TRANSDERMAL FENTANYL AS A FORM OF REBOUND PAIN REDUCTION IN FAST TRACK PROGRAMME IN PRIMARY KNEE ARTHROPLASTY. Prospective Observational Non-inferiority Study Versus Morphine PCA.

Total knee arthroplasty (TKA from now on) is a frequent surgery in routine clinical practice in constant evolution and improvement, with the objective being the earliest possible functional recovery of these patients, the role of the anaesthesiologist being fundamental in this sense throughout the perioperative period since, as we know, it is a surgery that improves the mobility and quality of life of the patient, but causes intense postoperative pain during the first 24 to 72 hours, it is a surgery that improves the patient's mobility and quality of life, but causes intense postoperative pain during the first 24 to 72 hours, a very important factor to take into account in these patients since it is known that adequate postoperative pain control promotes ambulation and the possibility of physiotherapy for the patient, This leads to an early recovery, a lower rate of complications (such as deep vein thrombosis or nosocomial infection) and a shorter hospital stay, allowing the strategy known as "fast-track" to be carried out, which would consist of achieving functional recovery and discharge as early as possible, this being the current trend in management by most trauma teams dedicated to this field, since it results in a decrease in morbidity and greater patient satisfaction. In relation to this approach, as has been mentioned, multiple studies have shown that one of the main causes of delayed hospital discharge is poor pain control and mobilisation problems.

Therefore, adequate postoperative pain control will be important in this surgery. This control could be achieved with a combination of locoregional techniques such as peripheral nerve blocks of the adductor canal (BCA) and the so-called "Infiltration between the Popliteal Artery and the Capsule of the Knee" (IPACK) together with conventional analgesia such as NSAIDs, paracetamol, metamizole and, if this is not sufficient, opioids.

The locoregional techniques mentioned (BCA + IPACK) have demonstrated their efficacy in controlling immediate postoperative pain in this type of surgery in multiple studies. The first of these, BCA, provides analgesia in the anteromedial area of the knee, avoiding the femoral nerve block with the consequent weakness of the quadriceps. The second of these, IPACK, is a relatively novel technique described to achieve analgesia in the posterior capsule of the knee that is achieved by blocking the popliteal plexus formed by the articular sensory branches of the tibial and obturator nerves arranged around the popliteal artery and vein without involving the motor branches of the tibial and peroneal nerves, avoiding the motor block resulting from performing a block of the sciatic nerve at the popliteal level.

From all of the above, we can deduce that both techniques are effective individually, but even more so, as is logical, in combination, allowing the entire joint territory to be covered with the least possible impairment of motor function, which will be of particular relevance in terms of starting the patient's mobilisation earlier, which is definitely an advantage over traditional femoral and sciatic nerve block techniques.

However, the duration of nerve blocks is less than 24h, which is a problem, since the period of maximum incidence of postoperative pain has not yet been completed and it is possible that just at this time (12-24h after the block, when the effect of the block ends) what is known as "rebound pain" may appear, considered a possible adverse effect of peripheral nerve blocks that is becoming increasingly important, as studies dedicated to its study emerge.