Infections in Total Hip and Total Knee Arthroplasty: Development of a Score To Assess Endogenous Risk of Surgical Site Infections

Abstract

Background:

Surgical site infections (SSI) are a dreaded complication of total hip (THA) and knee arthroplasties (TKA), and are a major public health concern. Risk factors are well known, but no endogenous risk assessment score exists. The objective of this study to develop a score to assess endogenous risk of infection after THA or TKA.

Methods:

All infections after TKA and THA implanted in the department of orthopedic surgery of a teaching hospital between January 2007 and December 2012 were included. Two control groups were matched to cases on the type of prosthesis (hip or knee; first-line or revision).

Results:

Twenty-four SSIs after THA and 21 after TKA were registered (respective incidence during the study period: 1.56 and 1.91%). Relevant endogenous risk factors found were: Smoking (adjusted odds ratio=3.9), a BMI greater than 35 kg/mÇ (1.8), inflammatory rheumatism (7.3), and the number of operations (prosthetic or not) on the involved joint (2.9 per additional surgery). The average score of endogenous infection risk on all analyzed subjects was 3.37±3.33 (median=3, range=0-17). Mean scores were substantially different among cases and control groups: Respectively 5.84±4.04 vs 2.13±2.01 (p<0.0001). With a five-point threshold, the sensitivity and specificity of the score are respectively 62 and 91%. ASA score greater than or equal to three was not found to be substantial risk factor in this study (p=0.15).

Conclusions:

Endogenous infection risk score studied here was found to be relevant in discriminating cases from control groups, but requires validation in a larger cohort.

Joint arthroplasties have consistently improved the quality of life of patients suffering from degenerative joint disorders [1]. The number of joint prostheses (JP) implanted annually in France is estimated at 120,000 for the hip and 40,000 for the knee. This figure is set to increase in the coming years, given the aging population, better management practices, and thus expansion of indications.

Surgical site infections (SSI) occurring after the implementation of this type of material, although infrequent [2], can have a major impact on the functional outcome of the joint and the quality of life. Moreover, the impact of these SSIs is of major concern about individual health (morbidity, mortality, social and professional consequences) or public health (increased length of hospitalization, multiplication tests, overuse of antibiotics, costs) [3]. Thus, prevention of SSI in orthopedic surgery is a major objective in terms of individual and public health, and requires knowledge of specific risk factors in order to adapt their management.

The patient-related (endogenous) risk factors for SSI after any surgery include male gender [4], age [5], obesity (body mass index (BMI) greater than 35 kg/m² [6 –8], malnutrition [9], diabetes mellitus (10] smoking [7], and immunosuppression [10]. The National Nosocomial Infections Surveillance Score (NNISS) score is usually used to evaluate the post-operative risk of infection [11–12], but it is rather designed as an epidemiological score, used to compare infection rates between different healthcare institutions. Other risk factors are more specific for orthopedic surgery: Inflasmmatory rheumatism (rheumatoid arthritis, spondyloarthritis) [3,12–13], history of trauma or joint infections [10,14], recent corticosteroid joint injections [15].

Some risk factors—such as the notion of prosthesis revision or comorbidities not included in the ASA classification—do not appear in scores of risk assessment of SSI commonly used (as the NNISS). Other risk scores or scales could be developed for this purpose [16–17].

The main objective of this study was to develop a score to assess endogenous risk of surgical site infection after scheduled total hip or knee arthroplasties.

Patients and Methods

Study design

This study was a retrospective case-control study of data collected in an orthopedic department of a teaching hospital.

Surgical site infection (SSI) prevention protocol

Protocol for SSI prevention in the orthopedic department consisted of: A skin preparation of the patient (shower with antiseptic soap the day before and the day of an elective arthrolasty, and two separate antiseptic with polyvidone), antibiotic prophylaxis with cefuroxime 1.5g 30 min to 60 min before surgery, with a double-dose when BMI exceed 35Kg/m², and a second injection of 0.75g if the duration of the intervention exceeds 2 h or vancomycin at the dose of 15mg/kg in case of allergy to betalactam or colonization with methicillin-resistant Staphylococcus aureus. The operating theater is equipped with a laminar airflow. Gentamicin-loaded cement is used for total joint arthroplasty. Active surveillance and multi-modal interventions are performed by the infection control unit.

Inclusion criteria, case, and control group definition

Cases were defined as deep SSI occurring within the year after scheduled total hip arthroplasty (THA) and knee arthroplasty (TKA) in the Department of Orthopedic Surgery, Teaching Hospital of Reims, between January 1, 2007, and December 31, 2012. Deep SSI was defined according to the U.S. Centers for Disease Control and Prevention/National Healthcare Safety Network guidelines [18]. Each SSI was validated by a surgeon and led to an investigation.

For each case, two control groups were matched to the type of prosthesis (THA or TKA), first-line or revision) and period of hospitalization.

