Search

Background & Aims: HBV coinfection is common among people living with HIV (PLWH) and is the most important cause of hepatocellular carcinoma (HCC). While risk prediction tools for HCC have been validated in patients with HBV monoinfection, they have not been evaluated in PLWH. Thus, we performed an external validation of PAGE-B in people with HIV/HBV coinfection.Methods: We included data on PLWH from four European cohorts who were positive for HBsAg and did not have HCC before starting tenofovir. We estimated the predictive performance of PAGE-B for HCC occurrence over 15 years in patients receiving tenofovir-containing antiretroviral therapy. Model discrimination was assessed after multiple imputation using Cox regression with the prognostic index as a covariate, and by calculating Harrell's c-index. Calibration was assessed by comparing our cumulative incidence with the PAGE-B derivation study using Kaplan-Meier curves.Results: In total, 2,963 individuals with HIV/HBV coinfection on tenofovir-containing antiretroviral therapy were included. PAGE-B was Conclusions: For individuals with HIV/HBV coinfection, PAGE-B is a valid tool to determine the need for HCC screening. Impact and implications: Chronic HBV infection is the most important cause of hepatocellular carcinoma (HCC) among people living with HIV. Valid risk prediction may enable better targeting of HCC screening efforts to high-risk individuals. We aimed to validate PAGE-B, a risk prediction tool that is based on age, sex, and platelets, in 2,963 individuals with HIV/HBV coinfection who received tenofovir-containing antiretroviral therapy. In the present study, PAGE-B showed good discrimination, adequate calibration, and a cut-off of

Rigid bronchoscopy remains the gold standard for the diagnosis of foreign body aspiration (FBA) despite high rates of negative bronchoscopies. The use of computed tomography (CT) imaging in the assessment of FBA has recently emerged and could help obviate unnecessary bronchoscopy in these patients. The aim of this study is to assess the diagnostic accuracy of CT in the diagnosis of pediatric FBA.A systematic literature review was conducted to identify studies reporting the use of CT imaging in suspected pediatric FBA. The search included published articles in Ovid MEDLINE, Ovid EMBASE, PubMed MEDLINE and Web of Science. The search strategy included all articles from inception of the database to January 2021. Manuscripts were reviewed and graded for quality using the QUADAS-2 tool. Subgroup analyses based on the use of virtual bronchoscopy (VB) and sedation was conducted. A meta-analysis evaluating the use of VB in the diagnosis of FBA was also conducted.Sixteen manuscripts met all inclusion criteria. In total, 2056 pediatric patients ranging from 0.3 to 15 years underwent CT for suspected FBA. The sensitivity and specificity of CT were 98.8% and 96.6%, respectively. VB was used in 71.4% (1391/1948) of patients while sedation during CT was required in 70.2% (1263/1800) of patients. Radiation dosing ranged from 0.04 to 2 mSv, 0.99-59.1 mGy-cm and 0.03-16.99 mGy.CT can accurately diagnose pediatric FBA and can help decrease the rate of unnecessary bronchoscopies with an acceptable dose of radiation.

Introduction Ultrasound imaging has many clinical applications, but there is a lack of data about its use by infectiologists. The aim of this study was to describe ultrasound performed routinely by infectiologists and to assess the diagnostic performance of ultrasound with aspirate and fluid analysis in prosthetic joint infections. Methods Retrospective study between 1st June 2019 and 1st June 2020 in an infectious and tropical diseases unit in a tertiary University Hospital. Results One hundred and thirty-one ultrasounds were performed on 127 patients by the infectious diseases team. These included 64 musculoskeletal ultrasounds (31 in native joints and 33 in prosthetic joints including 15 knees, 13 hips and 5 spacers) and 33 led to a fluid aspirate. Fourteen lung ultrasounds were done, 11 confirmed pneumopathy and 7 resulted in pleural puncture. Twenty-three vascular ultrasounds were done, 17 to insert a catheter, and four to perform a blood test. Five ultrasounds explored adenopathy, of which one node tuberculosis and one Bartonella infection were diagnosed. In prosthetic joint infections, sensitivity and specificity of ultrasound with fluid aspirate and analysis were respectively 100% and 100% for the knee and 40% and 100% for the hip. Conclusion Ultrasound performed by infectiologists is useful and contributes to a faster diagnosis. Furthermore, the specificity of ultrasound with aspirate and fluid analysis is very high in prosthetic joint infection. Ultrasound training courses should be considered for infectiologists including residents.

