Article Text

Download PDFPDF

Impact of department volume on surgical site infections following arthroscopy, knee replacement or hip replacement
  1. Elisabeth Meyer1,2,
  2. Doris Weitzel-Kage1,2,
  3. Dorit Sohr1,2,
  4. Petra Gastmeier1,2
  1. 1Nationales Referenzzentrum (NRZ) für die Surveillance von nosokomialen Infektionen, Berlin, Germany
  2. 2Institut für Hygiene und Umweltmedizin, Charité-Universitätsmedizin, Berlin, Germany
  1. Correspondence to Dr Elisabeth Meyer, Institute of Hygiene and Environmental Medicine, Charité University Medicine Berlin, Hindenburgdamm 27, Berlin 12203, Germany; elisabeth.meyer{at}


Objective To examine the association between surgical department volume and the risk of surgical site infections (SSI) after orthopaedic procedures.

Background A minimum volume regulation of at least 50 knee replacements per year was implemented in 2006 in German surgical departments.

Methods SSI rates were obtained from Krankenhaus-Infektions-Surveillance-System, the German national nosocomial infections surveillance system (January 2003–June 2008). The authors analysed the data by linear regression models. The adjusted ORs were estimated based on general estimating equation models to assess the independent effect of department volume (low, ie, ≤50, medium, ie, >50 and ≤100, and high, ie, >100 procedures annually).

Results A total of 206 surgical departments performed 14 339 arthroscopies, 63 045 knee replacements and 43 180 hip replacements during the 5.5-year study period. SSI rates were significantly higher in departments with a procedure volume of ≤50 arthroscopies and knee replacements. A higher threshold of 100 procedures per year did lead to a significant decrease in SSI rates for all three procedures in the univariate analysis. The multivariate analysis showed that the risk of SSI in low volume departments was sevenfold higher for arthroscopies and twofold higher for knee replacement than in medium volume departments. SSI risk after hip replacement was significantly lower in high volume centres.

Conclusion The authors' findings offer some support for recommendations to concentrate arthroscopy and knee replacement in surgical departments with more than 50 procedures and hip replacement in departments with more than 100 procedures per year in order to reduce SSI.

  • Operation volume
  • nosocomial infection
  • outcome
  • surveillance of surgical site infections
  • adverse event
  • morbidity and mortality
  • patient outcomes
  • surgery

Statistics from

Request Permissions

If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.


It was 1869 when Sir James Simpson sent out questionnaires to country surgeons in England and Scotland.1 2 He wanted to know whether experienced surgeons had better outcomes after amputating limbs and whether mortality was lower in small or large hospitals. Indeed, he observed outcome differences: inexperienced surgeons (<6 amputations) reported a mortality of 22.7% after thigh amputation, whereas the mortality was only 16.5% for experienced surgeons.3 Newer studies also show that surgeons with a higher operation volume have lower mortalities than their colleagues with lower volumes, and that a higher operation volume in large hospitals is associated with better outcomes.4–10 For example, Katz et al reported that only 25% of knee replacements were carried out by surgeons who performed 12 or fewer procedures in the Medicare population annually, and only 11% were carried out in hospitals with an annual volume of 25 or less. Based on their study results, patients treated at hospitals and by surgeons with a greater volume have a lower risk of perioperative adverse events (including infection).4 They also found an association between higher annual caseload, and lower mortality and selected complications (eg, for dislocation but not for deep infection) for total hip replacement. This indicates that hospitals with a higher volume have better results, better quality and higher patient safety. Patient safety and the quality of surgery are major concerns for a national healthcare organisation. That was the reason why a regulation requiring a minimum volume of at least 50 knee replacements per year was implemented in 2006, for German surgical departments. Exceptions were allowed for emergency surgery, the construction of new units, new directors of existing units and areas where access to care choice is limited—for example, in rural settings with limited providers. This minimum volume regulation does not apply for arthroscopy or hip replacement.

