The impact of preoperative single low-dose dexamethasone on in-hospital prognosis in geriatric intertrochanteric fracture patients: Analysis of secondary outcomes in a randomized controlled trial.

BACKGROUND: Intertrochanteric fracture in the geriatric population is associated with poor prognosis, which may be attributed to consistent stress and the systemic inflammatory response. Dexamethasone is an exogenous glucocorticoid commonly used in clinical practice for broad anti-inflammatory action. The purpose is to investigate whether a single preoperative low-dose dexamethasone can improve the in-hospital prognosis in geriatric intertrochanteric fracture patients undergoing internal fixation surgery. METHODS: Between June 2020 and October 2022, 219 eligible patients with intertrochanteric fractures were in this study. After meeting the inclusion and exclusion criteria, 160 patients were randomly allocated to the dexamethasone or placebo groups (80 patients who are geriatric with an intertrochanteric fracture in each group). The patients in the dexamethasone group received 10 mg (2 mL) of dexamethasone intravenously, whereas the patients in the placebo group received 2 mL of saline intravenously within 30 minutes before being sent to the operating room. The efficacy-related outcomes (the first bed-chair transfer ability, in-hospital mortality, and length of stay) and safety-related outcomes (infection events and hyperglycemia) were collected for analysis. RESULTS: There were no significant differences in the baseline characteristics between the 2 groups. The dexamethasone group had a significantly higher rate of the first bed-chair transfer than the placebo group (65.0% [52/80] vs 48.8% [39/80], relative risk = 1.46, 95% confidence interval = 1.02 to 2.11; P = .038). One patient in the dexamethasone group and 7 patients in the placebo group died during hospitalization (1.3% [1/80] vs 8.8% [7/80], relative risk = 0.92, 95% confidence interval = 0.86 to 0.99; P = .07). No differences were found in the length of stay, infections, and hyperglycemia between the 2 groups. CONCLUSION: A single preoperative low-dose of dexamethasone can improve the in-hospital prognosis (increase the ability of the first bed-chair transfer and potentially decrease the in-hospital mortality) in geriatric intertrochanteric fracture patients after internal fixation surgery.

A single preoperative low-dose dexamethasone may reduce the incidence and severity of postoperative delirium in the geriatric intertrochanteric fracture patients with internal fixation surgery: an exploratory analysis of a randomized, placebo-controlled trial.

OBJECTIVE: Postoperative delirium (POD) is a common complication along with poor prognosis in geriatric intertrochanteric fracture (ITF) patients. However, the prevention and treatment of POD remain unclear. Previous studies have confirmed that POD is essentially a consequence of neuro-inflammatory responses. Dexamethasone is a glucocorticoid with comprehensive anti-inflammatory effects, while a high dose of dexamethasone correlates with many side effects or even adverse consequences. Thus, this prospective study aims to discuss whether a single preoperative low-dose dexamethasone can reduce the impact of POD on geriatric ITF patients with internal fixation surgery. METHODS: Between June 2020 and October 2022, there were 219 consecutive ITF patients assessed in our department. Of the 219 ITF patients, 160 cases who met the inclusion and exclusion criteria were finally enrolled and randomly allocated to the dexamethasone group and the placebo group (80 geriatric ITF patients in each group) in this prospective study. The patients in the dexamethasone group received intravenous 10 mg (2 ml) dexamethasone while the patients in the placebo group received intravenous 2 ml saline in 30 min before being sent to the operating room, respectively. The baseline characteristics, surgical information, incidence and severity of POD as the efficacy-related outcomes, and infection events and hyperglycemia as safety-related outcomes (adverse events), were collected and analyzed between the two groups. The severity of POD was evaluated by Memorial Delirium Assessment Scale (MDAS) score. RESULTS: There were no differences in baseline characteristics and surgical information between the dexamethasone group and the placebo group. The dexamethasone group had a lower incidence of POD than the placebo group within the first 5 days after surgery [(9/80, 11.3% vs. 21/80, 26.3%, RR = 0.83, 95% CI 0.71-0.97, P = 0.015]. The dexamethasone group had lower MDAS scores (Mean ± SD) than the placebo group [13.2 ± 1.0 (range 11 to 15) vs. 15.48 ± 2.9 (range 9 to 20), P = 0.011, effect size = 0.514]. There were no differences in infection events and hyperglycemia between the two groups. CONCLUSIONS: A single preoperative low-dose dexamethasone may reduce the incidence and severity of POD in geriatric ITF patients with internal fixation surgery. TRIAL REGISTRATION: ChiCTR2200055281.