Data

Data were collected from patient records and collected on a standardized questionnaire developed for the study. Potential endogenous risk factors for SSI to collect in this work were determined after a review of the literature [3 –15]. Socio-demographic data (age, gender), co-morbidities (obesity, inflammatory rheumatism, diabetes mellitus, hypertension, immunosuppression), medical history on the involved joint (number of operations, number of prosthetic operations), and lifestyle (smoking, alcoholism, drug addiction) were studied.

Statistical analysis

The description of the data was performed using numbers and percentages for qualitative variables, averages, and standard deviations for quantitative variables. Comparison of cases and control groups was performed by Mantel-Haenszel Chi-square tests, stratified Student tests and Wilcoxon tests, for the univariate analysis; and by conditional logistic regression for multivariable analysis.

After developing the final model, a point scoring system was built, with a number of points assigned to each risk factor by using the odds ratio from the conditional logistic regression model. Endogenous risk scores were calculated for each subject by summing the odds ratio (OR) of relevant risk factors. Then these scores were compared by Mantel-Haenszel Chi-square tests and stratified Student tests.

Receiver Operating Characteristic Curve (ROC) was built to evaluate the score and determine the best threshold for discriminating cases and control groups.

The level of significance for all tests was 0.05. Data were entered into an EpiInfo database. Analyses were performed using SAS software 9.4 and R software 3.0.1 (SAS Institute Inc., Cary, NC).

Results

Infections

A total of 45 SSIs occurred at a mean of 17.2 d (10–365) after a THA (24/1679, 1.43%) or a TKA (21/1097, 1.91%) with an overall infection rate of 1.62%. Rates of SSI were greater after revision for THA (12/405, 2.96%) and for TKA (6/162, 3.7%) than in primary THA (12/1274, 0,94%) or in primary TKA (15/935, 1.6%).

Common causes of infections were Staphylococcus aureus (44% of isolates, none was methicillin resistant) and coagulase negative staphylococci (31% of isolates). Other organism included Enterobacteriaceae (6.7% of isolates), Propionibacterium acnes (9.9% of isolates), enterococci (6.7% of isolates) and corynebacteria (2.2% of isolates).

Each patient received two antibiotics for a total duration of 3 mo. Surgical treatment consisted of irrigation, debridement, and replacement of modular implant for 39 patients or of one- or two-stage exchange arthroplasty for six patients.

Risk factors

Results of the case-control analysis are summarized in Table 1. Substantial risk factors found with multivariable analysis were: Smoking (with an adjusted OR=3.91), BMI greater than 35 kg/m² (OR=1.84), presence of inflammatory rheumatism (OR=7.25), and the number of previous surgery (prosthetic or not) on the involved joint (OR=2.88).

Table 1.

Endogenous Risk Factors of Surgical Site Infections among Total Hip or Total Knee Arthroplasty

	Cases (n=45)	Control groups (n=90)	p	Adjusted odds ratio ^ [CI 95%]*	Point values of each item
Male (%)	30 (66.7)	40 (44.4)	0.92^$
Age >65	23 (51.1)	58 (64.4)	0.55^£
Severe obesity (IMC >35kg/m²) (%)	14 (31.1)	7 (7.8)	0.007^$	1.84 [1.11–3.05]	2
Number of prior operations of the joint (m±s)	0.84±0.80	0.50±0.57	0.018^£	2.88 [1.43–5.80]	3
Smoking (%)	18 (40.0)	9 (10.0)	0.032^$	3.91 [1.19–12.84]	4
Inflammatory rheumatism (%)	7 (15.6)	1 (1.1)	0.045^$	7.25 [1.06–49.61]	7
History of joint infection (%)	5 (11.1)	1 (1.1)	0.58^$
ASA ≥3 (%)	26 (57.8)	43 (47.8)	0.15^$
Immunosuppression (%)	5 (11.1)	4 (4.4)	0.68^$

Mantel-Haenszel Chi-square test.

Student test.

Conditional logistic regression.

CI=confidence interval; ASA=American Society of Anesthesiologists.

The point values assigned to each of the risk factors identified in the model were equal to their OR. Endogenous risk of infection after THA or TKA score was assigned to each participant by adding the points for each risk factor present. For instance, a non-smoker (zero), obese with BMI=37 (two) female with history of inflammatory rheumatism (seven), receiving a first THA (zero) would have a score of nine.

The average score of the whole analyzed population was 3.37±3.33 points (median score=three, range [0–17]). Table 2 shows the results of the comparison of cases and control groups: cases mean score was substantially greater than control groups mean score (5.84 vs 2.13, p<0.0001). Figure 1 shows the ROC curve built from these data.

FIG. 1.

Receiver Operating Characteristic Curve (ROC) curve.

Table 2.

Comparison of Endogenous Risk Factor Score for Deep Surgical Site Infection Cases and Control Groups

	Cases (n=45)	Control groups (n=90)	p	OR [CI 95%]
Mean score (m±s)	5.84±4.04	2.13±2.01	<0.0001	1.63 [1.30 - 2.06]
Median score [min–max]	5 [0–17]	3 [0–7]

OR=Odds ratio.

