Our work on the correlation between patient-related risk factors and SSSI or PJI includes a large cohort of primary hip and knee arthroplasties, with careful follow-up of each included patient. This study shows knee surgery, age over 65, high ASA and high BMI as independent and significant risk factors for developing SSSI or PJI.
The occurrence of SSSI may increase the risk of subsequent PJI by up to 35 times (9). If it is possible to identify high risk patients in order to optimise or even avoid patient-related risk factors preoperatively, we may be able to decrease the risk of subsequent PJI. Superficial wound complications are associated with patient morbidity and higher costs to the health-care system due to prolonged hospital stay, re-admission, ongoing treatments, and reduced patient satisfaction (9, 26, 27). The rate of SSSI (7%) and PJI (1,9%) shown in this study is consistent with international studies showing levels of SSSI ranging from 1% to 10% (7, 28) and PJI ranging from 0.2% to 2.23% (11, 12, 14).
The results in this current study where knee surgery was found to be a statistically significant risk factor for SSSI and PJI are in line with results from earlier studies, i.e. a meta-analysis including 2,000,000 patients (37 studies) by Resende et al. (29). Patients with knee prostheses have shown a higher rate of PJI and are known to be in greater need of revision surgery than patients with hip prostheses (12, 15, 18, 29). There is less soft tissue around the knee than around the hip, which means a shorter distance between skin and joint. Blood circulation around the knee area is more exposed to impact as opposed to around the hip area, and the perfusion is easier to disturb.
Age was also found to be a significant risk factor for SSSI, which is congruent with results from a retrospective study by Carroll et al., including 1,000 patients (7). It would be reasonable to assume that an elderly patient has pre-existing medical conditions and fragile skin that can impair wound healing and cause SSSI.
A high ASA class was shown to be a significant risk factor for SSSI in univariate analysis but not shown significant after adjustment for the other covariates. The correlation between a high ASA class and SSSI may be explained by the fact that ASA class encapsulates several other known risk factors including smoking, DM, and obesity. Each of these, on their own, have been associated with a higher risk of surgical site infection as a result of tissue hypoperfusion and subsequent impaired immunological function (19, 30). All of these factors may harm wound healing and skin perfusion. Skin impairment may lead to an SSSI, and skin perfusion must be kept robust to avoid wound complications (31).
Excess weight is generally a worrying factor, since obesity increases the risk of osteoarthritis, TJA and PJI (32). Multiple medical comorbidities, including DM type II, hypertension and cardiovascular diseases, are usually associated with obesity which consequently affects patients´ BMI and ASA class (33–35). In this study, BMI ≥ 35 was found to be a risk factor for SSSI and PJI. The association between BMI and postoperative wound complications may be explained by the need for a longer surgical incision, the increased risk of fat necrosis, prolonged or more complicated arthroplasty surgery (36), and prolonged postoperative wound drainage (37). The results in this current study are in line with large register-based studies, such as the study by Sayed-Noor et al. on 83,000 patients where the risk of re-operation within 2 to 5 years was shown to be increased for patients with higher BMI class (I-III) (38) and a study by Shohat et al. on 19,000 patients investigating the BMI cut-off threshold for association with an increased risk for PJI (39). No threshold for PJI was found (39) however a higher BMI class showed an increased risk of PJI.
Our study found male gender to be a risk factor for the progression of SSSI into PJI. In the meta-analysis by Resende et al. (29) male gender was found to be a risk factor for PJI and a subgroup analysis suggested that males were at greater risk of developing PJI, especially after TKA (29). This correlation may be attributed to some contributory behavioural factors such as smoking, diet, hygiene and alcohol consumption, but the reasons behind this are not clear. Gender-related differences in immune response due to bacteria such as Staphylococcus aureus and Pseudomonas aeruginosa are shown and septicaemia and bacteraemia are shown to occur more frequently in males than females (40) but whether or not this affects the development of SSSI or PJI has yet to be investigated.
Two of the main strengths of this study are the size of the cohort and the careful follow-up of each patient. This meticulous post-operative follow-up confirms that the number of recorded incidents of SSSI are accurate, and the follow-up time of five years, highlights any potential cases of PJI. Similar studies present a larger cohort but are based on registers only (41, 42) or have a shorter follow-up time (7, 8, 41). This study exclusively includes patients with primary elective joint surgery to minimise the influence of other risk factors concatenated with the initial trauma (hip fractures) or extended impact on the tissue (revision surgery). This is an additional strength to this study since the rate of PJI is known to be higher after trauma and revision surgery (43).
A potential limitation of this study lies in the fact that this is a retrospective study design and therefore it is possible that there may be inaccuracies or a degree of misinterpretation of information received from medical records. Another limitation is that SSSI is not culture-verified but determined by a medical assessment, which however reflects the clinical reality. An additional weakness of this study is that not all factors related to wound healing could be accounted for, such as nutrition, hygiene and wound care. This information was particularly difficult to obtain once patients had been discharged from the hospital. As in infection-related research in general, where the small number of infections is a well-known challenge, this study may have missed out on the correlation between a risk factor and postoperative infection, due to a statistical type II error. Though this cohort is large enough to represent the average number of elective hip and knee prostheses performed in a common hospital, but an even larger cohort would have been desirable.