Standardized Tip-Apex Distance (STAD): a modified individualized measurement of cephalic fixator position based on its own femoral head diameter in geriatric intertrochanteric fractures with internal fixation.

OBJECTIVE: To design a standardized Tip-Apex Distance (STAD) and analyze the clinical significance of STAD in predicting cut-out in geriatric intertrochanteric fractures with internal fixation. METHODS: Firstly, we designed STAD according to the rule of TAD. We measured the STAD individually based on its own femoral head diameter (iFHD) instead of the known diameter of the lag screw in calculating TAD, resulting in that the STAD is simply the relative quantitation relationship of iFHD (the times of iFHD). In this study, we assumed that all the iFHD was 6D (1iFHD = 6D, or 1D = 1/6 of iFHD) in order for complete match of the Cleveland zone system, easy comparison of the STAD, and convenient identification for artificial intelligence. Secondly, we calculated and recorded all the STAD of cephalic fixator in 123 eligible ITF patients. Thirdly, we grouped all the ITF patients into the Failure and Non-failure groups according to whether cut-out or not, and analyzed the correlation between the cut-out and the STAD. RESULTS: Cleveland zone, Parker's ratio (AP), TAD, and STAD were associated with the cut-out in univariate analysis. However, only STAD was the independent predictor of the cut-out by multivariate analysis. No cut-out was observed when STAD ≤ 2D (1/3 of iFHD). The Receiver Operating Characteristic (ROC) curve indicated that STAD was a reliable predictor of cut-out, and the best cut-off value of STAD was 2.92D. Cut-out rate increased dramatically when STAD increased, especially when STAD > 3D (1/2 of iFHD). CONCLUSION: Essentially, the STAD is a relative quantitation relationship of iFHD. The STAD is a reliable measurement of cephalic fixator position in predicting cut-out in geriatric ITF patients with single-screw cephalomedullary nail fixations. For avoiding cut-out, the STAD should be no more than a half of iFHD. LEVEL OF EVIDENCE: Level III, Prognostic Study.

Eccentric distance zone analysis system: New regional evaluation of cephalic fixator tip location for predicting cut-out in geriatric intertrochanteric fractures with internal fixation.

Objective: The aim of this study was to investigate an eccentric distance (ED) zone analysis system for regional evaluation of the cephalic fixator tip based on the ED of the cephalic fixator tip referenced to the radius of its own femoral head to predict cut-out in intertrochanteric fractures (ITF) with internal fixation. Methods: First, we assumed all the femoral heads were regular spheres with the radius (R FD) of "3" for a complete match of the Cleveland zone system and calculated the ED of the cephalic fixator tip by measuring the distances from the cephalic fixator tip to the geometric central axis in the femoral neck and head on both anteroposterior (AP) view and lateral view radiographs. Second, we defined the maximum transverse section of the femoral head into three zones named ED Zone A with ED less than "1," Zone B with ED ranging in "1-2," and Zone C with ED ranging in "2-3" in turns by concentric circles (circles A, B, and C) with the radius of 1/3, 2/3, and 3/3 times of R FD, respectively. Third, we evaluated the ED zones according to the ED and location of the cephalic fixator tip in the eligible 123 ITF patients with single-screw cephalomedullary nail (SCMN) fixation and then analyzed the correlation between the cut-out rate and the ED zones. Results: The cut-out rates in ED Zones A, B, and C were 4.17%, 38.46%, and 100%, respectively. Multivariate logistic regression indicated that ED Zone A had at least a 14 times lower rate of cut-out compared with ED Zone B. The cephalic fixator tip located in ED Zone A has a lower cut-out rate than that in Cleveland Zone 5. The cut-out rate in ED Zone A is significantly lower than that in the region inside Cleveland Zone 5 but outside ED Zone A. Conclusion: ED zone analysis system is a reliable regional evaluation of the cephalic fixator tip position for predicting cut-out in geriatric ITF patients with SCMN fixations and potentially an artificial intelligence measurement during surgery. For decreasing the cut-out rate, the cephalic fixator tip should be located in ED Zone A.

