intra - articular infections are deleterious for near - term joint function and cause pain . furthermore , they can lead to long - term osteoarthritic degradation , suggesting an association of acute inflammation and a catabolic cartilage metabolism . the regulatory mechanisms behind this are not yet fully understood . until now , most data available is about the association of chronic inflammatory diseases as rheumatoid arthritis and cartilage degradation , describing causal roles for proinflammatory cytokines such as tnf , il-6 , or death receptor 3 . furthermore , direct toxic effects of bacterial toxins on chondrocytes causing their death have been described . moreover , toll - like receptors ( tlr ) do exhibit both proinflammatory and catabolic effects mediated by the nf-b pathway in septic arthritis , an example for overlapping functions in regulatory pathways . the interactions between mediators upregulated during infection and cartilage metabolism need to be clarified in order to develop effective interventional strategies . therefore , the aim of this clinical trial was to correlate parameters defining the severity of inflammation with proteins characterizing cartilage degradation and metabolism . we hypothesized that there is a statistically significant association between certain clinical symptoms , synovial expression of inflammatory mediators , cartilage degradation , and cytokines with known importance in cartilage metabolism . the trial was registered ( missing , drks00003536 ) and approved by the ethics board of the university of freiburg ( an - ek - frbrg-50/11 ) . as already described in previous publications [ 4 , 5 ] , a consecutive series of 75 patients treated between april 2011 and november 2012 , presenting the clinical symptoms of bacterial joint infection , were recruited for the prospective collection of joint fluid . all patients suffered from pain , swelling , effusion , and elevated inflammatory serological parameters ( e.g. , c - reactive protein ) . infections of knee ( 75% ) , hip ( 6.6% ) , ankle ( 1.3% ) , and shoulder ( 17.1% ) were included . four other patients had to be excluded because of storage - protocol violations ( n = 3 ) or sudden death because of fulminant lung embolism ( n = 1 , no written consent ) . effusions were obtained within the first 24 hours after diagnosis during arthroscopy or preoperative puncture and immediately frozen . age ( time point of index diagnosis and puncture ) , sex ( male / female ) , body mass index ( bmi ) , and smoking habits ( yes / no ) were the epidemiological parameters characterizing the patients . for description of the septic constellation the following serum values were recorded : initial ( time point of diagnosis and puncture ) , maximal and final ( time point demission ) leucocyte counts , and c - reactive protein ( crp ) . furthermore , the degree of systemic inflammation was evaluated using the following parameters : necessity for an intensive care treatment , documented diagnosis of sepsis ( fulfilment of clinical sepsis criteria ) , fulfilment of clinical empyema criteria ( synovitis  grad 2 , detection of intra - articular bacteria , and clinical necessity of recurrent lavage ) , and necessity of in - hospital treatment . the comorbidity was assessed using the asa classification ( physical status according to the american society of anesthesiologists ) . furthermore , the preinfectious joint damage was recorded , which was defined as any articular lesion including osteoarthritis . the evaluation of the kellgren lawrence score   was based on conventional x - rays and done by 3 independent orthopedic surgeons , resulting in a consensus decision . 81% of isolated bacteria belonged to staphylococcus species ; in the subgroup of patients with knee infections , the portion of isolated staphylococcus species increased to 86% . cytokine and protein concentrations ( interleukin- [ il- ] 1 , il-10 , aggrecan [ acan ] , basic fibroblast growth factor [ bfgf ] , bone morphogenetic protein- [ bmp- ] 2 and bmp-7 , and collagen type 2 cleavage [ c2c ] ) in synovial joint fluids from acutely infected knee joints or coculture supernatants were analyzed by elisa ( rnd , minneapolis , mn , usa , and biosource deutschland gmbh , solingen , germany ) according to the manufacturers ' instructions . supernatants were applied to the wells and , after washing , an hrp - conjugated specific antibody was added to the wells . following the next wash , color development was proportional to the protein concentration and calculated by comparison with a standard . the bicinchoninic acid ( bca ) assay was available in kit form from pierce ( rockford , il , usa ) and used according to the manufacturer 's instructions . all data from the analyzed cytokines and proteins are reported as relative expression to the total protein content . concentrations of proteins and cytokines determined by the specific elisas and the bca assay were calculated according to the manufacturers ' instructions ( rnd , minneapolis , mn , usa ; thermo scientific , rockford , il , usa ) , creating a standard curve and reducing data using a four - parameter logistic ( 4-pl ) curve fit by using