Motor-Sparing Blocks for Knee Surgery

• Total knee arthroplasty is associated with severe post-operative pain in up to 36%


• Severe pain is associated with:
• Reduced patient satisfaction
• Delayed mobilisation and prolonged hospital stay
• Increased healthcare costs
• Increased rates of chronic pain


• Early mobilisation is central to ERAS e.g. physiotherapy reduces length of stay by ~1.8 days
• Femoral (± sciatic) nerve blocks provide excellent analgesia but cause quadriceps weakness (and foot drop), delaying mobilisation and increasing falls risk


• The goal is analgesia that preserves quadriceps strength, while avoiding the side-effects of systemic analgesics
• Achieved by targeting the sensory (articular) terminal branches around the knee rather than the motor trunks proximally
• Delivered as part of multimodal analgesia
• Can facilitate same-day discharge in selected patients
• Low concentrations of LA are preferred, e.g. 0.2% ropivacaine, 0.125-0.25% (levo)bupivacaine

• Femoral triangle borders:
• Superior: inguinal ligament
• Medial: adductor longus
• Lateral: sartorius
• Apex: where the medial borders of sartorius and adductor longus intersect; identifiable on ultrasound


• Adductor canal: runs from the apex of the femoral triangle to the adductor hiatus
• Roof: vastoadductor membrane and sartorius
• Lateral: vastus medialis
• Medial: adductor longus and magnus
• Adductor hiatus: gap in adductor magnus through which the femoral vessels pass posteriorly to become the popliteal vessels


• Contents: femoral artery and vein, saphenous nerve, nerve to vastus medialis (NVM) ± posterior and anterior branches of the obturator nerve and medial femoral cutaneous nerve

(a) Borders and apex of the femoral triangle; (b) sartorius reflected to show the adductor canal, saphenous nerve and NVM. Adapted from Motor-sparing regional anaesthesia for total knee arthroplasty: a narrative and systematic literature review

• Complex, variable and inconsistently described in the anatomical literature
• Follows Hilton's law: a joint is innervated by articular branches of the nerves supplying the muscles that move it
• Derived from the femoral, obturator and sciatic nerves (lumbar and sacral plexuses) and is usually simplified into anterior and posterior compartments, then quadrants

• Femoral nerve: divides below the inguinal ligament into:
• Anterior division: motor to sartorius + anterior femoral cutaneous nerves (AFCNs i.e. intermediate and medial femoral cutaneous nerves), supplying the skin of the anterior knee
• Posterior division: motor branches to quadriceps, which divide in the iliopectineal fossa (a proximal injection here causes weakness) and their terminal branches become sensory to the knee


• Femoral sensory contributions:
• NVM: superomedial quadrant as usually gives rise to the superomedial genicular nerve
• Nerve to vastus intermedius: medial and lateral articular branches to the suprapatellar bursa and both superior quadrants
• Nerve to vastus lateralis: superolateral quadrant
• Infrapatellar branch of the saphenous nerve (arises after the saphenous nerve exits the adductor canal): inferomedial quadrant and skin


• Sciatic contributions (despite their posterior origin, these mostly supply anterior structures):
• Superolateral genicular nerve: sciatic or common peroneal nerve
• Inferolateral genicular nerve: common peroneal nerve
• Inferomedial genicular nerve: tibial nerve or popliteal plexus
• Recurrent peroneal nerve: common peroneal nerve


• Genicular nerves supply the periosteum and anterior capsule, they travel with the genicular arteries, which act as ultrasound landmarks
• Obturator contribution is variable and inconsistent

Simplified innervation of the anterior knee. Adapted from Motor-sparing regional anaesthesia for total knee arthroplasty: a narrative and systematic literature review

• Supplied largely by the popliteal plexus, formed from articular branches of the:
• Tibial nerve (the consistent contributor)
• Posterior branch of the obturator nerve; runs in or below the adductor canal and passes through the adductor hiatus
• Common peroneal nerve