Despite numerous descriptive publications, the guidelines for treatment of cervical spinal tuberculosis (TB) are not very clear. The authors report a case of a young girl with cervico-thoracic spinal TB extending from C5 to T3 vertebrae presenting with weakness of the right hand and unsteady gait. An 11-year-old female who is an immigrant to the UK from Afghanistan, presented to our clinic with a 10-day history of difficulty in walking with an unsteady gait and 3-month history of progressive weakness in both her arms, the right side more affected than the left. Her immunisation history was unclear. Examination of the arms showed bilateral thenar and hypothenar wasting, more so on the right than the left. An MRI scan revealed a large para-spinal abscess extending from C3/4 to T4/5 with a significant anterior epidural cord compression from C5/6 to T2/3. Therapeutic/diagnostic aspiration was performed under ultrasound guidance and the aspirate was sent for microbiology. She was started empirically on multidrug anti-tubercular treatment and steroids. Although Ziehl–Neelsen stain was negative for acid-fast bacilli, microbiological confirmation of TB was obtained by positive TB culture sensitive to all first-line anti-TB drugs. She made a dramatic improvement within 3 weeks of anti-tubercular treatment. A follow-up MRI scan at 8 months showed complete resolution of the abscess. At 2 years of follow-up, she was a healthy looking child, back to her school with no residual clinical signs/symptoms of the disease. Our case of cervico-thoracic tuberculous abscess in a young child suggests that even with incomplete neurological deficit caused by epidural cord compression, ultrasound (or CT)-guided aspiration and anti-tubercular medication provide acceptable results at 2 years of follow-up.

We present an improved tropospheric nitrogen dioxide column retrieval algorithm (DOMINO v2.0) for OMI based on better air mass factors (AMFs) and a correction for across-track stripes resulting from calibration errors in the OMI backscattered reflectances. Since October 2004, NO2 retrievals from the Ozone Monitoring Instrument (OMI), a UV/Vis nadir spectrometer onboard NASA's EOS-Aura satellite, have been used with success in several scientific studies focusing on air quality monitoring, detection of trends, and NOx emission estimates. Dedicated evaluations of previous DOMINO tropospheric NO2 retrievals indicated their good quality, but also suggested that the tropospheric columns were susceptible to high biases (by 0–40%), probably because of errors in the air mass factor calculations. Here we update the DOMINO air mass factor approach. We calculate a new look-up table (LUT) for altitude-dependent AMFs based on more realistic atmospheric profile parameters, and include more surface albedo and surface pressure reference points than before. We improve the sampling of the TM4 model, resulting in a priori NO2 profiles that are better mixed throughout the boundary layer. We evaluate the NO2 profiles simulated with the improved TM4 sampling as used in the AMF calculations and show that they are highly consistent with in situ NO2 measurements from aircraft during the INTEX-A and INTEX-B campaigns in 2004 and 2006. Our air mass factor calculations are further updated by the implementation of a high-resolution terrain height and a high-resolution surface albedo climatology based on OMI measurements. Together with a correction for across-track stripes, the overall impact of the improved terrain height and albedo descriptions is modest (

An eight-year long reanalysis of atmospheric composition data covering the period 2003-2010 was constructed as part of the FP7-funded Monitoring Atmospheric Composition and Climate project by assimilating satellite data into a global model and data assimilation system. This reanalysis provides fields of chemically reactive gases, namely carbon monoxide, ozone, nitrogen oxides, and formaldehyde, as well as aerosols and greenhouse gases globally at a horizontal resolution of about 80 km for both the troposphere and the stratosphere. This paper describes the assimilation system for the reactive gases and presents validation results for the reactive gas analysis fields to document the data set and to give a first indication of its quality. Tropospheric CO values from the MACC reanalysis are on average 10-20% lower than routine observations from commercial aircrafts over airports through most of the troposphere, and have larger negative biases in the boundary layer at urban sites affected by air pollution, possibly due to an underestimation of CO or precursor emissions. Stratospheric ozone fields from the MACC reanalysis agree with ozonesondes and ACE-FTS data to within ±10% in most seasons and regions. In the troposphere the reanalysis shows biases of -5% to +10% with respect to ozonesondes and aircraft data in the extratropics, but has larger negative biases in the tropics. Area-averaged total column ozone agrees with ozone fields from a multi-sensor reanalysis data set to within a few percent. NO2 fields from the reanalysis show the right seasonality over polluted urban areas of the NH and over tropical biomass burning areas, but underestimate wintertime NO2 maxima over anthropogenic pollution regions and overestimate NO2 in northern and southern Africa during the tropical biomass burning seasons. Tropospheric HCHO is well simulated in the MACC reanalysis even though no satellite data are assimilated. It shows good agreement with independent SCIAMACHY retrievals over regions dominated by biogenic emissions with some anthropogenic input, such as the eastern US and China, and also over African regions influenced by biogenic sources and biomass burning. © Author(s) 2013., SCOPUS: re.j, info:eu-repo/semantics/published

