The Biology of Surgical Site Infection

Surgical site infection (SSI) is the third most common type of nosocomial (hospital acquired) infection, accounting for 38% of all infections in the 27 million patients who undergo surgery in the United States each year.  Following orthopaedic surgery, an SSI could be called a peri-prosthetic total joint infections, infected total joint, infected non-unions, infected fracture, osteomyelitis,  ”rejected” hardware, chronic wound drainage,  bone infection following an osteotomy, pin-tract infection, bone abscess, etc.    These infections are associated with substantial morbidity, mortality, and expense. Critical bacterial burden is by far the most significant of the many factors that influence surgical wound healing and determine the potential for infection and its incidence.

Unacceptable surgical site infection (SSI) rates still occur, despite the best efforts of infection prevention practitioners and improvements in surgical technique, antibiotic prophylaxis, methods of instrument sterilization, and “clean” operating room practices.  Bacteria can still gain access to a surgical wound from several sources. Endogenous contamination arises from: the patient’s skin or nares; the gastrointestinal, genitourinary, bronchial, or sinusoidal tracts; a concomitant remote-site infection (an infection existing somewhere else in the body).  Exogenous contamination comes from airborne or ‘transient pathogens’ transferred from instruments, implants, or personnel.   Most SSIs are caused by ‘commensal organisms’ from the patient’s skin or ‘transient organisms’ disseminated from health care personnel or surgical instruments.  Normal resident skin flora inhabit even the deepest layers of the dermis and are difficult to remove; these commensal organisms include Staphylococcus, diphtheroid, Pseudomonas, and Propionibacterium species.   Transient organisms are not consistently present but are easily exchanged between individuals: 30% of the contamin-ants are airborne and 70% transferred through instruments, personel or other contact surfaces. 

Although microorganisms gain access to all surgical wounds, only a small percentage of surgical patients develop a clinical infection (post-operative infection).  The take home point: the presence of micro-organisms in a wound is less important than the level of bacterial growth recorded during the first hours or days after surgery.  Whether an infection develops depends on the number and virulence of the bacteria present, the status or viability of the wound, presence or absence of a surgical implant, and the ability of host defenses to eliminate invading pathogens.  GC  9/04/09