Risk factors for surgical site infection in hip and knee arthroplasties in New Zealand: July 2013 – July 2015

Background: The New Zealand (NZ) SSI Improvement Programme began in 2013 and has prospectively collected data on hip and knee arthroplasties performed in the 20 District Health Board hospitals. The data base was analysed to determine risk factors for surgical site infections (SSI) in hip and knee arthroplasties.

Methods: Data captured included information on the patient (sex, age, height, weight and BMI), procedure (antimicrobial prophylaxis and skin antisepsis) and SSI. The CDC NHSN definitions for SSI were used. All patients were followed for 90 days. Only SSI occurring during the initial admission or resulting in readmission were included.

Results:

• 19,523 arthroplasties were performed; 48% primary hip, 43% primary knee and 7 and 2% hip and knee revisions. 3% were bilateral procedures.
• 243 SSI, 1.2% (95%CI 1.1-1.4); 25% superficial, 40% deep and 35% organ space.
•  44% of patients were obese (BMI>30) and 30% were overweight (BMI>25-30).
• ~50% of SSI were caused by staphylococci, 33% by S.aureus and 16% by coagulase-negative species.

Risk Factor	Significance	Odds ratio (95% confidence intervals)
Surgeon-specific Expertise (consultant vs registrar)	NS
Procedure-specific Unilateral vs bilateral procedure Revision procedure Use of gentamicin bone cement	NS P <0.001 P=0.003	3.1 (2.2-4.1) 0.66 (0.5-0.86)
Patient-specific Overweight (BMI >25-30) Obese (BMI >30)	NS P <0.004	1.9 (1.23-3.05)
Surgical antimicrobial prophylaxis Addition of gentamicin to cephalosporin Administration of prophylaxis > 60mins before knife to skin (KTS) or after KTS	NS P=0.002	2.2 (1.3-3.6)
Skin antisepsis Alcohol-based skin preparation povidone iodine vs chlorhexidine	P=0.04	0.67 (0.47-0.97).

Conclusion:

NZ patients share some of the known risk factors for orthopaedic SSI. In particular patients with BMI >30 and those in whom optimal timing of surgical antimicrobial prophylaxis is not achieved have higher SSI rates.

Other interventions may also lower SSI. The burden of staphylococcal SSI suggests that interventions aimed at reducing staphylococcal SSIs may also benefit patients undergoing arthroplasties in NZ.