Certain Foraminal Morphologies Are Associated with Indirect Decompression Failure in the Lumbar Spine: A Systematic Review and a Novel Classification

Introduction: Indirect decompression is a powerful technique that allows for central canal as well as foraminal decompression, all while avoiding bony resection and neural element manipulation. However, there remain instances when these techniques fail, and may even exacerbate symptoms resulting in unplanned surgeries. Numerous attempts have been made to help predict which patients will fail, but these remain imperfect. One understudied factor is the morphology of the intervertebral foramen. Indirect decompression works by distracting the disc space and increasing segmental lordosis. We hypothesized that some failure cases may result from foraminal morphologies whose cross-sectional areas may be compromised by these maneuvers. We therefore set out to collect all such cases of unfavorable foraminal morphologies felt to underlie indirect decompression failure. We aim to incorporate them into a classification system that allows a systematic preoperative evaluation to assess candidacy for indirect decompression.

Methods: A systematic review of PubMed, EMBASE, and the Cochrane database was conducted. We included all cases of failed indirect decompression secondary to abnormal foraminal morphology. Cases with failed indirect decompression due to other complications were excluded.

Results: Our review included thirteen studies describing 22 patients who have failed secondary to abnormal foraminal configurations. These cases displayed four distinct imaging patterns: 1. The prominent posterior inferior osteophyte variant, 2. The osseous ring variant, 3. The hypertrophic superior articular process variant, 4. The foramen crowded by disco-ligamentous material. (see Figure1)

Conclusion : We present our classification method to the reader as a simple, intuitive method to evaluate the foramen pre-operatively. Each of these variants is presented with cases in the literature implicating them in failed indirect decompression. Future studies will help better refine choice of surgical approach for each.