In the period from May through June 1994, an increase in the number of domestic cases of Shigella sonnei infection was detected in several European countries, including Norway, Sweden, and the United Kingdom. In all three countries epidemiological evidence incriminated imported iceberg lettuce of Spanish origin as the vehicle of transmission. The outbreaks shared a number of common features: a predominance of adults among the case patients, the presence of double infections with other enteropathogens, and the finding of two dominant phage types among the bacterial isolates. In Norway 110 culture-confirmed cases of infection were recorded; more than two-thirds (73%) were adults aged 30 to 60 years. A nationwide case-control study comprising 47 case patients and 155 matched control individuals showed that the consumption of imported iceberg lettuce was independently associated with an increased risk of shigellosis. Epidemiological investigation of a local outbreak incriminated iceberg lettuce from Spain, consumed from a salad bar, as the source. The presence of shigellae in the suspected food source could not be documented retrospectively. However, high numbers of fecal coliforms were detected in iceberg lettuce from patients' homes. Three lettuce specimens yielded salmonellae. The imported iceberg lettuce harbored Escherichia coli strains showing resistance to several antimicrobial agents, including ampicillin, ciprofloxacin, gentamicin, and trimethoprim-sulfamethoxazole. During the outbreak it is likely that thousands of Norwegians and an unknown number of consumers in other countries were exposed to coliforms containing antibiotic resistance genes.

The severe wildfires in western Russia during July–August 2010 coincided with a strong heat wave and led to large emissions of aerosols and trace gases such as carbon monoxide (CO), hydrocarbons and nitrogen oxides into the troposphere. This extreme event is used to evaluate the ability of the global MACC (Monitoring Atmospheric Composition and Climate) atmospheric composition forecasting system to provide analyses of large-scale pollution episodes and to test the respective influence of a priori emission information and data assimilation on the results. Daily 4-day hindcasts were conducted using assimilated aerosol optical depth (AOD), CO, nitrogen dioxide (NO2) and ozone (O3) data from a range of satellite instruments. Daily fire emissions were used from the Global Fire Assimilation System (GFAS) version 1.0, derived from satellite fire radiative power retrievals. The impact of accurate wildfire emissions is dominant on the composition in the boundary layer, whereas the assimilation system influences concentrations throughout the troposphere, reflecting the vertical sensitivity of the satellite instruments. The application of the daily fire emissions reduces the area-average mean bias by 63% (for CO), 60% (O3) and 75% (NO2) during the first 24 h with respect to independent satellite observations, compared to a reference simulation with a multi-annual mean climatology of biomass burning emissions. When initial tracer concentrations are further constrained by data assimilation, biases are reduced by 87, 67 and 90%. The forecast accuracy, quantified by the mean bias up to 96 h lead time, was best for all compounds when using both the GFAS emissions and assimilation. The model simulations suggest an indirect positive impact of O3 and CO assimilation on hindcasts of NO2 via changes in the oxidizing capacity. However, the quality of local hindcasts was strongly dependent on the assumptions made for forecasted fire emissions. This was well visible from a relatively poor forecast accuracy quantified by the root mean square error, as well as the temporal correlation with respect to ground-based CO total column data and AOD. This calls for a more advanced method to forecast fire emissions than the currently adopted persistency approach. The combined analysis of fire radiative power observations, multiple trace gas and aerosol satellite observations, as provided by the MACC system, results in a detailed quantitative description of the impact of major fires on atmospheric composition, and demonstrate the capabilities for the real-time analysis and forecasts of large-scale fire events.

