Updated: Oct 14, 2020
With the latest evidence showing the superiority of local anaesthetics over opioids for patient comfort, the following discussion focuses on regional anaesthesia for stifle surgery.
Successful regional anaesthesia (RA) techniques require thorough anatomical knowledge for the establishment of reliable landmarks, puncture sites, the direction and depth of needle insertion, and relevant structures to be avoided.
Options for RA for the stifle joint are blockade of the femoral and sciatic nerves or an epidural. This blog gives an introduction to the femoral and sciatic nerve blocks. Links are provided to videos demonstrating the techniques using nerve location to illustrate the approach to the nerve. A nerve locator is desirable, although not essential to perform these blocks.
Understanding the anatomy
The innervation of the canine pelvic limb has been examined in several cadaver studies and effective anaesthesia of the stifle requires blockade of the femoral, obturator and sciatic nerves.
Anaesthesia of the entire limb can be achieved with perineural administration of local anaesthetic to the main nerves of the lumbosacral plexus (L4-S2) which are the femoral (L4-L6) and the sciatic (L6-S2) nerves plus the lateral femoral cutaneous nerve and obturator (L4-L6).
In dogs the stifle is innervated by the medial articular nerve (saphenous [femoral] in origin) and the lateral articular nerve (both ScN in origin). The medial articular nerve occasionally receives branches from the obturator nerve. In dogs the caudal articular nerve is often absent but by contrast this is the largest of the articular nerves in the cat (O’Connor & Woodbury 1982).
The sciatic (ScN) exits the pelvis through the greater ischiatic notch and runs caudally towards the coxofemoral joint, passing caudal to the greater trochanter of the femur. It passes over the gluteus profundus muscle, deep to gluteus medius and gluteus superficialis muscles. It then travels distally between biceps femoris and the adductor muscles. With biceps femoris laterally and semitendinosus medially it divides into terminal branches, the tibial and peroneal nerves. The level at which this division occurs varies (Dyce et al. 1996).
The femoral (saphenous) nerve
The saphenous (SaN) nerve is a branch of the femoral nerve (FN) and provides sensory input from the medial thigh and stifle.
The FN (saphenous) block at the inguinal triangle is the easiest approach in practice to perform without a nerve locator. The femoral artery is palpated as a landmark and the needle is inserted just cranially to that point, to a depth of approximately 5mm. A dose of 0.2ml/kg 0.5% bupivacaine (0.1ml/kg for nerve location) is injected following aspiration to verify that a blood vessel has not been encountered. At this location the femoral nerve will be blocked but not the obturator. Mahler & Adogwa (2007) reported a lack of reliable anatomical landmarks to suggest the depth to which the needle should be advanced to block the FN when an inguinal approach is used, and therefore techniques such as nerve location and ultrasound guidance have been investigated. Above I have recommended a depth of 5mm.
To eliminate the variability in block caused by missing the obturator nerve, the FN can be targeted closer to the spinal cord in the lumbar plexus and a variety of approaches are described in this blog.
The sciatic nerve
Several approaches to anaesthetise the ScN have been described at various locations along its course from its origin to the division into common peroneal and tibial nerves.
The proximal ScN block is the simplest of these whereby the ScN is targeted between the greater trochanter and the ischiatic tuberosity. The needle is advanced perpendicular to the skin and has been described in laboratory and clinical studies (Campoy et al. 2008a, Campoy et al. 2008b, Vettorato et al. 2012). A volume of 0.1ml/kg 0.5% bupivacaine is advised for a blind technique compared to 0.05ml/kg for nerve locator use. Watch the sciatic block here.
Regional anaesthesia compared to epidural
Two studies have compared an inguinal SaN block combined with a ScN block to epidural local anaesthetic. Campoy et al. (2012a) compared epidural morphine-bupivacaine to RA bupivacaine and Caniglia et al. (2012) used a lidocaine bupivacaine combination either as a regional technique or administered epidurally. Each reported a comparable degree of analgesia to the same combinations used epidurally, with Campoy et al. reporting duration of analgesia of 10 hours with a SaN and ScN nerve block. Each of these studies used a small population of subjects and these promising results should be validated by larger studies.
Having taught numerous delegates over the years to perform femoral (saphenous) and sciatic nerve blocks the best feedback we hear is that they find them so easy to perform and notice such a huge difference they make during and after surgery. Of course as well as blocking sensory nerves we are also blocking motor nerves. Even for day procedures if these are conducted on the morning of surgery the dog or cat should be ambulatory that evening. If a larger volume of bupivacaine is used the effect will last longer – so you may wish to make a decision about whether to send the pet home the same day.