One High-Volume Posteromedial Knee Capsular Injection Can Provide Safe Anesthetic Infiltration Across the Entire Posterior Knee Capsule.

BACKGROUND

Anesthetic injection through the central or posterolateral knee capsule for analgesia after knee surgery risks popliteal vessel and tibial and peroneal nerve injury. We evaluated the distribution of a high volume of fluid injected through only the posteromedial capsule and compared it to a technique involving injections through the posteromedial and posterolateral capsules.

METHODS

Four fresh cadaveric knees were injected with 50 ml of Omnipaque 350 diluted 1:4 with normal saline through the posteromedial knee capsule. In four other specimens, we followed a published protocol by injecting 12.5 ml posteromedially and 12.5 ml posterolaterally. The knees were then ranged 20 times over 20 minutes before obtaining a computed tomography scan to evaluate the spread of injected contrast fluid.

RESULTS

Both techniques demonstrated broad distribution of injected fluid posterior to the capsule. An average of 80% of the capsular width at the level of the injection was within 5 mm of the injected contrast with the single injection, while 63% was with the two-injection technique.

CONCLUSIONS

One 50 mL injection through the posteromedial knee capsule can provide broad medial, lateral, superior, and inferior distribution of anesthetic without risking injury to neurovascular structures. This has become our preferred method of anesthetizing the posterior knee during knee surgery.