Surgical Position of Lateral-Tilted Supine is Suitable for Proximal Humeral Fracture Operations in Geriatric Patients.

Objective: To report a new surgical position of lateral-tilted supine (LTS) for geriatric proximal humeral fracture operations. Methods: Between January 2016 and December 2020, we adopted the LTS position for operations in 65 geriatric patients with proximal humeral fractures. Results: Sixty-five patients including 25 males and 40 females aged 80.3 ± 8.5 years. The LTS position could be used for almost all proximal humeral fracture surgeries, such as ORIF with plate, suture anchor, and other fixation in 4 patients, open reduction and internal fixation (ORIF) with multiLoc nailing in 48, and shoulder hemiarthroplasty (SHA) in 13. Surgical position setting times were 11.47 ± 2.14 min. The systolic blood pressure changes before and after positioning were 15.07 ± 8.72 mmHg. All of the C-arm X-ray directions, including the cephalic side, contralateral side, and ipsilateral side, can be used in the LTS position surgeries. No surgical complications or no surgical position-related complications were found in these 65 cases. Conclusion: The surgical position of LTS is suitable for geriatric proximal humeral fracture operations.

The correlation between cutout and eccentric distance (ED) of the cephalic fixator tip in geriatric intertrochanteric fractures with internal fixation.

BACKGROUND: The location of cephalic fixator tip with different eccentric distance (ED) should have different risks of cutout. This study aims to evaluate the cephalic fixator tip position by measuring ED of the cephalic fixator tip in geriatric ITF patients with single-screw cephalomedullary nail (SCMN) fixation and analyze the correlation between the cutout and the ED. METHODS: Firstly, we assumed all the femoral head was a regular sphere and standardized the radius of the femoral head (RFD) as "3" no matter how big the RFD was for complete match of the Cleveland zone system and convenient identification of artificial intelligence. Secondly, we measured the ED of the cephalic fixator tip by calculating the distances from the cephalic fixator tip to the geometric central axis of the femoral neck and head on both AP view and lateral view radiographs. Thirdly, we evaluated all the ED of the cephalic fixator tip in the eligible 123 geriatric ITF patients and analyzed the correlation between the cutout and the ED. RESULTS: The ED in cutout group (1.25 ± 0.43) is much bigger than that in non-cutout group (0.64 ± 0.34) with significant difference (OR = 50.01, 95% CI 8.42-297.19, p < 0.001). The probability of cutout increased with ED increasing, especially when "ED ≥ 1." The best cutoff value of ED for predicting cutout was "1.022" ("1.022" was just a little bit more than 1/3 times of RFD because "RFD = 3," sensitivity = 73.3%, specificity = 86.1%, and AUC = 0.867, p < 0.001). CONCLUSION: ED is suitable for evaluation of the cephalic fixator tip position for predicting cutout in geriatric ITF patients with SCMN fixation, and ED can potentially be used as artificial intelligence application during surgery. The smaller the ED, the lower the cutout rate. For avoiding cutout, the ED of the cephalic fixator tip should be less than one-third times of the radius of the femoral head.

CalTAD is the Key Evaluation Tool for Measurement of Cephalic Fixation Position for Predicting Cut-Out in Geriatric Intertrochanteric Fracture Patients with Internal Fixations after Achieving Acceptable Reduction.

