative errors between the two groups (subject-specific functions vs. cohort-specific function). The subject-specific functions resulted in a 6% lower overall prediction error and a 6% lower RMSE% than the cohort-specific function (p<0.001). The determined subject-specific relations were mostly linear, with variable a ranging from 9307 to 15673 and variable b ranging from 0.68 to 1.40. For the cohort-specific relation, the following power law was obtained: E(MPa)=12486ρqCT(1.16)(gK2HPO4/cm(3)). We conclude that individual density-elasticity relationships improve the accuracy of subject-specific finite element models. Future subject-specific finite-element-analyses of bones should include the individuality of the elastic properties by a stochastic density-elasticity relationship with mean and standard deviation of a and b.

Biomechanical evaluation of subtalar fusion: the influence of screw configuration and placement.

Common surgical procedures for subtalar fusion include joint resection, autologous bone grafting, and osteosynthesis with screws in a parallel screw configuration. Although fusion is a routine procedure, the reported rates of nonunion have been high. The present study assessed different screw configurations in terms of their rotational and bending stability in an artificial bone model and cadaver bone. Arthrodesis was always performed with 2 screws. Three different screw configurations were tested: parallel, counter-parallel, and a delta configuration. Two different screw designs were used: a cannulated, partially threaded screw (6.5-mm and 8.0-mm diameter) and a solid screw with a different thread design. Eight experimental groups were investigated as pilot studies in artificial bones and then 3 groups in cadaver bones. The parameters were the primary stiffness and deflection of the construct for loads simulating the internal-external rotation and supination-pronation. Delta positioning of the screws resulted in the greatest biomechanical stiffness and the lowest degrees of deflection of the arthrodesis in the artificial bones and cadaver bones. Increasing the screw diameter from 6.5 to 8.0 mm resulted in no additional stability of the arthrodesis in the artificial bones. The results of the present study have indicated that the delta configuration for arthrodesis results in the greatest construct stiffness and lower relative deflection between the talus and calcaneus in the positions tested.

An investigation to determine if a single validated density-elasticity relationship can be used for subject specific finite element analyses of human long bones.

Subject-specific FE-models of human long bones have to predict mechanical parameters with sufficient accuracy to be applicable in a clinical setting. One of the main aspects in subject-specific FE-models of bones regarding accuracy is the modeling of the material inhomogeneity. The goal of this study was therefore to develop FE-models of human femurs and investigate if a single validated density-elasticity relationship can be used for subject specific finite element analyses of human long bones, when the task is to predict the bone's mechanical response to load. To this aim, 23 human cadaver femurs were tested in axial compression with a load of 1000 N. Strains, local displacements, and axial bone stiffness were determined. Subject-specific FE-models were developed for each bone based on quantitative CT-scans. Three different density-elasticity relationships were retrieved from the literature, and were implemented in the FE-models. The predicted mechanical values depended largely on the used equation. The most reasonable equation showed a mean error of -11% in strain prediction, a mean error of -23% in local displacement prediction, and a mean error of +23% in axial stiffness prediction. The scatter of the predictions was very low in all three categories of measurements with a 1.96 standard deviation of about 30% to the mean errors. In conclusion, a framework for subject-specific FE-models was developed that was able to predict surface strains and bone deformation with good accuracy by using a single density-elasticity relationship. However, it was also found that the most appropriate density-elasticity relationship was specimen-specific.

Auxiliary locking plate improves fracture stability and healing in intertrochanteric fractures fixated by intramedullary nail.

BACKGROUND: Intertrochanteric fractures present a significant management challenge due to their low inherent stability. The objective of this study was to determine whether an auxiliary locking plate decreases interfragmentary motions and improves fracture healing in intertrochanteric fractures treated by intramedullary nail. METHODS: Biomechanical tests and a clinical retrospective study in intertrochanteric to subtrochanteric nonunions were performed. Six synthetic femurs were osteotomized intertrochanterically and fixated with a long gamma nail and an additional locking compression plate. Mechanical tests were conducted that simulated the hip joint force during gait cycle. Following the initial test, the locking compression plate (LCP) was removed from each specimen and the test was repeated. Interfragmentary motions, strains on implants and osteosynthesis stiffness were determined. For the clinical part of the study, 13 intertrochanteric to subtrochanteric nonunions were treated with revisional long gamma nail and additional locking compression plate. Complications and time to union were determined. FINDINGS: Biomechanically, interfragmentary rotation was 48% smaller (P=0.047) and interfragmentary shear movement was 42% smaller (P=0.007) with locking compression plate. Strains on the nail decreased by 20-27% (P<0.027) and the osteosynthesis stiffness increased by 23% (P=0.005) with locking compression plate. Clinically, fracture healing was achieved in eleven out of 13 patients after 9.0months (range 4 to 22months). INTERPRETATION: The findings of our study indicate that auxiliary locked plating considerably improves biomechanical performance and results in successful healing of unstable intertrochanteric to subtrochanteric femur fractures.

Evaluation of risk for secondary fracture after removal of a new femoral neck plate for intracapsular hip fractures.

