The risk of foraminal violation and nerve root impingement after anterior placement of lumbar interbody fusion cages

Study Design. Three groups of six embalmed cadaver spines underwent placeme nt of lumbar interbody fusion cages centered either at midline, 10% lateral of midline, or 20% lateral of midline. The spines were evaluated for evide nce of neuroforamen violation or nerve root impingement. Objectives. To determine the potential for foraminal violation or nerve roo t impingement after correct placement and lateral misplacement of lumbar in terbody fusion cages. Summary of Background Data. Radicular symptoms after anterior cage placement have raised some concern about the potential for in advertent device-related foraminal violation not adequately appreciated by intraoperative fluoroscopy. Methods. Preoperative computed tomography scanning and plain radiography wa s used to measure endplate dimensions at L4-L5 and to template the appropri ately sized interbody fusion cages. The cadaveric specimens were randomly d ivided into three groups of six (Groups I-III) and instrumented at L4-L5 ei ther at midline (I) or 10% (II) or 20% (III) lateral of midline. Postoperat ive computed tomography and plain radiography was evaluated for evidence of neuroforamen violation, followed by dissection of the specimens. Results. Foraminal violation occurred in one of six spines in group II (10% off midline) and in three of six spines in group III (20% off midline). Tw o of the three cadavers in group III with foraminal violation also were not ed to have nerve root abutment on computed tomography scans and spinal diss ection. Conclusions. Excessive lateral placement of lumbar interbody fusion cages m ay result in foraminal violation and possible nerve encroachment. The "safe zone" for centering the cages extends approximately 5 mm on either side of midline.