PERIPHERAL OPIOID EFFECTS AFTER KNEE SURG ERY - A REVIEW OF CLINICAL-TRIALS

In this article 34 randomized controlled trials examining peripherally mediated opioid effects after knee surgery are discussed. All studies examined small doses of morphine injected intraarticularly at the end of knee surgery, but not all studies did show an analgesic effect of the peripheral opioid. Because of differing study designs a metaanalys is of the data is not possible. Some important factors like those that the respective authors try to explain the contradictory results with are discussed here:The used concentrations, Volumes and doses of morph ine are not of major importance. An addition of adrenaline to the stud y drug, as well as the use of a tourniquet and the time interval betwe en the intra-articular injection and tourniquet release do not seem cr ucial either. The use of intra-articular drainage (which is opened 10 min after injection of the study drug) and of patient-controlled analg esia as an evaluative method also do not seem to play a major role. Bu t it is remarkable that peripheral opioid effects have often been desc ribed after general and local anesthesia but only once after regional anaesthesia. It may be that the activation or expression of peripheral opioid receptors is inhibited by the blockade of input to the central nervous system. Animal experiments are needed to clarify this issue. But it may also be that postoperative pain after regional anaesthesia does not reach a level high enough to make an analgesic effect measura ble. If patients after regional anesthesia are not considered, 20 stud ies out of 29 were able to demonstrate opioid effects whereas 9 were n ot:The work of HEARD and coll.[14] as well as RUWE and coll. [44] must be criticized because of methodological shortcomings. In the remainin g 7 studies the patients of the comparative groups only show low pain scores which may make it impossible to measure an analgesic effect by intraarticular morphine. In summary it can be concluded that very smal l doses of morphine injected intraarticularly after knee surgery do ex ert an analgesic effect. In some studies this effect lasted even up to 48 h. But at least during the first two hours the effect is small or else doubtfull. Therefore a combination with bupivacain, a local anest hetic which acts rapidly but only for some hours can be recommended. M ost authors testing this combination found it most useful.