The recent introduction of intra-articular local anaesthetic techniques following total joint arthroplasty have demonstrated improvements in post-operative pain control, early joint mobilisation and may contribute to early discharge. Following intra-operative infiltration, the CALEDonian Technique requires ropivicaine to be administered postoperatively via an epidural catheter and filter, 20 to 40 ml over 60 seconds, on three occasions. Epidural filter catheters are used to prevent bacterial contamination during injection and have demonstrated 100% efficacy when intact. However, on internal departmental audit we have become aware of a filter membrane rupture rate approximating 10%. We therefore investigated the variables of syringe size and rate of administration in the occurrence of filter ruptures.

Using a standard primed epidural catheter set (Perifix B Braun), pressure measurements were taken pre- and post-filter. Recordings using the filter with attached catheter tubing were undertaken using 5 ml, 10 ml and 20 ml syringes (n=10) during infusion of a standard 5 ml volume. Controlled (over 1 min and following departmental protocol) and forced (over less than 15 seconds) infusions were undertaken manually and the experiment also undertaken using an automated syringe driver with 40 ml infused at a rate of 400 ml/hr. Each experiment was repeated ten times. Infusion pressures were measured and filters examined for evidence of rupture.

Using departmental protocol, controlled infusions independent of syringe size generated consistently low pressures, averaging 115 kPa, with no filter ruptures. Forced administration, independent of syringe size, generated pressures averaging 625.1 kPa. This is above the filter threshold and resulted in almost universal filter rupture. An automated device infusing at a rate of 400 ml/hr again generated low pressures and no filter ruptures.

Our study demonstrates low infusion pressures and no filter ruptures, independent of syringe size, when departmental protocol is adhered to suggesting that a human element may be in-part responsible for filter rupture. Although the technique currently used is safe, our audit has raised awareness of a potential difficulty and has lead to re-education of staff involved in this process. Consideration is being given to the possibility of automating infusions in the future and a process of re-audit of filter rupture will be undertaken.