Seasonal effect on the incidence of post-operative wound complications after trauma-related surgery of the foot, ankle and lower leg

Baseline patient, injury and surgical characteristics were collected based on procedure. If one patient had multiple surgical procedures in the study period, these were counted separately. Bilateral surgery was also counted as two separate procedures since there would be two incisions with the potential to infect. The following patient’s baseline characteristics were collected: gender, age, height, weight, American Society of Anesthesiologists (ASA) classification, immune disorders or use of immunosuppressive medication, severe comorbidities (including Diabetes Mellitus, severe kidney failure defined as eGFR < 20), and intoxications (smoking, alcohol and drugs abuse).

The injury characteristics collected for fractures were: open/closed fracture, Gustilo classification and type of fracture, and for all injuries the inflicted body part:

Surgical characteristics were: previous procedure in same area, type of procedure (ORIF, arthrodesis, implant removal, other), the amount of days between trauma and procedure (for acute procedures only), duration of surgery in minutes, tourniquet use, the use/dose of prophylactic antibiotics, type of fixation (plates, nails and/or screws), use of bone void fillers, and amount of blood loss in ml. Post-surgical characteristics included the use of antibiotics, type of wound dressing (plaster cast, pressure bandage, vacuum system) and time to mobilization.

All procedures were categorized in the month of surgery and then coded as summer (June–July–August) or other seasons (September–May). Weather conditions for that time of year were obtained from the Dutch Weather Institute or “Koninklijk Nederlands Meteorologisch Instituut” (KNMI). Conditions entailed average temperatures, precipitation and hours of sun.

The primary outcome was the incidence of wound complications, defined as a SSI according to US Centers for Disease Control and Prevention (CDC) criteria [23] or another surgical wound healing complication (wound dehiscence), developed within 90 days after the surgical procedure. The term FRI was not used as a primary endpoint since this study also includes data from secondary/elective procedures (sometimes years after the initial fracture occurred) and the definition of FRI does not clearly describe a time frame for the diagnosis. Wound complications were diagnosed during follow-up visits, protocolled at 2 weeks, 6 weeks and 3 months after discharge from the hospital and retrospectively retrieved from the medical records. All patients were instructed upon discharge to contact the hospital sooner in case of suspicion of wound complications. In case of a complication, treatment was recorded as well, categorized as expectative treatment (wait and see), vacuum system, wound reopened (at outpatient clinic or ward), oral antibiotics, i.v. antibiotics or revision surgery.

Data were collected and analyzed using SPSS, Version 25.0 [24]. All clinically relevant patient, injury and (post-)surgical variables were univariately analyzed for differences between patients with and without wound complications to identify possible confounders. Chi-square analysis was used to identify differences for categorical variables. For continuous data that were normally distributed a student’s T-test was used, and when not normally distributed, a Mann Whitney U test. All (nearly) significantly differing variables (p < 0.2) were included in multivariate analysis to identify individual predictors for wound complications with manual backwards selection. To limit bias caused by missing data, multiple imputations was used for variables with (under 50%) missing data. Imputation was performed using predictive mean matching with 10 sets. Imputed variables were: height, weight, smoking, alcohol abuse, drugs abuse, ASA classification, previous surgery, prophylactic antibiotics, duration of surgery, tourniquet use, primary closure, wound dressing and days between injury and surgery (for acute procedures). The amount of blood loss was not imputed because of too many missing values. After imputation, the remaining individual predictors were added to multivariable logistic regression analyzing the effect of season of the number of wound complications. The pooled results of the ten imputed sets were used to estimate odds ratios and confidence intervals.

The aim of this study was to identify whether the season of surgery has an influence on the incidence of wound complications following trauma surgery of the lower leg, ankle and foot. This seasonality could not be confirmed in this study, contradictory to our hypothesis. No significant difference was found in the number of surgical wound complications between seasons. Moreover, in this cohort a (non-significant) peak of wound complications was seen in winter instead of summer.

To the best of our knowledge, there is only one other study which investigated seasonal effects in foot/ankle surgery. They retrospectively analyzed a large cohort of 17,939 patients undergoing orthopedic foot/ankle surgery and could not identify a statistically significant difference in wound complications between seasons, without any large confounding factors being present [15].