Objective: To discover the key evaluation tool of the cephalic fixation position for predicting implant failures in geriatric intertrochanteric fracture (ITF) patients treated with internal fixations after achieving an acceptable reduction. Methods: We measured the geriatric ITF patients undergoing single-screw cephalomedullary nailing (CMN) fixation surgery after obtaining the acceptable reduction (including anatomical reduction and positive medial cortex support reduction) in our treatment group between September 2016 and March 2020 by using four kinds of cephalic fixation position evaluation tools including Cleveland zone system, Parker's ratio index, tip-apex distance (TAD), calcar-referenced TAD (CalTAD), and analyzed which were the key evaluation tools for measurement of cephalic fixation position for prediction of implant failures in geriatric ITF patients with internal fixations. Results: Seventy-four ITF patients treated with single-screw CMN fixation after obtaining the acceptable reduction were enrolled in this study. Of the 74 patients, nine cases were observed with implant failures. There were six cases of cut-out and three of pending cut-out. We found that TAD (odds ratio (OR)=1.149; 95% confidence interval (CI), 1.00-1.32; P=.046) and CalTAD (OR=1.140; 95% CI, 1.00-1.30; P=.037) were risk factors for implant failures by univariate analysis, while only CalTAD (OR=1.200; 95% CI, 1.032-1.395; P=.018) was the independent risk factor for implant failures by multivariate analysis. The Kappa coefficient (κ) of CalTAD was .976 (95% CI, .966-.984) by ICC analysis. The ROC analysis showed that the best cut-off value of CalTAD was 23.76 mm with a sensitivity of 77.8% and specificity of 72.3% (area under the curve, AUC =.775; P = .001). Conclusions: CalTAD is the key evaluation tool for measurement of cephalic fixation position for predicting implant failures in geriatric ITF patients treated with single-screw CMN after obtaining the acceptable reduction.

Risk factors for cut-outs in geriatric intertrochanteric fractures with cephalomedullary nailing after obtaining acceptable reduction: a case-control study.

BACKGROUND: It is irresponsible if we disregard reduction quality to talk about cut-outs in intertrochanteric fractures (ITF) with internal fixation. The aim of this study is to analyze the risk-factors for cut-outs in geriatric ITF with cephalomedullary nailing after obtaining acceptable reduction. METHODS: In order to investigate the risk-factors for cut-outs in geriatric ITF after obtaining acceptable reduction, we retrospectively reviewed 367 patients who underwent cephalomedullary nail for ITF in our department between September 2016 and December 2021. Potential variables including demographic data and radiological parameters (namely the fracture type, Singh index, lateral wall fracture, cephalic nail position, Parker's ratio index, tip-apex-distance (TAD), and calcar-referenced TAD (CalTAD)) were collected. Logistic regression analysis was performed to identify the significant risk factors for cut-outs. RESULTS: One hundred twenty-one patients were suitable for this study. Of the 121 cases, nine cases (7.4%) were observed with cut-out or pending cut-out. We found that Age (adjusted odds ratio (OR) 1.158, 95% confidence interval (CI) 1.016 to 1.318, p = 0.028), lateral wall fracture (adjusted OR 11.07, 95%CI 1.790 to 68.380, p = 0.01), and CalTAD (adjusted OR 1.277, 95%CI 1.005 to 1.622, p = 0.045) were independent risk-factors for cut-outs. CONCLUSIONS: Age, lateral wall fracture and CalTAD are independent risk-factors for cut-outs in geriatric ITF with cephalomedullary nailing after obtaining acceptable reduction. In order to avoid cut-outs, an optimal CalTAD is necessary even obtaining acceptable reduction, especially in the over-aged patients with lateral wall fracture.

Early prediction of implant failures in geriatric intertrochanteric fractures with single-screw cephalomedullary nailing fixation.