The sensitivities and specificities depending on the cut-off (threshold for classifying subjects in cases or control groups) are shown in Table 3: Best values were four points (sensitivity=64.4% and specificity=85.6%) and five points (sensitivity=62.2% and specificity=91.1%).

Table 3.

Sensitivity and Specificity of Endogenous Risk Factor Score For Deep Surgical Site Infections

Cut-off	Expected number^ of cases*	Expected number^ of control groups*	Sensitivity (%)	Specificity (%)
2	86	49	88.9	38.9
3	42	93	82.2	45.6
4	36	99	64.4	85.6
5	30	105	62.2	91.1
6	23	112	48.9	91.1
7	9	126	42.2	96.6

“Case” expected if score>cut-off; “Control group” expected if score ≤cut-off.

In the study population, correlation between observed and expected numbers (depending on the individual endogenous infection risk score in relation to the score cut-off) strength was 77% (Table 4).

Table 4.

Agreement Between Observed and Expected Population (Cut-off=5).

	Observed (%)
	Cases ^*	Control groups ^*
Expected^*
Cases^*	82 (60.7)	8 (5.9)
Control groups^*	23 (17.0)	22 (16.3)

“Case” expected if score>5 ; “Control group” expected if score ≤5.

Discussion

This study confirmed risk factors for SSI on THAs and TKAs, according to current literature data. Relevant endogenous risk factors in the analyzed population are: Smoking [7], severe obesity (defined as a BMI greater than 35 kg/m²) [6 –8], presence of an inflammatory rheumatism (such as rheumatoid arthritis or spondyloarthropathy) [3,12–13], and the number of previous surgery (prosthetic or not) on the involved joint [10].

Some well known endogenous risk factors were not observed here: Age, malnutrition, or diabetes mellitus, for instance. Presumably the small number of subjects included in this study did not allow sufficient power to highlight them. In addition, the analysis of some risk factors, such as malnutrition, has been limited by a large number of missing data. Malnutrition was defined as a recent serum albumin less than 35g/L (30 g/L in more than 70 y) [19] but this assay was rarely achieved in practice (92% missing). Diabetes mellitus was also found to be non-substantially associated with SSI in the study of Namba et al. [8].

In our study, ASA score and NNISS did not appear to be substantially related to SSI risk. Although they are well known risk factors, their limitations were highlighted in several recent studies [16–17]. The ASA classification is an indicator of perioperative morbidity and mortality, assessing the risk of anesthesia rather than the risk of surgery itself. It is included in the calculation of NNISS, which is the most widely used to assess the risk of infection after surgery [20]. But NNISS has not been developed for individual risk assessment: It is rather an epidemiological indicator, used to standardized rates of infections on the risk level of patients enrolled. Moreover, authors have shown that some comorbidities not included in the ASA score are real risk factors for SSI, particularly in prosthetic joint surgery [17]. Our findings confirm the importance of developing new tools for endogenous infection risk assessment.

We propose a score based on the risk factors found and mentioned above, giving each risk factor a number of points equal to its odds ratio. Cases have a substantially different score from control groups, and the proportions of cases and controls with a score “at risk” (greater than five, the threshold determined by building a ROC curve, for a sensitivity of 62% and a specificity of 91%) was also substantially different.

Such a score would permit an individualized screening of risk factors for SSI, taking into account specificities of arthroplasties and specific risk factors of each patient. It could also allow adapting and prioritizing pre-operative support (nutritional management, optimizing the glycemic control in diabetic patients), and targeting prosthetic indications in relation to expected infectious risk level.

Some risk factors such as inflammatory rheumatism or previous surgery cannot be modified because they are specific to the patient's history. However, smoking and severe obesity can be tracked and controlled especially on patients with other risk factors. All active smoker patients must be referred to a tobaccology consultation in order to stop smoking before surgery. In the same way, all obese patients should be included in a multi-disciplinary team with endocrinologist, nutritionist, and bariatric surgeon.

With the help of such a risk score, the orthopedic surgeon can determine which patient is at high risk of infection and can incite the patients to limit their risks. This score should be a part of the detailed information transmitted to the patient.

However, this score, constructed from a case-control study, needs to be checked and should be validated in a wider multi-center prospective cohort.

Conclusion

With a frequency lower than 5%, infectious complications after joint prosthetic surgery are rare. Nevertheless, they constitute a major public health concern, because of their socio-economic impact. Prevention requires a thorough knowledge of potential risk factors, favoring screening and targeted supporting, centered on individual risks, and involving physician, hospital care team, and the patient him or herself. ASA score and NNISS showed limitations in the evaluation of post-operative infection risk after joint prosthetic surgey. The development and validation of other scores could improve the assessment, and therefore help optimize prevention.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

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