the incidence of heterotopic ossification ( ho ) after primary hip arthroplasty varies between 8% and 90% in literature . the reason why heterotopic ossification develops is unknown , but it has been assumed that the inflammatory reaction due to the surgical trauma is the major driving force . the fact that the incidence of ho in patients receiving non steroidal anti - inflammatory drugs ( nsaids ) after a total hip arthroplasty ( tha ) is much lower compared to patients not receiving nsaids is a confirmation of this assumption . nsaids are still the cornerstone of ho prophylaxis , which are sometimes used in combination with radiation therapy in high risk patients [ 15 , 23 ] . multiple cytokines are involved in inflammatory reactions . the main pro - inflammatory cytokines interleukin-1 ( il-1 ) , interleukin-6 ( il-6 ) and tumour necrosis factor- ( tnf- ) are involved in the inflammatory response after injury . after elective surgery a temporary elevation of il-6 levels in serum or plasma has been observed [ 2 , 9 , 17 , 21 , 25 , 26 , 28 , 30 ] . increased levels of il-6 were also observed in patients having had total hip arthroplasty more then ten  years previously . even more important for orthopaedic surgeons is the finding that higher il-1  and tnf are also elevated in prosthetic loosening . as one of the most important anti - inflammatory cytokines , elevated serum levels of interleukin 10 ( il-10 ) after trauma have been shown [ 11 , 12 ] . for il-10 even a positive correlation between higher levels immediately after trauma and the occurrence of sepsis later in the course was reported . most reports on the increase of cytokines after trauma or surgery are based on serum levels . there are limited data about the cytokine concentrations at the site of surgery [ 2 , 26 , 29 ] . however , as cytokines are very effective in cell - to - cell communication , the concentration of cytokines locally at the surgical site is of major interest . furthermore , in most studies only the pro - inflammatory cytokines are measured , resulting in a lack of knowledge about the elevation of anti - inflammatory cytokines in drainage fluid . this study was conducted to investigate if the levels of different pro- and anti - inflammatory cytokines are measurable in drainage fluid after total hip arthroplasty and whether a difference in cytokine concentration between one and six hours postoperatively could be detected . samples from the drainage system were collected from 30 patients following total hip arthroplasty . all patients were treated according to our local protocol which included pre - operative antibiotics ( cefazoline 2 g i.v . ) and non steroidal anti - inflammatory drugs to prevent infection and heterotopic ossification , respectively . to facilitate taking samples from the standard hip drainage system we inserted this connector , which originally is a part from a suprapubic catheter system ( cook ; limerick , ireland ) , to the drain , on which we placed a three - way connector from an intravenous infusion system . a standard collection bag was placed at one end of the three - way system , while the other connection was used to collect the samples . in this way we were able to take samples from a closed suction drainage system at different time intervals instead of taking samples from the collection bag , which would have given an average of the time interval after centrifugation of the fluid at 10,000 rpm for five minutes , the supernatant was taken and analysed using the bioplex 17 system ( bio - rad , herculus , usa ) . gm - csf , g - csf , ifn- , tnf , mcp-1 , il-1beta , il-2 , il-4 , il-5 , il-6 , il-7 , il-8 , il-10 , il-12 , il-13 and mip-1beta levels were measured in the drainage fluids using commercially available kits according to the manufacturer 's instructions ( bio - rad , hercules , usa ) . cytokine levels were measured and analysed using the bio - plex system ( bio - rad , hercules , usa ) . the differences between the samples taken after one and six hours were compared using the paired sample t - test . in three of the 30 patients it was not possible to take samples after one hour , because the drain production was too low . in another eight patients , only one measurement was done because the wound drains clotted and stopped draining before the collection at six hours . the average drain production after one hour was 55  ml ( sd 31  ml ) and after six hours it was 268  ml ( sd 134  ml ) . we found