OBJECTIVE: The implant failures of intertrochanteric fractures (ITF) after single-screw cephalomedullary nailing (CMN) were multifactorially associated with various related factors. However, a comprehensive scoring system for the early prediction of implant failures is still lacking. Thus, this study aims to establish a quantification scoring system (QSS) and verify whether the QSS is reliable for predicting implant failures in geriatric ITF patients. METHODS: We established the QSS of geriatric ITF with single-screw cephalomedullary nailing within three days after surgery. The QSS included eight points totally at eight parameters, including bone quality, fracture type, reduction quality, and internal fixation placement. Then we retrospectively analyzed seventy-seven ITF (seventy-six patients) with surgical treatment between October 2016 and July 2020 in our hospital to verify whether the QSS scoring is suitable for predicting implant failures in ITF patients. RESULTS: Implant failures were in fifteen fractures (fifteen patients), including six cases of cut-out, eight of pending cut-out, and one of cut-through. There were three cases with 2 points in QSS, three with 3 points, five with 4 points, four with 5 points in these fifteen fractures. No fractures were with 1 point in QSS, and no implant failures when scoring over 5 points in QSS. Except for QSS scoring, no significant difference was in the collected data by binary logistic regression analysis. QSS scoring was significantly associated with implant failures (Adjusted odds ratio (OR) = 7.312; 95% confidence intervals (CI), 2.561 to 20.871; p < 0.001). In the analysis of Spearman's correlation, there was a strong correlation between QSS scoring and the occurrences of implant failures (RQSS = -0.964, p < 0.001). The ROC result indicated that QSS was reliable in predicting implant failures at the cut-off of 5 points (AUC (the area under the curve) = 0.944; 95% CI, 0.866 to 0.983; p < 0.001). CONCLUSION: The QSS is a useful early prediction of implant failures in geriatric ITF with cephalomedullary nailing fixation. QSS scoring more than 5 points can effectively reduce the risk of implant failures.

Joint Salvage and Biological Repair of Massive-Cavity Bone Defects After Extensive Curettage of Campanacci Grade II or III Giant Cell Tumor Around the Knee With Vascularized Fibular Autograft and Cancellous Allograft.

OBJECTIVE: The aim of the study was to report the clinical outcomes of repair of massive-cavity bone defects after extensive curettage of Campanacci grade II or III giant cell tumor (GCT) around knee with vascularized fibular autograft and cancellous allograft. METHODS: There were 12 consecutive patients with Campanacci grade II or III GCT around knee treated in our department between 2004 and 2016. All the patients underwent clinical evaluation, plain radiography, and/or magnetic resonance imaging of the knee right after admission. To preserve their knee function, we repaired the massive-cavity bone defects after extensive curettage of GCT by vascularized fibular autografts and cancellous allograft. All the patients were evaluated through clinical examinations, plain radiography of the knee and chest, and Musculoskeletal Tumor Society (MSTS) scores of the lower extremity in the follow-ups. RESULTS: The follow-up ranged from 1.5 to 12.0 years (mean, 4.2 years). There were no local recurrences or lung metastasis in any of the 12 patients at the last follow-up. Ten patients had no pain or experienced occasional pain, and 9 were able to resume their previous work. The mean range of motion of knee flexion was 117 degrees, and the extension was -6 degrees. The mean MSTS score was 24.7, and a total of 10 patients had excellent or good MSTS scores. CONCLUSIONS: It is reliable to achieve knee joint salvage and repair massive-cavity bone defects after extensive curettage with vascularized fibular autograft and cancellous allograft in patients with Campanacci grade II or III GCT around the knee.

The analgesic evaluation of gabapentin for arthroscopy: A meta-analysis of randomized controlled trials.

INTRODUCTION: The efficacy of gabapentin for pain management of arthroscopy remains controversial. We conduct a systematic review and meta-analysis to explore the influence of gabapentin versus placebo on the postoperative pain intensity of arthroscopy. METHODS: We search PubMed, EMbase, Web of science, EBSCO, and Cochrane library databases through April 2020 for randomized controlled trials assessing the effect of gabapentin versus placebo on pain control of arthroscopy. This meta-analysis is performed using the random-effect model. RESULTS: Five randomized controlled trials are included in the meta-analysis. Overall, compared with control group for arthroscopy, gabapentin remarkably decreases pain scores at 24 hour (standard mean difference [SMD]=-0.68; 95% confidence interval [CI]=-1.15 to -0.02; P = .21), analgesic consumption (SMD = -18.24; 95% CI=-24.61 to -11.88; P < .00001), nausea and vomiting (OR = 0.42; 95% CI = 0.21 to 0.84; P = .01), but has no obvious influence on pain scores at 6 h (SMD = -1.30; 95% CI = -2.92 to 0.31; P = .11) or dizziness (OR = 1.12; 95% CI = 0.56 to 2.24; P = .75). CONCLUSIONS: Gabapentin is effective for pain control after arthroscopy.

