During a posterior superior alveolar nerve block, the needle is inserted near the posterior aspect of the maxilla to reach the nerve branches. The injection site lies close to the infratemporal fossa, a deep space located posterior to the maxilla. If aseptic technique is compromised, infection can be introduced directly into this anatomical space. The infratemporal fossa communicates with several vital spaces, increasing the risk of spread of infection. Thus, the location of the needle tip during this block predisposes the patient to infection in the infratemporal fossa rather than other adjacent areas such as the maxillary sinus or pterygopalatine fossa.

Reference: Oral and Maxillofacial Pathology, Neville et al., 4th Edition.