a significant elevation of almost all cytokines between the samples after one hour and six hours postoperatively . in particular , elevation in this time interval of the concentrations of il- 6 and il-8 by a factor of 31.6 and 49.3 , respectively , is striking . calculating the il-6 to il-10 ratio , this ratio increased from 304 ( sd 256 ) to 12,357 ( sd 6788 ) ( p  <  0.000 ) , which shows an increased predominance of the pro - inflammatory cytokines when comparing the measurements after one and six hours , respectively . table  1cytokine concentrations in drainage fluidcytokineconcentration after 1 hour ( pg)concentration after 6 hours ( pg)significanceil-23352861p  <  0.000il-45,50545,910p  <  0.000il-6313,9149,916,640p  <  0.000il-810,615523,257p  <  0.000il-107971,012p  =  0.215il-1b4988,126p  <  0.000il-546197p  <  0.000il-7551965p  <  0.000il-123461,126p  <  0.000il-13103560p  <  0.000gm - csf2,28522,058p  =  0.001ifn-5,47750,363p  < 0.000tnf-1,24111,559p  <  0.000g - csf20,200348,667p  <  0.000mcp-134,086529,352p  <  0.000mip-1b19,44816,454p  =  0.392 cytokine concentrations in drainage fluid no ( negative ) correlation between the total drain volume and cytokine level was found , so the higher levels of cytokines are not caused by a concentration effect . high levels of numerous cytokines can be detected in drainage fluid after total hip arthroplasty . furthermore , the il-6 to il-10 ratio is increased between these two measurements , suggesting a relative rise in the pro - inflammatory interleukins . these cytokine levels were found despite the fact that all patients received nsaids before surgery to prevent heterotopic ossification . it would have been ideal to compare the levels of cytokines in drainage fluid in patients who did not receive nsaids . however , this was considered unethical due to our local protocol . to measure cytokine levels it is possible to analyse up to 100 different analytes in a single microtiter well , with a high correlation using the traditional elisa method . as it is almost impossible to measure the interstitial levels of cytokines , we believe the levels in drainage fluid are a better representation of these levels than the levels in serum . although cytokines in general function in cell - to - cell communication   and not at long distances , many studies describe cytokine levels in serum [ 8 , 17 , 30 ] . these systemic levels could represent either an over- or under - representation of the actual interstitial levels . found a difference in cytokine levels in chest tubes , and in the same patients they did not detect a difference in the serum levels . most studies in which cytokine levels are measured both in drainage fluid and in serum show comparable results , i.e. the levels in drainage fluid are higher than in serum [ 13 , 16 , 26 ] . furthermore , in most studies only the rise in interleukin level in the drainage fluid was significant , the levels in serum were not , or only for a subset of interleukins [ 1 , 2 , 16 , 26 ] . as pro - inflammatory interleukins ( mainly tnf- , il-1 and il-6 ) initiate elaboration and release other interleukins , anti - inflammatory cytokines ( il4 , il-10 and il-13 ) serve to down - regulate synthesis of pro - inflammatory cytokines . taniguchi et al .   showed that the ratio of il-6 and il-10 was correlated with injury severity after major trauma and it was recommended as a useful marker to predict the degree of injury following trauma . il-6 levels in serum correlate with disease severity and patient mortality in patients with sepsis [ 5 , 10 ] . di vita et al .   showed that in patients who underwent surgical incisional hernia repair the local levels of cytokines differed between the group in which a mesh was used as compared to the group with a direct repair . in the group in which a mesh was implanted , the il-1 production was higher , whereas the direct repair group showed higher levels of il-1ra and il-10 , which are anti - inflammatory cytokines . in mice il-1 stimulates bone formation and il-1-antagonist completely abolishes bone formation . il-4 and il-13 induce bone formation and the anti - inflammatory cytokine il-10 does not induce bone . the high levels of these cytokines found in our study in the drain fluid ( though not measurable in serum in former studies ) can be a clue to better understanding the pathogenesis of ho . in further research it would be preferable to include a reference group not receiving any nsaids .