graphpad prism 5 software ( graphpad software , inc . , la jolla , ca , usa ) . all values were expressed as mean  standard error of the mean . regarding the scores and all numerical values , statistical significance multiple comparisons were calculated using a post hoc statistics based on the h - test according to kruskall - wallis . correlations were determined by calculating the spearman coefficient (  ) for the predominantly not normally distributed values . a cluster analysis with a simple agglomeration method was used for grouping values of registered cases on the basis of minimal distances between group members . this in combination with a percentile analysis or known boarders of values ( leucocyte counts and crp ) was used to group values . the average age was 59.7  2.5 years , the average bmi was 27.8  0.7 , and the ratio of female and male patients was 67.1%/32.9% . the distribution of the asa classification , characterizing the patient 's comorbidity , was as follows : group 1 : 26.3% , group 2 : 27.6% , group 3 : 42.1% , and group 4 : 3.9% . in order to gain a more homogenous population , the average age of this group was 57.2  3.1 years , the average bmi was 27.5  0.9 , and the ratio of female and male patients was 73.7%/26.3% ( no statistically significant difference between the analysis groups for each item ) . the distribution of the asa classification , characterizing the patient 's comorbidity , was as follows : group 1 : 29.8% , group 2 : 28.1% , group 3 : 38.6% , and group 4 : 3.5% . all included patients survived , and all infections were successfully treated ; two times an arthrodesis was necessary . based on a cluster analysis , the intra - articular il-1 levels were regarded as increased when the relative expression was higher than 0.005 . the necessity of intensive care or an in - hospital treatment was associated with increased synovial il-1 levels in all patients and in the subgroup of septic knee arthritis . in contrast , in case of a clinically documented sepsis , intra - articular il-1 concentrations were not elevated . if the patients fulfilled the clinical empyema criteria , the majority had synovial il-1 levels above the median . furthermore , a preinfectious joint damage , which was defined as any lesion including osteoarthritis , predisposed to higher intra - articular il-1 concentrations indicating a higher degree of inflammation . the comorbidity did not influence the degree of inflammatory response ; there was no correlation of any crp level or the synovial il-1 concentrations with the asa classification , and the comorbidity was equally distributed in the il-1 clusters in both all and knee infections . to describe the association of joint inflammation and the systemic inflammatory reaction , serum and blood levels of c - reactive protein ( crp ) and leucocyte counts were correlated with synovial il-1 levels . the course of septic arthritis was characterized using the initial , the maximal , and the final ( discharge ) systemic values . there was no association with the assessments at the time point of discharge , which was expected , because the inflammation was successfully treated . however , both initial and maximal crp serum concentrations and leucocyte counts correlated with intra - articular il-1 concentrations . nitrotyrosine ( no - tyr ) is a known marker of inflammation and no production , which is also known to be associated with cartilage destruction . no - tyr failed to show any correlation to clinical , serological , or other secretory inflammatory markers . degradation of cartilage is characterized by the release of extracellular matrix products as collagen ( collagen type 2 cleavage c2c ) or aggrecan . there was a statistically significant correlation of synovial aggrecan (  = 0.30 , p = 0.006 ) and c2c levels (  = 0.34 , p = 0.002 ) with initial serum leucocyte counts in all patients . in the subgroup of knee infections only correlations with c2c concentrations reached statistical significance (  = 0.45 , p = 0.0005 ) , which could also be shown for the leucocyte count maximum (  = 0.41 , p = 0.002 ) . increased serum leucocytes at the time point of diagnosis were also associated with higher c2c - levels . this association reached statistical significance analyzing all patients and the subgroup of knee infections . reported there was also a positive correlation of c2c concentrations with intra - articular il-1 levels (  = 0.34 , p = 0.001 ) , which also characterize synovial inflammation . an increasing kellgren lawrence score ( kls ) , which radiologically defines the progress of osteoarthritis ( oa ) , was associated with enhanced c2c levels in septic knee arthritis ( p = 0.049 , figure 2 ) . the distribution of kls was 18.0% grade 0 , 34.4% grade 1 , 27.9% grade 2 , 18.0% grade 3 , and 1.6% grade 4 . oa was considered as a preinfectious joint damage , which was associated with a higher degree of inflammation ( synovial il-1 concentrations ) . although the regulation pattern was similar for aggrecan , the values failed to reach statistical significance . since intra - articular concentrations of il-1 characterized the clinical relevance and severity of septic arthritis , the correlations to cytokines with known roles in chondrocyte anabolism or catabolism were analyzed , searching for the interaction between inflammation and cartilage metabolism . whereas the synovial expressions of bfgf (  = 0.42 , p < 0.001 ) , bmp-2 (  = 0.37 , p = 0.002 ) , and bmp-7 (  = 0.49 , p < 0.0001 ) were positively associated with il-1 , no correlation could be shown for cd105 (  = 0.16 , n.s . ) . the same association was further calculated based on the different clusters of il-1 ( figure 3 ) , showing increasing concentrations of bfgf , bmp-2 , and bmp-7 with rising il-1 levels . the h - test , defining statistical significance for the cluster differences , was statistically significant only in case of bmp-7 ( p = 0.03 ) , therefore demonstrating the strongest dependency on proinflammatory conditions . there was no statistically significant correlation of il-1 and il-10 (  < 0.01 , n.s . ) . in order to analyze the influence of the bacteria species on the inflammatory reaction and cartilage destruction 3 groups were defined : no detection of bacteria ( group 1 , n = 44 ) , staphylococcus species ( group 2 , n = 23 ) , and other bacteria species ( group 3 , n = 9 ) . infections with staphylococcus species induced the highest expression of il-1 ( figure 4(a ) ) . an anova was used as a screening test , showing statistically significant differences only between group 1 ( no ) and group 2 ( staph . species ) , which was confirmed by the direct comparison ( p < 0.001 ) . the anti - inflammatory il-10 showed a different regulatory pattern with highest levels in the group with other bacteria ( figure 4(b ) ) . although the anova showed statistically significant differences between group 3 and both other groups , the direct comparison could only confirm this for groups 1 and 3 ( p = 0.02 ) . c2c and aggrecan , both indicating cartilage breakdown , demonstrated the same associations as il-10 , showing the highest concentration after infections with other bacterial strains ( group 3 ) . although the anova showed again statistically significant differences between group 3 and both other groups , the direct comparison could only confirm this for groups 1 and 3 ( p = 0.02 and 0.024 , resp . ) . there were no differences for the expression of cd105 , bfgf , bmp-2 , and bmp-7 between the groups . the study 's main findings are that synovial il-1 levels appear to be a reliable tool to measure the severity and clinical relevance of septic arthritis . collagen type ii cleavage products ( c2c ) characterize cartilage degradation during acute joint infection better than other matrix breakdown products . moreover , there is a positive correlation of the degree of intra - articular inflammation with upregulation of bfgf , bmp-2 , and bmp-7 , representing markers of cartilage metabolism . bacterial septic arthritis is frequently found in patients with a high comorbidity , can lead to joint destruction , and is associated with a high mortality . synovial concentrations of the proinflammatory markers tnf , il-1 , and il-6 are elevated in the course of disease , in which tnf was a better indicator to discriminate bacterial arthritis from other inflammatory arthritis . the clinical relevance of intra - articular il-1 levels could be confirmed by our data , showing an association with systemic inflammatory parameters as serum leucocyte counts and serum crp - levels . this has been demonstrated before in the postoperative follow - up of patients undergoing surgical cartilage regenerating procedures . furthermore , elevated synovial il-1 levels were more frequently found in patients with necessity of intensive care or in - hospital treatment and clinical empyema signs . in contrast , this could not be shown for patients with clinical sepsis signs . this indicates that sepsis in general seems to be associated with physical conditions beyond joint infection . comorbidity , for example , associated with drug abuse or immunosuppression , which was not decisive in the course of joint infection , seems to play a more important role in these patients . the phenomena of synovial inflammation and cartilage destruction occur within the same time frame as shown by the parallel upregulation of inflammatory cytokines and breakdown products of the extracellular matrix . this was found in different studies investigating osteoarthritis , psoriatic arthritis , and trauma   in humans , and different other conditions including septic arthritis in animals . our data indicate that collagen type 2 cleavage products ( c2c ) , which possess pharmacodynamic and physiological relevance for cartilage degradation , are a more reliable parameter characterizing cartilage degradation in septic arthritis compared to aggrecan . however , the levels of this matrix component were closely related to the development of osteoarthritis without acute infection . the interactions between bacterial infection , upregulation of il-1 and il-6 , chondrocyte apoptosis , and cartilage degradation have been earlier described in vitro ; however , the analysis of the molecular basis of the interaction of proinflammatory parameters , matrix degradation , and cartilage metabolism in a