The posterior superior alveolar nerve is a branch of the maxillary nerve that supplies sensation to most of the upper back teeth, the surrounding gum tissue, and part of the lining of the maxillary sinus. If you have ever had a dental injection for work on an upper molar, this is one of the nerves your dentist was trying to numb. It sits in an anatomically busy neighborhood, running alongside an artery of the same name through a bony canal in the wall of the maxillary sinus, and that proximity to blood vessels, sinuses, and even the eye socket is what makes it both clinically important and occasionally tricky to anesthetize safely.
Where the Nerve Runs and What It Supplies
The posterior superior alveolar nerve, often shortened to PSA or PSAN, branches off the maxillary division of the trigeminal nerve just before it enters the infraorbital canal. From there it drops downward along the back wall of the maxilla, the bone that forms your upper jaw. As it descends, it travels through or alongside a bony canal in the lateral wall of the maxillary sinus. Imaging studies using three-dimensional CT scans have found that this canal appears as a tunnel-shaped structure in roughly 60 percent of patients, while dissection of dry skulls shows a tunnel format in about 65 percent of cases, with the remaining specimens showing either a fragmented canal or an open groove.
Once inside the bone, the nerve fans out into smaller branches that reach the roots of the upper molars and, in many people, the two roots of the upper second premolar that face toward the cheek. It also sends fibers into the periodontal ligament around those teeth, the buccal gum tissue overlying them, and the mucous membrane lining the back and side walls of the maxillary sinus. A dissection study examining how the first upper molar receives its nerve supply found that the posterior superior alveolar nerve contributed innervation in every single specimen, making it the most consistent source of sensation for the upper back teeth.
Other Nerves That Pitch In
Although the PSA nerve is the workhorse for the upper molars, it does not work alone. The middle superior alveolar nerve, when it exists, picks up some of the territory. That same dissection study found the middle superior alveolar nerve contributing to the first molar’s innervation in only about 28 percent of specimens.1Oral Surgery, Oral Medicine, Oral Pathology. Patterns of innervation of the maxillary first molar: A dissection study A meaningful fraction of people simply do not have a distinct middle superior alveolar nerve at all, which shifts more of the load onto the PSA nerve and its neighbor, the anterior superior alveolar nerve.
More recent research has shown that the greater palatine nerve and the nasopalatine nerve also send branches into the maxillary teeth through small openings on the palatal side of the jaw bone. These palatal alveolar branches supply not only bone and gum tissue but actual tooth structures, working alongside the superior alveolar nerves.2PubMed. The greater palatine nerve and artery both supply the maxillary teeth: An anatomic and radiologic study This overlap explains why a single injection aimed at the PSA nerve sometimes fails to completely numb a tooth: other nerve pathways are still delivering sensation from a different direction.
Perhaps the most surprising finding in this area is that the PSA nerve appears to contribute sensation even to the front teeth. A histological study that traced nerve fibers in three dimensions found that about 16 percent of the nerve bundles running through the canal toward the front of the mouth originated from the PSA nerve, with many of those bundles dispersing as they coursed toward the nasal floor. The researchers concluded that the PSA nerve partially contributes to pain sensation in the anterior teeth.3ScienceDirect. Posterior superior alveolar nerves contribute to sensation in the anterior teeth This is a relatively new observation that challenges the traditional textbook picture of neatly separated territories for the three superior alveolar nerves.