The Correlation Between Timing of Surgery and the Need for RBC Transfusions in the Geriatric Intertrochanteric Fracture Population.

OBJECTIVE: To identify whether the timing of surgery affects red blood cell (RBC) transfusion requirements in the elderly with intertrochanteric fractures. METHODS: We retrospectively studied all patients undergoing surgical fixation of their intertrochanteric fractures in our hospital between January 2009 and December 2018 and analyzed the relationship between the timing of surgery and RBC transfusion. RESULTS: A total of 679 patients were included in this study. The need for RBC transfusion was lower in the patients who underwent surgery within 12 h after admission (timing of surgery <12 h, <12 h group) than those who underwent surgery over 12 h after admission (timing of surgery >12 h, >12 h group) (P = 0.046); lower in the the patients who underwent surgery within 24 h after admission (timing of surgery <24 h, <24 h group) than in those who underwent surgery over 24 h after admission (timing of surgery >24 h, >24 h group) (P = 0.008), and lower in the <24 h group compared to the patients who underwent surgery within 48 h after admission (timing of surgery <48 h, <48 h group) (P = 0.035). Moreover, the need for RBC transfusion was lower in the <24 h group (in the first 24 h from admission to surgery) than in the 24-48 h group (in the second 24 h from admission to surgery) (P = 0.016), and also lower in the <24 h group compared to the 48-72 h group (in the third 24 h from admission to surgery) (P = 0.047). However, there were no differences between the <12 h group and 12-24 h group, between the <12 h group and <24 h group, and between the 12-24 h group and <24 h group, respectively. CONCLUSION: Timing of surgery within 24 h contributes to the reduction of RBC transfusion in the elderly with intertrochanteric fractures.

Repair of Small-Size Wound With Achilles Tendon Exposure With Proximal Pedicled Cutaneous Neurovascular Flap.

OBJECTIVE: To investigate the clinical outcomes associated with repairing of small-sized wounds of Achilles tendon exposure with proximal pedicled cutaneous neurovascular flap in the dorsolateral foot. METHODS: After thorough debridement, 16 cases with small-sized wounds of Achilles tendon exposure were repaired by proximal pedicled cutaneous neurovascular flap of the dorsolateral foot, and their clinical outcomes were observed. RESULTS: All the flaps in the 16 cases survived completely, excluding the marginal part necrosis in 1 case, and all the wounds were healed. The 2-point discrimination of the flaps was 14.53 ± 1.55 mm (range, 12-17 mm) in patients without sural nerve injury after 3 to 18 months follow-up. No discomfort was felt in wearing normal shoes by all the 16 patients. CONCLUSIONS: It is reasonable to repair the small-sized wounds of Achilles tendon exposure with proximal pedicled cutaneous neurovascular flap of dorsolateral foot due to its effective repair of the wound, relatively uncomplicated surgery, and had satisfactory healing recovery.

Growth inhibition effects of ent-11α-hydroxy-15-oxo-kaur-16-en-19-oic-acid on colorectal carcinoma cells and colon carcinoma-bearing mice.

The aim of the present study was to investigate the mechanism underlying the antitumor effects of ent-11α-hydroxy-15-oxo-kaur-16-en-19-oic-acid (5F) in colorectal cancer (CRC). 5F was isolated and used to treat C26 murine colon carcinoma cells, a xenograft tumor mouse model (induced by C26 cells) and a CRC mouse model [induced by 1,2-dimethylhydrazine (DMH)/dextran sodium sulfate (DSS)]. C26 cell growth was inhibited by 5F in a dose- and time-dependent manner in vitro. In addition, 5F induced cell apoptosis and cell cycle arrest in the G2 phase, increased the activity of caspase-3 and caspase-9, but did not affect the activity of cascase­8, suggesting that 5F induced apoptosis via activation of the mitochondrial signaling pathway rather than the death­receptor signaling pathway. Furthermore, treatment of C26 cells with 5F resulted in upregulation of cyclin­dependent kinase inhibitor 1A (p21, Cip1), Bcl­2­associated X protein, nuclear factor of κ light polypeptide gene enhancer in B­cells inhibitor, α and downregulation of B­cell lymphoma 2, nuclear factor κ­light­chain enhancer of activated B cells and survivin. In vivo animal models demonstrated that 5F treatment protected mice from carcinogenesis induced by DMH/DSS and markedly decreased the xenograft tumor weight with minimal side effects. Therefore, 5F may have potential as an anti-CRC therapeutic agent for use in the clinical setting.