human clinical trial was missing . on the other hand , the potential benefits of il-1 inhibition applying il-1 receptor - antagonist have been highlighted in a clinical trial attenuating the posttraumatic inflammation . furthermore , there are several studies investigating the possible beneficial effect for corticosteroids in septic arthritis showing promising data , but  similar to what is seen for specific inhibition of inflammatory mediators  current studies do not provide evidence of higher level for treatment efficacy . the hypothesis that there is a relation of acute bacterial inflammation and cartilage degradation as well as cartilage metabolism could now be confirmed presenting this biomarker analysis and may explain the deleterious long - term effects of acute inflammation on the joint integrity . previously , upregulation of both proinflammatory cytokines and cartilage metabolites has been shown in vivo following lps - stimulation of mononuclear cells . in this study not only inflammatory cells but also chondrocytes and synoviocytes have actively contributed to the development of inflammation . in fact , the prolonged effects of synoviocyte activation have been hallmarked as a reason for prolonged devastating cartilage effects after elimination of bacteria in other studies . our data indicate that joints with a preexisting intra - articular damage are more susceptible to inflammation . this is in line with results showing an anti - inflammatory effect of cartilage itself leading to fas ligand mediated protection of synovial fibroblasts . therefore , cartilage is not only target but also an attenuating effector in synovial inflammation . one advantage of in vitro studies is that they can describe cell - specific and time - dependent regulation patterns , which indicate early upregulation of il-1 and a delayed increase of bmp-7 levels . therefore , these cytokines represent initiation of inflammation and a secondary , postponed start of a regenerative phase . the results of our study support the idea that cytokine regulation during septic arthritis is specific for different strains of bacteria . with regard to statistical necessities , the infectious bacteria in this trial were summarized in groups , showing that inflammation caused by staphylococcus species is associated with a high il-1 expression , but less cartilage destruction . the mechanisms behind this effect can not be clarified based on this observational study . although the time of analysis was defined and prospectively correlated with clinical signs of inflammation , we only present values of a single sample without time courses . this cross - sectional character of the presented analysis certainly limits the interpretation of the results . furthermore , it is an observational study , which does not allow drawing conclusions with regard to biochemical or physiological interactions . the asa classification is a reliable instrument to classify the health status but does not evaluate different metabolic elimination characteristics , for example , seen in kidney or liver failure , which possibly also exhibits an influence on the concentrations of the measured cytokines . in conclusion , articular infection and synovial inflammation are bacteria - specific and have direct influence on cartilage metabolism . collagen type ii cleavage products reliably mark destruction , which is associated with upregulation of typical cartilage turnover cytokines as bfgf , bmp-2 , and bmp-7 .
background . 
 intra - articular infections can rapidly lead to osteoarthritic degradation . 
 the aim of this clinical biomarker analysis was to investigate the influence of inflammation on cartilage destruction and metabolism . 
 methods . 
 patients with acute joint infections were enrolled in a prospective clinical trial and the cytokine composition of effusions ( n = 76 ) was analyzed . 
 characteristics of epidemiology and disease severity were correlated with levels of cytokines with known roles in cartilage turnover and degradation . 
 results . 
 higher synovial il-1 concentrations were associated with clinical parameters indicating a higher disease severity ( p < 0.03 ) excluding the incidence of sepsis 
 . additionally , intra - articular il-1 levels correlated with inflammatory serum parameters as leucocyte counts ( lc ) and c - reactive protein concentrations ( p < 0.05 ) but not with age or comorbidity . 
 both higher lc and synovial il-1 levels were associated with increased intra - articular collagen type ii cleavage products ( c2c ) indicating cartilage degradation . 
 joints with preinfectious lesions had higher c2c levels . 
 intra - articular inflammation led to increased concentrations of typical cartilage metabolites as bfgf , bmp-2 , and bmp-7 . 
 infections with staphylococcus species induced higher il-1 expression but less cartilage destruction than other bacteria . 
 conclusion . 
 articular infections have bacteria - specific implications on cartilage metabolism . 
 collagen type ii cleavage products reliably mark destruction , which is associated with upregulation of typical cartilage turnover cytokines . 
 this trial is registered with drks00003536 , missing .