• Innervates the posterior capsule and intra-articular structures

Simplified innervation of the posterior knee. Adapted from Motor-sparing regional anaesthesia for total knee arthroplasty: a narrative and systematic literature review

• Surgical infiltration of large volumes of dilute LA into the periarticular tissues
• At least two stages: posterior capsule before component insertion, then anterior, medial, lateral and subcutaneous tissues afterwards
• Typical dose ~400mg ropivacaine for unilateral TKA (up to 800mg bilateral)
• Additives include adrenaline, NSAIDs (e.g. ketorolac), steroids, opioids, α₂-agonists, antibiotics and TXA


• Mechanism: direct sensory block, dilution of inflammatory mediators, direct anti-inflammatory effect and a depot providing ongoing systemic analgesia
• Recommended by ERAS, PROSPECT and NICE and is considered the benchmark against which motor-sparing blocks are compared
• LIA and peripheral nerve blocks are not mutually exclusive as adding blocks can reduce the total LA dose in at-risk patients

• Measured free ropivacaine concentrations usually remain below toxic thresholds, but some patients approach or exceed them
• Time to peak plasma concentration is unpredictable: as early as 20mins with plain ropivacaine, delayed to 2-6hrs by adrenaline
• Peak levels may therefore occur after discharge from recovery which is relevant when planning monitoring and rescue blocks
• Reduce doses at extremes of age and weight, in females and with organ dysfunction

• Historically confused with many trials describing 'adductor canal blocks' which were in fact mid-thigh femoral triangle blocks

• Technique:
• Supine, 'frog-leg' position with high-frequency linear probe transverse on the medial thigh
• Identify the femoral artery deep to sartorius and slide distally to find the apex of the femoral triangle
• In-plane, lateral to medial, beneath sartorius towards the artery; 5-20ml LA adjacent to the artery depending on desired effect
• Nerve stimulation can identify the NVM, either to block it selectively or to avoid injuring it


• Evidence:
• Femoral triangle, proximal and distal adductor canal blocks produce similar analgesia and mobility
• Comparable analgesia to femoral nerve block with better preservation of quadriceps strength
• Comparable analgesia to LIA and adding a block to LIA gives superior analgesia to either alone
• Covers only the anteromedial knee, hence the need for complementary techniques
• Catheters: conflicting meta-analyses with no clear benefit in the first 24hrs but may improve pain at 24-48hrs
• Not advised routinely, but consider in opioid-tolerant patients and as part of rescue analgesia


• The NVM contributes more to anteromedial capsule innervation than the saphenous nerve, so blocking both is desirable
• The muscular branch of the NVM enters vastus medialis just below mid-thigh, and blocking it may explain occasional quadriceps weakness after adductor canal block (BJA, 2021)

• The AFCNs supply the anteromedial thigh and knee skin and are usually missed by femoral triangle and adductor canal blocks
• The medial femoral cutaneous nerve may be caught by a proximal femoral triangle injection
• The intermediate femoral cutaneous nerve branches more proximally and needs a separate injection
• Technique: linear probe transverse at the proximal mid-thigh injecting 3-5ml LA into the fat-filled plane superficial to the fascia lata over sartorius
• Adding to a femoral triangle block improves dynamic pain for ~12hrs, incision pain at 24hrs and morphine consumption over 48hrs

• Targets: superomedial, superolateral and inferomedial genicular nerves
• The inferolateral genicular and recurrent peroneal nerves are usually avoided due to the risk of common peroneal nerve block and foot drop
• Technique:
  • Linear probe longitudinal over the metaphyseal-diaphyseal junction of the distal femur or proximal tibia
  • Identify the genicular artery and deposit 4-5ml LA per target onto the periosteum
  
  
• Evidence:
• Improve analgesia vs. placebo
• Sdding them to adductor canal block + iPACK more than halved opioid use in the first 24hrs
• Equivalent to high-volume LIA using only 20-40ml LA rather than ~150ml
• Lower doses allow more concentrated LA, additional blocks within safe limits, or bilateral TKA

