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Requirement of the mandatory surveillance of SSI in orthopaedic surgery

Surgical site infections (SSI) are defined as infections related to a surgical procedure that affect the surgical wound or deeper tissues handled during the procedure.

The requirements of the mandatory surveillance are that all NHS Trusts where orthopaedic surgical procedures are performed are expected to carry out a minimum of three months surveillance in at least one of the four orthopaedic categories.

A hip hemiarthroplasty is a surgical procedure in which the damaged or diseased head and neck of the femur are removed and replaced with a prosthesis.  The procedure is commonly carried out on elderly patients who have fractured the neck of femur as a result of a fall.  Open reduction of fracture is a surgical procedure to repair a fractured bone using plates, screws or rods to stabilise the bone.

Some Trusts that have more than one acute hospital may have chosen to collect data at one hospital only.  Some Trusts, in particular paediatric specialist hospitals, only carry out procedures in the open reduction of long bone fracture category and the throughput was too small to enable them to participate in the surveillance.  

Surveillance methods

It is not possible to reliably identify SSI from laboratory data alone as the diagnosis depends on the presence of signs and symptoms of infection in the wound.  The surveillance to detect SSI therefore requires active monitoring of patients from the time of their operation until they are discharged from hospital.  To ensure that as far as possible data collected in different Trusts are comparable they are expected to adhere to the standard method of collecting and reporting data described in the SSI surveillance protocol.  Currently there is no requirement to continue surveillance once the patient has been discharged from hospital.

Trusts participate in the surveillance for minimum three-month periods although they can choose to collect data for more than one period.   Each patient undergoing a procedure within a defined category is included in the surveillance and monitored for subsequent signs of SSI.

Trusts participating in the surveillance receive an individual report at the end of each surveillance period that contains their results compared to the data aggregated from all participating hospitals.  They use this data to monitor local practice and initiate further investigation and action should the results indicate that rates are unusual. 

National data

Data on 41 242 procedures has been collected by 146 Trusts in the first year of mandatory surveillance of SSI between 1st April 2004 and 31st March 2005.  Only NHS Trusts that undertake relevant orthopaedic procedures and who are registered with SSISS appear in the results.

The rates of SSI in orthopaedics were generally low and three-quarters of infections affected the superficial layers of the wound only.

More information about the data collected in this first year of mandatory surveillance of SSI in orthopaedics is contained in a separate report available from the Centre of Infections at the Health Protection Agency website.

Interpretation of the data

The rates of SSI in this report should be interpreted with caution.  They represent estimates made from the sample of procedures included in the surveillance.  In many cases they are based on small numbers of procedure and are therefore imprecise.

The number of procedures on which rates are based varies according to the throughput of the given type of surgical procedure at the Trust and the number of surveillance periods they have chosen to participate in.

Confidence limits are shown in order to give a guide as to how precise a particular estimate is.  The confidence limits for a Trust rate in a given category represent the range of rates between which their true rate could feasibly lie.  The confidence limits will be wider for those Trusts with rates based on fewer operations and narrower for those Trusts with higher numbers of operations.  Very low, or very high rates that are based on a small number of procedures should therefore not be taken at face value but should be interpreted in conjunction with the confidence limits.  The conventional method of calculating confidence limits could be misleading for rates based on less than 50 operations, as they will overestimate the true upper 95% limit.  Confidence limits have therefore not been shown for rates based on less than 50 operations.

The possibility that an SSI will be detected depends on the length of time that the patient spends in hospital post-operatively.  Some variation in rates may therefore be explained by differences in length of post-operative follow-up.  In addition, the rates included in these tables have not been adjusted for underlying risk factors related to the patient or their operation that could affect the risk of developing an SSI, for example age, underlying illness, complexity of the operation.  The effect of these risk factors and longer post-operative follow-up contributes to the generally higher rates of SSI seen in patients undergoing hip hemiarthroplasty.  Data on the effect of some of these factors is contained in the report of the first year of mandatory SSI surveillance in orthopaedics.

• Download Surveillance of surgical site infection in orthopaedic surgery: Mandatory surveillance of surgical site infection in orthopaedic surgery: Report of data collected between April 2004 and March 2005 (PDF, 103K)
• Download Mandatory surveillance of surgical site infection in orthopaedic surgery: Trust data collected between April 2004 and March 2005 (PDF, 208K)
• Health Protection Agency: Surgical Site Infection Surveillance Service (SSISS) (opens new window)