The accurate determination of nitrogen dioxide (NO2) tropospheric vertical columns from satellite measurements depends strongly on the airmass factor (AMF) used. A sensitivity study was performed with the radiative transfer model SCIATRAN to better understand the impact of aerosols on the calculation of NO2 AMFs. This influence was studied by varying the NO2 and aerosol vertical distributions, as well as physical and optical properties of the particles. In terms of aerosol definitions, the key factors for these calculations were identified as the relation between trace gas and aerosol vertical profiles, the optical depth of the aerosol layer, and single scattering albedo. In addition, surface albedo also has a large impact on the calculations. Overall it was found that particles mixed with the trace gas increases the measurements' sensitivity, but only when the aerosol is not very absorbing. The largest change, a factor of ~2 relative to the situation without aerosols, was found when a low layer of aerosol (600 m) was combined with a homogenous NO2 layer of 1.0 km. A layer of aerosol above the NO2 usually reduces the sensitivity of the satellite measurement. This situation is found mostly for runs with discrete elevated aerosol layers (representative for long-range transport) that can generate a decrease of the AMF values of up to 70%. The use of measured aerosol profiles and modelled NO2 resulted, generally, in much smaller changes of AMF relative to the pure Rayleigh case. Exceptions are some events of elevated layers with high aerosol optical depth that lead to a strong decrease of the AMF values. These results highlight the importance of aerosols in the retrieval of tropospheric NO2 columns from space and indicate the need for detailed information on aerosol properties and vertical distribution.

Rotatory subluxation of the atlantoaxial joint in adults is a rare, but potentially life-threatening injury. Delayed or missed diagnosis can lead to catastrophic consequences. Early recognition and prompt treatment is paramount to avoid long-term morbidity and disability. We report on an instance of bilateral atlantoaxial rotatory subluxation (AARS) in a 25-year-old female who presented to the Casualty Department with a painful neck, right-sided hemiparesis and torticollis after a road traffic collision. Reduction was achieved by controlled cervical traction followed by immobilization in a four-post Halo brace, with complete resolution of her symptoms. At the 12-month follow-up, she had normal neck movements with no signs of instability.

The conservation of foodstuffs by chemical or physical methods is now practiced on a wide scale. Certain procedures (drying, use of pesticides) are well proved and are used on an industrial scale, although in certain situations they are not without risk to human health. New techniques (controlled atmospheres, irradiation, fumigation) are in use and will undoubtedly see further development for conservation of seeds and cereals in both industrialized and non-industrialized countries. In the latter case, the use of cost-effective fumigants may be of particular value. Preventive measures for the control of molds in foodstuffs are discussed in this report.

We propose a state-space model-based approach to the simultaneous estimation of subcarrier fading and symbol quasicoherent detection of multilevel modulated signals in OFDM systems. The receiver uses two low-order Kalman-Bucy filters (KBFs) per mixed subcarrier (data+pilot symbols) and a minimization criterion based on the filters innovations to detect the symbol and estimate the (complex) channel gain. With this scheme we avoid the computational complexity of the conventional minimum mean-squared error solutions. Moreover, since the KBFs estimate the channel continuously no extra time-interpolation algorithm is needed and the amount of pilot symbols can be substantially reduced. Simulation results are presented for 16-APSK in Rayleigh channels affected by residual frequency offset (due to imperfect carrier frequency synchronization).

Little is known about carriage of Candida albicans, the predominant pathogenic yeast in oral infection, in children. We cultured buccal mucosal and gingival swabs from 150 Portuguese children to investigate the prevalence of C. albicans at baseline (before dental treatment), post-treatment, and 12, 24, and 36 months post-baseline. The children, aged 8 to 11 years at baseline, had no systemic disease or clinical symptoms of oral candidiasis. At each successive visit, respectively, 47, 32, 21, 27, and 28% of children were C. albicans positive, resulting in an almost 50% reduction in prevalence from baseline to post-treatment (P0.0005). Children who carried C. albicans at one visit had 3 to 20 times greater odds of carrying C. albicans at another visit. C. albicans was cultured from 12 children at all time-points and from 10 children at four time-points. Children with oral C. albicans frequently maintained carriage over time, even with regular dental care.