Why Dentists Block This Nerve
The PSA nerve block is a standard technique in dentistry for numbing the upper second and third molars and, in most patients, the buccal root of the upper first molar. The injection is given in the tissue behind and above the last upper molar, directing the needle toward the back surface of the maxilla. The goal is to deposit anesthetic solution near the point where the nerve enters the bone. In a study of 200 patients undergoing this block, positive anesthesia was achieved within five to ten minutes, with no pain reported during or after extraction and no visual complications recorded.4PubMed Central. Simple and safe posterior superior alveolar nerve block
Research comparing the PSA block to local infiltration, where anesthetic is simply injected into the gum tissue next to the tooth, has produced interesting results. One prospective study found that patients who experienced the worst pain intensity during the injection itself were more common in the infiltration group (about 19 percent) than the PSA block group. After extraction, a higher proportion of PSA block patients reported no pain. The authors noted that the PSA block consistently provided pulpal anesthesia to the upper back teeth, even when infection or widely flared palatal roots were present.5International Journal of Drug Delivery Technology. Comparison of Pain Outcome Between Posterior Superior Nerve Block and Infiltration in Multiple Maxillary Posterior Tooth Extraction: A Prospective Study
However, a recent double-blind randomized trial painted a more nuanced picture. It found no significant difference in pain scores between the PSA block and buccal infiltration for molar extractions, with mean pain ratings nearly identical between the two groups. Supplemental anesthesia was needed in roughly 11 percent of the PSA group and 12 percent of the infiltration group. The catch was that complications, including hematoma and trismus, occurred only in the PSA block group at a rate of 4 percent, a statistically significant difference. The researchers concluded that buccal infiltration provides equivalent pain control with fewer complications and may be the preferred choice for routine upper molar extractions.6PubMed. Re-evaluating the role of posterior superior alveolar nerve block versus buccal infiltration for maxillary molar extractions: a double-blind, randomized controlled trial
When Articaine Enters the Picture
The rise of articaine, a newer anesthetic that penetrates bone tissue more readily than older agents like lidocaine, has shifted the conversation about whether a nerve block is even necessary for upper molars. A study comparing buccal infiltration with 4 percent articaine against a PSA nerve block with 2 percent lidocaine for anesthesia of upper second molars found that about 79 percent of the articaine infiltration group reported no pain, compared with roughly 68 percent in the lidocaine nerve block group. Statistically the difference was not significant, but the trend is telling: a simple injection next to the tooth with a bone-penetrating anesthetic may work about as well as a deeper, more technically demanding nerve block with a traditional agent.7PubMed Central. The Efficacy of Buccal Infiltration of 4% Articaine and PSA Injection of 2% Lidocaine on Anesthesia of Maxillary Second Molars
This is one reason you may notice that some dentists skip the PSA block entirely for routine fillings or crowns on upper molars, relying instead on one or two infiltration injections near the tooth. The block remains valuable for cases involving multiple molar extractions or when infection has made local infiltration unreliable, but its role as an everyday technique is being reconsidered.
Complications of the PSA Block
The anatomy behind the upper jaw is what makes the PSA nerve block riskier than a simple infiltration. The posterior superior alveolar artery runs alongside the nerve, and the pterygoid venous plexus sits just behind the maxilla. If the needle nicks one of these vessels, blood can pool in the tissue of the cheek or the infratemporal fossa, the space behind the upper jaw.
A case report described a 20-year-old woman who developed massive swelling immediately after a PSA block, followed by a hematoma that spread into the oral mucosa.8PubMed Central. Edema and hematoma after local anesthesia via posterior superior alveolar nerve block: a case report The swelling can look alarming, ballooning rapidly across the cheek, but it typically resolves over days to a couple of weeks with conservative management such as cold compresses and, if needed, antibiotics to prevent secondary infection. Hematomas are probably the most frequently discussed complication of this particular block, and the 4 percent complication rate found in the randomized trial mentioned earlier fits with the general understanding that it is uncommon but not rare.9PubMed. Re-evaluating the role of posterior superior alveolar nerve block versus buccal infiltration for maxillary molar extractions: a double-blind, randomized controlled trial
Double Vision After a Dental Injection
A far rarer and more unsettling complication is diplopia, or double vision, occurring minutes after the injection. The mechanism involves anesthetic solution tracking backward through tissue planes or along blood vessels until it reaches the nerves that control eye movement. Case reports have documented this happening in different ways. In one case, a patient could not gaze laterally with the eye on the same side as the injection, recovering completely once the anesthetic wore off.10PubMed Central. Diplopia with local anesthesia In another, the anesthetic affected the fourth cranial nerve, causing temporary paralysis of the muscle that rotates the eye downward and inward, leading to vertical double vision.11PubMed. Ocular complications after posterior superior alveolar nerve block: a case of trochlear nerve palsy
These episodes are transient. They clear up as the anesthetic metabolizes, usually within a few hours. But they are genuinely frightening for a patient sitting in a dental chair who suddenly cannot see properly. A review of such cases emphasizes that recognition is the key: the dentist needs to understand that this is a known, self-limiting complication rather than a sign of something more serious like a stroke.12PubMed Central. Diplopia following posterior superior alveolar nerve block: a case report and review of literature Standard practice is to patch the affected eye to eliminate the double vision, reassure the patient, and wait for the effect to resolve. No dental procedure needs to be canceled because of it, though the patient should not drive home.
The Nerve and the Maxillary Sinus
The bony canal carrying the PSA nerve and artery is embedded in the lateral wall of the maxillary sinus, and this relationship becomes critically important during sinus lift surgery. A sinus lift is a procedure done to add bone to the upper jaw before placing dental implants, and it involves creating a window in the lateral wall of the sinus. If the surgeon cuts through the canal, bleeding from the posterior superior alveolar artery can complicate the procedure. Bleeding from this artery’s connection with the infraorbital artery has been described as the second most frequent intraoperative complication during sinus lifts.13PubMed Central. Alveolar antral artery isolation during sinus lift procedure with the double window technique
That said, a meta-analysis looking at the anatomic characteristics of the posterior superior alveolar artery concluded that severe hemorrhage from damage to this vessel during sinus augmentation is not a substantial clinical problem overall.14PubMed Central. Anatomic evaluation of the posterior superior alveolar artery using cone-beam computed tomography: A systematic review and meta-analysis The artery is small, and when bleeding does occur, it can usually be controlled with pressure, bone wax, or electrocautery. The risk is less about catastrophic blood loss and more about the bleeding obscuring the surgical field, which can make the delicate work of lifting the sinus membrane harder.