Previous literature on seasonality in orthopedic/trauma surgery has led to varying conclusions. Anthony et al. performed a large national retrospective cohort study on seasonal influence on 30 day infection rates after total knee (TKA) and total hip replacement (THA) surgery in the United States of America (USA). With over 750,000 included procedures, they concluded that the incidence of wound complications was highest in summer with an increase of SSIs of 24% in June compared to December, corrected for comorbidities and socioeconomic status [10]. Multiple authors have supported these findings in orthopedic and trauma surgery [9, 12, 14, 25]. However, there are also authors who did not find a difference in wound complication rates in orthopedic surgery [8, 20, 26]. This raises the question of whether or not the found effects of season are clinically relevant, even when they are statistically significant. Most of the mentioned studies reported on large populations, making it perhaps too easy to achieve statistical significance.

Possible seasonality in wound complications has been ascribed to skin colonization associated with warm weather [21, 25]. The incidence of gram positive but especially gram negative bacteria such as Escherichia coli or Klebsiella seems to correlate well with the temperature, with a higher incidence in warmer months [27‐31]. Sagi et al. compared the rate of wound complications in open fractures in seven different climate regions within the USA. With results of all seven regions combined they could not identify a difference in wound complications between seasons. For two out of seven regions a significant seasonal difference was found, with fall having the highest number of wound complications. Interestingly, the overall incidence of wound complications did vary significantly between climate regions (corrected for patient and injury characteristics) [19]. This leads to the hypothesis that a seasonal effect can only be found in certain climate regions. This theory is supported by Haws et al. who did not identify an overall difference in wound complications but did find an effect when comparing the wet-season with other seasons in tropical areas [26]. The temperate climate of the Netherlands might be an explanation for the fact that this study could not detect a seasonal difference in wound complications. Seasonality might only exist in climate zones with more extreme weather conditions.

Another possible explanation is the “July-effect”, a description for the start of the academic year when new (inexperienced) surgeons in training start. Previous studies have concluded that there is a significant difference in the amount of complications, morbidity, outcomes and mortality at the beginning of the academic year [32]. However, the same “July-effect” was also found in non-teaching hospitals, making training status less likely as a cause for the higher number of wound complications in summer [21]. In the presently studied hospital, surgeons in training start at varying time points during the year, making a “July-effect” unlikely to occur.

However, the reason for a difference in the incidence of wound complications between seasons is most likely multifactorial. Besides changes in temperature and bacterial flora of the skin, diet change, clothing, sunlight exposure (influence vit D or skin flora), exercise, amount of time spent inside with other people, duration and quality of sleep, exposure to diseases could also influence wound complications. Also the number of admissions of trauma surgery may vary due to weather influence such as temperature or precipitation [33].

Some limitations can be mentioned in this study. The first limitation is the retrospective study design, which limits the number of variables that can be reliably extracted from electronic patient records. This may have led to underreporting of confounding factors (such as “amount of blood loss”) but also of the primary outcome measure “wound complications”. Especially superficial SSIs or other minor wound healing problems may have been treated by a general practitioner and, therefore, not mentioned in patient’s hospital records. However, major wound complications needing re-admission or additional surgery were well documented. Another limitation is the heterogeneity of the study population. Although included patients have been limited to patients undergoing trauma-related surgery of foot, ankle or lower leg, there is still variation in the risk of wound complications between these locations of injury. Moreover, this study includes both elective and acute procedures and open as well as closed fractures. By reporting all potential risk factors and confounders of wound complications and incorporating them in the multivariate analysis we have tried to make it more transparent and limit the effect on the main research question.

This study may be subjected to a selection bias since it was carried out with data of a single center, specialized in complex foot/ankle traumas. This may have led to a higher overall complication percentage than one would expect. A single center study in a small country does have the advantage of similar weather conditions for each patient. Finally, due to the single center study design, the number of included patients might not be sufficient to achieve statistical significance. However, the odds that our findings with more wound complications in winter than in summer are purely coincidental and that with sufficient power we would have found an opposite effect (more wound complications in summer) are slim.