The aim of this study was to evaluate the short-term radiological and functional outcome of surgical treatment for symptomatic, low-grade, adult isthmic spondylolisthesis. Twelve patients underwent a monosegmental fusion for symptomatic spondylolisthesis. Posterior reduction with pedicle screw instrumentation was followed by second-stage anterior interbody fusion with a cage. All patients underwent a decompressive laminectomy. At an average of 2.1 (range 1.4–3.0) years following surgery, all patients completed the Oswestry questionnaire, VAS back pain score and a questionnaire detailing their work status. Radiographs were evaluated for maintenance of reduction and fusion. The patients (nine male, three female; mean age 42, range 22–54 years) had experienced preoperative symptoms for an average of 38 (range 6–96) months. An average preoperative slip of 21% (range 11–36%) was reduced to 7% (range 0–17%). Reduction of slip was maintained at latest follow-up, at which time the average VAS score was 2.8 (range 0–8) and the average Oswestry score was 13 (range 0–32). All patients achieved a successful fusion. There were no postoperative nerve root deficits. All patients stated that they would be prepared to undergo the same procedure again if required. Seventy-five percent returned to their pre-symptom work status. Our findings suggest that posterior reduction and anterior fusion for low-grade adult isthmic spondylolisthesis may yield good functional short-term results. A high fusion rate and maintenance of reduction with a low complication rate may be expected. Further follow-up is necessary to evaluate long-term outcome.

Twenty-nine patients with pyogenic vertebral osteomyelitis were reviewed retrospectively after an average follow-up of 3.7 years. We identified 17 patients with predisposing factors (10 diabetes, 4 urinary tract infection, 2 HIV-positive, 1 rheumatoid arthritis). No patient presented with a febrile illness. The lumbar spine was involved in 15 patients. Eighteen patients had neurological impairment at presentation. Eleven patients who were neurologically intact had needle biopsies and the remaining 18 patients who were neurologically compromised had an open decompression. Staphylococcus aureus was cultured in 14 patients. Although spinal tuberculosis is relatively common in our environment it is important to obtain a tissue diagnosis in order to exclude pyogenic vertebral osteitis.

Phosphine is a food fumigant, used until now as an insecticide and rodenticide. The present work researches the action of phosphine treatment on growth and sterigmatocystin production of several fungal strains. Production of sterigmatocystin decreased by a ratio of 100 to 500 or was abolished.

This paper describes a methodology for analysis of dual linearly polarized radar data for the prediction of path attenuation at 10–30 GHz from long term point rainfall data. Detailed knowledge of the physical structure of precipitation in conjunction with scattering theory is used to provide attenuation predictions.

Levels of sterigmatocystin in cultures of various fungal strains were determined by high pressure liquid chromatography (HPLC). This mycotoxin was well separated using acetonitrile / water / acetic acid (55/45/2) as solvent on a μ-Bondapak C18 column (Waters Associates). It was detected by its absorbance at 320 nm. Using this method, sterigmatocystin can thus be simply and rapidly determined in mould strain cultures.

Production of aflatoxins by Aspergillus ruber THOM and CHURCH was first reported by KULIK and HOLADAY (1967), although these results have lacked confirmation. In this paper we provide evidence that this fungal strain produces aflatoxins. This finding has implications for food hygiene, especially in countries where such moulds are used in the preparation of foodstuffs.

The examinations were performed in 418 patients with malignancy or an ulcer of the stomach. In all cases the Cancer Registry of Norway was asked for the final diagnosis and date of death. The diagnosis of malignancy was based on histology on operated or autopsy specimens, and the surviving cases were re-examined by answering a questionnaire. At the time of the primary examination a total of 103 cases of malignancies were found, of whom 41 were women. 84 patients had not been operated on previously. Among these 7 cases of early carcinoma were found. In 19 patients a partial gastrectomy had been performed. A false diagnosis of benign lesion was given on biopsy in 2 per cent, by gastroscopy in 8, and by X-ray examination in 31 per cent. A definite diagnosis of malignancy was made correctly by the same procedures in 95, 53 and 16 per cent, respectively. After 30 months of observation 16 cases of malignant lesions were reported among the patients (315) who had an ulcer of the stomach with benign histology on directed biopsy at the primary examination. A false benign diagnosis had been made in 33 per cent of 119 with malignancy by X-ray examination, in 10 per cent by gastroscopy, and in 15 per cent by histology on biopsy. The corresponding percentages in the total material of cases with benign and malignant lesions were 10.2, 2.9, and 4.3, respectively. In cases with previous partial gastrectomy, and in those with early cancers much higher rates of false negative diagnosis of malignancy has so far been suggested in 12 per cent of cases with benign ulcers, a gastroscopical diagnosis has been suggested in 22 per cent, and a bioptical in none. A combination of diagnostic procedures provided a correct diagnosis of malignancy in 118 out of 119 patients.

