Most joint replacements can be done under a spinal or a general anesthetic, and the choice is made with the anesthesiologist.

The kinds of anesthesia, and what each involves

The American Academy of Orthopaedic Surgeons describes three broad categories: local, regional, and general ¹. Local anesthesia numbs only the area treated and, for a joint replacement, is used alongside the main anesthetic rather than instead of it ¹.

Regional anesthesia blocks the nerves to part of the body without affecting the brain or breathing; because the patient stays conscious, sedatives are given to relax them into a light sleep ¹. Three forms are used in joint replacement. A spinal block is a single injection into the fluid around the spinal cord in the lower back, whose numbing effect wears off after roughly one and a half to six hours ¹. An epidural block uses a thin catheter in the lower back to deliver anesthetic continuously for as long as it is needed ¹. A peripheral nerve block places anesthetic around the major nerves of the thigh and numbs only the operated leg, either as a single injection or through a catheter that can stay for several days after surgery ¹.

General anesthesia acts on the brain and nervous system and makes the patient temporarily unconscious; the anesthesiologist gives it by injection or inhalation and places a breathing tube ¹. The American Association of Hip and Knee Surgeons describes the same two main categories, and notes that under a spinal the legs are numb and cannot be moved during the operation ².

What the comparisons found

The largest synthesis is the 2019 International Consensus on Anaesthesia-Related Outcomes after Surgery, which reviewed 94 studies comparing neuraxial anesthesia, meaning spinal or epidural, with general anesthesia for hip and knee replacement ³. Neuraxial anesthesia was associated with lower odds or no difference in virtually every reported complication, with urinary retention the one exception ³. For hip replacement the odds of death were 0.67 relative to general anesthesia; for both joints the odds of pulmonary complications, kidney failure, deep vein thrombosis, infection and transfusion were lower ³. The group recommended neuraxial over general anesthesia for hip replacement as a strong recommendation on moderate-to-low evidence, and for knee replacement as a weak recommendation on low evidence ³.

A 2016 systematic review restricted to randomized and prospective comparisons, 29 studies and 10,488 patients, was more cautious. Neuraxial anesthesia shortened the hospital stay by 0.4 days on average, but the review found no statistically significant difference in mortality, surgical site or chest infection, nerve injury, nausea and vomiting, or blood clots when clot prophylaxis was used ⁴. The authors concluded that the two are equally effective, with limited evidence that neuraxial anesthesia improves outcomes ⁴.

A national database study of 382,236 hip and knee replacements between 2006 and 2010 supplies the population picture. In that period 11 percent were done under neuraxial anesthesia, 14 percent under a combination, and 75 percent under general ⁵. Thirty-day mortality was 0.10 percent with neuraxial, 0.10 percent with combined, and 0.18 percent with general anesthesia, and neuraxial anesthesia carried the most favorable complication profile after adjustment ⁵. The differences are small in absolute terms and the study is observational, which is why the consensus group graded its evidence as it did.

Nerve blocks for the pain after surgery

Whatever the main anesthetic, pain control after the operation often uses a peripheral nerve block. The association lists the blocks used for knee replacement, including femoral and adductor canal blocks and the newer IPACK block behind the knee, and describes their purpose as reducing pain while preserving enough muscle function for a patient to walk early ². The academy notes that an epidural or nerve-block catheter can be left in place after surgery to continue pain control, with the patient often able to adjust the dose within preset limits and monitored for over-sedation and falls ¹.

Why general anesthesia is still sometimes chosen

The academy gives two reasons: the preference of the patient, surgeon or anesthesiologist, and a patient who cannot receive regional anesthesia ¹. The association names the situations that make a spinal difficult or unwise: previous back surgery or known arthritis of the spine can make the injection hard to place, and bleeding disorders can rule it out ².

Each option has its side effects. The academy lists headache, trouble urinating, allergic reaction and rare nerve injury for regional anesthesia; and for general, a sore throat and hoarseness from the breathing tube, headache, nausea and drowsiness, with a small risk of a serious complication such as heart attack or stroke that rises with existing heart or lung disease ¹.

• Spinal or epidural: awake or lightly sedated; breathing on your own; the reviews find equal or better outcomes ¹³⁴.
• General: unconscious with a breathing tube; chosen by preference or when a spinal is not possible ¹².
• A nerve block for the days after surgery is common with either ¹².

What to ask your anesthesiologist and surgeon

• Which anesthetic do you plan for me, and why that one?
• Have I anything in my history, including back surgery, spinal arthritis or a bleeding problem, that changes the choice?
• Will I have a nerve block, and how long will the leg stay numb?
• How will my pain be managed in the first days, and what is the plan for coming off opioids?

The academy calls the selection of anesthesia a major decision that deserves careful discussion with both your surgeon and your anesthesiologist ¹. The evidence here describes populations; the choice for you is made from your health, your history and your preferences.