Spinal anesthesia involves a single injection of solution in the intrathecal space. Most commonly, the solution contains a combination of long-acting local anesthetic (bupivacaine) along with an opioid (e.g., fentanyl, sufentanil, morphine, or hydromophone). Opioids can enhance the duration and quality of a surgical block and provide postoperative analgesia after the local anesthetic effect has subsided. Hydrophilic opioids (morphine and hydromorphone) have a longer duration of action, as they do not easily traverse from the intrathecal space through the blood–brain barrier for subsequent metabolism. Fentanyl is a hydrophobic opioid that will enhance the duration of action of an intrathecal injection; however, it does not afford the same duration of postoperative analgesia. Some anesthesiologists also add an alpha-2 agonist (e.g., clonidine) to enhance the duration and quality of the spinal anesthetic (8).

The onset of action of intrathecal local anesthetic with or without opioids is fast with a dense anesthetic block from approximately the T4 dermatome (nipple line) down within minutes after injection. The sympathectomy that occurs after intrathecal anesthesia can cause a profound hypotension, which can be a concern in patients with cardiac or cerebrovascular disease. However, if the hypotension is controlled, it does provide controlled hypotensive anesthesia that may be beneficial for THA in terms of blood loss. Epidural hematoma is a rare complication of neuraxial anesthesia; however, the smaller needle used is generally thought to cause less damage and be associated with a lower risk. Most anesthesiologists currently use a pencil point tipped spinal needle that is associated with a very low incidence of postdural puncture (spinal) headache; however, cases are still reported. Spinal headache can either be managed symptomatically by having the patient lay flat, hydrate, and drink caffeine or more with an epidural blood patch.

The epidural space is a potential space posterior to the dura mater that is composed of spinal nerve roots, connective tissue, blood vessels, and fat. A small epidural catheter can be placed in this space to infuse local anesthetic solutions for operative anesthesia and postoperative analgesia. For THA, catheters are normally placed in the middle to lower lumbar interlaminar spaces (e.g., L3–L4 or L4–L5). After placement of the catheter, a local anesthetic solution—often with epinephrine—is injected through the catheter to obtain a surgical level of anesthesia. Bupivacaine and ropivacaine can be used for anesthesia; however, some clinicians prefer lidocaine as the dense anesthetic will resolve more quickly. Lidocaine is also more predictable and therefore could be a great asset in minimally invasive hip arthroplasty with an intent of rapid discharge from the hospital by avoiding prolonged motor blockade. As with spinal anesthesia, patients will have anesthesia from the high thoracic dermatomes down.

Many of the potential adverse effects of spinal anesthesia are true for epidural anesthesia as well, including hypotension and postdural puncture headaches. The advantages of the indwelling epidural catheter for postoperative pain relief do come with some additional risks. The most feared complication of neuraxial anesthesia is epidural hematoma. Whereas it would seem that the removal of an existing epidural catheter would be associated with less bleeding risk than the insertion of the epidural needle and catheter placement, the removal of an epidural catheter can cause epidural bleeding and should therefore be treated the same as placement for anticoagulation management. However, recently a study examined the INR at removal of the epidural catheter and determined varying INR in 10,000 patients with no complications (9) and to be safe. Most orthopedic surgeons will begin anticoagulation in the immediate postoperative course, and the catheter should be removed at that time (with the exception of prophylactic heparin or enoxaparin). The collaboration between the orthopedic surgeon and the perioperative anesthesiologist is critical to determine the optimal risk–benefit ratio and formulate a protocol that can encompass the majority of the patients.

Given the higher complication rates for lumbar plexus blockade and challenges associated with placing lumbar plexus catheters, some studies have investigated the potential use of femoral nerve blockade as an alternative means of perioperative analgesia. In a randomized, controlled study of THA, Ilfeld et al. (17) found equivalent pain scores the day after surgery when patients receiving continuous femoral nerve local anesthesia (n = 25) were compared with lumbar plexus infusions (n = 22). Patients described equal satisfaction and used similar amounts of secondary analgesics; however, patients with femoral nerve catheters ambulated for a significantly shorter distance on the first postoperative day. Further study is needed to better understand the potential use of femoral nerve catheters in THA. Although the associated risks for femoral nerve catheters are less than that with lumbar plexus catheters, the risk of patient falls remains a concern (18).

Clinical equipoise is defined as a state of genuine uncertainty as to the advantages or disadvantages of each therapeutic arm. The choice of primary anesthetic has been a long-standing debate, as prospective studies are generally underpowered to detect a difference in short- and long-term outcomes. The time and cost of such randomized trials is a limiting factor, and it is genuinely impossible to blind patients, surgeons, nurses, and anesthesiologists to the primary anesthetic provided. Large datasets and meta-analyses are beginning to shed light on the acute perioperative effects of the choice of primary anesthetic. A meta-analysis of 10 randomized trials comparing neuraxial to general anesthesia for THA showed significant acute perioperative benefits to neuraxial anesthesia, including decreased risk for deep venous thrombosis and pulmonary embolism, decreased blood loss, and transfusion risk (19). In a series of publications from a multicentered, large, administrative dataset, Memtsoudis et al. have studied patient trends, acute perioperative outcomes, and mortality following lower extremity arthroplasty (7,20,21,22,23,24,25,26,27,28,29,30). One study demonstrated that patients receiving neuraxial anesthesia were less likely to require critical care services after arthroplasty (29). Neuraxial anesthesia was also shown to be associated with lower adjusted odds of mortality (odds ratio [OR] = 0.71) and pulmonary complications (OR = 0.75) in hip fracture patients (31).

There are some adverse effects that can be, at least in part, attributed to the choice of primary anesthetic independent of other surgical and perioperative factors.