Generally, infection is one of the most common complications after surgery.11 Surgical site infections (SSI) have dramatic consequences for patients and hospitals: depending on individual patient risk factors, they increase the morbidity and mortality of patients,12 prolong hospital stay and increase costs for patients and society.13 In contrast, death of the patient is the worst-case outcome of a surgical-site infection and, fortunately, rare. Nevertheless, many studies on hospital volume have analysed mortalities.14 15

For this reason, we analysed SSIs as a common complication after arthroscopy, and primary knee and hip replacement in the network of German hospitals that take part in the German surveillance system for nosocomial infections (Krankenhaus-Infektions-Surveillance-System).

The aim of the study was to examine whether hospital volume matters and has an effect on SSI after orthopaedic procedures.


The German national nosocomial infections surveillance system applies (with minor modifications) the definitions and methods used by the National Healthcare Safety Network (especially Centers for Disease Control definitions for SSI and risk stratification).16 17 Participation is voluntary. Active prospective surveillance of SSI is performed by trained staff who collect the following information on all patients: sex, year of birth, date of surgery, type of procedure, duration of procedure, American Society of Anaesthesiologists physical status clarification (score), wound contamination class (clean, clean/contaminated, dirty or infected wounds are classed as 1, 2, 3 and 4, respectively), elective or emergency procedure, date of infection and type of infection (superficial, deep or organ space). In addition, standardised SSI ratios are calculated as quotients of observed and expected nosocomial infections, all based on national reference data. The number of expected SSIs is calculated from the number of operations performed in the various risk categories, and reference data are calculated for the corresponding procedures.

The data analysed were provided by surgical departments, each of which performed at least 30 operations over a period of 5.5 years (from January 2003 to June 2008) in one of the following three orthopaedic procedure categories: hip prosthesis due to arthrosis, primary knee prosthesis and arthroscopy. Revision surgery was not included.

SSI rates were calculated for three groups according to the volume of procedures performed per year: (1) ≤50 procedures, (2) >50 procedures and ≤100 procedures, and (3) >100 procedures.

In the univariate analysis, SSIs per 100 procedures were stratified by volume of procedures per year and compared using the Fisher exact test.

Multivariate analyses were performed to control for potentially confounding variables: patient sex and age of the individual patients, NNIS risk index score and frequency of the operative procedure. The NNIS risk index is widely used and is applied specifically to each operation. It ranges from baseline (0) to 3 points, with 1 point given for each of the following: (1) a wound classified as a contaminated or dirty/infected wound; (2) duration of operation longer than the 75th percentile of length for the specific procedure; and (3) American Society of Anesthesiologists score of 3 or higher.

Logistic regression models represented the adjusted odds of acquiring SSI depending on patients' risk factors. Three thresholds of the department volume per year were used as exposure variables in the models. Since patients from one department are not statistically independent due to the same treatment policies, adjusted odds ratios (ORs) were estimated based on general estimating equation models which account for this clustering effect.18


The inclusion criteria were met by 206 surgical departments. They performed a total of 120 564 procedures during the 5.5-year study period (table 1).

Table 1

Number of procedures and number of procedures by the 25%, 50% and 75% percentile of the departments, January 2003–June 2008

The results of the univariate analysis (shown in table 2 and depicted in figure 1) revealed significant lower SSI rates in departments with a procedure volume of >100 for arthroscopy, knee and hip replacement. SSI after arthroscopy occurred more than nine times more often in departments with an operation volume below 50 per year compared with departments with an operation volume of more than 100 arthroscopies per year (2.16 vs 0.24). SSI after knee endosprothesis was more than twice as high in low-volume departments (<50 procedures). This cut-off was set because of the minimum volume regulation for knee replacement of 50 procedures per year. The SSI rate of hip replacement showed a concave curve: SSIs were highest in departments with a yearly volume between 51 and 100, were lower (although not statistically significant) in departments with a volume under 50 and were significantly lower in departments with more than 100 procedures per year.

