Sensory innervations contributing pain after total knee arthroplasty (TKA) include branches
of femoral, obturator and sciatic nerves. Branches of femoral nerve contribute the most pain
sensation in TKA including nerves to the vastus medialis, intermedius, and lateralis, medial
and intermediate femoral cutaneous, and saphenous nerves. Smaller contribution of pain
sensation from branches of fibular and tibial nerves, and posterior branch of obturator
nerve. Multiple techniques of nerve block could anesthetize some or all of the sensory
innervations, but analgesia with motor sparing is important for early recovery and
rehabilitation after TKA. For both pain reduction and motor function, adductor canal block
(ACB) combined with local infiltration analgesia is considered more feasible than other
peripheral nerve blocks.
ACB could anesthetize nerves beyond in adductor canal. Anatomical studies revealed the
extended spreading of local anesthetics (LA) beyond adductor canal when performing ACB, and
caudal spreading could reach popliteal fossa through adductor hiatus. Cephalad spreading of
LA in ACB is limited and rarely extending to femoral triangle even when injecting from
proximal adductor canal, but the cephalad spreading also depends on the volume of injectants
and using tourniquets.
In clinical studies, both ACB injection site and volume of injectants were investigated.
Clinical trials and systematic reviews revealed the similar efficacy of analgesia when ACB
injection at proximal and distal adductor canal, although the volume and pattern of injection
(bolus or continuous) were variable. Regarding to the volume of injectants, 20ml injectant of
local anesthetics would be adequate without prominent motor impairment compared with smaller
volume.
Previous systematic reviews and meta-analysis have confirmed better analgesia with continuous
infusion of ACB than single shot, but few studies explored the difference of intermittent
bolus and continuous infusion. One clinical trial compared continuous infusion and
intermittent bolus of ACB in patients receiving TKA, two other trials investigated the
difference in healthy volunteers and patients receiving knee arthroscopy. All these three
studies concluded no difference of analgesic efficacy. However, no consistent volume and
frequency of injection was studied. Whether longer interval of intermittent bolus was the
same with continuous infusion in analgesic efficacy is still need to be further verified.