Phosphine is a food fumigant, used until now as an insecticide and rodenticide. The present work researches the action of phosphine treatment on growth and aflatoxin production of 23 Aspergillus strains. Production of aflatoxins B1, B2, G1, and G2 decreased in almost all cases by a ratio of 10 to 100. Phosphine treatment therefore seems favorable to prevent growth of various Aspergillus strains, in the context of keeping food safe.

This paper proposes an importance sampling methodology based on large deviations theory for the error floor assessment of a class of open-loop receivers with random carrier phase tracking in additive white Gaussian noise channels. Our simulations exhibit remarkable time gains when compared with conventional Monte Carlo.

Bismuth preparations are used world-wide for the management of peptic ulcer disease, for eradication of Helicobacter pylori, and in the prevention and treatment of diarrhoea. However neurological toxicity of bismuth has always been a major concern and evidence has been found of the absorption of bismuth. Recent studies have suggested that the absorption of bismuth increases when bismuth salts are used with ranitidine hydrochloride. The absorption and deposition of bismuth as a result of the use of the new drug ranitidine bismuth citrate have not been yet clarified. After 15 days of twice daily oral gavage with bismuth subcitrate, 13·7 mg kg−1 day−1 to eight rats, deposition of bismuth was found in all the tissues studied, especially the kidney (30·81 ± 8·59 μg g−1 dry weight). A similar pattern of distribution and tissue concentrations was found when bismuth subcitrate was given with ranitidine hydrochloride 8·6 mg kg−1 day−1 to another eight rats, although this combination resulted in lower brain levels (3·12 + 1·31 μg g−1 dry weight) than after administration of bismuth subcitrate alone (4·77 ± 0·97 μg g−1 dry weight). When six rats were given ranitidine bismuth citrate by gavage at 22·8 mg kg−1 day−1 for 15 days, kidney levels were lower (4·24 ± 1·75 μg g−1 dry weight) and brain levels were below detection limits; the bismuth concentrations in the faeces from this group were also significantly lower (1603 ± 104·0 μg g−1 dry weight) than for the two other groups. After dosing with bismuth alone or in association with ranitidine hydrochloride, bismuth was detected in several organs and deposition was not influenced by gastric pH. Blood levels correlate poorly with organ deposition and brain deposition was not always associated with encephalopathy. After administration of ranitidine bismuth citrate, significantly lower concentrations of bismuth were found in the kidney and bismuth was not detectable in the brain, suggesting lower bismuth absorption. This was confirmed by higher levels in the faeces after dosing with ranitidine bismuth citrate. Thirty days after dosing with ranitidine bismuth citrate or bismuth subcitrate, bismuth could not be detected in any of the organs examined but could be found in the urine. In conclusion, bismuth was deposited in the kidney, brain, lung and liver of rats after oral dosing with bismuth subcitrate. After oral dosing with an equivalent amount of bismuth in the form of ranitidine bismuth citrate, significantly lower concentrations of bismuth were deposited in the kidney; in the brain bismuth was not detectable.

This paper explores how structural properties of small firms networks emerge from the ways firms exchange knowledge. In particular we are interested in analyzing if and under which conditions the need for knowledge exchange within a set of co-located small firms is able to generate a more or less stable structure of links among firms. The general research question we intend to answer is the following: is there any correlation between the determinants of firms knowledge exchange in a cluster and the structural characteristics of the network? More specifically, we focus on a specific kind of small firms networks called industrial districts (IDs). One of the peculiar characteristics of IDs is flexible specialization: small firms specialize in given phases of the production processes and join up production chains in a flexible and dynamic way depending on market opportunities. Consequently knowledge exchange is mainly related to the matching of complementary know-how and competencies. Our specific research question is related to the relation between knowledge complementariness and network structure: can knowledge complementariness explain the emergence of a stable networks of firms and under which conditions?