Table 2

Number of procedures, surgical site infections and surgical site infection rate by department volume

Figure 1

Surgical site infection rate by department volume per year.

To determine if there are small ‘centres of excellence,’ we examined the SSI rates and department volume found in figure 2 and calculated the observed and expected SSI rates for all departments. If only statistically significant results are analysed, all observed SSI rates in low-volume departments were higher than expected rates for knee replacement and arthroscopy. For hip replacement, there was only one department with a significantly lower SSI rate than the expected SSI rate.

Figure 2

Surgical site infection (SSI) rate and median number of knee prostheses per year. One point can include more than one department.

The multivariate analyses confirmed a significantly higher SSI risk in departments performing <50 arthroscopies or knee replacements per year (detailed results in table 3) but not for hip replacement. However, it was shown to be protective to have hip replacements in departments with a procedure volume of more than 100. As far as the patient-based variables were concerned, NNIS risk index variables were significant risk factors, as was male sex for arthroscopy and for hip replacement. These findings are in accordance with other studies.19

Table 3

Adjusted ORs of surgical site infections: analysis of risk factors by logistic regression model using the general estimating equation method


The results of our study showed that patients treated in departments with 50 or fewer annual arthroscopies or knee replacements had significantly higher rates of SSI than in departments with >50 and ≤100. Such an effect was not seen for hip replacement, but fewer SSIs were found in departments performing more than 100 hip replacements per year.

We did not expect these clear-cut results. The most important decision-making body of the self-government in Germany, consisting of the national associations of doctors and dentists, the German Hospital Federation and the federal associations of health insurance funds, is the Federal Joint Committee (G-BA). Since 2004, the G-BA set an obligatory minimum quantity of procedures for four invasive procedures (liver-, kidney- and stem cell transplantation and complex oesophagus surgery).20 In 2006, this was expanded to include knee replacement. Hospitals are allowed to perform knee replacement only if they have at least 50 procedures per year. In G-BA studies, the risk-adjusted odds for suffering a SSI or secondary bleeding were highest in the group of hospitals with 50–100 procedures per year and a lower risk of complications only in hospitals which performed more than 200 procedures annually.21 The decision to set the threshold at 50 procedures per year was therefore not based on their study results but was probably based on health-policy considerations. Therefore, we were surprised that our results provided evidence for setting a threshold of at least 50 procedures per year.

Numerous studies reported a relationship between adverse events and hospital volume or size—a finding that is often explained by ‘practice makes perfect.’4 5 10 22 23 Interestingly, this relationship was not linear, but differed in our study by type of procedure, and it was most prominent for arthroscopies. The risk for suffering an SSI in departments with <50 arthroscopies per year was almost nine times higher than in medium-volume departments according to the univariate analysis; it was more than twice as high for knee replacements but was not significant for hip replacements. This possible difference between arthroscopy, knee replacement and hip replacement may be explained by several factors. Arthroscopies are performed increasingly in the outpatient setting, as reflected by the relatively small amount of hospital departments (n=25) providing data. Small samples tend to overweight outliers. More importantly, antibiotic prophylaxis is standard care in joint (hip and knee) implantation but not in arthroscopic knee surgery. In response to a questionnaire of perioperative management on outpatient knee arthroscopy, only 62% of German surgeons reported the routine use of an antibiotic prophylaxis.24 However, antibiotic prophylaxis as an important and not routinely applied confounder was not included in our analysis, and so we urge caution with these results (although significant) for arthroscopy.