Pre-surgical imaging with cone-beam CT scans has become standard for mapping the canal’s exact position before a sinus lift. Knowing where the nerve and artery sit lets the surgeon design the bony window to avoid them. In the CT study described earlier, the canal was visible as a distinct tunnel in about 60 percent of scans, meaning that in a substantial minority it is either too small to see clearly on imaging or takes a less predictable path.15PubMed Central. Identification of the Bony Canal of the Posterior Superior Alveolar Nerve and Artery in the Maxillary Sinus: Tomographic, Radiographic, and Macroscopic Analyses
Anatomical Variation and Cleft Lip and Palate
The canal’s position and size are not the same in everyone. In a typical adult without craniofacial anomalies, the canal runs through the lower third of the maxillary sinus wall and has an average diameter of about 0.6 mm. But in patients with cleft lip and palate, the anatomy shifts substantially. A multicenter study comparing 150 cleft patients to 150 controls found the canal visible in all cases on both sides, but in cleft patients the canal sat higher, in the middle to upper third of the sinus wall, and its diameter averaged about 1.1 mm, nearly double the control group.16PubMed. Cleft lip and palate cause large variations in size and location of the posterior superior alveolar canal These differences matter because cleft patients often undergo multiple jaw surgeries, and a canal that is both larger and higher up than expected puts the nerve and artery at greater risk during procedures that would be straightforward in a patient with typical anatomy.
Even outside the cleft population, individual variation is wide enough that assuming a “standard” position for this nerve is unwise. Factors like age, whether teeth are present or missing, and even which side of the face you are looking at can influence where the canal sits. This is why retrospective imaging studies continue to catalog the canal’s position across large patient samples, building a reference database that surgeons can use when planning implant placement and sinus surgery.17University of Manitoba Repository. Prevalence, Position and Diameter of The Posterior Superior Alveolar Neurovascular Bundle on Cone-Beam Computed Tomography Scans – A Retrospective Study
Computer-Controlled Delivery and Reducing Injection Pain
One area of active development is the use of computer-controlled local anesthetic delivery devices, sometimes called CCLAD systems. These machines control the flow rate and pressure of the anesthetic solution more precisely than a human hand on a syringe can. When evaluated for blocks including the PSA nerve block, studies have found modestly lower pain scores during drug administration with CCLAD compared to traditional syringes, with mean visual analog scale scores around 11 for CCLAD versus about 15 for the traditional technique. Pain immediately after injection and during the dental procedure itself also trended lower with CCLAD.18ScienceDirect. Computer-Controlled Local Anaesthesia Delivery efficacy − a literature review The differences are not dramatic, but for patients with dental anxiety, even a small reduction in injection discomfort can make a meaningful difference in their willingness to accept treatment.
These devices are most often marketed under brand names and look somewhat like oversized pens. They are not yet standard equipment in every dental office, partly because of cost and partly because many dentists feel their hand-injection technique is already good enough. But for nerve blocks in anatomically sensitive areas like the PSA region, where controlling the speed of injection may also reduce the risk of accidentally pushing solution into a blood vessel, the technology has a reasonable rationale beyond just patient comfort.
When Numbness Does Not Work as Expected
Failed or incomplete anesthesia on upper molars is one of the most common frustrations in dental practice, and understanding the PSA nerve helps explain why. The supplemental innervation from palatine and nasopalatine branches means that even a perfectly placed PSA block may leave some pathways intact. When a patient says “I can still feel that” despite a good block, the sensation is often coming from the palatal side, and a supplemental injection on the palate may be needed.
Infection presents another challenge. Acidic tissue around an abscess can inactivate the anesthetic before it reaches the nerve fibers, and inflamed tissue may be hypersensitive, requiring more profound nerve blockade than a healthy tooth would. The PSA block, because it deposits anesthetic at a distance from the infected site, can sometimes succeed where local infiltration fails. This is one of the clinical situations where the block still clearly outperforms infiltration despite the general trend toward simpler techniques for routine cases.
Tooth anatomy matters too. Upper first molars sometimes receive innervation from the middle superior alveolar nerve on their front-facing root, which the PSA block does not reliably reach. And wisdom teeth, especially those that are impacted and sitting deep in the bone, may have nerve supply patterns that differ from fully erupted molars. In these situations, dentists often combine the PSA block with infiltration injections at adjacent sites to ensure complete coverage, a strategy that trades simplicity for reliability.