TKA Post Op Pain and Saphenous Adductor Canal Block

 San Joaquin River Gorge - photo by JoAnn Sturman

Scott Sturman, M.D.

We are achieving good post op pain control with a variety of surgeries, but TKA presents some unique problems.  Blocking the saphenous nerve in the adductor canal is one of the latest methods used to better control post op pain.  The block is straight forward, and in thin patients it's like shooting fish in a barrel.  However, in generously sized patients one gets the idea that the superficial in superficial femoral artery is a misnomer.
 
After watching two videos from the “Block Jocks” website, Ultrasound Guided Mid-Thigh Saphenous Nerve Block and Saphenous Catheter Placement, a couple questions came to mind:

How can blocking a purely sensory nerve cause motor weakness?

What is the purpose of using a nerve stimulator when placing a saphenous catheter?

Jack Vander Beek in his website Neuraxiom provides a beautifully illustrated saphenous nerve block.  After reviewing it, I wrote to him to review the function of the saphenous nerve.  The following is his response:

Dr. Sturman,

Thanks for writing. As I understand it the saphenous is the terminal branch of the sciatic and is sensory only covering the medial lower leg to the cutaneous, subcutaneous, fascia, muscle, and periosteum. 


Motor innervation of the v.medialis and adductors is derived from the femoral nerve more proximal to the terminal branch. If you're blocking the saphenous as it exits the canal (with the descending genicular artery), there is little chance of getting the motor branches which exit earlier.

I don't k now how you locate sensory nerves using a nerve stimulator.  Blocking the femoral nerve anywhere along the adductor canal will generally avoid blocking nerves (motor included) which arise proximal to the reach of the local solution. Using lower concentrations and ropivacaine lower the incidence of motor block as well. Blocking the femoral nerve in the lower third of the thigh in the canal will minimize the motor block and stand a good chance of preserving the quadriceps for safer ambulation.

Anyway, good luck and keep up the good work,
Jack Vander Beek

Ted Gingrich sent an informative summary from his European redoubt which is helpful in putting this information in context.

Adductor Canal Block or Subsartorial Saphenous Nerve Block

San Joaquin River Gorge - photo by JoAnn Sturman