In a dog model, trauma alone was insufficient to induce granulation tissue, but trauma and bacterial contamination led to granulation tissue. Human studies have shown an association between positive cultures and the presence of granulation tissue, and airway stenting has been found to be associated with frequent bacterial colonization. However, whether infections lead to granulation tissue or whether granulation tissue leads to obstruction and subsequent infection is less clear. This situation is termed reverse causation—when the outcome affects the exposure, rather than the reverse. Previous studies have been unable to address this issue because they correlated cultures with the concurrent presence or absence of granulation tissue. By using time-varying covariates, we were able to address the issue of reverse causation Kamagra Australia here. Our results demonstrate that prior infections are a risk factor for subsequent granulation tissue formation. 
This is consistent with our observation that Aero stents are associated with a higher incidence of both infection and granulation tissue. In the multivariable model for granulation tissue, after controlling for infection, the Aero stent was not associated with granulation tissue. This finding suggests that the infections associated with the Aero stent drive granulation tissue development, rather than some other structural issue causing trauma. In contrast, silicone stents, which are associated with increased migration, are associated with granulation tissue formation even after adjusting for infection. So both repetitive motion trauma and infection play a role.
This study has some limitations. As with all retrospective studies, residual confounding may be present. Selection bias is also a possibility. Expandable metal stents are easier to place than silicone stents, so it may be that sicker patients with more severe airway obstruction were more likely to have metal stents. Similarly, it may be that physicians placed silicone stents in patients they believed were likely to respond to treatment, since silicone stents are easier to remove.