• Nerve to vastus intermedius: its articular branches run between the femur and vastus intermedius in the distal third of the thigh; LA onto the anterior femoral periosteum just above the patella
• Nerve to vastus lateralis: targeted at mid-thigh, adjacent to the descending branch of the lateral circumflex femoral artery
• 3-5ml LA each may add benefit, but neither has been assessed in isolation in comparative trials

• Infiltration between the popliteal artery and capsule of the knee
• Targets popliteal plexus branches while sparing the tibial and common peroneal nerves
• Cadaveric studies suggest some spread to anterior articular branches too


• Technique:
• 'Frog-leg' position with curvilinear probe transverse over the distal medial thigh
• Trace the femoral vessels distally until the popliteal artery lies posterior to the femur, just proximal to the condyles
• In-plane, medial to lateral, parallel to the femoral shaft until the tip is ~2cm lateral to the artery
• ~20ml (10-30ml) LA injected incrementally as the needle is withdrawn
• Avoid a lateral-to-medial approach due to the risk of common peroneal nerve injury


• Evidence:
• Without LIA: improves analgesia and function for the first 24hrs
• Equivalent or better analgesia than LIA using half the LA dose
• No additional benefit when added to LIA, as both treat the posterior capsule

• Avoids foot drop from common peroneal block, but not fully motor-sparing
• Not superior to LIA, and slower mobilisation than iPACK, probably due to variable spread to the sciatic bifurcation

• Superior analgesia to femoral triangle and adductor canal blocks, but quadriceps weakness has removed it from routine ERAS pathways
• The analgesic gap is probably due to missed AFCNs and terminal branches of the nerves to vastus lateralis and intermedius
• Better analgesia and less nausea than opioid PCA, and similar analgesia to epidural and LIA (Cochrane, 2014)


• Remaining roles:
• Rescue analgesia in recovery using a short-acting LA (e.g. 2% chloroprocaine), which improved pain and opioid use
• Lidocaine femoral block for TKA under GA (without spinal) alongside motor-sparing blocks, to reduce recovery pain and opioid use
• Dense femoral ± sciatic blocks for complex revisions, planned delayed mobilisation or severe pre-operative pain

• The optimal combination remains unclear, with evidence strongest for subsartorial blocks and limited for other targets


• PROSPECT: single-shot adductor canal block + LIA is the standard approach


• Anatomically complete coverage would combine:
• Adductor canal/femoral triangle block (anteromedial capsule)
• AFCN block (skin)
• Genicular nerve blocks (anterior capsule)
• iPACK (posterior capsule)
• No RCT has compared all four against LIA although a small retrospective study of a multi-block regimen reduced 48hr morphine use by 68% vs. LIA + adductor canal block
• Reasonable routine options:
• Adductor canal block + LIA
• Adductor canal block + genicular nerve blocks, which may be preferred in those at risk of LAST
• Calculate the maximum LA dose meticulously when combining blocks with each other or with LIA

• Most trials exclude extremes of age and BMI, ASA 3-5, organ dysfunction, high-dose opioid use and substance use
• These patients are hardest to manage and stand to gain most from multi-block techniques and catheters

• Quadriceps weakness and falls:
• Proximal spread of large volumes to the femoral nerve motor branches
• Direct block of the NVM muscular branch
• Rarely, myositis from spread outside the vastoadductor membrane; profound weakness that resolves over months


• Foot drop from common peroneal nerve involvement:
• Distal spread of adductor canal injectate into the popliteal fossa
• Inferolateral genicular or recurrent peroneal nerve blocks
• Lateral-to-medial iPACK approach or spread to the sciatic bifurcation

• LAST, particularly with combined blocks and LIA
• Vascular puncture or intravascular injection: femoral, popliteal and genicular vessels
• Nerve injury
• Catheter problems: migration, leakage, obstruction, dislodgement and infection