Postoperative Fluid Collections after Lumbar Spine Surgery: Differential Diagnosis and Surgical Considerations.

Postoperative fluid collections are commonly encountered at imaging after lumbar spine surgery. While small collections usually resolve on their own, larger or more complex ones have a limited differential diagnosis. Varied surgical approaches to the lumbar spine may predispose patients to different types of fluid collections, including seromas, hematomas, pseudomeningoceles, abscesses, bone morphogenetic protein (BMP)-related collections, lymphoceles, and urinomas; thus, radiologists should be aware of the different types of anterior and posterior surgical approaches to lumbar spine interbody fusion. Hematomas and abscesses may occur with any surgical approach. Pseudomeningoceles most frequently result after posterior approach surgery, while urinomas and lymphoceles more likely develop after anterior approach surgery due to the close proximity of the surgical corridor to the nephroureteral system or lymphatics, respectively. Surgical implants or biologics used at surgery can lead to BMP-related collections; awareness of any implanted materials intraoperatively can be helpful at the time of interpretation. While MRI is most frequently used for identifying fluid collections in the postoperative spine, CT can serve as an important adjunct. CT myelography can be used to confirm or exclude the presence of a pseudomeningocele but may not be necessary if MRI shows a confirmatory flow jet, confirming leakage of cerebrospinal fluid into the pseudomeningocele. CT enhanced with intravenous contrast material during the excretory phase also allows confirmation of the presence of a urinoma. When the contents of a postoperative fluid collection remain uncertain, aspiration may be necessary. Accurate diagnosis of the type of postoperative collection is extremely useful in guiding patient management and determining prognosis. ^©RSNA, 2025.