Total knee arthroplasty is a surgical procedure that offers very good results with pain relief and restoration of function . Infection is one of the most feared complications and is the most important cause of TKA revision. The proper management of the infected TKA usually requires new surgical procedures, prolonged hospital stays and prolonged antibiotic treatment and therefore, it is important to know what are the risk factors of the infection for its control [16,17].
In our country, the infection rate of knee arthroplasties is higher than those reported in other countries [4,17,18]. The data of infection rates in primary TKA presented in arthroplasty registers from some countries show infection rates less than 2% [19–21] , in our country it is around 2% . The actual incidence of prosthetic knee infection may not be well analyzed. Several factors can introduce important biases  .Several risk factors have been linked to the onset of the knee prosthesis infection. Some factors are related to biodemographic characteristics. Many authors find that factors related to the patient are the most important risk factors in the onset of infection . The most important biodemographic factors related to TKA infection were ethnicity, obesity, diabetes, rheumatoid arthritis and comorbidities . Other factors that may be involved in the onset of infection are surgical factors, such as type of cement used, surgical time, ischemia time, antibiotic prophylaxis, surgeon experience and operating room environment . Several authors have conducted studies about the risks factors for infection in TKA. There is no consensus for all the factors and there are even some contradictory data, but, it seems evident that there are some risk factors for prosthetic infection that should be corrected. Some of these factors are related to a greater susceptibility to infection such as steroid treatment or rheumatoid arthritis where we know that there may be some immunosuppression.
We have found significant differences between the two study groups for the risk factors indicated in table 1. These risk factors that are related to infection are: cement type, obesity, ASA grade and diabetes. In the case of surgical time and ischemia time, when a regression analysis was performed, the statistically significant association disappears.
Patient risks factors
In the case of patient risks factors, we find that diabetes, obesity an ASA score are statistically significant with TKA infection. Glycemic control was an important factor to preventing surgical site infection. Some studies have pointed out the relationship between diabetes and the increased risk of infection of arthroplasties [6,8,24,25] . For the obesity we find also a relationship with infection. Obesity is also a factor related to the development of periprosthetic infection of the knee, especially if it is associated with diabetes [24,26]. In a large study about risk factors for PJI in Knee arthroplasty Namba et al  finds a statistically significant relationship between obesity, diabetes, ASA grade and the risk of periprosthetic infection of the knee. Other risk factors identified in this study were the male gender and age. In our study, we did not find a relationship between gender and age with a higher risk of periprosthetic knee infection. But it must be borne in mind that the ASA grade is a reflection in part of the comorbidity suffered by the patient. On these three factors, it is possible to establish control measures, especially the glycemic normalization and weight loss. Among the patient factors, age and male gender have been indicated by some authors . In our study we have not found that these factors have an influence on the appearance of the infection. We did not find a relationship between urban or rural patient origin and PJI.
Surgical risks factors
For the surgical factors we find that only cement type have effect in the risk of PJI in knee arthroplasty. It would seem logical to think that the antibiotic-laden cement had a protective effect on infection appearance, but this fact has not been verified. The relationship between the use of cement with antibiotics and the risk of periprosthetic infection has been analyzed by several authors. Namba et al  in his large study does not find that the antibiotic-laden cement has a protective effect on the appearance of the periprosthetic infection, rather on the contrary. This author indicates that perhaps the patients with the highest risk of infection are those who receive cement with antibiotics. Tayton et al.  in a study from New Zealand Joint Registry find similar results. This author finds that the use of antibiotic loaded cement is a factor associated with the appearance of a periprosthetic infection a six month after knee arthroplasty. Bhom et al  in a study with data from the Canadian registry finds that the use of cement with antibiotics has no effect on the revision rate after total knee replacement. Similarly, Hinarejos et al.  finds no difference between the use of cement with or without antibiotic and the risk of periprosthetic infection after total knee arthroplasty. Our results differ from those previously mentioned and it is difficult to find a cause that explains this disparity. In our study, the two groups presented similar basal characteristics and the use of cement with antibiotics or not depend on a decision of the main surgeon.
Other surgical risk factors studied were surgical time and ischemia time. In our study we have not found a relationship between both factors and the development of an periprosthetic infection when we conducted a logistic regression study. But,the duration of the surgical procedure has been linked to a higher risk of infection. Some authors have found a relationship between surgical time and PJI after knee arthroplasty [10,29]. These authors have even pointed out that times greater than 127 minutes may be associated with an increased risk of infection.
We think that modifiable risk factors were specifically relevant and for that reason we can reduce the infection rate.
This study presents some limitations. The size of the sample can be considered small, although this is a less important factor because it is a case-control study. Likewise, the type of statistical study can solve the smallest sample size. In the study of the risk factors related to infection, various statistical tools have been used. In some cases, some of the factors that were initially shown as relevant, lost their significance in another type of analysis. In this case we believe that the study of attributable risk fraction is interesting because it can offer us more evident data of the impact of a risk factor in the appearance or not of a clinical event, in this case the infection of the TKA. In our opinion, the findings of risk measures are interesting, especially the attributable risk that informs us of the impact of each risk factor in the incidence of infection, both for the study population and for the general population. To our knowledge, this is the first study that has presented this data. Our study provides evidence that diabetes, cement type, ASA and BMI were independently associated with increased risk of PJI for TKA patients in a Spanish population. These results allow us to establish some preventive measures of periprosthetic infection such as weight loss, glycemic control, comorbidity control, in addition to those already established in general terms such as antibiotic prophylaxis and skin preparation.