Trial to Compare Femoral Nerve Block With Local Anaesthetic Injection for Post-operative Pain After Knee Replacement.

A Randomised, Observer Blinded, Controlled Trial Of Femoral Nerve Block Versus Local Infiltration Analgesia for Post Operative Analgesia Following Total Knee Arthroplasty

Pain after a knee replacement can impair recovery and use of the new knee. Having an injection to numb the femoral nerve is known to give good pain relief after the operation but may lead to slower mobilisation as it also prevents the patient from moving the knee. Recent studies have shown that infiltration of local anaesthetic (LIA) within the new knee joint may also give good pain relief. The null hypothesis is that there is no difference in primary or secondary outcome measures between femoral nerve block and LIA, as anaesthetic techniques for knee replacement.

Knee pain and stiffness is a common problem which can sometimes be improved by inserting a replacement knee joint. An anaesthetist is a doctor who specialises in looking after patients undergoing surgery, and there are a variety of different anaesthetics which can be used for knee replacement surgery. These include general anaesthesia (going to sleep), and spinal or epidural anaesthesia (where pain killers are injected into the back, resulting in temporarily numb legs). Pain killers can also be injected around the nerves which supply the leg, or around the site of the operation itself, combined with general or spinal anaesthesia if required.

Over the years, multiple different combinations of these techniques have been tried. All have advantages and disadvantages. Generally, those which completely numb the leg after the operation often cause weakness which interferes with movement. Although the patient will have no pain, getting up and around with the physiotherapist is crucial and the weakness can delay recovery. However, excessive pain can also interfere with movement. There is therefore a balance to be struck between pain and weakness, and the choice of anaesthetic technique is key.

Researchers previously conducted a study at the Royal Devon and Exeter Hospital which compared the effects of two techniques; the use of diamorphine in a spinal injection, and the injection of pain killer around a nerve supplying the leg (femoral nerve block, FNB). Whilst the research showed that FNB gave better pain relief, there are still concerns that it causes weakness which may interfere with movement. A newer technique has evolved over recent years in which pain killer is injected directly around the knee during the operation. This is known as local infiltration analgesia (LIA) and the potential advantages are that it is simple, safe and does not cause leg weakness.

If research shows that LIA provides adequate pain relief without weakness, it may be a better option to use routinely, rather than FNB. The primary outcome measure is the amount of morphine used in the first 48 hours. The secondary outcome measures are the Total Pain Relief Score (TOTPAR), post operative pain scores, the ability to achieve set rehabilitation goals, readiness for discharge and qualitative data on patient recovery and satisfaction.

Inclusion Criteria:

All adult patients presenting for primary knee arthroplasty under the care of the Exeter Knee Unit Consultants Messrs Toms, Eyres, Cox, Mandalia, Schrantz.

Exclusion Criteria:

1. Total knee arthroplasty for trauma

2. Unicompartmental surgery

3. Bilateral surgery

4. Contra indication to spinal anaesthesia or peripheral nerve blocks (anticoagulation, hydrocephalus, raised intracranial pressure, peripheral neuropathy)

5. Allergy to local anaesthetics or morphine

6. Chronic pain:

   • Under active follow up by chronic pain team
   • Chronic strong opiate use (morphine, oxycodone, buprenorphine, pethidine, methadone). Codeine, dihydrocodeine and tramadol are not included
   • Other chronic pain medications (including gabapentin, pregabalin or amitriptyline)

7. Unable to adequately understand verbal explanations or written information given in English, or patients with special communication needs -