Most knee replacements use spinal anesthesia, sometimes combined with a nerve block for extended pain relief. General anesthesia is the other main option. Your surgical team will recommend one based on your health history, but understanding the differences can help you have a more informed conversation before your procedure.
The Two Main Options
Knee replacement anesthesia falls into two categories: general anesthesia, which puts you fully to sleep, and spinal anesthesia, which numbs you from the waist down while you remain technically conscious. Both are safe and widely used, with no difference in mortality rates up to 90 days after surgery.
With general anesthesia, you breathe through a tube while inhaled and intravenous medications keep you unconscious throughout the procedure. You won’t be aware of anything happening, and the anesthesiologist controls your breathing and monitors your vital signs continuously.
Spinal anesthesia works differently. A needle delivers numbing medication into the fluid-filled space around your spinal cord in your lower back, blocking pain and movement signals from reaching your brain. You lose sensation and the ability to move your legs temporarily. Most people who receive spinal anesthesia also get intravenous sedation, so you’re relaxed and drowsy (or fully asleep) during the surgery itself, even though you’re not under general anesthesia. The sedation level can range from light, where you’d respond to someone speaking to you, to deep, where you’re essentially unresponsive.
Why Spinal Anesthesia Is Often Preferred
For knee replacement specifically, spinal anesthesia has several measurable advantages. A large Canadian study of over 5,500 knee replacement patients found that those who received spinal anesthesia needed blood transfusions about half as often as those under general anesthesia (3.7% vs. 6.8%). They also went home about a third of a day sooner and were significantly more likely to be discharged home rather than to a rehabilitation facility: 94.1% compared with 91.2%.
A separate study from Singapore found an even larger gap in hospital stays, with general anesthesia patients staying nearly a full day longer on average after adjusting for other health factors. Nausea and vomiting after surgery are also less common with spinal anesthesia. Research across surgical procedures consistently shows that spinal anesthesia cuts postoperative nausea rates by more than half compared to general anesthesia.
That said, spinal anesthesia isn’t right for everyone. People taking certain blood-thinning medications, those with infections near the injection site, or patients with specific spinal conditions may not be candidates. If you can’t receive spinal anesthesia, general anesthesia is a perfectly safe alternative, with comparable complication rates overall.
Nerve Blocks for Pain After Surgery
Beyond the primary anesthesia that gets you through the operation, most knee replacement patients also receive a nerve block to manage pain in the hours and days that follow. This is a separate injection, guided by ultrasound, that delivers long-acting numbing medication to specific nerves around the knee.
The two most common nerve blocks for knee replacement are the femoral nerve block and the adductor canal block. Both control pain effectively, but they differ in one important way: the femoral nerve block significantly weakens your quadriceps muscle, the large muscle on the front of your thigh. In studies of healthy volunteers, quadriceps strength dropped by 49% after a femoral nerve block but only 8% after an adductor canal block. That difference matters because weak quads make it harder to stand, walk, and participate in physical therapy, and they increase your fall risk.
For this reason, the adductor canal block has largely replaced the femoral nerve block in current practice. It provides comparable pain relief while preserving nearly all of your leg strength. Patients who receive it consistently walk farther and mobilize more easily during the first 24 to 48 hours after surgery, which is the critical window for starting rehabilitation.
Long-Acting Pain Relief Options
The numbing medication used in nerve blocks also matters. Standard formulations wear off within several hours, but a longer-acting version encapsulates the medication in tiny particles that release it gradually. In a recent Phase 3 clinical trial, patients who received this extended-release formulation through an adductor canal block had lower pain scores and used 23% less opioid medication over the first 96 hours after surgery. They also went longer before needing their first dose of opioid pain medication (about 4.2 hours vs. 3.6 hours).
These differences may sound modest, but reducing opioid use in the days after surgery can meaningfully lower the risk of side effects like nausea, constipation, and excessive drowsiness, all of which slow your recovery.
Epidural Anesthesia
Epidural anesthesia is a third option you may hear about. Like spinal anesthesia, it involves an injection in your lower back, but the needle targets a slightly different space and typically leaves a thin catheter in place. This allows continuous delivery of numbing medication during and after surgery. Epidurals are less commonly chosen for knee replacement than spinal anesthesia because they take longer to set up and the catheter can limit early mobility. However, they remain an option in certain situations, particularly when prolonged postoperative pain control through the catheter is part of the surgical team’s plan.
What to Expect on Surgery Day
If you receive spinal anesthesia, you’ll sit or lie on your side while the anesthesiologist numbs the skin on your lower back, then places the spinal needle. The injection itself takes only a few minutes. Within 5 to 10 minutes your legs will feel warm, heavy, and then numb. Sedation through your IV will start around the same time, and most people remember very little of the actual procedure.
If your team uses general anesthesia, medication flows through your IV and you’ll fall asleep within seconds. A breathing tube is placed after you’re unconscious and removed before you fully wake up. Either way, your nerve block is typically placed before or immediately after the operation while you’re still sedated.
One side effect worth knowing about with spinal anesthesia is temporary difficulty urinating afterward. The numbing medication can affect bladder function, and the rate of urinary retention depends on which specific drug is used. With the most common long-acting option, about 9% of patients need a temporary catheter. Shorter-acting spinal medications bring that rate down to 1.5% to 2.8%, though they may not last as long through the surgery.
Your anesthesia team will weigh all of these factors, your medical history, medications, and preferences, to recommend the best combination for your specific situation. Knowing the options ahead of time helps you ask the right questions at your pre-surgical appointment.