OBJECTIVE: To determine whether a new femoral neck plate has a higher risk for secondary fracture after implant removal than the current standard treatment for intracapsular hip fractures. METHODS: Six pairs of human cadaver femora (age, 56 ± 5.6 years; range, 48-64 years; two female and four male donors) were instrumented with the femoral neck plate (FNP) or the compression hip screw combined with an antirotation screw (CHS) in a paired study design. After removal of the implants, axial compression tests to failure of the bones were conducted. Maximum force to failure of the bones after implant removal was determined. Axial stiffness of the bones before surgery and after implant removal was determined. RESULTS: The FNP resulted in a mean failure load of 4687 ± 1743 N (mean ± standard deviation) and the CHS resulted in a mean failure load of 4892 ± 1608 N with no significant difference between the two implant groups (P = 0.405). There was no significant difference in stiffness (P = 0.214) between the FNP (1240 ± 362 N/mm) and the CHS (1293 ± 304 N/mm). The cavities left by the surgery had no effect on the bone stiffness (P > 0.05). The mean failure load of all specimens correlated with the bone mineral density in the proximal part of the femurs by R² = 0.715 (P = 0.001). CONCLUSION: The FNP demonstrated a similar failure load after implant removal compared with the CHS, although the FNP left a 39% larger cavity in the bone.

Patient-specific finite element analysis of the human femur--a double-blinded biomechanical validation.

Patient-specific finite element (PSFE) models based on quantitative computer tomography (qCT) are generally used to "predict" the biomechanical response of human bones with the future goal to be applied in clinical decision-making. However, clinical applications require a well validated tool that is free of numerical errors and furthermore match closely experimental findings. In previous studies, not all measurable data (strains and displacements) were considered for validation. Furthermore, the same research group performed both the experiments and PSFE analyses; thus, the validation may have been biased. The aim of the present study was therefore to validate PSFE models with biomechanical experiments, and to address the above-mentioned issues of measurable data and validation bias. A PSFE model (p-method) of each cadaver femur (n = 12) was generated based on qCT scans of the specimens. The models were validated by biomechanical in-vitro experiments, which determined strains and local displacements on the bone surface and the axial stiffness of the specimens. The validation was performed in a double-blinded manner by two different research institutes to avoid any bias. Inspecting all measurements (155 values), the numerical results correlated well with the experimental results (R(2) = 0.93, slope 1.0093, mean of absolute deviations 22%). In conclusion, a method to generate PSFE models from qCT scans was used in this study on a sample size not yet considered in the past, and compared to experiments in a douple-blinded manner. The results demonstrate that the presented method is in an advanced stage, and can be used in clinical computer-aided decision-making.

Should extramedullary fixations for hip fractures be removed after bone union?

BACKGROUND: Osteosynthesis implants, which remain in the patient after fracture union to save additional surgery, may affect the strain distribution within the bone. A reduction of strain within the bone is known to result in localized bone loss ("stress shielding") and increased fracture risk. The purpose of this study was to examine whether extramedullary fixations for femoral neck fractures have to be removed after fracture union to prevent reductions in cortex strains. METHODS: In a biomechanical experiment, six pairs of human cadaver femora (mean age 56 years, range 48 to 64) were supplied with five strain gauges per bone. The bones were equally supplied with a compression hip screw or a femoral neck plate. Before surgery, after surgery and after removal of the implants, axial compression tests were conducted to measure surface strains during loading. FINDINGS: The compression hip screw reduced the amount of strain at the superior neck by 88% (P=0.015) and at the lesser trochanter by 51% (P=0.038). The femoral neck plate reduced the amount of strain at the superior neck by 89% (P=0.001), and increased the amount of strain at the inferior neck by 58% (P=0.02) and at the lesser trochanter by 63% (P=0.005). After implant removal, there was no significant difference in strain compared to pre-fracture levels, except for the compression hip screw with 21% less strain (P=0.047) at the superior neck. INTERPRETATION: Removal of osteosynthesis implants after bone union reverts bone strains to pre-fracture levels, and might prevent further bone loss induced by stress shielding.

Type of hip fracture determines load share in intramedullary osteosynthesis.

The choice of the appropriate implant continues to be critical for fixation of unstable hip fractures. Therefore, the goal of this study was to develop a numerical model to investigate the mechanical performance of hip fracture osteosynthesis. We hypothesized that decreasing fracture stability results in increasing load share of the implant and therefore higher stress within the implant. We also investigated the relationship of interfragmentary movement to the fracture stability. A finite element model was developed for a cephalomedullary nail within a synthetic femur and simulated a pertrochanteric fracture, a lateral neck fracture, and a subtrochanteric fracture. The femur was loaded with a hip force and was constrained physiologically. The FE model was validated by mechanical experiments. All three fractures resulted in similar values for stiffness (462-528 N/mm). The subtrochanteric fracture resulted in the highest local stress (665 MPa), and the pertrochanteric fracture resulted in a lower stress (621 MPa) with even lower values for the lateral neck fracture (480 MPa). Thus, intramedullary implants can stabilize unstable hip fractures with almost the same amount of stiffness as seen in stable fractures, but they have to bear a higher load share, resulting in higher stresses in the implant.