Revision for PJI after total hip replacement: more exploration is needed – Authors’ reply

We thank Chao Tu and colleagues for their interest in our paper. Measurement of intraoperative blood loss in total hip replacement is known to be unreliable. A wide variety of formulae can be used to calculate total blood loss (including hidden blood loss), but they are based on differing assumptions and yield heterogenous results.

The results of the 1991 study

cited in our Article, which reported higher blood loss was associated with the lateral approach, are consistent with the findings of the a 2004 Cochrane Review

comparing the posterior and lateral approaches. More recent studies

^, 

have focused on minimally invasive approaches or the anterior approach. In our experience, the more extensive muscle dissection of the lateral approach is associated with a higher blood loss than the posterior approach. Blood loss data is not collected for the National Joint Registry dataset and we are not aware of any national registries that do so.

Tu and colleagues state that generally older patients have less competent immune systems than younger patients, but the study cited by the authors to support this is a case control study comparing 45 patients who had total hip and total knee replacement with PJI with 252 controls in which no assessment of immune status was done. Older patients might have impaired immune status in comparison to younger patients, but younger patients who undergo total hip replacement might not be generalisable to young people without total hip replacement. We have included the American Society of Anaesthesiologists grade, which would reflect immune status, as an adjustment variable in our analysis. It is important to note that we have reported on the risk of revision for prosthetic joint infection (PJI) and the willingness to do major revision surgery might decline as age increases.

We agree with Tu and colleagues about the changing criteria for the diagnosis of PJI over time. There is no gold standard set of diagnostic criteria that could be applied over a period long enough to collect evidence on a cohort of this magnitude and criteria compliance data would be difficult to collect in this setting. We believe that the most important determinant for PJI diagnosis is that the patient had sufficient symptoms, signs, and diagnostic results for the surgeon to recommend this highly invasive procedure, which has substantial risks. As stated in our Article, the criteria applied would have been contemporary to the time at which the procedure was done.

The views expressed are those of the authors and not necessarily those of the National Institute of Health Research (NIHR), National Health Service, or the Department of Health and Social Care. MRW reports speaker fees awarded to his institution from Heraeus and DePuy outside the submitted work. EL reports grants from Becton Dickinson, Limbs & Things, and Ferrings. AWB reports grants from NIHR during the study, and grants from Stryker outside the submitted work. This manuscript presents an independent reply from researchers funded by the National Institute for Health Research under its Programme Grants for Applied Research programme (RP-PG-1210–12005), supported by the NIHR Biomedical Research Centre at the University Hospitals Bristol NHS Foundation Trust and the University of Bristol.