With respect to knee replacement, the results of our study might contribute to the existing evidence that in primary knee replacement, a higher hospital volume is indeed associated with better patient outcome with respect to SSI. Moreover, we did not find any indicators that there are small ‘centres of excellence,’ that is, low volume centres with very low SSI rates. A recently published study of German hospitals before and after the implementation of the minimum volume regulation demonstrated a risk reduction of 22.5% for postoperative wound infections.25 Volume–outcome relationships can be explained by more effective routine performance and by the implementation of policies and pathways in medium- or high-volume departments, although surgical practice can vary considerably.26 However, an explanation for the difference by type of procedure (≤50 procedures for knee endoprosthesis and >100 procedures protective for hip replacement) cannot be obtained from these data.

Prior studies of the relationship between hospital surgical volume and adverse events such as SSI have also provided conflicting results.27 Anderson et al called their study ‘Size matters.’22 After adjusting for differences between hospital category (low, medium and high volume of 48 different surgical procedures) and confounders (NNIS risk index), they found that the risk for SSI in low-volume hospitals was 1.5 times higher than medium-volume hospitals. However, it was also 1.3 times higher in large-volume hospitals, leading the authors to conclude that there is a U-shaped relationship between SSI and hospital surgical volume. In a recently published Canadian study, the authors did not see any association between provider volume of total hip or knee replacement and surgical complications (including SSI) during index admission.28 Findings for other outcomes were also mixed. For example, surgeon procedure volume was related to rates of revision of hip replacement but not to rates of revision of knee replacement. Data from the Dutch Network for Prevention of Nosocomial Infections through Surveillance (PREZIES) from 1996 to 2000 revealed that a high annual volume of hip arthroplasty was associated with a reduced risk of SSI.29 However, again, data from PREZIES from 1996 to 2003 did not show that hospital volume was associated with SSI risk (total hip arthroplasty and knee arthroplasty were analysed among others).19

A systematic review of MEDLINE from 1980 to 2000 reported strong volume–outcome relationships for surgeries for pancreatic or oesophageal cancer, and of a much smaller magnitude—although statistically significant—for orthopaedic surgery.9 A systematic review focusing on orthopaedic procedures and reviewing studies from 1966 to 2005 found an association between higher hospital or surgeon volumes and lower rates of hip dislocation.30 Recently, Marlow et al published a systematic review on the relationship between volume and outcome in knee arthroplasty. Their results supported the theory that improved outcomes were associated with increased hospital and surgeon volume. However, the authors stated a North American bias (which limits generalisability) and that the association between outcome and volume appeared tenuous, when results were separated by arthroplasty type (primary, revision).

Generally, volume is only a proxy measure of many parameters because it cannot directly produce favourable or unfavourable results, but as long as specific differences in clinical management or procedural techniques between high-volume providers with good outcomes and low-volume providers with poor ones are not exactly identified, it might be justified from a healthcare-policy perspective to implement minimum regulation volumes.

Our study has several strengths. First, the data result from a large network of hospitals (110 surgical departments provided SSI rates after hip replacement). Surveillance data from Krankenhaus-Infektions-Surveillance-System are representative and validated.31–33 Second, standard Centers for Disease Control-definitions for SSI surveillance were applied across all surgical departments. Third, these data are based on a comparatively long study period of 5.5 years.

Our study also has several limitations. We could not evaluate the volume of individual surgeons or the influence of the surgeon experience. Neither could we analyse other parameters such as antimicrobial prophylaxis, postoperative care or other complications such as bleeding, diabetes, immunosuppression, postoperative pneumonia or thrombosis. Furthermore, we did not perform a systematic postdischarge surveillance, which might explain why SSI rates from the Dutch surveillance system were higher (patients underwent active postdischarge surveillance for 30 days and 1 year).

We conclude that our findings offer some support for recommendations to concentrate on arthroscopy and knee replacement in surgical departments with more than 50 procedures and hip replacement in departments with more than 100 procedures per year in order to reduce SSI.


We thank R Plocher for his help in preparing the manuscript.



  • Funding The KISS Project is supported by the German Ministry of Health. The study received no additional support.

  • Competing interests None.

  • Provenance and peer review Not commissioned; externally peer reviewed.