Micromechanical Modeling of Fiber-Reinforced Composites with Statistically Equivalent Random Fiber Distribution.

Modeling the random fiber distribution of a fiber-reinforced composite is of great importance for studying the progressive failure behavior of the material on the micro scale. In this paper, we develop a new algorithm for generating random representative volume elements (RVEs) with statistical equivalent fiber distribution against the actual material microstructure. The realistic statistical data is utilized as inputs of the new method, which is archived through implementation of the probability equations. Extensive statistical analysis is conducted to examine the capability of the proposed method and to compare it with existing methods. It is found that the proposed method presents a good match with experimental results in all aspects including the nearest neighbor distance, nearest neighbor orientation, Ripley's K function, and the radial distribution function. Finite element analysis is presented to predict the effective elastic properties of a carbon/epoxy composite, to validate the generated random representative volume elements, and to provide insights of the effect of fiber distribution on the elastic properties. The present algorithm is shown to be highly accurate and can be used to generate statistically equivalent RVEs for not only fiber-reinforced composites but also other materials such as foam materials and particle-reinforced composites.

Strong adhesion and friction coupling in hierarchical carbon nanotube arrays for dry adhesive applications.

The adhesion and friction coupling of hierarchical carbon nanotube arrays was investigated with a hierarchical multiscale modeling approach. At device level, vertically aligned carbon nanotube (VA-CNT) arrays with laterally distributed segments on top were analyzed via finite element methods to determine the macroscopic adhesion and friction force coupling. At the nanoscale, molecular dynamics simulation was performed to explore the origin of the adhesion enhancement due to the existence of the laterally distributed CNTs. The results show interfacial adhesion force is drastically promoted by interfacial friction force when a single lateral CNT is being peeled from an amorphous carbon substrate. By fitting with experiments, we find that under shearing loadings the maximum interfacial adhesion force is increased by a factor of ~5, compared to that under normal loadings. Pre-existing surface asperities of the substrate have proven to be the source of generating large interfacial friction, which in turn results in an enhanced adhesion. The critical peeling angles derived from the continuum and nano- levels are comparable to those of geckos and other synthetic adhesives. Our analysis indicates that the adhesion enhancement factor of the hierarchically structured VA-CNT arrays could be further increased by uniformly orienting the laterally distributed CNTs on top. Most importantly, a significant buckling of the lateral CNT at peeling front is captured on the molecular level, which provides a basis for the fundamental understanding of local deformation, and failure mechanisms of nanofibrillar structures. This work gives an insight into the durability issues that prevent the success of artificial dry adhesives.

Friction and adhesion of hierarchical carbon nanotube structures for biomimetic dry adhesives: multiscale modeling.

With unique hierarchical fibrillar structures on their feet, gecko lizards can walk on vertical walls or even ceilings. Recent experiments have shown that strong binding along the shear direction and easy lifting in the normal direction can be achieved by forming unidirectional carbon nanotube array with laterally distributed tips similar to gecko's feet. In this study, a multiscale modeling approach was developed to analyze friction and adhesion behaviors of this hierarchical fibrillar system. Vertically aligned carbon nanotube array with laterally distributed segments at the end was simulated by coarse grained molecular dynamics. The effects of the laterally distributed segments on friction and adhesion strengths were analyzed, and further adopted as cohesive laws used in finite element analysis at device scale. The results show that the laterally distributed segments play an essential role in achieving high force anisotropy between normal and shear directions in the adhesives. Finite element analysis reveals a new friction-enhanced adhesion mechanism of the carbon nanotube array, which also exists in gecko adhesive system. The multiscale modeling provides an approach to bridge the microlevel structures of the carbon nanotube array with its macrolevel adhesive behaviors, and the predictions from this modeling give an insight into the mechanisms of gecko-mimicking dry adhesives.